GIFT OF
CYCLOPEDIC SCIENCE SIMPLIFIED.
SPECTRUM ANALYSIS,
Exhibited at the^Royal Polytechnic Institution, London.
i. SIMPLE SPECTRUM. 2. SILVER. 3. THALLIUM.
CYCLOPEDIC SCIENCE
SIMPLIFIED.
j. H. PEPPER,
Of the Royal Polytechnic Institution, Fellow of the Chemical Society, Associate of the Institution
of Civil Engineers ; Author of -various IVorks for Youth, &c.
EMBRACING
LIGHT
REFLECTION AND REFRACTION OF LIGHT
LIGHT AND COLOUR
SPECTRUM ANALYSIS
THE HUMAN EYE
POLARIZED LIGHT
HEAT
THERMOMETRIC HEAT
CONDUCTION OF HEAT
LATENT HEAT
STEAM
ELECTRICITY
VOLTAIC, GALVANIC, OR DYNAMICAL
ELECTRICITY
MAGNETISM
ELECTRO-MAGNETISM, MAGNETO-ELEC .
TRICITY, THERMO-ELECTRICITY
DIA-MAGNETISM
WHEATSTONE'S TELEGRAPHS
PNEUMATICS
THE AIR-PUMP
THE DIVING-BELL
ACOUSTICS
THE EDUCATION OF THE EAR
CHEMISTRY
ELEMENTS WHICH ARK NOT METALLIC
THE METALS
WITH NUMEROUS ILLUSTRATIONS.
BE7ISFD EDIT70JJT,
FREDERICK WARNE AND CO.,
BEDFORD STREET, COVENT GARDEN.
NEW YORK : SCRIBNER, WELFORD, AND ARMSTRONG.
, ,
INTRODUCTION.
T N the Author's earlier works, written only for the youthful student
* in Science, a promise was made that other books, to be regarded
as a series of steps in Science, should be forthcoming.
It is with this view that the present volume is offered; and as
the general reader in fact, "the Public" has not the time or the
inclination to study the very minute and laborious works of GMELIN,
WATT, MILLER, and other learned authors, it is hoped that the facts
contained in this more advanced but still elementary work will be
found sufficiently attractive to stimulate, at all events, the would-be
philosopher to further reading, and especially to perform correct scien-
tific experiments.
Brevity and simplicity have been carefully attended to in the
following pages; and when other authors are quoted, the writer has
preferred giving their exact language, instead of altering and para-
phrasing words, which frequently detracts from the sense of the
passage.
The reader will find portions of valuable papers written by
256328
vi INTRODUCTION.
FARADAY, DANIELL, WHEATSTONE, BREWSTER, TYNDALL, CROOKES,
BROWNING, SIEMENS, NOAD, STEWART, TAIT, MARLOYE, and others,
with a brief summary of Photography by JOHN SPILLER, Esq.
In a work like this, including such a multiplicity of subjects, the
kind indulgence of the reader is invoked for any errors that the most
painstaking supervision may have permitted to pass.
I dedicate this work, with all kindly feelings, to those
students at Harrow, . Eton, Hayleybury, and Cheam, to whom,
under the auspices of the Rev. Drs. Vaughan, v Goodford, Hawtrey,
Butler, Bradby, and Tabor, I have addressed many lectures on
Science.
I believe and trust that those lectures have not been alto-
gether unfruitful ; but that, they have aided in the establishment
of regular science classes for the present generation, instead of the
desultory lectures at rare intervals to which custom formerly con-
demned the teachers of popular science.
JOHN HENRY PEPPER.
CONTENTS.
LIGHT.
Page
LIGHT, AND THE ETHER SUPPOSED TO
PERVADE THE WHOLE UNIVERSE I
CORPUSCULAR THEORY OF LIGHT 2
EXPERIMENTS WITH BLACKED ALUMINIUM
Disc 4
SOURCES OF LIGHT 8
HEAT A SOURCE OF LIGHT 10
LIGHT THE FREQUENT ATTENDANT OF
ELECTRICAL PHENOMENA 10
CHEMICAL COMBINATION A SOURCE OF
LIGHT n
Is MECHANICAL FORCE TO BE REGARDED
AS A TRUE SOURCE OF LIGHT? 12
THE DIFFUSION OF LIGHT 14
MODIFICATIONS THAT LIGHT MAY UN-
DERGO 20
THE REFLECTION OF LIGHT 21
THE GHOST ILLUSION 23
IMAGES FORMED BY SILVERED MIRRORS 27
THE KALEIDOSCOPE 31
THE JAPANESE MAGIC MIRROR 35
BROWNING'S DESCRIPTION OF THE SIL-
VERED GLASS REFLECTING TELE-
SCOPES 41
THE REFRACTION OF LIGHT 49
DIOPTRICS 49
REFRACTION OF LIGHT THROUGH PLANE
GLASS 53
REFRACTION OF PARALLEL RAYS OF
LIGHT BY CONVEX SURFACES 53
REFRACTION OF PARALLEL RAYS BY
CONCAVE SURFACES 54
OTHER FORMS OF LENSES.... . 54
L I G H T -continued.
Page
OPTICAL INSTRUMENTS WHOSE
PROPERTIES DEPEND ON RE-
FRACTION" 55
THE SIMPLE AND COMPOUND MICRO-
SCOPE AND TELESCOPE 55
THE CAMERA OBSCURA 57
THE HUMAN EYE 64
THE STEREOSCOPE 68
PROFESSOR WHEATSTONE'S REFLECTING
STEREOSCOPE 68
DIRECTIONS FOR USING THE STEREO-
SCOPE 71
PERSISTENCE OF VISION 7 i
LIGHT AND COLOUR 86
SPECTRUM ANALYSIS 86
ABERRATION AND ACHROMATISM 86
PHYSICAL PROPERTIES OF THE SPEC-
TRUM 91
THE DARK OR FIXED LINES IN THE
SOLAR SPECTRUM 92
HOW TO USE THE SPECTROSCOPE 95
SPHERICAL ABERRATION 103
THE DISPERSION OF LIGHT, OR CHRO-
MATIC ABERRATION 104
THE INTERFERENCE OF LIGHT... 106
COLOURS OF THIN PLATES 106
DOUBLE REFRACTION AND THE
POLARIZATION OF LIGHT in
POLARIZATION BY REFLECTION AND BY
SIMPLE REFRACTION 114
POLARIZATION BY THE TOURMALINE 116
CONTENTS.
HEAT.
Page
THERMOMETRIC HEAT 123
THE COMMON EFFECTS OF HEAT 127
AMOUNT OF EXPANSION IN SOLIDS,
LIQUIDS, AND GASES 130
THE EXPANSION OF LIQUIDS 132
THE THERMOMETER 135
THE PYROMETER 139
THE EXPANSION OF GASES 142
CONDUCTION 147
"POTENTIAL" FORCE 151
"ACTUAL" FORCE, OR "ENERGY" 151
LATENT HEAT .. ... 160
H E A ^continued.
Page
ENERGY OR HEAT 161
CAPACITY FOR HEAT 164
STEAM 172
THE STEAM ENGINE 179
DESCRIPTION OF THE STEAM ENGINE ... 182
EVAPORATION 193
HYGROMETRY 194
RADIATION 196
TRANSMISSION OF HEAT 200
THE CONVERSION OF LIGHT RAYS INTO
HEAT RAYS, AND VICE VERSA, BY
CHANGE OF RfiFRANGIBILITY 2O2
ELECTRICITY.
ELECTRICITY, FRICTIONAL OR
STATICAL
THE ELECTROSCOPE....
THEORIES OF ELECTRICITY 213
EXPERIMENTS WITH THE ELEC-
TROSCOPE 214
ELECTRICAL MACHINES 220
ELECTRICAL ATTRACTION AND
REPULSION GOVERNED BY
CERTAIN LAWS 227
THE ELECTRIC WELL 234
ELECTRICAL INDUCTION 236
THE ELECTROPHORUS 245
THE LEYDEN JAR 247
EXPERIMENTS WITH THE ELECTRICAL
MACHINE, THE LEYDEN JAR, AND
LE.YDEN BATTERY 255
ELECTRICIT "Y continued.
THE HYDRO-ELECTRIC MACHINE 272
SUMMARY OF THE LAWS OF ELEC-
TRICAL ACCUMULATION 279
LATERAL DISCHARGE 284
VOLTAIC, GALVANIC, OR DYNAMI- .
CAL ELECTRICITY 285
DYNAMICAL ELECTRICAL PHENOMENA
OBTAINED FROM THE VOLTAIC BAT-
TERY 311
"FARADAY'S RESEARCHES" 315
ON A NEW MEASURER OF VOLTA-LEC-
TRICITY ; 318
OHM'S LAW 326
THE RHEOSTAT OF WHEATSTONE 329
THE CALORIFIC EFFECTS OF THE VOLTAIC
CURRENT 335
THE ELECTRIC TORPEDO 341
THE ELECTRIC LAMP 344
MAGNETISM.
THE MAGNET 349
DIA-MAGNETISM 364
ELECTRO-MAGNETISM 3 7 c
MAGNETIS TH- continued.
MAGNETO-ELECTRICITY 378
INDUCTION BY CURRENT ELEC-
TRICITY 378
CONTENTS.
XI
MAGNETIS ^.-continued.
THERMO-ELECTRICITY ....
Page
.. 389
WHEATSTONE'S TELEGRAPHS 392
IMPROVEMENTS IN ELECTRIC TELE-
GRAPHS, AND IN APPARATUS CON-
NECTED THEREWITH 407
MAGNETIS TUL-coutmued.
Page
SIR CHARLES WHEATSTONE'S LAST
TELEGRAPHIC APPARATUS 413
THE ATLANTIC TELEGRAPH CABLE 421
THE DIFFERENTIAL RESISTANCE MEA-
SURER 423
ON THE CONSERVATION OF FORCE 430
PNEUMATICS.
PNEUMATICS
THE AIR-PUMP
THE DIVING-BELL
433
434
442
EXPERIMENTS WITH THE AIR-
PUMP ...
PNEUMATICS
THE BAROMETER
ADMIRAL FITZROY'S WEATHER GUIDE.
WATER-PUMPS
THE PNEUMATIC LEVER
450
452
459
467
ACOUSTICS.
ACOUSTICS 472
MARLOYE'S INTRODUCTION TO CHEVAL-
LIER'S CATALOGUE 474
ON THE EDUCATION OF THE EAR 480
CONSIDERATIONS ON SOUND 484
PROJECT OF STUDY CONCERNING THE
ACOUSTICS OF PUBLIC BUILDINGS ... 489
"ON THE SOUNDS PRODUCED BY FLAME
IN TUBES, &c." 502
VIBRATIONS OF STRINGS, RODS,
PLATES, AND COLUMNS OF AIR 506
LONGITUDINAL VIBRATIONS OF STRINGS 507
LONGITUDINAL VIBRATIONS OF RODS ... 508
VIBRATING PLATES 509
ACOUSTIC ^continued.
TRANSVERSE VIBRATIONS OF BLADES AND
RODS 509
LONGITUDINAL VIBRATIONS OF COLUMNS
OF AIR 510
EMBOUCHURES 512
THE REFLECTION, REFRACTION,
&c., OF SOUNDS
THE TRANSMISSION OF SOUNDS
THROUGH GASEOUS, LIQUID,
AND SOLID MEDIA
TRANSMISSION OF SOUND THROUGH
LIQUIDS 520
TRANSMISSION OF SOUND THROUGH
SOLID CONDUCTORS 520
513
.... 517
CHEMISTRY.
CHEMISTRY 527
OLEOGRAPHY : BEING A PROCESS FOR THE
UTILIZATION OF TOMLINSON'S CO-
HESION FIGURES 531
CHEMISTR Y continued.
EXHIBITING COHESION FIGURES TO A
LECTURE AUDIENCE 533
CHEMICAL ACTION ... ... 537
Xll
CONTENTS.
CHEMISTR Y -continued.
Page
NOMENCLATURE 539
ELEMENTS WHICH ARE NOT ME-
TALLIC 544
OXYGEN .' 544
THE PROPERTIES OF OXYGEN 548
REMARKS 549
OZONE 549
NITROGEN 55i
HYDROGEN 552
NITROGEN AND HYDROGEN, AMMONIA... 566
THE HALOGENS
CHLORINE 568
IODINE 570
THE ART OF PHOTOGRAPHY 571
BROMINE 575
FLUORINE 576
CARBON 577
" ON THE PRESSURE CAVITIES IN TOPAZ,
BERYL, AND DIAMOND, AND THEIR
BEARING ON GEOLOGICAL THEORIES" 581
COMPOUNDS OF CARBON WITH OXYGEN 582
CARBONIC OXIDE 586
COMPOUNDS OF CARBON WITH HYDROGEN 586
BORON 587
SILICON 589
How GEMS ARE MANUFACTURED 592
SELENIUM ". 594
SULPHUR 595
COMPOUNDS OF SULPHUR WITH HYDROGEN 599
PHOSPHORUS 600
RED PHOSPHORUS 605
THE METALS.
TELLURIUM
608
ARSENIC 609
SOURCES OF ARSENIC 609
PHYSICAL QUALITIES OF ARSENIC 609
CHEMICAL PROPERTIES OF ARSENIC 610
ANTIMONY 613
SOURCES WHENCE DERIVED 613
PHYSICAL PROPERTIES OF ANTIMONY ... 614
CHEMICAL PROPERTIES OF ANTIMONY ... 614
BISMUTH ... 614
CHEMISTS, Y continued.
Page
SOURCES WHENCE DERIVED 614
PHYSICAL PROPERTIES OF BISMUTH 615
CHEMICAL PROPERTIES OF BISMUTH 615
CLASSIFICATION OF THE METALS 616
CLASS I.
POTASSIUM 617
SOURCES WHENCE DERIVED 617
PHYSICAL PROPERTIES OF POTASSIUM ... 618
CHEMICAL PROPERTIES OF POTASSIUM... 618
SODIUM 619
SOURCES WHENCE DERIVED 619
PHYSICAL PPOPERTIES OF SODIUM 619
CHEMICAL PROPERTIES OF SODIUM 620
RUBIDIUM 620
CESIUM 620
LITHIUM i 62*
AMMONIUM 621
CLASS II.
CALCIUM
STRONTIUM
BARIUM
ALUMINIUM ,
MAGNESIUM.
CLASS III.
622
622
623
... 623
CLASS IV.
626
ZINC 629
SOURCES WHENCE DERIVED 629
PHYSICAL PROPERTIES OF ZINC 630
CHEMICAL PROPERTIES OF ZINC 6x>
CADMIUM 630
CLASS V.
IRON 631
SOURCES WHENCE DERIVED 631
PHYSICAL PROPERTIES OF IRON 637
CHEMICAL PROPERTIES OF IRON 637
MANGANESE 638
COBALT 6 3 3
NICKEL 639
CHROMIUM 639
URANIUM 639
INDIUM.... ... 6 4 o
CONTENTS.
Xlll
CHEMISTR t continued,
CLASS VI.
TIN ..
Page
640
TITANIUM 640
NIOBIUM 640
TANTALUM 640
CLASS VII.
TUNGSTEN 641
CLASS VIII.
ARSENIC, ANTIMONY, AND BIS-
MUTH 641
CHEMISTS, 'X continued.
CLASS IX.
Page
LEAD 642
THALLIUM 642
CLASS X.
SILVER 6 44
COPPER 648
MERCURY 650
CLASS XL
PLATINUM 650
GOLD 651
How JEWELLERY is MAIJE BY MACHIN-
ERY 652
ORGANIC CHEMISTRY.
ORGANIC ANALYSIS , 657
DR RICHARDSON'S EXPERIMENTS IN OR-
GANIC DECOMPOSITION .. ... 660
ORGANIC CHEMISTRY-r*/zf.
EXPOSURE OF ANIMAL SUBSTANCES TO
WATER GAS AT A HIGH TEMPERA-
TURE 660664
ON LIGHT,
AND THE ETHER SUPPOSED TO PERVADE THE WHOLE
UNIVERSE.
ABOUT two hundred years ago Descartes, Hook, and Huygens, three of
the most celebrated mathematicians of their day, entertained the idea
that light was propagated by the vibrations and undulations of a subtile
elastic fluid called ether, which not only filled infinite space, but was con
tained in all solid, fluid, and gaseous bodies. The immortal Newton, who
was opposed to this theory, or at least created one of his own, usually called
the Corpuscular Theory of Light, appears to have entertained the opinion
(according to Enfield) that " All fixed bodies, when heated beyond a certain
degree, emit light and shine ; and this emission is performed by the vibrating
motion of their parts."
" The heat of a warm room is conveyed through a vacuum by the vibration
of a much subtiler medium than air, which, after the air is drawn out, remains
in the vacuum.
" It is by the vibrations of this medium that light is refracted and reflected,
and heat communicated. This medium is exceedingly more elastic and active,
as well as subtile, than the air ; it readily pervades all bodies, and is by its
elastic force expanded through the heavens. Its density is greater in free and
open space than in compact bodies, and increases as it recedes from them.
This medium, growing densei and denser perpetually as it passes from the
celestial bodies, may, by its elastic force, cause the gravity of those great
bodies towards one another, and of their parts towards the bodies. Vision,
hearing, and animal motion may be performed by the vibrations of this sub-
tile elastic fluid or ether."
\LIGHT.
These opinions would seem to show that Newton believed all emanations
of particles of light were attended by the undulations of an ethereal medium
accompanying it in its passage.
The theory, however, generally ascribed to him is, that rays of light are
small corpuscles emitted with exceeding celerity, travelling at about the rate of
one hundred and eighty-two thousand miles per second ; and these rays of
light, falling upon the eye, excite vibrations in the tunica retina, which, being
propagated along the solid fibres of the optic nerve to the brain, cause the
sense of sight.
Could Newton, who insisted so much on the importance of experimenting
before enunciating a theory, have been acquainted with the highly interesting
experiments connected with the inflection or diffraction of light, he would not
have opposed the notion of an analogy between the phenomena of light and
sound when he says : " The waves, pulses, or vibrations of the air, wherein
sound consists, are manifestly inflected, though not so considerably as the
waves of water ; and sounds are propagated with equal ease through crooked
tubes and through straight lines ; but light was never known to move in any
curve, nor to inflect itself ad umbram" This decided statement is directly
contradicted by actual experiment, because light can be bent into or towards
the shadow.
The corpuscular theory fails to explain that which is easily understood by
the undulatory theory, and by analogy to waves of water or air, that two rays
of light may come together in a special manner and produce darkness, just as
two waves of water may interfere with each other and form a smooth surface,
or two waves of sound produce silence. Dismissing the theory of Newton as
we might pass by the venerable ruins of some ancient edifice, with mingled
interest and regret, we may return to the consideration of the ether supposed
to fill all space.
The great Dr. Franklin, in a letter dated 23rd April, 1752, throws out the
suggestion that all the phenomena of light may be more conveniently solved
by supposing universal space filled with a subtile elastic fluid, which when at
rest is not visible, but whose vibrations affect that fine sense in the eye as
those of air do the grosser organs of the ear.
Thornbury, Mitchell, and others, endeavoured to prove the materiality cf
light by showing that the corpuscles had a power of momentum which might
affect other and very light substances. Could this fact have been really ascer-
tained, there would be nothing more to say against Newton's hypothesis ; but
their experiments were illusory and useless. On the other hand, the supporters
of the undulatory theory have within the last three years performed the most
elaborate and exact experiments to try to prove the real existence of the ether.
Mr. Balfour Stewart, F.R.S., superintendent of Kew Observatory, and Pro-
fessor P. G. Tait, M.A., of Edinburgh, whilst leaving other scientific men to make
their own deductions from the results they obtained, have called attention to
the subject by a paper read before the Royal Society in June, 1865, and
modestly entitled " On the Heating of a Disc by Rapid Rotation in vacuo."
The authors, having obtained certain results in air, were encouraged to construct
the apparatus as figured below, Fig. I, wherewith to procure rotation in vacuo.
" In this apparatus a slowly revolving shaft is carried up through a barometer
tube, having at its top the receiver which is to be exhausted. When the
exhaustion has taken place, the shaft connected with the multiplying gear
revolves in mercury. The train of toothed wheels causes the disc of alumi-
ON LIGHT.
nium to revolve 125 times for each revolution of the shaft. The thermo-electric
pile, the most delicate thermometer or test of heat, is connected by two wires
carried through two holes in the bed-plate of the receiver with a Thompson's
reflecting galvanometer needle (an instrument which is described and figured
FIG. i.
a, Fifs. i and 2, thermo-electric pile with reflecting cone attached ; ab, height 6 in. from bed-plate;
a c, length of cone, &c., jj in. ; c d, diameter of the aperture of the reflecting cone 2^ in. ; / h, the
disc of aluminum 13 in. diameter; eg, height from bed-plate to centre of the aluminum disc 8 in. ;
h e, distance of centre of the thermo-electric pile from the disc of aluminum 8 in. ; m, base containing
the multiplying gear; s i s, air-tight glass receiver, ig in. diameter and 16 in. high, covering the whole.
1 2
ON LIGHT.
in the article on Electricity in this work). The outside of the thermo-electric
pile and its attached cone was wrapped round with wadding and cloth, so as
to be entirely unaffected by currents of air.
" During these experiments the disc of aluminium was rotated rapidly for half
a minute, and a heating effect was, in consequence of the rotation, recorded
by the thermo-electric pile (an instrument described fully in the article on
Electricity).
" To obviate the objection that the electric currents which take place in a
revolving metallic disc might alter the zero of the galvanometer, the position
of the line of light was read before the motion began, and immediately after
it ceased, the difference being taken to denote the heating effect produced by
the rotation.
"The thermometric value of the indications given by the galvanometer was
found in this way : The disc was removed from its attachment and laid upon
a mercury bath of known temperature. It was then attached to its spindle
again, being in this position exposed to the pile, and having a temperature
higher than that of the pile by a known amount. The deflection produced
by this exposure being divided by the number of degrees by which the disc
was hotter than the pile, gives at once the value in terms of the galvanometric
scale of a heating of the disc equal to i on Fahrenheit's scale.
" The disc of aluminium being blackened with a' coating of lampblack, ap-
plied by negative photographic varnish, and rock salt inserted in the cone,
the following results were obtained :
No. of No. of observations Time at Heat indications
set. in each set. full speed. Fahrenheit.
I. 3 30 0-85
II. 4 30 0-87
III. 4 30 o'8i
IV. 3 30 075
" To ascertain whether the radiant heat recorded was derived from the rock
salt, or from heated air, or from the surface of the disc, the next series of
experiments were tried.
EXPERIMENTS WITH BLACKED ALUMINIUM Disc WITHOUT ROCK SALT.
No. of No. of observations Time at Heat indications
set. in each set. full speed. Fahrenheit.
V. 3 30 0-92
VI. 3 30 0-93
" With certain modifications of the above experiments it was satisfactorily
proved that the effect was not due to heating of the rock salt, or to radiation
from heated air ; it must therefore be due to the disc of aluminium, which
seemed to have rubbed against some matter which remained in the receiver
after the air was removed. The question being, was this ether?"
The authors further state that,
" i. It may be due to the air which cannot be entirely got rid of.
" 2. It is possible that visible motion becomes dissipated by an etherial
medium in the same manner and possibly to nearly the same extent
as molecular motion, or that motion which constitutes heat.
" 3. Or, the effect may be due partly to air and partly to ether.
" Not to leave the matter wholly undecided, it was suggested by Professors
ON LIGHT.
Maxwell and Graham that there is another effect of afr, viz., fluid friction,
the coefficient for which they believe to be independent of the tension.
" It would appear, however, that the fluid friction of hydrogen is much less
than that of atmospheric air, so that were the heating effect due to fluid fric-
tion it ought to be less in a hydrogen vacuum. An experiment proved that
the heating effect due to rotation in a hydrogen vacuum was 22*5, while in an
air vacuum it was 23*5, and the authors are inclined to consider these numbers
as sensibly the same, and that the experiment indicates that the effect is not due
to fluid friction ; at the same time they do not suppose that their experiments
have yet conclusively decided the origin of this heating effect, but they hope
to elicit the opinions of those interested in the subject, which may serve to
direct their future research."
These experiments are more satisfactory than any previously tried, and,
taken in conjunction with other facts, such as the temporary phosphorescence
of certain bodies by what is termed insolation or irradiation, or the action of
light in reducing certain salts to their metallic state, or the elaborate and
beautiful effects obtainable from thin films of solid, fluid, and gaseous bodies,
or the action of crystallized bodies on polarized light, they do altogether
impress the reasoning faculties with a conviction that a vibrating motion
accompanies the production of all light, which can only be propagated by
the communication of these vibrations or tremblings to a medium, itself a?
subtile, rare, and exquisite as the delicate mechanism that sets it in motion.
Starting with the proposition that all sources of light and luminous bodies,
like musical instruments, must first vibrate, it is not difficult to understand
by analogy how these vibrations may travel at the rate of 182,000 miles per
second, in straight lines, called rays.
FIG. 3.
A, tuning-fork struck on the leaden cone B, capped with leather, and applied to the end of the rod c,
whilst the other end is held against the sounding-board D.
A tuning-fork emitting sound might by analogy represent a source of light
like the sun, whilst a long rod communicating with it would stand in the place
of the theoretical ether, propagating the undulations from the sun through a
space of 92^ millions of miles, and if the other end of the rod communicates
with the sounding-board of a guitar, the audible sound obtained might com-
pare with the light falling on the earth and becoming apparent by radiation.
ON LIGHT.
The conversion of a continued series of mechanical impulses into waves is
beautifully shown by taking hold of the end of a long vulcanized india rubber
tube filled with sand, and having attached one end to the ceiling or other con-
venient place, it is easy by a jerk to produce the appearance of a wave, which
travels distinctly from the hand to the ceiling ; at the same time it demonstrates
the progressive nature of the wave or undulation, and as the portion held
by the operator cannot move from his hand to the ceiling, it shows how the
eye is deceived whilst looking at the motion of waves of water. Every wave
in water is propagated by the rising and falling of that which has preceded
it, and not because the volume of water representing the wave travels bodily
from the spot where it is first noticed to the shore where it breaks.
FIG. 4. The Vulcanized Tube attached to the ceiling, and tlirown
into protuberance or waves by the hand of the operator.
Dr. Tyndall has shown, by a modification of Dr. Young's experiments with
vibrating strings upon which light is thrown, a number of very beautiful
effects. A silvered cord attached to the iron arm of a curved spring band, one
end of which is made to vibrate by an electro-magnet, displays the divisions
of the cords into wave-like figures most perfectly when the cord is illuminated
by the lime or, better still, the electric light. (Figs. 5 and 6, p. 7.)
Using the brilliant light as before, a still more perfect and admirable experi-
ment may be conducted by attaching one end of a bright silvered chain to a
hook screwed into a vertical whirling table, and the other to a proper stand. The
chain being horizontal and the wheel vertical, it may be swung into one long
wave, or, by a still more rapid rotation, can be divided into three, four, or more.
The links of the chain flash in the light, and produce the most pleasing effects.
It must be remembered that if cords, chains, water, air, &c., can assume a
wave-like motion, the wonderful tension and elasticity of the hypothetical ether
ON LIGHT.
8 ON LIGHT.
would permit the latter to adapt itself to the most complicated movements
almost with the rapidity of thought. The very spiral, spindle-like, or cork-
screw motion observable in the chain and cord affords a -good idea of the
mechanism of the propagation of light, as the movement of e ach molecule
of ether is always perpendicular to the path of the ray or wave of light.
The astonishing rapidity of the periodic movements of the non-gravitating
molecules of ether becomes apparent, when it is stated that to produce white
light five hundred millions of millions of vibrations of the ether, 1,000,000,000,000
X 500 must occur in every second of time.
Or, taking the coloured rays at the extremities of the solar spectrum, viz.,
the red ray and the violet, the former demands the recurrence of four hundred
and fifty-eight millions of millions, 1,000,000.000.000 X 458 ; and the latter,
the violet, a still larger number, and greater rapidity of vibration, six hundred
and ninety-nine millions of millions, 1,000,000,000,000 X 699 per second.
The coloured rays of light are supposed, according to the undulatory theory,
to be distinguished from each by the breadths of the different waves, just
as the sound of a sti inged instrument may vary according to the diameter and
thickness of the strings. A tightly-stretched thin cord vibrating would be
the parallel to violet light. It is an axiom that, " The rapidity of vibration
is inversely proportional to the length and diameter of the string, and propor-
tional to the square root of the tension" A thicker cord 'not so tightly
stretched would be the oarallel to red light.
SOURCES OF LIGHT.
At the various instrument-makers cases containing four or five tubes, filled
with white powders and hermetically sealed, are to be obtained. When the
tubes are observed in a dark room (and, of course, before exposure to light),
ihey are invisible ; if, however, a piece of magnesium wire is now burnt close
to the tubes, they will be found to shine in the dark and to emit various
coloured rays of faint light. To this curious effect is given the name of phos-
phorescence ; and when the same result is obtained by exposing the tubes to
the light of the sun, the resulting phenomenon is denominated phosphorescence
after insolation, z'.^., after exposure to the sun. The chemical substances
which possess the property of developing light after exposure to light are
called phosphori, and the best are the diamond, Bolognian phosphorus, or
Bologna stone, made from sulphate of baryta, which occurs in nature as a
mineral, and is called heavy spar or barytine. It is prepared by heating this
mineral with charcoal to a dull red heat, or by the process of Margraf, in
which the mineral is powdered, mixed with flour, and made red hot ; or more
amusingly by the process of Daguerre, who uses a marrow-bone for his
crucible, and, after it is freed from fat and thoroughly dried, fills it with heavy
spar, powdered in any ;z0;/-metallic mortar. The bone is now closed with a
clay lute, and inclosed in an iron tube, which is surrounded with fine clay,
and the whole exposed for three hours to a red heat in a furnace. The sub-
stance which produces the effect is a sulphuret of barium. In the same manner
strontian phosphorus is obtained from ccelestin.
Canton's phosphorus is prepared by exposing a mixture of three parts of
sifted and calcined oyster-shells and one part of flowers of sulphur to a strong
SOURCES OF LIGHT.
fire for one hour. There are also many other phosphori ; amongst these may
be enumerated Osann's phosphori, Wach's phosphori, Homberg's phosphorus,
Baldwin's phosphorus, and many kinds of fluor spar.
The phosphorescence of these various bodies, unlike that of the curious
element phosphorus, is produced independently of any chemical change ; and
if inclosed in sealed glass tubes, and excluded from light, they may retain the
property of showing phosphorescence for many years, whilst the light emitted
from phosphorus is due to the slow oxidation of this element ; and if this is
arrested, by placing it in water, or in any gas, like nitrogen, the light is no
FiG. 7. The Phosphorescent Tubes.
longer produced. Upon what principle, then, is it possible to explain the cause
of the emission of light after exposing phosphori to the sun or any brilliant
artificial light?
The most rational theory which can be suggested is, that the undulations
of light convey their own vibratory motions to the phosphori, just as one
musical instrument may cause another to vibrate symphathetically with it,
and phosphorescence is observed so long as the substance continues to vibrate.
In a dark room, and without a constant accession or supply of vibratory power,
the light becomes fainter and fainter, until it is no longer capable of affecting
the eye ; the vibratory power, like any other mechanical motion, must come
to an end when cut off from its source of power, when, as in this case, it is
removed from the greater vibratory power, the sun or the burning magnesium,
which originally set it in motion. This opinion is further confirmed when we
take into account the large number of- substances which may become phos-
phorescent in a tolerably high degree. If this property was confined to a few
bodies, the theory might not be so applicable; but if it is agreed beforehand
io ON LIGHT,
that any particles may become luminous if they are capable of entering into
that state of vibration which we suppose belongs to the sun and artificial
sources of light, then it can be understood why the following organic or
inorganic substances are all considered to enjoy in a limited degree the pro-
perty of phosphorescence after exposure to the sun : crystallized boracic acid,
sal ammoniac, sulphate of potash, nitre, crystallized carbonate, borate, or
sulphate of soda, rock salt, witherite, radiating heavy spar from Bologna,
marienglas, fibrous gypsum, alabaster, artificial sulphate of lime, common
fluor spar, crystallized sulphate of magnesia, crystallized alum, arsenious
acid, pharmacolite, freshly prepared flowers of zinc, sulphate of mercury,
tartar, benzoic acid, loaf sugar, sugar of milk, bleached wax, white paper
(especially when it has been heated almost to burning), yellow and red paper,
which are nearly as phosphorescent as white paper, egg-shells, corals, snails,
pearls, bones, teeth, ivory, leather, and skins of men and animals, tartaric
acid, also seeds, grain, flour, starch, crumbs of bread, gum arabic, feathers,
cheese, yolk of egg, muscular flesh, tendons, isinglass, glue, horn, all well
dried ; moreover, the albumen of trees, bleached linen, bleached cotton yarn,
and other bleached vegetable fibres. The above is only a small instalment of
the different chemical bodies and common substances which Gmelin enume-
rates when he speaks of those things which become phosphorescent by irradia-
tion. Phosphorescence may also be further developed by heat, mechanical force,
and crystallization, all of which are modes of motion, and suggest the setting
up of a vibratory effect, resulting in the production of light. Chemical action,
another mode of motion, is concerned in the phosphorescence of live animals
and putrifying animal matter, and also in the production of the same effect
in living and decaying plants.
HEAT A SOURCE OF LIGHT.
When iron is heated to a temperature of 635 Fahrenheit, it emits a dull red
light, visible only in a darkened room. If the heat is further increased to 903
Fahrenheit, a bright red light is apparent, visible in a chamber fairly illumi-
nated. The light attains a greater intensity at the moment the iron is heated
to 1000 Fahrenheit. Thus, by the progressive increase of the heat of the iron,
what is called a dull red, a pale red, and a white heat is obtained. By
increasing the heat of a solid body a development of light or incandescence
is obtainable.
LIGHT THE FREQUENT ATTENDANT OF ELECTRICAL PHENOMENA.
The intense and dazzling brightness of lightning has been known to cause
temporary and permanent blindness. The immense electric spark, the result
of the discharge of thousands of acres of charged clouds, will probably be
more closely imitated than ever by an enormous induction coil, now being
constructed by Mr. Apps for the Royal Polytechnic, which is calculated to
give a spark 5 ft. in length, the usual length being from 5 to 18 in., or, in very
rare cases, 2 ft. At the moment of discharge the electricity may develop light,
heat, magnetical, mechanical, and chemical effects. Here is a correlation of
forces that might well excuse Oersted in proposing a theory of light in which
he regards light as the result of electric sparks.
SOURCES OF LIGHT.
ii
FIG. 8. The Inductorium of Mr, Apps, giving sparks 18 in. in length.
CHEMICAL COMBINATION A SOURCE OF LIGHT.
Finely divided lead or iron shaken from a tube into the air or oxygen
oxidizes rapidly, burns, and emits light. Finely powdered antimony unites
rapidly with chlorine gas, and glows with the intensity of light whilst the
FIG. 9. Blotting-paper upon which the Solution of Phosphorus in Sulphide
of Carbon has been poured, and then supported on an iron wire.
combination is taking place. A solution of phosphorus in sulphide of carbon,
poured upon blotting-paper, soon begins to evolve smoke, produced during the
formation of phosphoric acid, and then rapidly and spontaneously catches fire
12
ON LIGHT.
by the union of the finely divided phosphorus with the oxygen of the air.
The name of Greek modernized into Fenian fire is given to this solution,
which should only be made and used in small quantities.
Is MECHANICAL FORCE TO BE REGARDED AS A TRUE SOURCE OF LIGHT?
Since the numerous experiments made at Shoeburyness with iron plates and
heavy guns, it has been ascertained over and over again that heat and fre-
quently light are produced at the moment the impact or blow is given by the
shot. The mechanical force, in the abstract, may be regarded as the source
of light ; but not perhaps directly, as the blow develops heat, and the latter,
FIGS. 10 and n. The Shadow Blondin.
X^Tt-'
Arrangement of Mechanism and Oxy-Hydrogen Light required to produce the effect of the Shadow
Blondin. A, the mechanical figure; B', the lime-light ; c, the handles used to produce the movements
of the figure.
the figure
probably, the light. It is found that almost all bodies which acquire phos-
phorescence by exposure to the sun, or insolation, or by heat, also become
luminous by friction or percussion. Sometimes the light obtained by friction
is simply electrical. The sparks from a flint and steel are due to the com-
bustion of minute particles of metal accelerated by the heat eliminated at the
SOURCES OF LIGHT.
FIG. 12. Effect in front of the Curtain.
moment the particle is struck off. Mechanical force can only be regarded
as an indirect mode of producing light, because heat is first developed ; heat
is a source of light.
From what has been previously stated, it will be understood that all matter
may be divided in relation to light into luminous and non-luminous bodies.
The sun or a lighted lamp would represent the former, and the moon with the
other planets, or a piece of whitened board, the latter, because our satellite
shines by borrowed light from the sun, and not by any inherent self-luminosity;
the piece of board will reflect and scatter the rays of light from the lamp,
and whilst doing this appears very bright. At the same time the board
obstructs the light and casts a shadow behind it, and thus indicates another
relation of light to solid matter, called opacity ; the opposite to this property
being transparency, whilst the intermediate links between opacity and trans-
parency are termed semi-transparency, or opalescence. There are many very
amusing effects produced by casting shadows of living or inanimate objects
on a transparent disc by the oxy-hydrogen light. (Figs. 10, 1 1, 12.)
i 4 ON LIGHT.
The shadow pantomimic action of living figures visible on a transparent
disc with this strong light, and first introduced by the author at the Poly-
technic, has gone the round of nearly all the exhibitions and theatres in
London and New York.
There still remains, however, something new and amusing even in this
hackneyed branch of light. Mr. Walker, jun., constructed a very simple
and ingenious piece of mechanism, and giving it the outline of a human figure,
produced a good imitation of the bold feats performed by Monsieur Blondin
on the high rope. The shadow of the figure only was projected on to the disc
by the lime-light, and it simulated all the usual movements, such as standing,
walking, dancing, and sitting astride the rope. Indeed it did rather more
than the living prototype, for the figure stood on its head, and threw the most
unnatural but highly-amusing sommersaults. (Figs. 10, n, 12, pp. 12, 13.)
THE DIFFUSION OF LIGHT.
A luminous object evolves light from every visible point of its surface, and
if a single point of light were placed in the centre of a hollow globe, every
portion of the internal area would be equally illuminated.
FlG. 13. A Flame in the centre of a circle, throwing out rays in
every direction, like the spokes of a wheel.
Owing to the manner in which light is distributed and transmitted in
straight lines diverging from each other, its intensity diminishes as the square
of its distance from the luminous source increases, and it is on this principle
that the instruments called photometers, or light-measurers, are constructed.
A scale of 20 ft. in length, divided into feet and inches, may be used in con-
junction with a box somewhat like a stereoscope, containing two mirrors
placed at an angle of 45, and reflecting the rays from the two sources of
light which are to be confronted with each other. A candle, one of six to the
THE DIFFUSION OF LIGHT.
pound, and burning so many grains per minute, is fixed in a nozzle, which
slides on the scale. The box, which may also slide or be fixed in the centre
of the scale, reflects on one side the light from the lamp or gas-burner which
is being tested, on the other it reflects the light of the candle. The experi-
ment may be conducted either by placing the lamp and the candle at opposite
ends of the scale, and moving the box with the reflectors until the two spots
of light are equal ; or, the box being fixed in the centre, and the lamp under
examination placed at one end of the scale, the candle may be moved towards
the box till the lights are equal, the respective distances from the box being
then squared, and the greater number divided by the less, will give the quo-
tient which represents the illuminating power of the lamp as compared with
the candle.
FiGS. 14 and 15. Ritchie's Photometer.
Section of the box containing the mirrors A B, AC, openings po, EO, to admit the light which is reflected
from the mirrors on to two cncular apertures p p, covered with oiled paper, which are seen and com-
pared when looked at from the top at T T. The arrows indicate the direction of the rays from the
lamp, and L, the wax candle w c. Example : the distance of the lights from the box being respectively
12 ft. and 3 ft. 12 x 12 = 144 -f 3 x 3 = 9- Quotient, 16.
In the practice of photometry the standard used is a candle defined by Act
of Parliament " as a sperm candle of six to the pound, burning at the rate of
1 20 grains per hour." This standard would be a very simple one if every
candle could be made alike, but it unfortunately happens that the composition
and the wick are not always the same, and as important experiments have to
be made in various parts of the United Kingdom, it becomes difficult to
assimilate and compare them with each other. All authorities on this question
have condemned the use of test candles. The credit is due to Mr. Crookes,
the editor of the " Chemical News," of devising a standard test lamp-wick and
combustible fluid which could be made in every part of the civilized world,
and of inventing an improved photometer, in which the phenomena of
polarized light are employed. The following is the inventor's description of
the apparatus and materials used, commencing with the lamp and its fuel :*
"Alcohol of sp. gr. 0.805, and pure benzol boiling at 81 C, are mixed
together in the proportion of 5 volumes of the former and I of the latter.
This burning fluid can be accurately imitated from description at any future
time and in any country, and if a lamp could be devised equally simple and
invariable, the light which it would yield would, it is presumed, be invariable.
This difficulty the writer has attempted to overcome in the following manner.
" Chemical News, ' July i7th, iS68.
1 6 ON LIGHT.
" A glass lamp is taken of about 2 ounces capacity, the aperture in the neck
being 0*25 inch diameter ; another aperture at the side allows the liquid fuel
to be introduced, and, by a well-known laboratory device, the level of the' fluid
in the lamp can be kept uniform. The wick-holder consists of a platinum
tube I '8 1 in. long, and 0-125 m - internal diameter. The bottom of this is
closed with a flat plug of platinum, apertures being left in the sides to allow
free access of spirit. A small platinum cup 0.5 in. diameter and I in. deep
is soldered round the outside of the tube 0*5 in. from the top, answering the
threefold purpose of keeping the wick-holder at a proper height in the lamp,
preventing evaporation of the liquid, and keeping out dust. The wick consists
of 52 pieces of hard-drawn platinum wire, each o'oi in. diameter and 2 in. long,
perfectly straight, and tightly pushed down into the platinum-holder until only
o'l in. projects above the tube. The height of the burning fluid in the lamp
must be sufficient to cover the bottom of the wick-holder ; it answers best to
keep it always at the uniform distance of 175 in. from the top of the platinum
wick ; a slight variation of level, however, has not been found to influence the
light to an extent appreciable by our present means of photometry. The lamp
having the reservoir of spirit thus arranged, the platinum wires parallel, and
their projecting ends level, a light is applied, and the flame instantly appears,
forming a perfectly shaped cone 1*25 in. in height, the point of maximum
brilliancy being 0^56 in. from the top of the wick. The extremity of the flame
is perfectly sharp, without any tendency to smoke ; without flicker or move-
ment of any kind ; it burns, when protected from currents of air, at a uniform
rate of 136 gr. of liquid per hour. The temperature should be about 60 F.,
although moderate variations on ither side exert no perceptible influence.
Bearing in mind Dr. Franklin's observations on the direct increase in the
light of a candle with the atmospheric pressure, accurate observations ought
only to be taken at one height of the barometer To avoid the inconvenience
and delay which this would occasion, a table of corrections should be con-
structed for each o'l variation of barometric pressure.
" There is no doubt that this flame is very much more uniform than that of
the sperm candle sold for photometric purposes. Tested against a candle,
considerable variations in relative illuminating power have been observed ;
but on placing two of these lamps in opposition, no such variations have been
detected. The same candle has been used, and the experiments have been
repeated at wide intervals, using all usual precautions to ensure uniformity."
The results are thus shown to be due to variations in the candle, and not in
the lamp.
In Arago's "Astronomy," the author describes his photometer in the fol-
lowing words :
" I have constructed an apparatus by means of which, upon operating with
the polarized image of a star, we can succeed in attenuating its intensity by-
degrees exactly calculable after a law which I have demonstrated.'" It is
difficult to obtain an exact idea of this instrument from the description given ;
but from the drawings it would appear to be exceedingly complicated, and to
be different in principle and construction from the one now about to be de-
scribed. The present photometer has this in common with that of Arago, as
well as with those described in 1853 by Bernard,* and in 1854 by Babinet,f
* ' Comptes Rendus,' 1 April 2$, i8<3.
t " Proceedings of the British Association," Liverpool Meeting, 1854.
THE DIFFUSION OF LIGHT.
that the phenomena of polarized light are used for effecting the desired end.
But it is believed that the present arrangement is quite new, and it certainly
appears to answer the purpose in a way which leaves little to be desired. The
instrument will be better understood if the principles on which it is based are
first described.
" Fig. 1 6 shows a plan of the arrangement of parts, not drawn to scale, and
only to be regarded as an outline sketch to assist in
the comprehension of general principles. Let D repre- / N/" N
sent a source of light. This may be a white disc of ( -J- Y + )
porcelain or paper illuminated by any artificial or na-
tural light. C represents a similar white disc likewise
illuminated. It is required to compare the photome-
trie intensities of D and C. (It is necessary that neither
D nor C should contain any polarized light, but that the
light coming from them, represented on each disc by
the two lines at right angles to each other, forming a
cross, should be entirely unpolarized.) Let H represent c c
a double-refracting achromatic prism of Iceland spar; x \ x x ^ x
this \\ill resolve the disc D into two discs, d and d' ', ( "" Y Hh Yl J
polarized in opposite directions ; the plane of d being, V / V_^x V__^/
we will assume, vertical, and that of d' horizontal. The & A
prism H will likewise give two images of the disc c ;
the image c being polarized horizontally, and c' verti- ;
cally. The size of the discs D, c, and the separating
power of the prism H are to be so arranged that the
vertically polarized image d, and the horizontally po-
larized image <:, exactly overlap each other, forming, as
shown in the figure, one compound disc, c d, built up of /^ X /" X
half the light from D and half that from c. f 3 } if ^ J
" The measure of the amount of free polarization V__^_. . \_x
present in the disc c d, will give the relative photome- ^ 1G 1O
trie intensities of D and C.
" The letter I represents a diaphragm with a circular hole in the centre, just
large enough to allow the compound disc c d to be seen, but cutting off from
view the side discs c' d'. In front of the aperture in I is placed a piece of
selenite of appropriate thickness for it to give a strongly-contrasting red and
green image under the influence of polarized light. K is a doubly-refracting
prism, similar in all respects to H, placed at such a distance from the aperture
in I that the two discs into which I appears to be split up are separated from
each other, as at^- D. If the disc c d contains no polarized light, the images
g r will be white, consisting of oppositely polarized rays of white light ; but if
there is a trace of polarized light in c d, the two discs g r will be coloured
complementary, the contrast between the green and red being stronger in
proportion to the quantity of polarized light in c d.
" The action of this arrangement will be readily evident. Let it be supposed
in the first place that the two sources of light, D and C, are exactly equal.
They will each be divided by H into two discs, d' d and c c', and the two
polarized rays of which c d is compounded will also be absolutely equal in
intensity, and will neutralize each other and form common light, no trace of
free polarization being present. In this case the two discs of light g*D will be
colourless. Let it now be supposed that one source of light (D for instance)
2
i8
ON LIGHT.
is stronger than the other (c). It follows that the two images d' d will be
more luminous than the two images c c', and that the vertically polarized ray
d will be stronger than the horizontally polarized ray c. The compound
disc c d will therefore shine with partially polarized light, the amount of free
polarization being in exact ratio with the photometric intensity of D over c.
" In this case the image of the selenite plate in front of the aperture I will
be divided by K into a red and a green disc.
FIG. 17.
" Fig. 17 shows the instrument fitted up. A is the eye-piece (shown in
enlarged section at Fig. 3). G B is a brass tube, blacked inside, having a
piece, shown separate at D c, slipping into the end B. The sloping sides, D B,
B c, are covered with a white reflecting surface (white paper or finely ground
porcelain), so that when b c is pushed into the end B, one white surface, D B,
may be illuminated (as in Fig. 17) by the candle, and the other surface, B C, by
the lamp. If the eye-piece A is removed, the observer, looking down the tube
G B, will see at the end a luminous white disc divided vertically into two parts,
one half being illuminated by the candle E, and the other half by the lamp F.
By moving the candle E, for instance, along the scale, the illumination of the
half D B can be varied at will, the illumination of the other half remaining
stationary.
" The eye-piece A (shown enlarged at Fig. 18) will be understood by reference
to Fig. 1 6, the same letters representing similar parts. At L is a lens to collect
the rays from B D c, Fig. 17), and throw the image into the proper part of the
tube. At M is another lens, so adjusted as to give a sharp image of the two
discs into which I is divided by the prism K. The part N is an adaptation
of Arago's polarimeter ; it consists of a series of thin plates of glass capable
of moving round the axis of the tube, and furnished with a pointer and
graduated arc. By means of this pile it is possible to partially polarize the rays
coming from the illuminated discs in one or the other direction, and thus bring
to the neutral state the partially polarized beam c d (Fig. 16), so as to get the
images g D free from colour. It is so adjusted that when at the zero point it
produces an equal effect on both discs.
THE DIFFUSION OF LIGHT.
" The action of the instrument is as follows. The standard lamp being
placed on one of the supporting pillars which slide along the graduated stem
(Fig. 17), it is adjusted to the proper height, and
moved along the bar to a convenient distance,
depending on the intensity of the light to be
measured ; the whole length being a little over
4 ft., each light can be placed at a distance of
24 in. from the disc. The flame is then sheltered
from currents of air by black screens placed
round, and the light to be compared is fixed in a
similar way on the other side of the instrument.
The whole should be placed in a dark room, or
surrounded with non-reflecting screens ; and the
eye must also be protected from direct rays from
the two lights. On looking through the eye-piece
two bright discs will be seen, probably of diffe-
rent colours. Supposing E represents the stan-
dard flame, and F the light to be compared with
it, the latter must now be slid along the scale
until the two discs of light, seen through the eye-
piece, are about equal in tint. Equality of illu-
mination is easily obtained ; for, as the eye is
observing two adjacent discs of light, which pass
rapidly from red-green to green-red, through a
neutral point of no colour, there is no difficulty
in hitting this point with great precision. It has
been found most convenient not to attempt to get
absolute equality in this manner, but to move the
flame to the nearest inch on one side or the other
of equality. The final adjustment is now effected
at the eye-end, by turning the polarimeter one
way or the other up to 45, until the images are
seen without any trace of colour. This will be
found more accurate than the plan of relying
entirely on the alteration of the distance of the
flame along the scale; and, by a series of experi-
FIG. 1 8.
mental adjustments, the value of every angle through which the bundle of
plates is rotated can be ascertained once for all, when the future calculations
will present no difficulty. Squaring the number of inches between the flames
and the centre will give their approximate ratios ; and the number of degrees
the eye-piece rotates will give the number to be added or subtracted in order
to obtain the necessary accuracy.
" The delicacy of the instrument is very great. With two lamps, each about
24 in. from the centre, it is easy to distinguish a movement of one of them to
the extent of i-ioth of an inch to or fro ; and by using the polarimeter, an
accuracy considerably exceeding that can be attained.
" The employment of a photometer of this kind enables us to compare
lights of different colours with one another, and leads to the solution of a
problem which, from the nature of their construction, would be beyond the
powers of the instruments in general use. So long as the observer, by the
eye alone, has to compare the relative intensities of two surfaces respectively
22
20 ON LIGHT.
illuminated by the lights under trial, it is evident that unless they are of the
same tint it is impossible to obtain that absolute equality of illumination in
the instrument which is requisite for a comparison. By the unaided eye one
cannot tell which is the brighter half of a paper disc illuminated on one side
with a reddish, and on the other with a yellowish light ; but, by using the
above-described photometer, the problem becomes practicable. For instance,
on reference to Fig. 16, suppose the disc D were illuminated with light of a
reddish colour, and the disc C with greenish light, the polarized discs d 1 ' d
would be reddish, and the discs c' c greenish, the central disc c d being of the
tint formed by the union of the two shades. The analysing prism K, and the
selenite disc I, will detect free polarization in the disc c d, if it be coloured, as
readily as if it were white ; the only difference being that the two discs of
light g r cannot be brought to a uniform white colour v/hen the lights from
D and C are equal in intensity, but will assume a tint similar to that of c d.
When the contrasts of colour between D and C are very strong when, for
instance, one is a bright green and the other scarlet there is some difficulty
in estimating the exact point of neutrality ; but this only diminishes the
accuracy of the comparison, and does not render it impossible, as it would
be according to other systems.
" No attempt has been made in these experiments to ascertain the exact
value of the standard spirit-flame in terms of the Parliamentary sperm candle.
Difficulty was experienced in getting two lots of candles yielding light of
equal intensities ; and when their flames were compared between themselves
and with the spirit-flame, variations of as much as 10 per cent, were some-
times observed in the light they gave. Two standard spirit-flames, on the
other hand, seldom showed a variation of I per cent., and had they been
more carefully made they would not have varied 0*1 per cent.
" This plan of photometry is capable of far more accuracy than the present
instrument will give. It can scarcely be expected that the first instrument of
the kind, roughly made by an amateur workman, should possess equal sensi-
tiveness with one in which all the parts have been skilfully made with special
adaptation to the end in view."
MODIFICATIONS THAT LIGHT MAY UNDERGO.
1. In the same medium of the same density rays of light undergo no
change.
2. When rays of light pass out of one medium into another, or into one of
a different density, they may undergo the following modifications :
3. The rays of light may rebound from the surface of a solid, fluid, or
gaseous body, and are then .said to be reflected, the rebounding being
denominated Reflection.
4. A ray of light, after passing into a substance, may be bent from its
natural course, or Refracted.
5. A ray of light may be split into two portions when it enters certain
bodies, such as Iceland spar, and each portion of the light possesses
distinct properties.
6. A ray of light may be so checked in its passage that a portion may be
lost or absorbed.
7. A ray of light, by reflection, refraction, double refraction, and absorption,
may acquire new properties, and become what is termed Polarized Light.
THE REFLECTION OF LIGHT.
21
THE REFLECTION OF LIGHT.
Catoptrics is the name given to all effects produced by reflection. It is a
word taken from the Greek /caTOTnyHKos, " belonging to a mirror," and whilst
the laws which govern the reflection of light are remarkably simple, they give
rise to a most interesting series of phenomena.
Premising that the incident rays ,
are those which fall on the surface,
and that those sent off are called
reflected T&yS) it is soon ascertained
ist, that the incident and re-
flected rays always lie in the same
plane, i.e., if the incident ray falls
in a perpendicular plane or direc-
tion, the reflected one will also be
in the same plane or direction ; and
the like reasoning applies to the
horizontal position. 2nd, the in-
cident and reflected rays always
form equal angles, or when light
falls upon any surface, whether
plane or curved, the angle of re- K R fe the reflecting surfaces A'B is the incident ray ;
of
B c, the reflected ray ; A B p, the angle of incidence;
c B p, the angle of reflection.
flection is equal to the angle
incidence.
The luminous rays may be parallel to each other, like the lines in a copy-
book, or they may be divergent when they spread out in the same manner as
the sticks of a fan, or convergent when they gradually approach each other,
and end in a point like a spear-head.
FIG. 20. Reflection of Parallel or Equi-distant Rays.
R R R, the parallel rays incident on a plane or flat surface at T, and reflected in lines at equal distances
from each other. The rays of the sun are nearly parallel with each other, and will illustrate this fact.
22
ON LIGHT.
FIG. 21.
Parallel rays falling on a concave mirror, M M, converge or come to a focus or fireplace at F.
FIG. 22.
Reflection of parallel rays from a convex mirror. The rays which are reflected become divergent*
and are shown on the ceiling.
THE REFLECTION OF LIGHT,
A very large number of the waves of light are lost when they fall even upon
the most perfectly polished metallic mirrors ; thus light reflected from a clear
and bright surface of metallic mercury at an angle of 78 5' loses nearly
one quarter, and only 754 rays out of 1000 are reflected.
A transparent substance, like glass, reflects more light from the second sur-
face than the first ; and if the former is coated with an amalgam of tin and
mercury, the brilliancy of the reflection of the second or coated surface over-
powers that of the first, although if a candle is held opposite the best quick-
silvered mirror two images are apparent.
In the production of illusory effects by reflection from the surface of
glass, the image reflected from the surface of the first surface interferes with
the second ; but this may be prevented, as shown to the author by a friend, by
coating the first side with a very delicate film of collodion or varnish, such as
is used for photographic purposes. Thus
the intensity of the reflection of the
second surface is increased by a coat-
ing of amalgam, whilst the intensity of
the reflection from the first surface is
reduced by coating it with a substance
like collodion, having an absorptive
rather than a reflecting power on light.
Where objects are reflected from either
glass or silvered glass plane mirrors,
they appear to come from the back, and
the image is as far behind the glass -as
the real object is before it. It is this
physical truth that increases so amaz-
ingly the effect of what is familiarly
called "The Ghost Illusion." The
spectator looking, at the image does
not observe the glass which has pro-
duced it, because the former is so far
in advance of the latter. Had this FlG. 23.
physical fact in Catoptics been remem- o, the real object reflected from the glass A B, ft
bered, many scientific men would have R '. to ^ e eye at E ; s - behind ^ glass '.
sooner discovered the secret of the illu-
sion by looking in front of the image
for the glass or reflecting surface.
The same truth is still more apparent when divergent or convergent rays
are traced out in their reflections from a plane surface of glass.
To cause the image or ghost to appear, the lights are alternately thrown or.
or cut off the real figure. (See Fig. 24, p. 24.)
This mode of showing the ghost has to be modified when the angles of
vision are so different as seen from the pit, boxes, and gallery of a theatre.
Then it is advisable to sink a stage a few feet below the regular stage, and to
arrange a board at the same angle as the glass, on which the living figures
recline. The latter method allows only certain movements to be exhibited,
and is called the "spectroscope" and " phantoscope " by travelling showmen
who exhibit the ghost.
One of the prettiest stories which can be illustrated with this illusion is
that called " The Knight watching his Armour," and as many persons have
where the image appears to come from, and if
the whole distance from E to s o is measured, it
will be found equal to E R, R o.
ON LIGHT.
seen it at the Polytechnic, and doubtless might wish to entertain others with
this popular illusion the little tale is added as a sequel to the contrivance
itself.
FIG. ^.Exhibition of the " Ghost " at the Polytechnic, being a section of
the stage in the large Theatre. .
A, the real figure ; B, lime-light ; c c, looking-glass ; D D, plate glass ; G, the spectral image or ghost,
which would appear much farther behind the glass D D; s, spectators.
KNIGHT WATCHING HIS ARMOUR.
N.B. The spectral image described appears at all places marked with a star. *
The following is told of a knight, called Hubert de Burgh, who won his
spurs on Flodden Field :
King James was so pleased with his deeds of valour, that he promised to
dub him knight, on the following morning ; but told him that he would have to
go through the ancient ordeal of watching his armour throughout the previous
night.
Sir Hubert started with helmet and corselet to the church. Before entering,
he met his lady-love, fair Agnes, and telling her of his good fortune, begged
that their wedding might take place on the first day he wore his golden spurs.
The maiden consented, and told him that she would also watch with him
in spirit throughout the night, and bade him beware of the many temptations
held out by the evil spirits to all warriors during the period of their watching.
After a loving farewell, Hubert commenced his duties. And now, for the
first time, does he feel fatigue from his hard day's fighting; but remembering
the caution that he must neither eat, drink, nor sleep during his vigils, he
continues to watch and fast until the break of morn : sitting down, he thinks
of his good fortune in winning the prize so much coveted by all true warriors.
Whilst buried in thought he hears the sound of approaching foot-steps, and
THE REFLECTION OF LIGHT.
feels that the time has come when he needs all his energy to keep his armour
pure from evil touch. On looking up he beholds a Benedictine monk * standing
near, watching him most carefully.
" Peace be with you," says the monk.
" Amen, father," replies the knight.
" My son," continues the friar, "thou hast acted nobly this day, and deservest
the honours our gracious sovereign is about to confer on thee ; thou hast had
a weary day, and needest rest and sustenance ; sleep awhile, and I will keep
custody over these true steel arms."
" Nay, father," said Hubert, "my duty is to watch, and not take deputy for
this all-important work, neither will my instructions permit me to eat or sleep."
" My son," replies the priest, " as a brother of our holy order, I absolve thee
26 ON LIGHT.
of this heavy charge, and will keep watch ; and in that same capacity I bid
thee drink. See ! here is a cup of right good wine which will much relieve thee."
"Father," said Hubert, "sorry am I that mistrust enters my mind; I like
not to break the solemn right, and though I would gladly accept thy proffered
gift, I dare not, without you make the sign of your order over the wine."
The monk for some time hesitated, but at length in an angry tone replied
" Fool! drink or starve; what care I for such a coward loon?"
" Now, by St. Peter," ejaculates Hubert, " these sound not like a good
priest's words ; thou wearest the dress without the sign of thy calling. Who
art thou ? Answer quickly, or this good sword shall make short work of thy
disguised body."
Grasping his sword, he advances towards the friar, who, with a fiendish
laugh, vanishes from before him, and is gone.
Hubert felt it must have been an evil spirit who sought to destroy him,
and with firmer determination to resist, he again returns to his weary task.
Some time elapsed, when there comes before him, gliding out of the darkness,
a beauteous syren,* who speaks kindly and fairly to him of his great prowess
and feats of arms. She tells him she is an inhabitant of fairy-land in fact,
their queen that she loves him fondly, and beseeches him to come to their
fairy home, where he shall reign supreme.
She pictures to him the delight of being always young and gay of being
master of countless hosts flying through the night amidst the stars prince
of all the land ; and in such strains does she pour forth her eloquence, that
he flies with her in fancy through the realms she so beautifully describes ; but
the thought of his fair Agnes, and the promise made, recalls him to his duty,
and slowly advancing towards his armour, he lays his hand on the left side of
his corselet, saying, "If thou be a spirit of evil, thus do I destroy thy charm."
The temptress gives one faint sigh, and vanishes from his view.
Hubert, now relieved from a second temptation, watches with renewed
vigilance ; he now knows that the morn is not far distant that morn which
blesses him doubly, by giving him the name of knight and a fair bride.
The thought of Agnes causes him pain : " So soon shall I be forced to leave
her, to seek a fortune which I have not;" and for the first time he knew what
it was to wish for wealth. Whilst deep in thought how he should increase
his little store of treasure, a stately man, dressed in the garb of a wealthy
merchant, stands before him and questions him upon the sadness of his looks.*
" For one so young," said his visitor, " should ne'er be sad."
" Good sir," replies Hubert, "thou seemest kindly in thy manner, so will I
tell thee of my only grief. To-morrow, by the will of ur good king, I put on
the golden spurs of knighthood, I wed a noble lady, whom I shall drag down
to my own level of poverty ; though the world has given me an honoured
name, still do I lack the wealth to keep my wife in station that befits her, and
calm reflection tells me I did wrong to take her promise, and so, sir, do I
feel sad."
" Beshrew me, but thou art a noble youth," replies the merchant " noble
in thought as well as deed, and if it had been ordained that I was blessed
with such a son, he should not long need wealth."
"Ah!" said Hubert, "fate has not given me a parent's love, care, or assist-
ance ; my mother died when I was yet a babe, and ere many months my
father followed her, dying as a noble soldier should, upon the battle-field."
" Stay, said the merchant, " may I again question thee as to whether him
THE REFLECTION OF LIGHT. 27
you spoke of was the noble Ralph de Burgh, one of my most true and honest
friends?"
" 'T is so," said Hubert, " and if my father sought to win your friendship,
pray extend the same good fellowship to his son."
" That I will, right willingly," returns the merchant. " Stay," continued he,
" methinks you said you needed gold nay, turn not away I have enough,
too much for an old and childless man. Say, let me aid thee. I ask v it as
a favour; nay, I command it, as your father's friend. Here, take this purse to
meet your most urgent wants, and to-morrow shalt thou revel in as great
wealth as any son of our noble houses. Nay, I will take no denial,"
Hubert, who had been struggling within himself as to his right to take the
proffered gift, at last rises to approach the stranger, when he imagines he hears
sweet music passing through the air. He stops Lo listen, and fancies he hears
a well-known voice exclaim, " Beware ! keep to your trust, 't is almost morn."
Amazed, he steps back, and sees his fair Agnes beckoning him away,* and
waving the merchant back, who, with a frown and disappointed look, fades
into the darkness.
The maiden said, " Dear Hubert, thy task is finished ; for see, the morn is
breaking. Farewell ; we meet again at noon, never to be parted. I said I
would watch over thee in spirit; say, have I performed the task?"
As the warrior is about to embrace his beloved, she disappears from before
him.
The first tint of the morning sun soon glistened upon his helmet ; so this
true knight had watched from eve till sunrise to guard his armour from all
evil spirits.
IMAGES FORMED BY SILVERED MIRRORS.
Soon after the novelty of " the Ghost " had waned, another illusion was pre-
sented to the public called "Proteus; or, We are Here, but not Here," Mr.
Thomas Tobin and the author being co-inventors. A large and handsome box,
like a huge sentry-box on wheels, and raised from the floor so that the spec-
tators could see under, over, and all round it, is wheeled on to the platform
(Fig. 26). On being opened it appeared to be well lighted from the top by an
ordinary railway carriage lamp, and, of course, seemed to be perfectly empty.
The assistant being now invited to enter the box, the door is closed and locked,
and, after a few minutes have elapsed, is re-opened, when a skeleton appeared
to be standing in the very place where the living being had been formerly
observed (Fig. 27.) Again the door is closed, and the next time it is opened
the skeleton has vanished, and the assistant walks out of the box with a carpet
bag. The person explaining the apparatus now goes in, and sounds the walls
all round with his knuckles ; and, while doing this, the door is suddenly closed,
and being as quickly opened, he is found to have disappeared, again to appear
after the door is once more closed and opened. This illusion is produced by
two plane silvered mirrors, folding into the sides of the box, and when open
forming together an angle of 45. The mirrors when open reflect the two sides
of the box, and, as already explained, they appear behind the mirrors, and
cause the spectator to suppose that he is looking at an empty box. In the
angle formed by the mirrors the skeleton is concealed and brought out when
required, and in the same place the assistant and lecturer are alternately
hidden. Thus a box can be constructed in which the most elaborate tricks
of the Davenport Brothers may be performed.
28
ON LIGHT.
FIG. 26.
FIG. 27.
In the accompanying drawings, Fig. 26 is an exterior, and Fig. 27 an interior
view, and Fig. 28 a horizontal section of the box or chamber above referred
to. The sides may be made of wood, or papier mache, or sheet iron, but the
former is preferred.
a a are two doors hung at the angles of the
box, and capable of closing on the post d or of
lying back in a recess in the sides, as shown on
the right-hand side of the box. These doors a a
have crlass mirrors on the sides ffff, and a fresco
or design at the upper part of the box or chamber
suitable for the illusion to be represented. The
post d is set at the junction of the lines bisecting
the angles of the back and sides. The box or
chamber as shown is rectangular. If, for conve-
nience or for the purpose of any particular re-
presentation, the box or chamber is desired to be
wider at the front than at the back, the post will
still be placed at the junction of the two lines
bisecting the angles made by the back and two
sides, but any considerable departure from the rectangular form would be
found inconvenient, b is a door of clear thick plate glass ; c is the external
door. A lamp is hung at the top of the post d to light and assist in ventilating
FIG. 28.
THE REFLECTION OF LIGHT.
29
the box by promoting an upward current, and a mat or rug is placed at the
bottom of the box or chamber.
The same co-inventors, by placing a silvered-glass mirror at an angle,
and thrown back from the spectators, produced some very popular illusions,
one of which, called " The Modern Delphic Oracle," may be thus described.
The curtain being raised, a person dressed in the garb of an ancient Athenian
nobleman walks through and out of the entrance to a temple, across which a
curtain rolls as he passes. Walking in front, he throws incense on a Crazier
of charcoal, and invokes Socrates to appear. The curtain now rolls back and
Elevation showing the appearance presented by the illusion called "The Modern Delphic Oracle."
discloses the head of the sage floating in the air, the proof of its solidity being
that it casts a shadow on the wall behind. The Greek asks Socrates whether
the words he spoke on the occasion of his memorable trial accurately expressed
his real convictions whether the purpose of his life was as pure as we have
been taught to believe. The sage replies :
'It was my purpose ever to control
The stormy passions that perturb the soul ;
Averse from idle pomp and wealth,' to find
The only lasting treasure in the mind.
The truth I learned without reward to teach.
And show the falsehood hid by forms of speech;
The voice that warned within me to obey
That safest guide when doubtful was my way.
I learned to live as one prepared to die,
And calmly met my fate when death drew nigh j
Rejoiced to quit this troubled world, and rest
Immortal in the regions of the blest ! " *
* Written by John Oxenford, Esq,
ON LIGHT.
The curtain once more rolls before the entrance, and as it is re-opened to
allow the Athenian to pass through, the head has vanished, and nothing but
the bare walls are apparent.
This illusion is performed with the aid of a large silvered mirror, which is
placed at an angle across the small chamber in which the head appears, and
being perforated in the centre, the head of the actor is thrust through the
hole, whilst the rest of the large mirror conceals his body, and, reflecting only
the top of the room, painted to represent the back of the temple, induces the
spectator to suppose he is looking at a head suspended in an empty room.
FIG. 30.
Transverse section- A B, the silvered mirror; c, the hole through which the actor thrusts his head;
B D, the ceiling painted is reflected in the mirror, and appears behind the head at H H.
The mirror is carefully supported on a framework on wheels, and can be rolled
out of the way when the actor representing the Athenian walks through in
coming out and returning to the temple.
The exhibition of the Ghost at the Polytechnic took London by surprise
as a novelty. It is, however, evident from the next diagram, copied from
"Robinson's Recreative Memoirs," published in 1831, that he approached
very near to the arrangements necessary to produce reflected images from
plane surfaces. In the first place, Robertson remarks, it is necessaay to take
care that the angles of the mirror must not exceed 20. You may try in vain
to increase this angle by increasing the size of the mirrors a, b, c, which reci-
procally cause the rays to pass through the opening where a double-convex
lens is placed. Thus to obtain an image of the same size as the object say
6 ft. high it is necessary to place the figure 18 ft. distant' from the mirror r,
and to use a lens of 9 ft. focus, to have the image 1 8 ft. on the other side of
THE REFLECTION OF LIGHT.
Fl<J. 31. Robertson's proposed Apparatus for Ghost.
the partition, \\here it is projected on to the curtain or screen. You may place
the real figure on the lens side or the mirror side. Robertson then gives direc-
tions for altering the positions of the figures, according to the space the
operator has on either side of the partition. It is, however, difficult to con-
ceive that the image thrown upon a screen in this way could have been pro-
perly illuminated, unless sunlight was employed. The whole diagram betrays
theory instead of practice.
THE KALEIDOSCOPE.
One of the most philosophical and beautiful instruments ever constructed,
and, like the above illusion, wholly dependent on reflection, is the amusing
toy invented by the late Sir David Brewster, called the Kaleidoscope, from
the Greek words KaAos, beautiful, 1809, a form, and cr/coTre'w, to see. Sir David
Brewster says, " The first idea of this instrument presented itself to me in the
year 1814, in the course of a series of experiments on the polarization of light
by successive reflections between plates of glass, which were published in the
'Philosophical Transactions' for 1815, and which the Royal Society did me the
honour to distinguish by the adjudication of the Copley medal." " On the 7th
February, 1815, when I disco vered^ the development of the complementary
colours by the successive reflection of polarized light between two plates of
gold and silver, the effects of the kaleidoscope, though rudely exhibited, were
again forced upon my notice. In repeating, at a subsequent period, the very
beautiful experiments of M. Biot on the action of homogeneous fluids upon
polarized light, and in extending them to other fluids which he had not tried,
I found it most convenient to place them in a triangular trough', formed by
two plates of glass cemented together by two of their sides, so as to form an
acute angle. The ends being closed up with pieces of plate glass cemented
to the other plates, the trough is fixed horizontally for the reception of the
fluids. The eye being necessarily placed without the trough, and at one end,
some of the cement, which had been pressed through between the plates at
the object end of the trough, appeared to be arranged in a manner far more
symmetrical and regular than I had before observed, when the objects, in my
early experiments, were situated at a distance from the reflectors. From the
ON LIGHT.
approximation to perfect symmetry which the figure now displayed, compared
with the great deviation from symmetry which I had formerly observed, it was
obvious that the progression from the one effect to the other must take place
during the passage of the object from the one point to the other, and it
became highly probable that a position would be found where the symmetry
was mathematically perfect.
" By investigating this subject optically, I discovered the leading principles
of the kaleidoscope in so far as the inclination of the reflectors, the position
of the object, and the position of the eye are concerned.
" I found that in order to produce perfectly beautiful and symmetrical forms
three conditions were necessary :
" Firstly, That the reflectors should be placed at an angle which was an
even or an odd aliquot part of a circle when the object was regular and simi-
larly situated with respect to both the mirrors ; or an even aliquot part of a
circle when the object was irregular and had any position whatever.
" Secondly, That out of an infinite number of positions for the object, both
within and without the reflectors, there was only one where perfect symmetry
could be obtained, namely, when the object was placed in contact with the
ends of the reflectors. This was precisely the position of the cement in the
preceding experiment with the triangular trough.
" Thirdly, That out of an infinite number of positions for the eye there was
only one where the symmetry was perfect, namely, as near as possible to the
angular point, so that the circular field could be distinctly seen.
" The great step, however, towards the completion of the instrument re-
mained yet to be made, and it was not till some time afterwards that the idea
occurred to me of giving motion to objects, such as pieces of coloured glass,
&>c., which were either fixed or placed loosely in a cell at the end of the instru-
ment.
" When this idea was carried into execution, and the reflectors placed in the
tube and filled up on the preceding principle, the kaleidoscope in its simple
form was completed.
" When the kaleidoscope was brought to this degree of perfection, it was
impossible not to perceive that it would prove of the highest service in all the
ornamental arts, and would at the same time become a popular instrument for
the purposes of rational amusement. With these views, I thought it advisable
to secure the exclusive property of it by a patent. But, in consequence of
one of the patent instruments having been exhibited to some of the London
opticians, the remarkable properties of the kaleidoscope became known before
any number of them could be prepared for sale.
" According to the computation of those who were best able to form an
opinion on the subject, no fewer than 200,000 instruments were sold in London
and Paris during three months.
" In order to construct the kaleidoscope in its most simple form, we must
procure two reflectors about 5, 6, 7, or 8 in. long. These reflectors may be
either rectangular plates, or plates shaped like those in Fig. 32, having their
broadest ends, A o, B O, from I to 2 in. wide, and their narrowest ends, a E,
b E, half an inch wide.
" If the reflectors are of glass, the newest plate glass should be used. The
plate glass may be either quicksilvered or not, and its posterior surface may be
ground, or covered with black wax, or black varnish, or anything else that
reverses its reflecting power.
THE REFLECTION OF LIGHT.
33
FIG. 32.
" The proper application of the objects at the end of the reflectors is now
the only step which is required to complete the simple kaleidoscope. The
most simple method consists in bringing the tube about half an inch beyond
the ends of the reflectors. A circular piece of thin glass of the same diameter
as the tube is then pushed into the tube so as to touch the reflectors. The
pieces of coloured glass being laid upon this piece of glass when the tube is
FIG. 33. The Oxy-hydrogen Kaleidoscope as made by Mr. Darker.
Key pattern, produced from a key.
held in a vertical position, another disc, having its outer surface ground with
fine emery, is next placed above the glass fragments, being prevented from
pressing upon them by a ring of brass, and is kept in its place by burnishing
down the end of the tube." Such are the instructions given by Sir David
Brewster for the manufacture of the ordinary kaleidoscope ; he also speaks of
the application of the instrument to the magic lantern, but as the details were
not sufficiently complete to enable any one to throw the kaleidoscopic figure
3
ON LIGHT.
on the disc, the author was induced to urge Mr. Darker, of Paradise Street,
Lambeth, to persevere in the adjustment of the mirrors, lenses, and lighting
until perfection was obtained. During the Christmas of 1866 the oxy-hydrogen
kaleidoscope was exhibited daily at the Polytechnic with the greatest success,
and by its means the principle of the instrument could be better understood.
FIG. 34.
a, Figures obtained by putting a single figure, such as key, into the apparatus; b, c t other figures
produced by using the light only with 'an empty slide.
It is chiefly by the adjustment of the light that the original angular opening
is gradually multiplied by reflection eight times, and eight distinct sectors
or divisions become visible on the disc. When the tip of the finger is now
inserted, eight single reflections or four double ones are the result, and by
thrusting in all the fingers the curious figures shown at , Fig. 35, are obtained.
Not only are transparent bodies, such as glass, exhibited with success, but any
opaque object will produce the most distinct and symmetrical figures on the
FIG. 35.
Figures obtained on the screen from the oxy-hydrogen kaleidoscope with pins and needles, D; the
fingers, E; and F, a comb.
screen; in Fig. 35 the pattern ? is chiefly produced with a cell containing only
pins and needles. If glass be used, it should always be broken from coloured
glass rods with the hammer, in order to secure the conchoidal fracture, as the
wedge-shaped figures give gradual tones of colour, which are very pleasing to
the eye, and produce fair imitations of the colours and grouping of rubies,
emeralds., and sapphires when projected on the screen.
THE REFLECTION OF LIGHT. 35
A gentleman, who saw these and other patterns, and especially some obtained
by using ferns and other natural objects, was so pleased that he stated it was
his intention to have an oxy-hydrogen kaleidoscope fitted up in his calico-
printing establishment, in order to assist the artist who designed the patterns ;
and he stated that, although they had long used the ordinary kaleidoscope for
this purpose, the oxy-hydrogen one gave a much better notion of the effect
required to be produced, and would enable the manufacturer to select and
decide upon the best patterns for commercial purposes.
The phenomena of light produced by reflection, and the instruments which
have been constructed to demonstrate these effects, are too numerou& to be
detailed here, so that two or three examples must suffice. The property of
reflection is affected more by the condition of the surface than by the physical
nature of the substance used as a reflector. The kaleidoscope reflectors em-
ployed by Mr. Darker are made of the best plate glass, coated with metallic
silver, and it is extremely difficult to prevent a slight deposit of moisture upon
them. The watery particles greatly impair the kaleidoscopic figures, and
demonstrate how thoroughly the power of reflection depends on the state of
the surface, as this exquisitely thin film of moisture interferes with the perfect
illumination of the kaleidoscopic figure.
FIG. -fiBack of the Japanese Mirror.
THE JAPANESE MAGIC MIRROR.
Some mirrors made in Japan have a very curious property. The back is
usually ornamented with Japanese characters, also with flowers, vases, &c. ;
the front is polished in the usual manner, like any other metallic speculum,
and, if carefully examined, with or without a magnifying power, betrays nothing
more than the highly polished surface of the alloy, which appears to be com-
posed chiefly of tin and copper. When, however, the mirror is held in the
highly divergent rays emitted from an oxy-hydrogen light, it not only reflects
on to a disc the surface of the polished disc, but likewise all the Japanese
characters, vases, and flowers, which are in relievo on the back of the mirror.
3 2
ON LIGHT,
FIG. 37. Reflection from the front or bright side of the Japanese Mirror.
We have in the above experiment a scientific puzzle that is somewhat difficult
to explain. May it be supposed that much of the success of the effect obtained
is due to the nature of the alloy used in the casting of the mirror ? The
figures in relief on the back of the mirror, during the operation of casting,
must first enter the mould in the liquid state : are these first and quickly
congealed before the whole mass of metal ? and does the minute difference in*
the molecular condition of the metal produced by a greater rapidity of cooling,
extend through the thin metal to the front and polished side ?
Would careful heating and anfiealing destroy the effect ? Whatever may
be the method employed, it is certain that the figures reflected from the
surface are wholly invisible, and cannot be observed in the strongest light,
and with a good magnifying-glass. In all cases where metals are inlaid with
other metals the lines where the metals join are distinctly visible, and there-
fore it cannot be supposed that the Japanese mirror is made in this manner.
Are the mirrors cast in a double mould, one side of which is in intaglio and
the other in relievo, and after being cast do they grind down the sides of the
mirror in which the figures are sunk, until they get a plain surface, which is
then polished, leaving the other side and back of the mirror with the figures
in relief? The pattern die, conferred on both sides of the metal whilst soli-
THE REFLECTION OF LIGHT. 37
difying, might still further determine the molecular difference. It is a curious
circumstance that the Chinese mirrors, made in imitation of the Japanese
mirrors, do not answer the purpose, the former being much heavier than the
latter. Whatever may be the secret of success, it is certain that this is only
another instance of the remarkable ingenuity of the Japanese workers in
metal.
Sir D. Brewster explains the apparent anomaly by suggesting that the
design on the back is dexterously reproduced by careful engraving, which is
so lightly done that the figures traced are quite invisible after the mirror is
brought to the highest degree of polish, and it is only by submitting the
mirror to a powerful light, and casting the reflection of the surface on a wall,
that the design becomes apparent. The concealment of the most delicate
engraving, unless done in some way by Barton's ruling-machine, would be
extremely difficult, if not impossible. The Japanese know nothing of the
machine with which Barton ruled his steel patterns, and even if they did the
reflected patterns would give evidence of colour, which is not the case.
In the " Journal of the Asiatic Society," vol. i., page 242, there is a very
clever paper, by James Prinseps, on " The Magic Mirrors of Japan." He says:
" The Japanese mirror is a slightly convex disc of bell metal, about 6 in. in
diameter, and a quarter of an inch in thickness on the edge, ground and po-
lished on the convex face, and covered with a thin coating of silver to give it a
white colour. (Fig. 38, p. 39.)
" The back of the mirror is deeply curved or indented, with ornamental work
in circles and festoons, and it bears an inscription in the Japanese character
in high relief upon what may be termed the tympanum of the disc ; in the
centre there is a projecting knob, perforated laterally to receive a string for
suspending the mirror. The metal is highly sonorous when struck as a bell,
and is so soft as easily to be indented or scratched on contact with any hard
substance. I found its composition to be
Copper 80
Tin 20
100
with no traces 01 silver or arsenic, and a very slight indication of zinc*"
Mr. Prinseps then describes the curious property of the mirror, similar in
effect to those already mentioned and illustrated at Fig. 37, p. 36, He then
proceeds to discuss the cause of this seeming anomaly.
" It then occurred that the various parts of the Japanese mirror might be of
different density, supposing the pattern to be made by stamping, and that
either the rays of light might be more forcibly repelled by the denser metal
than by the lighter, or that parts of the surface would acquire different
degrees of polish, sufficient to cause the illusion, although imperceptible to the
eye. But in such case the thin parts, from being the hardest, should give the
stronger reflection.
" This supposition was also overthrown by experiment. A disc of silver,
having been annealed at a red heat so as to be quite soft, was stamped on the
back with a circular ring, deeply indented, so as to harden the silver in that
part only. The opposite surface was then ground and polished, when it was
found to give a clear and uniformly reflected spectrum.
" Another and, I believe, the true explanation is suggested by the well-known
3 8 ON LIGHT.
phenomenon of the reflection from a brass button, which every school-boy has
remarked when sporting his Sunday ' blue coat with metal buttons' in the sun-
shine of his tutor's parlour-window. The button throws a radiated irregular
image on the wall, exhibiting two bright concentric circles, one on the edge and
another about one-third within it, and there is generally a bright spot in the
centre : all of this seems but the picture of the stamp on the back of the button :
the radii resemble, and indeed coincide with, the letters of 'superfine' or 'trebly
gilt' inscribed within a double circle, and the central spot represents the
shank. There can be little doubt that the principle is in this case precisely
that of the Japanese mirror ; and, on a cursory view, the surface looks equally
smooth and unsuspicious. On minute examination, however, of several buttons,
I found them to be by no means plane ; their general surface is slightly convex;
there is a hollow in the centre and a projection in the position of the inscrip-
tion behind, caused no doubt by the blow necessary in stamping it. The polish
is probably given by a rotary motion, and consequently does not remove these
very small irregularities. To follow up the experimental investigation, I selected
one of the buttons which gave a good image, ground it on a flat hone, and
polished it: all of the magical figures vanished in a moment, and a plain,
bright disc appeared in their stead. Here, then, may be a key to the mystery
of the mirror: the deception is entirely produced by irregularities on the sur-
face, which are rendered the less perceptible to the eye because the surface is
convex instead of being plane. But it may be objected that the two circles
which appear bright in the reflected spectrum of the button represent the
indented or thin parts of the metal, whereas the thick parts of the Japanese
mirror are those which will appear illuminated. A short analysis of the facts
in either case will readily explain to what these discrepancies are attributable ;
but it will be necessary to have recourse to a diagram.
" Let A B, Fig. 38, be a plain mirror upon which the rays of light R impinge ;
they will be reflected uniformly to R', forming a clear image. Now let A B c D
E F G be another reflecting surface, having two convexities, B C, E F, and one
concavity in the centre D (the condition nearly of the brass button). In this
case the light reflected from the outer concave flexures of the protruding
portion of the surfaces B c, E F, will converge in the foci b c and ^/respectively,
at distances corresponding to the radius of their curvature ; the effect will, of
course, be visible within wide limits of the actual focus. In most of the buttons,
however, the central depression is so great that it collects the rays in a focus,
d t a few inches only in front of the surface ; and when the spectrum is thrown
farther off, the rays crossing from two less distinct luminous foci, d' d', it follows
from analogy that the thin parts or tympanum of the Japanese mirror are
slightly convex with reference to the rest of the reflecting surface, which may
have been caused either by the ornamental work being stamped or partially
carved with the hammer and chisel on its back, or, what is more probable,
that part of the metal was by this stamping rendered harder, so that in po-
lishing it was not worn away to the same extent."
Since the above was written, an English brass-finisher appears to have dis-
covered the secret. Taking ordinary brass, he finds that any figure stamped
upon it with a proper die, and ground down and polished, will not reflect the
figure impressed by the die; but if the process with the same die is repeated
three times, so that the figure intended to be projected from the surface is
stamped three times in the same place, and subsequently ground down and
polished after each stamping, then a molecular difference is established between
THE REFLECTION OF LIGHT.
39
FIG. 38.
the stamped and unstamped parts, which is not apparent to the eye, but is
shown directly the surface so acted on is used for reflecting light.
There can be no doubt that, until the magic lantern was invented, the only
optical apparatus used by persons who pretended to wield the " magic art "
consisted of plane and concave mirrors. The memoirs of Monsieur E. J.
Robertson, published in Paris in 1831, disclose some amusing applications of
surfaces that reflect light, and he describes' how the magician Nostrodamus
deceived the politic Marie de Medicis, and pretended to show the astute
queen the king for whom the throne of the Bourbons was destined. He states
that Marie de Medicis, disquieted by apprehensions regarding the succession
to the throne of France, went to consult Nostrodamus. This dealer in
miracles by the use of plain mirrors produced the effect shown in Fig. 39, p. 40.
La Boite Magique. The magic box is another amusing example of the
same kind, only in this case a concave mirror is employed instead of a plane
one. This experiment, Robertson declares, is charming, and having, he says,
told a lady the secret of several illusions which pleased her greatly, he
happened to be staying with the same individual in the country, at the time
that a most agreeable gentleman was paying his court to her; the latter
said to her lover, " If you do not fear apparitions, I promise you one this
evening which may please you. At twelve precisely open the box that you
ON LIGHT.
FIG. 39.
The throne, placed in the first apartment A, is reflected by a mirror concealed in the canopy B, Marie de
Medicis beholds the representation of the image in a mirror c, supported by a Cupid.
will find on your table, of which this is
the key, and my image will come out
of the box." This promise seemed only
an agreeable kind of banter to her gal-
lant, and, though he promised to open
the box, he feared to do so, lest he might
be made the dupe of some trick. At
first he would not touch it, but at last,
yielding to curiosity, he opened the
box, when the image of his lady-love
immediately appeared, with a very grave
and composed air ; but she, guessing
that the countenance of her gallant
must bear a strange a serio-comic,
though interesting expression, forgot
that silence was necessary, and, burst-
ing out into laughter, was thus disco-
vered in the adjoining room.
FIG. 40.
A, concave mirror; the head B, inclined towards c, appears to emerge from D, to an eye placed
at EJ the head, B, must be well illuminated, and the mirror in the shadow, so that it may not be
visible; G is the wall; at H a box to open, firmly fixed on a table K. The interior of the box is painted
black, and of course the \vall which separates the two apartments is open under the table.
THE REFLECTION Of LIGHT. 41
The ancients made use of concave mirrors to rekindle the vestal fires.
Plutarch says they employed o-Kac/>aa, or dishes, for that purpose. They were,
most likely, hemispherical vessels highly polished within.
As an illustration of the more refined uses and applications of silvered
mirrors, may be quoted the admirable instructions given by Mr. John Browning,
of in Minories, for adjusting and using reflectors for astronomical tele-
scopes with silvered-glass specula.
FIG. 41.
MR. BROWNING'S DESCRIPTION OF THE SILVERED GLASS REFLECTING
TELESCOPES.
These telescopes are of the kind called Newtonian, a form so well known,
that it is, perhaps, scarcely necessary to describe it ; but I append a plain
diagram (Fig. 41) and brief description, because it will assist in making
clearer the instructions I have given further on, of the method of adjusting
the instrument. The Newtonian telescope consists of a tube closed at the
lower end, which is occupied by a concave mirror, M. The cone of rays
reflected from this mirror is again reflected at right angles from the surface
of a small plane mirror, m n, mounted at an angle of 45, near the open end
of the tube, into the eye-piece, which is exactly opposite.*
In reflecting telescopes, as originally constructed, the concave mirror was
made of an extremely hard alloy, known as speculum metal. These metallic
mirrors possessed several disadvantages, so serious in character -that they
have for some time fallen out of general use. The principal defects were
the following :
1. From the extreme brittleness of the alloy, they were very liable to fracture,
sometimes breaking merely from a sudden change of temperature.
2. From their great weight it was extremely difficult to mount them in
such a way as to prevent flexure, the smallest amount of which greatly
injured their optical performance.
3. Their greatest drawback, however, consisted in the fact that the surface
of the metal, from damp or other causes, sometimes became very rapidly
tarnished, and this tarnish could seldom be removed, except by repolishing
and, consequently, refiguring the mirror ; and this involved nearly as great
an outlay as the purchase of a new speculum, besides incurring the serious
risk of a fine figure being irretrievably lost.
In the telescope now described, the metallic mirror is replaced by one of
* The mirror must not be worked to a spherical, but to a very perfect parabolic curve.
42 ON LIGHT.
glass, on the surface of which a coating of pure silver has been deposited by
Liebig's process, and described further on.
These glass mirrors are not at all injuriously affected by change of tem-
perature, and their lightness very considerably reduces their liability to flexure ;
indeed, mounted in the manner I shall presently describe, no flexure has ever
been observed in them. I may, however, state that I make the discs of the
specula, which Mr. With parabolizes for me, out of glass nearly twice the
substance of that generally used for the purpose. The coating of pure silver
reflects fully one-third more light than the best speculum metal, as the alloy
before mentioned is called. But the greatest superiority of silvered glass over
metallic mirrors consists in the fact that, should they become tarnished, their
brilliancy may readily be restored by gentle friction with soft leather and a
little of the finest rouge ; and even should the silver coating become utterly
spoiled, it may be easily removed without in any way impairing either the
figure or polish of the glass speculum, and a fresh one deposited at a trifling
cost, thus making the mirror equal to new ; and this may be repeated indefi-
nitely. Should the owner possess a little patience, he may renew the coating
himself at the cost of only a few pence. The silvering process is fully described
further on.
With this alteration these telescopes have latterly gained much ground in the
opinion of practical observers well known in the scientific world, who have
had considerable experience in working with them.
On figuring Specula. About three years since, the Rev. Cooper Key dis-
covered a more simple method of parabolizing the surface of specula than any
which had hitherto been employed, and by this process he produced two fine
specula of 12 in. diameter.
The process by which these specula were worked Mr. Key communicated to
Mr. G. With, and after having worked by Mr. Key's process until a few months
since, Mr. With at length contrived, another plan of working, by which he
considers still finer results are with greater certainty secured.
The wonderful perfection of Mr. With's specula is now generally admitted,
and it is almost certain that they surpass any that have previously been
produced. I have great pleasure in stating that specula of Mr. With's para-
bolizing are now only to be obtained from me.
On mounting Specula. It has elsewhere been suggested that much of the
dissatisfaction which has been expressed by those who have used reflectors
has arisen from their having been imperfectly mounted.
Because specula are much cheaper than achromatic object-glasses, it has
been supposed that they could be mounted at proportionately less cost than
that incurred in mounting reflectors. This is only true to the extent that cost
can be saved by reason of their shorter focal length.
It cannot be too strongly enforced that, to give the best performance,
reflectors require to be mounted more steadily than refractors, because by a
well-known law of optics the effect of any vibration will be multiplied many
times. Their tubes must also be carefully arranged, so as to avoid as much
as possible the interference of air-currents, which are the bane of reflectors
improperly mounted or badly situated. The specula in the telescopes now
described are mounted rigidly on a new plan, which ensures permanence in
adjustment and prevents flexure. This plan is represented in Fig. 42.
The bottom of the speculum A is a carefully prepared plane surface, and
the bottom of the inner iron cell B, on which it rests, is also a plane. The
THE REFLECTION OF LIGHT.
43
FIG. 42.
speculum is clamped in this cell by the ring G G, and it may be removed from
and replaced in the telescope without altering its adjustment. The elastic
methods of mounting the speculum, which have hitherto been employed,
generally required re-adjustment whenever the speculum had been removed.
The reflecting diagonal prism or mirror is mounted in the manner shown
in the diagrams Figs. 43 and 44.
FIG. 44.
In these B B B represent strips of strong chronometer spring steel, placed
edgewise towards the speculum, by which the prism or small mirror D is
suspended.
The mirror, thus mounted, does not produce such coarse rays on bright
stars as when it is fixed to a single stout arm ; it is also less liable to vibration,
which is very injurious to distinct vision, or to flexure, which interferes with
the accuracy of the adjustments.
If an observer determines to lay out a given sum in the purchase of a tele-
scope, he will find it to his advantage to have a smaller speculum completely
mounted, instead of a large speculum imperfectly mounted. With the smaller
and perfect instrument he will really do more work, and with much greater
comfort and satisfaction to himself. No matter how good a speculum may
be, nothing can be told of its performance on difficult double stars if it is
mounted on an unsteady stand.
44 ON LIGHT.
The alt-azimuth stand, represented in Fig. 45, is entirely of iron. The tube
of the telescope is of extremely stout block tin, coloured dark green, the stand
being coloured dark chocolate. The body is equipoised, so that it will remain
in any position, while the movements are so smooth, and the leverage so
arranged, that a star may be followed, even with a power of 300, without screw
motions. The instrument can be used on a table, at any window ; and a stand
is supplied with it, on which it can be supported at a convenient height when
it is used in the open air. This mounting is only adapted for a small-sized
speculum, say not exceeding 5 in. in diameter, as, if made of a larger size, it
FlG. 45. The small Alt-azimuth.
would be so heavy as not to be portable ; while with higher powers than 300,
such as specula of 6 in. and above will easily bear, the celestial bodies cannot
be followed without screw motions. By fastening the circular foot down on a
block of wood of a wedge form, the angle being the complementary angle to the
latitude of the place, this stand can very readily, and at a comparatively trifling
expense, be made to move equatorially, so that the heavenly bodies can be
followed with a single motion of the telescope. Such an arrangement is shown
in Fig. 45. A cheaper mounting is shown in Fig. 54.
The 4|-inch silvered-glass speculum, with powers from 100 to 150, will
divide
/3 Orionis. a Lyrae.
8 Geminorum. e Hydras.
Ursae Majoris.
Bootis.
v Ceti. Draconis.
The 6| will divide, with powers from 200 to 300
Arietis. a Herculis.
Bootis. 32 Orionis.
i Equulei. 77 Coronas Borealis.
36 Andromedas.
The 8 J, with powers from 300 to 350, in a favourable state of the air, will
divide
THE REFLECTION OF LIGHT.
45
y- Andromedas.
fj, Bootis.
These last-named double stars are both under half a second apart, and are
so difficult to divide as to have hitherto been considered good work for a
12-inch speculum.
TO SILVER GLASS SPECULA.
Prepare three standard solutions :
Solution A
Solution B
Solution C
90 grains
4 ounces
i ounce
25 ounces
ounce
Dissolve.
Dissolve.
Dissolve.
Crystals of nitrate of silver
Distilled water ....
Potassa, pure by alcohol
Distilled water ....
Milk-sugar, in powder
Distilled water 5 ounces
Solutions A and B will keep, in stoppered bottles, for any length of time ;
solution C must be fresh.
The Silvering Fluid. To prepare sufficient for silvering an 8-inch speculum,
pour 2 ounces 'of solution A into a glass vessel capable of holding 35 fluid
ounces. Add, drop by drop, stirring all the time with a glass rod, as much liquid
ammonia as is just necessary to obtain a clear solution of the grey precipitate
first thrown down. Add 4 ounces of solution B. The brown-black precipi-
tate formed must be just re-dissolved by the addition of more ammonia as
before. Add distilled water until the bulk reaches 1 5 ounces, and add, drop
by drop, some of solution A, until a grey precipitate, which does not re-dissolve
after stirring for three minutes, is obtained, then add 15 ounces more of dis-
tilled water. Set this solution aside to settle. Do not filter.
When all is ready for immersing the mirror, add to the silvering solution 2
ounces of solution C, and stir gently and thoroughly. Solution C may be filtered.
Perfectly pure chemicals may be obtained of Mr. Townson, 89, Bishopsgate
Within, London, E.G., and Mr. R. f Thomas, 10, Pall Mall.
To prepare the Speculum. Procure a circular block of wood 2 in. thick and
2 in. less in diameter than the speculum. Into this should
be screwed three eye-pins at equal distances, as in Fig. 46.
To these pins fasten stout whipcord, making a secure loop
at the top.
Melt some soft pitch in any convenient vessel, and hav-
ing placed the wooden block face upwards on a level table,
pour on it the fluid pitch, and on the pitch place the back
of the speculum, having previously moistened it with a thin
film of spirit of turpentine to secure adhesion. Let the
whole rest until the pitch is cold.
To clean the Speculum. Place the speculum, cemented to the circular
block, face upwards, on a level table ; pour on it a small quantity
of strong nitric acid, and rub it gently all over the surface with a
brush made by plugging a glass tube with pure cotton wool. (Fig.
47.) Having perfectly cleaned the surface and sfdes, wash well
with common water, and finally with distilled water. Place the
speculum face downwards in a dish containing a little rectified spirit
ef wine until the silvering fluid is ready.
To immerse the Speculum. Take a circular dish about 3 in.
FIG. 47. deep and 2 in. larger in diameter than the speculum. Mix in it
FIG. 46.
4 6
ON LIGHT.
FIG. 48.
the silvering solution and the solution C, and suspend the speculum, face
downwards, in the liquid, which may rise about a quarter of
an inch up the side of the speculum.
When the silvering is completed, remove the speculum
from the solution, and immediately wash with plenty of
water, using at least two gallons, and finally with a little
distilled water. Place the speculum on its edge on blotting-
paper to drain and dry. (Fig. 48.)
When perfectly dry, polish the film by
gently rubbing first with a piece of the
softest wash-leather, using circular strokes
(Fig. 49), and finally with the addition of
a little finest rouge.*
A " flat " may be silvered by fastening with pitch to a slice
of cork, cleaning as above described, and using as much sil-
vering fluid as will form a stratum about half an inch deep
beneath the mirror.
To separate the Speculum from the Block. Stand the speculum on its side,
insert the edge of a sharp half-inch chisel between the wood and glass, adminis-
tering two or three gentle blows, and the block and mirror will separate safely
and easily. It is preferable to obtain the aid of an assistant in this operation.
Any pitch which remains on the back of the mirror may be removed by
scraping and a little turpentine.
The cost of silvering an 8-inch speculum, exclusive of the cost of alcohol,
which may be used over and over again, will not exceed gd.,
Nitrate of silver being 45. per oz.
Potash . . 8d.
Milk-sugar . . 2d. n
Avoid all excess of ammonia, and be sure to use//m? distilled water.
ON WORKING GLASS SPECULA.
FIG. 49.
FIG. 50.
When parallel rays of light are allowed to fall upon the surface of a concave
mirror, if the surface be a spherical curve, the rays will not all be reflected to
a single point.
,In Fig. 50 it will be seen that the rays A A, falling on the mirror, would be
* The silvering will be completed in from 30 to 70 minutes, according to temperature ; Jo minute?
will be sufficient in summer.
THE REFLECTION OF LIGHT. 47
reflected and form an image at a ; while the rays B B would be reflected and
form an image at <, farther from the front of the mirror.
If the reflected images were viewed with an eye-piece placed anywhere in
front of the mirror, they would not be in focus at the same time, so that only
a blurred and indistinct image would be seen.
To make the mirror reflect rays falling on all parts of its surface to one
point, it is necessary that it should be fashioned into a parabolic curve.
FIG. 51.
Such a curve is snown in Fig. 51, which maybe considered as a spherical
curve, in which the curve has been made deeper or the outer portion flattened.
In practice the amount of this difference is so exceedingly minute as to be
inappreciable by actual measurement.
Sir John Herschel states that the utmost variation of a 4-foot speculum
from a spherical curve is less than than one 2i,oooth part of an inch. Yet it
is well known that for telescopic use a mirror with a spherical curve is, for the
reason just explained, totally useless.
In working the glass specula, a disc of hard crown glass, varying in substance
from three-quarters of an inch to one and a half inches, according to the size of
the speculum for which it is intended, is turned, and polished on the edge.
One side of this disc is ground to a truly plane surface. On this side the
speculum, when mounted on the writer's plan, rests in its cell. The other
side is then ground to a concave spherical curve of such a radius as will
produce the desired focus. This spherical curve is converted into a parabolic
figure somewhat thus :
An iron tool, similar to that on which the spherical curve has been ground,
fs covered with a layer of pitch, tempered to a certain consistency. This pitch
is warmed, and the speculum being laid upon it makes the pitch assume the
same curve. The speculum is then polished on the pitch with rouge. In this
polishing the speculum and polisher are not worked together equally all over
the surfaces, but the speculum is moved in such a manner that it is polished
away most towards the edge, and a parabolic curve is produced. During the
process both the speculum and the polisher continually revolve.
The diagram of Lord Rosse's machine, with which he figured his speculum
6 ft. in diameter, will give an idea of the action of the speculum and polisher
on each other.
This machine is represented in Fig. 52; A is the spindle, by turning which
the whole machine is set in motion ; H I is the speculum ; K L, the polisher ;
B, an excentric which carries the polisher backwards and forwards ; G, another
excentric Avhich moves the polisher from side to side slowly during the recipro-
ON LIGHT.
eating motion. The amount of motion given to the polisher, and the rapidity
of rotation of the speculum, can be changed at pleasure.
Fig. 52.
In Fig. 53 the dotted line represents the spherical curve of the mirror when
the polishing is begun, and the continuous line the parabolic curve it assumes
when the polishing process is finished. It will be, of course, understood that
in all the diagrams these curves are enormously exaggerated.
During the graduated polishing the speculum is repeatedly tested, until the
desired definition is attained. When completed, if accurately figured, the
marginal inch of the speculum should give equally sharp definition with the
centre, and have identically the same focus.
FIG. 54.
In figuring the mirrors of the telescopes herein described, an improved
method has been adopted of fashioning the parabolic curve; it is believed
this method gives superior results to any hitherto attained.*
* The reader who wishes for further information on this subject is referred to Sir John Herschel's
work on "The Telescope."
THE REFRACTION OF LIGHT. 49
THE REFRACTION OF LIGHT.
When a ray of light passes from one medium to another 01 tne same
density, and in a perfectly straight line, no alteration of its course takes place ;
but if the light passes in an oblique direction, its course is broken or refracted,
i.e., bent back from its natural path. To this branch of optics, which includes,
perhaps, the widest field of inquiry, and traces the propagation of light through
transparent, solid, liquid, and gaseous bodies, has been given the name of
DIOPTRICS.
To prove that a straight line representing a ray of light is really bent when
passing from a rare medium, air, into a denser one, such as water, nothing is
easier than to place a bright shilling on the end of an ivory paper-knife, which
is inclined in a large empty tumbler. On looking down the paper-knife a
straight line only is apparent, terminating with the coin ; but if the tumbler
is filled with water whilst the observer is still looking down the flat surface, he
FIG. 55. A simple demonstration of the property of Refraction.
will notice that at the point of juncture between the air and water a break
takes place, and the end of the paper-knife, or all that part immersed, appears
to be lifted up or bent upwards from its natural course or direction. If a small
pocket-pistol were now aimed at the coin and the bullet discharged it would
certainly miss, because every visible object appears to be in a direction repre-
sented by a straight line drawn from it to the eye. A straight line ruled to
the. shilling would not touch it, the line must be ruled to, or the pistol aimed
at, a point nearer to the spectator than the apparent position of the coin.
The bending of the ray is governed by certain laws known as " Descartes'
Laws.'"'
Firstly, Whatever the obliquity of the incident ray, the sine of the incident
angle and the sine of the angle of refraction are in a constant ratio for the
same two media, but vary with different media.
Secondly, The incident and the refracted rays are in the same plane, which
is perpendicular to the surface separating the two media.
A very complete French apparatus (Fig. 56), described in Ganot's " Physics,"
4
5
ON LIGHT.
is made for the purpose of proving those laws experimentally,, It consists of
a large and carefully graduated circle supported on a tripod stand. In the
centre is placed a semi-cylindrical glass vessel filled with water, or any other
fluid whose index of refraction it is required to ascertain, so that the level of
the fluid coincides with the height of the centre of the circle. From the
mirror A, a ray of light is reflected through a hole in the screen B, and falls
on the surface of the water at c. Passing through the water, the course of the
refracted ray is traced to a screen D, on which
the circular image is received. In the various
positions of the screens B and D, attached to
arms radiating from the centre C, the sines of
the angles of incidence and refraction are ob-
tained and measured by two graduated rules
E F, movable so as to be always horizontal,
and therefore perpendicular to the diameter
G H.
The numbers vary with the positions of the
screens, but the sines of the incident and re-
fracted rays are in a constant ratio to the same
two media, viz., air and water. If the sine of
the incident ray is doubled, the sine of the
refracted one will increase in the same ratio.
When another fluid is put into the trough,
a variation in the sines would occur, and it is
in this manner the first law is proved. By
moving the mirror and screen B, so that the
light falls perpendicularly on the surface of
the water, the instrument proves the second
law, as there cannot then be any angle formed,
or sines to record or measure.
Supposing the sine of the angle of refraction in the above experiment with
air and water to measure 12 in., and the sine of the angle of incidence 16 in.,
it would follow that in water the sine of the angle of incidence is to the sine
of the angle of refraction as 1*336 to I, or as nearly as possible i^ to I. The
number i'336, which expresses this ratio for water, is called the index: of re-
fraction for water, and sometimes its refractive power.
The determination of the refractive powers of various kinds of glass is of
great use in the manufacture of achromatic telescopes ; and sometimes the
purity of a liquid, and its freedom from adulteration, may be approximately
ascertained by taking the index of refraction.
In the chapter devoted to the consideration of
the reflection of light, it was thought to be the most
simple and instructive plan to trace the progress
of parallel rays when thrown off from plane, con-
cave, or convex surfaces.
The forms of refracting bodies, and their action
on light, are so numerous and well discussed in
the more elaborate works on Dioptrics, that it is
mere repetition to quote them all.
The laws of refraction being known, and the
refractive power of the glass used for experiment
FIG.
THE REFRACTION OF LIGHT.
being ascertained, the mathematician may work out on paper the exact direc-
tion of the light passing into or out of the most complicated forms. As an
illustration of this mode of investigation, the following instructions are given
by Brewster, in order to enable the student to study the refraction of light
through one of the most important optical instruments, viz., the Prism. (Fig.
570'
An optical prism, a solid having three plane surfaces. A B, A C, called its
refracting surfaces ; B C is called the base of the prism.
Let ABC (Fig. 58) be a prism of plate glass, whose index of refraction is
1-500, and let H R be a ray of light falling obliquely upon its first surface A B
at the point R. Round R, as a centre, and with any radius H R, describe the
circle H M b. Through R draw M R N perpendicular to A B, and H m perpen-
dicular to M R. The angle H R M will be the angle of incidence of the ray
H R, and H m its sine, which in the present case is 1*500. Then, having
made a scale in which the distance H m is
1*500, or \\ parts, take one part or unity
from the same scale, and having set one
foot of the compasses on the circle, some-
where about b, move it to different points
of the circle till the other foot strikes only
one point n of the line R N ; the point b
thus found will be that through which the
refracted ray passes, R b will be the re-
fracted ray, and n^b the angle of refrac-
tion, because the sine b 'n of this angle
has been made such, that its ratio to H ?/?,
the sine of the angle of incidence, is as
i to i '500. The ray R b thus refracted
will go on in a straight line till it meets
the second surface of the prism at R R', when it will again suffer refraction in
the direction R b'. In order to determine this direction, make R' H' equal
to R H, and, with this distance as radius, describe the circle H' b'. Draw
R' N perpendicular to A C, and H' m' perpendicular to R N, and form a scale on
which H' m shall be one part, or i 'ooo, and divide it into tenths and hun-
dredths. From this scale take in the compasses the index of refraction 1-500
as i^ of these parts ; and, having set one foot somewhere in the line R' n\
move it to different parts of it till the other foot falls upon some part of the
circle about b>, taking care that the point b 1 is such, that when one foot of the
compasses is placed there, the other foot will touch the line R' $', continued
only in one place, join R' b Then, since H' R' vv is the angle of incidence, or
the second surface A C and H' m its sine, and since n' &', the sine of the angle
b' R w, has been made to have to H' in' the ratio of 1*500 to i, b' R' n' will be
the angle of refraction, and R' b' the refracted ray. In the construction of the
figure (Fig. 58) the ray H R has been made to fall upon the prism at such an
angle that the refracted .ray R R' is equally inclined to the faces A B, A C ; or
is parallel to the base B C of the prism ; and it will be found that the angle of
incidence H R B is equal to the angle of emergence b' R 7 C. Under these cir-
cumstances, we shall find, by working the angle H R B either greater or less
than it is in the figures, that the angle of deviation H E D is less than at any
other angle of incidence. If we, therefore, place the eye behind the prism at
b\ and turn the prism round in the plane BAG, sometimes bringing A towards
42
5 2
ON- LIGHT.
the eye and sometimes pushing it from it, we shall easily discover the position
when the image of the candle seen in the direction b' D has the least devia-
tion. When this position is found, the angles H R B and b' R' C are equal,
and R R' is parallel to B C, and perpendicular to A F, a line bisecting the
refracting angle B A C of the prism ; but since B A F is known, the angle of
refraction B R N is also known ; and the angle of incidence H R B being found
by the preceding methods, we may determine the index of refraction for any
prism by the following analogy :
As the sine of the angle of refraction is to the sine of the angle of incidence,
so is unity to the index of refraction ; or the index of refraction is equal to
the sine of the angle of incidence divided by the
sine of the angle of refraction. By this method
we may readily measure the refractive power of
all bodies. If the body be solid, it must be shaped
into a prism ; and if it is soft or fluid, it must be
placed in the angle B A C of a hollow prism, ABC,
(Fig. 59) made by cementing together three pieces
of plate glass, A B, A C, B C. A very simple hollow
prism for this purpose maybe made by fastening
FIG. 59.
together at any angle two pieces of plate glass, A B, A c, with a bit of wax F.
A drop of the fluid may then be placed in the angle at A, when it will be
retained by the force of capillary attraction.
TABLE OF THE INDICES OF REFRACTION.
Vacuum .
I "OOOOOO
Lens, Crystalline
I-384
Air ....
1-000294
Vitrous .
1-339
Albumen .
1*360
Aqueous .
1-336
Alcohol
1-374
Nitrous Oxide Gas .
Ammonia Gas.
1-000385
Nitric Acid
1-410
Alum
i '45 7
Oxygen .
1-000272
Amber
1-547
Olefiant Gas .
1-000678
Bisulphide of Carbon
1-678*
Oil, Olive .
1-470
Carbonic Acid Gas .
i -000449
Turpentine.
1-475
Chlorine Gas
1-000772
Castor
1-490
Diamond .
2-439
Cloves
1-535
Ether
1-358
Cassia'
1-641
Fluid Spar
1-434
Phosphorus
2-424
Glass, Flint
1-605
Quartz
1-548
Plate .
1*543
Ruby
1779
Crown .
1*534
Sapphire .
1794
Garnet
1-815
Sulphur
2-115
Hydrogen .
1-000138
Sulphuric Acid Gas .
i -000665
Hydrochloric Acid Gas
i -000449
Sulphuric Acid .
1-434
Hydrochloric Acid .
1-410
Tabasheer
mi
Iceland Spar
Water
I-336
Ordinary ray .
1-654
Solid (Ice)
1-310
Extraordinary ray .
1-483 Zircon
1-961
The course of parallel
rays of light through plane, concave,
and convex
* Used to till prisms for spectrum analysis.
THE REFRACTION OF LIGHT.
53
surfaces of glass may now be considered, and they will be found to contrast
in the most simple manner with similar-shaped reflecting surfaces.
FIG. 60.
REFRACTION OF LIGHT THROUGH PLANE GLASS.
Let A B (Fig. 60) be a ray of light incident on the upper surface or side of a
piece of ordinary plate glass, marked C C, whose other or under side, D D, is
parallel to C C. On entering the glass the ray is refracted in the direction
B E, and it will be refracted again at its exit from the under side, D D, to the
same amount as at its entrance in the line E F ; consequently an eye placed at
F would see the ray as if it came from the pint A' along the line F E A 7 . The
light has undergone refraction, and an object seen through a window is not
seen in its true position ; but, as parallel rays falling upon a plane glass retain
their parallel lines after passing through it, the object does not appear to
undergo any change unless the two surfaces of the glass are uneven, and not
parallel with each other, when distortion takes place. Such an effect is
rarely seen now in looking through the windows of good houses, because they
are usually glazed with plate glass, the sides of which are nearly parallel. It
has already been shown that convex mirrors (page 22) render parallel rays of
light divergent ; precisely the reverse occurs with convex refracting surfaces.
REFRACTION OF PARALLEL RAYS OF LIGHT BY CONVEX SURFACES.
Fig. 6 1 represents a piece of glass cut into the form of a double-convex lens
A B, a figure such as would be pro-
duced by placing one watch-glass on
the edge of another having precisely
the same amount of convexity. Let
C D be a ray of light falling perpen-
dicularly on the refracting surface
^ E
and passing straight through the
glass, in obedience to the law al-
ready enunciated, that a ray of light
which falls perpendicularly on a re-
fracting surface undergoes no change
in its direction, and therefore C D
passes through the middle or axis
FIG. 61.
54
ON LIGHT.
of the crystal lens without deviation from a straight line C D E. The other
two rays, F G, H I, falling at an angle on the glass, undergo refraction, and are
bent towards and emerge from the other side, and meet at the point E, called
the principal focus, or focus for parallel rays. These parallel rays of light are
refracted by a double-convex lens, and become convergent, meeting at a point
called the focus. On the other hand, if E be considered as the luminous point
from which divergent rays are emitted, they become parallel rays when they
emerge from the double-convex lens A B.
REFRACTION OF PARALLEL RAYS BY CONCAVE SURFACES.
Let A B (Fig. 62) be a glass lens, whose two sides are hollowed out, or
concave, and C D a ray of light falling perpendicularly on the surface, and
therefore passing straight through
the lens. F G and H I are two other
rays impinging on the surface of the
glass at an angle ; these undergo re-
fraction, and are bent outwards in
the direction F G K and H I K.
Thus the property of a concave
lens is just the reverse of a concave
mirror, the former causing parallel
rays of light to become divergent,
the latter convergent; and if the rays
K K be regarded as convergent rays,
they become parallel when emerg-
FIG. 62.
ing from the concave lens A B.
OTHER FORMS OF LENSES.
For various optical purposes a variety of lenses, in addition to the prism,
the double convex, or the double concave lenses, is required, which may be
ground into the following forms :
a. A spherical lens, causing parallel rays to become
convergent .
d. A piano convex lens ; parallel rays become conver-
gent .....
THE REFRACTION OF LIGHT.
55
c. A plano-concave lens ; parallel rays become diver-
gent . . . . . o . . .
d. A meniscus ; parallel rays become convergent .
e. A concavo-convex lens ; when the concavity exceeds
the convexity, parallel rays become divergent
FIG. 63.
It is good practice for the student in physics to make careful drawings of
the above figures, and to trace the paths of imaginary rays of light through
them. The drawings may be varied by supposing the lenses to be made of any
of the solid transparent substances whose refracting indices are given in the
table at page 52.
OPTICAL INSTRUMENTS WHOSE PROPERTIES DEPEND
ON REFRACTION.
THE SIMPLE AND COMPOUND MICROSCOPE AND TELESCOPE.
It follows from the laws of refraction already explained, that when a double-
convex lens (Fig. 64) acts on rays proceeding from an object, such as a candle,
A B, that, as the rays are not all parallel, they will be collected into a focus
A' B' at a distance behind the lens somewhat greater than the focus for parallel
rays at E, and that an inverted image of the candle A B will be produced at
A" B', which may be received on any white surface. Thus a double-convex
lens becomes the most simple microscope which can be used, and it is some-
times used for that purpose in the examination of samples of wheat. The
ON LIGHT.
FIG. 64.
cheapest microscope the author has seen is that made by Me Culloch, of
Blucher Street, Birmingham, for half-a-crown. It includes a lens made,
seemingly, of a filament of glass melted into' a globule, fitted into a brass
tube which contains a plate of glass to be used as an object-holder (such as
for the eels in paste), and the opposite end of the
brass tube is closed with a diaphragm, which can
be unscrewed if more light is required. The whole
is fitted into a case, and might be made a very
amusing companion for young people when they
go into the fields ; and if lost, the value is not an
alarming consideration. Another marvel of cheap-
ness ib a telescope made by Solomon, of Albe-
marle Street, at a cost of five shillings. The latter,
of course, is not achromatic ; but its definition of
distant objects is really excellent, and the work-
manship good.
In the compound microscope the image A' B"
(Fig. 64) is still further magnified, and can be
more carefully examined by the addition of an-
other double-convex lens, say of an inch focal
distance. It is the image formed in the tube of
the compound telescope, which may be again
magnified by employing a second lens with a very
short focus. In these cases the first lens is called
Fig. 65. Simple Microscope, the object-glass, and the second the eye-piece or
glass. Of late years the most elaborate and per-
fect microscopes have been made in this country ;
so that England stands unrivalled in this branch
of optical instruments, whilst her microscopical societies have contributed
largely to our knowledge of those things which cannot be appreciated or
examined without the use of these contrivances.
in which the Lens is focused bv
turning the Screw.
C \j
FIG. 66. The Compound Telescope.
THE REFRACTION OF LIGHT. 57
B, The object-glass, which throws an inverted image into the dark tube ; C is
the eye-glass, which magnifies the inverted image. This telescope could only
be used for astronomical purposes ; but, by the addition of two other convex
lenses at D E, called erecting-glasses, an erect image is obtained.
THE CAMERA OBSCURA.
A dark chamber into which a double-convex lens is fitted. The invention
of this pleasing contrivance has been usually ascribed to Baptista Porta, as it
appears in his " Magica Naturalis," lib. xvii., cap. vi., first published at Frank-
fort about 1589 or 1591.
Fifty years ago the camera obscura was more popular than it is now, and
was frequently erected on elevated spots of ground by wealthy individuals,
the consequence being that the whole apparatus and the building to which it
was attached were most carefully made and adjusted to each other.
FIG. 67.
Fig. 67 represents a dome or cupola placed over a room erected for the
purpose of a camera obscura. The whole dome, which carries the box and
containing a mirror placed at an angle over a double-convex lens, may be
made to turn round on friction-wheels ; or, what is more simple, the box is
made movable in a groove upon the dome, and may be turned with a long
rod by a person inside. The box is recommended to be of a cubical
form, of about 6 or 7 in. square, and contains a carefully ground plain silvered
mirror, which should be made of parallel glass placed diagonally in the box ;
the mirror itself should be attached by hinges at the lower end, so that a
different angle may be obtained if required. Underneath the mirror, in a
round cell at the bottom of the box, is fixed a double-convex lens, about 6 or
8 ft. focus and 4 or 5 in. in diameter; this lens will form, upon a white table
ON LIGHT.
placed on the floor below, the image of the objects reflected by the mirror
above at the focal distance of the lens.
FIG. 68.- 772* Prism Camera.
D D D, section of a pyramidal box; M, a brass tube open
on one side, moving in another tube, and containing
the rectangular prism ABC, one side of which, A c, is
convex, and the other, c B, concave ; o, the framework
to support the sheet of paper.
The diameter of the table should be
2\ or 3 ft., and, in order to cor-
rect the indistinct images formed
at the edge by spherical aberra-
tion, it is usual to make the sur-
face slightly concave, and to form
it of the best plaster of paris or
stucco. The table should be sup-
ported by a pillar in the centre,
fitting into a tube provided with
a screw, so that the table may be
raised or lowered, and the images
exactly focused on its surface. A
still more perfect optical arrange-
ment for projecting brilliant
images of distant objects on to a
white surface for the purposes of
the artist is shown in the figure
annexed. (Fig. 68.)
In this camera the rays of light,
after falling on the convex sur-
face, enter the prism, and, being
totally reflected from the side A B,
pass into the box through the
concave surface, and fall upon a
sheet of paper laid out on a pro-
per framework. The picture thus
obtained has not the fault of
those produced by the ordinary
arrangement of the mirror or con-
vex lens, being free from spherical
aberration, which is neutralized
in this instance by the concave
surface of the prism. As these
prisms are difficult to make, the
same result is attained by care-
fully cementing with Canada bal-
sam a
piano -
convex
lens on
one side
of the prism, and a plano-concave on the other, whose
focal lengths are equal to each other. (Fig. 69.)
The magic lantern apparatus, the dissolving view
and the phantasmagoria lantern apparatus, are all
refracting optical instruments, very easily constructed.
The magic lantern was contrived, about the year
1650, by the celebrated Kircher, and is described in
his work entitled, "Ars Magna Lucis et Umbrae."
FlG. 69.
ABC, the prism, with plano-
convex and plano-concave
lens attached at A E and c E.
THE REFRACTION OF LIGHT
59
There is, however, a curious account of phantom figures or demons, made to
appear in the smoke of a fire and thrown upon walls, ascribed to Cellini, who
lived nearly a century before Kircher. If the story be true, it would seem to
show that phantasmagorial effects preceded the magic-lantern pictures, and
that Cellini must have been acquainted with the construction of the instrument,
or such effects as described could not have been produced. The magic lantern
consists of a box provided with a chimney, containing a good lamp, or, still
better, an oxy-hydrogen light ; when the former is used, a reflector is usually
FlG. 70. Common Magic Lantern.
B, the box ; c, the lamp and reflector ; A, the plano-convex lens ; c c, the tube sliding within the first
tube, and containing a double-convex lens, A'.
placed behind the flame, in order to increase the illumination of the pictures,
The lime-light is placed behind the lenses called condensers (Fig. 71); these are
usually composed of two plano-convex lenses, with the flat side placed towards
the lamp, and the convex side touching, or nearly so, the convexity of the other
lens, the flat side of which is towards the picture. The picture, painted or
carefully photographed on glass, is placed in front of the condensers, and the
whole projected and properly fo-
cused on a white screen by means
of two other plano-convex lenses ;
the flat side of one lens being to-
wards the picture, and the convex
side towards the flat side of the
second lens. The focusing lenses
are contained in a tube which slides
within the other, and is moved back-
wards and forwards with a simple
rack-work.
The dissolving view arrangement,
long kept a secret by Mr. Child, the
inventor, is nothing more than two
magic lanterns (Figs. 74, 75) placed
side by side, and provided with slid-
ing plates so arranged that, as one
picture is gradually cut off, the second
FIG. 71. Section of Superior Magic
Lantern.
diaphragm to reduce the aberration of light.
6o
ON LIGHT.
is disclosed; and by alternately throwing on one picture and cutting off the other,
the most pleasing effects are obtained, provided the two lanterns are precisely
similar. To save gas, it is sometimes usual to turn off the oxygen from one
lantern and to supply it to the other, and thus by alternately raising and
lowering the lights in the lanterns the same result is obtained. (Fig. 76.)
The phantasmagorial effects first ascribed to Cellini are produced by painting
in the figure-picture on glass, and then blackening out the whole of the
ground, and either by carrying the
lantern and moving backwards and
forwards behind the sheet, or by a me-
chanical arrangement in which the lan-
tern runs on a tramway, and is focused
as it approaches or recedes from the
transparent disc the pictures are made
to increase or diminish at pleasure. In
practice it is better to allow the lantern
and person showing it to be carried on
the same carriage, as the lever arrange-
ment shown in Fig. 72, and attached
to the focusing lenses is very apt to
get out of order.
One of the most useful instruments
for public exhibitions is that designed
by Messrs. Chadburn, of 71 Lord Street,
Liverpool, for the purpose of producing
enlarged images upon a screen (similar
to those of the magic lantern) from
opaque objects, such as photographs,
carte de visites, engravings, drawings,
relievos, natural objects in all their
colours, mechanical apparatus, or deli-
cate mechanism in motion, such as the
various parts of a watch or, still better,
of a repeating watch. The instrument
is simple in its construction, and con-
sists of a lantern box, containing in the
centre a pillar with adjusting screw,
upon which the lime cylinder is placed ;
behind it the metallic reflector, which
must be so adjusted that the picture is
evenly illuminated. The reflector can be raised or lowered, or moved back-
wards and forwards ; it receives the light, and throws it upon the condensing
lens, by which it is concentrated upon the picture placed in the sliding door
in the angular box joined to the square compartment. The light thrown off
from the highly illuminated picture is received by the achromatic objective
lenses (the focus of which is adjusted by the rack upon them), and projected
upon the screen. The angular compartment may be removed, and replaced
by a part with lenses for direct light and transparent pictures, as in the ordi-
nary magic lantern.
An oyster directly after it is opened, the 'half of an orange, particularly
if squeezed, as the effect is most ridiculous, the juice and pips appear to fall
FIG. 72.
THE REFRACTION OF LIGHT.
61
upwards all bodies being reversed in this instrument, the hand and orange
are shown upside down the human hand, the face of a watch, a gold or
FIG. 73. Part Section and Elevation of Chadburtfs Lantern,
A, the light; B, reflector c, condensing lens; D, the picture; E, the achromatic focusing lenses.
FIG. 74. Improved Dissolving View Apparatus by Highley, IOA Great
Portland Street.
silver coin, and photographs of distinguished persons, are all good objects for
this instrument.
In 1857 the writer introduced at the Polytechnic photographs- of original
ON LIGHT.
FIG. 75. Section of Highlefs Dissolving View Apparatus (Fig. 74).
drawings made by Mr. George Hine, the distinguished artist. The whole of
the pictures illustrating the amusing story of Blue Beard were done in this
FlG. 76. Arrangement for saving oxygen gas, which is supplied alternately
to one lime light and then to the other.
way, and were most effective and successful, as every touch of the original
artist is thus delineated in the photograph and subsequently thrown on the
THE REFRACTIOM OF LIGHT.
FlG. 77. Highley'' s complete Apparatus for Dissolving Views ; all packed in
two boxes,
screen. Messrs. Negretti and Zambra followed up the idea by using photo-
graphs of statuary, which they displayed at Manchester with astonishing
success, the Mechanics' Institution there realizing something like ^600 by the
exhibition in a few months. Mr. Highley has continued in the same track,
and deserves notice for the admirable photographs of natural objects which
he prepared for the dissolving-view apparatus his arrangement of the latter
contrivance, shown in Fig. 74 and in section Fig. 75, is good and convenient.
The arrangement for saving oxygen gas (Fig. 76) is also extremely useful where
the gas cannot be obtained easily. Portability and economy of space have
all been carefully studied by Highley in Fig. 77, where the gases (oxygen or
hydrogen) are condensed in separate strong copper cylinders which pack in
one box, and the lantern, the slides, and the stand upon which they are placed,
come out of and belong to the second box.
64 ON LIGHT.
THE HUMAN EYE.
This elaborate and wonderful work of the Creator, built up of the usual
constituents of animal substances, viz., albumen, gelatine, fibrine, with a little
fatty matter, all marvellously shaped and fitted to their purposes, repre-
sents an optical instrument which transcends every contrivance made by
the hand of man. The camera obscura is the nearest approach to an imita-
tion of the eye. It is fitted with a double-convex lens ; the rays of light
thrown off from any object placed before the apparatus are brought to a focus,
and received upon a sheet of paper or piece of ground glass. In the eye the
same result is brought about by the refraction of light in the crystalline lens
and the other humours ; the rays are brought to a focus, and impinge upon
a nerve, spread out as a delicate network to catch the beams, and to vibrate in
sympathy with those exquisite undulations which cause the propagation of
light, and thus to produce the sensation of vision. Anatomists have given
this organ their most careful attention, and published elaborate drawings of
the various parts of the eye. By the permission of Messrs. Chadburn, of Shef-
field, a copy of their instructive diagrams of the eye is added (page 65).
A. The Pupil, or circular opening in the iris, capable of being contracted
or enlarged, according to the amount and intensity of light.
B. The Iris, a flat circular membrane, of a grey, blue, or black colour,
forming the anterior and posterior chambers of the eye. It performs the
same functions as a diaphragm in an optical instrument.
C. The Sclerotic Coat, a tough white membrane, to which the muscles
for moving the eyeball are attached.
D. The Eyelids, containing the tarsal fibro-cartilages.
E. The Cornea, composed of tough transparent laminae, forming the front
of the eye ; the first surface, where the rays of light are refracted. Some
anatomists have considered the sclerotica and cornea as one and the same,
and have termed the cornea the transparent, and the sclerotica the opaque
cornea.
F. The Aqueous Humour, contained in a delicate membrane filling the
space from the cornea to the crystalline lens. The space occupied by this
humour is divided into two parts by the iris, forming, as shown at B, the
anterior and posterior chambers of the eye.
G. The Crystalline Lens, contained in a transparent membrane called the
Capsule, the principal refracting medium of the eye. The capsule adheres by
its edge to the ring-shaped body called' the Ciliary Circle or ligament, N.
H. The Vitreous Humour, contained in the hyaloid membrane a jelly-
like substance, resembling. the.white of an egg, filling the body of the eye.
I. The Retina, a membrane which receives the impression of light, and
transmits it to the brain through the optic nerve, K.
j. The Choroid Coat, a delicate membrane lining the sclerotica, covered
on its inner surface with a black substance (pigmentum nigrum, resembling
the colouring matter of the negro's skin) contiguous to the retina. The choroid,
by its vascular tissue, serves to carry the blood into the interior of the eye.
K. The Optic Nerve.
L. Canal of Petit.
M. Central Artery of the optic nerve.
U. Ciliary Circle or ligament.
THE HUMAN EYE.
FIG. 74. 77^ ////;;m ^. FIG. ^.The Eyeball, showing the Coats,-&.
of the Eye. FIG. 76. Longitudinal Section of the Eye and Orbit,
through the dotted tines on Fig. 74.
5
ON LIGHT,
66
U. Ciliary Nerves.
P. Vasa Vorticosa.
M. The Ciliary processes.
k. Tunica Conjunctiva.
k s. Tunica Conjunctiva collapsed, as when the eye is closed.
s. Elastic Muscle of the Eyelid.
T. Elastic Muscle of the Eye.
U. Superior Oblique Muscle.
v. Depressive Muscle of the Eye.
w. Section of Oblique inferior Muscle.
X. Nerves and Arteries.
Y. Tube conveying the optic nerve to the brain.
z. Bone forming the socket of the eye.
N.B ""The same letters apply to each figure.
Brewster found the following to be the refractive powers of the different
humours of the eye, the ray of light being incident upon them from air :
Aqueous humour . . i'336
Crystalline lens, surface I '3767
M ,i centre 1-399
Crystalline lens, mean . 13839
Vitreous humour . . i'3394
Water .... 1*3358
But the rays of light are not all incident upon them from the air, and as
the rays refracted by the aqueous humour pass into the crystalline, and those
from the crystalline into the vitreous humour, the indices of refraction of the
separating surfaces of their humours will be
From aqueous humour to outer coat of the crystalline . 1*0466
From i: ii to crystalline, using the mean index 1*0353
From vitreous to crystalline, outer coat I '0445
From i, to \\ using the mean index . . 1.0332
The eye,' as already described, consists of four coats or membranes, which
are disposed in the following order, viz., ist, the sclerotic; 2nd, the cornea,
which fits into it like the glass of a watch; 3rd, the choroid; and 4th, the
retina; of two fluids or humours, the aqueous and the vitreous, and of a lens
called the crystalline.
Over the cornea and sclerotic is expanded a delicate mucous membrane,
called the conjunctiva. The iris is suspended across the eye, and in its centre
is an opening, termed the pupil, which immediately opens when the light
diminishes, and closes if the light is too strong. The posterior convexity of
the lens is greater than the anterior. Sometimes, from a too great convexity
of the lens or the cornea, the rays of light which enter the eye come to a focus
before they impinge upon the retina, producing the defect called short-sighted
vision. Optical science corrects this inconvenience by the use of a concave
lens. If the crystalline lens is not sufficiently convex, the rays of light come
to a focus behind the retina ; this defect is surmounted by the use of a convex
lens, which diminishes the divergence of the rays. Such ingenious artificial
additions to the eye' are common enough at the present day, but it may be
asked, how did our forefathers bear these infirmities? Spectacles are supposed
to have been unknown to the ancients, and it is stated by Francisco Redi
that they were invented in the I3th century, between the years 1280 and 1311,
probably about the year 1299 or 1300; he gave the honour of the discovery to
THE HUMAN EYE. 67
Alexander de Spina, a monk of the order of Predicants of St. Catharine, at
Pisa. Muschenbroek, the old electrician who discovered the Leyden jar,
observes that it is inscribed on the tomb of Salvinus Armatus, a nobleman
of Florence, who died in 1317, that he was the inventor of spectacles. This
may have been the person who had the secret as well as the learned monk,
because Redi states that the latter only disclosed the secret upon learning that
another person had it as well as himself.
Mr. Acland makes the following practical and valuable observations on
defects of vision :
" On the Symptoms indicating a Necessity for Spectacles.
" The natural decay of vision occurs usually from thirty to fifty years of age,
varying according to habits and employment of the individual. Sometime
during this interval the refractive power of the crystalline humours of the eye
slightly alters its condition, whilst the crystalline lens and cornea change their
form, so that a difficulty of distinct vision is felt. The eye loses a portion of
its power of seeing at varying distances, or its power of adjustment ; and
near objects are no longer as easily seen as in youth. Reading small print
by candle-light is difficult, as the book requires to be held at a greater distance
from the eye than formerly, and a more powerful light is needed ; and even
then the letters appear misty, and to run one into the other, or seem double.
And still further, in order to see more easily, the light is often placed between
the book and the eye, and fatigue is soon felt, even with moderate reading.
" When these symptoms show the eye to have altered its primitive form,
spectacles are absolutely needed. Nature is calling for aid, and must have
assistance, and if such is longer withheld, the eye is needlessly taxed, and the
change, which at first was slight, proceeds more rapidly, until a permanent
injury is produced.
" There is a common notion that the use of spectacles should be put off as
long as possible, but such is a great mistake, leading often to impaired vision
for life, and is even more injurious than a too early employment.
" Timely assistance relieves the eye, and diminishes the tendency to flat-
tening, whereas should the use of spectacles be longer postponed, the eye
changes rapidly, and when the optician is at last consulted, it is found that a
deeper focus spectacle must be used than usual for the first pair, and even
these suit but a short time, and have to be again exchanged for those of still
deeper power; and these frequent changes become a matter of necessity
which, unless judiciously checked, continue during life.
" It must not be forgotten that, when first using spectacles, they are not
required during daylight, but only for reading, &c., by artificial light, and it
may be from six months to two years from the time of first adopting them ere
they will be required for day use.
" Spectacles for the Short-sighted. Short sight is often present at birth, but
is little noticed, nor its inconveniences felt, until study becomes imperative.
When this occurs, the power employed should be always slightly under that
needed to remedy the defect, otherwise the eye will gradually accommodate
itself to the lenses, and require constantly an increase of power. In all cases
leave some little for the adjustment of the eye to do, and then you may, after
a time, diminish the power of the lenses needed.
" The Optician's Knowledge. Having now shown when spectacles should
be employed, let us for a moment consider what are the requirements that
5 2
68
ON LIGHT.
should in all cases be possessed by the optician to whom the selection of
spectacle lenses is entrusted.
' These requirements are
' i st. An intimate knowledge of the anatomical structure of the eye, and
of the theory of vision.
' 2nd. An extensive acquaintance with the science of optics.
' 3rd. A sound mathematical knowledge.
'4th. A practical acquaintance with the manufacture of lenses and
spectacle frames.
" Having for the last fourteen years made the adaptation of spectacles my
especial study, at the establishment of Messrs. Home and Thornthwaite, 122,
Newgate Street, I have frequently met with cases where great injury has been
done to the weak-sighted by the ordinary optician's improper selection of
spectacles ; and I could heartily wish more of my medical brethren would
bring their knowledge to bear on this subject, which demands, and frequently
calls forth, all the science and skill we possess, to meet the requirements of
some abnormal cases that present themselves."
The knowledge which the eye conveys to the mind is boundless ; the rela-
tive condition of matter, large and small, of motion or rest, of colour, of
solidity, of transparency, of brilliancy, of opacity, of space or distance, are
only a few of the results attained by the exercise of the faculty of vision.
THE STEREOSCOPE.
This most valuable and instructive instrument, and now not only a " house-
hold word," but a piece of domestic apparatus without which no drawing-room
is thought complete, was invented by Professor Wheatstone, and subsequently
modified by Sir D. Brewster. It demonstrates that man must have two eyes
in order to enjoy the appreciation of distance, or, like the fabled Polyphemus,
we might only have had one eye. Mr. Woodward gives the following excel-
lent and familiar explanation of the phenomena produced by the stereoscope.
FIG. 77.
PROFESSOR WHEATSTONE'S REFLECTING STEREOSCOPE.
A familiar explanation of the phenomena produced by the Stereoscope.
" The name is derived from two Greek words, signifying to view solid things,
and the instrument is so constructed that two flat pictures, taken under certain
conditions, shall appear to form a single solid or projecting body.
THE STEREOSCOPE. 69
"A picture of any object is formed on the retina of each eye ; but although
there may be but one object presented to the two eyes, the pictures formed
on the two retinas are not precisely alike, because the object is not observed
from the same point of view.
"If the right hand be held at right angles to, and a few inches from, the
face, the back of the hand will be seen when viewed by the right eye only,
and the palm of the hand when viewed by the left eye only ; hence the images
formed on the retinae of the two eyes must differ, the one including more of
the right side and the other more of the left side of the same solid or pro-
jecting object. Again, if we bend a card so as to represent a triangular roof,
place it on the table with the gable end towards the eyes, and loolj; at it, first
with one eye and then with the other, quickly and alternately opening and
closing one of the eyes, the card will appear to move from side to side, because
it will be seen by each eye under a different angle of vision. If we look at
.the card with the left eye only, the whole of the left side of the card will be
plainly seen, while the right side will be thrown into shadow. If we next look
at the same card with the right eye only, the whole of the right side of the
card will be distinctly visible, while the left side will be thrown into shadow ;
and thus two images of the same object, with differences of outline, light and
shade, will be formed the one on the retina of the right eye, and the other on
the retina of the left. These images falling on corresponding parts of the
retina convey to the mind the impression of a single object j * while experience
having taught us, however unconscious the mind may be of the existence of
two different images, that the effect observed is always produced by a body
which really stands out or projects, the judgment naturally determines the
object to be a projecting body.
" It is experience also that teaches us to judge of distances by the different
angles of vision under which an object is observed by the two eyes ; for the
inclination of the optic axes, when so adjusted that the images may fall on
corresponding parts of the retina;, and thus convey to the mind the impression
of a single object, must be greater or less, according to the distance of the
object from the eyes.
" Perfect vision cannot then be obtained without two eyes, as it is by the
combined effect of the image produced on the retina of each eye, and the
different angles under which objects are observed, that a judgment is formed
respecting their solidity and distances.
" A man restored to sight by couching cannot tell the form of a body without
touching it, until his judgment has been matured by experience, although a
perfect image may be formed on the retina of each eye. A man with only
one eye cannot readily distinguish the form of a body which he had never
previously seen, but quickly and unwittingly moves his head from side to
side, so that his one eye may alternately occupy the different positions of a
right and a left eye ; and, if we approach a candle with one eye shiit, and then
attempt to snuff it, we shall experience more difficulty than we might have
expected, because the usual mode of determining the correct distance is
wanting.
"In order, then, to deceive the judgment, so that flat surfaces may represent
* That this is the correct theory of single vision with the two eyes is evident. For if, while looking
at a single object with both eyes, we make a slight pressure with the finger on one of the eyeballs, \ve
shall immediately perceive two objects j but, on removing the pressure, only one will be again seen.
70 ON LIGHT.
solid or projecting figures, we must cause the different images of a body, as
observed by the two eyes, to be depicted on the respective retinas, and yet to
appear to have emanated from one and the same object. Two pictures are
therefore taken from the really projecting or solid body, the one as observed
by the right eye only, and the other as seen by the left. These pictures are
then placed in the box of the stereoscope, which is furnished with two eye-
pieces, containing lenses so constructed that the rays proceeding from the
respective pictures to the corresponding eye-pieces shall be refracted or bent
outwards, at such an angle as each set of rays would have formed had they
proceeded from a single picture in the centre of the box to the respective
eyes, without the intervention of the lenses ; and as it is an axiom in optics
that the mind always refers the situation of an object to the direction from
which the rays appear to have proceeded when they enter the eyes, both
pictures will appear to have emanated from one central object ; but as one
picture represents the real or projecting object as seen by the right eye, and
the other as observed by the left, though appearing by refraction to have pro-
ceeded from one and the same object, the effects conveyed to the mind, and
the judgment formed thereon, will be precisely the same as if the images were
both derived from one solid or projecting body, instead of from two pictures,
because all the usual conditions are fulfilled ; and consequently the two
pictures will appear to be converted into one solid body.
"The necessary pictures for producing these effects, excepting those of geo-
metrical figures, which may be laid down by certain rules, cannot, however,
be drawn by the hands of man ; for, as Professor Wheatstone has observed,
' It is evidently impossible for the most accurate and accomplished artist to
delineate, by the sole aid of his eye, the two projections necessary to form the
stereoscopic relief of objects as they exist in nature, with their delicate dif-
ferences of outline, light, and shade. But what the hand of the artist was
unable to accomplish, the chemical action of light, directed by the camera,
has enabled us to effect.'
FIG. 78. Breivster's Refracting Stereoscope.
" Daguerreotype portraits and Talbotype pictures are therefore taken, usually,
by two cameras placed towards the object, with a difference of angle equal to
the difference of the angle of vision of the two eyes, which is about 18 when
the object is eight inches from the eyes ; hence, if these be carefully examined
and compared with the original projecting objects, they will be found to be
faithful representations of the object as seen by each eye respectively."
PERSISTENCE OF VISION. 71
DIRECTIONS FOR USING THE STEREOSCOPE.
" The objects must be so adjusted in the box, that only one picture may be
seen in the centre, care being taken that the pictures are not reversed so as
to be seen by the right eye instead of the left, and vice versa.
" The proper position of portraits, buildings, and similar objects cannot be
mistaken ; but where this is not readily perceived, it should be ascertained,
when the object can be marked so as at once to be properly placed.
"The eye-pieces, if allowed to turn, are marked with arrows, to indicate
their proper position, these must be placed inwards, and in a right line with
each other.
" The eye-pieces in some instances are made to draw out to suit the foci of
different persons. But those who use spectacles will generally see best with'
them on, bringing them forward so as to lie flat on the eye-pieces, which in
such cases should not be drawn out.
" Persons, however, with a defective sight in either eye will not be able to
perceive the astonishing effects of the arrangement, as two different images
will not be perfectly formed on the retinae of the respective eyes."
FlG. 79. Example of the zigzag path oj Lightning.
PERSISTENCE OF VISION.
There is a most interesting class of experiments that depend chiefly upon
another property or faculty of vision, by which we retain for a certain limited
period the images of objects presented before us. It may be premised that
the term image refers to that picture which remains upon the eye as long as
the object is present ; whereas the spectrum, which every one knows is the
Latin for spectre, is that lingering impression left upon the eye after the real
object has been removed. This property, like binocular vision, may be
satisfactorily proved in various ways. Thus, if a broom-stick be thrust into
the fire and burnt, so as to obtain a mass of ignited charcoal, and then
whirled rapidly round in a circle, a complete circle of light is visible. Now,
72 ON LIGHT.
it is evident that the hand or stick cannot be in every part of the circle
at the same -instant of time ; the mind is therefore obliged to confess, in
tracing the stick through the quarter, half, three-quarter, and whole circle,
that of course the impression of the train of light must have remained upon
the eyes, or else a single spot of light moving in a circle could only have been
visible. A planet, if it moved fast enough, would leave a train of light,
indicating, like the burning stick, its particular path or disc. The meteors
move with such amazing velocity that their trains of light are extremely vivid,
marked, and lengthened out, and show distinctly the direction or path they
lake. A discharge of natural electricity or lightning would, if it moved slowly,
be represented by a ball of fire travelling from one point to another ; it is,
however, usually represented by a lengthened-out zigzag. (Fig. 79.) It is then
called " forked lightning," and every part of its track remaining impressed on
the vision, the whole appears as a series of continuous lines of fire, which,
although diverted right or left, in a horizontal, perpendicular, or angular
direction, pursue their path to the point where the discharge occurs, they are
visible as a whole, and called a flash of lightning.
The act of winking the eye is another familiar example of the same truth ;
the eyelid closes and re-opens so rapidly, for the purpose of lubricating the
eyeball, that the object we may be looking at does not become invisible, but
remains impressed upon the eye. It has been ascertained that the impression
lasts for about the seventh or eighth part of a second, and although some-
times it may last for the third part of a second, it depends, no doubt, upon the
amount of sensitiveness belonging to the organ of vision. There are very
curious modifications of this property of vision, whereby colours and their
complementary tints are impressed upon the eye. Thus, if a red wafer is
placed on a sheet of black paper, and well illuminated by a sunbeam or any
brilliant light, it will appear again to a spectator looking from the black 'to a
white paper as a green one ; the red wafer being the real image, whilst the
green one is the spectrum. The experiment may be varied with a yellow
wafer on a black ground, which appears violet when the eyes are turned
rapidly away to a white surface. On this principle a very entertaining book
has been published. The reader, after staring at one of the illustrations, is
directed to look up to the ceiling or wall, to observe the spectral effect. Sir
D. Brewster explains these curious results, spoken of as accidental colours, by
supposing that the eyes, after staring at any particular colour, say a bright
red, become so fatigued or partially paralyzed that they cannot receive or
appreciate the wave of red light, but as white light is made up of various waves
of coloured light, the remaining sets of waves viz., blue or yellow-- can
impress the vision by producing the complementary green colour. The late
Dr. Golding Bird describes the following mode of demonstrating this fact,
giving the merit of the experiment to the late Professor Cowper, who invented
so many clever illustrations :
" Cut in a piece of cardboard a series of holes, so that when folded to-
gether they will exactly correspond, the whole resembling open lattice-work.
Provide some sheets of thin tissue-paper of various colours, selecting those
presenting strongly defined tints ; place one of them between the folds of the
cardboard and hold it up to a vivid light, keeping the eye fixed on the lattice-
work whilst the light penetrates the coloured paper ; in a few seconds the
white colour of the pasteboard will vanish, and be replaced by a strongly
marked tint complementary to that of the paper placed in it. Thus, with
PERSISTENCE OF VISION. 73
yellow paper the framework will appear violet, with blue it will be orange, and
with red it will be green. This illusion is so complete that it always excites
surprise in those who see it for the first time."
A little gunpowder placed on a block of wood, with iron filings sprinkled
over it, throws up a shower of brilliant sparks of burning particles of iron
when fire is applied ; and if the experiment is performed in a dark room, and
the eyes of those standing near the experiment are closed directly after
witnessing the real image of the burning particles of metal, they will see a
volume of faint light, sometimes coloured, which remains upon the retinae,
and forms a spectral image. If the colours of the solar spectrum are painted
FIG. 80. The Polytechnic Phenakistiscope.
on a glass disc, to which rapid motion may be imparted, after being fitted into
the oxy-hydrogen lantern, a large disc can be thrown upon the screen, which
changes to a greyish white directly it is set in motion. The change of the
disc of many colours to a grey is very impressive, and is probably understood
better by suggesting that the spectator should look through an aperture made
in some opaque screen at the coloured disc ; the red, orange, yellow, green, blue,
indigo, and violet pass before the aperture with such rapidity that they have
not time to impress the retina as single colours, succeeding each other one by
one, and they must therefore act collectively on the vision ; if collectively,
then synthetically ; or, in plainer terms, the colours are caused to unite and
reconstitute white light, or the nearest approach to it that can be produced
by a mechanical contrivance of this nature.
Many years ago the juveniles discovered that by twirling a halfpenny you
could see both sides of it ; not only the portrait of the reigning monarch, but
the usual figure of Britannia. This simple arrangement appears to have
been succeeded by a more elegant contrivance, invented by the late Dr.
74 ON LIGHT.
Paris, and called the Thaumatrope, or " Wonder-Turner," like many other
clever things, a " nine days' wonder," and succeeded and surpassed by a very
ingenious optical toy, invented by Plateau, called the Phenakistiscope.
In connection with the name of Plateau, the Rev. Mr. Shaw, in a letter to the
writer, says : " It may enhance the interest connected with the Phenakistiscope,
if not known to you or your auditory, to learn that this gentleman, now re-
siding in Ghent, Belgium, is and has been for years totally blind, carrying
out his discoveries and observations entirely through the intervention of his
wife. I mention this from personal experience, having assisted him some
years ago to translate a treatise on capillary attraction for English publica-
tion." Plateau's instrument, as arranged for the oxy-hydrogen light by Soleil
Duboscq, is a very complicated affair, consisting of the usual condensing
lenses, in front of which is the disc of glass with devices in regular order
painted upon it. The latter, of course, rotates, and at the same time another
wheel, containing four double-convex lenses set in the four quarters of the
wheel, supplies that intermittent and separate light to each picture, which,
when focused by the front lenses, produces all the effects of the popular
Zoetrope (Fig. 81).
FlG. 81. The Zoetrope at rest, showing tJie simple construction of the
Instrument.
In order to produce the best effect, it is absolutely necessary that each
picture should be impressed separately but quickly upon the vision; and
this is secured by the apertures followed by a certain opaque space, as
employed in Plateau's original device so long exhibited at the Poly
technic. This old-fashioned apparatus consists of a wheel perforated with
apertures, on the back of which the figures are painted, and when the
spectator looks through the slits into a plane mirror the figures appear to
move.
If the figures are painted in the usual manner on a disc, they all merge one
into the other when the disc is set in motion, and a series of circles and
eccentrics alone become apparent, which do not afford the slightest idea that
they represent the figures; but Sir Charles Wheatstone has^shown that by
constantly checking the motion, by a peculiar mechanism, so that each sepa-
rate figure is impressed momentarily on the vision, then the same effects of
motion are obtained without the intervention of the usual revolving slits or
PERSISTENCE OF VISION.
75
FIG. 82. The Zoetrope in motion, simulating exactly the motions of a little
girl playing with a skipping-rope.
apertures. This important experiment establishes the basis of this class of
illusions ; and the fact is further proved by the penny book now sold in the
streets. The little pages have printed on them a series of devices representing
any ordinary act of motion, such as a see-saw, and by rapidly passing the
pages over the thumb with the first finger the effect of apparent movement is
secured, as it would be with Plateau's apparatus, the Zoetrope, or Wheatstone's
disc, with the checking and arresting mechanism.
The best apparatus for showing to a large audience all the effects of per-
sistence of vision, and the curious and elaborate movements obtainable from
painted discs, is undoubtedly that devised by Mr. Thomas Rose, of Glasgow.*
But before explaining this contrivance it will be advisable to study Faraday's
paper.
One of the first and most interesting papers written on the effects which are
produced by persistence of vision is that of the late Dr. Faraday, and entitled,
" On a Peculiar Class of Optical Deceptions ;" and, as the apparatus used
chiefly consists of models constructed in cardboard, some copious quotations
from that paper are here made.f
" The preeminent importance of the eye as ah organ of perception confers
an interest upon the various modes in which it performs its office, the circum-
stances which modify its indications, and the deceptions to which it is liable,
far beyond what they otherwise would possess. The following account of a
* Fully described in article " Persistence " in a new edition of the " Popular Encyclopaedia." Blackie
"Journal of the Royal Institution," vol. i , p 205.
and Sons, London, Glasgow, and Edinburgh.
7 6
ON LIGHT.
peculiar ocular deception, which, in a greater or smaller degree, is not
uncommon, and which, if looked for, may be observed with the utmost
facility, may therefore prove worthy of attention ; and I am the more inclined
to hope so, because in some points it associates with an account and explana-
tion of an ocular deception given by Dr. Roget in the 'Philosophical Transac-
tions' for 1825, page 121.
" The following are some cases of the appearance in question. Being at the
magnificent lead-mills of Messrs. Maltby, two cog-wheels were shown me
moving with such velocity that if the eye were retained immovable no distinct
appearance of the cogs in either could be observed ; but, upon standing in
such a position that one wheel appeared behind the other, there was imme-
diately the distinct, though shadowy, resemblance of cogs moving slowly in
one direction.
" Mr. Brunei, junior, described to me two small similar wheels at the Thames
Tunnel ; an endless rope, which passed over and was carried by one of them,
immediately returned and passed in the opposite direction over the other, and
consequently moved the two wheels in opposite directions with great but equal
velocities. When looked at from a particular position, they presented the
appearance of a wheel with immovable radii.
" When the two wheels of a gig or carriage in motion are looked at from an
oblique position, so that the line of sight crosses the axle, the space through
which the wheels overlap appears to be divided into a number of fixed curved
lines, passing from the axle of one wheel to the axle of the other, in form and
arrangement resembling the lines described by iron filings between the oppo-
site poles of a magnet. The effect may be obtained at pleasure by cutting two
equal wheels out of white cardboard (Fig. 83 or 84), each having from twelve
FIG. 83.
FIG. 84.
to twenty or thirty radii, sticking them on a large needle two or three inches
apart, revolving them between the fingers, and
looking at them in the right direction against a
dark or black ground : the greater the velocity of
the wheels, the more perfect will be the appear-
ance. (Fig. 85.)
"When the dark-coloured wheel of a carriage is
moving on a good light-coloured road, so that the
sun shines almost directly on its broadside, and the
wheel and its shadow are looked at obliquely, so
that the one overlaps the other in part, then in the
overlapping part luminous or light lines will be
perceived, curved more or less, and conjoining the
axle and its shadow, if the wheel and shadow are
FlG. 85. superposed sufficiently, or tending to do so if they
PERSISTENCE OF VISION. 77
are superposed only in part. The more rapid the motion, the more perfect is
the appearance. The effect may be easily observed (Fig. 86) by making a
pasteboard wheel like one of those just described,
blackening it, sticking it on a pin, and revolving it
in the sunshine or candle-light before a sheet of
white paper.
" If the wheel be converted into a teetotum or top,
by having a pin thrust through its centre and spun
upon a sheet of white paper, the effect produced by
the wheel and its shadow will be obtained with
facility, and in form will resemble Fig. 85. In all
these cases no rims are required; the spokes or
radii will produce the effect. If a carriage wheel
running rapidly before upright bars, as a palisade FlG. 86.
or railing, be observed, the attention being fixed on
the wheel, peculiar stationary lines' will appear ; those perpendicular to the
nave or axis will be straight, but the others curved ; and the curve will be
greatest in those which are furthest from the upper straight line. These
curves are the same in form as those already described and explained by Dr.
Roget,* and the appearance itself is produced in a similar manner ; but the
phenomena are distinct, and the causes different. The effect at present re-
ferred to is best observed when the velocities are great, whereas that explained
by Dr. Roget takes place only when the velocities are moderate. It is pro-
bable that the effects briefly mentioned by an anonymous writer in the
'Quarterly Journal of Science,' first series, vol. x., p. 282, and already referred
to by Dr. Roget, were of the kind now to be explained ; for, though the de-
scription is not accurate, either for the effects which form the object of this
paper or that explained by Dr. Roget, and is, indeed, inconsistent with the
observation or explanation of any of the phenomena, it probably had its
origin in the occurrence of some of both kinds under the eyes of the writer.
" The effect is easily obtained by revolving a white pasteboard wheel before
a black or dark ground, and then, whilst regarding the wheel fixedly, traversing
the space before it with a grate also cut out of white pasteboard. By altering
the position of the grate and direction of its motion, it will be seen that the
straight lines in the wheel are always parallel to the bars of the grate, and
that the convexity of the curved lines is always towards that side of the grate
where its motion coincides in direction with the motion of the radii of the
wheel. By varying the velocity of the wheel and grate, the curves change in
their appearance, and the whole or any part of the system, as described and
figured by Dr. Roget, may be obtained at pleasure.
" I have had a very simple apparatus constructed by which these and many
other analogous appearances may be shown in great perfection and variety.
One board was fixed upright upon the middle of another, serving as a base ; the
upright board was cut into the shape represented in Fig. 87 ; the middle and
two extreme projections, forming points of support, were supplied with little
caps cut out of copper plate and bent into shape (Fig. 88), so that, when in
their places, they offer four bearings for the support of two axes, one on each
side the middle. The axes are small pieces of steel wire tapered at the extre-
mities ; each has upon it a little roller or disc of soft wood, which, though it
"Philosophical Transactions," 1825, p. ui.
ON LIGHT.
can be moved by force from one part of the axis to another, still has friction
sufficient to carry the latter with it when turned round. These axes are made
to revolve in the following manner: a circular copper plate, about 4 in. in
diameter, has three pulleys of different dia-
meters fixed upon its upper surface, whilst
its lower surface is covered with a piece of
fine sand-paper, attached by cement. A hole
is made through the centre of the plates and
pulleys, and guarded by a brass tube, so fitted
as to move steadily but freely upon an up-
right steel pin fixed in the middle of the cen-
tre wooden support ; hence, when the plate
is in its place, it rests upon the two rollers
belonging to the horizontal axes, whilst it is
rendered steady by the upright pin. It can
be easily turned round in a horizontal plane,
and it then causes the two axes with their
rollers to revolve in opposite directions ; and
the velocities of these can be made either
equal to each other, or to differ in almost any
ratio, by shifting the rollers upon the hori-
zontal axes nearer to or farther from the
centre of the stand.
FIG. 87.
FIG.
"To produce motions of the axes in the same
direction, an aperture was cut in the lower part of
the upright board; a roller turned for it, which
loosely fitted within the aperture ; a steel pin or rod
passed as an axis through the roller. The roller
hangs in its place by endless lines made of thread,
passing under it and over little pulleys fixed on the
horizontal axis. When, therefore, it is turned by the projecting pin, it causes
the revolution of the axes. The variation in velocities is obtained by having
the roller of different diameters in different parts, and by having pulleys of
different dimensions. This description will be easily understood by reference
to the figures 87 and 88.
FIG. 89.
" This apparatus had to carry wheels, either with cogs or spokes, which was
contrived in the following manner: The wheels were cut out of cardboard,
were about 7 in. in diameter, and were formed with cogs and sookes at pleasure.
PERSISTENCE OF VISION. 79
A piece of cork, being the end of a phial cork, about the tenth of an inch in
thickness, was then fastened by a little soft cement to the middle of the wheel,
and a needle run through both and then withdrawn. These wheels could at
any time be put upon the axes, and, being held sufficiently firm by the friction
of the cork, turned with them. By these arrangements the axes could be
changed, or the wheels shifted, or the velocities altered without the least delay.
" The beauty of many of the effects obtained by this apparatus has induced
me to describe it more particularly than I otherwise should have done. The
appearance which I first had shown to me by Mr. Maltby was exhibited very
perfectly : two equal cog-wheels were mounted (Fig. 89) so as to have equal
opposite velocities ; when put into motion, which is easily done by the thumb
and finger applied to the upper pulley and the horizontal copper plate, they
presented each the appearance of an uniform tint at the part corresponding to
the series of cogs or teeth, provided that the eye was so placed as to see the
whole of both wheels ; but when a position was taken up so that the wheels
were visually superposed, then, in place of an uniform tint, the appearance of
teeth or cogs were seen, misty, but perfectly stationary, whatever the degree
of velocity given to the wheel. By cutting the cogs or teeth in the wheel nearest
to the eye deeper (Fig. 90), the eye could be brought into the prolongation of
the axes of the wheels, and then the spectral cog-
wheel appeared perfect (Fig. 91). The number of
intervals thus occurring was exactly double the
number of teeth in either wheel ; thus a wheel with
twelve teeth produced twenty-four black and
twenty-four white alternations. When one wheel
was made to move a little faster than the other, by
shifting the wooden roller on its axis, then the
spectrum travelled in the direction of that wheel
having the greatest velocity, and with more rapi-
dity the greater the difference between the velocities
of the two wheels. When the wheels were looked
at so that they only partly visually superposed each FIG. 91.
other, the effect took place only in those parts;
and it was striking and extraordinary to observe, as it were, two uniform tints
mingling and instantly breaking out into the alternations of light and shade
which I have described. There are many variations in the curvature and other
appearances obtained by altering the position of the eye, which will be imme-
diately understood when observed, and which, for brevity's sake, I refrain from
describing.
"Wheels were then fixed on the machine, consisting of radii or spokes, each
having twelve, equal in length and width (Fig. 84). When revolving alone,
each wheel gave with a certain velocity a perfectly uniform tint ; but when
visually superposed there appeared a fixed wheel, having twenty-four spokes,
equal in dimensions to the original spokes. Variations of the position of the
eye, or of the relative velocity of the two wheels, caused alternations similar
to those I have referred to with the cog-wheels.
" In observing these effects, either the wheels should be black or in shade,
whilst the part beyond is illuminated ; or else the wheels should be white and
enlightened, whilst the part beyond is-in deep shade. The cog-wheels present
nearly a similar appearance in both cases, though in reality the parts of the
spectrum which appear darkest by one method are lightest by the other. The'
8o ON LIGHT.
spoke-wheels give a spectrum having white radii in the first method, and
dark radii in the second. * Placing the wheels between the eye and the clouds,
on a white wall, or a lunar lamp, answers very well for the first method ; and,
for the second, merely reversing the position, and allowing the light to shine
on the parts of the wheel towards the eye, whilst the background is black or
in obscurity, is all that is required. Strictly, the phenomena should be viewed
with one eye only, but it is not often that vision with two eyes disturbs the
effects to any extent.
" The cause of these appearances, when pointed out, is sufficiently obvious,
and immediately indicates many other effects of a similar kind, and equally
striking, which are dependent upon it. The eye has the power, as is well
known, of retaining visual impressions for a sensible period of time ; and in
this way recurring actions, made sufficiently near to each other, are percep-
tibly connected and made to appear as a continued impression. The lumi-
nous circle visible when a lighted coal or taper is whirled round, the beautiful
appearance of the Kaleidophone, the uniform tint spread by the revolution of
one of the spoke or cog wheels already described, are few of the many effects
of this kind which are well known.
" But during such impressions the eye, although to the mind occupied by
an object, is still open, for a large proportion of time, to receive impressions
from other sources ; for the original object looked at is not in the way to act
as a screen, and shut out all else from sight. The result is that two or more
objects may seem to exist before the eye at once, being visually superposed.
The school-boy experiment of seeing both sides of a whirling halfpenny at the
same moment, the appearance of the Thaumatrope, and the transparency of
the revolving cog or spoke wheels referred to in consequence of which other
objects are seen through the shaded parts are all effects of this kind ; two
or more distinct impressions, or sets of impressions, being made upon the
eye, but appearing to the perception" as one.
" So it is in the appearances particularly referred to in this paper. They
are the natural results of two or more impressions upon the eye, really, but
not sensibly, distinct from each other. If, whilst the eye is stationary, a
series of cogs, like those represented by the continuous outline (Fig. 92), pass
rapidly before it, they produce a uniform
tint to the eye ; and, for the purpose of
following out the description, let it be
supposed the cogs are in shade between
the eye and a white background, the tint
is then a hazy semi-transparent grey. If
another series of cogs, represented by the
dotted outline, and close to the first, so
as to give no sensible angular difference
to the dimensions of the cogs, pass with
equal velocity in the same direction, it will produce its corresponding tint.
If the two sets of cogs be visually superposed in part, as in the figure, there
will be no alteration in the uniformity of the tint. If the cogs of one set be
more or less to the right or left of the other, then the superposed part will
approach more or less to the tint of the shaded and uncut part of the card-
board wheel, and be less transparent. But if, instead of the motion being
equal, the velocities are unequal, then total changes of the appearance super-
vene ; the spectrum (if I may so call it) of the superposed parts becomes alter-
PERSISTENCE OF VISION. 81
nately light and dark, and the alternations take place more or less rapidly as
the velocities of the two sets of cogs differ more or less from each other.
" When the cogs move in opposite directions, the uniform tint which each
alone can produce is soon broken up in the superposed parts into lighter and
darker portions ; and when the velocities of both are equal, the spectrum is
resolved into a certain number of light and dark alternations, which are
perfectly fixed, and which to the mind offer a singular contrast to the rapidly
moving state of the wheels, and to the variations which their velocity may
undergo without altering the visible result.
" The effect, strange as it at first appears, will be easily understood by
reference to Fig. 91. Suppose the eye directed to the part / beyond the cogs,
and the sets of cogs to be moving with equal velocities in opposite directions,
indicated by the arrow-heads, the part / will be eclipsed by the cogs a and b
simultaneously, and for exactly the same time ; for they begin to cover it and
leave it together. /, therefore, is alternately open to and shut from the eye
for equal times ; for what these cogs have done will be performed by all the
other cogs in turn, and the cogs are equal in area to the spaces between ;
FIG. 92 A. FIG. 92 B.
half the light, therefore, from that part of the background comes to the eye,
and produces a corresponding impression. But with respect to the point d,
although the cog b is just leaving it exposed, the cog a is just beginning to
eclipse it ; and by the time the latter has passed over, the edge of the cog e
will be upon the spot, and that cog will therefore hide it until f comes up, so
that in fact the point d is always hidden ; no light comes from that part of
the background, and it consequently appears dark. /' is circumstanced just
as / was, for the cogs a and e cover it simultaneously, and so do all the other
cogs in pairs ; it is, therefore, a light space in the spectrum, d 1 is a repetition
in everything of d, and is a dark space. The parts intermediate between the
maxima of light and darkness will, by examination, be found to be eclipsed
for intermediate periods, and to appear more or less dark in consequence, so
that the appearance of the spectrum belonging to the visually superposed
parts of the two sets of cogs is as in Fig. 92 A.
" In the case of equal wheels with radii, the fixed spectrum produced when
the wheels superpose each other has twice the number of radii of either
wheel, that being, of course, the number of times which the radii coincide
with each other in one revolution.
" Fig. 92 B represents the fixed spectrum produced by two equal wheels of
eight radii each. When the radii or spokes are narrow, the difference in the
intensity of tint between the middle and the edges of each image of a spoke
is so slight as to be scarcely perceptible. But as this circumstance and many
others will explain themselves immediately they are experimentally observed,
82 ON LIGHT.
it is unnecessary to dwell minutely upon them here. A very simple experi-
ment will render the whole of these effects perfectly intelligible.
" If a little rod of white cardboard, five or six inches long, and one-thirtieth
of an inch wide, be moved to and fro from right to left
before the eye, an obscure or black background being
beyond, it will spread a tint, as it were, over the space
through which it moves. (Fig. 93, A.) A similar rod
held and moved in the other hand will produce the
same effect ; but if these be visually superposed, i.e., if
one be moved to and fro behind the other, also moving,
then in the quadrangular space included within the in-
tersection of the two tints will be seen a black line,
sometimes straight, and connecting the opposite angles
A of the quadrangle, at other times oval or round, or even
FIG. 93. square, according to the motions given to the two card-
board rods (Fig. 94, B).
"This appearance is visible even when the rods are several inches or a foot
apart from each other, provided they are visually superposed. It is produced
exactly as in the former case, and the black line is in fact the path of the
intersecting point of the moving rods. As their motions vary, so does the
course of this point change, and, wherever it occurs, there is less eclipse of
the background beyond than in the other parts, and consequently less light
from that spot to the eye than from the other portions of the compound spec-
trum produced by the moving rods.
"In this experiment the eye should be fixed, and the part looked at should
be between the planes in which the rods are moved. The variation produced
by using black rods, and looking at a white ground, will suggest itself. Those
who find it difficult to observe the effect at first, will instantly be able to do
so if the rod nearest the eye is black, or held so as to throw a deep shade
the line is then much more distinct ; but the explanation is not quite the same,
but nearly so it will suggest itself. Two bright pins or needles produce the
effect very well in diffused daylight ; and the line produced by the shadow of
one on the other, and that belonging to the intersection, are easily dis-
tinguished and separated.
"If whilst a single bar is moved in one hand several bars or a grate is
moved in the other, then spectral lines, equal to the number of bars in the
grate, are produced. If one grate is moved before another, then the lines are
proportionably numerous ; or if the distances are equal, and the velocity the
same, so that many spectral lines may coincide in one, that one is so much
the more strongly marked. If the bars used be serpentine or curved, the lines
may be either straight or curved at pleasure, according as the positions and
motions are arranged so as to make the intersecting point travel in a straight,
or a curve, or in any other line.
" The cause of the curious appearance produced, when spoke or cog wheels
revolve before each other, already described, will now be easily understood ;
the spokes and cogs of the wheels produce precisely the same effect as the
bars held in the hand, and the fixedness of the position of the spectrum
depends upon the recurrence of the intersecting or hiding positions, exactly in
the same place with equal wheels, provided the opposite motion of each be of
equal velocity and the eye be fixed.
" When the wheels were used in the little machine described (Fig. 87), having
PERSISTENCE OF VISION. 83
equal but oblique teeth, and the obliquity in the same direction, the spectrum
was also marked obliquely ; but when the obliquity was in opposite directions
the spectrum was marked as with straight teeth.
" When equal wheels were revolved with opposite motions, one rather faster
than the other, the spectrum travels slowly in the direction of the fastest wheel ;
when the difference in the velocity of the two wheels was made greater, the
spectrum travels faster. These effects are the necessary consequence of the
transference of the intersecting points already described, in the direction of
the motion of the fastest wheel. When one wheel contains more cogs than
the other, as, for instance, twenty-four and twenty-two, then with equal
motions the spectrum was clear and distinct, but travelled in the direction of
the wheel having the greatest number of teeth.
" When the other wheel was made to move so much faster as to bring an
equal number of cogs before the eye, or rather any one part of the eye, in the
same time as the other, the spectrum became stationary again. The explana-
tion of these variations will suggest themselves immediately the effects are
witnessed. When the motion of the wheels upon the machine is in the same
direction, the velocities equal, and the eye placed in the prolongation of the
axis of the wheels, no particular effect takes place. If it so happens that the
cogs of one coincide with those of the other, the uniform tint belonging to one
wheel only is produced. If they project by the side of each other, it is as if
the cogs were larger, and the tint is therefore stronger. But, when the velo-
cities vary, the appearances are very curious ; the spectrum then becomes
altogether alternately light and dark, and the alternations succeed each other
more rapidly as the velocities differ more from each other.
" When wheels with radii are put upon the machine, it is easy to observe,
in perfection, the optical appearance already referred to, as exhibited by car-
riage-wheels, &c. (Fig. 85). They should be looked at obliquely, so as to be
visually superposed only in part ; and, provided the wheels are alike, and both
revolving in the same direction with equal velocities, they immediately assume
the form described, passing in curves from the axis of one wheel to the axis
of the other, much resembling in disposition those curves formed by iron
filings between two opposite poles of a magnet.
"If the wheels revolve in opposite directions, then the spectral lines, origi-
nating at each axis as a pole, have another disposition, and, instead of running
the one set into the other, are disposed generally like the filings about two
similar magnetic poles, as if a repulsion existed; not that the curves or the
causes are the same, but the appearances are similar. A very little attention
will show that all these lines are the necessary consequence of the travelling
of successive intersecting points ; and any one of them may be followed out
by experimenting with the two pasteboard rods already described, these being
moved in the hand as if each were the spoke of a wheel.
" All these effects may be simply exhibited by cutting out two equal paste-
board wheels without rims, passing a pin as an axis through each, spinning
one upon a mahogany or dark table, and then spinning the other between the
fingers over it, so that the two may be visually superposed. If the appear-
ances are observed by a lamp or candle, the wheels should be so held to the
light that the shadow of the upper may not fall upon the lower ; otherwise the
effects are complicated by similar sets of lines which appear upon the lower
wheel, and are produced by the shadow of the upper.
" These are the same in form and disposition as in the former, and are even
62
ON LIGHT.
more distinct ; they should be viewed, not through the upper wheel, but
directly upon the lower; their explanation has in part been given, and will be
sufficiently evident."
Returning to the consideration of Mr. Rose's Photodrome, or "Light-
Runner," the construction is simple, and not likely to get out of order.
It consists of two parts. The first part (b,. Fig. 94) consists of a wheel about
four feet in diameter, provided with eight spokes, and wholly constructed of the
the best seasoned mahogany. The wheel is driven by a gut band proceeding
from a smaller flying- wheel, which is worked by .hand. This large wheel is so
FIG. 94. The two portions of Rose's Photodrome, viz., the large and small
wheels alluded to.
arranged on a platform, or other convenient place, that a strong light, arranged
in a lantern with a proper lens, casts its rays through one of two apertures
in a second disc (a, Fig. 94), about two feet in diameter, placed below and in front
of it, so that the shadow of the large wheel is distinctly thrown upon the white
screen behind. When the large wheel is set in motion, and a certain velocity,
from about 250 to 300 revolutions per minute, obtained, all the spokes and the
shadows of them disappear, and then the curious effect of the rim or ring of
the wheel is shown revolving without any apparent connection with the central
axis. Whilst this large wheel is going round, if the spectator looks obliquely
through the spokes of the real wheel to those of the shadow-wheel, he will see
the curved lines described by Faraday, as obtained by him with his cardboard
models in Fig. 86, p. 77. In a favourable position the whole distorted shadow-
wheel, with curve lines on a grey ground, becomes visible on a scale not
probably contemplated by Faraday with his small cardboard spokes.
These effects being shown with the large wheel, attention is now directed to
the second portion of the apparatus, consisting of a disc about two feet in
PERSISTENCE OF VISION.
FIG. 95. Exhibition of the Photodrome at the Polytechnic.
diameter, and provided with two apertures. With an ingenious sliding
arrangement six or eight apertures can be obtained, if required, but two are
preferred for this experiment.
The rays of light, as already described, pass through these apertures every
time they come round, and the large wheel being still in motion and the
spokes invisible, directly the small disc begins to move and attains a moderate
velocity all the spokes and their shadows return. At first they are very hazy
and indistinct, and almost semi-transparent ; but, as the velocity increases,
they become distinctly apparent, and the large wheel appears to be going
round slowly and nearly to stand still. The next change in the velocity of
the small disc throwing the flashes causes the spokes to be multiplied,
generally by five ; thus, forty spokes and forty shadows may be counted, the
latter being grey, and not black, like the original eight shadows. The next
and last increase of velocity in the small disc, which brings it up to about a
thousand revolutions in a minute, causes the large wheel the eight spokes
and eight shadows to appear quite distinct, and at that moment, although
the large wheel is going round three hundred times in a minute, it appears to
stand still.
The flashes of light perform the same duty as the slits or apertures in
Plateau's apparatus, and before the large wheel has time to move the light
arrives and passes away. If the large wheel was moving at the rate of one
thousand revolutions in a minute, no change would occur. It is the difference
in the two velocities which determines this curious form of the illusion.
Mr. Rose mentions a most amusing story in connection with the curious
illusions produced by the Photodrome, viz., that of the large wheel apparently
standing still when it is really moving very fast. It appears that whilst
86 ON LIGHT.
showing the experiment to a number of working men, at a lecture-hall in
Glasgow, one of them rose from his seat, and wanted to creep up quietly to
the large wheel, for the purpose of convincing himself by touch that it really
was moving. Fortunately, they stopped the man in time, or he would pro-
bably have received a blow from the spokes of the wheel which might have
broken some finger-bones. This incredulity was an interesting example of
the effect of that teaching which grows up with us, viz., that " seeing is
believing." Here was a man who had evidently never seen an optical illusion
before, and, doubtless, by the time Mr. Rose had finished his beautiful
experiments,, he discovered that the eye, like the ear, is easily deceived.
LIGHT AND COLOUR.
SPECTRUM ANALYSIS.
ABERRATION AND ACHROMATISM.
In the frontispiece of this book is shown the beautiful apparatus attached
to the Duboscq lantern, containing the electric lamp. Such a contrivance, with
its lenses and prisms, is a great contrast to the simple means employed by Sir
Isaac Newton, nearly two hundred years ago, to effect the same object viz.,
the decomposition of light. The great discovery made by Newton, about the
year 1672, that light is not of a simple but of a compound nature, was estab-
lished by the help of a prism (an optical instrument already described at p. 50)
through which a sunbeam was permitted to pass. No doubt, whilst moving the
prism about in the light the production of colour might have been accidentally
discovered, as it would have been by any other careful experimentalist ; but it
fortunately happened that the discovery fell to the lot of a mind already well
prepared to grapple with difficult phenomena, and Newton was soon able to
convince himself and others that he had analysed light, and resolved it into
seven colours viz., Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
Here was light, not only refracted or bent from its natural course, but spread
out a phenomenon to which the term dispersion is now given. Other lenses
or optical instruments possess the same property in a more limited degree,
and hence the edges of the pictures or images thrown by convex lenses from
the magic lantern show colours. In what are called achromatic lenses, the
disagreeable effect upon the eye produced by ordinary lenses is prevented, and
the colours neutralized and destroyed. The value of science-teaching as a
part of regular education is now fully recognized ; but schoolmasters have little
time to spare to superintend the manufacture and collection of oxygen in bags,
or to put together a voltaic battery, for the purpose of obtaining either the
oxy-calcium or the electric light ; consequently, the phenomena of light are
only taught theoretically instead of experimentally. If a master could teach
the leading principles of optics by merely closing the shutters of his room, and
allowing a sunbeam of a greater or lesser diameter, determined by different-
sized diaphragms, to enter through an aperture into the darkened room, he
would be more disposed to impart this kind of knowledge to his boys, because
LIGHT AND COLOUR.
FIG. 96.
The Heliostat, placed on a shelf outside the window, reflecting the ray of
light which passes through a hole in a sJmtter on to a prism, to show the
decomposition of light.
the sunbeam would cost nothing, and with the help of an instrument called
the Heliostat* (77X10$, the sun, and <rraTos, to stand still) the reflected ray of
light may always be retained in a fixed direction, notwithstanding the apparent
motion of the sun.
In order to obtain a solar spectrum of the most perfect kind, the aperture
through which the light passes should be a slit not more than the twentieth
part of an inch in breadth, and the length rather less than that of the prism,
placed at an angle of sixty degrees, and the spectrum thrown on the white
wall or screen, which should be about sixteen or nineteen feet distant. In the
frontispiece is represented the lantern containing the electric lamp, connected
with a powerful battery, which latter is placed outside the lecture-room. The
light is condensed by a plano-convex lens, and passed through a very narrow
slit of metal, capable of adjustment by a proper motion, so that it can be made
narrjwer if required. The slice of light, or thin electric light-ray, is now per-
mitted to fall on another double-convex lens, which causes the ray to converge
a little more, and to fall upon two hollow prisms filled with bisulphide of
carbon, which enjoys a high refractive power. After passing through the two
* These instruments are now sold at a very cheap rate bv Messrs. Griffin, and can he obtained for
'3 ot 4$.
88
ON LIGHT.
prisms, it is bent on to the screen in front ; and if the battery is in good order,
the most vivid colours are obtained.
The seven colours are easily caused to re-unite and form white light, either
by passing the dispersed rays through
a fish-globe full of water, or by receiving
them on to a double-convex lens (A, Fig.
97), or into a concave mirror (B, Fig.
97), or by allowing the spectrum formed
by one prism to fall on another, as at C,
Fig. 97, of the same nature and at the
same refracting angle, but in a reversed
position, so that the two outer faces of
the two prisms become parallel to each
other, and in fact are then equivalent
to a piece of flat or plane glass.
A very refined and beautiful experi-
ment, originated in Paris (Fig. 98), is
that in which seven mirrors are used,
and by arranging them at the proper
angles they may be made to reflect each
colour separately on to a disc, or the
whole may be brought together to pro-
duce one spot of white light.
In stating that light is made up of
seven colours, it must be borne in mind
A X
FlG. 98. Apparatus with Seven Plane
Mirrors for reflecting the seven
FIG. 97. The Recomposition of Light* colours of the Solar Spectrum.
that they are not considered to represent the ultimate, but proximate analysis
of light. One of Brewster's masterly essays is that in which he endeavours to
prove that the spectrum is entirely pervaded with the three simple colours,
red, yellow, and blue, from which the other colours, orange, green, indigo,
* Effected in three ways by a double-convex lens, A ; a concave mirror, B; or by a second prism, c.'
LIGHT AND COLOUR. 89
and violet, arise. By employing the absorptive power of a wedge of blue
glass, he was enabled to refute the conclusion deduced by Newton, " That t(
the same degree of refrangibility ever belongs the same colour, and to the
same colour ever belongs the same degree of refrangibility." Sir Isaac ex-
amined each colour separately by making a hole in the screen upon which the
spectrum fell exactly in the centre of each colour, and allowing that colour to
fall upon another prism ; and finding that this second refracting surface did
not change or decompose the special colour under examination into any other
colours, he concluded " That the light of each different colour had the same
index of refraction;" and he called such light homogeneous or simple light,
whilst ordinary or white light he termed heterogeneous or compound. It is
this enunciation of Newton which Brewster disproved by the following experi-
ments : He says, "If we take a piece of blue glass, like that generally used for
finger-glasses, and transmit through it a beam of white light, the light will be
a fine deep blue. This blue is not a simple homogeneous colour like the blue
or indigo of the spectrum, but is a mixture of all the colours of white light
which the glass has not absorbed, and the colours which the glass has absorbed
are those which the blue wants of white light, or which, when mixed with this
blue, would form white light. In order to determine what these colours are, let
us transmit through the blue glass the prismatic spectrum ; or, what is the
same thing, let the observer place his eye behind the prism, and look through
it at the sun, or rather at a circular aperture made in the window-shutter of a
dark room. He will then see through the prism the spectrum as far before
the aperture as it would be above the spot when shown on the screen. Let the
blue glass be now interposed between the eye and the prism, and a remarkable
spectrum will be seen, deficient in a certain number of its differently coloured
rays. A particular thickness absorbs the middle of the red space, the whole
of the orange, a great part of the green, a considerable part of the blue, a little
of the indigo, and very little of the violet. The yellow space, which has not
been much absorbed, has increased in breadth. It occupies part of the space
formerly covered by the orange on one side, and part of the space formerly
covered by the green on the other. Hence
it follows that the blue glass has absorbed
the red light which, when mixed with the
yellow light, constituted orange, and has
absorbed also the blue light which, when
mixed with the yellow, constituted the part
of the green space next the yellow. We
have therefore, by absorption, decomposed
green light into yellow and blue, and orange
light into yellow and red ; and it conse FlG. 99.
quently follows that the orange and green
rays of the spectrum, though they cannot be decomposed by prismatic refrac-
tion, can be decomposed by absorption, and actually consist of two different
colours possessing the same degree of refrangibility. Difference of colour is,
therefore, not a test of difference of refrangibility. Red, yellow, and blue
light exist at every point of the solar spectrum. The existence of these
primary colours in the spectrum, and the mode in which they produce, by
their combination, the seven secondary or compound colours which are de-
veloped by the prism, will be understood from Fig. 99, where M N is the
prismatic spectrum, consisting of three primary spectra of the same lengths,
9
ON LIGHT.
M N, viz., a red, a yellow, and a blue spectrum. The red spectrum has its
maximum intensity at R ; and this intensity may be represented by the distance
of the point R from M N. The intensity declines rapidly to M and slowly to
N, at both of which points it vanishes. The yellow spectrum has its maximum
intensity at Y, the intensity declining to zero at M and N ; and the blue has its
maximum intensity at B, declining to nothing at M N. The general curve which
represents the total illumination at any point will be outside these three curves,
and its ordinate at any point will be equal to the sum of the three ordinates
at the same point. Thus the ordinate of the general curve at the point Y will
6e equal to the ordinate of the yellow curve, which we may suppose to be 10,
added to that of the red curve, which may be 2, and that of the blue, which
FIG. 100. HerscheFs Direct-Vision Prism.
may be i. Hence the general ordinate will be 13. Now, if we suppose that
3 parts of yellow, 2 of red, and I of blue make white, we shall have the colour
at Y equal to 3+2+1, equal to 6 parts of white mixed with 7 parts of yellow;
that is, the compound tint at Y will be a bright yellow, without any trace of
red or blue. As these colours all occupy the same place in the spectrum, they
cannot be separated by the prism ; and if we could find a coloured glass which
would absorb 7 parts of the yellow, we should obtain at the point Y a white
light which the prism could not decompose." *
It may be useful to mention that, with Herschel's direct-vision prism, filled
with bisulphide of carbon, the trouble required in adjusting the lantern so as
to throw the spectrum on to the disc is obviated, and the lantern, with its
prism attached, may be placed directly in front of the screen, as in any other
ordinary optical experiment.
Helmholtz and Airy have thrown great doubts on Brewster's experiments and theory of the spectrum.
LIGHT AND COLOUR. 91
PHYSICAL PROPERTIES OF THE SPECTRUM.
It has been shown how a ray of light can be separated into its proximate or
ultimate colours. These various portions of coloured light have certain distinct
properties, which have been most carefully investigated by different physicists.
The illuminating power of the spectrum, as might be imagined, exists in the
most luminous portion of the band of colours, viz., in the yellow light ; and experi-
ments carefully conducted by Herschel and Frauenhofer confirm this fact, and
show that the greatest amount of light exists nearer the red than the violet
end of the spectrum. The calorific power of the spectrum increases gradually
from the blue colour ; it rises to its maximum in the red ; but, what is most
curious, it reaches its greatest elevation beyond the limits of the visible red
ray, or red end of the spectrum. The invisible rays of heat are, therefore,
more powerful than the other heat-giving rays of the spectrum accompanied
with light, as in the yellow, orange, or red colours ; the luminous radiations do
not give so much heat as the non-luminous ones ; and Tyndall, speaking of
this remarkable circumstance, says, "In the region of dark rays beyond the
red the curve shoots up in a steep and massive peak, a kind of Matterhorn of
heat, which dwarfs by its magnitude the portion of the diagram representing
the luminous radiation."
The chemical influence of the spectrum, unlike the heating and illuminating
rays, is at its minimum at the red end, and rises gradually in intensity towards
the violet. Light acts as a chemical agent not only with certain portions of
its luminous rays, but, like heat, with its non-luminous rays. Ritter, of Jena,
discovered that chloride of silver was acted upon and blackened beyond the
violet end of the spectrum. Dr. Herschel and Dr. Wollaston confirmed this
fact. These chemical or actinic rays have been carefully studied and most
industriously employed, so that a new art has been created, called Photography,
which, in a thousand different ways, is now made subservient to the require-
ments of man. Moser has discovered that certain rays have the power to set
up chemical change, and this once begun may be continued with other co-
loured rays, that could not in themselves produce chemical decomposition. An
iodized silver plate, with an engraving placed over it, was exposed to light until
the action had commenced ; if this plate was then placed under a violet glass,
the picture was soon obtained ; whilst a long time elapsed, and the result was
imperfect, when the same plate, after exposure to sunlight, was placed under
a red glass. If, however, the prepared plate was first exposed in a camera to a
blue light, and then placed under a red glass, the picture was speedily obtained.
In the early portion of this article phosphorescence has been considered, and
here it may be mentioned that Becquerel calls the rays capable of setting
up or commencing chemical action "exciting rays," and others which only
possess the power of continuing a chemical change " phosphoregenic " or
" continuing rays," and has identified the latter with the power possessed by
light of rendering certain bodies luminous. (See p. 9.) It is the phosphoregenic
rays, extending from the indigo to beyond the violet ray, which render certain
bodies phosphorescent by insolation. Becquerel has invented a most ingenious
instrument, called the Phosphoroscope, by which substances can be viewed,
directly after exposure to light, and the time of the duration of the phos-
phorescent power accurately determined. Thus several bodies, which are only
phosphorescent for some fraction of a second, have been added to the long
list of substances affected in a similar but more decided manner.
ON LIGHT.
When the bright rays from the electric lamp are passed through blue glass,
and then permitted to fall upon a plate of glass coloured yellow by the oxide
of uranium, the latter becomes self-luminous, and emits rays which are altered
in their vibratory power ; the original rays have undergone a change in their
refrangibility.
To these phenomena, which Professor Stokes has investigated with the
greatest care, the title of fluorescence, or internal dispersion, has been given.
Figures or letters painted with a strong solution of sulphate of quinine in tar-
taric acid become curiously self-luminous when the rays passed through blue
or, better still, violet glass are allowed to fall upon them. A little sulphur
burnt in a jar of oxygen emits rays which render paper painted with an
alcoholic solution of stramonium self-luminous.
A tube of uranium glass, conveying the coil-discharge in -vacuo, is similarly
affected by this peculiar electric light. It was ascertained that prisms made
of glass appeared to absorb a larger number of the more refrangible rays,
and Professor Stokes found that by using prisms made of quartz he could
obtain, with the electric light, a spectrum six or eight times as long as the
ordinary one ; and his experiments indicate that the chemical, the luminous, the
phosphorogenic rays, or rays of high refrangibility, are intimately connected
with each other, and are only so many effects of one and the same cause.
THE DARK OR FIXED LINES IN THE SOLAR SPECTRUM.
At the beginning of the present century, in the year 1802, Dr. Wollaston
announced that he had discovered two dark lines, one in the green and the
other in the blue space of the solar spectrum, formed by a prism of flint glass.
This very humble beginning, at first exciting little or no attention, has led on
to a series of most valuable experiments, which have not only been made with
terrestrial substances, but have even by analogy conducted the aspiring philo-
sopher to the far-distant celestial bodies, where, by the help of the light
emitted and reflected from them, certain conclusions as to their physical
nature and aspect have been arrived at. Wollaston also showed his great
sagacity as an observer, in discovering the bright lines in the spectrum of the
electric spark.
. .:' i ! SI .!:'.-. yf
FIG. lOl.Fraugnhofer's Seven Lines in the Solar Spectrum.
About the year 1814, the celebrated mathematical optician, Frauenhofer, of
Munich, repeated Wollaston's experiment, -and not only found the two lines,
but discovered that the spectrum was crossed throughout its entire length by
a great number of dark lines of different breadths. In consequence of the
industry with which Frauenhofer continued the investigation, and the care with
which he mapped out and measured the exact place of each most important
line in the spectrum, they have by universal consent been called Frauenhofer's
SPECTRUM ANALYSIS. 93
lines; of these seven have been particularly distinguished, and are marked B,
C, D, E, F, G, and H, Fig. 101.
Thus B is in the red space, near the end ; C is near the edge of the red ; D
is in the orange, and is a strong double line, separated by a bright one ; E is
in the green, consisting of several, the middle one being the strongest ; F is in
the blue ; G is in the indigo ; and H in the violet. These special lines, so care-
fully determined by Frauenhofer, have remained as fixed points of reference.
But they do. not give the student any idea of the immense number of lines
which are to be found throughout the whole length of the visible portion of
the solar spectrum, and even in the invisible rays rendered visible by the experi-
ments of Professor Stokes. Their name is legion, and they are to be counted
by hundreds and thousands; and so far back as the year 1814 Frauenhofer
had counted 600. In the year 1830 Simms constructed the first most import-
ant spectrum apparatus. In 1832 Brewster carefully examined the dark lines
produced by passing the spectrum obtained from an artificial source of light
through nitrous acid gas ; at first he thought they were identical with the dark
lines in the solar spectrum, but Professors Daniell and Miller proved that
this was not the case, and that they were produced by the absorptive power
of the gas. In the year 1835 Wheatstone observed that the incandescent
vapour of metals, obtained by the electric discharge through metallic poles,
gave certain coloured lines peculiar to each metal. He concluded that the
electric spark results from the volatilization and ignition, not the combustion,
of the ponderable matter of the poles itself, as the same phenomena were
observed in hydrogen; and he states that these differences of spectra
obtained from various metallic poles "are so obvious, that one metal may
instantly be distinguished from another by the appearance of its spark ; and
we have here a mode of discriminating metallic bodies, more readily appli-
cable even than a chemical examination, 'which may hereafter be employed for
useful purposes." How true this prediction proved is shown in the construc-
tion and use of the apparatus now employed to obtain the spectra of terres-
trial metals for the purpose of comparing their coloured lines with the black
lines obtained from the light of the sun, the fixed stars, &c. The apparatus
made by Huggins and Miller, and applied to the heavenly bodies, includes a
slit for the admission of light, and over one half of it is placed a right-angled
prism to receive the light from the electric sparks obtained from metallic
poles and sent by a mirror through an aperture to the prism. The lines
obtained from any given metal, such as sodium, could be at once compared by
exact measurement with similar black lines obtainable from solar light, and
the two identified with eax:h other. It remained for Bunsen and Kirchoff,
in 1859, to sum up all the labours of the clever men who had preceded them,
and, with the help of their own experiments, to read Frauenhofer's black lines
as if they were hieroglyphics, the key to which they had at last discovered by
elaborate experiments. Since Kirchoff's discoveries, Mr. Huggins and Dr.
Miller have steadily persevered in the same path of spectrum analysis, and
have given the world some remarkable facts, showing the nature of the
planets, fixed stars, nebulas, and comets.
The chief credit has fallen to Bunsen and Kirchoff, because they skilfully
grasped the whole phenomena, and reduced them to a perfect system ; it
should, however, be remarked that, as far back as the year 1752, Thomas
Melville investigated the nature of coloured flames, and specially observed
the yellow flame, which no doubt gave Brewster the idea of the monochro-
94
ON LIGHT.
matic lamp and light, obtained with alcohol and salt. In 1822 Sir John
Herschel remarks that, "The colours thus communicated by different bases
to flame affords in many cases a ready and neat way of detecting extremely
minute quantities of them." In 1834 Mr. Fox Talbot, speaking of his experi-
ments with the red tint of flame produced by lithium and strontium, says,
" I hesitate not to say that optical analysis can distinguish the minutest
portion of these substances from any other, with as much certainty as, if not
more than, by any other method." It will thus be seen that English philo-
sophers were not wholly ignorant of the primary truths which led to the grand
generalizations of Kirchoff. Since the first instrument used by Bunsen and
Kirchoff, other and more perfect instruments have been made for spectrum
analysis. Probably one of the most simple in construction is that made by
Mr. John Browning, of in Minories, with the assistance of Mr. Herschel,
and called by him the Herschel-Browning direct-vision spectroscope, in
which the direct vision is produced by a combination of two direct-vision
prisms.
FIG. 102. The Herschel-Browning Direct- Vision Spectroscope.
A, arrangement of the two prisms, B B being direct-vision prisms.
For quick examination of atmospheric lines, and for noting the changes
that occur near the horizon, or in any particular direction, this form of the spec-
troscope is one of the best yet devised, as it can be instantaneously and
accurately pointed at any cloud in any direction. Its dispersion and pre-
cision are so great as to divide Frauenhofer's line D with a magnifying power
of only 5.
Another form of spectroscope, which is exceedingly useful to the student,
has a prism of extremely dense glass of superior workmanship. (Fig. 103.)
The circle is divided, and reads with a vernier, thus dispensing with the incon-
venience of an illuminated scale. This arrangement possesses the very great
advantage of giving angular measures in place of a perfectly arbitrary scale.
The slit is also furnished with a reflecting prism, by means of which two spectra
can be shown in the field of view at once.
For elaborate researches a larger spectroscope (Fig. 104), containing four
dense glass prisms, and a telescope with object-glasses of i| in. diameter and
SPECTRUM ANALYSIS.
95
FIG. 103.
1 8 in. focal length, may be employed. A powerful train of eleven prisms was
arranged by Mr. Gassiot; the prisms were hollow, and filled with bisulphide
of carbon. It is described in the " Phil. Mag." [4] xxviii. 69.
Mr. Browning has had great experience in the construction of spectroscopes ;
he made the Kew Observatory spectroscope, furnished with nine glass prisms,
another of eleven fluid prisms, which he made for T. P. Gassiot, Esq., and
also the spectrum apparatus constructed for William Huggins, Esq., for his
important researches on the spectra of the fixed stars ; and therefore his direc-
tions for the use of the spectroscope are given here.
HOW TO USE THE SPECTROSCOPE.
" Screw the telescope carrying the knife-edges at the small end into the
upright ring fixed on to the divided circle, and the other telescope into the
ring attached to the movable index. Now place any common bright light
exactly in front of the knife-edges, and while looking through the telescope
on the movable index (having first unscrewed the clamping screw under the
circle), turn the telescope with the index round the circle until a bright and
continuous spectrum is visible.
TO OBTAIN THE BRIGHT LINES IN THE SPECTRUM GIVEN BY ANY
SUBSTANCE.
" Remove the bright flame from the front of the knife-edges, and substitute in
its place the flame of a common spirit-lamp, or, still better, a gas jet known as
a Bunsen's burner (Fig. 105). Take a piece of platinum wire, about the
substance of a fine sewing needle, bend the end into a small loop about the
eighth of an inch in diameter ; fuse a small bead of the substance or salt to
be experimented on, into the loop of the platinum wire, and, attaching it to
any sort of light stand or support, bring the bead into the front edge of the
ON LIGHT.
the flame be opposite the
telescope, the fixed lines
flame, a little below the level of the knife-edges. If the
knife-edges on looking through the eye-piece of the
due to the substance will be plainly visible. When minute quantities have to
be examined, the substance should be dissolved, and a drop of the solution,
instead of a soild bead, be used on the platinum wire.
SPECTRUM ANALYSIS. 97
" The delicacy of this method of analysis is very great. Swan found, in
1557 (Ed. Phil. Trans., vol. xxi., p. 411), that. the lines of sodium are visible
when a quantity of solution is employed which does not contain more than
i-2,5oo,oooth of a grain of sodium.
" To view Frauenhofer's lines on the solar spectrum, it is only necessary to
turn the knife-edges towards a white cloud, and make the slit formed by the
knife-edges very narrow, by turning the screw at the side of them. In every
instance the focus of the telescope must be adjusted in the ordinary way, by
sliding the draw-tube until it suits the observer's sight, and distinct vision is
obtained.
FIG. 105. A Bunsen Burner, with Ring-stand, supporting the Platinum
Wire
"It should be noted that lines at various parts of the spectrum require fi
different adjustment in focusing the telescope.
" The small prism turning on a joint in front of the knife-edges is for the
purpose of showing two spectra in the field of view at the same time. To do
this it must be brought close to the front of the knife-edges. Then one flame
must be placed in the position in which the flame of the candle is shown in
the small figure, and the other directly in front of the slit. On looking
through the telescope as before described, the spectra due to the two sub-
stances will be seen one above the other.
" When the slit is turned towards a bright cloud, and a light is used in the
position of the candle flame, the spectrum of any substance may be seen,
compared with the solar spectrum. In this manner Kirchoff determined in
the solar spectrum the presence of the lines of the greater number of the
elements which are believed to exist in the sun.
PROFESSOR STOKES'S ABSORPTION BANDS.
" The instrument is expressly adapted to the prismatic analysis of organic
bodies, according to the method recommended by Professor Stokes, in his
lecture at the Chemical Society, printed in the * Chemical News.'
" To observe these bands it is only necessary to place a very dilute solu-
7
9 8
ON LIGHT.
FIG. 1 06.
tion of the substance in a test-tube, then fix the test-tube in the small clip
attached to a ring, which slips on in front of the knife-edges. Upon bringing
any bright light in front of the tube, on looking through the telescope, if the
instrument has been properly adjusted, a bright spectrum will be seen, inter-
rupted by the dark bands due to the substance in solution.
" One of the simplest and most interesting experiments of this kind can be
made by preparing dilute solutions of madder, port wine, and blood.
"In these very dilute solutions no difference can be detected by the unas-
sisted eye; but on submitting them, in the manner already described, to the
test of spectrum analysis, very different appearances will be presented.
"The absorption bands may, however, be most conveniently examined,
and accurately investigated, by means of Sorby and Browning's new Micro-
spectroscope."
As will be seen from Fig. 106, it is a very compact piece of apparatus, very
ingenious in construction, and consisting of several parts. The prism is con-
tained in a small tube, which can be removed at pleasure. Below the prism
is an achromatic eye-piece, having an adjustible slit between the two lenses ;
the upper lens being furnished with a screw motion to focus the slit. A side
SPECTRUM ANALYSIS. 99
slit, capable of adjustment, admits, when required, a second beam of light
from any object whose spectrum it is desired to compare with that of the
object placed on the stage of the microscope. This second beam of light
strikes against a very small prism suitably placed inside the apparatus, and
is reflected up through the compound prism, forming a spectrum in the same
field with that obtained from the object on the stage
A is a brass tube carrying the compound direct-vision prism.
B is a milled head, with screw motion to adjust the focus of the achromatic
eye-lens.
C, milled head, with screw motion to open or shut the slit vertically. Another
screw at right angles to C, and which, from its position, could not be shown in
the cut, regulates the slit horizontally. This screw has a larger head, and when
once recognized cannot be mistaken for the other.
D D, an apparatus for holding small tube, that the spectrum given by its
contents may be compared with that from any other object on the stage.
E, square-headed screw, opening and shutting a slit to admit the quantity
of light required to form the second spectrum. Light, entering the round
hole near E, strikes against the right-angled prism which we have mentioned
as being placed inside the apparatus, and is reflected up through the slit
belonging to the compound prism. If any incandescent object is placed in a
suitable position with reference to the round hole, its spectrum will be obtained,
and will be seen on looking through it.
F shows the position of the field lens of the eye-piece.
G is a tube made to fit the microscope to which the instrument is applied.
To use this instrument, insert G, like an eye-piece is in the microscope-tube,
taking care that the slit at the top of the eye-piece is in the same direction as
the slit below the prism. Screw on to the microscope the object-glass required,
and place the object whose spectrum is to be viewed on the stage. Illuminate
with stage mirror if transparent, with mirror and Lieberkuhn and dark well if
opaque, or by side reflector, bull's-eye, c. Remove A, and open the slit by
means of the milled head, not shown in cut, but which is at right angles to
D D. When the slit is sufficiently open, the rest of the apparatus acts like an
-ordinary eye-piece, and any object can be focused in the usual way. Having
focused the object, replace A, and gradually close the slit till a good spectrum
is obtained. The spectrum will be much improved by throwing the object a
little out of focus.
Every part of the spectrum differs a little from adjacent parts in refrangi-
bility, and delicate bands or lines can only be brought out by accurately
focusing their own parts of the spectrum. This can be done by the milled
head B. Disappointment will occur in any attempt at delicate investigation,
if this direction is not carefully attended to.
When the spectra of very small objects are to be viewed, powers of from
^ in. to i-2oth, or higher, may be employed.
Blood, matter, aniline red, permanganate-of-potash solution (quite fresh),
are convenient substances to begin experiments with. Solutions that are too
strong are apt to give dark clouds instead of delicate absorption bands.
Mr. Browning makes small cells and other contrivances to hold fluids for
examination.
The spectra obtainable from solid, liquid, and gaseous incandescent bodies
may be arranged in three orders.
A spectrum of the first order is that which is produced by a solid incan-
72
100
ON LIGHT.
FIG. 107.
descent substance, such as charcoal. The band of colours is continuous from
red to violet, and therefore can teach little or nothing of the constitution of
the body producing the light ; such a spectrum could
not be employed for analytical purposes. A spectrum of
the second order differs essentially from the first, inas-
much as the colours are not continuous, but consist of
distinct coloured bands ; it can only be obtained from
light emitted from incandescent gases ; and any sub-
stance which can be converted into a gaseous state by
intense heat without undergoing decomposition will afford
distinct bands of colour, which are always the same.
The metal silver placed in a cup-shaped charcoal pole
and connected with the other pole, in the electric lantern
figured in the frontispiece, is converted into silver gas r
and produces on the disc two distinct green lines. (See-
Frontispiece.)
Thallium so cleverly discovered by Mr. Crookes, in
1 86 1, in certain kinds of iron pyrites, and so called from
the Greek because it produces a splendid green flame
Arrangement of charcoal would probably have been unknown but for this new
crucible A. containing method of analysis. The attention of Mr. Crookes was
wltr'theTharcVa^poie nrst directed to the splendid green line as obtained from
B, and the metal vapour- certain specimens of pyrites, and it was by following up
lzed - this simple fact this slender clue that he was at last
enabled to isolate the body that produces the green lines,.
and confidently pronounce it to be a metal. The spectrum of thallium is shown
in the frontispiece. In projecting metal spectra on to the disc, it must be
understood that for exact purposes of research they cannot be so truthful as
the spectra results obtained by the instruments described on p. 95. The
optical arrangements required to show the spectra of incandescent metals to
a large audience on the disc cannot be compared to the elaborate instruments
already mentioned. Moreover, the charcoal crucible and points contain ash
consisting of alkaline earths and salts, which must interfere with the spectrum,,
results. A spectrum of the third order is obtained when the regularity of the
spectrum is interfered with by black fixed lines. Such a spectrum is always,
obtained from the rays of the sun. As Mr. Huggins remarks, " These dark
spaces are not produced by the source of light." They tell us of vapours
through which the light has passed on its way, and which have robbed the
light by absorption of certain definite colours or rates of vibration. A very
simple mode of showing such a spectrum crossed by dark lines is to interpose
between the slit of the electric lantern and the double-convex lens a vessel
containing some nitrous acid gas. Directly this is done, all the visible indigo,
blue, and green colours vanish, and the remainder of the spectrum is crossed
with numerous dark lines. In using the electric lantern it must always be
borne in mind that if the aperture or slit is too widely opened the dark lines
are very indistinct. The slit should be very narrow indeed to display the
dark lines sharp and distinct. A more instructive mode is first to produce
the two yellow lines representing the spectrum of sodium, and then with a
peculiar-shaped crucible (Fig. 108).
It was by this and kindred experiments that Kirchoff showed that if vapours
of terrestrial substances come between the eye and an incandescent body >
SPECTR UM AN'AL $$&.+'
IOI
they cause groups of dark lines, and,
group of dark lines produced by each vapour is iden-
tical in the number of lines and in their position in
the spectrum with the group of lines of which the light
of the vapour consists when it is luminous.
The reversal of the spectrum of coloured flame,
and the mode in which he obtained the proof of the
identity between the terrestrial sodium line and the
dark lines similarily placed in the solar spectrum, is
thus described by Kirchoff :
" In order to test by direct experiment the truth of
the frequently asserted fact of the coincidence of the
sodium lines with the lines D (Frauenhofer), I obtained
a tolerably bright solar spectrum, and brought a flame
coloured by sodium vapour in front of the slit. I then
saw the dark lines D change into bright ones. The
flame of a Bunsen's lamp threw the bright sodium
lines upon the solar spectrum. In order to find out
the extent to which the intensity of the solar spectrum
could be increased without impairing the distinctness
of the sodium lines, I allowed the full sunlight to shine
through the sodium flame upon the slit, and, to my
astonishment, I saw that the dark lines D appeared
with an extraordinary degree of clearness."
With respect to this important experiment, showing the reversal of the
sodium lines, perhaps the most simple experiment is that of Roscoe, who
seals up some of the metal sodium in a vacuum tube, and on volatilizing the
metal the vapour is colourless by white light, but dark and opaque when the
monochromatic or yellow light of sodium is shown behind it.
It was by the exact reversal of the bright terrestrial lines, and the absolute
identity in position of the bright terrestrial and dark solar lines, that Kirchoff
discovered the elements that exist in the sun, viz., hydrogen, sodium, magne-
sium, iron, calcium, nickel, chromium, copper, zinc, barium, and probably
strontium, cobalt, cadmium.
At p. 92, and in Fig. 101, are shown the lines B, c, D, E, F, G, and H, which
are called Frauenhofer's principal fixed dark lines in the solar spectrum. The
labours of Kirchoff have now almost interpreted the whole of these lines,
which are read as follows :
FIG. 1 08.
The section of the Crucible to
be used for showing the re-
versal of the bright sodium
lines, of which A is the cen-
tral hole, and contains some
chloride of sodium, and B B
a ring or trench all round A,
in which metallic sodium is
placed; c, the upper char-
coal pole.
C, F, and G are Hydrogen.
D is Sodium.
E is Iron.
H, Aluminium.
C, Magnesium.
The limits of this work do not permit the consideration of stellar che-
mistry, and the extremely valuable researches of Mr. Huggins and Dr. Miller
in this direction ; but the reader is referred to Mr. Huggins's discourse " On
the Results of Spectrum Analysis applied to the Heavenly Bodies," published
by Ladd; or to Mr. Watt's "Dictionary of Chemistry," for a complete
resume of this subject This much may be said, that spectrum analysis
proves that the fixed stars are suns like our own a fact which could only be
assumed and taken for granted before the important experiments of Kirchoff,
Huggins, and Miller.
102
ON LIGHT.
FIG. 109. Star Spectroscope, with adjust ible Reflecting Prism and Mirror.
With finest object-glass micrometric apparatus tor measuring the lines of the spectrum to i-io,oooth of
an inch, extra eye-piece, and ivory tube to reader of vernier, as made for W. Huggins, Esq., F.R.S.,
and used during the observation of the red flames of the sun in India, August, 1868.
Moreover, the spectroscope has discovered the real nature of the " red
flames " or " prominences " of the sun, which are invisible under ordinary cir-
cumstances, being overpowered by the dazzling brilliancy of the rays which
proceed from the sun ; but visible during the few minutes that elapse during a
total eclipse of the sun, as in the one which created so much interest in August
of the present year, 1 868, visible only in the line or path of the shadow, which
fell in India. Four xpeditions went to India to observe the red flames; they
were all armed with the spectroscopic apparatus, and their united statements
all agree that the red flames belong to the sun, and that, as they give bright lines
which belong only to spectra of the second order, they must be enormous gas-
heaps, intensely ignited or self-luminous. The bright lines chiefly observed appear
to be those which belong to hydrogen gas and sodium, at least so far as we know
at present (September, 1868) ; and this interesting statement was made through
the telegrams from Major Tennant, Lieutenant Herschel, and M. Jannsen,
which arrived in England, and were all sent independently of each other. As
the red flames belong to the sun and show bright lines in the spectroscope,
are they. great volumes of the photosphere thrust out (like the pips and juice
'of a squeezed gooseberry) beyond the last or gaseous atmosphere, which
usually robs the light from the photosphere of its beautiful coloured bands r
LIGHT AND COLOUR.
103
and changes them to dark lines ? for where light is not, there can only be
darkness.
These and other facts are discoverable by another modification of the specr
troscopic arrangement (Fig. 109), as constructed by Mr. Browning.
SPHERICAL ABERRATION.
In using an ordinary concave mirror the experimentalist cannot fail to
notice that the rays reflected from the part near the circumference do not
come to the same meeting-point or focus as the rays reflected from parts near
the centre. (Fig. 1 10.) It is evident that the rays A B, A c, come to a focus at
G, which is further off than the focus F from the parallel rays D D, D D. The
distance between F and G, the two foci, is called the longitudinal spherical
FIG. 1 10. Concave Mirror, showing the Aberration of the Rays of Light.
aberration. The natural consequence must be that an image projected by
an ordinary concave mirror will be confused, because the eye has to look at a
double image, the one superposed on the other. To get rid of the rays from
the outer part of the mirror it is usual to employ a screen, so that the rays
D D, D D, from the central part of the mirror only are used.
FlG. in. Production of Caustic Curves.
FIG. 112.
Arising from this circumstance is the unequal illumination of a white
ground on which rays are reflected to different foci, and the production of.
symmetrical curves, termed caustic lines or caustic curves, in the study of
which mathematicians have been most industrious. Brewster lays claim to
the following method of exhibiting caustic curves. He recommends the use
io 4 ON LIGHT.
of a piece of steel spring highly polished, or, better still, polished silver, which
is to be bent into a concave figure and placed vertically on its edge upon a
piece of card or white paper, and when exposed either to the rays of the sun
or any good artificial light, the curves shown in Fig. 1 1 1 are well defined.
In the same way, passing from reflecting to refracting bodies, the spherical
figure of a convex lens causes the rays which fall near the outer edge to come
to a focus nearer the lens than the rays which are refracted from the centre.
The result, as might be expected, is just the reverse of the concave mirror.
The rays A B, A B, Fig. 1 1 2, falling on the margin of the double-convex lens are
refracted to a focus at F, whilst those rays, D D, D D, which fall near the axis of
the lens come together at a more remote point, viz., at C. Here again a
screen or diaphragm cutting off the rays refracted from the outer edge of the
lens gives a better image ; the picture produced by such a lens, provided with
a screen, can be focused more distinctly; hence telescopes, microscopes,
cameras, oxy-hydrogen lanterns, c., &c., are usually fitted with diaphragms,
which reduce the light, but cause the images to become more distinct. The
lens of the eye would, from this cause, project on to the retina a confused or
double picture, which might render vision extremely imperfect ; this, however,
is prevented by the iris, which acts as a diaphragm, thus the aberration of
sphericity is corrected.
THE DISPERSION OF LIGHT, OR CHROMATIC ABERRATION.
If light consisted of a series of coloured rays, every one of which possessed
the same index of refraction when they fall upon a glass lens, they would all
come together in the same spot, and white light only would be obtained ; but
this is not the case, and it is known in practice that lenses, and especially con-
densing lenses, project coloured rings, and give images with coloured edges.
And this is not remarkable when it is remembered that a double-convex lens
may be regarded as a series of prisms united at their bases, and therefore
capable of decomposing or dispersing light. It is a singular fact that Sir
Isaac Newton considered, from the experiments he had tried with various
prisms, that dispersion was proportioned to refraction, and he believed that
all substances had the same chromatic aberrations when formed into lenses,
and that any combination of a concave with a convex glass would produce
colour with refraction. Newton reasoned only from the facts he had acquired
on the dispersive powers of bodies, and pronounced the construction of
achromatic telescopes which should not project images with coloured edges
to be impossible. ' The fallibility even of his great mind is shown by the fact
that, a few years after his death, Hall in 1733, and Dolland, the famous
optician, in 1757, demonstrated that by using two media, viz., crown and
flint glass, of different refractive and dispersive powers, a lens may be formed
which is achromatic.
The principle of the achromatic lens is not complicated or difficult to
understand, provided the previous matter relating to compound and simple
colours (p. 89) has been already studied. Given a lens made of a certain glass,
and projecting, amongst other colours, a ring of red light, what colour, pro-
jected from another lens, is required to neutralize it ? The answer is obvious :
any colour which together with the red light would form white light. That colour
must be green, because it contains yellow and blue ; and, as already shown,
red, yellow, and blue form white light. In the adjustment of the two lenses
LIGHT AND COLOUR.
I0 5
forming the achromatic (Fig. 113), it is so arranged that the colours which would
be separately produced by each lens shall, when combined, by their unequal
dispersion fall together at the same spot and unite together. Any two colours
which unite and form white light are said to be complementary, and there is
a very conclusive experiment which may be performed with polarized light
passed through a selenite slide placed behind a Nicol's prism composed of
I.
FIG. 113.
No. I., Dolland's Achromatic Lens, consisting of one
double-convex crown glass lens, a, and another
concavo-convex lens of flint glass, b; No. II., Dr.
Blair's Achromatic Lens, composed of two double-
convex lenses of crown glass, enclosing a solution
of chloride of antimony.
FIG. n AT Complementary Colours
overlapping and forming IVhite
Light.
double-refracting spar. The two discs of light projected on to the screen
separately are green and red; but when caused to overlap each other by
enlarging the aperture through which they pass, the two colours unite in the
centre, forming white light, whilst red and green remain intact in those po-
sitions which do not overlap. (Fig. 114.)
Other complementary colours would be yellow and indigo, blue and orange.
FIG. 115. Arrangement of the Composite Lenses in an Achromatic Telescope.
Flint glass has a greater dispersive power than crown glass ; it will spread or
disperse the spectrum over a larger space. The dispersive power of the prism
used in decomposing light for showing the spectra of incandescent metal is
increased by rilling them with carbonic disulphide (bisulphide of carbon),
and the composition and dispersive powers of the three bodies is as follows:
Crown glass .
Flint glass
Carbonic disulphide
0-039
0-048
io6 ON LIGHT.
THE INTERFERENCE OF LIGHT.
COLOURS OF THIN PLATES.
About the year 1672, Dr. Hook, a very clever mechanician, and learned in all
the science of his day, discovered that by splitting mica, which is free from
colour, and sometimes used instead of glass, into very thin films, they
exhibited the most beautiful colours. But as they were less than the twelve-
thousandth part of an inch in thickness, Dr. Hook could not measure them,
and was therefore unable to determine the law that regulated the production
of any particular colour, according to the thickness of the film of mica. In
due course of time the experiments engaged the attention of Sir Isaac Newton,
and directly he touched the subject it was truly, so far as intellect was con-
cerned, with the hand of a giant, and he soon discovered a method of measur-
ing the films. He did not begin with mica, because it would have been very
troublesome, if not impossible, to split it into a graduated series of films of the
extreme thinness required to produce colour. Newton therefore commenced
with air, and having once determined the law, it was easy, knowing the index
of refraction of all other transparent bodies, to work out by calculation the
respective thicknesses required to produce the same colours. He took a plano-
convex lens, the radius of whose convexity was 14 ft., and placed it on a
double-convex lens, the radius of whose convexity was Soft., and by means of
proper clamps and screws the surfaces of the two lenses could be brought
closely together. The convexity of the lower lens being so extremely slight,
it might indeed be almost regarded as a flat surface, like any moderate area
on the surface of the globe, because the sphere of glass (of which the lens
would be a slice) had a theoretical diameter of 100 ft. (Fig. 116.)
FIG. 1 1 6. Instrument used by Newton to obtain the Rings of Colour from
Thin Plates of Air.
L L, upper lens pressed on the lower one, / I, by the thumb-screws p p p.
When the two lenses were pressed together, concentric rays of colour maae
their appearance ; indeed the same kind of effect is often produced acci-
dentally when a number of flat plates of window-glass are piled one above
the other, the enclosed air being then pressed by the weight of the superin-
cumbent'glass into a film sufficiently thin to show coloured rings.
The Hon. Robert Boyle first discovered that thin bubbles of the essential
oils, spirit of wine, turpentine, soap and water, produce the colours, and he
THE INTERFERENCE OF LIGHT.
107
succeeded in blowing glass so thin that, like the mica, it displayed varieties of
colour.
Lord Brereton observed the colour of thin oxidized or decomposed films, such
as are produced by the action of the weather during a prolonged period on the
ancient glass in church windows, or on glass which has been buried in the
earth. When steel is tempered, the regular gradations of colour produced by
the oxidation of a very thin outer film are a guide to the skilled workman
who tempers the metal.
Mr. De la Rue, by floating a very thin film of a quick-drying varnish on
the surface of hot water, and then receiving this on a sheet of paper, was
enabled to secure in the most perfect manner those lovely tints, which are
sometimes associated with the surface of ponds into which greasy matter or
oil may pass, or in the puddles after rain in the yard of a gas-works where
liquor containing coal oil has been spilt.
The variety of colours which Newton describes in his important "Table of the
Colours of thin Plates in Air, Water, and Glass," are given by him in the suc-
cession of spectra or order of colours, where he enumerates seven spectra or
orders of colours ; these are different from reflected and transmitted rays, and
are produced by thicknesses of air, water, or .glass, estimated from a scale of
'an inch divided into one-million parts.
FIG. 117. Woodward's Model of Waves, 'with movable Rods.
Newton measured the diameter of every coloured ring ; he did not depend
merely upon calculation, but tried a number of experiments with the colours
of the spectra, allowing each to fall separately on his apparatus, and dis-
covered that under these circumstances he no longer obtained a variety ol
coloured rings, but observed that the central dark spot was surrounded by
rings of the same colour as the light incident on the lenses alternating with
dark rings.
Thus, supposing Newton to have placed the apparatus for producing the
rings into the yellow part of the spectrum, there would be a dark spot in the
centre, then a yellow ring, now a dark, again a yellow ring, and so on ; he then
squared the diameters of the reflected coloured rays, and obtained the odd
numbers, I, 3, 5, 7, 9, &c., while the square of the diameters of the dark rings
were as 2, 4, 6, 8, 10, &c. When the rings were observed by transmitted
light, the order was reversed the coloured rings being at the even numbers,
and the dark ones at odd integers.
These effects Newton called fas of transmission andfas of reflection ; they
could not be reconciled or explained by his own favourite theory, and, to the
honour of this great philosopher, he did not attempt to press the corpuscular
theory, and compel it to his own use, but simply left behind him a record of
facts, only naming that which he had proved to exist, and giving the relative
thicknesses of the plates of air by which each colour is reflected.
io8
ON LIGHT.
The undulatory theory of light alone is adopted to explain these phenomena,
and by what is termed the interference of the waves the effects are supposed
to be produced. Ingenious models have been made to explain the law of
FIG. 1 1 8. A AT odd of Fixed Waves.
interference ; but those of Mr. Charles Woodward, the President of the I sling'
ton Scientific Society, are the most simple, and are thus described by him in
his admirable little work on the " Polarization of Light : "
A B (Fig. 1 17) represents a model with rods freely moving in a perpendicular
direction through the frame A B, and furnished with pins resting upon the
FIG. 119. Intensity of Waves doubled by the Superposition and Coincidence of
two equal Systems.
upper part of the frame, so that when at rest the whole may assume the
appearance of waves, as in the diagram.
CD (Fig. 1 1 8) represents a fixed model with waves of similar size and
intensity, and numbered so as to distinguish each half-undulation.
FlG. 1 20. Waves neutralised by the Superposition and Interference of two
equal Systems.
It will be seen that when the stars indicating the highest point of the waves,
as A B, correspond with the odd numbers of half-undulations on c D, each
system of waves will be in the same state of vibration; and, if so superposed, a
series of waves of doubled intensity will be the result, as in Fig. 119.
THE INTERFERENCE OF LIGHT. 109
If, on the other hand, the two systems be so superposed as that the stars
on A B shall coincide with the even numbers on C D, as in Fig. 120, there will
be a difference of half an undulation in the two systems ; the one will neutra-
lize the other by interference, and darkness will be the result.
If C D be continued so that A B may be moved forward indefinitely, it will
be obvious that the waves will be equally increased in intensity by a difference
in the two systems of any even number, and neutralized by a difference of any
odd number, of half-undulations. These models are, therefore, well suited to
teach matter of fact, viz., that two sets of waves of water may come together
and obliterate each other, as in the tides of the port of Batsha, described by
H alley and Newton, where the two waves arrive by channels of different
fengths, and produce a smooth surface; or two waves of light may come
together in such phases that in one case they exalt each other and produce
a wave of double intensity, and in the other phase they may destroy one
another and cause darkness. A wave of white light is, however, made up of other
waves of coloured light ; so that when such a complicated series of differer.t-
coloured waves interfere, it is easy to perceive that certain coloured waves may
coincide and extinguish each other, whilst the remaining colours may unite
and intensify each other.
That waves of light do so interfere is placed beyond all doubt by the expe-
riments of Dr. Young, and even more elaborately by the following beautiful
experiment devised by Fresnel :
FIG. i2i.--FresneFs arrangement to show the Interference of the
Waves of Light.
A sunbeam from the Haeliostat is passed through a narrow rectangular slit
in the shutter (as described at p. 87), covered with red glass to secure a mono-
chromatic light, or wave of simple light. The red light is brought, by a cylin-
drical lens of very short focus, to a point at A. The rays cross each other
and fall upon two mirrors of parallel glass B C, B D, placed at a very obtuse
angle, having their line of intersection parallel to the line of light. After re-
flection the rays proceed as if they came from the two points F F behind the
two mirrors ; they interfere at G, and at other points not marked out in the
no ON LIGHT.
diagram, and produce light and dark fringes ; but, if one of the beams of light
proceeding from the points E E be intercepted by a screen, the whole of the
alternations of red and dark fringes disappear, and the only light left is that
derived from the single ray of red light which remains after the other was
removed by the screen. In this diagram two sets of waves only are used, but,
of course, the same law applies to all.
It is this principle of interference which produces coloured fringes by inflexion
or diffraction, such as rays passing along the edge of a screen, or the fringes
at the edge of a plane mirror, or fringes produced by narrow rectangular
openings, fringes by two narrow slits very close together, and those obtained
through gratings or networks. The word grating might deceive the reader, and
lead him to suppose that the effect was caused by some rough arrangement ;
but these beautiful experiments were carried out by Frauenhofer by tracing
parallel lines on a film of gold leaf fixed on a plate of glass, and look-
ing through them with transmitted light. Nature supplies us with
striated bodies, which are in effect reflecting gratings. Brewster calculated
that there were three thousand lines in an inch of a piece of iridescent
mother-of-pearl. But this number has been surpassed by Barton, who ruled
from two to ten thousand lines on steel, which he afterwards hardened
and used as a die to stamp bright brass buttons. These, when illuminated
by the various rays emanating from the numerous lighted wax candles in a
ball-room, flashed with the splendid colours of the diamond. The colours of
Newton's rings are due to the interference of the light reflected from the upper
and under surface of the film of air ; for, however thin this may be, it must
have an upper and an under surface, like a sheet of paper.
Let Figs. 117 and 118, pages 107, 108, represent two equal systems of waves
from red light reflected to the eye from the upper and under surface of Newton's
thin plates of air. If they be superposed, as in Fig. 119, page 108, the waves
will coincide, and there will be red light, as in the first coloured ring. On
moving A B a distance equal to one half-undulation at Fig. 1 20, the waves will
be neutralized by interference, and there will be darkness ; on moving A B a
second half-undulation, there will be light, and so on ; for when the stars indi-
cating the highest part of the waves of A B coincide with the odd numbers of
half-undulations of C D, there will be light, as in Fig. 119; and when they
coincide with the even numbers, darkness will be caused by interference, as in
Fig. 120.
Dr. Young proved that each of Newton's fits of transmission and reflection
was equal to half a wave of each colour, and this is equal in length to the
thickness of the plate of air at which that colour is first reflected, and there-
fore a whole undulation would be equal to two of Newton's spaces or fits, or
what he termed the length of an interval between the fits of easy reflection.
Thus, the thickness of the plate of air required to produce red light being
determined by Newton to be 133 ten-millionths of an inch, double that number,
or the length of a wave of red light, would be 266 ten-millionths of an inch.
For orange
yellow
green
blue .
;, indigo
violet
240 ten-millionths of an inch
227
211
196
185
167
THE INTERFERENCE OF LIGHT
in
Herschel's table is, perhaps, the most complete record of the invaluable
work of Newton. The figures are Newton's, although the meanings of them
have been altered to comply with the undulatory theory.
Colours of the Spectrum.
Lengths of an Un-
dulation in parts
of an inch.
Number of Undu-
lations in an
inch.
Number of Undulations
in a second.
Extreme red
0'0000266
37,640
45 8,000000,000000
Red ...
0*0000256
39,180
477,000000,000000
Intermediate
o '0000246
40,720
49 5 ,000000,000000
Orange
0'0000240 ,
4I,6lO
506,000000,000000
Intermediate
0'0000235
42,5*10
5 17,000000,000000
Yellow .
0*0000227
44,000
535 ,000000,000000
Intermediate
0'00002I9
45,600
5 5 5,000000,000000
Green .
0*00002 1 1
47,460
5 77,000000,000000
Intermediate
O*OOOO2O3
49,320
600,000000,000000
Blue .
0*OOOOI96
51,110
622,000000,000000
Intermediate
0*0000l89
52,910
644,000000,000000
Indigo .
O*OOOOl85
54,070
658 ,000000,000000
Intermediate '
0*0000181
55,240
67 2,000000,000000
Violet . .
0*0000174
57,490
699,000000,000000
Extreme violet
0*0000167
59,750
727,000000,000000
A very good idea may be given of the effect of the law of interference by means
of a simple contrivance proposed by Sir Charles Wheatstone, called the Eido-
trope. It is made of two circular pieces of ordinary perforated zinc, one of which
is made to turn round in front of the other by means of a band and pulley, the
whole being arranged as an ordinary magic-lantern slide. Wire gauze or
perforated cardboard may be substituted for the perforated zinc. If the two
zinc plates were perforated exactly alike, little or no effect would be observed ;
but as one set of perforations is always a little in advance of the other, certain
shadows, which assume interesting forms, are perceptible when the instrument
is used in the magic lantern, and the figures projected on to the disc. The
dark shadows are caused by the mechanical interference of the zinc plates in
the proportion to represent the half-undulation, and in some positions are very
distinct. If wire gauze is employed, the shadows assume just the same
appearance as the surface of watered silk.
DOUBLE REFRACTION AND THE POLARIZATION OF LIGHT.
When a ray of light falls upon the surface of Iceland spar, it is divided into
two colourless rays, one of which is called the ordinary, and the other extra-
ordinary, ray of light ; both rays possess physical properties different from
those which belong to common light, and if reunited they would again form
common light.
In the year 1817, Dr. Young, the famous revivalist and supporter of the
undulatory theory whilst considering the results of the speculations of
112
ON LIGHT.
Huygens, Wollaston, and Brewster, and the cause of double refraction, was
led to believe that the effect must arise from a difference of elasticity in the
crystal of Iceland spar; and being aware that Newton had expressed the idea
that a ray of light possesses sides, he first proposed the hypothesis of trans-
versal vibrations of light. The theory is, that in the progress of a ray of
light the forward motion is made up of two sets of vibrations, which are either
longitudinal or transversal. The longitudinal vibrations represent the path or
direction of the ray, whilst the transversal ones take place at right angles to
the former. This peculiar motion may be compared to the particles of water
which move up and down whilst the wave advances horizontally. Dr. Young
illustrated these vibrations by the propagation of undulations along a stretched
FIG. 122. A Rhomb of Iceland Spar, showing the double Refraction of Light.
cord agitated at one end, which supposing a person to hold in his hand, and
by moving first quickly up and down, a wave will be produced, that will run
along the cord (see p. 6) to the other end, and then by a similar movement,
but from the right side to the left, another wave will be produced, which will
run along the cord as the former ; but the vibrations and undulations of each
will be in planes at right angles to each other, and independent of each other,
FIG. 123.
A, Woodward's cardboard model representing a ray of common light; B, transverse section, shewing
the figure of a cross.
one being in a perpendicular plane and the other in a horizontal plane, so that,
according to this theory, A (Fig. 123) may be considered to represent a ray of
ordinary or unpolarized light, a cross section of which would give the simple
figure B, it being understood that the vibrations take place in planes all round
the direction of propagation.
With the help of this hypothesis of transversal vibration, double refraction
is easily explained, and is put into the most concise terms by the editor of
THE POLARIZATION OF LIGHT.
XI 3
the late Dr. Young's lectures : " A ray of light falls on the surface of a crystal
the elasticity of which is. different in different directions. The motions conse-
quently are not all transmitted with the same velocity, and, as the index of
refraction depends on the velocity, one set of vibrations will, on emergence, be
totally separated from another. Moreover, the light, on emerging, is quite
different from common light. In each ray it consists only of vibrations in one
direction. Suppose, therefore, one of these rays to fall on a second crystal
placed in a similar position with the first ; it will not now be divided into two,
but will emerge just as it entered. Light which consists of vibrations in one
direction is called polarized light. In 1810 it was discovered by Malus, an
officer in the French engineers, that light reflected from the same face of unsil-
vered glass is more or less polarized, and Brewster ascertained that it is per-
fectly so when the tangent of the angle of incidence is equal to the refractive
index, and also that the transmitted ray is partially polarized."
But why called polarized ? The term, perhaps, is not a very happy one, but
was suggested by analogy to the poles of a magnet.
Dr. Whewell thus defines polarity: " Opposite properties in opposite direc-
tions, so exactly equal as to be capable of accurately neutralizing one another."
FIG. 124.
A, magnet made of watch-spring with north and south poles ; B, same magnet bent round, and polarity
neutralized; c, common light; D D, polarized light.
A piece of steel watch-spring, when magnetized, has a north and south pole
(see A, Fig. 124) ; but when the same piece of steel is bent round in a circle,
as at B, Fig. 124, the two forces neutralize each other, and the polarity is gone.
Such a circular piece of steel might be compared to common light : it is like
the section of a hoop-stick, c ; whilst polarized light may be compared to the
straight steel magnet A, or to a lath. A hoop-stick is the same all round ; but
a lath has a top and bottom and sides. The former may represent common
light, and the latter polarized light ; and thus polarization is simply the separa-
tion of the two sets of undulations or vibrations, D D, Fig. 124.
When common light is passed through transparent refracting bodies per-
fectly homogeneous in their structure, and of a uniform temperature throughout,
such as gases, common air, pure water, annealed glass, jelly, and many kinds
of crystallized bodies, the form of whose primitive crystal is the cube, the regular
octahedron, and the rhomboidal dodecahedron, such as alum, common salt, or
fluor spar, the beam of light is refracted singly ; but in nearly every other
crystalline body the rays undergo double refraction, and, although this is not
apparent at once, like it is with Iceland spar, the property of double refraction
is soon discovered by using polarized light.
ON LIGHT.
Polarized light may be obtained in four different ways, viz.
Firstly, by reflection ;
Secondly, by simple refraction ;
Thirdly, double refraction ;
Fourthly, by transmission through a plate of tourmaline, slit parallel to the
axis of the crystal.
Thirty years ago, Mr. J. F. Goddard, then of the Polytechnic, London,
received from the Society of Arts a silver medal for his apparatus for
experiments on polarizing light. The description which accompanied
the apparatus is so good and so little known, that the writer has quoted the
most important part of it, in order to explain, with the assistance of the appa-
ratus invented by Mr. Goddard, this most difficult branch of optical science.
POLARIZATION BY REFLECTION AND SIMPLE REFRACTION.
" Polarization may be effected with common crown glass, either by ordinary
reflection or refraction, each of which will exhibit the same effects. In order
to understand this, let b b (Fig. A 125) represent a bundle of plates of common
FlGS. A and B 125. Explanation of Polarisation by Reflection and Simple
Refraction.
glass, placed so that a ray of ordinary light, a a, may form an angle of incidence
of 56 45' with a line perpendicular to their surface; then the light reflected
and represented as passing off at a will be polarized light ; and if a proper
number of plates, which for the same angle of incidence is twenty-seven, be
employed, the light transmitted at c will be polarized also, the two rays pos-
sessing the same properties, but at right angles to each other.
" Thus in the reflected ray d the vibrations are supposed to take place in a
perpendicular plane, this being a bird's-eye view (Fig. B 125 being a horizontal
view of the same thing), whilst in the refracted ray c the vibrations are per-
formed in a horizontal plane. This will be easily understood on analyzing
either of the rays, which may be done by the same means as that by which the
original beam is polarized. Thus, supposing we experiment with, test, or
THE POLARIZATION OF LIGHT.
analyze the reflected ray </, in which the vibrations are in a perpendicular
plane, when it is made to fall upon a second bundle of glass, h h, at the same
angle of incidence, and the glass be so placed that the reflection may again
be in the same plane, it will be again wholly reflected, as at d' d', and none
will be transmitted or refracted through the second bundle of glass, for the
very same cause that produced its reflection from the first bundle, viz., that
the vibrations continue parallel to the reflecting surfaces. But if the second
bundle of glass is put in such a position that the vibration shall be performed
in a plane perpendicular to the reflecting surface (which may be done by
turning it round 90 in such a direction that the ray of light shall be the axis
on which it turns, and always making the same angle of incidence), then, as
soon as it begins to turn, the reflected light will begin to decrease in intensity,
and, as it decreases, a portion will begin to be transmitted or refracted through
the glass, which will increase in the same ratio as the reflected light decreases ;
and when the bundle of glass has turned 90, in which position it is shown at
C* jj.
FIG. 126.
A, Fig. 1 26, as a bird's-eye view, and at the horizontal view, B, Fig. 1 26, the light d
is wholly transmitted or refracted at c' <:', no portion being reflected. In such a
position the vibrations will be in a plane perpendicular to the reflecting surface ;
and such vibrations are always transmitted, and not reflected, as we also see
has taken place in the polarization of the original beam of common light at A,
Fig. 125, before referred to. Now let the second bundle h 7z, B, Fig. 126, con-
tinue to turn ; it will be seen that, as soon as it begins to move, the transmitted
c' will begin to decrease, a portion beginning to be again reflected, which, as the
glass turns, will increase in intensity in the same ratio as the transmitted light
decreases, until it has turned another 90, or reached 180 from the first posi-
tion, as seen at C, Fig. 126, when the plane of reflection is again parallel to the
plane in which the vibration takes place ; consequently the whole light is again
reflected at d' d', none being transmitted, from the same reason as before
stated. On continuing the revolution, the same thing occurs at each quadrant
of the circle. In Fig. D 126 the bundle of glass h h is represented as having
turned 270, or three-quarters of a circle, in which position the same thing occurs
at 90, when the light d is wholly refracted and transmitted through glass,
as at c e>\ so that it is evident, in these experiments, that there are two posi-
tions, shown in Figs. 125 and 126, in which the same ray of polarized light d is
wholly reflected, as at d 1 d\ and two other positions, A, D, Figs. 125 and 126, in
82
n6
ON LIGHT.
COMMON LIGHT
1 . Is capable of reflection at oblique
angles of incidence in every position
of the reflector.
2. Will pass through a bundle of
plates of glass in any position in
Avhich they may be placed.
3. Passes through a plate of tour-
maline, cut parallel to the axis of the
crystal, in every position of the plate.
which it is wholly transmitted by the analyzing bundle of glass, as at rV, all
of which are easily understood by bearing in mind the description of the
physical nature of common light according to the undulatory theory, and the
action of the first or polarizing bundle of glass, or transversal vibrations.
" Thus we obtain experimental data, which may be expressed as follows :
POLARIZED LIGHT
1. Is capable of reflection at oblique
angles of incidence in certain positions
only of the reflector.
2. Will pass through a bundle of
plates of glass only when they are
placed in certain positions.
3. Does not pass through a plate
of tourmaline cut parallel to the axis
of the crystal, except in certain posi-
tions; in others, the tourmaline, though
quite transparent, stops the whole of
the polarized light as if it was opaque.
" A bundle of plates of glass or a slice of tourmaline is consequently to be
regarded as a test of polarized light, and enables the physicist to distinguish
between the latter and common light, which he is said to analyze, the bundle
of glass or the tourmaline being called the analyser.
POLARIZATION BY THE TOURMALINE.
" Amongst crystallized minerals there .are many possessing the property of
polarizing the light transmitted through them, the most remarkable of which,
however, is the tourmaline. This mineral crystallizes in long prisms, whose
primitive form is the obtuse rhomboid, having the axis parallel to the axis of
the prism.
" It must be remembered also that the axis of crystals is not, like the axis of
the earth, a single line within the crystal, but a single direction through the
crystal; for supposing Fig. 127 to represent a crystal of any kind, the axis of
FIG. 128.
A, single plate of tourmaline ; B, superposition of
the second plate on the first.
which is in the direction A X, if we divide such a crystal into four along the
lines B B and C C, each separately will have its axis A O, O X, c B, and B c,
which, when united in one crystal, are all parallel ; every line, then, within the
crystal parallel to A X is an axis.
THE POLARIZATION OF LIGHT. 117
"If \ve cut a crystal of tourmaline of a proper kind parallel to the axis into
thin plates of an uniform thickness (about one-twentieth of an inch), and
polish each side, it possesses the property of polarizing light transmitted
through it in a remarkable manner. Fig. A 128 represents one of these plates,
the lines across which we may suppose to be parallel to the axis. Now, if we
hold such a plate before the eye, and look at the light of the sun, or flame of
a candle, or any artificial light, a great portion will be transmitted through the
plate, which will appear quite transparent, having only the accidental colour
of the crystal, which in specimens suited for these experiments is generally
brown or green; but the light so transmitted will be polarized light, and, on
being analysed by a second plate, which may be done by looking through
both at the same time, we shall find that when the axes of both plates coincide,
i.e., are parallel with each other, the light which is passed through the first
will also freely pass through the second, and they will together appear per-
fectly transparent ; but when one is turned round, so that the axes of each
plate are at right angles (across each other), as represented in B, Fig. 128,
not a ray of light will pass through they will appear perfectly opaque,
although we may be looking at the meridian sun. If we suppose the structure
of the crystal to be represented by a grating, the bars of which are the axis,
we may conceive that its action on ordinary light will be to transmit such
vibrations only as are performed in a plane parallel with the axis, and to stop
all others. Hence, the light transmitted through a single plate will be polar-
ized, and possess exactly the same properties as the light polarized by any other
means, as may be proved by analyzing it by any of the means which have
been described. But let us suppose a second tourmaline to be used, and, as
it is understood that in the light which makes its way through the first tour-
maline the vibrations are parallel to the axis, all other vibrations being stopped
when the axis of the second or analyzing plate is perpendicular to the first, as
represented in B, Fig. 128, the vibrations which have passed through the first,
being now perpendicular to the second, will also now be stopped by the second
plate in such a position; and, as it is turned round, there will be found two
positions in which it will not pass through, being wholly stopped, these posi-
tions being at right angles to each other, as will be understood by B, Fig. 128,
where a a is the first or polarizing plate, and c the second or analyzing plate,
overlapping the first."
Mr. Goddard then describes the instrument for which he received the silver
medal the oxy-hydrogen polariscope. (Fig. 129.)
" In this instrument A represents the hydro-oxygen blowpipe ; B, the lime
cylinder and diverging rays of light refracted by the condensing lenses c c c and
falling upon a mirror bb, composed of ten plates of thin flattened crown glass
placed in the elbow of a tube bent to the polarizing angle of crown glass ; d,
converging rays of polarized light reflected from the mirror ; h h, a bundle of
sixteen plates of mica, for analyzing the light previously polarized by reflec-
tion ; c, a double-refracting crystal (film of selenite) placed in the focus of the
object-glass I, which forms an image of the crystal upon a disc or screen at r.
As the analyzing bundle of mica, h h, 'is made to revolve (or turn round), the
image of the selenite upon the disc undergoes all the changes, and exhibits
alternately the primary and complementary colours at the same time, one being
reflected in the direction s, and the other transmitted and seen at r.
" The great advantage of polarizing the light from a number of plates is the
obtaining a beam of any required dimensions, of much greater intensity than
Tl8
ON LIGHT.
113 20
FIG. 129. Goddard's Oxy-hydrogen Polariscope.
by any other means; for whatever single surface may be employed that
polarizes light at the same angle as the glass used (which for crown glass is
56 45'), we obtain an additional quantity by laying on it a single plate of such
glass, and a further quantity by the addition of a second, third, or any further
number ; the quantity of light added by each succeeding plate being, how-
ever, less in proportion to the number of plates through which it has pre*-
viously to pass. In this respect the single-image (Nicol's) prism of Iceland
spar is decidedly the best for analyzing, as by this a great variety of objects
may be exhibited. Its application is shown in Fig. 130, where e, the selenite,
THE POLARIZATION OF LIGHT.
119
is placed in the rays, ddd,o>i polarized light, an image of which is projected by
the lenses ; h is the analyzing prism through which the rays of light r r are
refracted.
FlG. 130. Use of the Nicol's Prism as an analyzer.
" But there is one class of phenomena, viz., the rings seen to encircle the
optic axes of crystals, the number of which increases in some crystals (the
topaz, for instance) with the divergence of the rays of polarized light passing
through them. It will be evident, then, that the tourmalines enable us to
exhibit more of these rings, and upon a larger scale, than the prism, which
will be better understood by the arrangement shown in Fig. 131.
FIG. 131.
"ddd, converging rays of light polarized by reflection ; /, a lens of short focus
transmitting a cone of light with an angle of divergence from its ray r r of
45; e, a crystal, say topaz; /*, the tourmalines for analyzing; so that, even for
these purposes, the cost of the tourmalines is reduced one-half by Goddard's
polariscope, as only one need be used."
The writer frequently uses Goddard's instrument as made by Mr. Darker, jun.,
of Paradise Street, Lambeth, whose father before him earned so much credit
in the practical parts of this branch of optics. Darker also makes the most
elaborate and beautiful designs in selenite or sparry gypsum, being the
native crystallized hydrated sulphate of lime, from which plaster of paris can
.be made by driving off the water of crystallization. This mineral, split into
thin films, and cut under water, or oil, or turpentine, is laid upon glass with
Canada balsam. The greatest nicety is required in the manufacture of the
selenite slides, or else all the edges of the figures would be rough.
120
ON LIGHT.
A piece or film of selenite of unequal thicknesses exhibits the most varied
and beautiful colours when placed in the polariscope, the colours transmitted
by the analyzer being complementary to those reflected from the bundle of
glass plates. Any transparent substance in which unequal elasticities occur
will present phenomena of colour when placed in the polariscope. A piece
of plate glass, if well annealed, shows no colour until it is bent or sq
being placed in a strong frame provided with a screw.
it is bent or squeezed by
FIG. 132. Apparatus for compressing Glass.
A A, the press; B, the piece of plate-glass.
On the same principle, unannealed glass exhibits some of the most vivid
colours and figures. (Fig. 133.)
Or if a rod of plate glass is placed in the polariscope and heated with a
FIG. 133. Unannealed Glass.
red-hot copper bar, the unequal expansion of the particles causes that retar-
dation in the path of the rays which results in interference, and the produc-
tion of colours, and these disappear gradually when the hot copper bar is
removed. A little jelly allowed to solidify in a proper frame, the sides of
which are of glass, exhibits no double refracting power until it is subjected
to pressure.
A quill pen flattened out and arranged for exhibition in the polariscope will
give some very pleasing tints.
Water of an uniform temperature has no double refracting power, but when
frozen and converted into ice the particles exhibit unequal elasticities, and
colour is the result when it is placed in the polariscope.
If plates of selenite or any doubly refracting crystal of considerable thick-
ness be ground away on one edge, so as to give them a wedge-shape, they
will present bands or fringes composed of all the colours of Newton's table,
arising from the various thicknesses which such a shape possesses ; or by grind-
ing a concavity in a similar plate a number of concentric rings (reminding
THE POLARIZATION OF LIGHT.
121
the spectator of Newton's rings) are produced. Small crystals obtained by
evaporating single drops of solutions of acetate of zinc, chlorate of potash,
sulphate of soda, oxalic acid, oxalate of ammonia, sulphate of copper, borax,
ferrocyanide of potassium, &c., may be exhibited in the polariscope.
The lovely rings obtained by using uniaxial and biaxial crystals are well
shown by Goddard's apparatus, with a large Nicol's prism or a good tourmaline
as the analyzer. To exhibit these coloured rings a higher microscopic power
FIG. 134.
Appearance of the rings produced by Iceland spar cut perpendicularly to the principal or shortest axis,
and alternate appearance of the black and white cross with complementary colours, as the analyzer
is rotated.
is used. This is always supplied with the instrument, and is put on before
using the polariscope. For these experiments Iceland spar, rock crystal,
emerald, sapphire, beryl, ice, furnish good examples of uniaxial crystals.
A very large number of crystals are biaxial, and have two axes of double
FIG.
Double curves or sets of elliptical or oval-like rings produced by a plate of nitre 1-12 or 1-15 in. thick,
cut perpendicular to the prismatic axis.
refraction, which are more or less inclined to each other. These are termed
biaxial crystals, or crystals with two optic axes. Nitrate of potash exhibits
this optical property very perfectly, also Rochelle salt, selenite, sugar, borax,
and many others.
12,2
ON LIGHT.
The coloured bands obtained from biaxial crystals are not concentric, but
somewhat oval, with two centres, which represent the two axes of the crystal.
The splendid phenomena of colours produced by various substances in
polarized light are the results of transversal vibrations. When a single wave
or vibration in any one plane is divided into two, at right angles to each other,
one will of necessity be half a wave behind the other, the two being opposite
halves of the same wave ; and as each gf these again is divided or resolved
into two others, there will be four waves or vibrations produced from the
original one. Two of these in one plane coincide and strengthen each other,
while the two in the other plane oppose and destroy each other.
This difficult subject may be summed up and concluded with Woodward's
very instructive diagrams, exhibiting at one view
POLARIZATION,
ANALYZATION,
INTERFERENCE OF LIGHT.
FIGS. 136, 137.
FIG. 136. A, B, c, D, common light; E a plate of tourmaline, or a bundle of plates o.f glass, termed
the polarizer; F, polarized light; G, a plate of selenite; H, dipolarized light; i, a plate of tourmaline, or
a bundle of thin plates of glass, called the analyzer; K, coincidence of waves for red light; L, inter-
ference of waves for yellow, and M. of those for blue light; N, the result red light.
FIG. 137 i, the analyzer turned round 90; K, interference of waves for red light; L, coincidence of
waves for yellow, and M, those for blue light; N, the result green light.
HEAT.
THERMOMETRIC HEAT.
AMONGST the physical forces, the corellation of which has been so well
discussed by various philosophers, that termed caloric (at one time, like
Jight, considered to be a direct emanation of some rare and subtle form of
matter) has received the most careful attention. Light is discoverable by two
most sensitive inlets the eyes. The sensation termed heat is not more
appreciable by the eyes than by any other part of the human body, and yet the
mind may be easily deceived by sensations caused by heat or its absence,
termed cold. The body may experience the greatest torture by an excess of
heat or burning, and it may derive pleasure from the application of a moderate
amount of the same power, as in the use of the Turkish or other baths.
The nervous system distributed over the surface of the body cannot, how-
ever, distinguish properly degrees of heat, and we seem to be able to discover
only when heat is entering or leaving our bodies, and then the exclamations
referring to extremes, such as "how hot .-" or. "how cold ! " escape us. And even
this faculty is limited, because the sensations caused by touching a lump of
frozen mercury and a hot iron are the same. The unfortunate person who
does this will complain as if he were burnt with the intense cold of solid
mercury. We cannot, as with the eye or the ear with light and sound,
discern gradations of heat; hence artificial means have been invented to
supply this want.
It is not surprising that heat should have been considered to be a material
body, entering into combination with solids, fluids, or gases, because it is so
i2 4 HEAT.
readily evoked from ponderable substances. A clever blacksmith, with his
hammer, anvil, and a rod of good iron, will dexterously obtain, by hammering
the metal, enough heat to light his forge fire, provided a little sulphur is used
as the intermediate combustible body.
Heat travels with light from the sun ; and as Newton succeeded in con-
vincing his contemporaries that the latter was a material body, it came to
pass by a natural sequence of reasoning that the former should also be
regarded as a subtile rare form of matter opposed to cohesion. The material
theory of caloric the hypothesis of "emission" has given way to the more
rational theory of "undulation." If, as has been explained at p. I, an im-
ponderable elastic ether pervades all space, a peculiar vibratory motion set
up in the material particles of a body may be communicated to this ether;
and then, on the same principle that a glass trembles whilst producing sound
in air, so the minute particles or molecules of solid fluids or gases oscillate,
and these oscillations or vibrations are communicated to and transmitted by
the ether. Physicists, however, prefer to speak of their favourite hypothesis
as " The Dynamical Theory" (Sai/ot/Ais, power). The title at once shows that
heat, and not light, is intended to be expressed. Heat is in every sense of the
word a " power ;" the terms are mutually convertible the one into the other.
The combustion of coal produces heat, which generates steam, and the latter
is the greatest modern representative of power.
Power, as shown by the muscular force of the arm conveyed through a
hammer, generates heat when metals are beaten on the anvil. This connection
between heat and power is shown in the most perfect and masterly style by
Dr. Tyndall,* the industrious and worthy successor of Faraday. He has
enriched this branch of philosophy with a vast number of practical demon-
strations and experiments, giving quite anew and fresh appearance to a science
which seemed to have reached its limits in the stereotyped repetition of descrip-
tions of thermometers, pyrometers, calorimeters, and eternal disquisitions on
specific heat and latent caloric. Referring back to heat as the equivalent for
power, there is a telling experiment of Tyndall's, in which a brass tube con-
taining water is connected with a whirling table, and whilst it is going round
with great velocity, it is rubbed with the wood of a lemon-squeezer; the friction
soon generates enough heat to cause the water to boil, and to eject a cork with
which .the tube is closed. Power generates heat, and vice versa. If a mode-
rate-sized piece of lecture-table apparatus generating heat is to be regarded as
a power, what must be the energy of the sun ? what kind of force is at work to
produce so much heat ? Pouillet has carefully ascertained the total heating
effect of the sun's rays upon the earth, and estimating the whole heating
power of the sun as 2,300 millions of parts, he calculates that less than one
of those parts only reaches our earthi, and yet it would melt a layer of ice
thirty-five yards thick over the whole surface of our globe. This proportion
of heat is not all available : some of it is at once converted into power by
setting the air in motion, to create the winds; another portion raises the water
of the ocean into vapour, which, descending in the form of rain on high levels,
such as the mighty water-shed which supplies the great lakes (discovered by
Speke and Grant and Sir Samuel Baker), the sources of the Nile, flows down
to the lowlands, giving rise to water power, which is again the equivalent for
* Heat Considered as a Mode of Motion. By John Tyndall, F.R.S., etc. Longman, Green, Longman,
Roberts, and Green.
THERMOMETRIC HEAT. 125
heat ; another part stimulates and increases the growth of plants ; and thus,
in ages long since passed away, the heat of the sun's rays was not all lost, as the
older Stephenson insisted, but stored up ready for man to use in another form,
viz., coal, and therefore called potential heat. The plants, being the food of
animals, again contribute to the production of animal heat and muscular force.
The sources of heat are all connected with motion of some kind.
No. i. Friction is a notable illustration, and it was by causing two pieces of
ice to nib one against the other that Sjr Humphrey Davy generated heat,
liquified the ice ; and like Dr. Young, who proved that light could turn a
corner, and established by his experiments with inflection a sort of basis upon
which the undulatory theory of light was again reconstructed, so this famous
experiment of Davy supplied a great fact, and gave the first blow to the old
theory which said that the ice melted because latent heat was made sensible
heat, when it was well known that water at a temperature of 32 Fahrenheit
contains much more heat than ice ; how, then, could the ice, already deficient
in heat, supply enough to satisfy the condition of water? There are plenty of
illustrations of the generation of heat by friction, The flint and steel ; the
attrition of dried wood, as used by savage tribes ; the famous experiments of
Count Rumford whilst boring cannon, when enough heat was generated in two
hours and a half to cause two and a half gallons of water to boil ; the friction
of railway- wheel axles, which have been known to become red hot and to set
fire to the woodwork of the carriage. In North America, a case is quoted
where heat was intentionally generated by waste water power and used for
heating purposes, the generator being two flat plates of iron which rubbed
against each other.
No. 2. Percussion. It was said formerly that metals when struck with a
hammer, or with a die in the coining-press, became hot because their density
was increased, and 'therefore their capacity or containing power for heat was
altered ; but it is clearly shown that this is not the true explanation. Lead,
for instance, which becomes hot by percussion, does not increase in density
and yet becomes hot so hot that when projected from the steam gun in the
form of bullets against a wrought-iron target, a flash of light is apparent in a
darkened room. The heavy shot used for battering iron plates always become
very hot after they have struck the plate.
No. 3. Chemical Action. The bringing together of a number of atoms,
however small, the clashing together (as Tyndall calls it) of particles to pro-
duce new compounds, as in the heating and combustion of finely powdered
antimony when it is brought in contact with chlorine gas, or the heat gene-
rated by combustion or from other chemical changes, are all to be regarded as
the result of motion which the eye cannot detect, but which must occur before
the elements come in contact, combine, and form new compounds. There are
many chemical changes accelerated by motion, and hence the stirring-rod is
an important mechanical means to secure the more rapid ^nion of particles.
No. 4. Electrical Action. The very essence of the existence of electrical
power is circulation or motion. The intense heat generated by the discharge
of a powerful Leyden battery through a thin iron wire seems to be increased
by the resistance offered to the passage of the current, and thus work is con-
sumed. The ignition of a platinum wire by a current of voltaic electricity
affords a further instance of resistance ; whilst another wire of the same size
made of silver, offering less resistance an*d consuming less work, does not
become red hot. We speak of a current of electricity : a current is something
126 HEAT.
flowing; it is of course motion. Here again the two forces are similarly con-
vertible. The heat generated by the passage of a current of electricity through
a platinum wire will set up another current of electricity, if the heat is applied
to a series of bars of bismuth and antimony arranged properly, and thus
called a thermo-battery or multiplier a most delicate indicator of heat, which
in connection with the galvanic needle is usefully and extensively employed
in experiments where heat, inappreciable by a thermometer or other ordinary
means, is generated.
No. 5. Vital Power, impossible without food, appears to be the result of a
kind of slow combustion, or change of carbon and hydrogen into carbonic
acid and water, and furnishes another illustration of heat generated by
chemical action. The muscular power of a horse, as sagaciously observed by
Count Rumford, might certainly be used to produce by friction (as in the boring
of iron) enough heat to cause water to boil for the purpose of cooking victuals,
if a quicker -and more advantageous mode were not suggested by the direct
combustion of the fodder which the horse must eat to maintain the animal
heat, in order to be able to exert his muscular energy.
To work out the relation between heat and mechanical power, it has been
found necessary to establish a standard of comparison, or unit of work, which
latter in England is defined to be " the force required to overcome the pres-
sure of one pound through the space of one foot."
By a very extensive series of experiments Dr. J. P. Joule determined that
772 foot-pounds, or units of work, have to be performed to raise a pound of
water at about 50 Fahrenheit one degree; 772 units of work would, therefore,
be called the mechanical equivalent of heat, and an equivalent to a force that
would raise one pound 772 feet high; or,, if we reverse the statement, and
imagine the same water falling through 772 feet, it would be raised one degree
Fahrenheit. The power or force used was measured by the descent of weights,
which caused the apparatus, viz., an iron paddle-wheel, to rotate in water or
mercury, and, by the friction of the iron and mercury or water, to eliminate
heat, which was estimated in the most careful manner. "Joule's equivalent"
is, therefore, a standard of the most valuable and truthful kind, verified by
another great man, Dr. Mayer, who, by different means and by calculation,
makes out the equivalent to be 771*4 foot-pounds, instead of 772, and thus
? roved how correct had been the previous experiments and calculations of
oule.
Dr. Young says, " If heat is not a substance, it must be a quality; and this qua-
lity can only be motion. It was Newton's opinion that heat consists in a minute
vibratory motion of the particles of bodies, and that this motion is communi-
cated through an apparent vacuum by the undulations of an elastic medium,
which is also concerned in the phenomena of light. It is easy to imagine that
such vibrations may be excited in the component parts of bodies by percussion,
by friction, or by the destruction of the equilibrium of cohesion and repulsion,
and by a change of the conditions on which it may be restored in consequence
of combustion or of any other chemical change." Further on, he says, " The
effect of radiant heat in raising the temperature of a body on which it falls re-
sembles the sympathetic agitation of a string, when the sound of another
string, which is in unison with it, is transmitted to it through the air.
" All these analogies are certainly favourable to the opinion of the vibratory
nature of heat, which has been sufficiently sanctioned by the authority of the
greatest philosophers of past times and of the most sober reasoners of the
THERMOMETRIC HEAT. 127
present. Those, however, who look up with unqualified reverence to the
dogmas of the modern school of chemistry will probably long retain a par-
tiality for the convenient, but superficial and inaccurate, modes of reasoning
which have been founded on the favourite hypothesis of the existence of caloric
as a separate substance ; but it may be presumed that in the end a careful and
repeated examination of the facts which have been adduced in confutation
of that system will make a sufficient impression on the minds of the cultivators
of chemistry to induce them to listen to a less objectionable theory."
These anticipations of Young have been fulfilled : the re-establishment of
the undulatory theory of light, by his exertions, has been slowly followed by
the reception of the dynamical theory of heat.
THE COMMON EFFECTS OF HEAT.
When a solid is raised in temperature, either by percussion or by the direct
application of heat, the vibratory motion supposed to be set up in the mole-
cules or atoms of the substance appears to overcome for a time their cohesive
force, and they are separated : they occupy more space ; they expand, and, im-
perceptible as that expansion must be to the eye, it may still be made apparent
by a proper instrument. A miniature house, fitted with a number of movable
metallic tiles, is so arranged that, when the outer walls are driven apart by any
means, the roof and tiles fall in. Between the parts of the model representing
the walls of the house is arranged a broad band of brass, which is nicely
adjusted by a screw, so that it just touches the sides, which are held together
with a spring. When a series of spirit-lamps are lighted under the bar, it
FIG. 138.
a, the spirit-lamp; b, the brass ring; c, the brass ball.
soon expands with great force, and, overcoming the springs which hold the
sides together, they are pushed out, and the rattle of the falling roof and tiles
shows to the eyes and ears the catastrophe that might happen on a larger
scale. The expansion of the brass rod is thus indicated in a simple and
effective manner. When the contents of warehouses provided with great
iron girders take fire, the latter expand, push out the walls, and ultimately
bend themselves (when they become red hot) with the superincumbent weight
above them.
What is called Gavesande's ball (Fig. 138) is a simple and effective mode of
showing cubical expansion. A brass ball is carefully turned and polished,
so that it exactly fits, and will pass through a metal ring ; but, when heated,
128
HEAT.
expansion takes place, and, instead of falling through the ring, it is held up as
in a ring-sta.nd, and will no longer pass through the opening.
The expansion of a fluid body is also shown by placing some coloured water
in a flask ; to this is fitted a cork and tube with a
small bore, which is bent round at the top, so that
.any liquid ejected by expansion may fall into a
shallow dish containing some bits of potassium ;
the rise of the liquid in the tube may be watched
by placing a piece of cardboard behind it, and
directly the full expansion occurs the liquid is
ejected and the potassium takes fire.
The expansion of gases by heat is readily shown
by various simple, experiments. The neck of an
empty retort is placed under water, and directly
the body is heated the air expands and passes in
bubbles through the water; before removing the
lamp a little ink should be stirred into the water,
so that when the heat is withdrawn the amount
of expansion may be shown by the rise of the
coloured water to fill the space at first occupied
with air, but now lost by expansion.
The Montgolfier or fire-balloon has never ceased
to please, because its inflation is so simple and
rapid. The only difficulty seems to be to avoid
setting the paper on fire ; this is easily prevented
i, the flask with cork and tube, by using a metal funnel with coarse wire gauze at
iied with water coloured with *the top, and inside of which the large piece of tow,
P n omt r frffcuV conSming Etc w e"ed with spirits of wine, is allowed to burn ; the
potassium ; d, the ring-stand and inverted funnel may be supported on three legs,
spirit-lamp. an( j the mouth. should be at least 3 in. in diameter.
in order to allow the heated air to pass rapidly into
the paper bag. By attaching a thin string, the balloon may be let up and
down any number of times. When the balloon is intended for the amusement
of young people, at an out-door fete, the balloon can be sheltered from the
wind by a blanket stretched between two poles ; and if the balloon, when
nearly ready to start, is blown by a sudden gust of wind across the heating
apparatus, it does not catch fire, because it is protected from the flame by the
funnel and wire gauze.
This mode of sending up a fire-balloon is the safest, because it does not carry
any fire. The late accidental and total destruction by fire of the immense fire-
balloon in the grounds of the Crystal Palace sufficiently indicates the danger
of these aeronautical machines, and how soon they may ignite ; indeed, no
Montgolfier balloon on the large scale should be used on this principle without
first rendering the material of which it is composed incapable of combustion,
by preparing it with a solution of phosphate of ammonia.
Robertson, in his " Recreative Memoirs," gives a very interesting account of
the construction and ascent of an enormous Montgolfier or fire balloon at
Vienna, in the year 1781. It was the first experiment of the kind tried there,
and was carried out in a most fearless, not to say reckless, manner by Gaspard
Stuver. ' The length of the balloon (Fig. 141), which was constructed like a
cylinder closed at both ends with cones, amounted to 60 ft. It was made of
FIG. 139.
THERMOMETRIC HEAT.
129
FIG. 140. The Paper M outg
The sheet-iron funnel, with coarse wire gauze at the end, supported on three legs about i ft. from
ground. An iron ladle containing tow moistened with spirits of wine, or a small fire fed with
shavings, will do.
canvas lined with sized paper. Three persons ascended with Stuver in a
Danube boat arranged as a car, and attached to the balloon by proper cords.
They entered the car with the greater courage because they did not intend to
allow the balloon to travel where the winds migh't direct it, but to retain it as
a " captive balloon " (like the one at the Crystal Palace) by a strong cord ;
but, unfortunately, the rope was not strong enough : it broke, and away went
the balloon with immense rapidity, not without considerable peril to the
unfortunate passengers. The shock was so violent that the boat tilted on
9
L'30
HEAT.
one side, and the fire was thrown out on the canvas. By a happy forethought,
the men were provided with water and long rods to which were attached large
sponges, and with these they courageously stopped the further progress of
the flames. The voyage was not very long : the un wieldly machine descended
a little, and knocked down a large wooden framework prepared for some
pyrotechnic display : it then reascended, grazed the tops of the trees in the
Prater or park, and fell on the grass on the other side of the Danube.
AMOUNT OF EXPANSION IN SOLIDS, LIQUIDS, AND GASES.
MEASURES OF HEAT THERMOMETRY.
The fact that the particles or molecules of a solid body are pushed away
from each other by heat, and suffer a certain increase in dimension, called
expansion, has been already mentioned, and in every-day life examples are not
wanting. The moulds for casting in metal are always made larger than the
size required, in order to allow for the expansion of the metal when in the
liquid state. The iron hoops of carriage or cart wheels are put on red hot,
and being cooled suddenly by the application of cold water, they contract
with great force and draw all parts of the wheel firmly together. In bottles
the stopper is often fixed tight, and cannot be removed by any force that is
applied ; when this is the case, the outer part of the neck should be carefully
heated over the flame of a spirit-lamp ; expansion takes place, and then the
neck is tightly grasped with the hand, protected by a duster in case of
FIG. 142.
A B, cast-iron frame; c c, red-hot iron bar, passed through the holes in A B, and fitted tight by the
screw D. E represents the same iron frame broken by the contraction of the bar.
accidental breakage; a slight effort, and particularly moving the stopper
backwards and forwards in one direction only, and carefully avoiding a motion
which would cause the stopper to turn round, will soon be rewarded by the
extrication of the stopper from the tight embrace of the neck of the bottle. A
piece of iron, cast with two elbow-pieces, each bored so as to allow an iron
bar to be placed through them when red hot, and then screwed up with a
thumb-screw as tight as possible, instantly breaks off one or both the elbow-
pieces when the red-hot bar is cooled by being suddenly plunged into water.
The amount of expansion, or coefficient of expansion, of solids is, however,
exceedingly small, and requires the utmost nicety of experiment to discover
its amount. The difference between linear expansion, or the increase of
length, and superficial expansion, or the increase in the area of a surface,
must of course be remembered.
THE EXPANSION OF SOLIDS.
The unequal expansion of metals is shown by riveting together two flat
plates of iron and brass ; the latter, expanding in a greater degree than the
former, causes the compound bar to take the form of an arch or curve when
heated ; the brass side being uppermost, the arch ascends, and a convex figure
is observed ; but if downwards, the reverse or concave form is produced.
The rise or arching of the riveted bars is easily shown if united with an
electrical battery ringing a bell with which contact only is made when the
curve is produced. Attention is further directed to the result of unequal
expansion when the spirit-lamp is withdrawn and the bar cooled with ice or
cold water ; the bell ceases to ring, and the bar again becomes straight.
FIG. 143.
A A, The compound bar of brass and iron, heated by a spirit-lamp, and rising in a curve towards the
wire B, connected with the bell c and battery D, of which the otheu wire is attached to the compound
bar at E. When the spirit-lamp is removed, the bar contracts, and the contact is broken at B.
It is on this principle that Breguet constructs his most delicate metallic
thermometers. The solid affected by heat consists of a thin metallic ribbon,
composed of three strips of platinum, gold,, and silver, passed through a
rolling-mill together. The ribbon is then coiled in a spiral form, the platinum
being outside and the silver inside ; one end is fixed to a proper support, and
the other is attached to a copper needle. The spiral unwinds when the heat
increases, and the contrary result occurs if heat is withdrawn, and it cools.
The needle moves round a scale which is graduated by direct comparison
with a standard mercurial thermometer.
The following table shows the comparative increase of length or linear
expansion in bars or rods of various substances when they are heated, from
the freezing to the boiling point of water viz., from 32 to 212 Fahrenheit,
according to the authority of Graham :
Zinc (cast) .
Zinc (sheet) .
Lead .
Tin .
Silver .
Copper
Brass .
on 323
340
35i
5*6
524
,,58i
584
Pure gold .
Iron wire .
Palladium .
Glass without lead
Platinum .
Flint glass
Black marble .
on 682
812
,,i,ooo
1,142
" 1^248
2,833
92
i,32 HEAT.
In this table it will be noticed that glass and platinum elongate nearly the
same, and this fact explains why platinum wire can be melted into glass tubes
made of glass not containing lead, such as the hard German glass, without
causing the tubes to crack, either by expansion or contraction, at the points
where the wires are inserted. The minute linear expansion of black marble,
only one on 2,833, is of course the reason why it is used as the pendulum-rod
in the clock of the Royal Society of Edinburgh.
Even a bar or rod of ice elongates by an increase of temperature, and is
found even to surpass zinc ; the ice will elongate I part on 267, the zinc I on
323 parts. Ice will also contract when exposed to a temperature lower than
its freezing-point, and the amount of contraction has been carefully observed
up to 30 or 40 below the freezing-point of water.
A solid may expand at a uniform rate up to a certain point, and then, if the
heat is increased, the elongation becomes more rapid.
Amongst metals, platinum is found to expand with the greatest uniformity,
most probably in consequence of its great infusibility.
Certain crystals of the same nature throughout expand very unequally in
their several dimensions of length, and breadth, and height, and they are
found to elongate in a greater degree in the direction of one axis than in
another. Iceland spar possesses this property ; and was found, by Professor
Mitscherlich, to expand in one direction, the crystallographic axis, and to
contract in the other at right angles to the former, so that the anomaly of
expansion and contraction in one body was apparent. Another remarkable
anomaly of the same kind will be noticed presently in connection with a fluid
viz., water when reduced to a certain temperature. The difference between
linear, surface, and volume expansion is determined by the geometrical prin-
ciple, that when a solid increases in magnitude without undergoing a change
in figure, taking the linear expansion as the unit, or say 100, the superficial
expansion will be twice the linear, 100x2 = 200, and cubic expansion three
times the linear, 100X3 = 300.
Thus the linear coefficient of expansion of glass being 0-000,008, the cubic
expansion of the same will be 0-000,024 ; the dilatation of volume and surface
of solids being calculated from linear expansion.
THE EXPANSION OF LIQUIDS.
It has already been noticed that the expansion of solids by heat is so con-
trolled by the antagonistic force of cohesion, that it is not perceptible to the
vision without the use of secondary means, such as those described at page 127.
With liquids the amount of expansion is very perceptible when they are
inclosed in narrow glass tubes, and in fact is much greater than that of solids,
because the force of cohesion is diminished so much that every particle is
free to move upon its neighbouring particles. Some fluids expand more than
others. Alcohol is more expansible than water, and water more than mercury ;
in fact, alcohol expands six times more than mercury. Messrs. Dulong
and Petit employed the most refined means to ascertain the rate of, and
absolute expansion of, mercury, and they found that the coefficient of the
latter was 1-5550 between the freezing and boiling point of water, the rate
being as follows :
THE EXPANSION OF LIQUIDS.
133
From o to 100 Centigrade, mercury expands i measure, or 55^
100 to 200 i 54^
200 to 300 i 53
Liquid carbonic acid expands four times more than air, and, when heated
from 32 to 86 Fahrenheit, 100 measures expand to 140.
There are other fluids, such as liquid sulphurous acid, hyponitrous acid,
cyanogen, and the chloride of ethyle, which also expand very considerably
when heated.
Alcohol and bisulphide of carbon expand uniformly, which is another
curious fact, because their boiling-points are so different, the former alcohol
being 173, the latter 116.
Liquids, as a rule, expand by heat, and contract by cold.
There is, however, a remarkable exception, probably more apparent than real
when the theory of the expansion of liquid is better understood, that water
which becomes solid in all parts of the globe at the level of the sea at 32
Fahrenheit, or of o Centigrade, expands instead of contracting when the water
reaches a temperature of 40 Fahrenheit, and falls to 32 : the amount of expan-
sion is not very great, being one part in ten thousand at 32.
But the fact, which at first was thought illusory, is indisputable, as proved
by the experiments of Dr. Hope. He placed two
thermometers in a large vessel of water, the one being
at the top, and the other at the bottom. Up to a
temperature of 40, the cold water contracted, and,
being the heavier, sank to the bottom, and the lower
thermometer registered the greatest cold. After 40
was passed, the water evidently expanded ; the coldest
water was found to be at the top, and duly recorded
by the thermometer sinking to 32, whilst the warmer
water, which ought, according to the law of expansion,
to have been uppermost, remains at the bottom, and
therefore was heavier, bulk for bulk, than the water
about to crystallize. It is this remarkable exception
that preserves the fish in the lakes and rivers. During
the severe winters of Siberia the water is frozen many
feet thick; but it is related by one of the exiles in this
roomy but severe prison, that part of their amusement
in certain seasons consisted in fishing in great holes
in the ice, and all they caught they partially but imme-
diately ate raw and living, biting out a piece of the
back, which was declared to be a most agreeable tit-
bit.
It is evident that the fish, if frozen, could have no
power of locomotion they must die ; so that on the
arrival of winter the Siberian waters would throw up
their dead fish, as all would be killed if the water, which is a very bad con-
ductor of heat, did not remain at 40 at the bottom of the lakes, rivers, and
seas.
Bismuth is said to possess the same curious property of expanding whilst
it is being cooled, and thus iron bottles filled with melted bismuth, and plugged
with a screw, burst at the moment the metal assumes the solid state. A
bomb-shell or cast-iron bottle filled with water, and screwed up, bursts in the
FIG. 144.
s experiment.
i 3 4 HEAT.
same manner if surrounded with a freezing mixture of pounded ice, or snow
and salt. With respect to bismuth it is right to state that Professor TyndalPs
conclusion on this similarity, stated in his work on " Heat as a Mode of
Motion," in which it is asserted "that the anomalous expansion of water in
the act of cooling below 40 Fahrenheit is by no means an isolated instance of
the kind, but that other bodies, and particularly melted bismuth, participates
in this extraordinary property of expanding near the point of solidification,"
is opposed by Mr. Alfred Tribe, who, after making experiments upon this
FIG. 145. Siberian Exiles fishing.
subject, considers that the analogy between water and bismuth is imperfect,
since in the case of the molten metal there is no perceptible range of tem-
perature through which it expands on cooling. The act of solidification is
itself accompanied by an increase in bulk ; but there is no evidence of this
expansion taking place prior to the act of crystallization. When the crystal-
lization of any salt is exhibited on the disc with the oxy-hydrogen microscope,
the visible illustration of the motion of the particles is very decided, as the
crystals shoot out and interlace with each other. The act of solidification is,
therefore, one of motion, and heat is produced, and very decidedly so in the
case of sulphate of soda or glauber salt. A large flask filled with a satu-
rated solution of sulphate of soda, and carefully closed with a cork and-
bladder, so that the air is excluded, does not solidify when cold. Crystalliza-
tion only begins when the air is admitted ; the solution of a minute volume of
air liberates a tiny crystal, and, the nucleus once formed, cohesion sets in with
rapidity ; the molecules are set in motion, and sufficient heat is produced to
be felt by the hand, and becomes still more apparent if a delicate air-ther-
mometer is used. India-rubber caoutchouc, when stretched and apparently
expanded, becomes warm instead of cold ; is it possible to suppose that the
expansion in the direction of the length may cause contraction in the trans-
verse direction in the breadth, and the sum of this violent motion is in favour
of the contraction, and thus heat is the result?
The contraction of stretched or expanded caoutchouc by heat is another
THE THERMOMETER.
135
remarkable anomaly. It was suggested by Mr. Thompson that it might
shorten if heated, and the fact was proved by Tyndall, who first stretched a
vulcanized india-rubber tube by a ten-pound weight, and surrounding it with
hot air, the caoutchouc tube contracted and lifted the weight. In this case,
motion, the stretching of the caoutchouc, eliminates heat, which again pro-
duces motion when the stretched caoutchouc is warmed and lifts the weight
by the contraction of its substance.
The expansion of fluids by heat, and the reverse, is taken advantage of in
the construction of those useful instruments called thermometers, or heat-
measurers.
A glass tube with a small bore, sufficiently so to be capillary, is selected
with care, in order to secure the same diameter throughout. The bores of
some tubes are like an elongated cone, and, if they
were used, the mercury would expand much more in
some parts of the tube than in others, and hence the
indications of such a thermometer would be incorrect.
A little mercury, amounting to an inch in length, is
allowed to enter the tube, and being moved from one
end of the tube to the other, it is soon discovered
whether the mercury increases or decreases in length,
or remains, as is usually the case, of the same linear
dimensions in all parts. The proper length having
been cut off, one end is melted and blown out into a
bulb, the other being formed into a cup or funnel-
shape form, to hold the mercury, which is forced in;
the tube is now inclined slightly, and the air in the
bulb expanded by heat; it is afterwards allowed to
cool, and, as the air cools and contracts, the mercury
from the upper funnel is forced in by the pressure of
the air, and enters to supply the place of the air driven
out by expansion. To get rid of the rest of the air,
the mercury is alternately boiled and cooled until the
bulb and part of the tube are full of mercury.
Having thus filled the bulb and one-third of the
tube, the next step is to seal it hermetically, which is
done by heating the mercury to the boiling-point, and
at the moment the mercury is overflowing at the
summit the glass is fused with a flame, urged by a blow-
pipe (Fig. 147), before the mercury has had time to
contract ; and if this operation has been skilfully per-
formed, a perfectly void space, or vacuum, free from
air, is obtained as the mercury sinks or contracts in the bulb and tube.
The instrument in its present state will show an increase or decrease of
heat by the rising or falling of the mercury ; but such indications would be
useless, and it would be impossible to compare the observations made by any
two ungraduated bulbs or tubes filled with mercury in the manner already
described.
The graduation of the instrument is, therefore, of paramount importance,
and standard-points must be obtained such, that they shall 'be the same in
every thermometer, whatever may be the scale.
Sir Isaac Newton enjoys the merit of having selected the temperature of
FIG. 146.
136
HEAT.
ice, which is dissolving and liquifying, for one point, and boiling water, emit-
ting steam freely and without pressure, for the other. Ice always melts at the
same temperature, and pure water invariably boils at the same temperature,
when the barometer stands at 29 8 in.
It is only necessary to immerse the thermometer alternately in melting ice
and in boiling water, with certain precautions, and to mark the point at which
the mercurial column stands one being called the freezing-point, and the
other the boiling-point.
The instrument must be immersed in the melting ice until the mercury
becomes perfectly stationary. The immersion in boiling water requires the
greatest care, and a time should be selected when the barometer stands at
29'8 or 30 in. The depth of the water in the vessel should not exceed 2 in.
FIG. 147. Blow-pipe work in Negretti and Zambra's Thermometer Room.
The vessel must not be a shallow one, but sufficiently deep to contain the
bulb and nearly all that part of the tube up which the mercury will rise when
placed in boiling water. Distilled water should be used, and brisk ebullition
maintained, and the steam allowed to escape freely, as any confinement of it
would raise the temperature above that of boiling water. The space or
interval between the two points is now divided into any number of equal
parts, which vary according to the scale used (Fig. 148.)
In England, the interval according to Fahrenheit is divided into 180 parts,
the zero being 32 below the freezing-point. On the Continent, the interval is
divided by Celsius into 100 parts, and is called the Centigrade scale, the zero
commences with the freezing-point ; sometimes into 80 parts, called Reau-
mer's scale, the zero, as before, being the freezing-point of water. Of the
v .hree, that of Celsius is the most simple, and will be gradually adopted
throughout the civilized world.
THE THERMOMETER.
FlG. 148. Graduation, by a Machine, of the Tubes after the freezing and
boiling points have been determined.
These scales are easily reduced from one to another by ascertaining their
numerical relation.
Thus 1 80 is to 100 as 9 to 5 ;
1 80 80 9 4.
Fahrenheit's is, therefore, reduced to the Centigrade scale by multiplying by
5 and dividing by 9 ; or to that of Reaumer by multiplying by 4 and dividing
by 9.
The Celsius or Centigrade and Reaumer's scales are reduced to Fahrenheit's
scale by reversing the process : the multiplier in both cases being 9, the divisor
will be 5 with Centigrade and 4 with Reaumer.
The reduction is, however, a little complicated when it is remembered that
Fahrenheit's zero is 32 below the freezing-point of water, so that in all these
calculations 32 must be first subtracted when Fahrenheit is reduced to Centi-
grade or Reaumer, and added when the contrary is required.
ist Example. To reduce 212 Fahrenheit to Centigrade: 212 32=i8oX
5 =900-^9= 1 00 C.
2nd Example. To reduce 100 Centigrade to Fahrenheit: iooX9'=9oo-^
5 = I 8o +32 =2i2F.
The freezing-point of water is therefore designated and known in all books
by the following expressions : o C, o R., 32 F.
The boiling-point of water, by 100 C., 80 R., 112 F. ; C. being Centigrade,
R. Reaumer, and F. Fahrenheit.
The limits to the use of the mercurial thermometer are the points at which
the metal solidifies, or is frozen, viz., at 39 below zero F., or at which the
metal boils* 662 F., or 450 above the boiling-point of water; hence in the
one case degrees of extreme cold are registered by thermometers filled with
138
HEAT.
alcohol, which has never been known to freeze at the greatest known cold ;
and, in the other case, all temperatures above 662 may be registered, to a
certain point, by the air-thermometer ; but all temperatures which soften glass
and go beyond that point can be estimated only by the pyrometer. The air-
thermometer will be explained in treating of the expansion of gases ; and in
ending the description of the ordinary mercurial thermometer, it may be
stated that the bulbs are liable to a permanent change of capacity, which
displaces the zero ; hence it is usual to keep standard thermometers three
or four years before they are graduated.
Thermometers are constructed for a variety of purposes, and have, there-
fore, different names given to them. In illustration of this statement, we give
a drawing of Negretti and Zambra's maximum thermometer for registering
the highest daily temperature of the air, or degree of heat at any particular
hour of the day
FIG. 149. The Maximum Thermometer ,
the construction of which is as follows : A small piece of glass is inserted
in the bend near the bulb, and within the tube, which it nearly fills : at an
increase of temperature, the mercury passes this piece of glass ; but on a
decrease of heat, not being able to recede, it remains in the tube, and thus
indicates the maximum temperature. After reading, it is easily adjusted.
Hitherto every series of meteorological observations has been more or less
broken by the frequent plunging of the steel index into the mercury, or be-
coming otherwise deranged. Messrs. Negretti and Zambra have, in their
maximum thermometer, supplied a want long felt.
FIG. 150. The Alcohol or Minimum Thermometer
consists of a glass tube, the bulb and part of the bore of which is filled
with perfectly pure spirits of wine, in which floats freely a black glass index. A
slight elevation of the thermometer, bulb uppermost, will cause the glass index
to flow to the surface of the liquid, where it will remain, unless violently
shaken. On a decrease of temperature, the alcohol recedes, taking with it the
glass index ; on an increase of temperature, the alcohol alone ascends in the
tube, leaving the end of the index furthest from the bulb indicating the
minimum temperature.
THE PYROMETER.
139
Directions for using the Minimum Thermometer for determination of the
Minimum Temperature of the Air. Having caused the glass index to flow to
the end of the column of spirits, by slightly tilting the thermometer bulb upper-
most, suspend the instrument in the shade, with the air passing freely to it on
all sides, by the two brass plates attached for that purpose, in such manner
that the bulb is about half an inch lower than- the upper, or the end of the
thermometer furthest from the bulb, then, on a decrease of temperature, the
spirits of wine will descend, carrying with it the glass index ; on an increase of
temperature, however, the spirits of wine will ascend in the tube, leaving that
end of the small glass index furthest from the bulb indicating the minimum
temperature. To re-set the instrument, simply raise the bulb end of the ther-
mometer a little, as before observed, and the index will again descend to the
end of the column, ready for future observation. The same instrument may
be used as a terrestrial radiation thermometer, and when in use is to be placed
with its bulb fully exposed to the sky, resting on grass, with its stem supported
by little forks of wood.
By no means jerk or shake an alcohol minimum thermometer when re-setting
it, for by so doing it is liable to disarrange the instrument, either by causing the
index to leave the spirit, or by separating a portion of the spirit from the
main column.
As alcohol thermometers have a tendency to read lower by age, owing to
the volatile nature of the alcohol allowing particles in the form of vapour to
rise and lodge in the tube, it becomes necessary to compare them occasionally
with a mercurial thermometer whose index error is known ; and if the differ-
ence be more than a few tenths of a degree, examine well the upper part of
the tube to see if any alcohol is hanging in the bore thereof ; if so, the detached
portion of it can be joined to the main column by swinging the thermometer
with a pendulous motion, bulb downwards.
THE PYROMETER.
One of the most celebrated contrivances for estimating high temperatures
was that of Mr. Wedgwood; but, as the indications depended on the con-
traction of clay cylinders, which will contract as much by the long continuance
of a comparatively low heat as by a short continuance of a high one, they
were enormously exaggerated, and could not be correct. The late Professor
Daniell improved greatly upon Wedgwood's instrument, and, by using the
linear expansion of bars of metal, arrived much nearer to a correct estimate
of temperatures above a dull red heat. Daniell calls his instrument the register
pyrometer, and describes it as follows : " It consists of two parts, which may
be distinguished as the register and the scale. The register is a solid bar of
blacklead earthenware, highly baked. In this a hole is drilled, into which a
bar of any metal, six inches long, may be dropped, and which will then rest
upon its solid end. A cylindrical piece of porcelain, called the index, is then
placed upon the top of the bar, and confined in its place by a ring or strap of
platinum passing round the top of the register, which is partly cut away at the
top, and tightened by a wedge of porcelain. When such an arrangement is
exposed to a high temperature, it is obvious that the expansion of the metallic
bar will force the index forward to the amount of the excess of its expansion
over that of the blacklead, and that, when cooled, it will be left at the point
of greatest elongation. What is now required is the measurement of the
HEAT.
distance which the index has been thrust forward from its first position ; and
this, though in any case but small, may be effected with great precision by
means of the scale.
" This is independent of the register, and consists of two rules of brass
accurately joined together at a right angle by their edges, and fitting square
upon two sides of the blacklead bar. At one end of this double rule a small
plate of brass projects at a right angle, which may be brought down upon the
shoulder of the register, formed by a notch cut away for the reception of the
index.
** A movable arm is attached upon this frame, turning upon its fixed extremity
upon a centre, and at its other carrying an arc of a circle, whose radius is
FIGS. 151, 152.
exactly 5 in., accurately divided into degrees and thirds of a degree. Upon
this arm at the centre of the circle another lighter arm is made to turn, one
end of which carries a nonius with it, which moves upon the face of the arc,
and subdivides the former graduation into minutes of a degree ; the other end
crosses the centre, and terminates in an obtuse steel point, turned inwards at a
right angle.
" When an observation is to be made, a bar of platinum or malleable iron
is placed in the cavity of the register ; the index is to be pressed down upon
it, and firmly fixed in its place by the platinum strap and porcelain wedge.
The scale is then to be applied by carefully adjusting the brass rule to the
sides of the register, and fixing it by pressing the cross piece upon the shoulder,
and placing the movable arm so that the steel point of the radius may drop
into a small cavity made for its reception, and coinciding with the axis of the
metallic bar.
" The minutes of the degree must then be noted, which the nonius indicates
upon the arc. A similar observation must be made after the register has been
THE PYROMETER. 141
exposed to the increased temperature which it is designed to measure, and
again cooled, and it will be found that the nonius has been moved forward a
certain number of degrees or minutes, as shown at Figs. 151 and 152."
Fig. 151 represents the register; A is the bar of black lead; a the cavity
for the reception of the rnetallic bar; cc 1 is the index, or cylindrical piece of
porcelain ; d, the platinum band, with its wedge, e.
"Fig. 152 is the scale by which the expansion is measured: f is the greater
rule, upon which the smaller, g, is fixed square. The projecting arc h is also
fitted square to the ledge under the platinum band d.
D is the arm which carries the graduated arc of the circle E, fixed to the rule
f, and movable upon the centre i.
C is the lighter bar fixed to the first, and moving upon the centre k.
H is the nonius at one of its extremities, and m the steel point at the other.
The rule^ admits of adjustment ony^ so that the arm h may be adjusted
to the centre z, in order that at the commencement of an experiment the nonius
may rest at the beginning of the scale.
The term " nonius," used by Daniell, is only another name for vernier, a
contrivance for measuring intervals between the divisions of graduated scales
on circular instruments.
The scale of this pyrometer is readily connected with that of the thermo-
meter by immersing the register in boiling mercury, whose temperature is as
constant as that of boiling water, and has been accurately determined by the
thermometer.
The amount of expansion for a known number of degrees is thus deter-
mined, and the volume of all other expansions may be considered as propor-
tional.
The melting-point of cast iron has been thus ascertained to be 2786, and
the highest temperature of a good wind-furnace about 3300 points which
were estimated by Mr. Wedgwood at 20,577 and 32,277" respectively.
Mr. Wedgwood, indeed, makes an observation which is calculated to throw
suspicion upon the accuracy of his results ; for he says, " We see at once how
small a portion (of the rays of heat) is concerned in animal and vegetable life,
and in the ordinary operations of nature. From freezing to vital heat is barely
i -5ooth part of the scale a quantity so inconsiderable relatively to the whole
that in the higher stages of ignition ten times as much might be added or
taken away without the least difference being discoverable in any of the
appearances from which the intensity of fire has hitherto been judged of.".
Now this, remarks Daniell, "is utterly unlike the gradual progression by
which the operations of nature are generally carried on ; and the fact is, that
a regular transition may be traced from one remarkable point of temperature
to another."
Thus from the freezing of water, 32, to vital heat in man is 60.
60 X 3= 1 80 Boiling water.
60 X 7= 420 Melted tin.
60X10= 600 Boiling mercury.
60 X 15= 900 Red heat.
60x31 = 1860 Melting silver. .
60x45=2700 Melting cast iron.
60 x 5 5=13300 Highest heat_ of wind-furnace.
Before the invention of the register pyrometer, the expansion of solids
had never been ascertained beyond the temperature of 527 : the following
I 4 2
HEAT.
table exhibits the progressive amount of several metals to their point of fusion,
as determined by Daniell's pyrometer:
PROGRESSIVE DILATATION OF SOLIDS.
One millon parts at 62 .
At 212.
At 662.
At Fusing-point.
Blacklead ware .
1,000,244
I,OOO,7O3
Wedgwood ware .
1,000,735
1,002,995
Platinum
1,000,735
1,002,995
1,009,926
(maximum, but not fused).
Iron, wrought
1,000,984
1,004,483
1,018,378
to the fusing-point of cast iron.
Iron, cast .
1,000,893
1,003,943
1,016,389
Gold .
1,001,025
1,004,238
Copper
1,001,430
1,006,347
1,024,976
Silver .
1,001,626
1,006,886
1,020,640
Zinc .
1,002,480
1,008,527
1,012,621
Lead .
1,002,323
1,009,072
Tin ...
1,001,472
...
1,003,798
Professor Daniell concludes his dissertation by the following passage,
which is quite in accordance with those notions which Tyndall has so ably
contended for viz., that heat is a mode of motion: "The amount of the force
which produces these expansions and contractions, measured by any oppo-
sing force, that of cohesion, for instance, is enormous.
" Some idea may be formed of it, when it is understood that it is equal to
the mechanical force which would be necessary to produce similar effects in
stretching or compressing the solids in which they take place. Thus, a bar
of iron heated so as to increase its length a quarter of an inch, by this slow
and quiet process exerts a power against any obstacle by which it may be
attempted to confine it, equal to that which would be required to reduce its
length by compression to an equal amount. On withdrawing the heat, it
would exert an equal power in returning to its former dimensions."
M. Molard used this great moving force to restore the walls of a building to
the perpendicular which had been bulged, and the same principle was used at
the Cathedral of Armagh.
THE EXPANSION OF GASES.
We now come to the most expansible bodies viz., the gases; and, although
at first there was considerable doubt whether they all expanded alike, because
the experimentalists had neglected to remove the moisture the aqueous
vapour from them, it was finally discovered, not only by Gay-Lussac in
Paris, but by our own countryman, the illustrious Dr. Dalton, that all gases
expand alike with the same amount of heat, and that the rate of dilatation
continues uniform for all temperatures. In discovering the expansibility
THE EXPANSION OF GASES.
of liquids it was found that cohesion was not quite overcome, and that there
was still a considerable amount of that force which tended to keep the par-
ticles in contact. This, however, is not the case with gases ; the cohesive
power is for the time completely overcome by the motion of heat. Sir H.
Davy speaks emphatically upon this motion in his " Chemical Philosophy."
" It seems possible to account for all the phenomena of heat, if it be supposed
that in solids the particles are in a constant state of vibratory motion, the
particles of the hottest bodies moving with the greatest velocity and through
the greatest space; that in fluids and elastic fluids, besides the vibratory
motion, which must be conceived greatest in the last, the particles have a
motion round their own axes with different velocity, the particles of elastic
fluids (gases) moving with the greatest quickness ; and that in ethereal sub-
stances the particles move round their own axes, and separate from each other,
penetrating in right lines through space. Temperature may be conceived to
depend upon the velocity of the vibration, increase of capacity in the motion
being performed in greater space ; and the diminution of temperature during
the conversion of solids into fluids or gases may be explained on the idea of
the loss of vibratory motion in consequence of the revolution of particles
round their axes at the moment when the body becomes fluid or aeriform, or
from the loss of rapidity of vibration in consequence of the motion of the
particles through space."
It has been proved that gases expand by i-49oth of their own volume lor
every degree o Fahrenheit's scale between the freezing-point, 32, and the
boiling-point of water, 212, and so on at higher or lower temperature, pro-
vided the pressure of the air remains the same. If the Centigrade scale is
used, the ratio of expansion of any gas will be 1-27 3rd of its volume for every
degree.
490 cubic inches of air at 32 become 491 at 33
491 33 492 34
492 34 493 35
From a most careful series of experiments it has been determined that
" the coefficient of expansion " of all gases, expressed in decimals, is 0-00,366.
These figures are near enough for all ordinary calculations, although it must
be observed that, speaking rigidly, this is not exactly the 'case, except probably
with the three permanent gases, oxygen, hydrogen, and nitrogen, in all the
other gases and vapours the expansion being greatest for those which are
most readily condensible.
M. Regnault has made the most elaborate and careful experiments, and
determined that one thousand volumes of certain gases at o C. or 32 F.
(the pressure of the air remaining unchanged) become expanded in the fol-
lowing proportions when heated to 100 C., or 212 F. :
Air .
Carbonic acid
Carbonic oxide
Cyanogen .
1,367-06
1,370-99
1,366-88
1,387-67
Hydrogen . . . 1,366-13
Hydrochlorine acid . 1,368-12
Nitrogen . . . 1,366-82
Nitric oxide . . i,37i'95
It will be apparent that hydrogen expands the least, and, as might be
expected, cyanogen, which is liquified with comparative ease, is much higher
viz., 1,387*67. It is, therefore, apparent that if the coefficient of expansion
remains the same with all gases, that cyanogen should have been represented
144
HEAT.
by the same figures as those which belong to air instead of being 0*00,387 to
0-00,367 atmospheric air. The conversion of this property of expansion into
power or motion is well described by Tyndall : " Suppose I have a quantity
of air contained in a very tall cyliYider (A B, Fig. 153), the transverse section
of which is one square inch in area. Let the top, A, of the
cylinder be open to the air, and let P be a piston, which, for
reasons to be explained immediately, I will suppose to weigh
two pounds one ounce, and which moves air-tight and without
friction up or down in the cylinder. At the commencement of
'the experiment let the piston be at the point P of the cylinder,
and let the height of the cylinder from its bottom B to the point
P be 273 inches, the air underneath the piston being at a tem-
perature of o C. Then, on heating the air from o to i C, the
piston will rise one inch; it will now stand at 274 inches above
the bottom. If the temperature be raised two degrees, the pis-
ton will stand at 275 ; if raised three degrees, it will stand at
276; if raised ten degrees, it will stand at 283; if 100 degrees,
it will stand at 373 inches above the bottom; finally, if the tem-
perature were raised to 273 C, it is 'quite manifest that 273
inches would be added to the height of the column ; or, in other
words, that by heating the air to 273 C. its vohime would be
doubled. -The gas in this experiment executes work. In expand-
ing from P upwards, it has to overcome the downward pressure
of the* atmosphere, which amounts to 1 5 Ibs. on every square
inch, and also the weight of the piston itself, which is 2 Ibs. I oz.
Hence, the section of the cylinder being one square inch in
area, in expanding from P to P' the work done by the gas is
equivalent to the raising a weight of 17 Ibs. I oz., or 273 ounces,
to a height of 273 inches. It is just the same as what it would
accomplish if the air above P were entirely abolished, and a
piston weighing 17 Ibs. i oz. were placed at P.
" Let us now alter our mode of experiment, and, instead of
allowing our gas to expand when heated, let us oppose its ex-
pansion by augmenting the pressure upon it ; in other words,
let us keep its volume constant while it is being heated.
" Suppose, as before, the initial temperature of the gas to be
o C., the pressure upon it, including the weight of the piston
P, being as formerly 273 ounces. Let us warm the gas from o
C. to i C. ; what weight must we add at P in order to keep its
volume constant ? Exactly one ounce.
" But we have supposed the gas at the commencement to be under a pres-
sure of 273 ounces, and the pressure it sustains is the measure of its elastic
force ; hence, by being heated i, the elastic force of the gas has augmented
by i-273rd of what it possessed at o. If we warm it 2, two ounces must
be added to keep its volume constant ; if 3, three ounces must be added ;
and if we raise its temperature 273, we should have to add 273 ounces,
that is, we should have to double the original pressure to keep its volume
constant.
"In the first case marked out, it is shown that by heating the air to 273 C.
its volume would be doubled. In the second, that by compressing the air with
273 ounces we may heat it to 273 C., and have, consequently, double the
FIG. 153.
THE EXPANSION OF GASES.
original pressure to keep the air confined to the same volume. In fact, trie-
volume being kept constant, the elastic force is doubled. ^
" But are the absolute quantities of heat imparted in both cases the same ?
By'no means. Supposing that to raise the temperature of the gas, whose
volume is kept constant, 273, ten grains of combustible matter are necessary;
then to raise the temperature of the gas, whose pressure is kept constant, an
equal number of degrees would require the combustion of 14^ grains of the
same combustible matter. The heat prodttced by the combustion of the addi-
tional 4^- grains in the latter case is entirely consumed in lifting the weight.
Using the accurate numbers, the quantity of heat applied when the volume is
constant is, to the quantity applied when the pressure is constant, in the pro-
portion of i to 1*421.
" This extremely important fact constituted the basis from which the
mechanical equivalent of heat was first calculated."
Various methods have been contrived to determine the amount of expansion
of gases when subjected to a uniform pressure, and one of the most simple is
that of Monsieur Pouillet (Fig. 154), described by Lardner.
"An iron syphon tube, D c, is formed
with short legs, from the bottom of which
proceeds a pipe with a stop-cock F, under
which is placed a cistern or reservoir G. In
the legs of the syphon D c are inserted two
glass tubes, D E and C B, of more than thirty
inches in height. The tube D E is open at
the top ; the tube C D is closed at the top,
' but has a horizontal branch united to it, at
B, which is connected with a tube, A B,
made of platinum, which terminates in a
hollow globe or ball, A, also made of pla-
tinum. In the tube B A is fixed a stop-
cock in order to communicate at pleasure
with the atmospheric air.
" The stop-cock F being closed, and the
stop-cock in the tube B A being open, mer-
cury is poured into the tube D E, so as to
fill the glass tubes D E and C B nearly to
the top. Since the tubes D E and C B both
communicate with the external air, the
columns of mercury in them will stand at
the same level.
" To determine the expansion which air FlG - J 54. Pouillet s Apparatus.
suffers when raised from the freezing-point
to the boiling-point under uniform pressure, let the ball A be immersed in
a bath of melting ice, so as to reduce the air included in it to the freezing-
point. Let the stop-cock in the tube B A be then closed, and let the bulb A be
removed to a bath of boiling water. The air in the bulb, expanding, will
press down the column of mercury in B c, and will cause the column in D E to
rise ; so that the levels of the two columns will no longer coincide. But they
may be equalized by opening the stop-cock F, and allowing mercury to flow
into the reservoir G from the syphon until the levels in the two legs come to
the same point. When that is accomplished, the pressure upon the expanded
10
146
HEAT.
air included in the bulb A and the tube communicating with it will be equal to
that of the atmosphere, and equal to that which the same air has when at the
freezing-point.
" The capacity of the tube C B being known, the volume which corresponds
to any length of it will be also known ; also the increment of volume which
the air has suffered by expansion will be indicated by the height through
which the mercury has fallen in the tube C D. This increment, therefore, will
be the dilatation of the air included in the bulb A and the communicating tube
between the freezing and the boiling points. In the same manner, by this
apparatus, the dilatation corresponding to any change whatever of tem-
perature under a given pressure can be ascertained."
The expansion of air by heat, and the uniformity with which it takes place,
suggested at a very early period of science the use of air-thermometers,
which are the most delicate and, with certain precautions, the most reliable
in certain cases where high temperatures have to be determined. The first is
supposed to have been constructed by a learned Italian
physician, named Sanctorius, about the year 1590. It
is sometimes attributed to Cornelius Drebel, who in-
troduced it in the year 1610; but this is a mistake.
Drebel followed Sanctorius, and therefore cannot be
the first inventor, although there is every reason to
suppose that he made his air-thermometer in perfect
ignorance of what Sanctorius had already done.
The construction is very simple : it consists of a
glass tube at the end of which a bulb or ball is blown;
this tube, with its ball, is then fitted into some conve-
nient glass vessel or bottle, containing a little coloured
water. On the application of heat, either from the
palm of the hand or the flame of a spirit-lamp, a por-
tion of the air in the tube is expelled, and, when cold,
the water ascends to fill its place ; the rise or fall of
this column of coloured water by the expansion or
contraction of the air in the bulb is supposed to indi-
cate the difference of temperature.
It was soon discovered that this air-thermometer
was not correct in its indications, and was, in fact,
affected by the pressure of air : when the barometer
fell, the air expanded in the bulb, and the coloured
fluid was driven downwards; or, on the contrary, if the
barometer rose, the air, contracted by the increased
pressure on the liquid, was pushed higher up the tube.
Sir John Leslie greatly improved upon the rude appa-
ratus already described, and invented a very elegant in-
FlG. 155. strument, called the Differential A ir Thermometer (Fig.
The Air-Thermometer 156), which has been of the greatest use in the refined
of Sanctorius. experimental researches made for the elucidation of
the more obscure properties of the force called heat.
It consists of two glass bulbs or balls connected together by a tube bent
twice at right angels. The balls contain air, and, just before they are her-
metically closed, a little sulphuric acid, coloured with carmine, is introduced,
so that it rises to about half the height of the two tubes bent at right angles.
CONDUCTION.
147
The ball left open for the introduction of the
coloured fluid is now finally closed, and as both
bulbs must be equally affected by changes of tem-
perature in the surrounding air, the liquids in the
tubes remain in equilibrium.
If, however, one of the balls is grasped by the
hand, the air expands, and the fluid is driven up
the other tube, which is provided with a proper
scale ; thus at any moment, by placing one ball in
a particular spot where heat is to be discovered,
the expansion of the air becomes a most sensitive
and delicate means of appreciating any small
amount of heat.
TIG. 156.- Leslies Differ-
ential Thermometer.
FlG. 157. Differential Tfiermometer used.to
discover Focus of Heat Rays.
CONDUCTION.
Our ideas of this property of heat, of travelling along and through material
substances, are quickly tormed and put in practice. If a bar of iron and a rod
of glass are thrust between the bars of a grate containing burning fuel, we
soon learn which we may first touch or take out with impunity. The iron
rapidly becomes so hot throughout its length and breadth, that we cannot
lay hold of it ; the glass rod may be quite softened within a few inches of
the hand, and yet the heat is not sensibly felt or becomes so great as to
prevent the rod of glass being held in the hand : in the one case there appear
to be regular stepping-stones across which the heat may, as it were, take its
way ; in the other there is no regular path provided, and the travelling power
of the heat is interfered with, and so greatly impeded that a considerable
time must elapse before any sensible progress or travelling of the heat can
be recorded. Thus in early days the wise men of the period rudely divided
all substances into conductors and non-conductors of heat. Such a division,
however, is not in accordance with nature ; there are intermediate conditions
of conductivity, and thus we come to speak of good and bad conductors of
heat.
102
148 HEAT.
In regarding heat by the dynamical theory, the student can have no diffi-
culty in understanding that the position of the solid substance under exami-
nation in the list of good or bad conductors must depend greatly upon its
physical structure. The metals are good conductors ; there is uniformity of
internal structure, and the vibratory movement necessary to set the heat-
waves in motion is regular and not interfered with ; moreover, the particles
are in close contact. Glass is a bad conductor, because those conditions
which are necessary for the setting up of molecular motion are not fulfilled ;
the vibrations are not communicated steadily from molecule to molecule, but
broken up and thrown into confusion ; the glass has no regular molecular homo-
geneity it is too heterogeneous. Any substance which can transmit molecular
motion is a good conductor of heat, and those bodies which do not transmit
this motion readily are bad conductors.
FIGS. 158 and 159. Griffith? experiment.
The difference oetween the conducting power of a metal, an earth, and art
earthy compound may be illustrated by the following simple and instructive
experiment : *
Provide solid cylinders of these three materials, viz., iron, sandstone, and
chalk ; let these be i in. in diameter and 6 in. long, and perfectly flat at each
of their ends.
Place a cup, containing an ounce of tallow, upon the warm hob of the grate;
and when the tallow is perfectlymelted, dip into it for about half an inch one
end of the iron cylinder, and then lift it out ; a portion of tallow will adhere,,
and quickly become solid, because the iron, by good conducting power, deprives
it of the heat of fluidity.
Dip one end of each of the other cylinders in the same way; they will
attract or absorb a considerable portion of the melted tallow, and some time
will be required before it will become equally solid with that on the iron
cylinder, because sandstone and chalk have not sufficient conducting power
to deprive it of heat in a similar degree.
Dip the end of all three cylinders again, and lift them out, and, when the
tallow becomes solid, dip them again, and lift them out until they have all
obtained an equal coating of tallow ; then allow them to cool. Pour boiling
water into a " hot- water plate," and place the three cylinders to stand upon it
at equal distances, with their coated ends uppermost, as shown in Fig. 158.
* " Chemistry of the Four Seasons," Griffiths.
CONDUCTION. 1 49
In the course of a few minutes, the iron will again prove its good conducting
power by melting the tallow ; but the sandstone and chalk will prove their
bad conducting power by the tallow remaining solid during the whole time
that the water is cooling down to common temperature.
By reversing the arrangement of the last experiment, namely, by applying
heat above, instead of beneath, the cylinders, it can be proved that neither
the conducting power of the iron nor the non-conducting power of the sand-
stone and chalk are in the least degree affected or modified.
Let the iron cylinder be again coated with tallow, but pare away all from
its circular extremity, that it may now stand firmly upon this, and have only
a ring of tallow, about half an inch wide, around its circumference ; do the
same with the cylinders of sandstone and chalk; then set the three at equal
distances within a circle similar in diameter to the bottom of the hot-water
plate, that they may form a tripod for its support (this arrangement must be
made upon a steady table) ; then remove the plate, without disturbing the
cylinder, fill it with boiling water, and carefully replace it to stand upon them,
.as represented in Fig. 159.
The three cylinders will now be subjected to heat applied from above, instead
of from below, as in the last experiment (Fig. 158); but this arrangement will
-cause no difference in their conducting power, or non-conducting power, as
will be proved in the course of a few minutes by the ring of tallow melting
from the iron cylinder, whilst that upon each of the other cylinders remains
solid as before.
Starting with gold, and taking it as the type of a good conductor, and
.giving it the first place in a scale amounting to 100, we have the following
tabulated results obtained by Franklin and Igenhausz, by watching the rate
at which wax was melted at the end of bars of
Gold 100*00
Platinum .... 98*10
Silver . . . . . 97*30
Copper 89*82
Iron 37-41
Tin . . . . . 30*38
Lead 17*96
Marble ..... 2*34
Porcelain .... 1*22
Brick-earth . . . . 1*13
Zinc 36*37
The metals are evidently the best conductors ; but even these differ remark-
ably, gold being 100, whilst lead has not one-fifth of the conducting property
and power of transmitting molecular motion possessed by the first-named
metal. Brick-earth is constituted of a number of distinct bodies; it is a
mechanical mixture of a variety of compounds, each of which has an exact
chemical composition. The particles are not only different from each other,
but are widely apart ; the substance is of a porous nature. Asbestos, pumice-
stone, charcoal and especially animal charcoal sand, are all porous, and
well-known bad conductors, so much so that a red-hot ball of iron can be
held in the hand for a certain time, provided a layer of either of the above-
named substances intervene between the skin of the hand and the heated
metal.
By a more careful mode of experimenting, the conductivity of the various
metals has been determined by Despretz, Wiedemann, and Franz. In this
table it will be seen that silver occupies the first place, instead of gold, which
is third. Platinum, again, which stands second in the first table, is very low
down in the scale of conductivity ; and bismuth is the lowest of all.
Silver
. ICO
Iron
Copper
Gold
. 74
53
Lead
Platinum
24
German silver .
Tin .
K
Bismuth .
HEAT.
9
8
6
2
Franklin and Igenhausz must therefore have committed some gross errors
in their experiments, or the second table quoted here is wrong.
Dr. Tyndall explains the cause of the difference with a very pretty experi-
ment. He takes a short prism of bismuth, and another of iron, of the same
size, and having coated the extremities with wax, they are both placed on the
lid of a vessel filled with boiling water. Strange to say, the wax on the
bismuth melts first, although it has six times less conductivity than iron.
Here is a para*dox which requires explanation, and shows why the experiments
conducted by Franklin and Igenhausz cannot agree with those of more
modern physicists. In the first place, the test of conductivity employed by
the earlier experimenters was the rapidity with which the wax and tallow
coating a bar of any given substance melted in comparison with another
just as Tyndall used the prisms of bismuth and iron.
In the second place, the mode of experimenting employed by Despretz
was not simply a determination of the rapidity with which the thermometer
inserted in the bar was affected, as shown in
a 5 c d e f
illli
FlG. 1 60. Desprettfs Mode of determining the Conductivity of Metals;
A, the bar containing the thermometers, a, b, c, d, e,f; B, glass supporting A; D, the spirit-lamp.
but he waited until the bar showed a stationary condition of heat, and the
thermometers no longer continued to rise, and, by estimating the difference
between each thermometer, he soon discovered that the best conductors
produced the least amount of difference between the thermometers, and that
the worst conductor gave the contrary result.
Why did he wait until the heat of the bar became stationary ?
To avoid the error caused by the difference of " specific heat," which varies
with every substance. This difference is readily explained by the following
experiments :
A pint of water at 50 F. mixed with a pint at 100 F. will amount to a
quart, which will have a mean temperature of 75 F.
CONDUCTION. 151
50 F.
100 F.
2)150
Here the molecules are exactly the same ; it is water mixed with water, and
the particular heat required to raise any given bulk to a certain temperature
cannot alter. If, however, a pint of water at 100 F. is mixed with a pint of
mercury at 40 F., the resulting temperature is not the mean, 70, but 80;
the water has only fallen 20, whilst the mercury has risen 40. The 20 of
heat from the water has been sufficient to heat the mercury 40. Hence it is
apparent that mercury has a less " capacity for heat" (keeping to old expres-
sions) than water, and it requires a smaller amount of heat to raise it to a
given temperature, viz., 80. For the term, " capacity of heat," or " specific
heat," substitute, according to the dynamical theory, the term, "power to
get into molecular motion," or " capacity for molecular motion."
We may once more return to Tyndall's paradox with the bismuth and iron.
The "capacity for heat," or "specific heat," of iron is o'lisS; that of
bismuth is only 0*0308 : like the mercury and the water experiment, it takes
less heat to warm any given mass of bismuth than it does to heat an equal
bulk of iron.
The molecular motion which can be set up in bismuth occurs much
quicker than it does in iron : one might almost say that the " inertia of heat "
in iron was greater than that of bismuth. But this inertia once overcome,
and each metal transmitting all the molecular motion which can be conferred
from the vessel containing the boiling water, it will soon be found, according
to the table quoted by Tyndall, that iron transmits six times more vibratory
power, or motion of heat, than bismuth; it has less power to get into
molecular motion than bismuth, but, once in motion, it sends vibration after
vibration from molecule to molecule, and soon outstrips the bismuth in the
race of conductivity.
In this place it is desirable to speak of certain terms which have arisen
and are used in conformity with the dynamical theory of heat.
i. "POTENTIAL" FORCE.
Potential force may be defined as a power waiting and ready to be used ;
" the sword of Damocles suspended by a hair ;" the giant standing motion-
less, but capable, at the word of command, of exerting great physical power.
It is, in short, stored-up energy the gold in the bank cellars, potential, but
not in circulation or use. Substitute for the word " force " heat, and you have
potential heat.
2. "ACTUAL" FORCE, OR "ENERGY."
As the first was dormant or passive, the second is " actual " or real, and
makes itself apparent the hair broken, the sword in the act of descending.
They are mutually convertible : as actual heat appears, potential heat is used
up and disappears. You cannot store gold in a cellar and use it at the same
time.
The stored gold would represent potential heat ; the gold in use or circula-
152
HEAT.
tion, actual heat. A country in a state of peace would have gold stored, and
ready to pay an army ; but the latter, once formed and in actual service, must
be paid ; and as the army becomes active, the potential energy the gold-
disappears.
One pound of hydrogen and eight pounds of oxygen contain potential
energy which is enormous ; when they unite, they form nine pounds of water,
and the mechanical value of the heat, or actual energy, set free is equivalent
to a force that would raise forty-seven millions of pounds weight one foot
high.
The change of one pound of hydrogen, by combination with eight pounds of
oxygen, into nine pounds of water would be an example of "chemical action."
Action and reaction are equal, but contrary; and therefore Dr. Odling's
admirable lecture " On Reverse Chemical Action," delivered before the last
FIG. 161.
A, the Hask of water boiled by spirit-lamp, and delivering steam to the platinum tube B, coiled round and
placed in a hollow made in a firebrick, and subjected to the intense heat of the oxy-hydrogen blow,
pipe c. D, small pneumatic trough and tube for collection of the two gases, oxygen and hydrogen,
meeting of the British Association, held at Norwich, is most welcome, because
it supplies the reasoning for the opposite effect viz., the conversion of "actual
energy, or heat," into potential energy.
By passing the vapour of water through a spiral platinum tube, made white-
hot by the oxy-hydrogen flame, the vapour is divided again into its elements,
oxygen and hydrogen. This beautiful experiment, so worthy of the author of
the " Correlation of the Physical Forces," Professor Groves, is shown at Fig.
161.
The platinum tube has no power to unite with the oxygen or the hydrogen ;
it is simply the vehicle for the application of the intense heat of the oxy-
hydrogen blowpipe. The potential energy of the mixed gases produces actual
energy or heat, and the latter again stores up potential energy by the repro-
duction of hydrogen and oxygen. Nothing can be more perfect as a train of
experimental reasoning, or more decidedly illustrate the conversion of poten-
tial into actual energy, and vice versa. It is a true illustration of " conserva-
tion of energy," and enables the student to realise the magnificent principle
which destroys nothing, nor admits the destruction of anything, because through-
out the universe the sum of these two energies, called "potential" and
CONDUCTION. 153
" actual," is equal. The conclusion of Dr. Odling's brilliant address, " On
Reverse Chemical Action," admirably expresses these grand truths :
" Reverse chemicaliActions are those which do not take place of themselves,
but only by the appfication of some external force or agency, which force
becomes as it were stored up in the product of the reaction ; in other words,
it is attended by a conversion of potential into actual energy. It is an instance
of winding up, and not of running down. Direct chemical action takes place
of itself by virtue not of an innate tendency of the bodies, which acts, but of
an energy which has been put into the bodies at some time or other ; it takes
place of itself, and is attended by the liberation of pent-up forces contained in
the reacting bodies, in other words, it is attended by a conversion of potential
into actual energy. Every direct chemical combination has been preceded
by some reverse chemical action, just as the falling down of a weight has been
preceded by the winding of it up. When we consume wood and coal in our
fires, or bread and wine in our bodies, we merely effect a combination whereby
their potential is converted into actual energy, this potential energy having
been stored up in them at the period of their formation ; this energy being, in
fact, the robbing of the sun's rays, and the storing up the heat of these rays
in these articles of fire and fuel. Under the action of the sun's rays the de-
composition is effected of the carbonic acid and water into oxygen gas,
restored to the atmosphere, and carbon-hydrogen, which is accumulated in
the vegetable tissue. When we burn these tissues in our fires or bodies, we
are simply restoring in the form of actual energy the potential heat of the
sun's rays or its mechanical equivalent. We have all read of the Bourgeois
Gentilhomme who had been talking prose all his life without knowing it. We
have all our lives, and some of us without knowing it, been realising that
celebrated problem of extracting sunbeams from cucumbers."
It should be mentioned that Wiedemann and Franz did not employ ther-
mometers ; they used a more refined arrangement with the thermo-electric
pile and galvanometer needle a most delicate measurer of heat, which will
be more fully explained presently. Wool, chalk, stone, fire-clay, ivory, are
all bad conductors of heat." Asbestos, powdered pumice-stone, charcoal, saw-
dust, and snow are still worse conductors of heat. The subdivision and
pulverization of the substance increase porosity, and decrease conductivity.
The wool and fur of animals, the plumage of birds, and especially the down
(made into eider-down quilts), are all good examples of the wondrous care
with which a superintending Creator has foreseen the various wants of the
animal kingdom, and protected them even against the vicissitudes of tem-
perature.
The kettle-holder made of wool, the pieces of ivory which break the metallic
communication between the good-conducting silver teapot and its handle and
the soot charcoal covering the bottom of a kettle, which allows the vessel
to be taken direct from the fire and, though full of boiling water, held upon
the palm of the hand, are good and familiar examples of the application of
bad conductors.
One of the most interesting novelties displayed in the department devoted
to Norway, in the French Exhibition of 1867, was the Self-acting Norwegian
Cooking Apparatus,, constructed in the most simple manner, of a wooden box
lined with four inches of felt, in which the saucepans containing the food,
previously boiled and maintained at the boiling-point for five or ten minutes,
according to the nature of the food to be cooked, are placed. The heated
154 HEAT.
saucepans are covered with a thick felt cover, and, the lid of the box being
fastened down, the rest of the cooking is done by slow digestion, no more heat
being added.
The heated vessels containing the food will retain a high temperature for
several hours, so that a dinner put into the apparatus at 8 in the morning
would be quite hot and ready by 5 in the afternoon, and would keep hot up
to 10 or 12 at night, because the felt clothing so completely prevents the
escape of the heat ; and as the whole is enclosed in a box, there are no currents
of air to carry off any other heat by convection.
FlG. 162. The Norwegian Self -Acting Cooking Apparatus.
A, the box, lined with felt; B B, saucepans fitting into box; c, the felt cover to be placed on the
top of the saucepans.
The principle on which this cooking apparatus acts is that of retaining the
heat ; and it consists of a heat-retainer or isolating apparatus shaped somewhat
like a refrigerator, and of one or more saucepans or other cooking-vessels
made to fit into it. Whereas in the ordinary way of cooking the fire is neces-
sarily kept up during the whole of the time required for completing the cooking
process, the same result is obtained, in using this apparatus, by simply giving
the food a start of a few minutes' boiling, the rest of the cooking being com-
pleted by itself in the heat-retainer away from the fire altogether.
Directions for use. Put the food intended for cooking, with the water or
other fluid cold, into the saucepan, and place it on the fire. Make it boil, and
when on the point of boiling skim if required. This done, replace the lid of
the saucepan firmly, and let it continue boiling for a few minutes. After the
expiration of these few minutes, take the saucepan off the fire, and place it
immediately into the isolating apparatus, cover it carefully with the cushion,
and fasten the lid of the apparatus firmly down. In this state the cooking
process will complete itself without fail.
By no means let the apparatus be opened during the time required for
cooking the food. The length of time which the different dishes should remain
in the isolating apparatus varies according to their nature. It may, however,,
be taken as a general rule that the same time is required to complete the
cooking in the apparatus as in the ordinary way on a slow fire.
The advantages of this apparatus are thus detailed by Herr Sorensen, the
patentee, whose attention was first directed to the subject by the Norwegian
CONDUCTION. 155
peasants, who heat their food in the morning, and whilst away in the fields
keep the saucepan hot by surrounding it with chopped hay :
1. Economy of fuel varies according to the length of time required for
cooking the different sorts of food. For those requiring, in the ordinary way,
only one hour's cooking, the saving is about 40 per cent. ; two hours, 60 per
cent. ; three hours, 65 per cent. ; six hours, 70 per cent. In the case of gas
being used, the saving would be greater still.
2. Economy of Labour. A few minutes' boiling is sufficient. No fire is
necessary afterwards. The cooking-pot once in the apparatus, the cooking
will complete itself. Over-cooking is simply impossible, and the process of
cooking is infallible in its result. The food will be cooked in about the same
time as if fire had been continuously used. But the food need not be eaten
for many hours after the cooking process is complete; so that half-an-hour's
use of a fire on a Saturday night, for example, will give a smoking hot dinner
on Sunday.
3. Portability. The weight of the apparatus complete varies from 18 to
5olbs. The apparatus can, in proportion to its dimensions, be carried about
with great facility, without interfering with the cooking process. By means
of a large apparatus for instance, following on a cart a detachment of soldiers
on the march it is possible to provide them with a hot meal at any moment
it might be found convenient (as may be proved by official reports from the
officers of the Royal Guard at Stockholm, in the possession of the patentee).
Again, fishermen, pilots, and others whose small vessels are not generally
so constructed as to enable them to procure hot food while at sea, may easily
do so, by taking out with them in the morning an apparatus prepared before
their departure. It is, in short, a thing for the million, for rich and poor ; for
the domestic kitchen, as well as for persons away from their homes. It cooks,
and keeps food hot, just as well when carried about on a pack-saddle, on a
cart, or in a fisherman's boat, as in a coal-pit or under the kitchen table.
4. Quality and quantity of the food prepared. Where other plans of cook-
ing waste one pound of meat, this apparatus, properly used, wastes about one
ounce. The unanimous testimony of those who have used it pronounces the
flavour of food cooked in this manner incomparably superior to that which is
ordinarily produced.
5. Simplicity of use. One of the greatest advantages of this invention is,
no doubt, its simplicity and practical application. There is no complication
of hot-water or air pipes to retain the heat, no mechanical combination what-
ever for producing a high degree of heat by steam pressure ; consequently
there is no necessity for steam-valves or other combinations which would
render the use of the apparatus difficult and dangerous. Any person will,
without difficulty, be able to use the apparatus to advantage after once having
witnessed it in operation. No special arrangement is required in the kitchen
for using the apparatus. Any fuel will do for starting the cooking.
6. In addition to all these advantages, the complete apparatus constitutes
the * Simple Refrigerator' for the preservation of ice, which has attracted so
much notice (see Letters in Times, July 30, 31, August 4, 1868), and had such
warm approval from medical men. It will keep ice in small quantities for
many days.
In the organization of our bodies there are chemical changes going on
which maintain a certain temperature. It matters not whether the living
being, man, is a resident of tropical or polar regions ; the temperature required
156 HEAT.
to promote and carry on vitality remains the same, or nearly so. If the cold or
absence of heat is likely to be dangerous, man uses the skins and furs of
animals for his clothing, and takes care to lose little or no heat. On the
other hand, if the heat is excessive, increased action of certain powers throws
out perspiration, which carries off the heat that might accumulate and prove
dangerous. Solid bodies convey their heat rapidly to the human body, and
the reverse. Somebody said that a frog could not be killed by any extreme
of cold ; but when the animal was carefully dressed in tinfoil and then sub-
jected to the cold produced by a freezing mixture, the conducting power of the
metal was too much for the animal powers of the frog to resist, and he was
killed. The air during the summer months is often very hot upwards of
1 00 F. in the glare of the sunlight ; but the heat from air is very slowly com-
municated to the body, and the latter has time to neutralize the otherwise
burning heat by consuming it in work, i. e., by forcing water through the
pores of the skin, and converting it in part into vapour.
The very low conductivity of the gases is shown by some very interesting
experiments, performed by Tillet in France, and by Dr. Fordyce and Sir
Charles Blagden and others in England, and thus related by Sir David
Brewster in his charming little book called " Letters on Natural Magic :"
" Sir Charles Blagden, Dr. Solander, and Sir Joseph Banks entered a room
in which the air had a temperature of 198 F., and remained ten minutes ; but,
as the thermometer sank very rapidly, they resolved to enter the room singly.
Dr. Solander went in alone, and found the heat 210 F., and Sir Joseph entered
when the heat was 21 1 F. Though exposed to such an elevated temperature,
their bodies preserved their natural degree of heat. Whenever they breathed
upon a thermometer, it sank several degrees : every expiration, particularly
if strongly made, gave a pleasant impression of coolness to their nostrils, and
their cold breath cooled their fingers whenever it reached them.
" On touching his side, Sir Charles Blagden found it cold like a corpse; and
yet the heat of his body, under his tongue, was 98 F.
" Hence they concluded that the human body possesses the power of destroy-
ing a certain degree of heat when communicated with a certain degree of
quickness. This power, however, they concluded, varied in various media.
" The same person who experienced no inconvenience from air heated to
211 could just bear rectified spirits of wine at 130, cooling oil at 129, cooling
water at 123, and cooling quicksilver at 117. A familiar instance of this
occurred in the heated room. All the pieces of metal there, even their watch-
chains, felt so hot that they could scarcely bear to touch them for a moment,
while the air from which the metal had derived all its heat was only unpleasant.
" Messrs. Duhamel and Tillet observed, in France, that the girls who were
accustomed to attend ovens in a bakehouse were capable of enduring for ten
minutes a temperature of 270.
"The same gentlemen who performed the experiments above described
ventured to expose themselves to a still higher temperature.
" Sir Charles Blagden went into a room where the heat was i or 2 above
260 F., and remained eight minutes in this situation, frequently walking about
to all the different parts of the room, but standing still most of the time in
the coolest spot, where the heat was above 240 F.
" The air, though very hot, gave no pain, and Sir Charles and all the other
gentlemen were of opinion that they could have supported a much greater
heat.
CONDUCTION. 157
" During seven minutes Sir C. Blagden's breathing remained perfectly good;
but after that time he felt an oppression in his lungs, with a sense of anxiety,
which induced him to leave the room. His pulse was then 144 double its
ordinary quickness.
"In order to prove that there was no mistake respecting the degree of heat
indicated by the thermometer, and that the air which they breathed was ca-
pable of producing all the well-known effects of such a heat on inanimate
matter, they placed some eggs and a beef-steak upon a tin frame, near the
thermometer, but more distant from the furnace than from the wall of the
room. In the space of twenty minutes the eggs were roasted hard ; and in
forty-seven minutes the steak was not only dressed, but almost dry. Another
beef-steak similarly placed was rather over-done in thirty-three minutes. In
the evening, when the heat was still more elevated, a third beef-steak was laid
in the same place, and, as they had noticed that the effect of the hot air was
greatly increased by putting it in motion, they blew upon the steak with a pair
of bellows, and thus hastened the cooking of it to such a degree that the
greatest portion of it was found to be pretty well done in thirteen minutes.
" Sir Francis Chantrey, the late eminent sculptor, exposed himself to a tem-
perature still higher than any yet mentioned.
" The furnace he employed for drying his moulds was about 14 ft. long,
12 ft. high, and 12 ft. broad. When raised to its highest temperature with the
doors closed, the thermometer stood at 350 F., and the iron floor was red hot.
The workmen entered it at a temperature of 340, walking over the iron floor
with wooden clogs which had become charred on the surface. On one occasion
Sir Francis, accompanied by five or six of his friends, entered the furnace, and,
after remaining two minutes, they brought out a thermometer which stood at
320. Some of the party experienced sharp pains in the tips of their ears and
in the septum of the nose, while others felt a pain in their eyes."
In this very interesting account we see it was assumed by the observers that
the power of resisting the high temperature was due to some natural power or
vitality, and yet it is stated that the tips of the ears and the septa of the nose
were painfully affected. Certainly a live body resists a heat that would cook
a dead one; therefore, in the abstract, vitality or the maintenance of the
various processes inseparable from the living being, must not be wholly dis-
regarded, as without vitality none of those changes of matter could occur which
enable the living- tissues to resist the great heat ; but, after all, the " actual "
heat is converted into " potential " heat, perspiration is secreted and escapes
from the natural outlets of the body, the pores of the skin, and the lungs.
Time, of course* is an important element in these experiments, and even the
living body must succumb to any lengthened application of the great heat
already described.
Heated gases impart their heat very slowly to surrounding objects, because
the gases are bad conductors of heat. If, for the sake of discussion, we could
imagine an atmosphere composed of minute and rare atoms of silver, such an
atmosphere, if it could be breathed, would impart its heat with dangerous
rapidity to the body.
Liquids, like gases, conduct heat very slowly. The hand may be placed
within a short distance of a quantity of boiling water, and is wholly unaffected
by its dangerous neighbour. The experiment is easily tried by first placing
round a cylindrical glass, that will easily admit the hand, a large tube of
caoutchouc.
158
HEAT.
The large tube can be made in the usual manner, by cutting the edges of
the sheet of caoutchouc first, and then winding it twice or thrice round some
cylindrical vessel ; the whole, being kept together with tape, is then boiled and
allowed to cool; a large india-rubber tube is then obtained, which can be
stretched over one end of the glass cylinder and properly fixed with string ;
the hand is then inserted, and the india-rubber tube tied round the wrist.
The glass, containing the hand, is now held upright, and cold water poured
in, so that the clenched hand is covered with one inch of water. Some boiling
FIG. 163. The Hand placed in Water 'which is boiling above if.
A, section of glass cylinder, made a little funnel-shaped at the top, with the caoutchouc tube B B attached
by string to the lower part; c c, apparatus attached to the arm, and tied round tightly, so that the
water cannot escape : this must be carefully attended to, because if the cold water runs away the
boiling water will come down upon and scald the hand ; D, the red hot iron (the half of a dumb'- bell
with a hole bored through it) held by a hook.
water, coloured with a solution of indigo, is now carefully poured in down the
sides of the glass, or, better still, on a thin disc of cork, floating on the cold
water above the hand. The line of demarcation is readily seen by the differ-
ence between the colourless cold and the coloured hot water. A red-hot ball,
held by a hooked iron, is now applied to the top of the coloured water, which
will soon enter into a violent state of ebullition : the water boils at the top, but
does not communicate its heat by conduction downwards to the hand.
After the experiment has been tried, of course the arm must not be reversed
to pour out the water, or else the hand may be scalded. A syphon, protected
by a fold of flannel or paper, may be filled with cold water in the usual way,
CONDUCTION. 159
and the boiling water run off quickly. If the syphon was not covered with
some bad conducting substance, the person helping to run off the boiling water
might be inclined to leave go, when the hand inside would run a great risk of
feeling the temperature of boiling water. It is, of course, one of those experi-
ments which succeed thoroughly if all the manipulations are properly carried
out from the beginning to the end.
Another and very delicate proof of the bad conductivity of water can be
shown by fixing a differential thermometer in a cork placed in the mouth of
an inverted gas jar, and then heating the water at the top with a red-hot iron.
FIG. 164.
A A, inverted gas jar with neck c stopped with a good cork, through which the stem attached to the
differential thermometer B B passes. The jar is rilled with cold water, and heated from the top by a
common urn-heater, D.
Although the thermometer is unaffected, it does not follow that water will not
conduct heat. M. Despretz has ascertained that water will conduct heat very
slowly. The motion of the particles which is immediately set up when the
water is heated from the top must tend to destroy that similarity of molecules
which seems so desirable to secure good conductivity.
Directly any portion of the water is heated, its gravity is -altered, and it
becomes lighter; this perpetual motion of the individual particles must inter-
fere with the steady propagation of dynamical force, which has been shown to
be essential to good conductivity. It appears to be doubtful whether gases do
conduct heat : the molecules are too wide apart, and have greater mobility than
liquids. Both with liquids and gases, circulation is a necessary condition if
either are to be warmed, and hence, in speaking of the application of heat to
these forms of material substances, another term is employed, viz., "convection,"
or carrying power.
To heat a vessel of water to the boiling-point, the fire must be applied at
the bottom ; a circulation of particles immediately commences ; the expanded
or lighter particles rise by reduced specific gravity to the top, and, as they
travel upwards, convey the heat " by convection " through the other and colder
160 HEAT.
particles, which descend to take their place, and thus a constant circulation
is set up until the whole is brought to one temperature, viz., the boiling-
point, 212 F.
When Sir Joseph Banks and others experimented with the atmosphere
heated to 260, they found that if the heated air was set in motion and caused
to travel rapidly, with the aid of the bellows, over the skin, the heat soon
became disagreeable, and with dead matter (the beef-steak), at a higher tem-
perature, it was distinctly shown that the process of cooking was more rapidly
carried on when the hot air was kept in motion and its carrying power made
use of.
The same fact was observed by the Arctic discoverers, who could bear the
most intense cold, viz., minus 55, or 14 below the freezing-point of mercury,
when the air was still ; but, if set in motion, the wind, the current of air, or
the cold blast dangerously affected the extremities, which were rapidly deprived
of heat by this power of convection, and frozen or " frost-bitten."
In all schemes for ventilating and supplying heated air, circulation must, of
course, be maintained, either to impart or carry off heat.
It is said that, if the hand is kept perfectly still in water heated to a tempera-
ture of 150 F., the nerves are not disagreeably affected ; but directly the hand
is moved, then the heat becomes painful, and cannot be borne.
As an illustration of convection, or the carrying of heat, on the grand scale,
there are the trade winds and the Gulf Stream.
In the tropics the heated earth imparts some of its force to great volumes
of air, which ascend and flow towards the poles; upper currents from the
equator to the poles must be succeeded by under currents from the poles to
the equator.
The constantly ascending warm air is thus a carrier of heat to colder cli-
mates, and vice versa. These currents are modified by the various physical
conditions of the earth's surface.
In like manner, a great current of warm water, which leaves the Straits of
Florida at a temperature of 83 F., passes across the Atlantic in a north-
easterly direction. It washes the north-western shores of Europe, and makes
itself, or rather its heat-giving power, apparent by flowing round the coast of
Ireland. In mild winters in England it is the diffusion of heat by certain
winds, and the good offices of the Gulf Stream, which mitigate the severity
of the season ; and these carriers of heat are only neutralized when similarly,
but contrarily, enormous masses of ice, icebergs, are detached from the polar
regions, and rob the water of its heat on its journey to our shores.
LATENT HEAT.
CAPACITY FOR HEAT SPECIFIC HEAT HEAT OF ATOMS ATOMIC HEAT.
These somewhat difficult terms or titles, referring to truths that the young
student does not, perhaps, fully appreciate at first, nay, to speak plainer,
which he never will comprehend without industrious application to study,
are set forth in the following chapters.
In all the old standard works upon natural philosophy it is usual to state
that there are two kinds of heat that may be resident in a body, viz., one kind
LATENT HEAT. 161
called " sensible heat," which is designated as temperature, and is capable
of measurement by the thermometer and other kindred instruments ; another
and more subtle condition, not apparent to our nervous system, called " latent
heat," and incapable, whilst in that condition, of affecting any measurer or
test of " sensible heat." The dynamical theory substitutes the terms " actual
energy," or force, for that of " sensible heat," and "potential energy" for that
of " latent heat."
The one, actual heat or energy, is in use ; the qther, potential heat or energy,
is in store. A horse-shoe nail may be warmed by any convenient source of
heat, and as long as it remains above the temperature of the air we have
evidence of " actual heat."
When cold it may be hammered on an anvil, by an expert blacksmith, and
then becomes so hot it will set fire to sulphur or phosphorus. The heat thus
evoked was formerly called " latent heat," and was supposed to be combined
with the material substance of the iron ; the dynamical theory rejects the
idea of its being a distinct subtle fluid, but ascribes the heat to the motion of
the particles of the iron. It may be useful here to tabulate the new terms
used by Clausius, Rankin, Tyndall, and others, in their exposition of the
dynamical theory of heat.
ENERGY OR HEAT.
Defined to be the power of performing work. It may be latent or sensible.
Latent.
Possible energy, or work to be
done.
Potential energy is energy in store.
Sensible.
Actual energy, or work is being
done.
Dynamic energy is energy in action.
One column of terms is the exact antithesis of the other. There is no
mechanical machine by which we can tear asunder or separate the ultimate
molecules of bodies. Cohesion, or molecular force, is too potent to be over-
come by mechanical energy. Heat, another kind of energy, will, however,
act where the former fails ; therefore heat is the equivalent for mechanical
energy.
When a metal is expanded by heat, every molecule is separated or forced
asunder; the energy of heat must be enormous to overcome the force of
cohesion. When a mass of metal is heated, there is not only the motion
imparted the vibratory power set up to produce sensible or actual energy
(heat) but the molecules or atoms of the metal are pushed asunder, as
shown by their expansion. This work, which goes on inside and throughout
the mass of the metal, is not visible, and therefore may be called " interior
work."
Tyndall compares this interior work to the raising of a weight from the earth
the overcoming of the force of gravity, which attracts all things, and keeps
all terrestrial bodies in their places. The raising of a weight by a cord from
the earth, it is clear, confers " a motion-producing power." The weight can
fall, and in its descent can perform work. Whilst hanging in the air, it
represents possible energy, or " potential " energy.
The pull, or attraction of gravity, causes this possible or "potential"
energy. If there were no attraction between the substance and the earth,
there would be no " possible " energy.
Substitute the ultimate atoms of bodies for the weight and the earth :
11
102 HEAT.
remember that the atoms of solid bodies are held together with molecular
force (cohesion), and it must be evident that whenever they are separated,
although the distance to which they are separated cannot be measured it is
too minute still the fact remains, and when the atoms come together it is
like the fall of the weight to the earth, and the result must be the production
of actual energy, or heat.
This is what Tyndall means when he speaks of the clashing together of
the atoms.
The heating of the cold horse-shoe nail by hammering, or the heating of
cold bars by rolling, is simply the conversion of mechanical energy into
molecular motion ; if the approach of the molecules of a body will produce
actual energy, a still nearer approach must increase that energy, or heat.
Indeed, the experiment already quoted, of heat produced by hammering and
bringing the atoms nearer together, is a good illustration of the above
argument.
The " specific heat" (a term that must be carefully considered presently) of
a metal like copper is altered when a nice, soft, well-annealed piece is ham-
mered: heat is produced, and the specific heat changes from 0*09501, 0*09455,
to 0*09360, 0*09330; and its specific gravity or density becomes higher.. When
again heated red hot and allowed to cool slowly, as is done in the process of
annealing, its specific heat returned to 0*09493, 0*09479, or very nearly
the same that it was at first. Thus by alternately hammering and then
heating or annealing a metal, the atoms are brought more closely together
or pushed further apart. When the atoms are pushed further apart, the
heat becomes potential or latent ; when advanced nearer to each other, the
heat is actual or sensible. Nearly every philosopher selects a particular
subject to which he devotes his special attention. Let us read what Dr.
Tyndall says of latent heat in his standard work, " Heat a Mode of Motion."
" We shall now direct our attention to the phenomena wjtiich accompany
changes of the state of aggregation. When sufficiently heated, a solid melts ;
and when sufficiently heated, a liquid assumes the form of gas. Let us take
the case of ice, and trace it through the entire cycle. This block of ice has
now a temperature of 10 C. below zero. I warm it ; a thermometer fixed in
it rises to o, and at this point the ice begins to melt ; the thermometric
column, which rose previously, is now arrested in its march, and becomes
perfectly stationary. I continue to apply warmth, but there is no augmenta-
tion of temperature ; and not until the last film of ice has been removed from
the bulb of the thermometer, does the mercury resume its motion. It is now
again ascending ; it reaches 30, 60, 100 C. ; here steam-bubbles appear in
the liquid ; it boils, and, from this point upwards, the thermometer remains
stationary at 100. But during the melting of the ice, and during the evapo-
ration of the water, heat is incessantly communicated. To simply liquefy the
ice, as much heat is imparted as would raise the same weight of water 79*4 C,
or as would raise 79*4 times the weight one degree in temperature ; and to con-
vert a pound of water at 100 C. into a pound of steam at the same temperature,
537*2 times as much heat is required as would raise a pound of water one
degree in temperature. The former number, 79*4 C. (or 143 F.), represents
what has been hitherto called the latent heat of water ; and the latter number,
537*2 C. (or 967 F.), represents the latent heat of steam.
" It was manifest to those who first used these terms, that throughout the
entire time of melting, and throughout the entire time of boiling, heat was
LATENT HEAT. 163
communicated ; but inasmuch as this heat was not revealed by the ther-
mometer, the fiction was invented that it was rendered latent. The fluid of
heat was supposed to hide itself in some unknown way in the interstitial
spaces of the water and the steam.
" According to our present theory (the dynamical), the heat expended in
melting is consumed in conferring potential energy upon the atoms : it is
virtually the lifting of a weight. So likewise as regards steam, the heat is
consumed in pulling the liquid molecules asunder conferring upon them a
still greater amount of potential energy.
" When the heat is withdrawn, the vapour condenses, the molecules again
clash with a dynamic energy equal to that which was employed to separate
them, and the precise quantity of heat then consumed now re-appears.
" The act of liquefaction consists of interior work expended in moving the
atoms into new positions. The act of vaporization is also, for the most part,
interior work; to which, however, must be added the exterior work of
forcing back the atmosphere, when the liquid becomes vapour
Let us then fix our attention upon this wonderful substance, water, and trace
it through the various stages of its existence. First, we have its constituents
as free atoms of oxygen and hydrogen, which attract each other, fall or
clash together. The mechanical value of this atomic act is easily determined.
The heating of i Ib. of water i C. is equivalent to 1,390 foot-pounds ; hence
the heating of 34,000 Ibs. of water i C. is equivalent to 34,000X1,390 foot-
pounds.
" We thus find that the concussion of our i Ib. of hydrogen with 8 Ibs. of
oxygen is equal, in mechanical value, to the raising of forty-seven million
pounds one foot high.
" I think I did not overstate matters when I stated that the force of gravity,
as exerted near the earth, is almost a vanishing quality, in comparison with
these molecular forces.
" The distances which separate the atoms before combination are so small
as to be utterly immeasurable ; still it is in passing over these spaces that
the atoms acquire a velocity sufficient to cause them to clash with the tre-
mendous energy indicated by the above numbers. After combination, it is in
a state of a vapour, which sinks to 100 C, and afterwards condenses into
water. In the first instance the atoms fall together to form the compound ;
in the next instance the molecules of the compound fall together to form a
liquid. The mechanical value of this act is also easily calculated. 9 Ibs. of
steam, in falling to water, generate an amount of heat sufficient to raise
537*2X9 4,835 Ibs. of water iC, or 967X9=8,703 Ibs. iF. Multiplying the
former number by 1,390, or the latter by 772, we have in round numbers a
product of 6,720,000 Ibs. as the mechanical value of the mere act of con-
densation.
" The next great fall is from the state of liquid to that of ice, and the
mechanical value of this act is equal to 993,564 foot-pounds. Thus our 9 Ibs.
of water, at its origin and during its progress, falls down three great pre-
cipices ; the first fall is equivalent in energy to the descent of a ton weight
down a precipice 22,320 feet high ; the second fall is equal to that of a ton
down a precipice 22,900 feet high ; and the third is equal to the fall of a ton
down a precipice 433 feet high.
" I have seen the wild stone-avalanches of the Alps, which smoke and
thunder down the declivities with a vehemence almost sufficient to stun the
11 2
1 64 HEAT.
observer. I have also seen snow-flakes descending so softly as not to hurt the
fragile spangles of which they were composed ; yet to produce from aqueous
vapour a quantity, which a child could carry, of that tender material, de-
mands an exertion of energy competent to gather up the shattered blocks of
the largest stone-avalanche I have ever seen, and pitch them to twice the
height from which they fell."
CAPACITY FOR HEAT.
This term, which is most simple and useful, expresses a fact that has been
forced upon observers by numerous experiments made with the thermometer.
The thermometer is usefully applied to determine the temperature of any solid,
fluid, or gaseous matter ; but it will not tell the observer how much heat or
actual energy is contained in different measures of the same fluid. A gallon
of water in one vessel, and a pint of water in another, may be shown by the
thermometer to have a temperature of 212 F. ; but the quantity of energy or
heat must be much greater in the larger measure the one gallon than in the
single pint. The thermometer fails to show the quantity of energy, whilst it
gives relatively the "relative actual heat" the "temperature." A photo-
meter, or measurer of light, will demonstrate the relative illuminating power of
any given source of light ; but it cannot give the number of vibrations per
second producing the light. A thermometer can tell us truthfully how m^.ch
hotter or colder than 32 or 212 F. a substance may be ; but it cannot inform
us what may be the amount of vibratory power given, and the molecular force
detached, which', according to the dynamical theory, must be the equivalent
for the expression or quantity of heat. There are certain facts, explain them
how we will, which are indisputable. If 10 Ibs. of water (one gallon) at 100
F. are mixed with the same weight of oil at 50 F., the resulting temperature
will not be the mean, 75 F., but 83-^ F. The water, therefore, has lost
"actual energy" equal to i6|; but the same energy has caused the oil to
rise 33f .
If the experiment is reversed, and 10 Ibs. of oil at 100 are mixed with 10 Ibs.
of water at 50, the mean will be 66|: the 33^ actual heat or energy given
out from the oil is only able to raise the temperature of the water i6f.
The actual energy which will raise the temperature of oil 2 will raise an
equal quantity of water only i. The heat that will raise any given substance
from o C. to i C., compared with the amount of " energy" required to heat
an equal weight of water to the same point, is called its "specific heat."
Therefore the specific or potential heat of oil will be a half, '5, as compared
with the unit or one viz., water.
As the oil has been quickly heated, so it will rapidly cool ; it has only
half the "energy of heat" possessed by water to give up. If the water
require one hour to cool to any given temperature, the oil would reach the
same point in half-an-hour.
Hence "time" is the test used sometimes to determine the specific heat of
bodies the time required by a substance to cool. Or the process may be
reversed by ascertaining the quantity of ice which exactly equal weights of
other bodies can melt in falling from one temperature to another, say from
the boiling-point to the freezing-point of water. As the process of mixture
already described with the oil and water may be employed, there are there-
fore three methods by which the specific heat of bodies may be determined :
LATENT HEAT. 165
1. The direct method by mixture.
2. Time required to cool, and rate of cooling.
3. Heating of ice, and quantity liquefied by a given weight of the substance
heated to 212 whilst falling to 32.
By the first method viz., mixture or immersion the distinguished phy-
sicist, Regnault, arrived at the following results :
"SPECIFIC HEATS OF EQUAL WEIGHTS BETWEEN O C AND IOO C.
Water .... rooooo
Oil of turpentine . . 0*42593
Charcoal. . . . 0*24150
Brass . . . . 0*09391
Silver .... 0*05701
Tin . . . 0*05623
Glass .... 0*19768 I Mercury .... 0*03332
Iron , . . 0*11379
Zinc .... 0*09555
Copper .... 0*09515
Aluminium . . . 0*21430
Platinum . . . 0*03243
Gold . . . . 0*03244
Lead . . . . 0*03140
Bismuth .... 0*03080
For a lecture-table experiment there are none better than that devised by
Tyndall, to show the time required by equal spheres of various solids, heated
to the same temperature, to melt their way through a cake of beeswax.
The metals used are iron, lead, bismuth, tin, copper : these are shaped as
balls or spheres, and each furnished with a hook for conveniently removing
them from the oil, in which they are heated to a temperature of i8oC.
A framework of wood, shaped like the spokes of a wheel, with five strings,
to which the balls are attached, may be used in order to remove the whole of
the balls at once from the heated oil.
When they are laid upon a cake of beeswax, 6 in. in diameter and half an
inch thick, supported on the ring of a tripod or other convenient means of
support, the iron and the copper balls go through first, the tin next, while
the lead and bismuth are retained. If they contained the same amount of
heat, or had the same " actual energy," they would all go through the wax in
the same time : the difference in their specific heats determines the rate at
which they perforate the wax.
Messrs. Dulong and Petit have shown that the specific heat of bodies
increases as their temperature rises. Any given substance will require more
heat to raise it a certain number of degrees when at a high than at a low
temperature. The variations of specific heat according to temperature are well
shown in the case of iron.
SPECIFIC HEAT OF IRON (DULONG AND PETIT).
From 32 to 212 . . . 0*1098
392 . . . 0*1150
572 ... 0*1218
666 ... 0*1255
In a similar manner the specific heat of the gases has been carefully deter-
mined, the methods employed involving one of the three modes already
described. De la Roche and Berard caused a measured volume of the gas
under examination, when heated to a fixed temperature and kept at a uniform
heat, to pass through a spiral glass tube surrounded with water (this plan
would be equivalent to the " mixture " of oil and water), and, by observing the
increase of the temperature of the water surrounding the spiral tube, and other
data, they determined the specific heat of certain gases.
i66
HEAT.
Dr. Apjohn devised another method, viz., that of vaporizing water by a
current of the heated gases, and, by inverse proportion, viz., the greater the
specific heat of the gas, the less time required to cool it, and vice versa, he
has given the specific heats of gases already examined by De la Roche ; but
unfortunately the figures of the two experimentalists did not agree, and there-
fore a more careful investigation was made by Regnault, who, taking the
specific heat of an equal weight of water as the unit of comparison, commences
with air, and gives the following table of the specific heats of a number of
gases and vapours with which he experimented ; and, what is still more valu-
able, the table gives the specific heat of equal volumes and weights of the
bodies examined :
SPECIFIC HEAT OF GASES AND VAPOURS.
GAS OR VAPOUR.
Equal
GAS OR VAPOUR.
Eq
Vols.
ual
Weight.
Vols.
Weight.
Air ...
0-2375
0-2375
Sulphurous an-
Oxygen
0'2405
0-2175
hydride .
0-341 1 0*1540
Nitrogen .
0-2368
0*2438
Hydrochloric
Hydrogen .
0-2359
3-4090
acid
0-2352
0-1842
Chlorine
0-2964
0"I2IO
Sulphuretted hy-
Bromine .
0-3040
0-0555
drogen .
0-2857
0-2432
Nitrous oxide .
0-3447
0-2262
Water
0*2989
0-4805
Nitric oxide
0-2406
0-23I7
Alcohol
0-7171
0-4534
Carbonic oxide .
0-2370
0-2450
Wood spirit
0-5063
0-4580
Carbonic anhy-
Ether .
I-2260
0-4796
dride
0-3307
0-2163
Ethyl chloride .
0-6096
0-2738
Carbonic disul-
Ethyl bromide .
0-7026
0-1896
phide
0'4I22
0-1569
Ethyl di sulphide
I '2466
0-4008
Ammonia .
0*2996
0*5084
Ethyl cyanide
0*8293
0-4261
Marsh gas .
0-3277
0-5929
Chloroform
0-6461
0-1566
Olefiant gas
0-4106
0-4040
Dutch liquid
079II
0-2293
Arsenious chlo-
Acetic ether
1*2184
0-4008
ride
07013
0'II22
Benzol
I'OII4
o*3754
Silicic chloride .
07778
0'1322
Acetone
0*8341
0-4125
Titanic chloride
0*8564
0*1290
Oil of turpentine
2.3776
0-5061
Stannic chloride
0-8639
0*0939
Phosphorous
chloride .
0*6386
0*1347
Regnault's experiments confute those of De la Roche and Berard, and
deny that the specific heat of air and all gases rises with the temperature.
Regnault's experiments were carried on with air between the limits of tem-
perature expressed by 30 C. and 200 C. The same result was obtained with
gases like hydrogen, which cannot be easily liquefied ; and the specific heat
was not found to increase with the temperature, at least between 30 C. and
200 C. A gas which can be easily condensed, such as carbonic acid, shows, in
accordance with the statement of De la Roche and Berard, an increased
specific heat with an increased temperature.
LATENT HEAT. 167
SPECIFIC HEAT OF CARBONIC ACID AT DIFFERENT TEMPERATURES.
Between 30 and 8 C. . .' specific heat 0-18427
- 8 100 . . . 0-20248
- 8 210 ... 0-21692
Regnault also discovered that the specific heat of a given volume of a gas
increases directly as its density is increased; and his valuable experiments
show that the specific heat of the same liquid varies with the temperature.
There exists a remarkable connection between specific heat and atomic
weight, which has given rise to another term "atomic heat." This expression
means the product obtained by multiplying the specific heat of a body by its
atomic weight.
The specific heat of an elementary body is inversely as its combining pro-
portion. Regnault discovered in upwards of twenty bodies chemically pure,
that the atomic heat ranged between 3-31 and 2*93, giving a mean of 3-13.
Hence, if the above number 3*13 is divided by the number expressing the
specific heat of iron, lead, mercury, tin, &c., the quotient gives very nearly
the atomic weight of the metal.
The term " atomic weight" must not be confounded with the term " chemical
equivalent:" the latter is obtained by direct experiment, and means the com-
bining proportion of the various elements, as, for instance, i being taken as
the combining proportion or equivalent for hydrogen, 16 will be that of
oxygen; or I of hydrogen may displace 65 of zinc: hence the former is equiva-
lent to the latter.
Atomic weight is a product arrived at by calculations carried out in various
ways, as, for instance, when the number 3-13 is divided by the specific heat of
a metal.
Atomic weight is also arrived at by other methods ; it may sometimes coin-
cide with the combining proportion, or equivalent number, or it may be a
multiple of it. ,
"Actual energy" (heat) disappears during liquefaction. When matter
passes from the solid to the liquid state, " actual " is converted into " poten-
tial energy;" and the heat is said to disappear, and cold is produced. It is
the enormous amount of actual heat, so slowly converted into potential heat,
that prevents the sudden liquefaction of ice or snow, and the great damage
which would occur to property if the snow could be quickly melted. Con-
versely, when a liquid is changed to the solid state, the closer proximity of the
molecules, the merging together of the particles by cohesion, converts the
" potential " into " actual " heat ; and thus the very change of water into
snow or ice produces actual energy, or heat, and helps to mitigate the effect
of a sudden frost.
Taking the fact (irrespective of theory) that liquefaction will produce cold,
there are various solids and mixtures of solids which will produce a sufficiently
low temperature, when quickly dissolved in water, to freeze water contained
in a vessel surrounded with the mixture. The mere solution of nitre alone
will lower the temperature of water from 50 to 35 F. Four ounces of nitre
and four ounces of common sal ammoniac dissolved in four ounces of water
reduce the temperature from 50 F. to 10 F. A mixture of equal parts of
snow, or powdered ice, and salt will sink the thermometer from 32 F. to o,
or 32 degrees below the freezing-point of water ; and two of snow and one of
salt reduce the temperature to 4 F. A mixture of three parts by weight of
i68
HEAT.
chloride of calcium and two of snow will reduce the temperature from 32 F.
to 50 F. ; and by powdering and carefully cooling the chloride to 32 F., and N
using very thin vessels, mercury can be frozen. The liquefaction of a met-.llic
alloy, composed of
'207 parts by weight of lead,
118 ' tin,
284 bismuth,
in 1,617 parts of mercury, will sink the thermometer from 63 to 14; and, of
course, water can be frozen by this process.
One of the most interesting experiments is that of Mousson, who contrived
an apparatus by which ice was subject to a pressure equal to thirteen thousand
atmospheres, and by which its bulk was reduced by thirteen-hundredths of
that which it occupied at o C. (32 F.).
The temperature of the ice was first reduced 20 C. ( 4 F.), and then
subjected to the pressure of a copper rod, worked by a very powerful screw.
FIG. 165. A Still, with " Still Head? and the Worm surrounded by Cold
Water.
Instead of increasing the solidity of the ice, the mechanical compression and
motion of the molecules liberated the equivalent in actual energy or heat ; the
ice liquefied, and the copper rod was found to have fallen to the bottom of
the water, which again solidified directly the pressure was removed.
The freezing-point of water is lowered to a minute extent by pressure.
IV. liquid alloy of sodium and potassium is easily obtained by pressing
pieces of the two metals together : if this liquid be brought into contact with
mercury, the amalgam instantly solidifies and becomes hard ; at the same
time so much heat is liberated that incandescence is apparent at the point
where the metals come in contact, and any combustible fluid, such as naphtha,
may be set on fire. Liquefaction produces cold ; congelation or solidification,
heat.
If liquefaction is pressed further by the addition of more heat, the water
is converted into vapour, the molecules are thrust wider apart, and " actual
heat" disappears.
EBULLITION. -169
This is demonstrated very conclusively in the distillation of water. The
heat is applied to the bottom of the vessel containing the water, and when it
has once reached the boiling-point, 212, the steam the vapour (also at 212)
carries off all the heat of the burning coals ; the heat disappears ; the ther-
mometer, inserted in the still, remains stationery. When the steam is passed
through the condensing apparatus the coil of pipe, called the worm, sur-
rounded by cold water, and contained in what is called the worm-tub the
heat or energy which it carries off from the fire becomes apparent ; the stored
heat is so large in quantity that it soon raises the temperature of the water in
the worm-tub, and the quantity of water in the tub, which may be raised to
212 F., is much larger than the water condensed. The stored "heat"
(already so often spoken of as " potential heat ") in the steam becomes
" actual " energy when the vapour passes to the liquid condition of matter ;
and this heat, as already described, is so great, that it may be conveniently
applied in the warming of buildings.
The conversion of water into vapour by the method already described is
progressive, and unattended with danger. If the water could be suddenly
converted into steam, and the specific heat of steam was not so high, the
attempt to boil water must always end disastrously, because it would be
generated suddenly and explosively ; the steady " ebullition," or escape of
bubbles of steam, as the cohesion of the molecules is gradually overcome,
would not be maintained. The escape of air from water, heated to 212 F., is
very apparent when it is boiled in a flask. Tyndall says the air acts as a
kind of elastic spring, pushing the atoms of the water apart, and thus helping
them to take a gaseous form.
The cohesion of the particles of water appears to be greatly increased when
the foreign matter viz., atmospheric air is removed. Thus, water allowed
to fall through a tube from which the air has been ejected by boiling the
water, and melting the glass and hermetically sealing the end, falls col-
lectively, making a noise, and would break through the end of the glass tube
like a solid substance. The vacuum-tube containing the water is called " the
water-hammer," and if altered in shape by bending it into a V-shaped
figure, nicely rounded off at the bend, some very amusing illustrations of the
modification of the cohesion of the water and adhesion to the glass can be
displayed.
The mechanical nature of the interior of a vessel in which steady "ebulli-
tion " is to be maintained greatly affects the escape of the vapour or steam.
If the interior surface is too smooth, like that of a flask, and distilled water
boiled therein, the flask is said to bump, i.e., the temperature of the boiling
water rises a degree or so above the boiling-point, and every time steam is
formed it escapes with a sudden jerk, as if it were a slight explosion, and the
temperature falls to 212, again rising and falling with each rush of vapour.
When this occurs, it may be instantly corrected by dropping in any metallic
filings, zinc or copper, or by placing in the flask a bit of crumpled platinum-
foil. The rough edges break up the continuity of the smooth surface of the
glass, and serve to conduct the heat of the lamp into the particles of the water,
and thus to hasten the disruption of their cohesive power. It is easy to follow
out the idea further by lining a copper vessel with shellac. Water placed in
a vessel prepared in this manner will not boil until it attains a temperature
of 219 F.," /.<?., seven degrees above the ordinary boiling-point. Bursts of
steam occur, the temperature falling after each escape of vapour to 212. The
170
HEAT.
hard copper is not in direct communication with the water ; there is an
intermediate non-metallic body which adheres to it and becomes soft It is
this intermediate physical condition, neither solid nor fluid, but partaking of
the physical nature of both, which interferes with the energy of heat, and
assists to maintain the cohesion of the water.
Monsieur Donny, of Ghent, has studied this subject very carefully, and has
shown that the presence of air is of great importance in maintaining steady
ebullition and escape of vapour from water ; the tiny volumes of air expand by
heat, and into these bubbles the steam passes, expands, and rises.
All spring and river water contains air in solution, and, as steam-boilers
are constantly fed with fresh water, the supply of bubbles of air goes on con-
tinually.
That the presence of air in solution does assist the escape of the steam is
proved by the explosive nature conferred on water after it has been boiled for
a lengthened period, so as to get rid of and drive off the dissolved air.
Under these circumstances the temperature of the water rises to 360 F., or
148 above the boiling-point ; and such was the violence with which the
steam escaped, that an open glass vessel was shattered with a loud report.
FIG. 166. Faraday's Experiment Soiling Water deprived of Air
under Oil of Turpentine.
A, the tube containing the oil and ice; B, the spirit-lamp; c, the screen of blotting-paper to receive
the water and oil when ejected explosively.
In great manufactories, boilers " banked up," and kept gently boiling from
Saturday night to Monday morning by a slow expenditure of fuel, have
exploded without warning, and without the engineer having the slightest
conception of any dangerous accumulation or pressure of steam. Amongst
the precautions taken to prevent accidents is one suggested by the recollection
of this property of water; and means should be taken to allow a small
quantity of fresh cold water to pass continually into all boilers during the
intervals of rest, and especially into locomotives which are sometimes kept
" banked up " and ready for service.
When water freezes, the air, by the compression of the particles, is squeezed
out, and none remains in solution. If a piece of Wenham or clear Norwegian
BOILING-POINT OF WATER.
171
ice is placed in a tube and surrounded with oil of turpentine, and then care-
fully-melted and heated, the boiling-point is raised very high, and, directly
steam is generated, the whole contents of the tube are ejected. This experi-
ment was first shown by Faraday at the Royal Institution. (Fig. 166.)
On the principle that the more we increase cohesive force, the greater must
be the power of resisting the energy of heat, is explained the rise in the
boiling-point of saline solutions. A saturated solution of nitrate of soda boils
at a temperature of 249*5 F. ; the quantity of salt being 224*8 parts in 100
of water, or more than double the weight of the solvent. Faraday and
Magnus have both shown that the steam arising from the boiling saline solution,
although escaping at a temperature of 249*5 F., speedily
and almost instantaneously adjusts itself to the atmo-
spheric pressure indicating only the ordinary tempera-
ture of steam 212 F. When it is said that water boils
at 212 at the ordinary pressure, it is meant that the
energy of heat, represented by the steam, cannot exert
itself, cannot even help the vapour to escape, until it
has overcome the pressure of the air, or weight equal
to fifteen pounds upon the square inch. The lifting
power or energy of heat is well illustrated by this sim-
ple fact ; and directly the pressure is partly removed,
the amount of energy, or heat, represented by the boil-
ing-point is reduced, and the water will enter into
ebullition at a lower temperature. The pressure of the
air is represented by the height at which a column of
mercury is supported: when the mercury is 16*6 inches
high, water boils at 184 F. ; if the pressure is doubled,
and the barometer, the column of mercury, stands at
32 '3 inches, water boils at 216 F. The difference be-
tween 1 6*6 inches and 32*3 inches is very great, and it
might be thought that such a fall in the barometer could
only be demonstrated by artificial means, and by the
creation of a partial vacuum with an air-pump. But it
must be remembered that there are certain spots on
the surface of the globe where the adventurous traveller
may ascend nearly three miles above the level of the
sea.
The famous De Saussure ascended to the summit of
Mont Blanc, which is 15,650 feet above the level of the
sea, and where water boils at a temperature of 185*8 F.,
FlGS. 167 and 1 68. Apparatus for determining Elevations by the Tempera-
ture of the Boiling-point of Water.
172 HEAT.
and the barometer stands at about 17 inches. The boiling-point of water is
lowered about one degree for every 590 feet. Dr. Saussure's observations
were verified by Tyndall in August, 1859, when the temperature of boiling
water at the summit of Mont Blanc was found to be 184-95 F.
It is by the careful observation of the temperature at which water boils
that the height of any hill or mountain may be determined. Since Dr Wol-
laston constructed his instrument for measuring heights by the observation of
the boiling-point, improvements have been made, as shown in Fig. 167.
The Barometrical Thermometer, or Hypsometrical Apparatus, as con-
structed by Negretti and Zambra, is intended to meet the requirements of
travellers in circumstances where the mercurial barometer cannot be conve-
niently employed. The instrument is very portable, and affords a ready and
accurate means of measuring heights by observation of the temperature of
boiling water. The apparatus is shown in Fig. 167. It consists,
First, of a very delicate thermometer, about 12 in. long, the scale ranging
from 1 80 to 212, having each degree subdivided, so as to show distinctly o'i.
Secondly, a copper boiler, C, attached to a small tripod stand. From the
boiler proceeds three double tubes, E E E and D D D, open at top ; screwed
on the top of the boiler; the outer tube has two openings, one at the top,
through which the thermometer E E is inserted, passing down to within an
inch of the water in the boiler, and supported by means of an india-rubber
washer, as shown in Fig. 167, the second opening forming an outlet for the
steam, as shown at G. The object of the double tube is to insure a steady
boiling-point, which it would be impossible to obtain in open-air experiments,
were only a single tube employed. A is a metallic spirit-lamp, surrounded
with wire gauze, B, to prevent the flame being extinguished when experi-
menting in the open air. The whole instrument, when packed for travelling,
is shown, drawn to a smaller scale, in Fig. 168. Each instrument is furnished
with a carefully computed set of tables, from which may be obtained, by an
easy calculation, the elevation corresponding to any observed boiling-point
between the temperatures of 180 and 212.
To use the boiling-point apparatus, it is simply necessary to pour into the
boiler, through the small opening F, on its surface, a sufficient quantity of
water to fill it about one-third, and afterwards close it by means of the screw
for that purpose ; the lighted spirit-lamp is then applied, and when the water
is made to boil, the steam rises, surrounding the bulb and tube, and, descend-
ing between the two tubes, issues from the opening at G. After a few seconds,
the mercury in the thermometer will rise and become stationary ; the degree
indicated by it must then be noted, when, by reference to the tables, the
elevation of the spot where the experiment has been performed may be obtained.
STEAM.
If water boils at a lower temperature when the ordinary pressure of the air
is reduced, it should, of course, indicate a higher temperature when the pres-
sure is increased.
Steam, escaping from an open vessel, is usually at a temperature of 212;
but it must always be remembered that the barometer shows that the pressure
of the air is constantly varying, and, even within the limits of the range of. the
STEAM.
173
barometrical indications in our climate, the boiling-
point of water may vary nearly five degrees.
The temperature of steam is always the same as
that of the water from which it is evolved. Conse-
quently, if water is confined in a closed and strong
vessel, the temperature of the water may be raised as
high as the strength of the vessel will permit.
Marcet's boiler is a very useful and safe piece of
apparatus for demonstrating the rise of the tempe-
rature of the steam as the pressure is increased. When
the water has been poured into the boiler, and the
heat of the spirit-lamp applied, it soon boils ; and, if
the stop-cock remains open, the temperature is shown
to be 212 F., and, of course, no mercury rises in the
barometer tube. If, however, the stop-cock is closed,
the rise of the mercury in the barometer is simultane-
ously Accompanied with an elevation of temperature,
indicated by the thermometer; and when the mercury
rises to thirty inches, it demonstrates that the pressure
is doubled, and amounts to thirty pounds upon the
square inch, because there is not only the pressure of
the air, but the weight of the mercury to be over-
come, before the latter can be pushed up the open
tube ; and looking at the thermometer, it will now be
found to stand at 250-5 F.
The question of the exact pressure which accom-
panies a rise of temperature in the boiling-point of
water, and simultaneously of the steam escaping from
it, was very properly made the subject of careful
scientific inquiry by the Academy of Sciences at
Paris, many years ago, by MM. Dulong and Arago.
They obtained facts by experiment up to 25 atmo-
spheres, and from the data so obtained calculated the
temperature and pressure up to fifty atmospheres, or
50 X 1 5 =750 pounds upon the square inch; giving, by
calculation, a temperature of 510-4 F.
FIG.
A, a strong brass globe, made of two hemispheres screwed together with flanges, and supported on
a tripod stand; B, the barometer tube passing through a steam-tight collar, and touching the bottom
of the boiler, in which sufficient mercury to fill the tube and cover the end of the barometer tube is-
placed; c, the thermometer graduated to 400 F., and passing, like the barometer tube, through a
steam-tight collar. D is the stop- cock ; E, a spirit-lamp.
FORCE AND TEMPERATURE OF STEAM.
Atmosphere. Temperature. Atmosphere. Temperature.
1 2I2'00 F. 9
2 250-52 10
3 275-18 ii
4 29372 12
5 307*50 13
6 320-36 14
7 33170 15
8 34178 16
35078
358-88
366-85
374-00
380-66
386-94
392-86
398-48
i 7 4 HEAT.
Atmosphere,
Temperature.
Atmosphere.
Temperature.
I?
403-82 F.
22
427-28 F.
18
408-92
23
43 1 '42
19
41378
24
435 '56
20
418-46
25
439'34
21
422-96
The above temperatures and pressures apply only to steam in contact with
water. " Dry steam " is affected by heat precisely in the same manner as the
permanent gases.
The energy called heat is, as we have seen in the remarkable experiment of
Groves, capable of application until a body is decomposed into its elements
(seep. 152, "The Decomposition of Steam by Heat intoOxygen and Hydrogen").
It is not then surprising that the instrument called Papin's digester should
exert such a powerful solvent action upon matter subjected to the high tempera-
ture of steam produced by confining and heating water in a very strong vessel.
In using an ordinary still for obtaining distilled water, supposing one
gallon of distilled water to be obtained, and the steam representing that mea-
sure of water to have been passed into five gallons and a half of ice-water
viz., water at a temperature of 32 F., the energy or heat carried up from the
fire, and converted for a brief space, in passing from the still to the worm,
into potential or stored force, is so great that it will raise the 5|- gallons of
water at 32 to 212 F. when condensed or converted into actual energy or
heat. The elasticity of the molecules of water must be enormous, to permit
the vibratory power or energy called heat to separate them so widely apart.
By the same amount that they are separated, so they must return. The act of
unlocking, or conversion into steam, is followed by condensation the locking
of the molecules, and the production from that motion of an enormous amount
of heat, usually spoken of as "latent heat" a term that may be usefully re-
tained so long as the cause, " motion," is not lost sight of. The " latent heat "
of vapour is a question of considerable importance. The illustrious Watt
observed by experiment that the same weight of steam, whether it escapes at
212 or 300 F., exhibits very nearly the same amount of heating power or
latent heat ; and although Regnault, by more elaborate experiments, has
determined " that the total quantity of heat necessary for the evaporation of
water increases with the temperature," it is found in practice that Watt's con-
clusion, that the latent heat of steam is increased in the proportion that the
" sensible heat " is absorbed, is sufficiently correct for ordinary working
purposes.
A given weight of steam at 212, condensed at
32 F., evolves 180 sensible heat,
950 latent heat.
1130
The same weight of steam at 250 . , .218 sensible heat,
912 latent heat.
1130
STEAM.
175
The same weight of steam at 100
68 sensible heat,
1062 latent heat.
1130
Regnault's experiments show that the total quantity of heat necessary to
evaporate water at 100 C. (212 F.) is equal to 637 ; at 120 C, it is 643 ; at
150 C., it is 651. These conclusions are at variance with those arrived at by
Watt, but, as already stated, are too minute to affect the main question.
To work out the figures representing the latent heat of steam, a simple
arrangement of apparatus may suffice.
FlG. 170. Flasks arranged to show the Latent Heat of Steam.
Thus, supposing each flask to contain eight ounces of water at 60 F., and
the steam from one of them be conducted into the other until the temperature
is raised to 188 F., or increased 128, it will be found that one flask has lost
one ounce of water, which the other has gained.
The whole heat carried over with the one ounce of steam into the eight
ounces of water will, therefore, be I28x8 = 1024.
But the 1024 cannot be all regarded as latent heat, because the steam, whilst
condensing, should have raised the water to 212 F. ; therefore, 188 F. must
be deducted from 212 F., which will leave 24; and now 1024 24= 1000,
the latent heat of steam.
When the steam is allowed to escape from the Marcet boiler (p. 173) at a
pressure of two atmospheres, and at a temperature of 250*5 F., it would be
imagined that the steam must severely scald the hand if held in the jet whilst
escaping under these circumstances. Curious to say, this is not the case : the
steam,"as it escapes, is comparatively cool, and the hand may be held in it with
perfect impunity.
i 7 6
HEAT.
Here expansion takes place ; the particles of the vapour of the water are
closely packed and squeezed together; the steam, whilst inside the boiler, is of
greater density ; the heat apparent is " actual force," but directly it escapes
work is consumed in the return of the vapour to its normal state of pressure,
and thus the heat becomes " potential," or is rendered latent or insensible.
Any gas or vapour in the act of expanding, that performs work, consumes,
heat.
If air is compressed in a strong cylinder and allowed to escape, the elastic
force which pushes out the air represents work ; and as heat is consumed, the
stream of air is found to be cold. When air is forced out of the nozzle of a
common pair of bellows, the air is slightly warmer than the external air, be-
cause it is the human muscles that do the work: it is the stoppage of the
motion of the air from the bellows that produces the slight increase of heat.
It was not the elastic force of the air behind the escaping portion (as with the
compressed air in the iron vessel) that caused the air to escape from the bellows;
it was human strength, and if that had not been sufficient no air would have
escaped : a baby cannot work a pair of bellows. It was fprmerly taken for
granted that in every case where gases and vapours expand cold must be pro-
duced ; but Gay-Lussac and J. P. Joule have clearly proved that you may have
expansion without producing cold, provided no work is performed.
FIG. 171. Gay-Lussac 's Experiment.
A, B, two copper cylinders; c, the connecting-pipe and stop-cock.
This experiment proves very beautifully that where no work is performed
there is no cold ; and in this experiment gas is allowed to expand without doing
work.
The vessel A is first exhausted, and the other, B, left full of air ; when the cock
is turned, the air rushes out of B into A. The air which formerly filled B is
now divided between B and A, and, if pumped back, would again fill B. The
half, in expanding from B into A, has performed work, and consumed heat. It
is cold ; but, striking against the interior of the copper vessel A, its motion is
stopped, and heat is generated. The heat produced in A by the arrest of motion
is exactly equivalent to the loss sustained in B by work, by the exertion of the
elastic force; hence the two effects of cold and heat neutralize each other, and
STEAM. 177
the temperature of the air in the two vessels, when thoroughly mixed, remains
unaltered. There is no work performed, and no heat lost.
A still more satisfactory experiment was performed by J. P. Joule. He com-
pressed air with a force equal to twenty-two atmospheres into a metallic vessel
he had twenty-two atmospheres squeezed into a space usually containing one
only ; he pumped the air out of a similar metallic vessel, producing a vacuum.
The vessels were connected, like Gay-Lussac's, with a tube and stop-cock, and
surrounded with water. On turning the cock, the air expanded from one vessel
into the other, and, by keeping the water surrounding both vessels properly
stirred, no increase or decrease of heat was observed in the water. The heat
or cold in one vessel exactly balanced the + heat in the other, reminding one
of plus or positive and minus or negative electricity, which exactly neu-
tralize one another. These experiments are very satisfactory, and support
greatly the dynamical theory of heat.
Professor Rankine, in his valuable " Manual of the Steam-engine," examines
the question, whether latent heat be a materiality or not, very clearly. He says,
" The term ' latent heat] when freed from hypothetical notions, means an
amount of that condition of matter called heat which has disappeared in pro-
ducing physical effects different from heat such as expansion, fusion, evapo-
ration, and chemical changes and which may be made to reappear by re-
versing the changes in which such physical effects consisted ; that is, by com-
pression, congelation, liquefaction of vapours, and inverse chemical changes.
The progress in the true theory of thermo-dynamics, to which this discovery
might have led, was for a long time retarded by a fallacious principle, arising
from the hypothesis of substantial caloric, in the following manner: Let a
substance change from a less bulky to a more bulky condition, or from
the liquid to the gaseous state, or generally from the state A to the state B,
that change being of such a nature that, according to Black's discovery, heat
disappears, and some physical effect different from heat is produced.
" Let this operation be called A B, and let H! be the amount of heat which
disappears.
" Next, let the substance change back from the state B to the original state
A : let this change be called B A. It will cause a certain quantity of heat,
H , to reappear. If the series of intermediate changes undergone by the
substance during the process B A be exactly the reverse, step by step, with
those undergone during the process A B, everything done by the first process
will be exactly undone by the second : no permanent physical effect will ensue
from the combined processes ; and the amount of heat which reappears, H ,
must necessarily be equal to the amount of heat, H,, which formerly dis-
appeared. This was understood from the time of the first discovery of latent
heat; and so far there is no fallacy, but an important truth. But it was further
assumed that heat has a substantial existence, and that, consequently, H =
H, under all circumstances, even although the processes, A B and B A, should
differ in their intermediate steps. This assumption leads to the following
paradoxical result, which shows it to be fallacious : It is known that the
process B A may be made to differ from A B in its intermediate steps in
such a manner that a permanent mechanical effect shall be produced by the
combined processes. Now, if under such circumstances H is assumed to be
still = HI, it follows that, by employing the mechanical effect of the com-
bined processes in developing heat by friction, we may increase the amount of
heat in the universe, or create caloric a consequence opoosed to the
12
173 HEAT.
original assumption of the substantiality of caloric, and proving that assump-
tion to be self-contradictory/'
Further on, Professor Rankine, speaking of the hypothesis of molecular
vortices, remarks that, "In thermo-dynamics, as well as in other branches
of molecular physics, the laws of phenomena have, to a certain extent, been
anticipated, and their investigation facilitated by the aid of hypotheses as
to occult molecular structures and motions with which such phenomena are
assumed to be connected.
" The hypothesis which has answered that purpose in the case of thermo-
dynamics is called that of ' molecular vortices,' or otherwise the ' centrifugal
theory of elasticity.' On this subject, see the ' Edinburgh Philosophic
Journal,' 1849; 'Edinburgh Transactions,' vol. xx., and 'Philosophical
Magazine,' passim, especially for December, 1851, and November and
December, 1855 ' Science of Energetics.' Although the mechanical hypo-
thesis just mentioned may be useful and interesting as a means of anticipating
laws, and connecting the science of thermo-dynamics with that of ordinary
mechanics, still it is to be remembered that the science of thermo-dynamics
is by no means dependent for its certainty on that or any other hypothesis,
having been now reduced to a system of principles and general facts, express-
ing chiefly the results of experiments as to the relation between heat and
motive power.
" In this point of view, the laws of thermo-dynamics may be regarded as
particular cases of more general laws applicable to all such states as con-
stitute * energy] or the capacity to perform work; while more general laws
form the basis of the 'science of energetics' a science comprehending, as
special branches, the theories of motion, heat, light, electricity, and all other
physical phenomena."
A cubic inch of water, converted into steam under the ordinary pressure of
the atmosphere, expands into 1696 cubic inches, or nearly one cubic foot.
It is the change of water into vapour, converted in its turn into mechanical
motion, which constitutes " energy," or heat, the first of " prime movers,"
and now bringing us to the Steam Engine.
THE STEAM ENGINE.
179
FIG. 172. Portrait of Watt, after Sir W. Beachy, and Waffs Autograph.
ON THE STEAM ENGINE.
The limits of this work will not permit of any lengthened description of
the various ingenious extensions, modifications, and improvements of the
original and successful steam engine of Watt invented and constructed by
him between 1759 and 1784. Omitting the history of the steam engine before
the period of 1759, which the reader will find fully described in the works of
Tredgold, Farey, Lardner, Bourne, and others, we find, according to the
" Memorials " of Watt, carefully collected and published by Mr. George
Williamson, late Perpetual President of the " Watt Club," of Greenock, that
" it is in the little town of Crawfordsdyke, about the middle of the seventeenth
century a small burgh in the parish of Greenock, and closely adjoining the
town of this name that we first meet with the name of Thomas Watt.
At what period of his life he
ettled here cannot now be known.
12 2
i8o HEAT.
" His object, no doubt, was to establish himself in some locality where those
branches of scientific knowledge connected with the mathematics, such as
astronomy and navigation, might be rendered available as a profession. This
Thomas Watt was the grandfather of the great mechanician, and he was
born during the civil wars between Charles I. and the Parliament; the exact
date of his birth appears to be doubtful, but it must have been between
1639 an d 1642. After coming to Crawfordsdyke, he became a teacher of the
mathematics and the principles of navigation ; and on his tombstone (for
he died on the 2/th February, 1754, aged 95) he is styled ' Professor of the
Mathematics.'
" Thomas had two sons, the elder John and the younger James Watt ; the
latter, a merchant of Greenock, the father of the great engineer, was raised to
office of bailie or magistrate of Greenock in the year 1757. He died in 1782,
at a good old age, having attained to his eighty-fifth year. The flat tombstone,
placed by his illustrious son, James Watt, records the deaths of his father,
mother, and brother, John Watt ; and the inscription ends with these words:
'TO HIS REVERED PARENTS,
AND TO HIS BROTHER, JAMES WATT
HAS PLACED THIS MEMORIAL.'
" Of the mother of Watt it was said by another lady, who knew her, that she
was ' braw, braiu woman; none now to be seen like her'
" From her he received his first lessons in knowledge ; and although, by their
very gentleness, he may have been rendered doubly sensitive under the ruder
and more popular methods of the public school to which he was afterwards
sent, there is every reason to believe that the very aversion occasioned in his
mind to the rough sports and hard usage of his less exquisitely refined play-
mates conspired with other causes to further rather than impede the steady
development of his future powers. The truth in regard to young Watt's first
years in the public school is, that, owing doubtless to infirm health, the suffering
and depression which affected his whole powers, he was unfitted for a consider-
able time for displaying even a very ordinary and moderate aptitude for the
common routine of school-lessons, and that during these years he was re-
garded by his schoolmates as slow and inapt.
"At thirteen years of age young Watt, like that other giant of Timnath when
the Philistines were upon him, woke tip into something of his real strength
on being put to the study of the mathematics.
" This the author observes to be the true date of his intellectual birth the
happy moment when he took into his hands the mystic key of all scientific
knowledge, with which in after-years he was successively to unlock so many
of the secrets of nature, and lead mankind to the participation of some of her
most precious treasures."
We pass on through the philosopher's boyhood, his sober pastimes, and his
cultivation of the learning of a sage, mathematics and astronomy, until we
arrive at his first studies in the practical mechanics "the making and fashion-
ing of such miniature pulleys or blocks, pumps and capstans, with their levers
or bars." These objects were all in course of manufacture on his father's
premises, who was not only a merchant, but a " master-wright," and made
such carpentry as the outfit and supply of the shipping demanded gun-car-
riages, blocks, pumps, capstans, dead-eyes, figure-heads, and the first "crane"
at Greenock, for the convenience of "the Virginian tobacco-ships" then fre-
quenting the harbour.
THE STEAM ENGINE.
.181
FIG. 173. Boyhood of Watt.
"A scene of useful labour such as this was a fitting school for the genius of
him who afterwards was to become the leading mechanician of the day.
" In clearing out an attic room used by Watt when a youth in his twelfth
or thirteenth year, it is stated by a late master-shipwright of Greenock that he
found a quantity of ingenious models, and among these models he remembered
in particular a miniature crane and a barrel-organ. Watt is known subse-
quently to have constructed several musical instruments, particularly an organ
of some dimensions and power, while he was in Glasgow, which, it is said,
produced the most remarkable harmonious effects, so as to deligh: even pro-
fessional musicians; the more remarkable because it is added that he could
not distinguish one note from another, and was wholly insensible to the charms
of music.
"Having completed his attendance at the grammar-school, young Watt was
for a year or more industriously occupied about his father's premises, either as
an amateur or in the way of intentionally acquiring an accurate knowledge of
the various nautical and scientific instruments left with his father for adjust-
ment. At all events, he had a small forge erected for his particular use. It
is probably to this period that his fabrication, for one of his friends, of ^punch-
ladle out of a large silver coin is to be referred.
" In the year 1753, and after the death of his mother and the altered cir-
cumstances of his father, at the age of seventeen or eighteen, he 'was sent to
Glasgow to reside with his maternal relations; and in the year 1755 went to
London with the view of perfecting himself in the profession which it would
appear the inclination of the time, as well as the circumstances in which he
had been brought up, dictated as the most expedient.
" 111 health compelled him to leave London, and he returned to Greenock in
182 HEAT.
1756, and in the course of this year probably settled in Glasgow for the pro-
secution of his business as a mathematical-instrument maker.
" Watt arrived in Glasgow in his twenty-first year ; but the Corporations of
Arts and Trades, the Corporation of Hammermen, grounding upon their ancient
privileges, looked upon the young artist from London as an intruder, and obsti-
nately denied to him the right to open even the most humble workshop. Every
means of conciliation having failed, the Institution of Glasgow interfered,
arranged and put at the disposal of the young Watt a small apartment within
its own buildings, allowed him to establish a shop, and honoured him with the
title of its instrument maker. Here the young mechanician made the acquaint-
ance, and then acquired the sincere friendship, of a most distinguished and
benevolent man, the founder of the Andersonian Institution of Glasgow, who,
in addition to the labours of his own class, which were strictly academic and
philosophical, instituted a class and lectures for workmen, and for those whose
pursuits did not allow of their conforming to the prescribed routine of uni-
versity studies ; to which anti-toga class, as he designated it, he continued
throughout a long life, terminated only at the advanced age of seventy, to
lecture twice every week during the session of college.
"Such a man, one would say, was eminently he under whose inspiriting influ-
ence it were to be desired that the adventurer in the philosophical instrument
business should have fallen. It was Professor Anderson- who put into the hands
of Watt the famous model of Newcomen's engine, which belonged to the
apparatus of the professor's class, and wanted repairing.
"In his little university room Watt now speculated-and experimented ; his
workshop became the resort of learned professors, as well as students 'a
kind of academy/ says Arago, * whither all the notabilities of Glasgow
repaired to discuss the nicest questions in art, science, and literature.' It
was here, as stated in the note appended to the model of the Newcomen
engine in the Hunterian Museum of Glasgow, that 'in 1765 James Watt, in
working to repair this model, belonging to the Natural Philosophy Class in
the University of Glasgow, made the discovery of a separate condenser, which
has identified his name with that of the steam-engine.'"
Watt had accomplished his grand discovery, the "separate condenser,"
and now formally registered his patent for " A Method of lessening the Con-
sumption of Steam, and consequently of Fuel, in Fire Engines."
He enrolled in Chancery his threefold specification of an effective, work-
able steam engine, a high-pressure engine and a horizontal rotatory engine.
Money (//) only now was wanting to give to his country and the world the boon
for which science and labour were alike waiting. This was not denied to
genius in this case, because industry was not wanting. The young workman
falls in with Smeaton ; their histories were similar ; and -now the young mathe-
matical-instrument maker becomes a surveyor and civil engineer. Mr. Watt
was next employed in the experiments and improvements going forward at the
Carron Iron Works, under the famous Dr. Roebuck, who first defrayed the
expense of carrying out Watt's invention.
For a series of years prior to the failure of Dr. Roebuck's magnificent under-
takings, and Mr. Watt's consequent settlement, with the famous Matthew
Boulton, at Soho, near Birmingham, about 1774-5, hi s principal professional
occupations were those connected with the business of civil engineering, or
surveying, as it then continued to be called. He was employed in 1769 to
survey the River Clyde. " We see, in fact," remarks Arago, " the creator of an
THE STEAM ENGINE. 183
engine destined to form an epoch in the annals of the world undergoing,
without murmur, the undiscerning neglect of capitalists during eight years
turning the lofty power of his genius to the getting up of plans, to paltry
levellings, to wearisome calculations of excavations and embankments and
courses of masonry."
Time, however, works wonders. Watt is now invited to join Mr. Boulton,
of Birmingham, who had taken Dr. Roebuck's place, and received from him
the most generous and hearty assistance in the further prosecution of the
manufacture of the steam engine. It was the energy of Boulton which ren-
dered the genius of Watt practically available; and Watt, in his " Notes on
the Steam Engine," says
" As a memorial due to that friendship, I avail myself of this, probably a
last, public opportunity of stating, that to his friendly encouragement, to his
partiality for scientific improvements, and his ready application of them to the
processes of art, to his intimate knowledge of business and manufactures, and
to his extended views and liberal spirit of enterprise, must, in a great measure,
be ascribed whatever success may have attended my exertions."
Watt, at the period of his leaving Scotland, was about thirty-eight or thirty-
nine years of age ; and " had Watt," says Playfair, " searched all Europe, he
could not have found another man so calculated to introduce the invention
to the public in a manner worthy of its importance."
Watt, by the advice of BouHon, applied to Parliament for an extension of
his patent. ,50,000 had already been expended in the manufacture of engines
and defence of the patent by Boulton and Watt before any return was realised.
The extension was granted for a term of twenty-five years, dating from 1775.
This important concession being secured, Boulton and Watt invited the utmost
publicity. Mechanics and scientific men crowded to see the capabilities of the
new machines. The Cornish and other miners, and all employers of power,
were shown the working and economy of the new system. The patentees
themselves said, in their prospectus, " All that we ask from those who choose
to have our engines is the value of one-third part of the coals which are saved
by using our improved machines, instead of the old. With our engines it will
not, in fact, cost you but a trifle more than half the money you now pay to do
the same work, even with one-third part included, besides an immense saving
of room, water, and expense of repairs.
" The machine itself which we supply is rated at that price which would be
charged by any neutral manufacturer of a similar article. And, to save all
misunderstanding, to engines of certain sizes certain prices are affixed."
The dates of Watt's inventions are as follows :
1769. The first patent involving the saving of steam and fuel the invention
of the " cutting off of steam," to enable it to work expansively.
1776. The invention of the " double-acting steam engine," and the applica-
tion of the crank to it ; also the adaptation of this engine to the production
of rotatory motion.
1784. Other patents of invention, viz., the parallel motion, the counter
which registered the strokes of the engine, the governor, the throttle valve,
the indicator for ascertaining the power of an engine, and a locomotive engine,
the latter never practically tested. By the time this work is printed and circur
lated, a hundred years will have elapsed since Watt took out his first patent.
How many patents for steam engines have been taken out during that period
it would be hard to say. Every requirement which steam power can fulfil is
i8 4 HEAT.
now satisfied ; and, classing the various forms with reference to their purposeSj
we find, according to Rankin, the following classification :
" I. Stationary engines, such as those used for pumping water, for driving
manufacturing machinery, &c.
"II. Portable engines, which can be removed from place to place, but are
stationary when at work.
"III. Marine engines, for propelling vessels.
" IV. Locomotive engines, for propelling vehicles on land."
After the time granted to Watt by Parliament had expired, he retired from
the firm, leaving his son and his partner's son to continue in the same path
of honourable industry.
The patent expired in 1800; and Watt died in the house which he occupied
at Heathfield during his sojourn at Soho, on the 23rd of August, 1819. He lies
buried in the parish church of Heathfield, at Handsworth, where a Gothic
chapel, containing a marble statue by Chantrey, was erected to his memory.
The next figure (Fig. 174), taken from Walker's "System of Familiar Philo-
sophy," published in 1801, will give the reader a good notion of the con-
struction of one of Watt's single-acting engines, and what was even then
called " Boulton and Co.'s new-invented patent fire engine."
A. The boiler, about half filled with water.
B. The steam-pipe, that conveys the steam into the cylinder.
C. The door, where a person may enter to clear out the boiler.
D. The loaded or safety valve ; forced open by the steam when too strong,
or to be opened by the handle c,
E. Feeding-pipe, from the warm-water cistern s.
F. Fire-door, opening to the fire under the boiler.
G. The ash-hole.
H. The cylinder, having a piston in it on the end of the rod d, which works
through the air-tight stuffing-box o.
I. Nozzles, where the steam is let out.
K. Plug frame, to open and shut the valves in its rising and falling, thereby
suffering the steam to pass to the condensing-pump Q.
L. Beams that support the cylinder.
M. The exhaustion-pipe, that conveys the steam through the cold-water
well o to the pump Q.
N. Injection-pipe in the cold well, to throw a little cold water into the
exhaustion-pipe M.
O. The blowing-pipe, to let out the air that might accumulate in the air-
pump Q.
P. The barometer, to compare the strength of the steam with the pressure
of the atmosphere.
Q. The air-pump, immersed in a well of cold water. When its piston ascends
by the chain, it draws the steam out of the cylinder, and condenses it by the
coldness and the vacuum in the pump. The steam becoming water by this
means, the piston descends into it, the piston-valve is opened by the water (as
in a common pump), and the next ascent of the piston forces that warm water
through the box R, up the pipe r, into the cistern S (which pipe is cut short in
the drawing, but it begins at the box R). This water supplies the boiler.
R. The box of the pipe r.
S. The cistern of ditto.
T. A forcing-pump, whose solid piston is forced down by the weights J, and
THE STEAM ENGINE.
FIG. 174
the piston of the cylinder H drawn up. When the steam from the boiler
forces down the piston of H, the piston in T rises, and rarefying the air in the
inside or barrel of the pump, the pressure of the atmosphere on the surface
of the well forces up the water, as in a common pump ; but, by the descent of
the piston in T, the water is forced through the pipe x to the place where it is
wanted.
w is an air-vessel, to prevent the bursting of the pipes.
Y. The great lever-beam.
z. The pipe to feed the condenser cistern, O N Q.
i86
HEAT.
The above is Walker's description of one of the first engines made by Watt
for pumping water from mines. They are still used for this purpose in Corn-
wall, and are called " single-acting engines," in which the steam performs its
work by its action on one side of the piston only ; a counterpoise fixed to the
other end of the beam causes the piston to rise.
Walker finishes his description by saying, " that this excellent machine is
sometimes made to .vork by the pressure of steam both upwards and down-
wards ; z>., the steam can be made to press the piston up, as well as down.
This adds considerably to the first expense and the continued expense of fire."
This Cornish engine, made by Watt, is a singular contrast to the engines of
the present day, in which the consumption of fuel, and consequently the work
performed, is carried to the most refined and absolute degree of perfection.
Engines have been made to perform the duty of raising one hundred million
pounds of water one foot high by the consumption of a single bushel of coals.
The essential portions of the steam engine are better studied in Watt's
" Double-action Engine." In this, as in the single-acting engine, the
" cylinder " holds the first place.
This consists of a cylinder of metal, A D, provided with a piston, B, the end
of which passes through a stuffing-box, c, and is connected with the beam by a
beautiful arrangement called the parallel mo-
tion (Fig. 177). The steam is passed into this
cylinder both above and below the piston with
the utmost regularity, by means of a sliding
valve, E. This valve opens a communication
between the interior of the boiler and the
cylinder, and the condenser and the cylinder,
in such a manner that, whilst the steam is
using its power on one side of the piston, it is
at the same time creating a vacuum on the
other side, by passing into a box called the
condenser, F the famous "separate con-
denser" of Watt to which an air-pump is
attached to remove any air that may collect,
the condensed water, and also that used for
injection.
The sliding of the valve upward and down-
ward is effected by means of another admi-
rable mechanical arrangement, called the
" eccentric."
In nearly every kind of engine there is
attached to the beam and piston-rod a " pa-
rallel motion," in order that the piston-rod
may always move in a straight line. This
simple mechanical arrangement is one of the
happiest of the inventions which seem to
have come, as it were, intuitively to the well-
educated mind of Watt.
To render the working of the double-acting
engine as perfect as possible, and to prevent
the bad effects of sudden and violent work-
ing by excess of steam, Watt caused his engine to regulate its own motion by
FIG. ij$.The Cylinder,
Valve., and Condenser .
THE STEAM ENGINE.
187
FIG. 176. The Eccentric.
This was not wholly the invention of Watt,
as the same principle had been previously used in the regulation of sluices of
water-mills, under the name of the " lift-tenter ;" but the merit due *o Watt is
FIG. 177. j. ne Parallel Motion.
a a, the beam ; ft, the piston-rod ; c, the air-pump rod ; d d <i, the links ; e, rod fixed to the cylindei
to support the guiding arm or radius rod,/.
that of accurately adjusting the contrivance to the opening and shutting of the
steam-pipe from the boiler by a valve called the " throttle valve," so that when
the engine is inclined to go fast, and use too much steam, the balls of the
governor fly out by centrifugal force, and, acting on the throttle valve, the
steam is cut off, and the velocity of the engine reduced.
i88
HEAT.
FlG. 178. The Governor and Throttle Valve, as used in a i\ horse-power
High-pressure Steam Engine, by Belliss &> Seekings, Great Exhibition, 1 862.
The governor in the above engine acts upon an equilibrium or double-
beat throttle valve, through the intervention of only a single lever; and the
comparative absence of resistance renders its action peculiarly sensitive.
The most important features of the " vacuum " or " condensing engine " of
Watt having been discussed, the high-pressure steam engine, such as that
delineated at Fig. 178, may next be considered. Their form is legion ; they
may be beam engines or horizontal or vertical engines. The machinery
comprised in their construction can be fitted up in a much smaller space ;
and they differ from the " vacuum or condensing engine " by the absence of
those parts which give the name to Watt's engine. The air-pump and condenser
are removed, and the steam, after performing its work, is allowed to escape
directly into the atmosphere. An illustration of a small engine is given,
because the high-pressure principle is well adapted for nearly all small
THE STEAM ENGINE.
189
engines, and it is especially to be noted in the locomotive. Portable engines,
which can be removed from place to place, but are stationary when at work,
are all worked on the high-pressure principle. At the great French Exhibi-
tion of 1867, our manufacturers of portable steam engines for agricultural
purposes, such as for working thrashing-machines, ploughing, &c., &c.,
received many more gold medals than those of other nations ; and the excel-
lence of the machinery used by advanced and intelligent farmers in England
has created a trade with foreign countries which, in spite of the low wages of
the engineers of the Continent, is still most thriving and lucrative.
FIG. 179. Howard's Patent Steam Ploughing and Cultivating Apparatus.
The apparatus delineated in the above engraving includes the engine, the
windlass, the wire rope, the cultivator, the anchors, and pulleys. The young
people for whom this book is intended have so many opportunities of studying
the locomotive engine at the various railway stations, that it is presumed the
general outline of this most important class of engines must be sufficiently
known to all.
The interior of a locomotive can hardly be thoroughly understood without
one of those valuable sectional models made by Messrs. Elliott Brothers, of
Charing Cross. The models have sectional working gear, and accurately
define the various parts and their respective uses;, and all good schools should
possess sectional models of the Watt condensing engine and of the locomo-
tive or high-pressure engine.
At the Exhibition of 1862 was exhibited a locomotive engine, built for the
London and North-Western Railway Company by Mr. Ramsbottom, their
locomotive superintendent, at Crewe, being a good specimen of a first-class
passenger engine. It was fitted with patent pistons, duplex safety-valves, and
lubricators, and adapted for burning coals with great economy.
An engine of this class ran the American express, on the 7th January,
r862. a distance of 130^ miles without stopping, at an average speed of 54
FIG. 1 80. Apparatus for Supplying Water to Tenders whilst in motion.
miles per hour. The tender attached (Fig. 180) was fitted with Mr. Rams-
bottom's most ingenious apparatus for taking up water whilst running.
The plan has been in daily operation on the Chester and Holyhead Railway
since it was first adopted in the winter of 1859-60. By it various quantities
of water, from 1,200 gallons downwards, can be picked up, at speeds ranging
from 22 miles to 50 miles and upwards per hour. In the running of the Irish
mails, the arrangement has the effect of reducing the dead weight of the
tender about six tons, equal to the weight of a loaded carriage.
These engines are added to the enormous screw engines manufactured by
Messrs. James Watt & Co. The latter consist of four cylinders, each of 84
inches diameter.
The paddle-wheels are driven by four engines, each of 72 inches diameter
of cylinder and 14 feet stroke, and rated collectively at 1000 nominal horse-
power.
In the Exhibition of 1862 some good examples of high and low pressure
marine condensing engines, with surface condensers, were shown by George
Rennie & Son.
The advantage of two cylinders in direct-acting marine screw engines is
that of working steam expansively, whereby economy of steam and fuel is
obtained, depending on the pressure of the steam and the relative volumes of
the high and low pressure cylinders. These engines are fitted with surface
condensers, with copper tubes and improved centrifugal pumps for circulating
the water in the condensers, these pumps being made on a double-curvature
principle of least resistance to the flow of water occasioned by the centrifugal
force generated by the angular velocity of the pump.
Engines on this principle are fitted with boilers in proportion. Apparatus
for superheating steam and feed- water heaters may be made to consume not
more than two pounds of coal per actual horse-power.
The important principle of working steam expansively has been applied
THE STEAM ENGINE.
19*
FIG. 1 8 1. The P addle-Wheel Engines of the Great Eastern.
with the greatest success in large engines, made like the Cornish ones, for
pumping enormous quantities of water for the use of great cities like London.
Steam of high pressure is used, and when admitted to the piston it is cut
off at one-eighth or one-tenth of the stroke. At the Kent Waterworks the
Cornish engines used are two with cylinders of 70 inches and 10 feet stroke,
two with cylinders of 60 inches, and two smaller ones ; in these engines the
expansion was not more than one-fifth. The high-pressure, condensing,
double-cylinder engines erected at Ditton for the Lambeth Waterworks, and at
Kingston for the Chelsea Waterworks Company, can accomplish, according
to Mr. Simpson, in ordinary work, 90 million pounds raised I foot high per
one hundredweight of coals consumed.
The returns of the work performed by the Cornish pumping engines have
been given from an early date, and are very interesting.
1769, John Taylor gave the return at only 5^ millions.
In 1800 . . 20
1815 50
l8 35 .125
The latter 125 millions was at Fowey's Consols Mine, where Austen's engine
was used. It might almost be disputed whether such an amount of duty was
ever done ; but it was well authenticated by the report of the committee ap-
pointed, and they reported that the work was done with a bushel of coals,
weighing 94 pounds.
HEAT.
The principle of expansion was used by Watt, but without any very good
result ; but Woolf and Trevithick applied the system with high-pressure
steam, and realised the economical results already referred to. With respect
to the double-cylinder engine, this was invented by Jonathan Hornblower,
and not by Watt. It was patented by the former in 1781. The first and
second patents of Hornblower contain the following :
" First, I use two vessels in which the steam is to act, and which in other
steam engines are generally called cylinders.
" Secondly, I employ the steam, after it has acted on the first vessel, to
operate a second time on the other, by permitting it to expand itself, which I
do by connecting the vessels together, and forming proper channels and
apertures, whereby the steam shall occasionally go in and out of the same
vessel."
The third invention was for " surface condensation," a term already used,
and meaning the application of cold water on the other side of a plate forming
the side of the box containing the steam. The more perfectly the circulation of
the cold water can be maintained, the better is the condensation. A surface-
condenser represents the worm attached to a common still, and this invention
evades the " separate condenser " in the patent of Watt.
FIG. 182. A Cornish Boiler.
Professional men have discussed the respective merits of the single and
double cylinders, and Mr. Hawksley stated, as the result of his experience,
" that when raising water from a pit, the Cornish engine (single cylinder)
would work well ; but it would perform best when pumping out of a deep pit,
and when it had a large amount of heavy rods to continue its action and
diminish its initial velocity. In the case of dear coal he would employ the
double cylinder ; in the case of cheap coal he would employ the single
cylinder ; and either not cut off the steam at all, or not much before it gets to
the end of the stroke.
With the double cylinder he would use eight expansions ; beyond that, so
little was gained by the system of expanding steam, that it was not worth
carrying it further.
In order to economise coal, and, of course, to increase the stowage qualities
E VAPOR A TION. 193
of a vessel, the " Combined Vapour Engine" was invented by M. du Trembley.
This ingenious arrangement provided that, after the steam had done its work
in the cylinder, it passed to the surface-condenser, which was surrounded
with ether, and, causing this fluid to boil, the vapour passed to another
cylinder, where it exerted its elastic force ; and after the vapour of the ether
had done its work, it was finally condensed and pumped back again to the
box surrounding the external condenser of the steam engine, the condensa-
tion of the vapour of water causing another fluid (ether) to boil. This clever
arrangement met with considerable approval, and has been tried on an exten-
sive scale in the propulsion of vessels.
Superheated steam, or steam passed through a coil of iron pipe placed in
the furnace, has been proposed and used successfully in the working of marine
engines in order to economise fuel. Rankin calls this superheated steam
^' steam-gas ;" and the Hon. John Wethered, of the United States, modified
this superheated steam by mixing it with ordinary steam from the boiler,
because he found that when the steam was heated sufficiently high to develop
the full power, it destroyed the cylinder and slides. He considers the differ-
ence between superheated steam and combined steam consists in this that
the former, being of a gaseous nature, was a bad conductor of heat, and parted
with it with difficulty ; whereas combined steam, being pure vapour and a
better conductor of heat, parted with it more readily, and left more in the
cylinder of the engine to be converted into mechanical power.
Wethered claimed an economy of combined steam over ordinary steam of
52*5 per cent., and over superheated steam of 25 per cent. According to
more recent experiments on the large scale, made by the eminent firm of
Messrs. Penn, of Greenwich, with superheated steam, it is conclusively
determined that an economy of 20 per cent, of fuel was realized in the working
of marine engines, when the steam at a pressure of 20 pounds per square inch
was raised by the superheating apparatus 100 Fahrenheit.
The Cornish boiler, to which allusion in connection with the Cornish
engine has already been made, is shown at Fig. 182. It consists of a double
cylinder, the fire being placed on bars inside it, and is one of the most
useful forms that can be employed, and is the kind of boiler used for working
the steam engine at the Polytechnic.
EVAPORATION.
It is so common an act to boil water and convert it into steam, that non-
scientific minds are sometimes puzzled when the more learned talk of steam, or
the vapour of water, being always present in the air we breathe; they begin to
ask themselves mentally for the visible presence of great cauldrons of boiling
water to supply the vapour ; and, failing these proofs, subside into a sort of
wondering doubt.
The great evaporating surfaces of the oceans, rivers, lakes, &c., are always
silently at work ; and Faraday, in one of his popular discourses, said that sixty
sacks of coal must be burnt to produce an amount of steam such as would
pass away gradually from the surface of an acre of ground during an ordinary
summer's day.
The oroof that the atmospheric air does contain invisible steam is shown
13
t94
HEAT.
by the water deposited outside a tumbler containing iced water, or water drawn
from a deep well a few degrees below the temperature of the air.
Evaporation is confined to the surface of the liquid exposed to the air ; and
that may be stopped, as in the case of water when oil is poured upon it.
If, during evaporation, vapour forms under the ordinary pressure of the
air, it is necessarily increased when produced in a vacuum, because there is
no resistance to be overcome ; as the first is the slow production of vapour at
the surface of a liquid, so the second is the quick production of vapour.
If a number of barometer-tubes are filled with mercury, and placed in a
proper vessel or trough, also containing mercury, they all exhibit a height cor-
responding to the existing pressure of the air ; when, however, a few drops of
water, alcohol, or ether, or a small lump of ice, are introduced respectively
into the separate tubes, the mercury is depressed immediately, showing the
evaporation which instantaneously takes place in the Toricellian vacuum, or
space above the level of the mercury in the barometer.
FIG. 183. The Catgut Hygrometer.
The amount of depression, showing the elastic force of the vapour, varies
with each liquid. At the same time, the above experiment shows that all vola-
tile liquids are instantaneously converted into vapour in a void space, or
vacuum.
Faraday found that there was a limit even to evaporation, and, experi-
menting with mercury, he noticed that a slip of gold leaf, suspended in the
neck of a bottle containing mercury, was whitened by the evaporation and
condensation of the quicksilver upon the gold. This effect did not, however,
take place at a temperature of about 39'2 F. With sulphuric acid, which
is a very permanent fluid, the temperature of the limit of evaporation was
found to be much higher, viz., about 86 F.
Various instruments have been devised, from the earliest times of scientific
investigation, to determine the quantity of invisible steam or moisture in the
air. All cords, and especially catgut (a string made from the peritoneal linings
of the intestines of the sheep;, lengthen or shorten according to the state of
the moisture in the air.
If a piece of catgut, made fast at one extremity, .be conveyed, as in Fig.
HYGROMETRY.
'95
183, over a series of pulleys, A, B, c, D, E, F, G, so as to make several turns
backwards and forwards, and if a weight, P, be suspended from the other
extremity, the latter will fall as the string lengthens in damp weather, and
rise as the air becomes drier. This is shown better by attaching an index
or pointer, H K, turning on a pivot I, in such a manner that the length I K
shall be greater than I H, and pointing to a graduated arc, L L.
Saussure employed a human hair for the same purpose; but all such arrange-
ments infallibly become deteriorated by time.
M. Le Roi was the first to suggest that the temperature at which dew begins
to be deposited should be employed as the measure of the moisture of the
air. De Luc also proved that the quantity and force of vapour in vacua are
FIG. 1 84. Regnaulfs Condensing Hydrometer ; by Negretti and Zambra.
the same as in an equal volume of air of the same temperature, or that these
two elements of vapour depend upon the temperature.
The determination of the exact temperature at which dew is formed, and at
which, in the open air, the dew disappears or ceases to be formed on the sides
of the vessel producing it, is of the utmost importance, and was carefully
investigated by the late Dr. Dalton. The observation is rendered more exact
with a bright metallic vessel, as in Regnault's elegant apparatus.
Regnault's Condenser Hygrometer consists of two highly polished silver
cylinders, into the upper part of which are cemented thin glass tubes ; these
have brass covers, arranged to receive and support two delicate standard
thermometers, the bulbs of which descend nearly to the bottom of the silver
portion of these chambers. Each chamber has a small internal tube carried
13 2
i 9 6 HEAT.
down from the brass to within a short distance of the bottom, to admit the
passage of the air, which is drawn through both chambers by an aspirator,
connected to the base of the hollow upright and arms supporting the cylinders.
To use this hygrometer, ether is poured into one chamber sufficient to cover
the bulb of the thermometer, and then the thermometers being inserted into
both cylinders, the instrument is now connected to the aspirator, and by it the
air is drawn through both cylinders down the internal tubes, passing in one
chamber in bubbles through the ether, and in the other chamber simply around
the thermometer. The tube in this empty cylinder is of such a diameter as to
ensure similar quantities of air passing through each chamber.
After a short time the passage of the air through the ether will cool it down
to the dew-point temperature, and the external portion of the silver chamber
containing the ether will become covered with moisture. The degree 'shown
by the thermometer in the ether at that instant will be the temperature of tho
dew-point ; the second thermometer showing the temperature of the air at the
time of observation.
The late Professor Daniell, who paid much attention to the construction of
hygrometers, and, indeed, constructed one of the best and most simple, says :
" The more accurate mode of expressing the moisture of the air from an
observation of the temperature and dew-point is by the quotient of the divi-
sion of the elasticity of vapour at the real atmospheric temperature by the
elasticity at the temperature of the dew-point ; for, calling the term of satura-
tion 1000, as the elasticity of vapour at the temperature of the air is to the
elasticity of vapour at the temperature of the dew-point, so is the term of
saturation to the observed degree of moisture. Thus, with regard to the obser-
vation in the Deccan, where, with a temperature of 90 F., the dew-point has
been seen as low as 29, making the degree of dry ness 61.
Force at 90. Force at 29.
1-430 0194 looo : 135
The fourth term is the degree of moisture on the hygrometric scale."
RADIATION.
It is not at aH surprising that the philosophers who first commenced experi-
ments with heat, or caloric, should have regarded it as an imponderable and
highly elastic fluid, which clothed, as it were, the material particles of solids,
fluids, and gases ; the latter attracting the former, and sometimes emitting
or throwing out their caloric, which was also supposed to be repulsive of its
own particles. The material theory of heat is, however, not tenable : when
we consider it as radiant matter, we are reminded at once of its analogy to
light, and we understand that the undulations of the same ethereal medium
may propagate heat as well as light: ,it is not necessary to suppose that the
ether which gives us light is interpenetrated by another kind of ether that
may give us heat. The examination of the invisible heat rays in the solar
spectrum assist us greatly in taking a correct view of the phenomena.
Whilst enjoying the social pleasures of the fireside, we are always reminded
that heat can travel, like light, through space.
At night, if travelling in a steamboat across the Channel, we approach the
funnel, from which invisible heat is constantly radiating, we see no fire, and
RADIATION.
197
yet we can understand by the pleasant warmth experienced that waves of
heat may be impinging upon us, just as the waves of water dart against the
sides of the vessel. We shall find presently that light-waves may be sepa-
rated from heat-undulations ; and even when they travel together, and the
light only is apparent, the heat may be rendered evident in various ways, as
in the use of the burning-glass, or, by permitting the rays of the sun to pass
through a glass containing some ether, the rays are freely transmitted, but if
a piece of charcoal is placed in the ether, the heat rays are arrested, and
vibratory power is soon conferred upon the charcoal, which in its turn com-
municates motion to the ether, and raises it to the boiling-point.
The intensity of the heat rays decreases or increases according to the same
law which affects light, viz., as the square' of the distance inversely. The in-
tensity of heat is less, the greater the obliquity of the rays with respect to
the radiating surface. Avoiding a source of heat which may be accompanied
with light, and using a canister filled with boiling water, and placing it in the
focus of a polished concave metallic reflector, the rays are collected, and can
be thrown off to another reflector, when they are again brought to a focus,
discoverable by an air-thermometer (p. 147.)
At the Polytechnic a small bit of meat can be cooked when placed in the
focus of a large concave reflector, and opposite to another standing 100 ft. away,
and containing in its focus a large wire cage full of burning charcoal.
FlG. 185. The large Polytechnic Metallic Reflectors.
A fire in the focus of one, and the meat in the foeus of the other.
The power of reflecting heat rays is influenced by the condition of the sur-
face. Polished metals possess the property in the highest degree ; and if the
bit of meat were covered with gold leaf it would not be warmed through,
whilst the opposite effect of first blackening the meat, by dusting finely
powdered charcoal over it, assists the absorption of the heat rays very greatly.
Melloni discovered that out of 100 rays
Silver reflects 90
Bright lead '. 60
Glass 10
Hence, if a glass concave mirror is used to reflect the rays of an ordinary
fire towards the face, little or no warmth is experienced ; on the other hand, a
concave tin plate will reflect the heat very sensibly.
I 9 8 HEAT.
It was formerly supposed that the power of a body to absorb heat was in
die inverse proportion of its power to throw off or reflect heat that the two
properties exactly accounted for the heat originally falling upon any given
surface. This, however, is not found to be the case. The heat waves which
are incident upon any given surface are disposed of in three ways :
I. Some portion is absorbed.
II. Another portion is reflected according to the ordinary laws which
govern the reflection of light.
/I I. A third portion is scattered, and is then called diffused heat.
The thinnest film of gold leaf will protect the parts of a sheet of paper
exposed to radiation from some red-hot surface, whilst the blackening of any
portion of the same sheet of paper will hasten its destruction.
Radiation and absorption, according to the experiments of Leslie, are
directly proportioned to each other ; a blackened tin vessel full of hot water,
that will radiate heat freely, and soon fall to the temperature of the air, will,
on the other hand, as rapidly increase in temperature if held near any good
source of heat.
The relation between radiation, absorption, and reflection, and the manner
in which the two first may balance each other, was elegantly shown by the
late Dr. Ritchie. He used for his experiments a metallic vessel filled with hot
water, and a differential thermometer, one bulb of which was shielded by a
bright metallic disc, and the other with a blackened one ; one surface of
the metallic box containing the water was also polished, and the other
blackened. When the blackened side of the box was placed opposite to the
bright metallic screen, no effect was produced on the thermometer, because
the radiating power of the black surface was neutralized by the non-absorp-
tive and good reflecting power of the bright metallic disc. If, however, the
same side was opposed to the blackened disc, then the thermometer was
affected. Similarly, but reversely, when the polished side of the box was
opposed to the blackened disc, little or no effect was perceptible, because the
highly polished surface did not radiate heat easily.
If boiling water be poured into two tea-pots, one of which is of bright
block tin, and the other of black japanned tin, the latter cools more quickly
than the former.
The air exercises a retarding power on the waves of heat which are absorbed,
and, as proved by Sir H. Davy, they travel much easier through a vacuum.
Davy ignited charcoal points by a current of electricity, and, placing them in
the focus- of a concave mirror, discovered that, when the receiver was exhausted
to i- 1 20th, the effect upon a thermometer placed in the focus of another reflector
was nearly three times as great as when the air was at its ordinary pressure.
The absorptive power of bodies was supposed to depend greatly upon the
particular colour used. Franklin placed pieces of coloured cloth in the
sun's rays on the snow, and found they sank into the snow or melted it in the
following order : black, blue, green, purple, red, yellow, white. Tyndall, how-
ever, has explained the cause more correctly, and has discovered that the
colour has not so much to do with the effect produced as the nature of the
material used for the colouring agent. Although it has been stated by Leslie
that white surfaces generally reflect heat well, and absorb it indifferently,
there is the curious fact, ascertained by Melloni, that white lead has quite as
great an absorbent power as lampblack ; and if the heat comes from boiling
water (column i), it will absorb twice as much as it would do if it came from an
RADIATION. 199
incandescent platinum wire. Melloni (p. 201) filled a copper canister with water,
and kept it at the boiling-point, and by means of a very delicate instru-
ment, called the thermo-multiplier, obtained the following relative absorptive
powers, as shown in column I. If, however, the heat is derived from an incan-
descent platinum wire, as in column 2, the figures are different ; and white lead
is found to absorb a less quantity of the rays of heat when they are luminous,
and Indian ink more.
No. t. No. a.
Lampblack . . . . 100 . . . 100
White lead .... 100 ... 56
Isinglass .... 91 ... 54
Indian ink .... 85 ... 95
Shellac .... 72 ... 47
Metals 13 ... 13*5
Leslie's principle does apply to clothing, and it appears that if we imitate
nature, and, like the Polar bear, wear white, we shall be warmer in winter and
cooler in summer.
In running streams, and even in the Rhine, what is called "ground ice" is
frequently found. This is no contradiction of the laws already explained
with reference to the cooling of water. The ice is formed at the bottom of the
stream, because the stones and other earthy matters forming the bed of the
river emit or radiate heat when the sky is very clear ; and as the water of the
stream is mixed by the current, and the temperature of the bed of the river is
lowered by radiation, the ice forms in spongy masses, which may rise to the
surface, carrying stones and even the anchors of ships with them. The rays
of heat are more readily absorbed when they fall upon bodies at angles near
the perpendicular ; hence the rays of the sun are hotter in summer than in
winter, when they are more oblique.
If the bulb of an air-thermometer be brought near a burning hydrogen
flame, its radiating power is found to be very low, although, as is well known,
the heat of the flame is so great that it will quickly ignite a spiral of platinum
wire ; when the heat waves are set in motion, emission or radiation takes
place, which will promptly affect the thermometer. Tyndall has investigated
the radiating and absorbing powers of gases and vapours, and, although they
are feeble, he has been able to discover that vapours and compound gases
have a much greater absorbing and emitting power than any simple or
elementary gas, such as oxygen or nitrogen, or when they are mechanically
mixed, as in atmospheric air. Had our globe been surrounded with a gas
like olefiant gas, the absorbent power would have been 240 times greater than
that of oxygen. Amongst gases, those which absorb heat the most also
radiate it freely.
As might be expected from the analogy between light and heat waves, the
latter may be reflected, refracted, may undergo double refraction, be absorbed,
and even polarized ; the latter fact being proved by the use of tourmaline
plates or bundles of plates of mica.
200 HEAT.
TRANSMISSION OF HEAT.
Melloni's name will ever be associated with all the more important experi-
ments in which the course of heat-waves is traced through various media.
As with light there are bodies called transparent, diaphanous, translucent or
transparent, opalescent, and opaque, so with reference to the power of trans-
mitting heat, bodies generally are divided into two classes :
I. Diathermanous or diathermic bodies (Sia, through, and tfepyuos, heat),
permitting heat-waves to travel through their substance. Examples
rock salt and certain elementary gases.
II. Athermanous or adiathermic bodies, which arrest or stop the progress of
the heat-undulations. Examples all liquids in variable proportions ;
alum in crystal and solution.
Mr. B. Stewart has shown that bodies of the first class are bad radiators of
beat, but that those of the second or adiathermic class are eood radiators.
It does not follow, because substances like the diamond, glass, ice, &c.,
ermit light-rays to pass through them, that they will also allow the heat-
rays to travel through in the same proportion. Glass permits the light to
pass freely through its substance, but stops a considerable number of the
heat-undulations ; and alum, nearly all. Rock salt is the only substance which
is entitled to be placed in the first or true diathennanous class, and although
it does, according to Krupland and Stewart, absorb certain of the heat-rays
more than others, still at present it stands first, and is therefore used in the
form of plates, prisms, and lenses for these delicate experiments. Melloni
found that certain solids, cut into plates one-tenth of an inch in thickness,
allowed the following percentage of heat waves from an Argand lamp to
pass :
Rock salt ..... 92, transparent
Plate glass and Iceland spar . . 62,
Smoky quartz 57, nearly opaque
Transparent carbonate of lead . 52, transparent
Selenite ,20,
Alum 12,
Sulphate of copper. ... o, deep blue
With liquids, when the source of heat was an Argand oil lamp, and the
fluids enclosed in a glass cell, the results given in Table I. were obtained.
Table II. shows the results obtained by Tyndall from liquids enclosed in a
rock-salt box, the source of heat being an ignited platinum wire :
Table I. Table II.
Bisulphide of carbon 63 83 transparent
Olive oil 30
Chloride of sulphur ... 63 red
Ether 21 41
Sulphuric acid .... 17 41
Alcohol 15 30
Solution of alum or sugar . .12 30
Water (distilled) 11 30
Water saturated with salt . 26
Rock salt stands in the same relation to heat, so far as transparency to
TRANSMISSION OF HEAT.
201
FlG. 1 86. Mellon? s Apparatus.
Argand oil lamp without a glass; spirit-1 imp and platinum wire , the copper box, blackened, to con-
tain water at 212 F. ; stand, to place the ob)ec's upon, screen, with apertures of various sizes j the
thermo-multiplier current, with the galvanometer needle.
heat-rays is concerned, as colourless glass does to the light-rays. When a
hot metallic ball is placed between the bulbs of a differential thermometer,
the liquid remains stationary, because both are equally heated ; if, however,
a plate of rock salt is interposed as a screen on one side of the ball, and a
plate of glass on the other, the thermometer is immediately affected, as more
rays pass through the rock salt than through the glass.
Melloni's apparatus for these investigations may be regarded as the model of
perfection. It includes the various sources of heat, such as a naked flame, an
ignited platinum wire, a blackened copper vessej containing water at 100 C.
(212 F.), or a copper plate heated 10400 C. (752 F.), and is plainly shown
in Fig. 1 86.
The delicacy of the thermo-multiplier as an indicator or measurer of
heat is most remarkable, and it will be fully explained in another part of this
work. The minute electrical currents set up in the thermo-multiplier are
recorded by the galvanometer needle.
It has already been shown that in bodies which arrest partially or wholly
the heat-waves, the nature of the heat, or rather the particular source from
which it is obtained, has a great influence upon the result. Thus fluor-spar
permits 33 per cent, of the heat-waves derived from boiling water to pass
through its substance, whilst the power rises to 78 per cent, when the source
of heat is a burning lamp. Heat-waves which have passed through one plate
of glass will also pierce another, with a small amount of loss ; the same waves
are nearly all stopped b) alum.
202 HEAT.
Tyndall's discovery, that the vapour of water absorbs thirteen times more
obscure heat than air, is a most important fact, and shows why the air con-
taining vapour nearer the earth is warmer than that which is dry and found
on the summit of lofty mountains. The dry air allows the obscure heat-
waves to travel through, and is too diathermanous, whilst air charged with
moisture has considerable athermaneity for obscure rays, which are produced
when the rays of the sun have passed through our atmosphere and fallen
upon the earth. When the rays of the sun fall upon the earth to warm it,
they are radiated and then diffused ; a change in their quality takes place, and
they become obscure rays of heat. It is these obscure rays which melt snow,
and perform other useful offices.
THE CONVERSION OF LIGHT RAYS INTO HEAT RAYS, AND VICE VERSA,
BY CHANGE OF REFRANGIBILITY.
At the meeting of the British Association, held at Newcastle, in 1863, Dr.
Akin proposed three experiments for the conversion of rays of light into heat-
rays ; of these one is deserving of notice, viz., the proposal to collect the rays
of the sun in a concave mirror, and then to cut off the light with " proper
absorbents," and to bring platinum foil into the focus of invisible rays.
Although Dr. Akin was the first to propose definitively to change the refran-
gibility of the ultra-red rays of the spectrum by causing them to raise platinum
foil to incandescence, yet the chief merit, in connection with this branch of
heat, is due to Dr. Tyndall, because, in the spirit of Lord Bacon, he was not
content with a theory which merely suggested that a certain result might be
obtained, but industriously worked out the crude idea, and proved that it was
substantially true, by devising a number of clever and original experiments,
which had never been shown before.
In the article on Light (p. 92), the change of refrangibility of certain rays "at
the violet end of the spectrum,, and the beautiful experiments with " fluorescence,"
by Professor Stokes, have already been specially considered. And just as he
obtained a large proportion of these r?ys, existing in and beyond the violet,
by using prisms of quartz, so Melloni, by using a prism of rock-salt, was en-
abled to prove that the ultra-red rays discovered by Sir W. Herschel formed
an invisible heat spectrum as long as the visible one. Other experimentalists
continued the -investigation, especially Professor M tiller, of Freiberg, who
worked out a curve expressing the heating power of the whole spectrum ; but
it was left for Tyndall to complete the investigation, and directly isolate the
invisible or obscure rays of heat ; and as Stokes, by lowering the refrangibility
of the invisible ultra-violet rays, rendered them visible, so Tyndall, by raising
the refrangibility of the ultra-red rays, rendered them also visible. The instru-
ments he used, to quote his own words,* " consisted of the electric lamp of
"Duboscq and .the linear thermo-electric pile of Melloni.
" The spectrum was formed by means of lenses and prisms of rock-salt ; it
was equal in width to the length of the row of elements forming the pile; and
the latter being caused to pass through its various colours in succession, and
also to search the space right and left of the visible spectrum, the heat falling
upon it at every portion of its march was determined by the deflection of an
extremely sensitive galvanometer.
* "Proceedings of the Royal Institution of Great Britain," vol. iv., partj. Professor Tyndall, "On
Combustion by Invisible Rays."
INVISIBLE HEAT RAYS. 203
D REO.OMNCE. YEUOW. GREEN ^BLVE
FIG. 187. Dr. TyndalVs Diagram.
" As in the case of the solar spectrum, the heat was found to augment from
the violet to the red, while in the dark space beyond the red it rose to a maxi-
mum. The position of the maximum was about as distant from the extreme
red in the one direction as the green of the spectrum in the opposite one.
" The augmentation of temperature beyond the red in the spectrum of the
electric light is sudden and enormous. Representing the thermal intensities
by lines of proportional lengths, and erecting these lines as perpendiculars
at the places to which they correspond, when we pass beyond the red these
perpendiculars suddenly and greatly increase in length, reach a maximum,
and then fall somewhat more suddenly on the opposite side of the maximum.
When the ends of the perpendiculars are united, the curve beyond the red,
representing the obscure radiation, rises in a steep and massive peak, which
quite dwarfs by its magnitude the radiation of the luminous portion of the
spectrum.
" Interposing suitable substances in the path of the beam, this peak may
be in part cut away. Water, in certain thicknesses, does this very effectually.
" The vapour of water would do the same; and this fact enables us to account
for the difference between the distribution of heat in the solar and in the
electric spectrum. The comparative height and steepness of the ultra-red
peak in the case of the electric light are much greater than in the case of the
sun, as shown by the diagram of Professor Miiller. No doubt the reason is,
that the eminence corresponding to the position of maximum heat in the solar
spectrum has been cut down by the aqueous vapour of our atmosphere..
Could a solar spectrum be produced beyond the limits of the atmosphere, it
would probably show as steep a mountain of invisible rays as that exhibited
by the electric light, which is practically uninfluenced by atmospheric absorp-
tion.
" Having thus demonstrated that a powerful flux of dark rays accompanies
the bright ones of the electric light, the question arises, ' Can we not detach
the former, and experiment on them alone?'
" In the author's first experiments on the invisible radiation of the electric
light, blaok glass was the substance made use of. The specimens, however,
204 HEAT.
which he was able to obtain destroyed, along with the visible, a considerable
portion of the invisible radiation.* But the discovery of the deportment of
elementary gases directed his attention to other simple substances. He exa-
mined sulphur dissolved in bisulphide of carbon, and found it almost perfectly
transparent to the invisible rays. He also examined the element bromine, and
found that, notwithstanding its dark colour, it was eminently transparent to
the ultra-red rays. Layers of this substance, for example, which entirely cut
off all the light of a brilliant gas-flame, transmitted its invisible radiant heat
with freedom. Finally, he tried a solution of iodine in bisulphide of carbon,
and arrived at the extraordinary result, that a quantity of dissolved iodine
sufficiently opaque to cut off the light of the mid-day sun was, within the
limits of experiment, absolutely transparent to invisible radiant heat.
" This, then, is the substance by which the invisible rays of the electric
light may be almost perfectly detached from the visible ones. Concentrating
by a small glass mirror, silvered in front, the rays emitted by the carbon points
of the electric lamp, we obtain a convergent cone of light. Interposing in the
path of this concentrated beam a cell containing the opaque solution of iodine,
the light of the cone is utterly destroyed, while its 'invisible rays are scarcely,
if at all, meddled with. These converge to a focus, at which, though nothing
can be seen even in the darkest room, the following series of effects may be
produced :
" When a piece of black paper is placed in the focus, it is pierced by the
invisible rays, as if a white-hot spear had been suddenly driven through it.
The paper instantly blazes, without apparent contact with anything hot.
u A piece of brown paper placed at the focus soon shows a red-hot burning
surface, extending over a considerable space of the paper, which finally bursts
into flame.
" The wood of a hat-box similarly placed is rapidly burnt through. A pile
of wood and shavings, on which the focus falls, is quickly ignited, and thus a
fire may be set burning by the invisible rays.
" A cigar or a pipe is immediately lighted when placed at the focus of invi-
sible rays.
" Discs of charred paper placed at the focus are raised to brilliant incan-
descence ; charcoal is also ignited there.
" A piece of charcoal, suspended in a glass receiver full of oxygen, is set on
fire at the focus, burning with the splendour exhibited by this substance in an
atmosphere of oxygen. The invisible rays, though they have passed through
the receiver, still retain sufficient power to render the charcoal within it red hot
" A mixture of oxygen and hydrogen is exploded in the dark focus, through
the ignition of its envelope.
" A strip of blackened zinc-foil placed at the focus is pierced and inflamed
by the invisible rays. By gradually drawing the strip through the focus, it
may be kept blazing with its characteristic purple light for a considerable time.
This experiment is particularly beautiful.
" Magnesium wire, presented suitably to the focus, burns with almost into-
lerable brilliancy.
"The effects thus far described are, in part, due to chemical action. The
substances placed at the dark focus are oxidizable ones, which, when heated
sufficiently, are attacked by the atmospheric oxygen, ordinary combustion
1 "The glass in thin layers had a greenish hue: 1 have since found black glass far more diathermic."
INVISIBLE HEAT RAYS.
205
being the results. But the experiments may be freed from this impurity. A thin
plate of charcoal, placed in vacuo, is raised to incandescence at the focus of
invisible rays. Chemical action is here entirely excluded. A thin plate of
silver or copper, with its surface slightly tarnished by the sulphide of the metal,
so as to diminish its reflective power, is raised to incandescence either in -vacua
or in air. With sufficient battery-power and proper concentration, a plate of
platinized platinum is rendered white hot at the focus of invisible rays ; and
when the incandescent platinum is looked at through a prism, its light yields
a complete and brilliant spectrum. In all these cases we have, in the first
FIG. 1 88. TyndalVs Apparatus for showing the heating-power of the
Invisible Rays.
A, the lantern containing the electric lamp and silvered mirror; B, the plate-glass trough, having an
outer jacket, through which cold water circulates, to prevent the solution of iodine in bisulphide of
carbon boiling ; c, the cistern of water and pipe passing to jacket, B, and flowing away to D -, E, stand
to cairy zincfoil.
place, a perfectly invisible image of the coal-points formed by the mirror ; and
no experiment hitherto made illustrates the identity of light and heat more
forcibly than this one. When the plate of metal or of charcoal is placed at
the focus, the invisible image raises it to incandescence, and thus prints itself
visibly upon the plate. On drawing the coal-points apart, or on causing them
to approach each other, the thermograph of the points follows their motion.
By cutting the plate of carbon along 'the boundary of the thermograph, we
might obtain a second pair of coal-points, of the same shape as the original
ones, but turned upside down ; and thus by the rays of one pair of coal-points,
ao6
HEAT.
which are incompetent to excite vision, we may cause a second pair to emit
all the rays of the spectrum.
" The ultra-red radiation of the electric light is known to consist of ethereal
undulations of greater length, and slower periods of recurrence, than those
which excite vision. When, therefore, those long waves impinge upon a plate
of platinum, and raise it to incandescence, their period of vibration is changed.
The waves emitted by the platinum are shorter and of more rapid recurrence
than those falling upon it; the refrangibility being thereby raised, and the
invisible rays rendered visible."
ELECTRICITY,
FRICTIONAL OR STATICAL.
'"THERE is no branch of science more fascinating to the youthful mind than
. this most curious form or mode of motion.
By motion it is evoked. There is nothing more to do than to rub some
body, such as glass or sealing-wax, with silk or flannel, or to lay a warm
sheet of brown paper on a tea-tray, and rub it well with india rubber ; and the
electric force becomes apparent, either by creating motion again, causing
light substances, such as feathers or the down of feathers, to move towards
the surface on which the force has been set free, or if observed in a darkened
room, the sheet of brown paper is found to give light, a crackling sound is
heard, and small sparks are visible as the sheet of paper is drawn up from
the tea-tray.
This can be done over and over again. It is only necessary to dry the
paper by holding it before the fire, and the same attractive power, the same
curious fire, is apparent. The sheet of paper itself, after being well rubbed,
will move towards the body of the person who holds it up by one corner, and
is said to be attracted because it is electrified or electrized.
One of the " seven wise men of Greece," named Thales, from whose school
at Miletus, in Ionia, came Socrates and his disciples, has always been con-
sidered as the first who introduced a scientific method of philosophising
among the Greeks, 600 years before the Christian era.
To this philosopher is ascribed the following :
" That God is the most ancient being, who has neither beginning nor end ;
that all things are full of God ; and that the world is the beautiful work of
God."
A principle of motion, wherever it exists, is, according to Thales, mind.
208 ELECTRICITY.
Hence he taught that the magnet and amber (r/AeKrpov) are endued with a
soul, which is the cause of their attracting powers.* It is from the Greek
name of amber, a fossil resin, that the science derives its name " Elec-
tricity."
There are many substances which are electrized by friction gutta-percha,
the skin of a cat, sulphur, the different resins, and especially shellac, the
chief constituent of good sealing-wax, glass, and the greater number of
crystals, &c. On the other hand, there are many bodies, such as the metals,
in which, apparently, the power cannot be developed.
The earlier experimentalists divided all bodies into electrics and non-electrics:
the former they considered could be electrized by friction; the latter, apparently,
not so. It was then discovered that this classification was not a correct one,
and that the reason the so-called non-electrics did not show any electrical
energy when rubbed was because of their " conductivity ; " as fast as the
electricity was produced, it was conducted away to the earth and lost. Finally,
they discovered, by cutting off the conducting communication with the earth
by attaching the so-called non-electrics, such as a rod of metal to one of
glass and then rubbing it, that now the metal could attract light particles
down, pith of the elder, gold leaf, c. and was then said to be " insulated."
An instrument had now to be invented to indicate the disturbance of electrical
equilibrium : this instrument was appropriately called an " electroscope," or
instrument for showing electrical excitation. Commencing with the more
simple forms, we may trace them up to the most refined and delicate instru-
ments.
FlG. 1 89. A simple form of Electroscope.
A, the needle and cork; B, the cup attached to the feather.
I, The mouth of a clean, dry, empty wine-bottle is closed with a cork,
through which a short needle has been passed, the point being up-
* " Enfield's History of Philosophy," p. 82.
ELECTRICITY.
209
wards. On this point is balanced an eagle's feather, to which a little
cup made of glass, or any other convenient hard substance, has been
fixed. The glass cup, or cap, with the feather attached, resting on the
point of the needle, offers little or no resistance or friction, and hence
the feather moves freely like a suspended magnet in any direction.
When a stick of sealing-wax is rubbed and advanced towards the
feather, the latter is immediately attracted, and will follow the sealing-
wax round with great rapidity.
After the feather has been touched several times by the electrized
wax, it is now found, on approaching the electrified sealing-wax, that the
feather is repelled not so energetically as it was attracted, but quite
sufficiently so as to be distinctly apparent.
" Attraction " and " repulsion " are thus illustrated :
II. A glass tube or rod is bent at right angles, and the end fixed to some
convenient support, viz., a round or square piece of wood. A pith-
ball suspended from it by a silk filament becomes a sensitive and
simple electroscope or electric pendulum.
FIG. 190. An Electroscope.
A A, the glass support j B, the pith-ball suspended; c, the electrized glass.
If two balls are suspended side by side, and the electrified wax or
glass brought towards them, they are found, after being attracted to
and touching the electrized glass, to repel each other.
"Attraction" and "repulsion" are again demonstrated:
Another modification of the above may be arranged by making two
similar supports, like that in Fig. 190, and suspending a pith-ball from
each. If the two balls placed close together are electrized, they repel
each other; but if the two stands are moved a little way from each other,
14
110
ELECTRICITY.
and one electrized with the rubbed glass and the other with the rubbed
wax, the two balls attract each other.
FIG. 191. The two Stands and Pith-balls.
G is electrified with the rubbed glass ; w, with the rubbed wax.
N.B. A little tinfoil neatly pasted round the joints where the threads
are suspended assists the accumulation of electricity ; and if the pith-
balls are gilt and suspended by very fine hair-like wires of silver or
gold, the effects are more decided the pith-balls do not cling together.
In this experiment it would appear that the electricity from glass
attracts that from the wax; whilst separately (Fig. 190) they are mutu-
ally repulsive of their own particles, and hence one electricity was
called vitreous and the other resinous.
III. A very delicate electroscope is that in whicn the material to be moved
by the electrical force is itself remarkably light, and must be screened
from the air to prevent it being agitated or blown off by any current
of wind suddenly impinging upon it. The material is gold leaf, which
can now be purchased in books cut ready for use. It is usual to attach
two gold leaves to the opposite sides of a thin plate of brass, or card
covered with gold paper ; this is held by a pair of pincers, at the end of
a brass rod passing through a glass tube cemented in a brass cap, at-
tached to a bell-glass. By this mode of suspension the brass wire,
which terminates with a circular brass plate or table, is supported
on the glass tube (a bad conductor of electricity), and the tube and
cup are again supported by the bell-glass, so that good insulation is
secured. When great refinement is required, it is usual to place a
glass shade over the whole ; the latter is perforated at the top with a
hole, about one inch in diameter, through which the brass rod and
table are passed, and lumps of lime being placed in both glasses, the
air is kept dry, and, the aqueous vapour being absorbed, there is no
deposit of dew-like moisture under either of the glasses. (Fig. 192.)
A, brass table or disc, with wire attached, and pincers P, to hold the
. gilt card to which the gold leaves are attached ; C, the inner bell-glass,
ELECTRICITY
211
upon which the cap carrying the glass tube D, through which the brass
rod passes, is cemented ; E E, the outer glass shade, perforated with a
hole in the top, about I in. in diameter, to allow the brass rod to pass
through. N.B. The table or round plate unscrews from the wire, in
order to allow this to be done : both the inner bell-glass and the outer
glass shade fit nicely into grooves made in a square mahogany stand,
G G, neatly fitted with a drawer to hold quicklime. The part of the
stand covered with the two glasses is perforated with holes, in order
that the desiccating power of the lime may take full effect on the air
enclosed by the two glasses. It is sometimes usual in this electroscope,
called Bennet's, to place two rods and balls in the stand; so that, if the
gold leaves are too highly charged, they may not be torn off, but, by
touching the brass rods, the excess of electricity, which might damage
FIG. 192. A more delicate Electroscope.
the instrument, is carried off to the earth. For other reasons, the brass
rods connected with the earth exalt the power of the electricity applied,
however feeble it may be.
An electrized glass rod, brought towards the cap of the instrument,
causes the gold leaves to diverge or repel each other ; when left diver-
gent with the electricity from glass, they instantly fall on the approach
of an electrized piece of wax. The little table is convenient for stand-
ing any object on, or else a plain ball would perhaps be a better ter-
minal, as the edges of the table, unless nicely rounded, are apt to dis-
sipate the electricity.
It is not necessary to touch the cap of the electroscope with the
14 2
212
ELECTRICITY.
electrized bodies, in order to pass into or on the rod connected with the
gold leaves the electricity we wish to examine. By an influence called
" induction," to be more fully explained hereafter, the gold leaves are
found to possess the same kind of electricity as that enjoyed by the
electrized body.
Another electroscope invented by Dr. Robert Hare, of the University
of Pennsylvania, in which one gold leaf only is used, is worthy of par-
ticular notice here, and is described in Noad's " Manual of Electricity:"
" The leaf, about 3 in. long and 3-ioths of an inch wide, is suspended,
according to Singer's method, in the centre of a globular or other
shaped glass vessel from a brass wire surmounted with a brass cap. A
similar rod of brass, carrying at each end a small disc of brass or gilt
wood, about half an inch in diameter, passes through the side of the
vessel, so that the internal disc shall be immediately opposite the lower
end of the suspended leaf. This wire slides freely through a socket, so
that the internal disc may be adjusted at any required distance from
the leaf.
" When it is employed to detect electricity, the
lateral wire is uninsulated by hanging a wire from
it to the earth, and the body to be tested is brought
into contact with the cap. If the distance between
the gold leaf and the disc B is very small, the most
minute force of attraction is rendered apparent.
When it is required to determine the kind of elec-
tricity with which a body is charged, the insulated
disc B is brought as near as possible to the leaf,
and electrified vAhzv positively (with excited glass)
or negatively (with excited wax) ; the gold leaf is
first attracted, and then repelled. Under these
circumstances the body to be tested is brought
into contact with the cap or with D : if its elec-
tricity be of the same nature as that with which
the leaf is charged, the latter will diverge more
freely; if of the contrary nature, it will collapse
towards B.
" By placing a gilt disc on each side of the gold
leaf, Mr. Gassiot obtained signs of electrical exci-
tation from a single cell of the voltaic battery."
From the preceding experiments the following conclusions may be arrived at:
I. That an electrified body has the power to attract another which is not
electrical.
II. That two bodies similarly electrified repel each other.
III. That the electricity derived from glass is different from that obtained
from wax ; and that, being dissimilar, they attract each other.
IV. The two electricities have names to distinguish them from each other:
one is called vitreous, because obtained from glass ; and the other
resinous, because usually obtained from sealing-wax. The whole is
summed up in the two simple statements : Similar electricities repel
each other ; dissimilar electricities attract each other.
V- The electricity a substance gives out by friction is not always the same,
but depends on the nature of the rubber used, and other circumstances.
FIG. 193.
Dr. Robert Hare's single-
leaf Electroscope'
THEORIES OF ELECTRICITY. 213
Glass, when rubbed with a cat's skin, gives resinous electricity, and
vitreous if rubbed with silk. Polish and temperature, as shown by De
la Rive, exercise a remarkable influence. When bodies are highly
polished, they have a greater tendency to give by friction vitreous elec-
tricity, or to acquire it ; by elevating the temperature of bodies, they
have a greater tendency to acquire resinous electricity.
A piece of roughened or ground glass, rubbed against a smooth and
highly polished piece of glass, becomes resinous, whilst the smooth
glass is negative.
VI. No single electricity can be evolved without an equal excitation of the
other or opposite electrical force ; the rubber and the substance rubbed
are always in opposite states the silk handkerchief being resinous, the
glass vitreous.
Electricity being, as it were, a resident in all substances, it is said to be
quiescent when the two opposite forces have neutralized each other. It is then
called the static state of electricity; and this state is supposed to be the normal
condition of all bodies before they become electrical.
When the two electricities travel towards each other, or pass in sparks
through intervals of air, or move insensibly along a wire or other conductor,
it is said to be in a dynamic state, or condition of motion or circulation, which
becomes very evident in watching the motion of an electrical machine, or the
single voltaic circle of zinc and copper placed in acid and water. The dynamic
state is sometimes spoken of as electric tension, and an electric current as a
continuous dynamic state.
THEORIES OF ELECTRICITY.
By the theory of Du Fay, as altered by Symmer, it is supposed that two
forces, called fluids, exist in every substance, whatever may be its nature
solid, liquid, or gaseous.
Each of the two fluids is supposed to be very subtile and rare, quite impon-
derable, and consisting of particles that repel each other.
When the two fluids are separated, electrical effects are obtained ; and when
they unite, the electrical power ceases, for they have now combined to form
neutral fluid, or natural electricity. As before stated, one electricity is called
vitreous, and the other negative.
The repellent nature of the electrical particles is supposed to cause them
to arrange themselves on the surface of conducting bodies, where they remain,
because they are checked in their movement by the non or badly conducting
air with which they are surrounded.
Non or bad conductors are supposed to retain the fluids, and to interfere
with their movements.
This theory of Symmer is a most convenient and simple one for the young
student, and will help him to fix the main experimental truths of electricity in
his mind.
The second theory, devised by Benjamin Franklin, supposes that one fluid
only exists, the particles of which mutually repel each other. The electrical
fluid is supposed to be combined with all matter : matter without electricity is
supposed to be repulsive of its own particles. When a body is in a quiescent
2i4 ELECTRICITY.
electrical state, then the matter is exactly saturated with electricity, and it is
in a natural condition.
If the substance is rubbed, it either gains or loses the electrical fluid. The
acquisition of more electricity is said to confer a plus or positive state of elec-
tricity : the loss of the electricity places the substance in a minority with
regard to electricity; it is now said to be indued with minus or negative
electricity.
What Symmer terms vitreous electricity Franklin calls positive electricity;
what Symmer styles resinous electricity is called by Franklin negative electri-
city.
It is of little consequence which theory we adopt, for one or the other must
be wrong; most likely, both are untrue. We have seen that a certain vibra-
tion of particles will produce invisible heat rays, and, when they are quickened
in their pulsation, light rays ; as in TyndalFs experiments, the concentrated
invisible rays of heat, falling on a piece of platinum-foil, are converted into
visible or light rays. The same wave theory will doubtless be ultimately applied
to electricity, which may only be some remarkable vibratory state of the ether
pervading all matter and space. And this opinion was held, forty years before
Galvani, by Sultzer, who first experimented with pieces of silver and lead. By
placing them on opposite sides of the tongue, and then bringing the two in
contact, he noticed a peculiar metallic taste, like vitriol.
Here again it will be understood why so much space was devoted to the
consideration of the "universal ether," at the commencement of the article
on Light.
EXPERIMENTS WITH THE ELECTROSCOPE.
An electroscope is easily made with a wide, clean lamp-glass. A cork is
fitted into it, and through the cork is passed a wire, one end of which is beaten
out, so as to give a sufficiently large and flat surface; a pair of small gold
leaves are attached to this end of the wire, and to the other is fixed a round
piece of cardboard, covered with tinfoil or gold paper. When the wire is
passed through the cork, the gold leaves may be attached by moistening the
flattened end of the wire with a little gum, and bringing it carefully down
upon the cut gold leaves in the book. The second gold leaf is the most diffi-
cult to get on. When both leaves are in their places, the cork, wire, and leaves
may be placed in the lamp-glass, and the cardboard table fixed on the wire.
I. A little coffee, quickly ground in a mill, received in a warm dry beaker
glass, and then sprinkled upon the table or plate of the electroscope,
causes the leaves to diverge.
II. Some whiting or chalk, dried and put into the valve of a pair of bellows,
and then forced out upon the electroscope with the wind, very soon
causes the leaves to be deflected.
III. A large lump of sugar held over the electroscope, and sawed in various
places with a saw, affects the instrument as the sugar-dust falls upon it.
IV. After playing a tune on a violin with a dry and well rosined bow, if
the latter is passed lightly over the electroscope, electrical excitation
is apparent.
V. A roll of dry warm flannel rubbed against a stick of sealing-wax
EXPERIMENTS WITH THE ELECTROSCOPE. 215
causes the leaves of the electroscope to stand out, and repel each
other ; but they fall directly the sealing-wax is applied, because the
two electrical and opposite forces vitreous from the flannel and
resinous from the wax neutralize each other, the rubber and the
substance rubbed giving always the opposite states.
VI. While the leaves are divergent with the rubbed wax, bring an excited
glass rod or tube towards the electroscope, as before ; the leaves
fall immediately.
VII. Mr. Symmer, whose name has already been mentioned in connection
with one of the theories of electricity, tried some very amusing ex-
periments with silk stockings. He put upon the same leg a worsted
stocking, and over that a silk one, and rubbing the outer stocking
before a fire, he slipped the silk one suddenly off, and, the sides re-
pelling each other, the stocking appeared to be inflated, and to retain
the same shape as if the leg were in it ; and of course, if the silk
stocking had been carefully approached towards the electroscope,
the leaves would have been rendered powerfully divergent.
VIII. A crystal of Iceland spar cemented to an insulating glass rod, then
pressed in the hand, and placed immediately on a very delicate elec-
troscope, will cause a slight divergence.
IX. A disc of insulated cork, gently warmed and simply pressed against
another one of the same material, will show a certain minute amount
of electrical energy when applied to the electroscope, the warm disc
being usually resinous, and the cold one vitreous.
X. A stick of sealing-wax broken, and the fractured portion applied to the
electroscope, gives abundant evidence of electrical excitation.
XI. On a sheet of mica place the end of a stick of sealing-wax whilst in
the melted state, and as hot as possible ; allow the stick of wax to
cool and to adhere to the mica. If now the wax is suddenly pulled
so as to tear away a film, the fracture will disturb the electrical
quiescence of the mica, and it affects the leaves of the electroscope.
XII. A roll of sulphur broken across, and the bits powdered up in a mortar,
produce a very lively effect upon the gold leaves when brought in
contact with the cap or table of the electroscope.
XIII. The crystals of tartaric acid, boracite, and the tourmaline all become
electrically excited when heated, and affect the electroscope. Choco-
late fresh from the mill, as it curls in the tin pans in which it is
received, becomes strongly electrical. When turned out of the pans,
it retains this property for some time, but soon loses it by handling.
Melting it again in an iron ladle, and pouring it into the tin pans as
at first, will, for once or twice, renew the power ; but when the mass
becomes very dry, and powdery in the ladle, the electricity is revived
no more by simple melting ; but if then a little olive oil be added,
and mixed well with the chocolate in the ladle, on pouring it into
the tin pans, as at first, it will be found to have completely recovered
its electric power.
M. Becquerel's experiment with heating the tourmaline is performed
as follows : The crystal of tourmaline is supported in a stirrup of
paper, attached to a few filaments of silk, hung on to an insulating
rod of glass, attached to an upright pillar, so that it can be moved up
or down. The crystal is lowered so as nearly to touch a plate of copper,
2l6
ELECTRICITY.
heated below with a spirit-lamp ; and resting on the plate is a cylin-
drical glass, open top and bottom, like a wide but short lamp-glass.
Two pieces of covered bent wire, each carrying a little disc of gilt
paper, are placed over the top edge of the cylinder, and so arranged
that each disc shall nearly touch the end of the crystal ; or, better
still, the cylinder is perforated with two holes, opposite each other,
and the wires cemented in with their discs, and made to face the
poles or ends of the tourmaline. If each wire is separately con'
FIG. 194. BecquereVs experiment with the heated Tourmaline.
A, the suspended and heated tourmaline; B+, the wire conveying the + or vitreous electricity to
the electroscope c + ; B , conveying the or negative electricity to the electroscope c ; D, the
spirit-lamp heating the copper plate E.
nected with a delicate electroscope having very small gold leaves,
and the crystal warmed and then raised so as to be opposite to and
just touching the little gilt discs, one end of the crystal will give
vitreous or -j- electricity, the other resinous or electricity. The
effect is most powerful whilst the temperature is rising; when tr?
temperature becomes fixed, the electrical effect ceases. On reversing
the experiment and allowing the tourmaline to cool, the electricity
again becomes apparent ; but the electrical poles of the crystal are
reversed, the end that was -f- whilst being heated becoming in
EXPERIMENTS WITH THE ELECTROSCOPE. 217
the act of cooling. If the crystal is broken, the fragments, like the
parts of a broken magnet, each exhibits the opposite electricities at
their extremities. M. Gaugain states that the crystal should not be
heated beyond about 302 F. If raised to 752 F., the tourmaline
becomes a conductor of electricity ; it recovers its insulating powei
on cooling, but is then rendered hygroscopic ; this property it again
loses on being washed and dried at 302 F.
XIV. In the article on Electrical Induction, a still more delicate electroscope
called Volta's condenser electroscope, and another termed Peclet's
Multiplying Condenser, will be described. With the first of these
instruments the electricity derived from " chemical action " is dis-
tinctly shown. A clean platinum capsule, containing some distilled
water, is placed upon the Volta electroscope ; into this is immersed
a plate of zinc connected by a wire with the earth. The liquid
acquires a very feeble charge of + or positive electricity, and the
metal is found to be or negative : the very slight oxidizing power
of the water upon the zinc is supposed to produce this result. There
is no advantage gained by the addition of a little sulphuric acid,
because the conducting power of the water is increased, and the
two electricities have a tendency to re-unite directly they are
liberated : hence pure water is the best for this experiment.
XV. With the same electroscope (Volta's) the electricity eliminated by
combustion may be rendered apparent. The carbonic acid is allowed
to impinge upon a metallic plate placed in conducting communica-
tion with the instrument, the charcoal being burnt in connection with
the earth. The electricity is extremely feeble, but is found to be
definite, the carbon being --or negative, whilst the carbonic acid is
+ or positive. The combustion of hydrogen gas produces water ;
and in this combination of the former with oxygen, the hydrogen is
found to be or negative, and the steam + or positive.
XVI. It was contended by Pouillet to whom we are indebted for a large
number of these delicate experiments that when water is evaporated
electricity is always liberated ; if the water was alkaline, it charged
the electroscope with positive, if acid, with negative electricity;
hence it was easy, and seemed feasible, to propose a theory which
should account for the accumulation of electricity in the clouds,
the enormous amount of evaporation going on from the surface of
rivers, lakes, seas, being supposed to be a constant source of electric
power. Peltier has shown that the electrical effects are most likely
due to friction of the evaporating fluid against the sides of the vessel,
as the electricity is only liberated at the last moment, when the alka-
line matter is crackling against the vessel in the act of becoming
solid. Moreover Faraday demonstrated that the steady evaporation
of water from a platinum dish did not produce electricity ; if, however,
the dish was made very hot, and a large drop of water allowed to
fall into it, the latter assumed the spheroidal state, and no electricity
was apparent until the temperature of the platinum dish was allowed
to fall, and the drop of water to boil violently and to rub against the
sides of the vessel. It will be seen presently that the further develop-
ment of this idea led to the construction of the powerful steam hydro-
electric machine at the Polytechnic Institution.
218 ELECTRICITY.
XVII. The slow oxidation of zinc by the air has been used by De Luc, who
contrived the dry pile. The dry pile is, however, useless if allowed
really to become dry; it has been found that, when the moisture
naturally present in all paper is thoroughly removed, the action of
the dry pile diminishes and almost ceases, but is easily restored
by the admission of damp air, which gives back to the paper its
natural amount of moisture. The dry pile is usually made by
arranging, in a tube capped at both ends with brass, discs of thin
sheet-zinc paper or silver-foil, and the following are Mr. Singer's
directions for the construction of a dry pile :
" The materials I prefer for these piles are thin plates of flatted
zinc, alternating with writing or smooth cartridge paper and silver
leaf.
" The silver leaf is first laid on paper, so as to form silvered paper,
which is afterwards cut into small round plates by means of a hollow
punch.
"In the same way an equal number of plates are cut from thin
flatted zinc and from common writing-paper.
" These plates are then arranged in the order of zinc paper, silvered
paper with the silver side upwards, zinc 'upon the silver, the paper,
and again silvered paper with the silvered side upwards, and so on ;
the silver being in contact with zinc throughout, and each pair of zinc
and silvered plates separated from the next pair by two discs of paper.
" An extensive arrangement of this kind may be placed between
three thin glass rods, covered with sealing-wax, and secured in a tri-
angle by being cemented at each end into three equidistant holes in
a round piece of wood ; or the plates may be introduced into a glass
tube, previously well dried, and having its end covered with sealing-
wax and capped with brass ; one of the brass caps may be cemented
on before the plates are introduced into the tube, and the other after-
wards. Each cap should have a screw pass through its centre, which
terminates in a hook outside. This screw serves to press the plates
closer together, and to secure a perfect metallic contact with the
extremities of the column."
FlG. 195. De Luc's "Dry Pile" connected with two Electroscopes.
If a tube containing one thousand alternations is laid upon two
electroscopes, as in Fig. 195, the zinc end is found to be positive,
and the silver negative. Mr. Singer continues :
u I found a series of from twelve to sixteen hundred groups,- which
EXPERIMENTS WITH THE ELECTROSCOPE. 219
are arranged in two columns of equal length, which are separately
insulated in a vertical position : the positive end of one column is
placed lowest, and the negative end of the other, their upper extremi-
ties being connected by a wire, they may be considered as one
continuous column. A small ball is situated between each extremity
of the column and its insulating support ; a brass ball is suspended
by a thin thread of raw silk, so as to hang midway between the balls ;
and at a very small distance from them.
" For this purpose the balls are connected during the adjustment
of the pendulum by a wire, that their attraction may not interfere
with it ; and when this wire is removed, the motion of the pendulum
commences. The whole appararus is placed
upon a circular mahogany base, in which a
groove is turned to receive the lower edge
of a glass shade, with which the whole is
covered."
Mr. Singer directs that, in order to preserve
the power of the columns, the two ends should
never be connected by a conducting sub-
stance for any length of time. It is there-
fore necessary, when laid by, that it should
be placed upon two sticks of sealing-wax, and
that the terminal balls be half an inch or so
from the table.
If a column which appears to have lost its
power be thus insulated for a few days, it will
recover. There is another cause of deteriora-
tion, which is more fatal: this is too much
moisture. The paper discs therefore should
be made as hot as possible before they are ~, , , .
put together ; or even subjected to a con- The per P etlial Chune,,
tinued but gentle heat for some time before
they are inclosed in the glass tube, and, that
being heated also, the plates may be inclosed
without the presence of any appreciable moisture.
The size of the plates may be f ths of an inch in diameter, or less.
With a column of 20,000 alternations a Leyden jar may be charged,
and minute sparks are visible when contact is made with the fine
points of wire connecting the two extremities.
When the dry pile is attached to the electroscope of Hare by sub-
stituting the poles of two of De Luc's columns for the gilt disc (Fig.
193, p. 212), the instrument is made wonderfully delicate, so much so
that Mr. Sturgeon describes an arrangement of this kind, the delicacy
of which he states to be such that, the cap being of zinc, and of the
size of a sixpence, the pendent leaf is caused to lean towards the
negative pole by merely pressing a plate of copper, also the size of a
sixpence, upon it, and when the copper is suddenly lifted up the
leaf strikes. The different electrical states of the inside and outside
of various articles of clothing were readily ascertained by this deli-
cate electroscope. Bohnenberger has the credit of making the first
of these instruments.
FIG. 196.
constructed with De
Luc's cohtmns.
220
ELECTRICITY.
FIG. 197. Bohnenberger's
Electroscope.
Tfye gold leaf, being in equilibrium, and neither
attracted or repelled, is instantly moved to one
side or the other when the very smallest amount
of electricity is evolved on the cap of the instru-
ment.
From these various experiments with electro-
scopes it maybe learned that friction under every
circumstance, and even when disguised, as in the
rapid evaporation of water from a hot surface, is
an important source of electricity ;
That there are two kinds or conditions of elec-
tricity which exactly neutralize each other, and
they are always evolved together ;
That pressure, or any modification of mecha-
nical motion, such as fracture, rending, or tear-
ing, all cause electrical quiescence to be dis-
turbed ;
That heat, as applied to various crystals, sets
their particles in motion, and causes the evolu-
tion of electric force ;
That chemical action is a source of electricity,
of a tension similar, though not equal, to that of
ordinary friction, as shown in De Luc's column.
A, the gold-leaf suspended between the two poles, B B, of the dry pile.
ELECTRICAL MACHINES.
In the year 1777, Tiberius Cavallo, a thoroughly practical and learned elec-
trician, describes, in his " Complete Treatise on Electricity," the construction
of the cylinder electrical machine of his day. It will not be found to differ
materially from that made in 1868. He remarks
" The principal parts of the electric machine are the electric, the moving
engine, the rubber, and the prime conductor, i.e., an insulated conductor which
immediately receives the electricity from the excited electric."
The electric formerly used was made of different substances, as glass, resin,
sulphur, sealing-wax, &c.; and in different forms, as cylinders, globes, sphe-
roids, &c. (Fig. 198.)
The three glass globes are made to rotate and rub against three cushions.
The conductor, a piece of metallic pipe or a gun-barrel, was suspended from
the ceiling by silken cords, and connected with the globes by unravelled gold
lace hanging down, the latter being used for the same purpose as the points
now attached to all conductors of electrical machines.
"This diversity," continues Cavallo, speaking of the various shapes and
nature of the electric used, " then obtained on two accounts : first, because it
was not ascertained which substance or form would answer best; and, secondly,
on account of producing a negative or positive electricity at the pleasure of
the operator ; for, before the electricity of the insulated rubber was discovered,
sulphur, rough glass, or sealing-wax was generally used for the negative elec-
ELECTRICAL MACHINES.
221
FlG. 198. Dr. Watson's Electrical Machine,
Showing the first application of the cushion as a rubber, instead of the hand.
tricity." The reader will perceive that Cavallo adopts the Franklinian theory.
" At present smooth glass only is used ; for, when the machine has an insu-
lated rubber, the operator may produce positive or negative electricity at his
pleasure, without changing the electric.
" In regard to the form of the glass, those commonly used at present are
globes and cylinders.
" The cylinders are made with two necks ; they are used to the greatest
advantage without any axis (or rod passed through from neck to neck) ; and
their common size is from 4 in. diameter and 8 in. long to 12 in. diameter and
2 ft. long, which are perhaps as large as the workmen can conveniently make
them.
" The glass generally used is the best flint, though it is not yet absolutely
determined which kind of metal is the best for electrical globes and cylinders.
The thickness of the glass seems immaterial, but perhaps the thinnest is
preferable.
" It has often happened that glass globes and cylinders in the act of whirling
have burst in innumerable pieces with great violence, and with some danger
to the bystanders. Those accidents are supposed to happen when the globes
and cylinders, after being blown, are suddenly cooled.
" It will, therefore," prudently remarks Cavallo, " be necessary to enjoin the
workmen to let them pass gradually from the heat of the glass-house to the
atmospherical temperature."
The author prefers a single handle, instead of the multiplying gear, which
is very apt to get out of order, and the cord to stretch or break when most
222 ELECTRICITY.
wanted. The various parts of the machine just described were gradually in-
vented and applied by various clever electricians, Otto Guericke, Hawkesbee,
Abbe* Nollet, Dr. Watson, Wilson, Nairne, Dr. Priestley; and many years
elapsed before the machine attained anything like the perfection of that em-
FlG. 199. Cylinder Electrical Machine, used by Cavallo in 1777,
Showing the glass cylinder A A, with a pulley attached to one neck, B, round which an endless cord
passes to a large or multiplying wheel, c; the cushion E, and silk flap F: the cushion, placed
on a glass pillar let into a piece of wood, moves backwards and forwards in a groove, c, and is se-
cured by a screw ; before use, is covered with amalgam. The machine is clamped to the table at H.
The prime conductor i i, with collecting points K, is supported on glass legs, L L, let into a maho-
gany stand. The amalgam used by Cavallo consisted of two parts mercury and one of tinfoil, with
a little powdered chalk, all rubbed up with grease.
ployed by Cavallo in his experiments. A more elegant and compact form is
now given to the cylinder machine by Messrs. Elliott, of the Strand.
The most convenient form is undoubtedly the plate electrical machine. Of
this Cavallo says
"Next to Dr. Priestley's machine, I shall describe another, which was
invented by Dr. Igenhouz, and which, from its simplicity and conciseness,
makes a fine contrast with the former.
" This machine consists of a circular glass plate, about i ft. in diameter,
which is turned vertically by a winch fixed to the iron axis that passes through
its middle ; and it is rubbed with four cushions, each about 2 in. long, situated
at the opposite ends of the vertical diameter."
Fig. 200 is a drawing of the- large plate electrical machine in use at the
Polytechnic. The plate glass is 7 ft. in diameter, and rather more than fths
of an inch thick ; it has two large rubbers, and, when these are well amalga-
mated, and the weather is propitious at least dry very long sparks -of great
intensity may be obtained; when the atmosphere is damp, in spite of the
rapidity and power with which it is turned round by a four-horse power steam
engine, it will hardly give a spark an inch in length.
The prime conductor is a large globe, about 3 ft. in diameter; and inserted
into this is a large ring of wood, 4 ft. in diameter, and raised 6 ft. from the
globe being an arrangement first proposed in connection with the Austrian
electrical machines exhibited in the Great Exhibition of 1862. The ring, no
doubt, theoretically speaking, should act as a condenser, and assist, by induc-
tion, to increase the tension of the electricity ; but whether it be due to the
ELECTRICAL MACHINES.
223
FlG. 200. The great Plate Electrical Machine at the Royal Polytechnic.
height of the building in which the ring is placed, or from other causes, the
effect produced did not appear to be increased by this addition to the apparatus.
The power of an electrical machine is greatly influenced by the nature of the
glass. There is a very fme-looking machine at the Polytechnic, constructed
on the plan of the late Sir William Green Harris : the plate is 3 ft. in
diameter; but, in consequence of the alkali of the plate glass, its power is
very slight, and not half so good as that of many small cylindrical machines.
_ The best -amalgam for an electrical machine is made of I part of tin, 2 of
zinc, and 6 of mercury. Melt the zinc and tin together, and, when approaching
solidification, add the mercury, and stir till the whole is solid : if the latter is
added when the alloy of zinc and tin is too hot, much of it may be dissipated
in vapour ; and the amalgam should be made under a chimney, to avoid the
fumes of mercury. Sometimes the above are rapidly melted together, and
then placed in a wooden box and shaken until quite cold. The shaking
reduces the greater part to a fine powder, which may be sifted out and used
224
ELECTRICITY.
with grease. The author always lays a coating of tallow-grease on the
cushion first, and then carefully sifts the amalgam upon it, laying all smooth
with a clean broad knife or spatula.
Very cheap machines can be made frc-m common window-glass, to the
centre of which, and on the opposite sides, two wooden caps, turned convex,
may be cemented without any perforation of the plate, the axle being made
of glass rod fitting into holes in the wooden caps. Mr. Goodman recom-
mends that the cement used should be made of equal parts of black resin
and beeswax ; but the writer recommends the use of less beeswax, because
it renders the cement liable to melt easily if the electrical machine is placed
before a fire to be warmed ; the quantity may be I Ib. of black resin to 3 oz:
or 4 oz., at the most, of beeswax. A plate of this kind would cost half-a-crown.
Sometimes two circles of common window-glass are cemented together to
increase the thickness, and prevent the chance of breakage. The common
window-glass, from its hardness, gives a large quantity of electricity when
the friction is properly and equally applied. Very excellent machines are
now made of plates of vulcanite, and, in fact, are used for mining purposes.
The plate is enclosed in a box of vulcanite, and turned by a handle on the
outside. It contains one or more Leyden plates ; and after these are charged
by a few turns of the machine, the discharge may be sent through covered
wire into one of Professor Abel's fuses
at a distance of many hundred yards.
The writer has used such a machine
on a damp night in November, in the
grounds of Haileybury College, Herts,
with the greatest success, exploding
three separate charges one of gun-
powder, directed against a heavy gate;
another, a mine, blowing many tons
of earth into the air ; and a third, a
keg of 9 Ibs. of gun-cotton, made by
Messrs. Prentice, of Stowmarket,
which nearly emptied a pond in which
it was exploded, and, sad to relate,
broke a great many windows in the
chapel by the terrific concussion of
the air, although the building was at
least three hundred yards from the
pond where the explosion took place.
Mr. Hart, of Edinburgh, describes a
very compact and well-arranged ma-
chine, called Winter's Electrical Ma-
chine.
Winter's Electrical Machine is one
of the most perfect forms of the plate
friction machine that has hitherto
been made. It distinguishes itself from
other machines by the extraordinary length of the spark that it gives, by sim-
plicity of construction, and by the uniformly good results that are obtained
from it, even when the state of the air is not favourable to the display of electrical
phenomena. The annexed drawing represents one of these instruments. It
FIG. 201.
Winter's Electrical Machine.
ELECTRICAL MACHINES. 225
will be seen from it that the glass plate is fixed into an axle, which revolves in
two upright supports. One of these, in which the shorter wooden end of the
axle revolves, is made of glass, and the other, in which the longer glass end
of the axle revolves, is made of wood. By this means the electricity formed
upon the plate cannot on either side reach the ground, for on the one side the
insulating glass pillar, and on the other the insulating glass axle, prevents it,
and thus complete insulation of the plate forms one of the elements of the ex-
cellence of Winter's machine. The friction in this, as well as in all friction-
machines, is caused by pressing on the plate of glass a flat surface of leather,
covered with an amalgam of mercury, zinc, and tin, which is put on with
the aid of a little grease. The frame standing on the low glass support to
the right of the figure is the wooden rubber frame, into the notches of
which fit two flat pieces of wood covered in front or on the s4de next the plate
with leather and a very little stuffing, and provided on the other side with
springs, which, acting against the frame, keep the front surface uniformly
pressing against the plate. There is only one pair of rubbers, not two, as in
ordinary machines, and this enables them to be placed at a greater distance
from the prime conductor of the machine.* The brass ball standing on the
tall glass support to the left is the prime conductor. For the sake of more
perfect insulation, this ball is fitted on to the support by means of a. trumpet-
shaped opening made in it, thereby preventing the dispersion of electricity that
would arise from the sharp edge of a hole exactly large enough for the rod.
There are three other openings in this ball, one on each side and one at the
top. The two small rings which are seen projecting upon the plate fit into
one of these by means of a T-shaped piece of brass. They are made of wood,
and have a groove cut in them on the side turned towards the plate, into which
a row of fine pin-points is fixed for collecting the electricity formed upon it.
These points are connected with the prime conductor by means of a strip of
tinfoil which lines the bottom of the groove. Two wings of oiled silk attached
to the rubbers stretch between them and these rings, so as to prevent the
electricity from dissipating itself before reaching them. The opening on the
top of the ball is made to receive the stalk of the large wooden ring, which is
seen surmounting it, and which forms the most peculiar feature of the instru-
ment. An iron wire forms the core of this ring, and is in metallic connection
with the prime conductor. The function performed by this remarkable appen-
dage is to lengthen the sparks given by the machine. In a 24 in. plate, for
instance, with the aid of the ring, the sparks are 14 in. in length, and without
it scarcely two. The remaining opening in the prime conductor is for the
stalk of the small brass ball from which the sparks are obtained. To the left
of the figure is the spark-drawer for receiving the sparks from the machine.
The length of the spark given by an electrical machine is by far the most
severe test of the excellence of its construction, and, in this respect, Winter's
machine is entitled to hold the first rank among friction-machines. A machine
12 in. in diameter costs ^5.
Another and most interesting electrical machine, by which the apparent
anomaly of frictional electricity without friction is realised, was exhibited in
the Great French Exhibition of 1 867, and described by a careful observer, in
the Mining Journal.'
" In appearance, Holtz's machine resembles the ordinary plate machine;
in fact, the most prominent part is a glass disc, which is mounted and
.revolved in the usual manner. But the plate is thinner the thinner the
15
22 6 ELECTRICITY.
flG. 202. The Holtz Electrical Machine, giving "frictional electricity"
without friction.
better and as it is desirable to revolve it very rapidly, a multiplying wheel
is connected with the plate, so that the speed may be increased to the extent
desired. The machine, however, has really but little resemblance to the
plate machine, for it has no rubbers ; it produces torrents of frictional elec-
tricity, but the electricity is not generated by friction ; there is no friction
about the machine, except at the axle bearings. The plate revolves in free
air, and nothing should touch it. In the place of rubbers are what are called
inductors, which are strips of paper 3 or 4 in. long, and about i in. wide.
They are supported and insulated on pieces of glass, which are of spear-head
form. The inductor is made complete by Dasting on to the paper pointed
pieces of cardboard, which project beyond the glass spear-heads an inch or
two. The spear-heads are attached to the framework of the machine, so that
they shall be parallel, and as near as possible, to the plate on its crank side.
Opposite the inductors, at the front of the plate, are the comb points, which
serve to collect the electricity, and convey it to the conductors for use. Each
inductor is furnished with its set of points. The combs are attached to brass
rods, terminated at their other ends by brass balls. The rods are fastened
to the framework of the machine, and are insulated from it. The balls at
the ends of the rods may be connected to each other in any desired order by
means of bent wires.
" To obtain the electricity, one of the inductors is slightly charged, by
means of an excited rod of hard rubber, glass tube, or otherwise, and turning
the crank. Its power progressively increases for about a minute, and until it
reaches the maximum, when it furnishes a steady supply of electricity as long
ELECTRICAL ATTRACTION. 227
as the disc is revolved. The amount of electricity which a disc of only 2 ft.
in diameter will yield is enormous.
" To explain the action of the machine three elements must be considered
the inductor, the plate, and the comb points. If a pointed wire be brought
opposite an electrified body as, for example, a prime conductor the positive
electricity of the prime conductor attracts the negative of the wire, and repels
its positive, and a stream of negative electricity flows out of the wire at its
point, while the positive flows to the opposite direction. Now, suppose a
sheet of glass be interposed between the point and the conductor. The
attraction of the positive electricity of the conductor for the negative of the
wire is by no means lessened ; the negative is accumulated towards the point,
and, by reason of its higher tension, flows out on to the glass. But the glass
is impervious to the electricity, and it remains on its surface; the glass
becomes electrified. In Professor Holtz's machine we have the electrified
body in the inductor, the wire point opposite, and the glass plate interposed.
Suppose inductor No. i electrified positively, this positive electricity attracts
negative electricity out of the comb points on to the interposed plate. The
plate moving on the part electrified negatively comes opposite card points of
inductor No. 2. Here the negative electricity of the plate draws out of the
card points positive electricity on to the glass, and inductor No. 2 becomes
charged negatively, while the glass is negatively charged on the further side,
and positively charged on the near side. Inductor No. 2, being charged
negatively, draws positive electricity out of comb points No. 2, and neutra-
lizes the negative drawn from comb points No. i. Card points No. 3 dis-
charge negative electricity on the plate, and inductor No. 3 becomes positive,
and, like No. i, draws negative electricity out of the corresponding comb
point. It will be seen that the alternate inductors are oppositely electrified,
and that their corresponding comb points give out or receive accordingly.
By varying the manner of connecting the balls at the extremities of the comb
points a considerable variety of changes in the relation of the quantity and
intensity may be obtained. These variations are somewhat similar to those
which are secured by varying the order of connecting the elements of the
galvanic battery. The greatest intensity is obtained by connecting the
inductors as they stand in numerical order round the disc. By connecting
one of the poles with the ground, the other may be used as a prime conductor
for charging Leyden jars, &c. It is found advisable, in order to secure more
perfect insulation, to varnish the plate and the inductors with shellac varnish."
ELECTRICAL ATTRACTION AND REPULSION GOVERNED BY
CERTAIN LAWS.
The electroscopes already described are merely intended to indicate the
development of electricity ; their construction does not permit any calculation
to be made as to the quantity of the force present in or upon any given surface.
In using a common magnet to attract a needle, it is evident that distance
regulates the intensity of the power, which increases rapidly as the two are
brought in closer proximity, or decreases as quickly by increasing the distance
between them.
15 2
228
ELECTRICITY.
The influence of distance is particularly shown in experiments with static
electricity, and the phenomena were carefully examined by Coulomb, who
determined the laws which bear his name.
First Law of Coulomb. Two electrified bodies attract or repel each other
with a force which is inversely proportional to the square of the distance that
separates them.
Example : An electrified body at a certain distance exerts a force which
may be called unity or one ; at half that distance the power is four times
greater; at one-third, nine times; one-fourth, sixteen times greater, and so on.
Second Law. The distance remaining the same, the attractions or repul-
sions are in the compound ratio to the quantities of electricity which the two
bodies possess.
Example : A fixed electrified ball, which will repel another and movable one
to a certain distance, called unity or one, will have only half the power if con-
nected with another ball of the same size, the charge distributed over one
surface is now spread over double the surface ; and if this again is connected
with another ball, the force is halved again, and possesses only a quarter of
its original power.
FIG. 203.
A, an insulated ball electrized with a force to be called unity, i ; A B, the same ball touching another
ball of the same size, B. The charge is now spread over twice the surface, and the force is reduced
one-half. B, the ball with one-half the charge ; B c, the same ball touching another, c. The original
charge is again spread over twice the surface, and the force reduced at A B to one-half is now reduced
at B c to a quarter.
On the same principle, by reversing the previous experiments and increasing
the charge, if a series of balls, gradually decreasing in size, are attached to
any given-sized ball, they must end in a very small ball, or that to which it is
equivalent, viz., a point ; consequently the charge increases in intensity, instead
of diminishing : and hence the use of points, which discharge electricity very
rapidly ; or receive it, as in the points attached to the prime conductor of an
ordinary electrical machine. The electric force tends to escape from the sur-
face of conductors by virtue of the repulsion of its own particles.
The force it exerts is considered proportional to the square of the quantity ;
hence, if the accumulation of electricity in eight balls decreasing in size be
taken as I, 2, 3, 4, 5, 6, 7, 8, the force will be the square, as shown in Fig. 204.
The last ball, which is eight times less in area than the first, is charged eight
times more than the first, and the force, or desire to escape or polarize the
surrounding particles of air, is increased by the square, viz., sixty-four times.
ELECTRICAL ATTRACTION.
229
64 4;9 36 25
16
FIG. 204. The rationale of a Point, and why it gives off Electricity.
The lines show how a point is arrived at from a series of spheres gradually decreasing in size.
These laws, and the applications which flow from them, were discovered by
Coulomb'with the very delicate instrument called the Torsion Electrometer, or
Torsion Balance (Fig. 205). It consists of a cylindrical or cubical glass box,
carrying upon, but communicating through, the upper pane of glass a vertical
tube 15 or 20 in. high. The box may be 12 in. high.
FIG. 205.
At the top of the tube is a graduated circle and pointer, and inside this, and
exactly in the centre, is attached a fine silver or platinum wire, stretched by a
little weight.
The wire suspended from the top of the tube is long enough to reach to the
centre of the box ; and through the weight that stretches it is passed a hori-
zontal needle of gum lac or glass. The needle is not suspended like a balance ;
but one arm is longer than the other, and carries a little disc of gold paper or
a small gilt pith-ball.
In order to measure the space traversed by the needle, a proper scale is
230 ELECTRICITY.
placed in the centre of the front glass pane ; and, before commencing experi-
ments, the zero of the circle carried by the tube is made to correspond with
the zero of the scale in the box ; and this can be done by carefully moving
round the top scale, to the inside of which is attached the metallic wire carry-
ing the little weight and needle.
The needle is affected by the electricity from a ball or disc of exactly the
same size as that attached to the needle, which is supported by an insulating
stem. It is introduced vertically in another hole made through the top pane
of glass, and the whole is so arranged that, when the ball of the needle is in
contact with the other, the needle is in the direction corresponding to the zero
or o of the two scales.
In the cylindrical glass Coulomb balance the vertical ball is suspended
by a metallic rod, which goes through the top, and is attached to another ball.
It is not then removed as in the square-box balance ; but a " proof plane " on
an insulating handle is applied to the electrified body under examination, and
this is caused to touch the outer ball of the balance.
In the square-box balance the ball is removed by its insulating handle, and
the electrified body under examination is touched with it, and the ball placed
inside the box. According to the first law, it immediately divides its electri-
city with the ball of the needle, which latter, being repelled, describes a larger
or smaller arc, according to the intensity of the charge.
Directly the needle is repelled, the wire must be twisted ; and this is called
the force of torsion. Coulomb ascertained that the forces of torsion are propor-
tional to the angles of torsion ; or, in other words, the force that causes the
torsion or twisting of the wire is exactly proportional to the arc described by
the needle.
Supposing the needle to be repelled to the distance of 36, in order to
compel the needle to come to 18, the top circle on the tube must be moved
round 1 26 degrees ; from which it follows that the wire, if twisted 1 8 below
and 126 above, makes up a torsion equal to 144. Under the same circum-
stances, to reduce the arc to 9, an angle of 576 of torsion must be used.
The relation of the 36, 1 8, and 9 are to each other as the angles of torsion,
36, 144, 576, or these angles are to each other as I 14: 16; hence, if the
distances are to each other as I : \ : , the repulsive forces are to each other
as i 14: 1 6, and the first law of Coulomb is proved.
The late Sir William Snow Harris employed a delicate brass scale-beam,
suspended from a curved brass rod fixed to an insulating support ; the beam
carried a circular gilded plane from one arm, and the scale from the other ;
the gilded plane is suspended over another plane of the same size, which can
be raised or depressed at pleasure. The attraction between the two planes
was estimated by the weights raised, and the instrument is known as Harris's
Balance Electrometer (Fig. 206).
It is with these apparatus (the bifilar balance and balance electrometer),
and by greatly varying his experiments, says De la Rive, " that Sir W. Harris
found that the law of the inverse of the square of the distance is not exactly
sustained, except when the balls or the discs are charged with an equal quan-
tity of electricity, when this quantity is not too feeble, and, finally, when the
angular distance that separates them is greater than 9.
" Otherwise, and especially if the electric charges of the two bodies are very
different, the force becomes the inverse of the simple distance, within certain
limits. The same causes equally modify the second law, which establishes
ELECTRICAL ATTRACTION.
231
the relation existing between the quantities of electricity and the attractive or
repulsive forces. Thus in one experiment, the respective quantities of elec-
tricity being successively on each of two discs in turn i and 2, the corre-
sponding repulsive forces, instead of being i and 4, were i and 5. This devia-
tion from the law was due to the absolute intensity of the electricity being too
feeble. But it is much more sensible when there is inequality in the electric
charges of two bodies, and when this -inequality is very great.
" These numerous exceptions to Coulomb's laws are in a great part due to
there occurring to electrised bodies,
when in presence of each other, impor-
tant modifications in their electric state,
by the effect of influences whose action
we shall study further on influences
which are the more sensible as the elec-
tric charges are more different.
"They depend also upon its being
very probable that the laws in question
are general only for points almost ma-
thematical, and not for bodies of any
forms or dimensions.
" Now we conceive that they must be
so when we employ, as Coulomb did,
small equal spheres for electrised
bodies ; for, as is demonstrated in me-
chanics, the action of a sphere is always
the same as that which would be exer-
cised by its centre, supposing all the
forces with which the sphere is en-
dowed were concentrated in this centre.
We see, therefore, that Coulomb's laws
may be regarded as general by restrict-
ing them to the cases of electrised
molecules or points ; and that in other
cases they maybe regarded as deviating
less from the truth as the bodies are of
smaller dimensions, and as the forms
approach more or less the spherical
form."
When a sponge, or any other porous
matter, is dipped in water, the latter is
taken up by the whole mass, and dif-
fused through it. Similar ideas might
be formed with respect to a charge of electricity that it spreads itself through
the whole body of the conductor on which it was rendered evident ; this, how-
ever, is not the case ; the electricity arranges itself on the surface of electrified
bodies.
Hence balls and cylinders used in the construction of electrical apparatus,
such as conductors, are always made hollow : solidity is not necessary. The
most conclusive experiment to prove the fact already stated was that made by
Coulomb. Having insulated a sphere of metal, it was charged with electricity,
and on the application of two hemispheres, supported, of course, by glass rods,
FIG. 206.
Harris's Balance Electrometer.
2 3 2
ELECTRICITY.
the whole of the charge was removed when they were taken away and applied
to an electroscope ; whereas the original ball first charged did not exhibit the
slightest charge when tested with the same instrument.
FIG. 207. Coulomb's Experiments, showing the distribution of the Electricity
on the surface of insulated Conductors.
The ball being first electrified, and the hemispheres applied, which remove the charge.
Faraday, whose name is so completely identified with the subject of electri-
city, devised many clever experiments to show the fact.
One of the best is where he uses a conical muslin bag attached to an insu-
lated ring. At the apex of the cone, both
outside and inside the net, is a silken thread
for the purpose of turning it inside out.
When the bag is charged, a "proof plane,"
i.e., a metallic plate attached to a glass or
wax rod, or a small disc of gilt paper fixed
to a thin rod of shellac or glass, is placed in
the interior, and then applied to a delicate
electroscope, which remains unaffected.
If, however, the proof plane touches the
outside of the bag, a charge is obtained,
which is rendered evident directly the elec-
troscope is used. By turning the bag inside
out (whilst insulated and charged) with the
dry silk string, silk being a non-conductor,
the condition is reversed ; that which was
FIG. 208. the outside is now the inside, and gives no
Faraday's Experiment with the evidence of electrical excitation, whilst that
conical Muslin Bag. which was inside is now outside, and, of
ELECTRICAL ATTRACTION.
233
course, will deliver a charge to the proof plane. A bird, a white mouse, some
gunpowder, and a delicate electroscope, placed inside a wire-gauze cover, such
as might be used for protecting meat, standing on a stool with glass legs, and
connected with the electrical machine when in full action, will give an abun-
dance of sparks from the outside, which do not affect the living things, the
gunpowder, or the electroscope in the slightest degree.
FIG. 209. The Mouse, the Bird, Gunpowder, and Electroscope under a Wire
Gauze Cover.
When two gilt pith-balls, hanging side by side, and suspended from an in-
sulating stand, are electrised, they stand out and repel each other, with a force
indicated by the distance at which they are separated. The distance is a
rough measurer of the intensity or energy of the charge.
By attaching the pith-balls to an insulated cylinder (Fig. 210), round which a
riband is wound up close, and conveying a charge from the electrical machine,
they repel each other for some time, and remain in that state in dry and moder-
ately warm air. If, however, the flap or riband of silk is unwound by the glass
handle, the electricity is spread over a larger surface, the intensity of the ori-
ginal charge is diminished, and this is shown by the pith-balls falling together,
and again returning to their original distance, or nearly so, when the glass is
again wound up.
This experiment is quite in accordance with the laws of Coulomb, already
explained at page 228. Instead of the proof plane, a " carrier ball," as Fara-
day termed it, may be used ; this is made of some nicely turned light wood,
covered with gold paper, and supported by a silk thread, well dried, and satu- '
rated with shellac. The latter is easily dissolved in methylated spirit, by
digestion in the cold for a day or so. Of course, warming the spirit by putting
the bottle on a piece of wood standing on the hob of a grate will accelerate
the solution ; but care must be taken to avoid the chance of its taking fire.
Most of the above experiments were devised by Faraday ; but it is easily
shown that the principle of distribution and the proof that electricity resides
234
ELECTRICITY.
FlG. 210. The Cylinder charged, and the flap unwound.
on the surfaces of metallic electrified bodies was well known and shown by
Cavallo in his book published in 1777. The experiment quoted is called
THE ELECTRIC WELL,
" Place upon an electric stool a metal quart mug, or some other conduct-
ing body nearly of the same form and dimension ; then tie a short cork-ball
electrometer at the end of a silk thread proceeding from the ceiling of the
room, or from any other proper support, so that the electroscope may be sus-
pended within the mug, and no part of it may be above the mouth; this done,
electrify the mug by giving it a spark with an excited electric, or otherwise,
and you will see that the electroscope, whilst it remains in that insulated
situation, even if it be made to touch the sides of the mug, is not attracted by
it, nor does it acquire any electricity ; but if, whilst it stands suspended
within the mug, a conductor, standing out of the mug, be made to communi-
cate with or only presented to it, then the electroscope acquires an electricity
contrary to that of the mug, and a quantity of it which is proportionable to
the body with which it has been made to communicate ; and it is then imme-
diately attracted by the mug. Cavallo explains the cause in his own quaint
language, and his theory is in accordance with that taught in these days, only
the technical names are changed ; thus, in modern style, the fact would be
explained by stating that " polarity cannot be set up when opposing actions
are at work in different directions, as in the inside of an insulated metallic
vessel." Cavallo says, " The reason why, in this experiment, the electroscope
contracts no electricity whilst suspended entirely within the cavity of the mug
is because the electricity of the mug acts upon the electroscope on all sides,
and this has no opportunity of parting with its fluid when the mug is electri-
fied positively, nor of receiving' any when the mug is electrified negatively.
But, as soon as any conductor communicates with it, the electroscope becomes
immediately possessed of the electricity contrary to that of the mug ; for, if
the mug be electrified positively, the fluid belonging to the electroscope will
be repelled to that body which communicates with it, and which, being out
THE ELECTRIC WELL.
235
of the mug, cannot be affected by its electricity ; and if the mug is electrified
negatively, it will attract the fluid of the electroscope, which actually receives
an additional quantity of it from that conducting body with which it com-
municates.
FIG. -211. C avail Js Electric Well
A, quart mug insulated, and containing the electroscope inside; B, the threads raised above the
edge of the vessel, or, still better, touched with an insulated brass rod extending into the air.
In A, opposing forces, + and +, oppose in different directions In B, polarity can be set up j because the
inside is 4-, the electroscope , and the extremity of the rod in the air +.
" The electroscope, therefore, becoming always possessed of a contrary elec-
tricity, must necessarily be attracted.
" If, by raising the silk thread a little, part of the electroscope, i.e., of its linen
threads, are lifted just above the mouth of the mug, the balls will be immedi-
ately attracted ; for then, by the action of the electricity of the mug, it will
acquire a contrary electricity by Diving to or receiving the electric fluid from
the air above the cavity of the mug.
" It has been supposed by some that the electroscope in the above experi-
ment (or any other small insulated body), hanging in the cavity of an electrified
vessel, or the like, is not attracted by the sides of the vessel because the attrac-
tion of electricity, being as the squares of the distances inversely, cannot affect
the electroscope one way more than another ; it being demonstrable that if to
every point of a spherical concave surface equal centripetal forces are directed,
decreasing as the squares of the distances from those points, a small body-
situated anywhere within that surface would remain there without being at-
tracted one way more than another.* But to this it may be replied that the
demonstration of the above-mentioned proposition, if it is applicable to sphe-
rical or cylindrical concave surfaces, cannot, however, be applied to every kind
of irregular cavities, with which, if they exceed not a certain size, the above
experiment succeeds as well as with the cylindrical cavity of the mug."
Cavallo proceeds to give what he considers to be the proper theory, which
in the main is right ; but, as before observed, the explanation is simplified by
stating that, as polarity cannot be set up inside a vessel, so a charge cannot
be maintained.
* Newton's " Principia," Book I., prop. Ixx.
236 ELECTRICITY.
ELECTRICAL INDUCTION.
In studying the phenomena of light and heat, it will be necessarily observed
that these forces have a radiant power. A heated body may be brought towards
another which is not heated, and impart Jo it a certain amount of its warmth ;
the latter gains what the former loses : the vibratory power set up in the heated
body is supposed to be conveyed by the undulations of the ether to the body
which is not heated, and setting up therein similar vibrations ; the result is
that heat is produced in a cold substance by the approach of a heated body,
which loses its vibrating energy in warming the other.
Loss of power, independent of any conducting power of damp air, curious
to say, is not observed when an electrified body is gradually brought towards
another which is not electrified ; and yet the electrical quiescence of the latter
is disturbed, and may give rise to large quantities of electricity, as in Holtz's
electrical machine (Fig. 202) ; the effect thus obtained is called " induced elec-
tricity."
The fact is well shown by using a cylindrical conductor, the two halves of
which can be separated with their respective insulating glass columns. On
the underside of the conductor small rings or hooks may be inserted for the
convenience of attaching pairs of gilt pith-balls, which should be as light as
possible.
FIG. 212.
A, the electrified ball approached to the conductor, B c, made in two halves to separate at D; each half
to have one suspended pith-ball at r3, so that, when joined together, they form a pair of balls as in the
ordinary electroscope ; also each to have a pair at the extremities B and c.
Directly the charged ball A has approached sufficiently near to the conductor
B c, the pith-balls show by their mutual repulsion that its electrical quiescence
is disturbed, and that, in fact, if the ball has been charged with positive or
+ electricity, it will cause negative or electricity to become apparent at
B, whilst positive or + electricity will be found at C. The pith-balls hanging
at D will hardly be disturbed, if at all, showing that there is a neutral point,
like the centre of a bar magnet, where the forces are balanced. When the
disturbing cause A is removed, the separated electricities rush together again,
the electrical equilibrium of the cylinder is restored, and the pith-balls no.
longer repel each other.
ELECTRICAL INDUCTION. 237
No advantage, therefore, so far as the production of a permanent charge of
electricity, has been obtained in the above experiment, which, it must be re-
membered, is performed with a conductor of electricity. If, however, the expe-
riment is repeated, and, whilst the conductor is under induction from the ball
A, the two halves are separated, then it will be found that each half is per-
manently electrified.
FIG. 213.
A, the electrified ball ; B, the half of the cylinder, separated from the other, and showing a charge of
negative or electricity ; C, the other half, showing positive or + electricity ; p, the single balls, sus-
pended from B and c, attract each other, as they represent the opposite electricities, + and .
The separation of the halves of the conductor whilst under induction has
prevented the opposite forces reuniting; the pith-balls remain deflected on
each half, and the single balls, suspended at the place where the two halves
are separated, incline towards each other, because dissimilar electricities attract.
The equality of the electrical disturbance is again beautifully shown by bring-
ing the halves together, when the electrical excitation set up entirely ceases, as
the two opposite forces exactly neutralize each other.
The experiment may be once more repeated, and the two halves separated
whilst under induction. If a stick of excited wax is approached to the half
of the cylinder marked B, minus, the pith-balls are deflected still further from
each other ; but when the same stick of excited sealing-wax is brought towards
C, or plus electricity, the balls drop down.
In the first case, the increased deflection shows that the electricity on B is
negative, because the wax is negative, and exalts the previous charge. In the
second, the diminished deflection and falling down of the balls show that the
electricity on c is positive, as it is neutralized for the time being by the in-
fluence of the negatively electrified wax.
Two electroscopes, one placed in connexion with each half of the conductor,
may be substituted for the pith-balls, and are, perhaps, more certain and
truthful in their indications ; moreover, they are more delicate, and would show
a smaller amount of electrical disturbance.
These experiments demonstrate that, in conductors, polarity, i.e., the sepa-
ration of the electricities, the production of opposite properties in opposite di-
rection, may be set up by induction, but is not maintained; and this is, in fact,
as contended by Faraday, the essential difference between conductors and non-
conductors : in the former polarity is not maintained ; in the latter, as we shall
2 3 8
ELECTRICITY.
now see, polarity, being set up, is maintained, or it would be impossible to
charge a Leyden jar.
When a plate of glass is held against the ball attached to the prime con-
ductor of an electrical machine, and a pith-ball, suspended on a glass sup-
port, is approached towards it, the ball is energetically attracted towards the
glass ; and yet the latter, being called a non-conductor, ought not to have per-
mitted the electricity to have apparently travelled, like heat, through its sub-
stance.
FIG. 214.
A, one side of the glass plate, which may be one foot square, and is held against the ball of the elec-
trified conductor ; B, the ball suspended on the glass stand, and attracted to the other side of the glass plate.
The electricity does not travel through the glass plate, but, like the brass
conductor (Fig. 212), is thrown into an electro-polar state, the one side touch-
ing the conductor being positive, and the other side, to which the pith-ball is
attracted, being negative ; a very slight charge is thus conferred upon the glass
plate, which will not rise higher until one side is put in conducting communi-
cation with the ground. The small charge, however, is retained when the
glass is removed, and thus the polarity is shown to be maintained by non-con-
ductors^ constituting the essential difference between them and conductors of
electricity.
The sheet of glass cannot be charged properly unless both surfaces are
ELECTRICAL INDUCTION.
239
coated with tinfoil, within, say, 2 in. of the outer edge. On a sheet of glass
i ft. square, the tinfoil will be 8 in. square. If this plate is supported on
an insulating stand, by being placed in the cleft or groove of a piece of
mahogany fitted on the top of a glass rod, fixed in a proper foot, the charge,
as before stated, is very slight, because the force called positive electricity
applied to one side of the plate, which may polarize the particles of the glass,
is opposed by the positive electricity resident on the other side of the glass,
and a balance is arrived at a dead lock ; the particles cannot increase their
charge, because the order is broken, and instead of the continuity being
represented by Fig. 215, where + is at one end and at the other, the regu-
FIG. 215.
larity is destroyed by the last particle being -f- instead of , as shown at
Fig. 216; and the molecules are now + at one end and + at the other, and
must therefore oppose (and thwart, as it were) each other.
FIG. 216.
The difficulty is, however, overcome by connecting one side of the plate
with the earth, when the order shown in Fig. 215 is restored, and the + elec-
tricity is said to escape to the ground, which latter, in its turn, represents a
vast series of particles all polarized ad infinitum, but decreasing in intensity
as the distance from the disturbing source is increased, according to the law
already explained at page 228.
Faraday insisted that electrical induction was an action of contiguous par-
ticles, whether it took place through a metal, or glass, or air ; he opposed the
" emission " theory of electricity, as others had done before with respect to
the emissive theories of light and heat.
Formerly .it was supposed that electricity travelled through air without
affecting the particles of the air ; it was imagined to be a subtle form of matter
of its own kind. Faraday laboured to prove that every particle of air becomes
polar, and takes part in the propagation of the force, just as the particles of
the glass become polar when charged with electricity.
A thin leaf of gold may be -f- on one side and on the other, so long as
it is subjected to the inductive action of an electrified body brought near it ;
40
ELECTRICITY.
that which occurs in a large conductor, as shown in Fig. 212, may occur,
microscopically, as it were, in a gold leaf*
If once the student grasps the idea of the polarity of each minute and con-
tiguous particle,, the difficulties of Faraday's inductive theory vanish. It is
well, here, to dwell on the condition of the surfaces of a glass plate whilst
under induction, and receiving a charge of electricity. The late Professor
Darnell's diagrams are very excellent*
FlG. 217. Danulfs L
Explaining the condition of the surfaces of an insulated
catted wife tuftfoO,
d plate of glass,
** Upon the molecular hypothesis of induction, No. I may represent a plate
of glass with its metallic Coatings, a b and c d^ in its neutral state. In No. 2,
we suppose the same plate, with its metallic coating, a , in contact with the
charged conductor of an electrical machine. Its other coatings we also
suppose to be insulated; and, as we know, the plate cannot be charged. The
coating a , however, being positive or +, not only will the particles of the
glass be thrown into a polar state, but the coating c d win also be polar, H ,
by induction to surrounding objects ; but the charge will not rise to any
degree of intensity, because the + electricity of the latter cannot be carried
off ? or diffuse itself upon the earth, but will react upon the glass. But if we
uninsulate this coating, then will No. 3 represent the high state of tension
(charge) which the forces will assume under the inductive process, when a
high charge of + electricity upon a b will sustain an equal charge of elec-
tricity upon d c by the polar arrangement of the particles of the interposed
dielectric (glass).
"In the above diagrams the unshaded circle represents the particles of glass
in a state of electrical quiescence ; the shaded circles represent polarity, the
shaded half being supposed to be + (plus), the unshaded half (minus)
electricity.*
When explaining the cause of electricity residing on the surface of an insu-
lated conductor, it was stated that the interior of the vessel (Fig. 21 1) was not
found to give the slightest charge to the proof plane, because polarity could
not be set up properly in consequence of opposing forces in different directions :
we may trace out the latter in the next diagram (Fig. 218). Suppose a set of
molecules in a polar state starting from A are met by another column of
particles in the opposite direction B, which virtually undo all that might '.
Darnell's "Xnttodnctkm to Cb
ELECTRICAL INDUCTION.
241
been done by A. The polar state cannot be set up in the carrier-ball or. in
fact, in the particles of air contained in the vessel under examination by the
carrier-ball or proof plane.
The same reasoning applies to all sets of molecules coming in the direction
c as opposed to c, Das opposed to D, E as opposed to E.
The nomenclature of the phenomena of
induced electricity is thus expressed by Fa-
raday : -
1 . The excited body, glass or wax, is catted
the inductric or inductive body.
2. The effect of the inductric on a distant
body, and where no loss of electricity is sus-
tained, as by contact, is called induction.
3. The electricity thus obtained is called
inducd electricity.
4. The body subjected to the action of the
inductric is caned the inducteout body.
5. The medium, such as air, through
which the electric may act upon the induc-
teous body is termed the dielectric {&<*,
through, and rjX&cirpov, electricity). A di-
electric may be solid, fluid, or gaseous.
When the above principles are once com-
prehended, it is easy to conceive that every
kind of electrical attraction must be pre-
l ceded by induction; to demonstrate this
fact, Harris attached a gold leaf to a disc of FIG. 219. >$"/> William Sassr
gilt paper, neatly fixed on a filament of shel- Harri/s Experiment dtmw-
lac, and suspended by a silk thread. The
disc may be attached to one end of a. wen-
dried straw suspended by a thread ; a little
bit of tinfoil on the other end will balance
strating that Attraction is
prec&led by Inductum.
'
242 ELECTRICITY.
the gilt disc. Directly another disc electrified is brought towards the sus-
pended disc, the little gold leaf on the other side stands out and is repelled,
showing distinctly that the opposite kind of electricity to that which is the
disturbing cause must be eliminated, or the gold leaf would not move until
the suspended disc touched the electrified disc.
At page 211, whilst explaining the construction of the electroscope, it was
stated that the instrument could be made more delicate by the introduction
of a simple arrangement, through which inductive action is brought to bear
upon the cap, and through that to the gold leaves. The part attached to the
electroscope-stand is called the condenser, and assists in increasing any minute
evolution of electricity that would otherwise be insufficient to overcome the
weight of the gold leaves, and cause them to repel each other.
FIG. 220. The Electrical Condenser.
A, plate supported on glass stem ; B, plate on a conducting stem, jointed at bottom so as to move to any
position c.
It (Fig. 220) consists of two circular brass plates, one supported by a glass
insulating stem, and the other resting upon a conducting stem jointed at the
bottom. When the plate on the insulated stem is connected by means of a
wire with the cap of the electroscope, which may be very feebly excited, as with
the pressure of Iceland spar, on the removal of the uninsulated plate, the gold
leaves of the electroscope indicate the minute electrical disturbance.
It is evident that between the two plates there must be a dielectric air, the
particles of which we have already seen are capable of assuming the electro-
polar state.
The electricity from the tourmaline on the cap of the electroscope has
charged the insulated plate A, Fig. 220; this throws the intervening air into
a polar state, so that the air is in the same condition as the glass plate with
its coatings of tinfoil, the latter being represented in this apparatus by the two
brass plates. If both plates were insulated, there would be opposing forces, as
shown at p. 239; but one, plate B, is connected with the earth. At first, and
whilst the plates are near each other, the electricity is said to be disguised.
All this time, if the electricity on the cap of the electroscope is positive (+),
it has, by induction through the film of air, thrown the second plate into the
opposite condition, negative or .
The two electricities on the two plates are, as it were, engaged to each other ;
the desire to unite, or their tendency towards one another, is simply arrested
by the intervening air, and this for the time disguises the electrical energy
which really exists ; but when the second plate is removed, and the two elec-
tricities are separated, then it is found that the feeble original charge has been
ELECTRICAL INDUCTION. 243
exalted ; for, as the feeble charge from the cap, connected with the insulated
brass plate, acted on the other uninsulated brass plate, the latter, by con-
nection with the earth, like the outside of a Leyden jar, reacts upon
the insulated plate ; so that, when the two are separated, a greater elec-
trical effect is perceptible. By the repeated application of the pressed Iceland
spar, and the withdrawal and return of the plate B to A, the charge is virtually in-
creased or condensed on A. The closer the two plates can be brought together,
the better the effect ; but, as the particles of air are soon broken through by
a disruptive discharge in the shape of a spark, and particularly so if the air is
at all humid, it is found better, as in Volta's condenser, to use a thin plate of
some non-conducting material, such as shellac, instead of air. The disguise
of the two electricities is the more complete when the metallic discs are brought
very close to each other, because the attraction of the two electricities becomes
stronger as the distance is diminished. The inductive power of the electrified
plate must be increased, and the reactionary force of the second plate, con-
nected with the earth, also rises to a more exalted state.
FiG. 221. The Gold- Leaf Condenser
Is so called because it is adapted to a gold-leaf electroscope. The nicety of
manipulation required in order to use the instrument properly is described by
M. de la Rive, in his "Treatise on "Electricity," translated by Mr. Charles V.
Walker :
" It is composed of two metal plates, nicely adjusted, of not less than 6 in.
nor more than i ft. in diameter. On of these plates is screwed on the exterior
extension of the metal stem of the electroscope by which the gold leaves are
supported, and has a wire and ball attached to it, A ; the other, B, is provided
with an insulating handle, c, fixed vertically at its centre, and is placed upon
the former so as exactly to cover it.
162
244 ELECTRICITY.
" The two plates have been coated on their surfaces in contact with several
layers, successively applied, of a very liquid varnish, formed of a solution of
shellac in alcohol. This varnish, in drying, forms a pellicle whose thickness
does not exceed i-25oth part of an inch, but which is sufficient to prevent the
recomposition of the electricities when they are not very strong.
" The plates are thus almost in contact, and the disguise of the electricity
is as complete as possible ; and the condensing power of this apparatus is very
considerable ; but it can only support very feeble charges, which, indeed, are all
it is intended to receive. It is important that the two plates be fitted to each
other as accurately as possible, and, consequently, that their surfaces be very
even. For this reason there is a limit to the size of these surfaces that cannot
possibly be exceeded, because their construction would become too difficult,
in consequence of the conditions we have pointed out. The manipulation also
would be very troublesome ; for it is essential that we should be able to raise
the upper plate easily, and should take care to raise it perpendicularly, with-
out exercising 2^ friction against the other, which of itself would be a source
of electricity, and would consequently interfere with the results.
" This reservation being once made, it is advantageous to have the largest
possible surface, because the quantity of electricity accumulated is propor-
tional to this surface.
" Experiment has demonstrated that we cannot exceed a foot in diameter,
without falling into the inconveniences that we have just pointed out. The
plates are generally of brass, and, if possible, of gilt brass, so as to be pro-
tected against the chemical action of the moist air, and of the vapours and
liquids with which they may have occasion to come in contact.
" Electrical signs are sometimes found on separating the two plates, even
although there may be no electrical source in communication with either of
them. This error is due to a small quantity of electricity arising from preced-
ing experiments, which has penetrated into the layers of varnish, and which
is not got .rid of without some difficulty.
"In order to remove it, we must place a very thin sheet of tinfoil between the
two discs, and leave it there until we have satisfied ourselves that, after having
been placed in immediate contact with each other, the plates liberate no
trace of electricity by the mere fact of their separation. It is essential always
to determine this absence of spontaneous electrical signs before making an
experiment.
" For greater convenience, the source of electricity is generally placed in com-
munication with the upper plate of the condenser B, which is termed the col-
lector; and the lower plate, or its connected brass ball, is touched with the
finger.
" When the two plates are separated, it is the electricity of the lower plate,
now become free, that affects the electroscope ; but we must not lose sight
of the fact of its being of a contrary nature to that of the upper one, and, con-
sequently, to that of the source subjected to experiment.
" Before beginning a second experiment, we must not forget to discharge both
the plates by touching them with the fingers ; and generally we must never
leave them charged, especially when they are in contact, because the electri-
city that they retain penetrates into the layers of varnish, from which, as we
have seen, it is a very difficult matter to expel it.
" By the assistance of this instrument Volta succeeded in showing that a plate
of zinc, when held in the hand and put into contact with the upper plate,
THE ELECTROPHORUS.
245
charged it with negative electricity an experiment that was the origin of the
voltaic pile. When this experiment is made, care must be taken that the
zinc plate be well cleansed, especially in the points where it touches the disc.
" In like manner, we can charge the plate with positive electricity by inter-
posing between the plate and the zinc plate, which is still held in the hand, a
disc of cloth or paper slightly moistened with salt water. In each case we
must not neglect to touch the lower plate with one of the hands, whilst the zinc
plate is held by the other in contact with the upper plate. The experiments
that we have just quoted, and the other delicate experiments in which the con-
denser is used, require the air of the room in which the operation is carried on
to .be as dry as possible, or at least the electroscope and all the pieces of which
it is constructed to be well protected from moisture. With this view, the whole
is covered with a glass cage, in the interior of which chloride of calcium is
placed, in order to produce the dryness."
Space does not permit us to describe Peclet's instrument, which is still
more sensitive, but requires precautions to be taken in its use that almost
negative its other valuable qualities.
If the condenser cannot be understood, the youthful student is supplied with
fresh ideas, which will help him to do so, in the old-fashioned and most useful
instrument, called the
ELECTROPHORUS
, electricity; <o/>os, carrying).
FIG. 222. The Electrophorus.
A B, the tin dish, with the sides sloping inwards, so that the composition cannot fall out; c c, the upper
metallic plate and glass handle, D ; E E, two spots of sealing-wax, dropped and mtlted on to the
lower side of the metallic plate, to keep it opposed to, but not touching, the resinous plate.
This instrument is spoken of by Cavallo as "a machine for exhibiting per-
petual electricity ;" though he explains afterwards that, being only an excited
electric, it must gradually lose its power like all other excited electrics, but
being flat it is not exposed to currents of air which may circulate around a
stick of sealing-wax, and carry off the charge more quickly.
To make an electrophorus, a circular tin, with a rim f in. deep, may be pro-
vided, about I ft. in diameter, and into this, whilst warm, should be poured a
mixture of two parts of shellac and one part of Venice turpentine, after they
are carefully melted and well incorporated together. When cold, the surface
246 ELECTRICITY.
has a bright polish, and is, of course, remarkably smooth ; indeed, care should
be taken not to scratch it. The second part of the apparatus for it consists
only of two parts is the circular flat plate, 10 in. in diameter, made of tin, or
gilt copper, or cardboard covered with tinfoil, in the centre of which is a glass
rod so fixed that it will lift the metallic plate.
The instrument is charged by gently rubbing or striking the resinous plate
with a cat's skin or a warm piece of flannel, and, like the charged pane of
glass, described at page 238, the thinner the resinous plate can be cast, the better,
as after being rubbed, and always supposing the tin dish is in conducting com-
munication with the earth, it acquires a charge like the Leyden jar, to be
described presently. The electro-polar plate having been set up in the resinous
plate, the metallic plate with the glass handle (which, in common with all the
glass supports of electrical apparatus, should be varnished with shellac var-
nish) is brought down upon the excited resinous plate ; no direct transfer of
electricity takes place except when the plate happens to touch the excited wax,
and this is prevented, in a great measure, by the two little studs of sealing-
wax, E E, already spoken of in Fig. 222.
When the plate is in position, and held by the glass handle, the two elec-
tricities, positive and negative, naturally resident in the metal, separate, as
already described in the explanation of the phenomena of induction, at page
237 ; because induction may not only take place in a long conductor, but on
the opposite sides of a tin, copper, or other metallic plate.
If the plate is now removed and examined, it is not found to have acquired
any charge of electricity ; conductors do not retain polarity; and the two forces,
separated whilst the metallic plate was in the neighbourhood of the excited
resinous plate, come together again, as already described fully at page 236.
The metal plate is again laid upon the lower excited resinous plate, and
now, if touched by the finger just the moment before it is raised by the glass
handle for the act of touching and raising should be almost simultaneous,
and is soon learnt with a little practice then, on applying the knuckle to the
edge of the metallic plate or to the brass ball, a spark immediately passes ;
and thus, by continually touching, raising, and applying the top metallic plate
by its glass handle to a small Leyden jar, the latter is speedily charged.
The rationale of the necessity for touching is easily explained. When the
plate is under induction, the lower side facing the negatively electrified
resinous plate is positive, and the upper side negative ; on touching the plate,
positive electricity passes to the negative, and the upper surface receives a
charge in excess of its natural quantity, and, instead of the two sides being
represented by +, plus, and , minus, the plate, when removed, is found to
be H K Here is an excess of electricity, which passes to the knuckle in
the form of a spark, and again restores the equilibrium to + and
It is in this way that the metallic plate can be charged any number of times
by alternately touching and raising, and the resinous plate loses no electrical
power whatever.
Holtz's electrical machine, described at p. 226, is another and very notable
instance of the same kind.
If the resinous plate in its tin dish, before being rubbed, is placed on an
insulating stand, so as to be well insulated, and is then rubbed, care being
taken not to touch the metallic dish, it acquires little or no charge. The under
side of the resinous plate must be in conducting communication with the
ground, like the glass plate with the tinfoil coatings, described at p. 239.
THE ELECTROPHORUS, 247
When the whole apparatus, previously excited and ready for use, is placed on
the insulating stand, and the metallic plate raised, it acquires so slight a charge
that it will not give a spark, and would only affect an electroscope, which,
Cavallo says, " shows that the electricity of this resinous plate will not be con-
spicuous on one side of it, if the opposite side is not at liberty to part with or
acquire more of the electric fluid."
The original electrophorus invented by Volta was a circular glass plate,
covered with a composition made of equal parts of shellac, rosin, and sulphur;
and these plates, no doubt, from their thinness, would answer the purpose re-
markably well.
Cavallo, who is always so thoroughly practical in his electrical experiments,
FIG. 223. Electrophorus^ made of Glass, and covered with Sealing-wax. '
says that he made one of a glass plate, and no more than 6 in. in diameter:
when once excited, it could charge a coated Leyden phial several times suc-
cessively, so strong as to pierce a hole through a card with the discharge.
Sometimes the metal plate, when separated from it, was so strongly electrified
that it darted strong flashes to the table upon which the electric plate was laid,
and even into the air, besides causing the sensation of the spider's web upon
the face brought near it, like an electric strongly excited.
" The power of some of my plates " (which he covered with sealing-wax,
second quality), he says, " is so strong, that sometimes the electric plate adheres
to the metal, when this is lifted up ; nor will they separate, even if the metal
plate is touched with the finger or othe^ conductor."
Thus, with a circular piece of window-glass, covered with sealing-wax melted
on to it, a circular piece of wood or card covered on both sides with tinfoil,
and fixed by a pasteboard tube to a glass rod, a very serviceable and cheap
electrical machine can be made by young people.
The Leyden jar is nothing more than the coated glass pane (p. 238) rolled
up or made into a cylinder.
It was discovered by three philosophers, who were working together at
Leyden, viz., Muschenbroeck, Allaman, and Cuneus. They were attempting
to collect and store electricity in a bottle, containing some water, through the
cork of which was thrust a nail, touching the water ; the first shock was re-
ceived when Muschenbroeck, holding the bottle in one hand, touched the nail
with the other accidentally. One smiles, thinking of personal experience in
248 ELECTRICITY.
these matters, to imagine the half-frightened wonderment of the worthy sage,
who might have supposed that he had invoked the demon or " genius," good
or evil, of the bottle.
Of course, everybody throughout Europe was made acquainted with the
electric shock by travelling electricians, who, like the travelling "ghost" show-
men of the present day, relieved Muschenbroeck of any trouble in communi-
cating his discovery to the world in general.
As the water was found to be inconvenient, in consequence of the vapour
condensing in the upper part of the bottle, and thus reducing the distance
between the outer and inner surface of glass, so that a small charge only could
be obtained, brass filings, fixed on with some varnish, were next tried ; and
Cavallo devotes more than a page of his " Complete Treatise on Electricity "
FIG. 224. The Leyden Jar and Discharger.
to the narrative of a grand explosion and smoke arising from the interior of
his Leyden bottle, prepared with varnish and brass filings, in consequence of
the latter taking fire with the electric spark, which, darting from point to
point of the filings, set the inflammable mixture of air and spirit vapour from
the varnish on fire ; and he adds, regretfully, that, after it had burnt out, all
the brass filings fell to the bottom of the bottle, because the adhesive quality
of the varnish was destroyed by fire.
The older electricians sometimes used mercury instead of water; but this
was soon found to be very expensive, and not applicable to large jars, in con-
sequence of the great weight of the metal.
The principle of the Leyden jar being once understood, viz., that the water
accidently used by Muschenbroeck in his bottle was the inner conducting
coating that conveyed the electricity to all parts of the interior surface of the
glass, and that the undesigned application of the hands on the outside served
for the outer coating, a little more consideration brought electricians to the
use of tinfoil, no doubt suggested by the use of this metal in the art of
silvering looking-glasses.
There are no better directions for coating and preparing Leyden jars and
batteries than those given by Cavallo, who says, " When glass plates or jars,
THE LEYDEN JAR. 249
having a sufficiently large opening, are to be coated, the best method is to
coat them with tinfoil on both sides, which may be fixed upon the glass with
varnish, gum-water, paste, beeswax, &c. ; but in case the jars have not an
aperture large enough to admit the tinfoil, or an instrument to adapt it to the
surface of the glass, then brass filings, such as are sold by the pin-makers,
may be advantageously used, and they may be stuck with gum-water, bees-
wax, &c. ; but not with varnish, for this is apt to be set on fire by the discharge.
Care must be taken that the coatings do not come very near the mouth of
the jar, for that will cause the jar to discharge itself (now called a spontaneous
discharge.
" If the coating is about two inches below the top, it will in general do very
well ; but there are some kinds of glass, especially tinged glass, that, when
coated and charged, have the property of discharging themselves more easily
than others, even when the coating is five or six inches below the edge.
" There is another sort of glass, like that of which Florence flasks are made,
which, on account of some unverified particles in its substance, is not capable
of holding the least charge. On these accounts, therefore, whenever a great
number of jars are to be chosen for a large battery, it is advisable to try
some of them first, so that their quality and power may be ascertained.
" If a battery is required of no very great power, as containing about eight
or nine square feet of coated glass, I should recom-
mend to make use of common pint or half-pint phials,
such as apothecaries use. They may be easily coated
with tinfoil, sheet lead", or gilt paper on the outside,
and brass filings on the inside. They occupy a small
space, and, on account of their thinness, hold a very
good charge; but when a large battery is required,
then these phials cannot be used, for they break very
easily, and for that purpose cylindrical glass jars of
about fifteen inches high, and four or five inches in
diameter, are the most convenient."
It is easily shown, by charging a Leyden jar fitted
with, shifting coatings, made of light tin-work or of
wire gauze, that they have nothing to do with the
maintenance of the charge ; they simply act as chan- _
nels for the conveyance of the electricity to all parts * .\ ^
of the glass. It is the polarity of the particles of the with s]n J tm
glass, which is kept up as long as the jar is charged,
and is only destroyed when the interior of the jar is brought in conducting
communication with the exterior by means of the useful instrument called the
discharger, already shown in Fig. 224.
^The Leyden jar with shifting coatings, having been charged, is discharged
with a loud snapping noise, by bringing one ball of the discharger to the
outside, and the other to the ball coming from the inside.
The jar is again charged, and the arm of the discharger is used to take out
the interior coating. Directly that is removed, the jar may be lifted out of its
outer coating, and, if the air of the room is dry, may be left some time without
fear of its losing the charge. The charged Leyden jar would keep its elec-
trical polarity still longer, if put under a dry glass shade, as the air around the
Leyden jar would then remain still, and would thus retard the slow discharging
of a charged glass surface, when the air of the room is in constant motion, by
250
ELECTRICITY.
reason of the warmth of the fire, or the movements of persons about the room
who are engaged in making the experiments.
After waiting a reasonable time, the jar may be lifted into its outer coating,
and the inner one can be quickly and dexterously returned, by the assistance
of the discharger, to the interior; and now, on applying the discharger as before,
a loud cracking and brilliant spark prove that the charge was confined to the
particles of the glass.
B
FIG. 226.
A, the conical glass jar; B, the outer coating ; c, the inner coating; D, the discharger.
An insulated Leyden jar, like the coated pane described (page 238), cannot
sustain a charge. Franklin soon discovered this fact, and hence the experi-
ment is usually called " Franklin's experiment with the Leyden jar."
The jar may be supported on a stand with a long glass support, which of
course must be dry, and insulate perfectly.
It should always be remembered that a steady gentle warmth is far better
than roasting the apparatus before a large fire ; indeed, a great deal of damage
THE LEYDEN JAR.
251
is done to electrical apparatus by foolishly exposing to a strong heat instruments
which are partly put together with cement : the latter melts, and the symmetry
and perfection of a piece of apparatus is often entirely spoilt ; because it requires
some experience to cement a brass cap on to a glass vessel, and the young
electrician can do little or nothing with his apparatus when the cement is
melted and running down the inside or outside of it.
FIG. 227. Franklin's Experiment with the Insulated Jar.
The interior of the jar is now connected with the ball of the prime con-
ductor of the electrical machine, and, after receiving some sparks, it will be
noticed that they cease to pass, and that the conductor is showing, by its
electrical brushes and dischaYges through the air, that there is no charge
passing into the Leyden jar.
When removed from the conductor, by pushing the insulating stand on one
side, and tested with the discharger, little or no spark is perceptible. If,
however, the wire and ball on which the Leyden jar stands usually inserted
into and made movable on the top of the insulating stand is now connected
by a chain with the ground, the jar is very quickly charged, when sparks are
received from the prime conductor.
The rationale has already been explained at page 240, but may be repeated
here.
When insulated, the positive electricity naturally resident on the outside of
the glass opposes any accumulation of positive electricity in the interior; the
chain of particles is not continuously charged in the order of plus and minus,
but is interrupted by plus coming in the wrong place ; the order, however, is
restored when the outside qf the jar is connected with the ground, as the
252
ELECTRICITY.
natural positive electricity finds a channel through which it can escape, and
no longer opposes the accumulation of the positive charge inside the jar.
When a number of jars are insulated on glass stands and placed in regular
order, the knob of the first being connected with the prime conductor, the
knob of the second to the outside of the first, the knob of the third in con-
<*-~imrm 4*=aSWM ^BB^^e -
FlG. 228. Charging t)ic j^eyaen Jar Uy Cascade.
tact with the outside of the second, and the outside of this last connected
with the ground, the whole series is charged by the first, because the first
loses exactly the proportion of positive electricity which enters its interior ;
this passes to the second, which in its turn loses the equivalent from the out-
side, and finally passes or flows, as it were, into the third jar, the outside of
which is connected with the ground. Thus the positive or plus electricity of
the first jar, like a continuous cascade, flows from one jar to the other, and, all
being charged, they cannot be discharged together ; to effect this, the interior
of all the jars must be connected together, and the same must be done with
the exteriors.
FlG. 229. The Jars turned round by their Insulating Glass Supports.
A A, brass rod, laid on the wires and knobs connected with the interior of the jnrs, not by the hand, but
with a silk thread; B B, brass rod, laid on outside of jars with hand; c, discharger, bringing the end*
of two rods in conducting communication, and spark discharged.
Each jar can be turned round at right angles, and a brass rod, with balls at
each end, suspended by a silk thread, can be laid across all the wires and
knobs of the jars, and another wire laid along the exterior of the jars ; then,
THE LEYDEN JAR. 253
if the two extremities of the rods in conducting communication with the out-
sides and insides of the jars are brought in contact with the discharger, a
brilliant spark and louder noise announces the discharge of the series of three
jars which had been charged with electricity according to the original method
discovered by Franklin.
Mr. I sham Baggs displayed some very brilliant experiments at the Poly-
technic with Leyden jars, charged in the manner already described ; and, by a
particular mode of arranging them in positive and negative series, a very long
and brilliant spark was obtained.*
It has been shown by the Franklin experiment that a jar cannot be charged
unless the outside is placed in communication with the ground ; it has also
been pointed out that Leyden jars are usually charged by passing the electri-
city to the interior. A Leyden jar can, however, be charged from the exterior ;
and the arrangement for this purpose is shown at Fig. 230.
FIG. 230. The Leyden Jar .charged from the exterior.
A brass disc, c, is screwed on the top of the ball of the large jar A, in
order to carry the smaller one B. When A is charged, B becomes polarized,
but cannot accumulate a charge until the positive electricity from the inside is
allowed to escape ; this is done by touching the knob of B and the outside of
A with the two balls of the ordinary discharger. A flash takes place when this
is done, and no\v both A and B are charged. The inner surface of B is nega-
* " Journal of the Royal Society," Jan. 13, 1848.
2 54
ELECTRICITY.
live, the inside of A is positive ; the outside of A is negative, the outside of B is
positive.
Both jars may be discharged by using two dischargers : one connects the
outside of A with the inside of B, thus bringing together the two negative sur-
faces ; and the other discharger by touching the first one and then being
advanced to the stage C, which represents the positive electricity, the usual
flash and discharge follow directly the discharger comes within the striking
distance.
A collection of Leyden jars, fitted up with wires and balls communicating
with each other, and placed on a sheet of tinfoil, so that the exterior of the
jars, like the interior, may be in conducting communication, constitutes what
is termed a Leyden battery. (Fig. 223.)
FIG. 231, The Leyden Battery.
The five large jars are coated with tinfoil, and the brass balls belonging to
each jar are supported by a method proposed by the Rev. F. Lockey, and
recommended because it sometimes occurs that a jar will break during the
discharge of the battery, although the electricity may pursue the path intended
for it. Jars are more likely to break if the wire to which the ball is attached
is carried down to the tinfoil inside. Direct metallic communication with one
point of the interior of the jar is not so safe as having four contacts, and this
is secured by the bar of wood, covered with tinfoil, and connected with two
cross-pieces of thinner wood laths, also covered with tinfoil, and shown at A B,
Fig. 231. It is evident that contact is made at two places, A, B, at the top,
and two at the bottom, C, D.
The writer has in his possession two very large jars, which he coated with
EXPERIMENTS.
2 55
\v
tinfoil, after first pasting a coating of paper, such as paper-hangers use, on the
jars, and allowing the paper to rise one inch above the tinfoil coatings. The
jars expose a surface of six square feet of glass,
and have been in use, without fracture, for the
last twenty-five years, although frequently very
highly charged, to break square pieces of maho-
gany, to demonstrate the mechanical power of
the electric discharge. Young experimentalists
would do well to avoid these trying experiments,
as the electricity may prefer to break through the
glass, instead of travelling only through the fibres
of the wood.
Henley's electrometer, shown at H, Fig. 231,
should always be inserted in one of the balls of
the battery whilst it is being charged, as it indi-
cates, by the rise of the arm carrying a light pith-
ball, the amount of charge, and when it reaches
90 the jars are fully charged.
It is sometimes convenient to keep a jar
charged for a considerable time, and particularly
if the electricity is required for medical purposes;
this is done by passing a glass tube through the
wooden cover of the Leyden jar: the tube is lined
half-way up from the bottom with tinfoil, and
terminates at the top with a brass cap ; to con-
nect this with the interior of the jar, a wire with
a loop at the top passes through the brass cap,
and, after the jar is charged, may be removed by
turning the jar upside down, when it tumbles
out ; or, better still, it may be taken away with a
curved wire and ball, supported on a glass handle.
FIG. 232. The ordinary
Leyden Jar, coated with
Tinfoil,
And containing the glass tube A B, capped with brass at A, and passing through the wooden top, which
is usually cemented in and well varm.shed. c shows the height to which the tube is lined with tinfoil ;
D is the wire, with ring at the top, removable by the insulated curved wire w.
EXPERIMENTS WITH THE ELECTRICAL MACHINE, THE LEYDEN JAR,
AND LEYDEN BATTERY.
I. The charging and discharging of a Leyden jar is beautifully shown
by coating the inside and outside with diamond and spotted coatings,
or little bits of tinfoil cut in the form of diamonds or spots, and pasted
on so that an interval of, glass surface may occur between each of
them. When connected with the prime conductor, the jar presents a
brilliant and most pleasing appearance during the time it is being
charged, and also at the moment when the discharger is used ; and
the jar so coated is usually called a spangled jar.
II. Similar spots or small circles of tinfoil pasted round a glass tube show
a brilliant spark between each interval or space left between the spots,
when held to the prime conductor, or at the moment that the charge
of a Leyden jar is sent through them. The tube is usually capped
with brass at each end.
2 5 6
ELECTRICITY.
FIG. 233.
A Spangled Jar.
FIG. 234.
A Spangled Tube.
FIG. 235.
1 1 1. Narrow strips of tinfoil are arranged in parallel lines on a plate of glass,
so that a continuous conducting strip, commencing with a ball at the
top of the glass and ending with one at the bottom, is obtained.
The strips are then neatly cut out, so as to leave a small interval
sufficiently wide to show the spark, and delineate in a succession of
sparks any word, such as the name of
FIG. 236.
EXPERIMENTS. 257
IV. A glass bottle may be coated inside and outside with weak glue and
rather large brass filings shaken inside and sifted over the glue out-
side. Of course, one side must be done first viz., the inside ; and the
coating should be carried about as high as the usual coating of tinfoil.
It should terminate top and bottom with a band of tinfoil, and it exhibits
a very pretty effect when hung on to the conductor of the electrical
machine, the outside being connected with a wire or chain with the
ground. The intervals between the filings give rise to the most varied
and beautiful appearances of lines and forked electric sparks ; and
as the jar discharges itself when the accumulation reaches a certain
point, measured by the distance between the wire from the inside
and the outside coatings, the effect is continuous as long as the elec-
trical machine is turned.
V. A little tow wrapped round one of the balls of the discharger, and dipped
in alcohol or ether, is set on fire directly the spark of the Leyden jar
passes through it.
VI. A person standing on a stool with glass legs, and holding in one hand
the chain from the prime conductor of an electrical machine in motion,
may set on fire spirit or ether (held to him by some one else) in a
metallic spoon, by merely allowing a spark to pass from his finger to
the inside of the edge of the spoon. The hair of the person standing
on the stool, and connected with the electrical machine, stands out in
a very fantastic manner, if the hair is fine, silky, and well combed out
previously.
VII. When a blunt wire, say -f in. thick, and nicely rounded off at the end,
is fixed into the conductor of an electrical machine (there are holes
drilled expressly for putting in wires), as the handle is turned, a feeling
like a gentle current of air is felt, when the face is approached to it,
and, if the room be darkened, very pretty brush-like discharges are
seen.
FIG. 237.
The brush discharge from a positively electrified wire. The reverse : the concentration of the same
brush into a glow or star when positive electricity is drawn towards the negative conductor. The one
is the reverse of the other.
If the same blunt wire is placed in the negative conductor and the
electrical machine put into rapid motion, a sort of glow or star is seen
on the end of the blunt wire. In the first case, the positive electricity
is escaping from the wire ; in the second, it is going into and towards
the wire.
VIII. An egg-shaped glass vessel, provided with a ground-glass plate, a collar
17
258
ELECTRICITY.
of leather at the top, and through which a brass rod and ball move
so as to approach to or recede from another ball fixed into the lower
cap, cemented on to the glass and provided with a stop-cock, is first,
FlG. 238.
exhausted of air with the air-pump. Directly it (Fig. 238) is con-
nected with the electrical machine, a beautiful glow of delicate violet-
coloured light is seen to pass between the balls.
IX,
FIG. 239.
The electrical inclined plane (Fig. 239) is formed by two inclined wires
stretched between four glass pillars. When a very light rod of wood,
covered with burnished gilt paper, having fine wires inserted at right
EXPERIMENTS.
2 59
angles, with their ends all bent exactly alike, is placed on the wires
which are connected with the conductor of the electrical machine, the
rod revolves by reason of the reaction of the dispersed particles of
electrified air upon those which are still, and it rolls up the inclined
plane. If the experiment is tried in a darkened room, all the points
exhibit pretty brushes of electric light.
X. If the inside of a clean dry tumbler or, better still, a German beaker
glass, is held over the brass rod and ball of the conductor, and, after
being well electrified, is put down over a number of light pith-balls
FIG. 240. The Electrical Dance of Puppets.
placed on a metallic plate ; the latter are attracted and repelled in the
most amusing manner, and, if the glass will take a good charge,
the effect lasts some time, and, when apparently stopped, may be often
renewed by drawing the finger over various parts of the outer surface.
XI. Light pith figures, if well made and balanced, perform a sort of dance,
by jumping up and down between a flat brass plate connected with
the conductor and suspended opposite another plate connected with
the ground (Fig. 240). When the shadow of the figure is cast on a
disc, everybody can see the experiment, which then assumes gigantic
proportions.
XII. A bell (Fig. 241) may be constantly struck with clappers, so arranged
that, whilst the bell is insulated and electrified, the clapper is alternately
attracted and repelled. Or, if the bell is placed in connection with
the inside of a Leyden jar (Fig. 242), and the outside with another
bell, the two being opposite to each other, and having between them
a suspended clapper, the bells will continue to ring until the jar is
discharged.
XIII. A very elegant experiment devised by Lichtenberg, and called after
him Lichtenberg figures, is thus described by De la Rive:
"Lichtenberg figures make manifest without an electroscope, and in
17 2
260
ELECTRICITY.
FlG. 241. The Electric BelL FlG. 242. The Ley den Jar and Bells.
a directly visible form, the nature of the electricity with which the inner
coating of a jar is charged. This experiment consists in slowly passing
over a cake of resin (or flat plate of vulcanite) the knob of a Leyden
jar, while the outer coating is held in the hand : we may even trace
figures with the knob.
" The free electricity of the inner coating, which is constantly renewed
in proportion as it escapes, because the other coating is held in the
hand, remains adhering to all the points of the cake which the knob
has touched.
" If, after having thus traced out lines with the knob of a jar charged
interiorly \vith positive electricity, we trace others beside them with
the knob of another jar.charged with positive electricity, we may render
each of them visible and distinct by powdering the cake with a powder
formed of a mixture of sulphur and red lead that have been rubbed
together. We perceive that all the particles of sulphur place them-
selves on the positive lines, and all those of red lead upon the negative ;
and they remain adhering there, even when we blow them or shake
the cake strongly, so as to make the portion cf the powder disappear
which is not upon the parts of the surface that had been touched by
the knob.
" The effect that we have just described arises from the particles of
sulphur, during their mutual trituration, having acquired negative
electricity, and those of red lead positive, which causes the former
to pass upon the positive traces, and the latter upon the negative.
We also remark that the sulphur forms a small tuft round each of the
positively electrified points, whilst on each of the negative points the
red lead leaves only a circular spot. This phenomenon, establishing,
as it does, a very remarkable difference between the two electricities,
is due to a more general cause.
" The property that we have thus recognised in resin, of retaining both
ELECTRICAL DISCHARGERS.
261
FIG.
electricities adhering to its surface, is not peculiar to this substance
alone : all bodies that are insulators possess it in a more or less marked
degree. We have already seen that it exists in glass, when we elec-
trized the interior of a glass jar, to produce the dance of pith balls.
A Leyden jar, the coatings of which are movable (see Fig. 226), fur-
nishes a further proof of this.
" The jar is charged as usual ; then with an insulating handle the
inner coating is lifted away, and afterwards the glass itself is lifted
out: the two coatings, being thus detached, manifest no electrical
signs. The two electricities have, in fact, remained adhering to the
glass, the positive on the interior surface, and the negative on the ex-
terior.
. Leyden Jar, with Lane's
Discharger.
FIG. 244. Harris's Improved
Lane's Electrometer.
" These two electricities are recovered again by replacing the jar
within its outer coating, and placing within it its inner coating ; the
discharge takes place between the two coatings as if they had not been
deranged. The fact just pointed out explains why a Leyden jar always
retains electricity after a first discharge, even when the latter has
given rise to a strong spark. We can obtain a second discharge,
much weaker, it is true, than the former, but yet very sensible, and
sometimes, indeed, exceedingly violent, if the jar is large, and has
been strongly charged.
"This second discharge arises from a portion of the two electricities
having remained adhering to the glass after the first discharge, not-
withstanding the contact of all the points of the two surfaces of the
262 ELECTRICITY.
jar with the metal surfaces ; but the second discharge is generally
sufficient to make all the remaining traces disappear."
XIV. A very portable and simple apparatus for obtaining electricity and
charging a Leyden phial was arranged by Mr. Adams, an optician of
the same date as Cavallo. It consists of a half-pint phial, coated inside
with brass filings, and outside with tinfoil, and is charged by a var-
nished silk ribbon, which is rubbed by being passed through hare-skin
rubbers placed, like finger-stalls, on the first and middle fingers of the
left hand. The following directions are given for the proper manipu-
lation of the silk rubbers : Place the two finger-caps of hare-skin on
the proper fingers ; hold the phial at the same time at the edge of the
coating, on the outside, between the thumb and first finger of the left
hand ; then take the ribbon in your right hand, and steadily and gently
draw it between the two ribbons, over the two fingers, taking care at
the same time that the brass ball of the jar is kept nearly close to the
ribbon while it is passing through the fingers.
By repeating this operation thirteen or fourteen times, the electrical
fire will pass into the jar, which will become charged, and, by placing
the discharger against it, you will see a sensible spark pass from the
ball of the jar to that of the discharger. If the apparatus is dry and in
good order, you will hear the crackling of the sparks when the ribbon
is passing through the fingers, and the phial will discharge at about
the distance of half an inch from the balls.
XV. In order to regulate the proper discharge of single Leyden jars and
batteries, very useful contrivances have been invented.
The arrangement (Fig. 243) consist of a bent glass arm, which is
fixed to the rod and ball passing to the inside of the jar; the arm carries
a tube through which a rod, with balls at both ends, slides. The dis-
tance between the two balls, one of which represents the interior and
the other the exterior of the jar, is regulated according to the scale
graduated on the sliding rod, so that a discharging spark of any re-
quired length (confined within the limits of the charged surface of
the jar) may be obtained. Sir W. Snow Harris improved the arrange-
ment of Lane's discharging electrometer, by making it an indepen-
dent piece of apparatus, that might be adapted to one or more jars.
The exploding balls of this instrument (Fig. 244) are supported be-
tween a bent glass arm and a vertical tube of brass, and may be set
at any given distance by means of a graduated slide. The bent arm
of glass is attached, and is movable on a stout glass cylindrical rod,
so as to insulate the whole, if required, and adjust the ball to be con-
nected with the inside of the jar or battery to any given height. These
and other pieces of electrical apparatus are made most correctly and
elegantly by Messrs. Elliott Brothers, of 5 Charing Cross, the worthy
successors of the old firm of Watkins and Hill, so long celebrated for
their electrical apparatus.
XVI. Cuthbertson's Balance Electrometer is an extremely useful contri-
vance, where large Leyden batteries are required to be rapidly and
uniformly discharged, as at the Polvtechnic, where the deflagration
of metallic wires* is displayed. The apparatus consists of a wooden
stand, in which two glass rods or supports are fixed : one of the insu-
lating rods or pedestals supports a arass ball, which has a little hook
ELECTRICAL DISCHARGERS. 263
below it, for the convenience of attaching the chain passing from the
outside of the Leyden battery ; the other and higher glass pedestal
supports a large brass ball, in which is arranged a long brass rod,
supported on knife-edges, and acting like a balance ; above this, and
proceeding from the same large brass ball, is another rod and ball,
placed so that the ball of the latter is exactly over, and almost touching,
the other and lower one, that works on knife-edges.
FlG. 245. Cuthbertson's Balance Electrometer.
A, B, glass supports. The hook of A is connected by a chain with
the outside of the battery. B carries the large ball through which the
balance-rod, D, works. The sliding weight, E, like that of a steel
rod, enables the experimenter to adjust the balance perfectly. H, the
upper and fixed wire and ball, which, when sufficiently electrified by
contact with the inside of the battery, by the hook and chain at K,
repels the movable balance D, and, making the circuit complete
(as shown by the dotted lines) by touching the brass ball on A, the
whole discharge of the battery is sent through any substance.
With Cuthbertson's compound universal discharging electrometer,
the experimenter may always have notice when the battery is nearly
charged and ready, by inserting in the upper ball a Henley's quadrant
electrometer, with graduated arc. The oscillation of the balance,
when the battery is almost ready, will likewise serve to warn the person
using it that he may expect the discharge to occur.
XVII. In connection with the Leyden battery, a Cuthbertson balance elec-
trometer and Henley's universal discharger and press are always
employed when a variety of substances are to be subjected to the
powerful effects of a large charged surface of glass. The mechani-
cal arrangements are such that the direction of the charge is certain
and precise.
The annexed figure (246) hardly requires any explanation, as the
parts are so simple. It consists of two glass legs, which support, by
264 ELECTRICITY.
hinged joints, two brass rods and balls with glass handles attached.
The latter slides through tubes, and may be caused to advance or
recede from each other, or they move right or left, as the hinged
joints work in sockets.
The balls meet either on the little table, in which a piece of ivory
is inserted, or the little table can be removed and the press substituted
for it ; as, for instance, when it is required to show the immense
FlG. 246. Henley's Universal Discharger and Press.
mechanical force of the electrical discharge by putting gold leaf
between glass plates, and passing a charge through them, which
shatters the glass to fragments, and frequently forces the gold leaf
into the body of the glass. In this experiment, it is usual to put the
glass plates in the press, and, to prevent accident from the pieces of
glass flying about, it is better to cover the whole with a dry clean
duster.
XVIII. Unscrew by a turn or two the balls attached to the arms of the Hen-
ley discharger ; take some very fine iron wire, such as is used by
silversmiths for making scratch-brushes, and having twisted a little
in the crack or opening left by unscrewing the balls, screw them up
again, when the thin wire will be held tightly, and, the length having
been adjusted to the power of the Leyden battery employed, the whole
is dispersed in minute white-hot globules when the electric charge
is sent through the wire.
XIX. Place the balls of the Henley discharger on the little table, about one
inch apart ; put some gunpowder between them. When the dis-
charge of the Leyden battery is sent across and through the gun-
powder, it is not ignited, but every grain is dispersed and thrown
away by the mechanical violence of the discharge, which occurs so
rapidly, that the heat of the electric discharge does not appear to
have time to affect the gunpowder.
When the great steam hydro-electric machine was in use at the
Polytechnic, it was possible, by directing from a point the whole dis-
charge of the mammoth machine for some minutes into a heap of gun-
powder, to accumulate heat and set it on fire ; but it was always very
troublesome to do, and a great deal of steam had to be used to effect
this object. If, however, a damp string formed part of the conduct-
ing arrangement, then the powder fired almost instantaneously, as
EXPERIMENTS. 265
the damp string exercises a retarding action on the velocity of the
current of electricity, and it then appears to have time to give its
heat to the powder.
To fire gunpowder by the Leyden jar, a little cardboard tray may
be placed on the table of the Henley discharger ; and in order not to
spoil the polish of the balls, two copper wires are thrust through the
sides of the tray containing the powder, and the brass balls of the
Henley discharger connected with them. A wet string may be tied to
one rod and also to an ordinary discharger, the other rod being con-
nected by a chain with the exterior of the Leyden jar; the ordinary
discharger with the wet string is made to touch the knob, and, although
it sometimes fails, the powder is very generally ignited directly con-
tact is made. To fire gunpowder, a wet string must form part of
the circuit. The powder may be placed in a closed case or cartridge,
so that it cannot be scattered by the mechanical violence of the dis-
charge.
Sturgeon retards the velocity of the discharge by placing the gun-
powder in a boxwood cup which is insulated and connected with the
outside of the jar. An insulated brass wire and ball is placed
directly over the cup, and, directly contact is made with this and the
interior of the Leyden jar by the ordinary discharger, the powder is
usually fired.
XX. A piece of mahogany, about two inches long and in. square, may
be split by passing the discharge into and through it by two copper
wires inserted about half an inch, one at each end. The softer the
wood, the safer the experiment so far as the jars are concerned, and,
as already observed at page 255, this experiment must not be pushed
too far by using larger and thicker pieces of wood.
XXI. When a lighted composite candle is blown out carefully, there rises
from it a column of gas and smoke, which is inflammable. If such a
candle is placed on the table of the Henley discharger, and the balls
adjusted so that the spark will go through a point just above the
burning wick, and the whole connected with a charged Leyden jar,
the spark will relight the candle, if, simultaneously with the blowing
out of the flame, contact is dexterously made with the Leyden jar.
FIG. 247. The Electric Bomb.
XXII. The expansion which air undergoes during the passage of an electric
discharge through it is shown by a very nicely constructed mortar,
to the mouth of which is accurately fitted a ball of some light wood.
266 ELECTRICITY.
When the discharge passes, the ball is forced out ; and if the whole
is made of ivory(Fig. 246), the effect is very certain. The expansion
of the air in this experiment will help the student to understand why
so much noise (thunder) is heard, when the electrical discharge
takes place from hundreds of acres of charged clouds. ,
XXIII. The experiment called the " Shooting Star" is extremely beautiful, but,
like many other illustrations, requires considerable pains to be taken in
order to obtain a good result. In the first place, a long tube must be pro-
vided at least four feet in length ; this is properly capped, and provided
with a stopcock at one end and a plain cap on the other, which should
be nicely rounded off, and inside the cap a small ball may be screwed.
The electrical machine being in good order, and the Leyden battery,
of six square feet of glass, warm and dry, one assistant may proceed
to charge it gradually, whilst another may be pumping the air out of
the long tube. When the electrometer shows that the battery is nearly
charged, one end of a chain is attached to one of the balls of the
discharger, and the other end to the top of the long tube. The air-
pump or stopcock end of the tube is, of course, in conducting com-
munication with the outside of the battery jars. The circuit is now
suddenly completed, and sometimes a continuous flash through the
whole length of the tube marks the discharge of the battery ; but it
may occur that it discharges itself in a brush, and that the battery
must be recharged, and the experiment tried again. To insure per-
fect success, the experiment should be tried with a barometer attached
to a pump, and then it- will soon be ascertained what vacuum is the
best for the experiment. Success greatly depends on the right
management of the vacuum, which must not be a perfect one.
XXIV. The velocity of electricity, and the consequent amazing rapidity of
the spark-discharge, and appearance or disappearance of the light,
is admirably shown by Mr. Rose's photodrome apparatus described
at p. 85, Fig. 95.
The writer uses the disc four feet in diameter, having a series of black
balls painted on a white ground ; when this is rotating three hundred
times in a minute, and the black balls have all merged one into the
other, according to the law of persistence of vision, already explained
at p. 84, they produce (instead of twelve distinct black balls) three
continuous rings, dark in the centre, and lighter towards the edges,
because there the greatest surface of the white disc is exposed. The
disc should be illuminated with a lime light and lens, and, directly
this is cut off, a Leyden jar, provided with a Lane's discharger, is per-
mitted to discharge itself regularly, by keeping the electrical machine
in motion ; all the black balls now return, and the disc, though going
round three hundred times in a minute, appears frequently to stand
still.
The same fact is observed during a storm at night, accompanied
with thunder and lightning : all objects seen by the light from the elec-
tric flash appear to stand still, although they may be in rapid motion
at the time. Captains of ships have frequent opportunities of noticing
this : a storm comes on suddenly, and some, if not all, the sails of the
ship require to be furled; the command is given, up. fly the sailors,
and the deck and rigging swarm with men who are actively engaged ;
EXPERIMENTS.
267
but if at this moment the ship is illuminated with a flash of lightning,
every officer, every man, the ship tossing about, and the waves of the
sea, all appear at rest, as if they were parts of a magnificent stereo-
scopic picture.
The fact is, that the light from a flash of lightning, as proved by
Sir Charles Wheatstone, comes and goes in the millionth part of a
second ; so that before the wheel, going round three hundred times in
a minute, has time to move, the electric light has arrived and passed
away. The same thing occurs with all other movements viewed with
the electric flash, and the fleetest racehorse even, under these circum-
stances, would actually appear to be standing still.
XXV. Many years ago, Sir Charles Wheatstone invented a most ingenious
arrangement for measuring the velocity of electricity through a
copper wire, and it was from these experiments he deduced the
almost instantaneity of the light from the electric spark.
His apparatus consisted of a Leyden jar, which was charged in
every experiment to the same amount, and the discharge sent through
a copper wire about half a mile long.
FIG. 248.
The copper wire was insulated and interrupted at three points, viz.
one, A A, within a few inches of the inner coating, one at the middle
of the circuit, B B, and one at the same number of inches of the
outer coating, c C, of the Leyden jar as the first which was in con-
tact with the inner coating. A very cleverly arranged insulated disc
(Fig. 248) contained the three breaks in the circuit, where the spark
discharges took place ; so that when the Leyden jar was discharged,
all the sparks could be seen at once, and were reflected in a small
268
ELECTRICITY.
revolving mirror. If observed without the mirror, the three sparks
appeared to occur simultaneously ; but when looked at in a small
revolving steel mirror through a plate of glass, the sparks, accord-
ing to the law of persistence of vision, become lines of light, of
which two are equal, whilst the third, representing the middle of the
circuit, is sufficiently delayed to give a shorter line, and, as the
velocity of the steel mirror is known, by a proper register, the exact
angular deviation of the image of the central spark is easily ob-
tained ; and from these data the retardation of the current by the
long copper wire is correctly calculated.
FlG, 249. Apparatus, made by Messrs. Elliott, to show the time occupied by
the transmission of an Electric Current by reflection.
B, the revolving mirror.
The three sparks, when seen in the revolving mirror, appear as
three straight bright lines; and, if the motion is very fast, the lines
assume the appearance A, Fig. 250, when the mirror is rotated to the
:B
FlG. 250. Lines of Light reflected from Revolving Mirror.
right ; but, if reversed, then they appear as in B, Fig. 250 : but the
lines were nevei seen as at c or D, Fig. 250, which should have been
the case according to the Franklinian theory of a single fluid. Thus
EXPERIMENTS.
269
Wheatstone's ingenious and beautiful experiment supports most
powerfully the theory of the two fluids, which seem to meet in the
centre of the wire, as if they rushed with equal speed to unite with
and neutralize each other.
The spark disc (Fig. 249) was placed 10 ft. away from the re-
volving mirror, and the summing-up of the experiments gave the
following conclusions :
1. That electricity travels, through a copper wire arranged as in
the experiment described, faster than light in its passage from the
sun.
2. That the electricities of the two kinds, viz., that from the interior
of the jar and the other from the exterior, travel at the same velo-
city, and meet in the middle of the wire.
3. That the light from the electric flash or spark-discharger does
not last longer than the millionth part of a second.
4. That the delicate optic nerve is capable of appreciating an
interval of that duration, or, in other words, can see objects which
are only illuminated for the millionth part of a second.
FIG. 25 1. Appearance of the Card after sending the discharge through Silver
Wire \-yx>th of an inch thick.
XXVI. Very fine .^old, silver, copper, brass, and iron wires can be obtained
of Messrs. Johnson and Matthey, at their assay office in Hatton
Garden. About three inches of either metallic wire is stretched
across a plain white card by making a small cut in the card at the
opposite ends, and then placing the wire in the cuts, which may
be neatly closed with little slips of tinfoil.
The card with the wire is then covered with another card, and
placed between the boards of the little press attached to Henley's
universal discharger (Fig. 245, p. 264). When tightly screwed up
and the brass balls of the discharger brought in contact with the
ends where the tinfoil marks the termination of the two ends of
the wire, the discharge from the Leyden battery can then be sent
through it. The result is that the wire is completely disintegrated,
270
ELECTRICITY.
and so perfectly divided that nothing remains upon the two cards
but certain curious marks (Fig. 251), which are no doubt caused by
the finely divided metal being driven bodily into the card,
although it is usually ascribed to oxidation, and this may be the
case with metals which unite easily with that element. When a very
thin iron wire is deflagrated alone by passing the battery discharge
through a length of nine or twelve inches, the effect is very beau-
tiful, as it is dispersed in a shower of red-hot globules, which are
well displayed in a darkened room.
The dissipation of gold by a powerful electrical discharge can
also be shown in a similar manner. The metal is vaporized, and
disappears in the form of a red vapour.
By receiving the vapour from gold on a piece of silk, a portrait
or other figure may be printed upon it. To obtain these portraits
a likeness of any known personage is cut out in a small piece of
cardboard, so that, if held against the wall with a candle behind
it, the shadow cast indicates that the portraiture is successful ; the
portrait-card is now laid upon a sheet of gold leaf pasted to another
card ; and, as the electrical discharge would act unequally upon
the gold if merely conveyed through the brass balls of the dis-
charger, it is usual to paste a slip of tinfoil on the opposite edge of
the gold leaf, thus bringing all the gold at once in conducting
communicating with the brass balls.
FIG. 252.
A, card covered with gold leaf, and edges prepared with tinfoil ; B, portrait- card ; c, the two cards, A and
B, in press, and in contact with the brass balls of the discharger.
XXVII. With the powerful hydro-electric machine at the Polytechnic (p. 273)
(to be hereafter described) a most beautiful effect was produced by
sending the discharge through a long chain composed of beads of
glass and copper strung on a stout silk cord ; and as the latter
was at least forty feet in length, the effect was very imposing.
On the smaller scale a piece of brass chain, hung in festoons on
a plate of glass blackened at the back, affords a very pretty
experiment, being illuminated throughout its entire length when
the electrical discharge is sent through it.
XXVIII. To imitate and demonstrate the effects of discharges of natural
electricity, or lightning, on buildings, &c., many ingenious models,
such as the gable end of a house, a pyramid, a powder-magazine,
EXPERIMENTS.
271
a ship, or mast of a ship, are made by Messrs. Elliott, of the Strand,
London.
FIG. 253.
All these models act upon one principle, viz., that as long as the
conductor is continuous throughout and unbroken, no harm or
damage occurs to the model ; but directly the conducting chain is
broken, by removing or altering the position of some part of the
conductor, then tha following results occur. In the first place, the
charged cloud is represented by Sir William Snow Harris's thunder-
cloud needle (Fig 254), formed by a brass horizontal rod or needle
balanced and movable upon the point of a vertical metallic rod
connected with the interior coating of a large Leyden jar.
FlG. 254. Harries Thunder-cloud Needle.
One end is covered with the finest cotton wool : a little good
gun-cotton increases the effect, as it may be so arranged that every
time the flash occurs the cotton shall ignite, and the sudden flash
with the crack and light of the spark is remarkably telling. The
cotton is intended to represent a cloud hovering over the chimney
or highest part of a house or church-steeple ; and, when the jar has
been sufficiently charged, it is attracted, according to the law of
induction, to the nearest object, and the simulated cloud descends
upon the top of the model, at the same time discharging the jar
272 ELECTRICITY.
through the parts of the models. As before stated, when the
lower portion of the conductor attached to either of the models
is connected with the outside of the jar by a chain, the Harris's
thunder-cloud needle being in connection with the interior of the
jar, the discharge causes no change in the disposition of the parts
of the toy model; but if, as in A, Fig. 253, the little piece of
square wood at B is turned round at right angles, the continuity of
the wire is broken, and it is blown out when the discharge takes
place. The model B maintains its erect position if the conductor
is undisturbed ; but when a little bit of tinfoil is removed from
D, it topples over in the most natural fashion when the miniature
thunder-cloud is discharged upon it.
The model E affords a good bang, and the roof is blown off when
the powder in the tube F is ignited ; but care must be taken not to
use too much gunpowder. The writer well remembers helping
poor young Mr. John Cooper, many, many years ago, at a lecture
delivered at the Southwark Institute, and, being directed sotto -vocc
to give them a " good one," he attended too implicitly to his in-
structions. Luckily, this was the concluding experiment : the
powder-house blew up with astounding effect ; but, unfortunately,
the roof descended into the middle of a large cylindrical electrical
machine, and the result, of course, was total annihilation. The
audience, it is believed, thought it was all part of the experiment,
and applauded in the most cheering manner ; but the glances ex-
changed between the lecturer and his assistant were of the most
desponding kind, considering that the large electrical machine had
only been borrowed for the occasion.
G, Fig. 253, is called the "fire-house," and exhibits the heat of
the electrical discharge, and its power to set fire to gun-cotton or
tow dipped in ether or alcohol ; and, as it is made of tin and
glazed with glass windows, the conflagration inside betrays the
lamentable effects that might and do occur when houses are struck
and set on fire by lightning.
XXIX. A lightning conductor, if intended to last, should be made of copper
rod, at least half an inch better three-quarters in diameter. It
should be carried above the highest chimney-top, and be well
pointed and doubly gilt ; the lower end must be carried down to the
clay, and must enter the first stratum of earth known to be always
damp. If the building is a long one, it is better to have a light-
ning conductor at each end, as a cloud, in coming up to a lightning-
conductor, is always discharged through the shortest road ; and if
a chimney-pot at the other end of the building rises as high as the
lightning conductor at the other end, it may divide the honours
and dangers of the discharge with the conductor, provided the
cloud arrives at the side opposite to that where the metallic
safety-rod is fixed.
XXX. The hydro-electric machine affords a magnificent example of
electricity derived from friction, and it continued for a lengthened
period to be one of the greatest attractions at the Polytechnic.
In the " Philosophical Magazine," vol. vii., appeared a letter from
Mr. (now Sir William) Armstrong, giving a curious and most inte-
EXPERIMENTS.
273
FIG. 255. The Hydro-Electric Machine at the Polytechnic.
resting description of the accidental production of the electric
spark by high-pressure steam escaping through a fissure or crack
in the cement by which the safety-valve ought to have been fitted
in steam-tight to the boiler of a locomotive standing at Sedgehill,
six miles from Newcastle. Every time the engine-man passed
his hand through the steam he received an intense electric spark,
which he spoke of as " fire." Mr. Armstrong investigated the
phenomena, and, continuing a very laborious and clever series of
experiments, arrived by gradual steps to the production of a per-
fect steam machine, in which the particles of water, impelled by
steam, rubbing against the interior of a series of jets lined with
partridge-wood, produced effects which have never been surpassed
in England. At that time Dr. Bachoffner was the very popular
lecturer on Natural Philosophy at the Polytechnic, and he assisted
at and conducted most patiently the vast number of experiments
which had to be carried out before the ponderous machine was
considered ready to be exhibited to the public. With Dr. Bach-
offner were of course associated the contriver, Mr. Armstrong, and
Mr. Walker ; and fearful that our readers may think the writer too
prone to talk of Polytechnic doings, he has preferred to take Dr.
Noad's account of the machine as exhibited fifteen years ago at
that Institution:
18
274 ELECTRICITY.
" Shortly after these experiments were made, the directors of the Polytechnic
Institution determined on constructing a machine, on a large scale, for the
purpose of producing electricity by the escape of steam ; and under the supcr-
intendance of Mr. Armstrong, assisted by Dr. Bachoffner, the ' Hydro-
Electric Machine' was finished, and placed in the theatre of the Institution,
where by its extraordinary power it soon excited the astonishment of all who
beheld it. The machine consists of a cylindrical-shaped boiler, similar in
form to a steam-engine boiler, constructed of iron plate f in. thick ; its extreme
length is 7 ft. 6 in., one foot of which being occupied by the smoke-chamber
makes the actual length of the boiler only 6 ft. 6 in. ; its diameter is 3 ft. 6 in.
The furnace and ash-hole are both within the boiler. When it is required
entirely to exclude the light, a metal screen is readily placed over these. By
the side of the door is the water-gauge and feed-valve. On the top of the
boiler, and running nearly its entire length, are forty-six bent iron tubes,
terminating in jets having peculiar-shaped apertures, and formed of partridge-
wood, which experience has shown Mr. Armstrong to be the best for the pur-
pose ; from these the steam issues. The tubes spring from one common pipe,
which is divided in the middle, and communicates with the boiler by two
elbows. By this contrivance the steam is admitted either to the whole or part
of the tubes, the steam being shut off or admitted by raising or lowering the
two lever Handles placed in the front of the boiler. Between the two elbows
is placed the safety-valve for regulating the pressure, and outside them, on
one side, is a cap covering a jet employed for illustrating a certain mechanical
action of a jet of steam, and on the other a loaded valve for liberating the
steam when approaching its maximum degree of pressure. At the further
extremity of the boiler is the funnel-pipe or chimney, so contrived that, by the
aid of pulleys and a balance-weight, the upper part can be raised and made
to slide into itself (similar to a telescope), so as to leave the boiler entirely
insulated. To prevent as much as possible the radiation of heat, the boiler
is cased in wood, and the whole is supported on six stout glass legs, 3! in.
diameter and 3 ft. long. In front of the jets, and covering the flue for con-
veying away the steam, is placed a long zinc box, in which are fixed four rows
of metallic points, for the purpose of collecting the electricity from the ejected
vapour, and thus preventing its returning to restore the equilibrium of the
boiler. The box is so contrived, that it can be drawn out or in, so as to bring
the points nearer or further from the jets of steam : the mouth or opening can
also be rendered wider or narrower. By these contrivances the power and
intensity of the spark is greatly modified. A ball-and-socket joint, furnished
with a long conducting-rod, has been added to the machine, so that by its aid
the electricity can be readily conveyed to the different pieces of apparatus
used to exhibit various phenomena. The pressure at which the machine is
usually worked is 60 Ibs. on the square inch.
" As it is now fully established that the electricity of the hydro-electric
machine is occasioned by the friction of the particles of water, the latter may
be regarded as the glass plate of the common electrical machine, the partridge-
wood as the rubber, and the steam as the rubbing power. The electricity
produced by this engine is not so remarkable for its high intensity as for its
enormous quantity. The maximum spark obtained by Mr. Armstrong in the
open air was 22 in., the extreme length under present circumstances has been
12 or 14 in.; but the large battery belonging to the Polytechnic Institution,
exposing nearly 80 ft. of coated glass, which under favourable circumstances
THE HYDRO-ELECTRIC MACHINE. 275
was charged by the large plate machine, 7 ft. in diameter, in about 50 seconds,
is commonly charged by the hydro-electric engine in 6 or 8 seconds. The
sparks which pass between the boiler and a conductor are exceedingly dense
in appearance, and, especially when short, more resemble the discharge from
a coated surface than from a prime conductor. They not only ignite gun-
powder, but even inflame paper and wood shavings when placed in their
course between two points. In the I5ist number of the 'Philosophical
Magazine,' a series of electrolytic experiments made with this machine are
described by Mr. Armstrong. True polar decomposition of water was effected
in the clearest and most decisive manner, not only in one tube, but in ten
different vessels, arranged in series, and filled respectively with distilled water,
acidified with sulphuric acid, solution of sulphate of soda tinged blue, and
red solution of sulphate of magnesia, c., c., and the gases were obtained
in sufficient quantities for examination.
" The following curious experiments are likewise described :
" Two glass vessels containing water were connected together by means of
wet cotton. On causing the electric current to pass through the glasses, the
water rose above its original level in the vessel containing the negative pole,
and subsided below it in that which contained the positive pole, indicating the
transmission of water in the direction of a current flowing from the positive to
the negative wire. Two wine-glasses were then filled nearly to the edge with
distilled water, and placed about 4-ioths of an inch from each other, being
connected together by a wet silk thread of sufficient length to allow a portion
of it to be coiled up in each glass. The negative wire, or that which com-
municated with the boiler, was inserted in one glass, and the positive wire, or
that which communicated with the ground, was placed in the other. The
machine being then se-t in action, the following singular effects presented
themselves :
" i. A slender column of water, inclosing the silk thread in its centre, was
instantly formed between the two glasses, and the silk thread began to move
from the negative towards the positive pole, and was quickly all drawn over
and deposited in the positive glass.
" 2. The column of water, after this, continued for a few seconds suspended
between the glasses as before, but without the support of the thread; and when
it broke, the electricity passed in sparks.
" 3. When one end of the silk thread was made fast in the negative glass,
the water diminished inthe positive glass, and increased in the negative glass,
showing, apparently, that the motion of the thread, when free to move, was
in the reverse direction of the current of water.
" 4. By scattering some particles of dust upon the surface of the water, it
was soon perceived by their motions that there were two opposite currents
passing between the glasses, which, judging from the action upon the silk
thread in the centre of the column, as well as from other less striking indica-
tions, were concluded to be concentric, the inner one flowing from negative to
positive, and the outer one from positive to negative. Sometimes that which
was assumed to be the outer current was not carried over into the negative
glass, but trickled down outside of the positive one, and then the water, instead
of accumulating, as before, in the negative glass, diminished both in it and in
the positive glass.
' 5. After many unsuccessful attempts, Mr. Armstrong succeeded in causing
the water to pass between the glasses without the intervention of a thread for
18 2
276 ELECTRICITY.
several minutes, at the end of which time he could not perceive that any
material variation had taken place in the quantity of water contained in either
glass. It appeared that the two currents were nearly, if not exactly, equal,
while the inner one was not retarded by the friction of the thread. Mr. Arm-
strong likewise succeeded in coating a silver coin with copper, in deflecting the
needle of a galvanometer between 20 and 30, and in making an electro-
magnet by means of the electricity from this novel machine.
" Extraordinary as is the power of the Polytechnic machine, it was after-
wards entirely eclipsed by a similar apparatus constructed at Newcastle under
the direction of Mr. Armstrong, and sent out to the United States of America.
In the arrangement of this machine, the boiler of which is not larger than that
at the Polytechnic Institution, Mr. Armstrong introduced certain improve-
ments, suggested by the working of the latter, and which had reference to
those parts of the apparatus more immediately concerned in the production of
the electricity, viz., the escape apertures and the condensing pipes. It was
found to be a matter of extreme nicety to adjust the quantity of water depo-
sited in the condensing pipes, so as to obtain the maximum excitation of elec-
tricity. If, on the one hand, there be an excess of water, then two results
will ensue, each tending to lessen the electricity produced: ist, the mean
density of the issuing current of steam and water is increased, which causes
the velocity of efflux, and consequent energy of the friction, to be diminished ;
and, 2iidly, the ejected steam-cloud is rendered so good a conductor by the
excess of moisture, that a large proportion of the electricity manifested in the
cloud retrocedes to the boiler, and neutralizes a corresponding proportion of
the opposite element. On the other hand, if the quantity of water be too
small, then, although each particle of water may be excited to the fullest extent,
the effect is rendered deficient, in consequence of the insufficient number of
aqueous particles which undergo excitation.
" In the Polytechnic, arrangement for condensation of the steam in the
tubes is effected by contact with the external air ; and when the density of the
steam in the boiler is diminished rapidly, they do not cool down with sufficient
rapidity to condense the' requisite quantity of water. To remedy this defect
in the American machine, Mr. Armstrong adopted a method of condensing
by the application of cold water. A number of cotton threads were suspended
from each condensing pipe into a trough of water, from which, by capillary
attraction, just as much water was lifted as was required for the cooling of the
pipe, since it was easy, by increasing or diminishing the quantity of cotton,
to increase or diminish the supply of cold water ; and this method of keeping
down the temperature proved so effective, that two or three times the number
of jets that were before used could now be employed. The number in the
American machine was 140, ranged in two horizontal rows, one above the
other, on the same side of the machine. The sparks obtained, though not
longer than those upon the London machine when it stood in the open air,
succeeded each other with three or four times the rapidity, and, even under
unfavourable circumstances, charged a Leyden battery, consisting of thirty-six
jars, containing 33 ft. of coated surface, to the utmost degree that the battery
could bear, upwards of sixty times in a minute, being equivalent to charging
nearly 2000 ft. of coated surface per minute, which is at least twenty times
greater than the utmost effect that could be obtained from the largest glass
electrical machine ever constructed."
The Polytechnic apparatus, itself unique, enormous, and powerful, was well
THE HYDRO-ELECTRIC MACHINE. 277
adapted for the purposes of the Institution, but could not be carried about or
fitted up in another lecture-room. The writer had a portable apparatus fitted
up, which gave safely, on the small scale, all that could be witnessed with the
great hydro-electric steam machine. It consisted of a cylindrical furnace
and strong copper boiler, supported on a stool with stout glass legs, each of
which rested on a disc of shellac. The boiler was provided with a safety-valve
and all necessary taps, and proceeding from it, and fitted with a ball-and-socket
joint, was a copper tube, I in. in diameter, curved round, and having a hollow
copper ball at the end, to which three stop-cocks were fitted. Whilst steam
was getting up, the copper tube was left off the boiler, and only screwed on
just before the experiments were shown.
The chimney of the furnace was so arranged that the portion connected
directly with the furnace could be removed, disclosing a square iron box, into
which a few pieces of burning charcoal were placed, so that, when the copper
tube and ball were screwed on, the first stop-cock exactly faced the iron box
containing the charcoal; and, of course, when the steam was turned on, it
blew out of the latter into the charcoal, and, causing the charcoal to burn
with greater rapidity, created a good draught, which carried off the steam, and
prevented it doing harm to the other electrical apparatus, which had to be
kept dry and warm.
FlG. 256. Portable Apparatus for showing the Electricity of Watery Steam.
A is the copper boiler, safety-valve, copper curved tube, with hollow ball and
three stop-cocks ; the lower one enables the operator to remove condensed
water, the upper one to introduce any different fluid ; the third contains the
jet made of hard partridge-wood (Fig. 257), from which the watery steam
escapes into the charcoal-box and chimney, D D. The dotted lines, C c, show
the portion of the chimney removable before the experiments commence, in
order to insulate the furnace B, which stands on a stool with strong glass legs,
resting on plates of shellac.
The operator must remember to keep a sufficient quantity of damp sand in
the bottom of the ash-pit, which should be regularly wetted by the assistant, or
the stool may catch fire, and great confusion caused by this untoward result.
The chimney D D is rendered independent of all extraneous support by being
attached to a strong iron pillar with claw feet, screwed to the floor with (E E)
278 ELECTRICITY.
stage-screws, i.e., spiral screws with handles, much used for theatrical purposes,
to support small bits of scenery on a stage.
The boiler being insulated, and the steam up to a pressure of at least 30 Ibs.
on the square inch, a number of interesting experiments may be performed.
FlG. 257. Section of the Jet used for the Hydro-Electric Machine,
Being a conical plug of hard wood (partridge-wood is preferred), terminated by a brass mouth-piece. The
shaded parts are brass.
I. Mere emission of dry steam produces no electricity, and will hardly
affect the gold leaves of an electroscope.
II. The copper ball is now purposely cooled a little by pouring cold water
and applying a wet flannel to it, so as to obtain some condensed
water; and now, when the steam is turned on, the usual signs of
electrical excitement become apparent, and sparks are easily procur-
able. The handles of the stop-cock must be covered with flannel, or
the operator will be unable to manipulate the opening and shutting
of them. The watery steam, rushing through the tube, evolves elec-
tricity, because the particles of water forced through by the jet of steam
rub against the inside of the jet, thus proving in a satisfactory manner
that friction is the exciting cause, and not the mere change of form
of water into steam. The copper boiler, whilst the steam is issuing,
is negatively electrified; the issuing steam, positively.
III. The steam being raised to 50 Ibs. on the square inch, the electric spark,
the inflammation of combustible matter, and the charging of the
Leyden jar, can be displayed, the boiler and steam remaining in the
same state of electricity.
IV. Altering the rubbing fluid, by substituting oil for the water in the copper
globe (easily done by pouring in a few drops of oil of turpentine
through the upper stop-cock), changes the state of the electricity of
the boiler from negative to positive, and the steam from positive to
negative, because the globules of water become coated with oil, and
thus expose a different surface against the rubber, viz., the inside of
the hard partridge-wood jet.
V. The electrical exaltation is destroyed for a time by putting a solution of
common salt into the copper globe, because the particles of water
are then made good conductors, and as fast as the electricity is ob-
tained it is neutralized (returned again to the boiler), just like rubbing
a piece of sealing-wax with a damp flannel. The gradual rise and
return of the electrical force is shown, as the conducting matter, the
salt, is blown out of the copper globe, as if the damp flannel had been
dried, and thus lost its conducting power.
VI. Dry steam or dry air will not excite electricity whilst rushing through a
tube ; this is easily proved by getting the copper tube and globe as
hot as possible, and then allowing the steam to issue from the jet.
6 VMMAR Y OF LA WS. 2 7 9
So also with air : the mere fact of allowing air to rub against the
inside of the nozzle of a common pair of bellows will not eliminate
the electric force ; but if a little whitening or powdered chalk is in-
troduced, as a substitute for the watery particles in the steam experi-
ment, the electricity is produced, and is shown distinctly if the
whitening is blown out on to the cap of the electroscope.
VII. By connecting an insulated platinum capsule, containing water, by a
wire, with an electroscope, and evaporating the water, no electricity
can be rendered evident ; if, however, a piece of red-hot charcoal is
placed in the platinum capsule, and a little water suddenly poured
upon it, and provided the ebullition is sufficiently violent to cause the
particles of water to rub against the sides of the capsule, then elec-
tricity is sometimes eliminated.
From these experiments it may be concluded that evaporation unattended
by friction, as from the surface of the oceans, rivers, lakes, is not a source
from whence electricity in nature is obtained, and we must therefore look to
some other cause for the explanation of the production of atmospherical elec-
tricity.
SUMMARY OF THE LAWS OF ELECTRICAL ACCUMULATION.
The young students who wish to travel easily through the chapters on voltaic
electricity, magnetism, and electro-magnetism will do well to make themselves
well acquainted with the laws which relate to frictional electricity, as they will
find them reproduced in more complicated forms as they proceed with the
consideration of the most important branches of science, with which all well-
educated persons should be acquainted.
I. Experiments would show, and especially those which relate to the
velocity of the passage of an electrical discharge through a copper wire
half a mile in length, performed by Sir Charles Wheatstone, p. 267,
that the idea of the existence of two forces, the one called " vitreous "
and the other "resinous" electricity, seems to be more rational and
better capable of proof than the Franklinian theory that supposes
the existence of one fluid only; and this idea is further supported by
Armstrong's curious experiments with the Polytechnic hydro-electric
machine, paragraphs I to 5, page 275.
II. Similar electricities repel, dissimilar attract, each other.
III. There is no absolute difference between insulators and conductors, it
is shown that they may both assume polarity ; but, in the former case,
the polarity lasts only so long as the disturbing cause exists ; in the
latter, as with glass and resin, the polarity set up is maintained.
These are called dielectrics, because they are capable of polarization.
IV. Electrical induction means that disturbance of electrical equilibrium
which occurs when an electrified body is brought towards another
which is in a quiescent state.
V. Faraday's theory of induction has overturned all previous hypotheses.
" Electrical induction is an action of contiguous particles." Every
particle of air between a piece of excited glass and the cap of an
electroscope is supposed to be in a polar state.
2 8o ELECTRICITY.
As long as the particles maintain their polarization, insulation is
secured ; but when the particles discharge themselves one into the
other, then a neutralization occurs, and the non-maintenance of
polarization is called conduction.
Even a Faraday could occasionally write vaguely. It is sometimes
better to take the epitome of a philosopher's assumptions through
another mind, and this want is admirably supplied by the late Pro-
fessor Daniell, of King's College, London :
" Up to the date of his discovery, the phenomena of induced elec-
tricity were supposed to arise from an action of a charged body upon
others at a distance, in straight lines, through non-conducting media,
the particles of which were assumed to be unaffected by it ; he has
shown induction, on the contrary, to be an action of contiguous par-
ticles throughout, capable of propagation in curved lines, and to be
concerned in all electrical phenomena ; having in reality the character
of a first, essential, and fundamental principle. . . It was formerly
supposed that the electric fluid was confined to the surfaces of bodies
by the mechanical pressure of the non-conducting air, in the midst
of which all our experiments are carried on ; but the fact is that the
electric force, originally appearing at a certain place, is propagated
to, and sustained at, a distance through the intervention of the con-
tiguous particles of air, each of which becomes polarized, as in the
case of insulated conducting masses, and appears in the inducteous
body, i.e., the body under induction as a force of the same kind
exactly equal in amount, but opposite in its directions and tendencies."
VI. Electricity is found to reside on the surface of an insulated metallic
conductor a natural sequence of the polarization of particles. The
difference in form, as between a ball and a point, so far as their rela-
tion to an electrical charge is concerned, is explicable by the theory
of contiguous particles.
" It was," says Daniell, " by an apparatus constructed on similar
principles to the electrophorus (p. 245) that Faraday brought to the test
of experiment his theoretical anticipation that inductive action, taking
place invariably through the intermediate influence of intervening
matter, would be found to be exerted, not in the direction of straight
lines only, as had always been assumed, but also in curved lines.
" A cylinder of solid shellac, of about I in. in diameter and 7 in. in
length, was fixed in a wooden foot ; it was made concave, and capped
at its upper extremity, so that a brass ball or hemisphere could stand
upon it. The upper half of the stem having been excited resinously,
by friction with warm flannel, a brass ball was placed on the top, and
then the whole arrangement examined by the carrier ball or proof-
plane and Coulomb's electrometer (p. 229). For this purpose the
carrier ball was applied to various parts of the ball ; the two were
uninsulated whilst in contact, or in position, then insulated, separated,
and the charge of the carrier examined as to its nature and force.
Of course, whatever general state the carrier acquired in any place
where it was uninsulated and then insulated, it retained on removal
from that place, and the distribution of the force upon the surface of
the inducteous body while under the influence of the inductive was
ascertained. The charges taken from the ball in this its uninsulated
SUMMARY OF LAWS.
281
state were always vitreous, or of the contrary character to the elec-
tricity of the lac. When the contact was made at the under part of
the ball, the measured degree of force was 512; when in a line with
its equator, 270; and when on the top of the ball, 130."
FIG. 258. Faraday's Experiment,
Proving that the polarization of the particles of air
may occur in curved as well as in straight lines.
FlG. 259. Faraday's Apparattisfor de-
termining the specific or particular
inductive power belonging to various
substances.
A, B, the two brass spheres, one within the other, A
being supported by a brass wire, c, passing through
a shellac rod, which latter insulates A, and prevents
it communicating with n. The space between A
and B can be rilled with any solid, liquid, or gas-
eous dielectric. E, the stop-cock, which screws
into the air-pump, if necessary.
The shellac electrophorus with its ball is here exhibited (Fig. 258),
together with the positions of the carrier ball referred to. When
placed at d, the effect produced was 512 ; at c, 270 ; at , 130. Even
in the position e the proof or carrier ball became inducteous ; and at a
it was affected in the highest degree, and gave a result above 1000.
VII. Specific Induction. If one body capable of maintaining polarization
can assume this condition quicker than another, it must be apparent
that a resisting force of some kind exists, which causes insulating
substances to vary in this respect.
Faraday ascertained this variable resistance by means of an appa-
ratus (Fig. 259) consisting essentially of two brass spheres, placed
one within the other, conducting communication between them being
prevented by proper means. The intervening space between one
sphere and another could then be filled with a variety of substances,
solid, fluid, and gaseous.
Faraday used two of the instruments (Fig. 259), and a certain
charge having been given to one of these, after the intervening space
had been filled with the substance under investigation, it was con-
nected with the second instrument, containing air; thus the latter
became the standard of comparison used throughout the experiments;
and the intensity, as before, was estimated by the carrier or proof
282
ELECTRICITY.
VIII
ball and Coulomb's electrometer. The inductive apparatus was in
effect a Leyden jar, with the advantage that the dielectric, represented
in the latter case by glass, could be removed at pleasure, and other
bodies substituted. With this apparatus Faraday determined the
inductive powers of a number of substances, and his experiments
have been extended and verified by Sir William Snow Harris.
Substance.
Air_ .
Rosin
Pitch .
Beeswax
Glass .
Sulphur
Shellac
Comparative Specific Inductive Power.
00
77
80
86
90
'93
'95
All gases, whatsoever may be their nature, have the same specific
inductive power- as air; no variation in the moisture, or temperature,
or density of the gases affects the uniformity of their property in this
respect.
Electricity stored in a Leyden jar can be measured into it, if neces-
sary, by a beautiful contrivance of Harris, called the unit or standard
jar; it is, of course, similar in principle to Lane's discharging electro-
meter, page 261. The unit Leyden jar is a very small one, and,
mounted on a glass rod, the outside has a brass cap carrying a brass
rod, which is placed at any required distance from the wire and ball
coming from the interior of the miniature jar. According to the
Franklinian experiment, page 251, every charge sent to the outside
of the unit jar sets free from the inside an equivalent proportion of"
vitreous electricity; and directly the charge in the little jar is of suffi-
cient intensity to break through the intervening thickness of air, it
discharges itself with the usual snapping noise.
IX. With Harris's unit jar (Fig. 260) and balance, the following facts have
been ascertained :
FlG. 260. Harris's Unit Jar.
c, the conductor of the electrical machine connected with the outside of the unit jar, b ; the inside,.
a, bting connected ith a large Leyden jnr, every time the little jar discharges itself between b and a,
a unit or definite quantity of electrical force has passed into the larger jar.
The area of the charged surface remaining constant, the attraction
SUMMARY OF LAWS. 283
between the two discs of the balance (see page 231) increases as the
square of the quantity. The intensity of the charge being main-
tained at one fixed point, and the distance between the discs altered,,
the attractive force varies inversely as the square of the distance.
Coulomb's laws, already detailed, can only be regarded as general
when they are confined to electrized molecules or points ; they are
again repeated here for the sake of the student, who may wish to
remember the chief laws. First law, " Two electrized bodies attract
and repel each other with a force which is inversely proportional to
the square of the distance that separates them."
The force with which two bodies that possess different electricities
attract each other is inversely proportional to the square of the dis-
tance by which they are separated.
X. The discharge of an electrical accumulation may take place in various
ways; viz.,
1. By conduction,
2. By disruption,
3. By convection.
The first is the most simple, as when a brass rod is held in the-
hand, and laid upon the conductor of an electrical machine in full
action.
The second involves the charge of particles, and their displace-
FlG. 261. A Current of Air set in motion from the Electric Point,
And, by convection, carrying the electricity to the flame of the candle, when it is dissipated and lost by
the heated and rarefied air.
ment in a gradual and steady manner, as by brushes or glow; or in
a violent degree, as with a spark passing through the air, or causing
the fracture of a thin Leyden jar, which has been too highly charged.
The third is special and peculiar, and involves motion ; it is, there-
fore, called a " carrying discharge." Faraday illustrated it by insu-
lating and electrifying a large copper boiler, 3 ft. in diameter, to a
limit just within that which would produce the brush or moderate
disruptive discharge. A brass ball, 2 in. in diameter, when sus-
pended by a silk thread and held within 2 in. of the boiler, became
charged, although insulated the whole time. As its electricity was
contrary to that of the boiler, the effect would be, with a light ball,
that it will be attracted, and then fly off to the nearest conductor, and
284 ELECTRICITY.
thus, like dust or any small particles capable of easy motion, would
gradually, by convection, carry away the charge.
A brush discharge may be frequently changed to a glow, by setting
up a current of air in the same direction as that taken by the brush
discharge ; and this effect may be reversed, and a glow converted
into a brush, by preventing the access of currents of air.
LATERAL DISCHARGE.
In consequence of the resistance offered, even by metals, to the progress, of
electricity, there is always a tendency in any electrical discharge to divide
itself if there are many contiguous conductors in the same line or path;
and thus sparks or flashes will occur when least expected, and, in the case of
ships of war or powder-magazines, may do some harm if they are struck by
lightning, although they may be supplied with lightning-conductors. The
subject of lateral discharge received considerable attention from the late Sir
William Snow Harris and Mr. Charles V. Walker, and the result of their dis-
cussions was the more careful protection of Her Majesty's ships by taking care
to connect all masses or bars of metal with the main conductor, so that no
accidental division shall occur anywhere ; and thus all chance of flashes or
sparks are prevented. The following experiment of Dr. Miller* serves to
illustrate this point :
FIG. 262.
Charge a Leyden jar, and arrange a metallic wire, w, from 120 to 150 ft. in
length, so as to act the part of discharger ; at the same time open a short path
for the discharge to the outer coating, by bringing the balls a, b within a short
distance of each other. Under this arrangement a portion of the electricity
takes the shorter course from a to b, and overcomes the high resistance of the
stratum of air interposed between the balls, owing to the resistance experienced
by the discharge to its passage along the continuous conducting wire w.
Miller's "Elements of Chemistry," vol. i., p. 432.
VOLTAIC ELECTRICITY.
285
FlG. 263. Galvani's Experiment with the Nerves and Muscles of the dead
Frog
(As exhibited on the disc at the Polytechnic).
VOLTAIC, GALVANIC, OR DYNAMICAL
ELECTRICITY.
It always seems quite natural, and taking things in their right order, to com-
mence this subject by speaking of that famous illustration of animal electri-
city primarily discovered by Galvani, who ascertained that by touching the
lumbar nerves of a frog, or lower part of the spine of a frog, recently killed,
with a clean copper wire, and the muscles with a zinc wire, and then bringing
the two metals in contact, that a current of electricity was evolved, which was
instantly rendered evident by the frog-electroscope, the limbs being always
convulsed in the most curious manner.
Galvani thought that the nerves and muscles of all animals were in oppo-
2-S6 ELECTRICITY.
site states of electricity, and that the effect occurred only at the moment when
the two opposite forces rush together and neutralize each other; but it was
soon shown that the convulsions were due to the effect of a current of elec-
tricity, however feeble, set up when the two metals touched each other in the
presence of a third element, viz., the liquid, containing chloride of sodium, with
which the limbs of the recently killed frog would necessarily be moistened ; it
was, in short, the oxidation of the zinc wire that produced the current, and
the prepared limbs of the frog represented only the electroscope that rendered
the electrical disturbance evident.
The biographer of Lewis Galvani, in " Rees's Cyclopaedia," states that he was
born in 1737, at Bologna.
In his early youth he showed a great propensity to religious austerities ; but,
being dissuaded from entering into an order of monks, whose convent he fre-
quented, he directed his attention to the study of medicine. He pursued this
study under able masters, and gained their esteem, especially that of Professor
Galcazzi, who received him into his house and gave him his daughter in mar-
riage. In the year 1762, after having sustained an inaugural thesis, " De
Ossibus," he was appointed public lecturer in the University of Bologna and
reader in anatomy to the Institute in that city. By the excellence of his
method of teaching, he obtained crowded audiences.
FIG. 264. The prepared Frog's Limbs.
Then follows the story of the soup made of frogs, which had been recom-
mended to his dearly loved wife, who was in a declining state of health, and
the accidental discovery that the limbs of the frog were affected by the point
of a scalpel held near the prime conductor of an electrical machine in action.
Matteuchi,. however, denies the originality of the experiment, and declares
that it was performed many years before the time of Galvani, in the presence
of the Grand Duke of Tuscany, by the celebrated Swammerdam.
His first publication on the subject was printed for the Institute at Bologna,
1791, and entitled " Aloysii Galvani de viribus Electricitatis in motu musculari
Commentarius." This work immediately excited the attention of philosophers,
both in Italy and other countries, and the experiments were repeated and
extended.
In conjunction with his physiological inquiries, the duties of his professor-
ship and his employment as a surgeon gave full occupation to the industry of
Galvani. In addition to a number of curious observations on the organ of
hearing in birds, which were published in the memoirs of the Institute of
ALDINPS EXPERIMENTS. 287
Bologna, he drew up various memoirs on professional topics, which have re-
mained unedited.
He regularly held learned conversations with a few literary friends, in which
new works were read and commented upon. He was a man of a most amiable
character in private life, and possessed of great sensibility, insomuch that the
death of his wife, in 1790, threw him into a profound melancholy.
His early impressions on the subject of religion remained unimpaired; he
was always punctual in practising its minutest rites ; and from this cause, no
doubt, he steadily refused to take the civic oath exacted by the then new con-
stitution of the Cis-Alpine republic, and was consequently deprived of his
posts and dignities. In a state of melancholy and poverty, he retired to the
house of his brother James, a man of very respectable character, and fell into
an extreme debility.
The republican governors, probably ashamed of their conduct towards
such a man, passed a decree for his* restoration to his professorial chair and
its emoluments ; but it was too late.
He expired on the 5th of November, 1798. But the good philosopher's name
and works were not to lie dead and forgotten : his nephew, the Professor
Aldini, of Bologna, seeing the grief and the sad end of his uncle, determined
to rescue his name from obscurity, and to defend Galvani's theories, which had
been attacked and repudiated.
For this purpose, Aldini travelled through France and England, demon-
strating the remarkable physiological experiments of Galvani, and so pleased
the professional authorities at Guy's Hospital, in 1803, that they presented
him with a gold medal.
For a very complete epitome of organic electricity, the reader is referred to
another work.* It may be sufficient here to state that Aldini maintained
that
" Muscular contractions are excited by the development of a fluid (electric)
in the animal machine, which is conducted from the nerves to the muscles,
without the concurrence or action of metals.
" All animals are endowed with an inherent electricity, appropriate to their
economy, which electricity, secreted by the brain, resides especially in the
nerves, by which it is communicated to every part of the body.
" The principal reservoirs are the muscles, each of which he regarded to
have two sides in opposite electric conditions.
." When a limb is willed to move, the nerves, aided by the brain, draw from
the interior of the muscles some electricity ; discharging this upon their sur-
face, they are thus contracted and produce the required change of position."
It is a remarkable fact, that when an acid and an alkaline solution are
so placed that their union may be effected through the substance of an animal
membrane or, indeed, any other porous diaphragm, a current of electricity is
evolved, the causes of which disturbance of electric equilibrium have already
been investigated. Now, with the exception of the stomach and caecum, the
whole extent of the mucous membrane is, in the human subject, bathed with an
alkaline mucous fluid, and the external covering of the body, the skin, is as
constantly exhaling an acid fluid, except in the axillary and, perhaps, pubic
regions. The mass of the animal frame is thus placed between two great
* "The Elements of Natural Philosophy," by Golding Bird, M.A., and Charles Brooke, M.A. John
Churchill, New Burlington Street.
288 ELECTRICITY.
envelopes, the one alkaline and the other acid, meeting only at the external
outlets. This arrangement has been shown by Donne to be quite competent
to the evolution of electricity, and, accordingly, he found that if a platinum
plate, connected with the galvanometer, be held in the mouth, whilst a second
be pressed against the moist perspiring surface of the body, the needles will
instantly traverse, as they did in the experiment just shown with an acid and
an alkali.
The current thus detected by Donne' at once explains the cause, and con-
firms the accuracy, of the celebrated experiment of Aldini, in which he excited
convulsions in a frog by holding its foot in the moistened hand, and allowing
the sciatic nerve to touch the tongue. There is also another remarkable expe-
riment of Aldini, explicable on the same principle, and shown in Fig. 265.
FIG. 265. Aldini's Battery,
Formed of the heads of recently decapitated oxen. A, c, c.
One of the ears of the first head, A, is well moistened with salt and water,
and connected, through the tongue, by a silver wire with the ear of B ; the
tongue of B is in like manner connected with the ear of C.
The ear of A and the tip of the tongue of c form the terminals of this
"bovine battery;" silver wires brought round from both are now connected
with the prepared limbs of a frog, just killed, so that the portion of the spine
still connected with its lumbar nerves touches the wire from the tip of the
tongue, which had been previously drawn out of the mouth of the ox, and
the skinned legs touch the wire from one of the ears. The frog's legs instantly
contract, and the contraction ceases when the circuit is broken.
Dr. Wilkinson estimated that the irritable muscles of a frog's leg were no
less than 56,000 times more delicate, as a test of electricity, than the most
sensitive condensing electroscope (p. 243).
" About forty years prior to Galvani's discovery,* a person of the name of
Sultzer gave an account of the following fact :
* "Rees's Cyclopaedia," article Galvanism.
VOLTAIC ELECTRICITY. 289
" If a piece of lead and a similar piece of silver be laid together, and the
edges of both be brought in contact with the tongue, a taste is perceived similar
to that of vitriol of iron ; at the same time that the metals applied separately
produce no effect.
" The observer of this fact does not appear to have been surprised at the
effect. At that time the doctrine of vibrations was employed to explain all
natural phenomena.
" He, therefore, concluded that some peculiar vibration took place from the
contact of the metals, which produced the peculiar sensation on the tongue.
" All the world were satisfied with this explanation ; and thus a prominent
fact had slept in obscurity from the time of Sultzer to the time of Galvani."
The excitation of galvanic electricity is traceable to chemical action. It has
already been stated that the combustion of a piece of charcoal will eliminate
the electric force, and can be discovered by a delicate condensing -electro-
scope. In galvanic experiments another instrument is required, in order to
detect the feeble currents of electricity of low tension or intensity.
This instrument admits of wonderful refinement, as will be seen presently
in the description of Sir William Thompson's reflecting galvanometer needle ;
but for ordinary experiments an instrument constructed as follows will suffice :
FIG. 266. The ordinary Galvanometer Needle.
It will be seen presently, that a single wire conveying an electric current
causes a magnetic needle to be deflected, and to take up a position at right
angles to the current. If one wire can produce this result, it is clear that,
by twisting the wire and increasing the number of convolutions, the effect of
the single wire is multiplied ; and by covering the wire with silk or cotton, so
as to prevent lateral communication, a much greater surface of electrified wire
is brought to bear by induction upon the magnetic needle. These conditions
are fulfilled in Fig. 266, which will answer remarkably well for any ordinary
lecture-table experiment: it consists of a magnetic needle, c, properly sus-
pended and placed inside a coil of wire, d, the two ends of which terminate
at a b. The whole is levelled by three screws.
The instrument, Fig. 267, is carefully levelled by three screws and spirit-
levels; it contains a coil of fine wire,-the two ends of which are brought out
to two screw connections. The magnetic needle is made astatic
19
290
ELECTRICITY.
just balanced), by being connected with another magnetic needle, the north
pole of which is placed opposite the south pole of the other, and vice versa,
and is thus unaffected by the earth's magnetism.
When a current of electricity, however feeble, is passed through the coil,
the astatic needle is deflected according to laws which will be fully explained
in the article on Electro- Magnetism.
FIG. 267. The Galvanometer Multiplier.
With this instrument the following experiments, all demonstrating that
chemical action is a source of electricity, can be performed :
Into a small clean iron ladle, well scraped inside with a file to secure a
metallic and not a rusted surface of iron, are placed some crystals of nitre ; the
ladle is supported by a tripod stand above a Bunsen's burner, and, when
melted by the heat, a wire is wound round the clean metallic surface of the
handle, and connected with one of the connecting screws of the galvanometer,
and the other with a second wire, bound round a piece of hard charcoal, such
as would be used for the electric lamp.
Of course all metallic connections must be bright and clean, and, directly
the charcoal is dipped into the nitre, the oxidation of the charcoal occurs ; the
nitre gives oxygen to the charcoal, and converts it into carbonic acid, which
unites with the remaining potash, producing carbonate of potash, and at the
same moment a current of electricity is liberated, which violently affects the
galvanometer needle.
The writer gives a drawing of the arrangement which will always be found
most simple and effective at the lecture-table. Moreover, it illustrates another
fact that one of the elements of a voltaic series must be in a liquid state, if
a notable current of dynamical electricity is desired to be shown or used. The
writer has always felt that when coal or charcoal could be oxidized, and used
VOLTAIC ELECTRICITY.
291
FIG. 268. The Oxidation of Carbon,
An instance of the evolution of electricity by true chemical action. A, iron ladle, containing the
nitre; B, the charcoal ; c, the Bunsen burner; r>, the galvanometer needle.
in the galvanic battery, the cheapest source of electricity will have oeen
attained ; and he learns from Mr. Crookes that a plate of platinum and one of
charcoal placed in fused soda or potash give a very good current.
The same experiment repeated, and a condensing electroscope used as the
test of electrical excitation, with the precaution of supporting the charcoal on
a glass rod, is very satisfactory ; and thus, by the oxidation and slow burning
of charcoal, both current or dynamical electricity and static electricity may be
obtained.
The usual mode of showing that charcoal in a state of combustion elimi-
nates electricity is by twisting a piece of copper wire round a bit of charcoal
some inches in length, and then connecting it with the lower plate of the con-
densing electroscope, whilst the upper plate is connected with the ground.
The charcoal is now ignited by a spirit-lamp, and if blown on with bellows,
and the top plate of the electroscope raised and lowered several times, any
rubbing of the two plates one against the other being carefully avoided, the
gold leaves will be seen to diverge with negative electricity ; and sometimes
one movement of the upper plate of the condensing electroscope is found to
be sufficient.
The experiments already quoted form a sort of connecting link between
frictional and voltaic electricity, and are further supported by some excellent
experiments of Faraday, who shows by a simple arrangement that the electri-
city of high tension obtained from the electrical machine will do all that a
voltaic circuit may effect. Faraday says :*
" Chemical Decomposition. The chemical action of voltaic electricity is
characteristic of that agent, but not more characteristic than are the laws
under which the bodies evolved by decomposition arrange themselves at the
*" Experimental Researches in Electricity " by Michael Faraday.
19 2
2 9 2
ELECTRICITY.
poles. Dr. Wollaston showed * that common electricity resembled it in these
effects, and that i they are both essentially the same ; ' but he mingled with
his proofs an experiment having a resemblance, and nothing more, to a case
of voltaic decomposition, which, however, he himself partly distinguished;
and this has been more frequently referred to by others, on the one hand, to
prove the occurrence of electro-chemical decomposition, like that of the pile,
and, on the other, to throw doubt upon the whole paper, than the more nume-
rous and decisive experiments which he has detailed.
" I take the liberty of describing briefly my results, and of thus adding my
testimony to that of Dr. Wollaston on the identity of voltaic and common
electricity as to chemical action, not only that I may facilitate the repetition
of the experiments, but also lead to some new consequences respecting electro-
chemical decomposition.
" I first repeated Wollaston's fourth experiment,t in which the ends of
coated silver wires are immersed in a drop of sulphate of copper. By passing
the electricity of the machine through such an arrangement, that end in the
drop which received the electricity became coated with metallic copper. One
hundred turns of the machine produced an evident effect ; two hundred turns
a very sensible one. The decomposing action was, however, very feeble. Very
little copper was precipitated, and no sensible trace of silver from the other
pole appeared in the solution.
FIG. 269.
"A much more convenient and effectual arrangement for chemical decompo-
sitions by common electricity is the following:
" Upon a glass plate (Fig. 269) placed over, but raised above, a piece of
white paper so that shadows may not interfere put two pieces of -tinfoil, , b ;
connect one of these by an insulated wire <r, or wire and string, with the
machine, and the other, g, with the discharging train, or the negative con-
ductor ; provide two pieces of fine platina wire, bent as in Fig. 270, so that
the part d f shall be nearly upright, whilst the whole is resting on the three
bearing points,^, e,f; place these as in Fig. 269 ; the points, /, ;/, then become
the decomposing poles. In this way surfaces of contact, as minute as possible,
can be obtained at pleasure, and the connection can be broken or renewed in
a moment, and the substances acted upon examined with the utmost facility.
'Philosophical Transactions," 1801, pp.427, 434.
t Ibid., 1801, p. 429.
VOLTAIC ELECTRICITY.
2 93
" A coarse line was made on the glass with solution of sulphate of copper,
and the terminations/ and n put into it; the foil, a, was connected with the
positive conductor of the machine by wire and wet string, so that no sparks
passed; twenty turns of the machine caused the precipitation of so much
copper on the end, /, that it looked like copper wire ; no apparent change
took place at ;/.
FIG. 270.
" A mixture of half muriatic acid and half water was rendered deep blue by
sulphate of indigo, and a large drop put on the glass (Fig. 269), so that/ and
n were immersed at opposite sides; a single turn of the machine showed
bleaching effects round /, from evolved chlorine. After twenty revolutions no
effect of the kind was visible at nj but so much chlorine had been set free at/,
that when the drop was stirred the whole became colourless.
" A drop of solution of iodide of potassium mingled with starch was put
into the same position at / and n; on turning the machine, iodine was evolved
at /, but not at n.
" A still further improvement in this form of apparatus consists in wetting
a piece of filtering paper in the solution to be experimented on, and placing
that under the points / and n, on the glass ; the paper retains the substance
evolved at the point of evolution, by its whiteness renders any change of
colour visible, and allows of the point of contact between it and the decom-
posing wires being contracted to the utmost degree. A piece of paper
moistened in the solution of iodide of potassium and starch, or of the iodide
alone, with certain precautions, is a most admirable test of electro-chemical
action, and, when thus placed and acted upon by the electric current, will '
show iodine evolved at / by only half a turn of the machine. With these
adjustments, and the use of iodide of potassium on paper, chemical action is
sometimes a more delicate test of electrical currents than the galvanometer.
Such cases occur when the bodies traversed by the current are bad conductors,
or when the quantity of electricity evolved or transmitted in a given time is
very small.
" A piece of litmus paper, moistened in solution of common salt or sulphate
of soda, was quickly reddened at /. A similar piece, moistened in muriatic
acid, was very soon bleached at/. No effects of a similar kind took place at n.
" A piece of turmeric paper, moistened in solution of sulphate of soda, was
reddened at n by two or three turns of the machine, and in twenty or thirty
turns plenty of alkali was there evolved. On turning the paper round, so that
the spot came under /, and then working the machine, the alkali soon dis-
appeared, the place became yellow, and a brown alkaline spot appeared in the
new part under n.
294 ELECTRICITY.
" On combining a piece of litmus with a piece of turmeric paper, wetting
both with solution of sulphate of soda, and putting the paper on the glass, so
that p was on the litmus and n on the turmeric, a very few turns of the
machine sufficed to show the evolution of acid* at the former, and alkali at the
latter, exactly in the manner effected by a volta-electric current.
" All these decompositions took place equally well, whether the electricity
passed from the machine to the foil, a, through water or through wire only,
by contact with the conductor or by sparks there, provided the sparks were
not so large as to cause the electricity to pass in sparks from^ to ;z, or towards
n; and I have seen no reason to believe that, in cases of true electro-chemical
decomposition by the machine, the electricity passed in sparks from the con-
ductor, or at any part of the current, is able to do more, because of its tension,
than that which is made to pass merely as a regular current.
" Finally, the experiment was extended into the following form, supplying
in this case the fullest analogy between common and voltaic electricity:
FIG. 271.
" Three compound pieces of litmus and turmeric paper were moistened in
solution of sulphate of soda, and arranged on a plate of glass with platina
wires, as in Fig. 271. The wire, ;;*, was connected with the prime conductor
of the machine, the wire, /, with the discharging train, and the wires, r and
.v, entered into the course of the electrical current by means of the pieces of
moistened paper ; they were so bent as to rest each on three points, ?/, r, p,
it, s, p) the points, r and s, being supported by the glass, and the others by the
papers ; the three terminations, p, p, p, rested on the litmus, and the other
three, , ?/, , on the turmeric paper. On working the machine for a short
time only, acid was evolved at all the poles or terminations, p, p, p, by which
the electricity entered the solution, an " alkali at the other poles, , , , by
which the electricity left the solution.
' In all experiments of electro-chemical decomposition by the common
machine and moistened papers, it is necessary to be aware of and to avoid
the following important source of error :
" If a spark passes over moistened litmus and turmeric paper, the litmus
paper (provided it be delicate, and not too alkaline) is reddened by it ; and if
several sparks are passed, it becomes powerfully reddened. If the electricity
pass a little way from the wire over the surface of the moistened paper, before
it finds mass and moisture enough to conduct it, then the reddening extends
as far as the ramifications. If similar ramifications occur at the termination
n, on the turmeric paper, they prevent the occurrence of the red spot due to
VOLTAIC ELECTRICITY. 295
the alkali, which would otherwise collect there ; sparks or ramifications from
the points, , will also redden litmus paper. If paper, moistened by a solution
of iodide of potassium (which is an admirably delicate test of electro-chemical
action), be exposed to the sparks or ramifications, or even a feeble stream of
electricity through the air from either the point p or 72, iodine will be immedi-
ately evolved.
" These effects must not be confounded with those due to the true electro-
chemical powers of common electricity, and must be carefully avoided when
the latter are to be observed. No sparks should be passed, therefore, in any
part of the current, nor any increase of intensity allowed by which the elec-
tricity may be induced to pass between the platina wires and the moistened
papers, otherwise than by conduction; for, if it burst through the air, the
effect referred to ensues.
" The effect itself is due to the formation of nitric acid by the combination
of the oxygen and nitrogen of the air, and is, in fact, only a delicate repetition
of Cavendish's beautiful experiment. The acid so formed, through small in
quantity, is in a high state. of concentration as to water, and produces the
consequent effects of reddening the litmus paper, or preventing the exhibition
of alkali on the turmeric paper, or, by acting on the iodide of potassium,
evolving iodine.
" By moistening a very small slip of litmus paper in solution of caustic
potassa, and then passing the electric spark over its length in the air, I gradu-
ally neutralized the alkali, and ultimately rendered the paper red ; on drying
it, I found that nitrate of potassa had resulted from the operation, and that
the paper had become touch-paper.
" Either litmus paper or white paper moistened in solution of iodide of
potassium offers, therefore, a very simple, beautiful, and ready means of illus-
trating Cavendish's experiment of the formation of nitric acid from the
atmosphere.
" I have already had occasion to refer to an experiment by Dr. Wollaston,
which is insisted upon too much, both by those who oppose and those who
agree with the accuracy of his views respecting the identity of voltaic and
ordinary electricity. By covering fine wires with glass or other insulating
substances, and then removingonly so much matter as to expose the point
or a section of the wires, and by passing electricity through two such wires,
the guarded points of which were immersed in water, Wollaston found that
the water could be decomposed even by the current from the machine, without
sparks, and that two streams of gas arose from the points, exactly resembling
in appearance those produced by voltaic electricity, and, like the latter, giving
a mixture of oxygen and hydrogen gases. But Dr. Wollaston himself points
out that the effect is different from that of the voltaic pile, inasmuch as both
oxygen and hydrogen are evolved from each pole ; he calls it * a very close
imitation of the galvanic phenomena,' but adds, that ' in fact the resemblance
is not complete,' and does not trust to it to establish the principles correctly
laid down in his paper.
" This experiment is neither more nor less than 'a repetition, in a refined
manner, of that made by Dr. Pearson, in, 1797,* and previously by MM.
Pacts van Troostwyk and Deiman in 1789, or earlier. That the experiment
should never be quoted as proving true electro-chemical decomposition is
"Nicholson's Journal," 4to, vol. i., pp. 241, 299, 349.
296 ELECTRICITY.
sufficiently evident from the circumstance, that the law which regulates the
transference and final place of the evolved bodies has no influence here. The
water is decomposed at both poles independently of each other, and the oxygen
and hydrogen evolved at the wires are the elements of the water existing the
instant before in those places. That the poles, or rather points, have no mutual
decomposing dependence may be shown by substituting a wire, or the finger,
for one of them, a change which does not at all interfere with the other, though
it stops all action at the changed pole. This fact may be observed by turning
the machine for some time ; for, though bubbles will rise from the point left
unaltered, in quantity sufficient to cover entirely the wire used for the other
communication, if they could be applied to it, yet not a single bubble will
appear on that wire.
" When electro-chemical decomposition takes place, there is great reason to
believe that the quantity of matter decomposed is not proportionate to the
intensity, but to the quantity of electricity passed. Of this I shall be able to
offer some proofs in a future part of this paper. But in the experiment under
consideration this is not the case. If, with a constant pair of points, the elec-
tricity be passed from the machine in sparks, a certain proportion of gas is
evolved ; but, if the sparks be rendered shorter, less gas is evolved ; and if no
sparks be passed, there is scarcely a sensible portion of gases set free. On
substituting solution of sulphate of soda for water, scarcely a sensible quantity
of gas could be procured even with powerful sparks, and almost none with the
mere current ; yet the quantity of electricity in a given time was the same in
all these cases.
" I do not intend to deny that with such an apparatus common electricity
can decompose water in a manner analogous to that of the voltaic pile; I be-
lieve at present that it can. But when what I consider the true effect only
was obtained, the quantity of gas given off was so small that I could not ascer-
tain whether it was, as it ought to be, oxygen at one wire and hydrogen at
the other. Of the two streams one seemed more copious than the other, and
on turning the apparatus round, still the same side in relation to the machine
gave the largest stream. On substituting solution of sulphate of soda for pure
water, these minute streams were still observed ; but the quantities were so
small that on working the machine for half an hour I could not obtain at either
pole a bubble of gas larger than a small grain of sand. If the conclusion
which I have drawn relating to the amount of chemical action be correct, this
ought to be the case.
" I have been the more anxious to assign the true value of this experiment
as a test of electro-chemical action, because I shall have occasion to refer to
it in cases of supposed chemical action by magneto-electric and other electric
currents and elsewhere. But, independent of it, there cannot be now a doubt
that Dr. Wollaston was right in his general conclusion, and that voltaic and
common electricity have powers of chemical decomposition alike in their
nature and governed by the same law of arrangement.
"Physiological Effects. The power of the common electric current to shock
and convulse the animal system, and when weak to affect the tongue and the
eyes, may be considered as the same with the similar power of voltaic elec-
tricity, account being taken of the intensity of the one electricity and duration
of the other. When a wet thread was interposed in the course of the current of
common electricity from the battery charged by eight or ten revolutions of
the machine in good action, and the discharge made by platina spatulas
VOLTAIC ELECTRICITY.
297
through the tongue or the gums, the effect upon the tongue and eyes was
exactly that of a feeble voltaic circuit.
" Spark. The beautiful flash of light attending the discharge of common
electricity is well known. It rivals in brilliancy, if it does not even very much
surpass, the light from the discharge of voltaic electricity; but it endures for
an instant only, and is attended by a sharp noise like that of a small explosion.
Still no difficulty can arise in recognizing it to be the same spark as that from
the voltaic battery, especially under certain circumstances. The eye cannot
distinguish the difference between a voltaic and a common electricity spark, if
they be taken between amalgamated surfaces of metal, at intervals only, and
through the same distance of air."
The simple voltaic circuit may be variously modified, but usually consists
of three elements, viz., two solids and one fluid, or one solid and two fluids.
The first is well represented by a plate of copper, a plate of zinc, and some
water acidulated with sulphuric acid ; the second by a single plate of zinc, one
half of which is immersed in salt and water, and the other in weak nitric acid.
FIG. 2J2.A simple Voltaic Circuit, consisting of two Metals and one Fluid.
<., zinc ; b, copper. The liquid represents the acid, and the arrows show the direction of the current.
In the above figure it is seen that the zinc fulfils the part of the glass in the
electrical machine; the acid, the rubber or excitant; the copper, the con-
FlG. 273. Magnetic Needle, suspended over two Plates 'which are immersed
in acid and water.
298
ELECTRICITY.
ductor. By using a galvanometer, it is found that a current of + electricity
flows from the zinc to the copper ; and when the wires attached to the plates
are brought in contact, this is called a closed circuit.
Every part of the circuit exercises an influence upon the magnetic needle.
If a needle is suspended over the two plates (Fig. 273) lying in any convenient
glass or porcelain dish, the needle, which should be arranged so that its direction
is at right angles to the immersed plates, is then deflected parallel with them.
If we take one cell of a Cruikshank battery, we find a plate of zinc soldered
to one of copper, these compound plates forming the sides of the cell, in which
dilute acid is poured.
FIG. 274.
2, c, zinc and copper, soldered together and cemented into a trough, A A. The current does not pass until
a bent wire, w, connects the two sides containing the dilute sulphuric acid.
In Volta's crown of cups a simple voltaic circuit, formed of slips of zinc and
copper, soldered together and placed alternately in separate glass vessels,
represents another arrangement for producing the same result.
A A A, the trough, divided by a water-tight partition ; z, the zinc plate ; c, the copper plate. The cur-
rent circulates in the direction of the arrows when the wire, w w w w, is bent over and dips into the
dilute acid.
A plate of zinc, inserted water-tight into a wooden trough, usually lined with
VOLTAIC ELECTRICITY.
: 99
a cement made of rosin and tallow, filled on one side with a solution of common
salt, and on the other with dilute nitric acid, gives a current if two wires are
inserted on either side, but not touching the metal.
FlG. 276. Zinc Plate, cemented into a Mahogany Trough.
The arrows show the direction of the current.
Or the arrangement may be varied by placing a long strip of clean copper in
a cylindrical glass (Fig. 277) ; into this is poured dilute nitric acid until half the
vessel is filled ; then, with a tube and funnel, a strong solution of sulphate of
copper is poured down to the bottom of the glass, which, gravitating by its
weight, raises the weak nitric acid above it, and thus the copper is immersed
in two solutions, the shaded one, A, being the solution of copper, and the one
above it, B, the dilute nitric acid.
A wire, G, covered with gutta percha, but exposing an inch or two of the
rim, is now let down quickly into the glass vessel, so that the gutta-percha
covered portion passes through the upper stratum, and the exposed wire only
is in contact with the solution of sulphate of copper.
An uncovered copper wire, H, is put into the dilute nitric acid ; and when
the ends of the two wires, G and H, are brought into metallic contact, a cur-
rent circulates in the direction of the arrows, and for every atom of copper
dissolved in B an equivalent proportion of the metal is deposited out of A on
to the lower part of the copper plate, c C.
Professor Matteucci, of Pisa, has shown that dissected legs of frogs, so
arranged that the half-thighs, skinned and laid alternately upon each other,
the inner half touching the outer, and vice versa, produce a current of elec-
tricity with which all the ordinary effects are produced, viz., deflection of
galvanometer, decomposition of iodide of potassium, and divergence of the
gold leaves of an electroscope. The electricity is that which belongs to the
animal, and, as proved by Matteucci, circulates from the interior to the exterior
of the muscle. He found that -f electricity always circulates from the inside
to the outside of the muscles of all animals, whether of birds, mammals,
fishes, or cold-blooded reptiles.* Thus it is shown that metals may be dis-
;ucci has suggested that the true muscular fibre, which is oxidized, represents the zinc ; the
sarcolemma of the animal body, the platinum ; whilst the exciting fluid is the blood.
Matteucci has
3 oo
ELECTRICITY.
FIG. 277.
pensed with, and the exclamation ascribed to Napoleon I., by Chaptal, whilst
looking at the action and power of his voltaic battery, derives additional force
on reviewing the last-named fact. The remark of the august man was this :
" Voila, docteur, 1'image de la vie : la colonne vertdbrale est le pile, la vessie
le pole positif, et le foie le pole ndgatif."
Sir H. Davy endeavoured to protect the copper sheathing of vessels by
attaching a metal which was more rapidly oxidized than the copper. His ex-
periments appeared at first to be thoroughly successful, a bit of zinc as large as
FlG. 278. Original Experiments of Sir H. Davy,
Given to the late Professor Griffith by Davy, and passing to the writer, being pieces of copper and their
protectors, arranged by Sir Humphrey Davy's own hands in his first experiments on the protection of
copper sheathing.
a small-bore bullet being sufficient to protect a surface of copper 40 or 50 inches,
square ; indeed, it may be said that Davy's experiments were too successful,
for directly the action of the chlorides in the sea-water was stopped, and the
VOLTAIC ELECTRICITY. 301
poisonous salt of copper no longer produced, the living things the barnacles,
the sea-weed, &c. attached themselves, like Sinbad to the floating island,
and, whilst making themselves uninvited passengers, they impeded the motion
of the vessel by fouling its coppered sides.
The drawing, Fig. 278, represents the actual slips of copper, with bits of zinc
tied round them with silk, used by Sir H. Davy in his first experiments.}
The principle of* the action of the simple voltaic circle zinc, copper, acid
is the use of dissimilar metals, one being acted upon more than the other.
The principle is so true, that a current may be obtained from two plates
of zinc, provided they differ in their mechanical or chemical state. By
melting zinc repeatedly and pouring it into cold water, a metal is obtained
which is remarkably pure, and upon which dilute sulphuric acid acts very
feebly.
If now a plate of this pure metal is made the opposite one to another of
ordinary zinc, and the two connected with wires, a current is obtained, which
distinctly deflects the galvanometer needle.
The writer, whilst making a great number of experiments on the probable
effect of the water contained in the various docks on the copper sheathing of
vessels, found that the plates of the sheathing lost weight, whilst the nail
increased in weight in the exact proportion lost by the sheathing as if the
soft copper sheathing became the zinc element, and the harder pure copper
nails the copper element.
The experiments were twenty in number, a bit of sheathing and a nail in-
serted through a hole in the metal being suspended by a silk thread in twenty
different samples of London water. With hardly one exception, the sheathing
lost weight, which the nail gained.
The writer's experiment with fused nitre and charcoal (p. 291) demonstrates
that water can be entirely dispensed with, thus proving, as in Matteucci's ex-
periments, that metals are not absolutely necessary. Fluidity of some kind
is, however, indispensable to the production of current or dynamical electricity;
and this fact conducts us to an assemblage of simple voltaic circles, or what
is termed a voltaic pile or battery.
In 1819 a writer on Voltaism says:
"In the galvanic battery there appear to be two sources from which the
electricity is obtained. The one is that which arises
from the contact of the metals, and the other from the
chemical action between the interposing fluid and the
zinc surface. The first does not require even the pre-
sence of moisture, as is shown in the electric column of
De Luc. The second is rendered greatly conspicuous
by introducing between the opposite surfaces any sub-
stance capable of oxidating and dissolving the zinc."
It is well to mention here that Faraday has shown,
by one of his simple and original experiments, that an
electric current can be set up independent of all con-
tact.
It consists of a piece of zinc, b, bent as in the figure,
and a plate of platinum, a, to which is soldered a pla-
tinum wire. A little piece of bibulous paper is moistened
with a solution of iodide of potassium and starch, and
laid upon b. When the two metals are placed in
FIG. 279.
3C2 ELECTRICITY.
the glass vessel containing diluted sulphuric acid, and the end of the wire
pressed upon it, iodine is liberated, which, uniting with the starch, produces
a purple compound ; and thus proves satisfactorily that a current of electricity
has passed through the salt, and that true electro-chemical decomposition has
taken place.
"Acids are the great promoters of the energy afforded by chemical action,
because they dissolve the zinc after it has been oxidated by the oxygen of the
water.
" This is more especially the case with the sulphuric and muriatic acids,
because these acids are not decomposed by the zinc.
" The nitric acid produces a still greater galvanic effect, because the acid is
decomposed and oxidates the zinc with greater facility than water.
" The water is also decomposed when this acid is used, and hydrogen is
always evolved."
These views of the rationale of the action of the acids in the voltaic bat-
tery are substantially correct, although written forty-eight years ago.
The same writer * anticipates the porous material required in Daniell's and
Grove's batteries ; and, indeed, the more frequently we consult old works, the
more difficult do we find it to disprove the words of Solomon, " There is
nothing new under the sun." The- writer remarks :
" When the fluids are required to be strictly separate, a bladder answers
very well as a separating medium. Animal and vegetable substances, how-
ever, abound with so many elements that in nice experiments they would be
objectionable. A vessel, divided into a proper number of cells, of earthen-
ware, in the state of biscuit, would be best calculated for these experiments.
" This vessel should be made of pure silex and pure alumina.
" Should it ever become an object of manufacture to separate acids and
alkalies from neutral salts, a vessel of wood, with a separation in the middle
of imglazed earthenware, would answer very well."
Here we have porous cells anticipated distinctly.
The important discovery of accumulating the effects of single voltaic circles
was made by Volta in 1 800, and the first apparatus constructed with that view
was called the voltaic pile.
The apparatus as first made by Volta (Fig. 280) consisted of a certain number
of pairs of zinc and silver plates, separated from each other by pieces of wet
cloth. Hence the arrangement was as follows: zinc, silver, and wet cloth;
zinc, silver, wet cloth, and so on. The silver plates were chiefly silver coins,
the plates of zinc and the pieces of cloth being of the same size. He found
this pile much more powerful when the pieces of cloth were moistened with a
solution of common salt instead of pure water. A pile consisting of forty pairs
of plates he found to possess the power of giving a very smart shock similar
to that of an electric jar, and that this effect took place as often as a commu-
nication was made between each end of the pile, and as long as the pieces
of cloth remained moist. An account of this discovery was communicated to
the Royal Society, and published in the " Philosophical Transactions."
We do not hear of this celebrated philosopher making any further discovery
after the invention of the pile and ascertaining the nature and extent of its
effect upon animals.
Mr. Cruikshank improved upon Volta's apparatus by cementing the plates
* " Rees's Cyclopaedia," article Voltaism.
VOLTAIC ELECTRICITY.
33
FIG. 280. The Voltaic Pile.
of zinc and copper into a wooden box, which was then called the galvanic
trough. In fact, the trough was Volta's pile placed horizontally, the cells
being for the reception of the fluid to answer the purpose of pieces of wet
cloth.
FlG. 281. Babingtoris improved Voltds " Couronne ds Tasses"
The plates lift in and out of the acid.
The learned Dr. Wollaston improved upon Cruikshank's arrangement by
increasing the area of the conducting element, viz., the copper, by doubling
this over the zinc ; and, in fact, surrounding the latter with copper, he increased
the power immensely. (Fig. 283.)
All his arrangements were so peculiarly neat and compact. An apparatus
is sold at Elliott's (Fig. 282 A), called Wollaston's calorimeter, consisting of one
pair of 4-in. zinc and double copper relates, movable in and out of a mahogany
ELECTRICITY.
trough. By this simple arrangement the calorific effect of an electric current
is shown by the ignition of fine platinum wire, stretched between the ter-
minals of the two metallic elements, zinc and copper.
FIG. 282.
A, Wollaston's calorimeter; B, Grove's constant Wollaston wire-gauze calorimeter. The gauze facili-
tates the escape of hydrogen, and this form is more constant.
We now come to the first important change in the adjustment of the ele-
ments and the choice of fluids, which originated with the late Professor
Daniell. In this work, the writer prefers that each author and inventor quoted
here should speak for himself: the enthusiasm of an inventor supplies expres-
sive language, which may be paraphrased, but can rarely be improved.*
FIG. 283.
A, the single cell, Daniell's; B, a Daniell's battery.
" The liquid employed in the voltaic batteries, when it has been desired to
excite them to the utmost, has generally been a mixture of sulphuric and nitric
acids diluted with water, in which case much local action takes place from the
zinc plates, which contributes nothing to the force which circulates, and which
* Daniell's "Introduction to Chemical Philosophy."
VOLTAIC ELECTRICITY.
305
rapidly destroys them. Their power, moreover, speedily declines by the zinc
which forms upon the copper plates ; and they are very inconstant in their
action. These defects are obviated^in the construction of the constant bat-
tery, the contrivance of the author, which consists of a series of single cir-
cuits, constructed upon the principle of a central disposition of the active
metal with regard to the conducting surface, as formerly explained. A cell of
this battery consists of a cylinder of copper, 3^ in. in diameter, which expe-
rience has proved to afford the most advantageous distance between the gene-
rating and conducting surfaces, but which may vary in height according to the
power which it is wished to obtain. A membraneous tube, formed of the gullet
of an ox, is hung in the centre by a collar and circular copper plate, resting
on the rim placed near the top of the cylinder ; and in this is suspended, by a
-. 1
; \
IP
I *
o o[
1
O !
i o
; O O
O !
''
1 Q o
O
o i
jo o
9
1|1 I '
(A/-
FIG. 284.
wooden cross-bar, a cylindrical rod of amalgamated zinc, half an inch in dia-
meter. The cell is charged with a .mixture of 8 parts of water to I part of
oil of vitriol, which has been saturated with sulphate of copper, and portions
of the solid salt are placed upon the upper copper plate, which is perforated
like a colander, for the purpose of keeping the solution always in a state of
saturation. The internal tube is filled with the same acid mixture, without the
copper. A tube of porous earthenware may be substituted for the membrane,
with great convenience, but probably with some loss of power. A number of
such cells admit of being connected together very readily into a compound
circuit, and will maintain a perfectly equal and steady current for many hours
together, with a power far beyond that which can be produced by any other
arrangement of a similar quantity of the same metals. The surface of the
conducting metal is thus perpetually renewed by the deposition of pure copper,
and the counteraction of zinc or any other precipitated metal effectually pre-
vented. The minor affinity of the copper for the acid, however, still remains ;
and snch an opposition could only be effectually avoided by the employment
20
ELECTRICITY.
of platinum plates, perpetually renewed by the decomposition in the circuit
of chloride of platinum. Such an arrangement would be perfect, but too costly
for ordinary applications.
" One of the cells of the constant battery is represented (Fig. 284). a b c d is a
copper cylinder, in which is placed a smaller cylinder of porous earthenware.
Upon the upper part of the copper cylinder rests a perforated colander, h /,
through which the earthenware cylinder passes ; / m is a cast rod of amalga-
mated zinc, resting upon the top of the interior cylinder by a cross-piece of
wood, and forming the axis of the arrangement. The cell is charged by
pouring into the earthenware cylinder water acidulated with one-eighth part
of its bulk of oil of vitriol, the space between the earthenware tube and the
copper being filled with the same acidulated water, saturated with sulphate of
copper, and solid sulphate being placed in the colander. A number of such
cells may be connected into a compound circuit by wires attached to the copper
cylinders, and fastened to the zinc by clamps and screws, as shown below.
"A more powerful combination upon the same principle, though not so
constant in itjs working or conveniently applicable to such extensive operations
as that of the constant battery, has been contrived by Professor Groves (Fig.
285), who makes use of conducting plates of platinum-foil, immersed in strong
FIG. 285. A Grovels Battery.
nitric acid, separated from the dilute sulphuric acid, in which the zinc is
plunged, by a diaphragm of porous earthenware. The conducting power of
the liquid portion of the combination is of the most perfect kind, and, the
hydrogen which travels in the circuit is immediately absorbed by the acid, upon
the conducting plate, and, reacting upon it, decomposes it with the evolution
of copious fumes of nitrous gas. It has been already seen that a single cell of
this construction is capable of overcoming the exterior resistance of a volta-
meter ; and a very efficient series may thus be made with the bowls of tobacco-
pipes and corresponding pieces of platinum-foil."
Mr. Warren De la Rue and Hugo Miiller have invented another entirely
new form of constant battery, which the authors recommend strongly * to the
chemist and physicist. " As a ready source of dynamic electricity always at
hand, and that especially when from a few himdreds to several thousand ele-
ments are requisite, it will be found to be valuable, handy, and compact. In
* " Journal of Chemical Society," November, 1868.
VOLTAIC BATTERIES. 307
its construction no porous cell is needed, and the electrolyte is solid and very
nearly insoluble, so that practically the electro-positive metal is scarcely at-
tacked, even when the elements are left immersed with the electrodes discon-
nected for several weeks. In our battery the generating or electro-positive
metal is zinc, which it is better to amalgamate, although it is not essential to
do so ; the negative metal is silver, and the electrolyte solid chloride of silver,
the whole being immersed in a solution of chloride of sodium or chloride of
zinc. The solution we generally use contains 25 grammes of common salt to
a like quantity of distilled water (219 grains to a pint). It is not desirable to
use common water for dissolving the chloride of sodium, as the carbonates
present cause a cloudiness by precipitating the zinc as carbonate when the
battery is in action. The form of the battery which we have adopted is repre-
sented in Figs. 286 and 287 ; but where a very large number of elements is
wanted, it is more economical and convenient to employ a modification, pre-
sently to be described. The zinc element is formed of Belgian zinc wire
(English zinc being too impure to be used advantageously), 2f in. (6 centi-
metres) long and o'2 in. (5-1 mm.) diameter. The electro-negative element
consists of a wire of pure silver 0*03 in. (077 mm.) in diameter, and round
this is cast* a cylinder of chloride of silver, 0*22 in. (5 '6 mm.) in diameter.
FIG. 286.
The silver wire projects about o - 2 in. (5 mm.) beyond the bottom end of the
chloride of silver, and about i^ in. (3'8 centimetres) beyond the top end of it,
so as to permit of its connection with the zinc of the next pair of elements.
The cells are conveniently formed out of i oz. vials, by cutting off the necks by
a diamond or an ignited splint-coal.
" The zinc and chloride of silver rods pass through, and are supported by, a
lath or bar of varnished mahogany, A A, which is pierced for that purpose,
The ends of this bar are also pierced with two larger holes, through which
two supporting glass rods, B B, pass ; it slides up and down these rods freely,
and is retained in any required position by means of the vulcanized caoutchouc
* " In making these cylinders a mould which was designed for casting rods of lunar caustic (nitrate of
silver) was found to be convenient. The mould contained a series of recesses which permitted of several
rods being cast at a time. The silver wire was held firmly in the centre of the cylindrical recess by
passing through a hole in the bottom of the mould, and by a series of arms projecting o\er the mouth
of each recess at a sufficient distance to permit of the fused chloride being poured mto them."
20 2
3 o8
ELECTRICITY.
collars, C C, on which it rests ; these grip the rods, B B, with adequate firmness
to support the bar, but at the same time permit of its being moved up and
down with sufficient freedom to immerse the element partially or wholly, as in
Fig. 286, or to raise them entirely out of the liquid, as in Fig. 287. The raising
is conveniently performed by placing the two forefingers of each hand under
the collars, C C, and pressing the thumbs on the top of the glass rods, B B.
The lowering of the bar can be effected by pressing down the two ends. These
glass rods should not be varnished on that portion over which the vulcanized
collars have to slide, as the varnish causes too much friction and a liability
of jerking ; below this point they may be varnished with advantage. They
FlG. 287.
are cemented into the base of varnished mahogany, D D, in which is made a
series of recesses to fit the cells, E, and keep them in their places. This base
rests on feet of vulcanite to increase the insulation. The rods of zinc and
chloride of silver are prevented from falling through the holes in bar A A by
means of heads formed in the zinc by hammering the wire while it is held in
a properly shaped tool, and on the chloride of silver by suitably shaping the
upper end of the mould into which it is cast. A collar of caoutchouc is placed
on the lower end of the zinc element to prevent contact between it and the
rod of chloride of silver. Another plan of support is, however, more advan-
tageous when a very numerous series of elements is used, as shown in Fig,
288, for it permits both of economizing the chloride of silver and of readily
renewing it from time to time. Pieces of gutta percha or ebonite, I I, are
well fitted into the bar A ; they are pierced with a hole just large enough to
permit of the silver wire, M, being drawn through them. The zincs are held
in position by means of the vulcanized collars, N, while a second collar, O,
serves as a clip for making connection with the silver wire, M, which is done
by passing the wire between the zinc and the collar, O.
" It should be observed that, as the chloride of silver becomes reduced, the
resulting spongy silver is of greater diameter and less regular in form than the
original rods of chloride. It is evident, therefore, that the reduced silver can-
not be withdrawn through the holes in the bar A A with the arrangement shown
in Figs. 286 and 287 ; moreover, that portion of the chloride which remains
out of the liquid in the arrangement (Figs. 286 and 287) is not reduced; and
although no silver is ultimately lost, yet a portion of the useful effect of its
chloride is sacrificed ; and, consequently, the arrangement of Fig. 288 is both
VOLTAIC BATTERIES.
39
more economical and convenient. When the chlorine is more or less com-
pletely exhausted by the reduction of the cylinders through their entire thick-
ness, the resulting rods of spongy silver should be placed in a vessel of water
acidulated with hydrochloric acid and some rods of zinc, in order to reduce
FIG. 288.
any undecomposed chloride, especially at their upper ends. After removal of
the zinc, the spongy silver must be treated with dilute hydrochloric acid, and
well washed to remove all traces of zinc. Very little, if any, loss of silver
pccurs, and the cost of renewal of the electrolyte is chiefly one of labour.
" There are many other forms of batteries. Professor Hare, of Philadelphia,
devised an enormous calorimeter. Mr. Alfred Smee's battery, A, Fig. 289, is
a most useful and popular form ; it consists of a plate of silver, covered with
black powder of platinum, and surrounded with amalgamated zinc. It is in
form a reversed Wollaston battery. The conductor, the platinized silver, is
placed inside, and the amalgamated zinc outside."
Sturgeon's battery, B, Fig. 289, is a cylindrical modification of Wollaston's
battery. It consists of two copper cylinders brazed on to a foot, so as to form
a hollow cylindrical vessel. Into this is placed a cylinder of zinc, which is
made movable, so that the surface can be scraped and cleaned, or the whole
cylinder renewed.
If amalgamated zinc is used, the mercury must be used sparingly, or else
tke excess will fall to the bottom of the copper cylindrical' vessel, and, amal-
gamating the copper, will soon pass through the metal, rendering it so
brittle that any hard substance, even the finger, may be thrust through it ;
indeed, very pure zinc cylinders should be used in preference to amalgamated
zinc in this particular instance.
3 io
ELECTRICITY.
FIG. 289.
A, Smee's single cell; B, Sturgeon's cylindrical battery , c, Mullins's sustaining battery; D, Sturgeon's cast
iron and amalgamated zinc battery.
C, Fig. 289, is a single cell of Mullins's sustaining battery. It consists of
a narrow cylindrical slip of sheet zinc, surrounding a copper vessel closed at
the bottom with wood, and a shelf provided at the top to carry crystals of the
blue sulphate of copper. The copper vessel is enveloped with a membrane,
and the whole arrangement placed in a stoneware jar. Sulphate of copper is
used inside the membrane, and chloride of ammonium or dilute sulphuric
acid outside the membrane. The title of sustaining battery is well maintained.
The writer has seen them in use, and rough use too, and found that they gave
a distinct current for months ; always, of course, taking care that water is
added, so that the salts do not dry up.
D, Fig. 289, Sturgeon's battery of cast iron and amalgamated rolled zinc.
It consists of two cylinders, one of cast iron and the other of amalgamated
zinc ; they are placed one within the other, in dilute sulphuric acid, contained
in a stoneware jar. This arrangement is well adapted for quantity effects ; but
its intensity is, of course, very low. This form is remarkably economical, and
when made on the large scale is powerful.
All these batteries can be obtained from Messrs. Elliot, of Charing Cross,
and the youthful student in looking at so many forms is apt to be puzzled with
regard to selection, and naturally asks, Which is the best ? The answer should
be, What do you want to use the battery for? If you wish to make electro-
types, and to throw down silver or gold upon other surfaces by the voltaic
current, you cannot select a cheaper, more convenient, and constant battery
than that of Smee or, better still, Daniell. If the battery is required for the
more brilliant effects, such as heating platinum wire, deflagrating the metals,
and producing the electric light, there is no battery yet constructed which
surpasses Professor Groves's for certainty and steadiness of results.
If you require a battery to work a small telegraphic system, or to move
electro-magnetic machines, use a few cells of Smee, arranged on a bar of
wood, and dropping into a trough made of stoneware and divided into cells,
all of which contain dilute sulphuric acid (by making a stand and two up-
rights with pulleys, the metals can be drawn out by catgut cords and counter-
poise weights at the sides when not in use, and immediately placed in position
when required to perform the above-named work) ; or, still better, the im-
proved bichromate battery, which is one of the best forms that can be em-
VOLTAIC ELECTRICITY.
FIG. 290.
ployed: its advantages are freedom from smell, economy, and it is always
ready for immediate use. Apps's patent battery is the best of all forms where
the bichromate solution is used : the proportions are a saturated solution of
bichromate of potash in water, with one-seventh part of sulphuric acid. A
two-cell battery on this principle is shown in Fig. 290, and consists of two
plates of zinc, surrounded by three plates of carbon, so arranged that the plates
can be placed in, or out of, the bichromate solution.
DYNAMICAL ELECTRICAL PHENOMENA OBTAINED FROM THE
VOLTAIC BATTERY.
The effects obtainable from the current of electricity flowing or in motion
from pole to pole, and through the whole system of a voltaic battery, can be
summed up under four heads :
1. Chemical phenomena chemical action.
2. Calorific and lighting effects heat and light.
3. Magnetic phenomena magnetism.
4. Dynamical effects mechanical motion.
As the action of a voltaic battery depends on " chemical action," it will be
most interesting to commence the inquiry by speaking first of the chemical
phenomena which may be rendered evident during the passage of a current
of electricity through any given substance, the conditions of success being
312 ELECTRICITY.
first understood to be the fluidity of the matter under decomposition and its
power of conducting the electric current.
"The first experiments," says a clear-headed writer in 1819, "made upon
the pile in this country appear to have been made by Messrs. Nicholson and
Carlisle. After observing the effects then already ascribed to the piles, on
bringing the wires from each end of the column in contact with a drop of
water, they observed a disengagement of bubbles of some elastic fluid; on
close examination, they took the gas to be hydrogen. They then took a glass
tube, about half an inch in diameter, into each end of which a cork was in-
serted, the tube being filled with water. Through each cork was introduced
a brass wire, so that the ends of the wires in the glass were about if of an
inch.
"The pile employed consisted of thirty-six half-crowns, and as many pieces
of zinc and wet pasteboard. The zinc end of the pile was then connected
with one of the wires in the tube, and the silver end to the other, so that the
circuit formed by the wires was separated by the water in the tube placed be-
tween them. A stream of bubbles was observed at the end of the wire, in the
tube connected with the silver end of the pile. No gas was disengaged from
the opposite wire, but it speedily became tarnished, first of an orange colour and
ultimately black. The tube was reversed, when it was observed that the wire
which in the first experiment became black gave out bubbles, while that which
previously gave out bubbles, in its turn, became tarnished. The emission of
gas from the wire connected with the silver end of the pile was constant and
uniform, except when a metallic current was formed between the ends of the
pile, during which no gas whatever appeared. It was observed that when this
metallic conductor was removed the appearance of the gas was not immediate,
since there was an interval of two seconds between removing the wire and the
appearance of the bubbles. After the process had continued two and a half
hours, a bulk of gas was produced equal to two-thirds of a cubic inch. This
gas was mixed with an equal bulk of common air, and exploded on the appli-
cation of a lighted taper.
" These ingenious experimenters, supposing the phenomena to arise from
the decomposition of the water, thought it surprising that the hydrogen should
make its appearance at a distance of if in. from the point where the oxygen
was disposed of. They made the experiment with a longer tube, but no appear-
ance of gas was observed at the distance of 36 in. When they introduced an
infusion of litmus, instead of pure water, they observed that the fluid in the
vicinity of the wire connected with the zinc end of the pile became red,
and hence were led to suppose that an acid was produced. The fluid at the
other wire was not changed; but gas, as usual, was evolved. Mr. Nicholson
ascertained that the zinc end of the pile was in the plus state of electricity,
and the opposite end was in the minus state.
" They next varied the experiment by inserting in the tube of water wires
of platina, instead of brass. Under these circumstances both the wires gave
out gas, but neither of them was tarnished. There appeared to be a larger
volume of gas from the silver end than from the zinc. The apparatus was so
arranged that the gases were separately collected. On examination, the gas
from the silver end was found to be hydrogen, as before, and that from the
zinc end oxygen. Their proportions were found to agree with the component
parts of water.
" The galvanic energy evinced in the decomposition of bodies was further
VOLTAIC ELECTRICITY. 313
prosecuted by Mr. Cruikshank, of Woolwich ; he employed in his experiments
a pile consisting of from 40 to 100 pairs of plates of silver and zinc, about i^
in. square. He also provided a glass tube, into each end of which was inserted
a cork, one of which was closely cemented, so as to be air-tight. Through
each of the corks a silver wire was passed, the ends in the tube being a certain
distance from each other. The tube, being filled with water, was placed per-
pendicularly in a cup containing water, with the uncemented cork downwards.
On the ends of the wires being connected with the ends of the pile, bubbles
began to appear at the* wire connected with the silver end of the pile ; at the
end of the other wire bubbles also appeared, and at the same time a white
cloud, which became of a darker colour, and ultimately purple and black.
The gas was collected, and found to consist of oxygen and hydrogen in the
proportion of one to three. The wire from the zinc end of the pile was much
corroded and even dissolved, which accounted for the deficiency of oxygen in
the gaseous form. Mr. Cruikshank very truly conjectured that the cloud that
became black was muriate of silver, the muriatic acid having been derived
from some muriatic salt in the water employed.
" With a view to ascertain how far his conjecture was right, he filled the
tube with distilled water, containing an infusion of litmus. The appearance
with regard to the evolution of gas was similar to the last experiment ; but the
fluid in the vicinity of the wire coming from the zinc end of the pile became
of a red colour, while the fluid about the other gradually lost its purple tinge
and became of a deeper blue. In short, an acid appeared to be produced
about the former wire, and an alkali about the lattet. An infusion of Brazil
wood underwent similar changes to those observed by an acid and an alkali.
" In all these experiments a quantity of silver was oxidated, and where water
was employed a portion was always dissolved, some of which was precipitated
at the wire from the silver end of the pile by the alkali which was produced.
This ingenious experimenter, knowing that hydrogen in its nascent state
was capable of reducing most metallic oxides, filled the tube with a solu-
tion of acetate of lead, and found that the hydrogen all disappeared, being
employed in the reduction of the metal ; by this means he also obtained pure
oxygen gas. The same was observed when sulphate of copper and nitrate of
silver were employed. When a solution of muriate of ammonia was employed
in the tube, the silver became oxidated, the oxide combined with the muriatic
*acid of the salt, and the liquor afterwards smelt strongly of ammonia. In a
similar way the muriate of soda and nitrate of magnesia were decomposed.
Mr. Cruikshank repeated the above experiments ; but, instead of silver wires, he
inserted into the tubes gold wires. The proportion of oxygen gas was now
much greater than with the silver wires, the gold not being susceptible of oxi-
dation in the process.
"His next attempt was to collect the gas separately; this he effected by
a tube about 10 in. long, which was bent into the form of the letter V; the
wires were passed through corks firmly cemented into the ends of the tubes,
coming near to the angular point. A small hole was made in the angular point
of the tube, by which it was filled with water. The tube was then inverted
in a cup of water, and the connection made with the other ends of the wires
and the pile. By this contrivance the hydrogen gas ascended into one leg of
the tube, and the oxygen into the other. He next filled the tube next employed,
instead of water, with muriate of lime : the rapidity of the process was much
increased ; the gold wire on the zinc side became partly dissolved, and the fluid
3 i4 ELECTRICITY.
in its vicinity assumed a yellow colour. When the tube was opened, a strong
smell of aqua regia was perceived. Similar phenomena were observed when
muriate of soda was employed. Many very anomalous facts were known in
chemistry long previous to the discovery of galvanism. All those chemical
phenomena under which the appearance called arborescence was observed
were inexplicable till it was shown by some experiments, published in 'Nichol-
son's Journal ' (vol. xv., p. 94), that galvanism is the cause of these singular
phenomena. In the experiment where lead is so beautifully precipitated by
suspending a piece of zinc in a solution of acetate of lead, the zinc first receives
a small portion of lead, which with the lead forms a galvanic combination.
The lead, if no solution of lead were present, would now give out hydrogen
gas ; but the hydrogen, instead of appearing in that form, combines with the
oxygen of the oxide, and the metallic lead is formed at the same point. Hence
the lead appears to grow from the last point formed, which gives the appear-
ance of vegetation. That this effect does not depend upon the presence of
zinc may be proved by the following experiment :
" Tie on one end of a glass tube, about half an inch wide, a piece of bladder,
so that it may hold water, and fill it with a solution of acetate of lead ; into
the other end insert a cork loosely, and through the cork let a platina wire
pass within about half an inch of the bladder. Into a wine-glass put some
diluted muriatic acid, in which place a zinc wire. When the tube with the
bladder is immersed in the wine-glass, if that part of the zinc wire that is
without the glass be brought in contact with that part of the platina wire with-
out the tube, beautiful crystals of metallic lead will appear upon the platina
wire. If the acetate of lead be removed, and a dilute acid put in its place,
bubbles of hydrogen will appear on the platina wire.
" Another experiment, similar to that of the lead tree, and equally anomalous,
has been long known in chemistry. If a plate of glass be smeared over with
a solution of nitrate of silver, and a brass pin or a piece of zinc wire be laid
in the middle of the plate, beautiful ramifications of silver will soon appear
as if growing out of the pin. very much resembling vegetation. By observing
the process by a magnifying-glass, each branch of this arborescence may be
seen to grow from the side or end of another, which proves that the silver
forming the vegetative appearance is not reduced by the oxidable metal laid
on the plate, but by something at the successive points of the silver branches.
With a view to ascertain this fact, one half of the plate of glass should be
smeared with nitrate of silver and the other half with dilute muriatic acid.
If a piece of zinc wire be tied to a piece of platina wire, and the compound
wire be so bent that the zinc may touch the dilute acid and the platina the
nitrate of silver, the ramifications of silver will soon appear on the platina
wire. That the silver is reduced by the hydrogen carried in the galvanic
current is probable from varying the experiment as follows :
"If, instead of smearing the plate with nitrate of silver, the whole be
covered with dilute acid, and the same compound arc be laid upon it, the platina
will give out bubbles of hydrogen. In the common way of making this expe-
riment with the pin, as well as the variation above stated, it appears that the
process is kept up by the galvanic current which furnishes the hydrogen. The
pin first reduces a small portion of silver, which forms a galvanic combination
with the pin. The hydrogen, which, but for the presence of the remaining
nitrate of silver, would appear in the gaseous form, is employed to deprive the
silver of its oxygen. With the compound arc the zinc does not require to
FARADAY'S RESEARCHES.
touch the nitrate of silver, because the platina with the zinc is already a gal-
vanic combination. The theory of whitening common pins can be explained
only on this principle. The tin in a small proportion is dissolved in the tar-
trate of potash ; pieces of metallic tin with the pins are also present. The
two latter form the galvanic combination, and a portion of tin is reduced from
the solution upon the pins, to which they owe their whiteness. We may gene-
rally conclude that, in all cases where one metal becomes the precipitant of
another, the precipitation is much facilitated by the agency of the galvanic
combination formed between the precipitating and the precipitated metals,
and the consequent presence of hydrogen.
" If a piece of zinc be introduced into a solution of sulphate of copper, the
zinc in the first instance becomes covered with copper, and the effect appears
to stop. If, however, a very small excess of sulphuric acid be added, the
process will go on with such rapidity that the copper becomes precipitated in a
very short time. By minutely observing the process, the copper will be seen to
be reduced upon that already produced, which is a proof that it is not done by
the mere agency of zinc. It appears very evident that, when a galvanic com-
' bination of zinc with any lesser oxidable metal is placed in a dilute acid,
a much larger quantity of hydrogen will be evolved from the lesser oxidable
wire than could possibly be produced by any electrical intensity generated by
the contact of the bodies employed ; but that, independently of this, there is an
immense quantity of electricity generated during the chemical action, by
which the hydrogen is transported from the greater oxidable surface to the
lesser one. If the quantity of hydrogen produced depended upon the attrac-
tions of the wires for the elements of the water, this power would depend upon
the electrical intensity alone, and, of course, upon the series of the galvanic
battery, whatever might be its surface; but it is found that the power of
galvanism to decompose water is much increased by an increase of surface
only."
Any clever experimentalist, reading this account carefully, would at once
perceive that these experiments were capable of great extension, and, thus
stimulated, his mind might pass, like that of Daniell and, later, of Warren De
la Rue, to indicate the discovery of the electrotype, which Jacobi in Russia
and Spenser in England brought before the scientific world, under the names
of " Galvano-plastic " and " Electrography."
But it was left for the genius of Faraday to put " electro-chemical decom-
position " in a clear light, and, in fact, to devise new instruments and a new
nomenclature, which are set forth in the seventh series of his " Experimental
Researches in Electricity:" "On Electro-chemical Decomposition;" "On
a new Measurer of Voltaic-electricity ;" " On the Absolute Quantity of Elec-
tricity associated with the Particles or Atoms of Matter."
The simplicity of Faraday's diction, and the clearness with which he de-
scribes the phenomena observed, are most remarkable, and supply a ''standard
of excellence " which scientific writers may well try to imitate. The following
are some of
"FARADAY'S RESEARCHES."
The theory which I believe to be a true expression of the facts of electro-
chemical decomposition, and which I have therefore detailed in a former
series of these Researches, is so much at variance with those previously
advanced, that I find the greatest difficulty in stating results, as I think,
316 ELECTRICITY.
correctly, whilst limited to the use of terms which are current with a certain
accepted meaning. Of this kind is the term pole, with its prefixes of positive
and negative, and the attached ideas of attraction and repulsion. The general
phraseology is, that the positive pole attracts oxygen, acids, &c., or, more
cautiously, that it determines their evolution upon the surface ; and that the
negative pole acts in an equal manner upon hydrogen, conbustibles, metals,
and bases. According to my view, the determining force is not at the poles,
but within the decomposing body ; and the oxygen and acids are rendered
at the negative extremity of that body, whilst hydrogen, metals, &c., are
evolved at the positive extremity.
To avoid, therefore, confusion and circumlocution, and for the sake of
greater precision of expression than I can otherwise obtain, I have delibe-
rately considered the subject with two friends, and, with their assistance and
concurrence in framing them, I purpose henceforward using certain other
terms, which I will now define. The poles, as they are usually called, are
only the doors or ways by which the electric current passes into and out of
the decomposing body ; and they, of course, when in contact with that body,
are the limits of its extent in the direction of the current. The term has been
generally applied to the metal surfaces in contact with the decomposing
substance; but whether philosophers generally would also apply it to the
surfaces of air and water, against which I have effected electro-chemical
decomposition, is subject to doubt. In place of the term pole, I propose
using electrode*, and I mean thereby that substance, or rather surface,
whether of air, water, metal, or any other body, which bounds the extent of
the decomposing matter in the direction of the electric current.
The surfaces at which, according to common phraseology, the electric
current enters and leaves a decomposing body, are most important places of
action, and require to be distinguished apart from the poles, with which they
are mostly, and the electrodes, with which they are always, in contact. Wish-
ing for a natural standard of electric direction to which I might refer these,
expressive of their difference and at the same time free from all theory, I have
thought it might be found in the earth. If the magnetism of the earth be due
to electric currents passing round it, the latter must be in a constant direction,
which, according to present usage of speech, would be from east to west, or,
which will strengthen this help to the memory, that in which the sun appears
to move. If in any case of electro-decomposition we consider the decompo-
sing body as placed so that the current passing through it shall be in the same
direction, and parallel to that supposed to exist in the earth, then the surfaces
at which the electricity is passing into and out of the substance would have
an invariable reference, and exhibit constantly the same relations of powers.
Upon this notion we purpose calling that towards the east the anode-\, and
that towards the west the cathode^ ; and whatever changes may take place in
our views of the nature of electricity and electrical action, as they must affect
the natural standard referred to in the same direction, and to an equal amount
with any decomposing substances to which these terms may at any time be
applied, there seems no reason to expect that they will lead to confusion, or
tend in any way to support false views. The anode is therefore that surface
and 6Sos a it<ay.
t ava. upwards, bSo<; a. way ; the way which the sun rises.
$ Kara do r wn<wardi, 6605 a 'way; the way which the sun sets.
FARADAY'S RESEARCHES. 317
at which the electric current, according to our present expression, enters ; it
is the negative extremity of the decomposing body ; is where oxygen, chlorine,
acids, &c., are evolved, and is against or opposite the positive electrode.
The cathode is that surface at which the current leaves the decomposing body,
and is its positive extremity ; the combustible bodies, metals, alkalies, and
bases, are evolved there, and it is in contact with the negative electrode.
I shall have occasion in these Researches, also, to class bodies together
according to certain relations derived from their electrical actions ; and wish-
ing to express those relations without at the same time involving the expres-
sion of any hypothetical views, I intend using the following names and terms :
Many bodies are decomposed directly by the electric current, their elements
being set free ; these I propose to call electrolytes*. Water, therefore, is an
electrolyte. The bodies which, like nitric or sulphuric acids, are decomposed
in a secondary manner are not included under this term. Then for electro-
chemically decomposed I shall often use the term electrolysed, derived in the
same way, and implying that the body spoken of is separated into its com-
ponents under the influence of electricity ; it is analogous in its sense and
sound to analyze, which is derived in a similar manner. The term electroly-
tical will be understood at once. Muriatic acid is electrolytical ; boracic acid
is not.
Finally, I require a term to express those bodies which can pass to the elec-
trodes, or, as they are usually called, the poles. Substances are frequently
spoken of as being electro-negative or electro-positive, according as they go
under the supposed influence of a direct attraction to the positive or negative
pole. But these terms are much too significant for the use to which I should
have to put them ; for though the meanings are perhaps right, they are only
hypothetical, and may be wrong ; and then, through a very imperceptible but
still very dangerous, because continual, influence, they do great injury to science,
by contracting and limiting the habitual views of those engaged in pursuing it.
I propose to distinguish these bodies by calling those anions-\ which go to the
anode of the decomposing body ; and those passing to the cathode, cations^. ;
and when I have occasion to speak of these together, I shall call them ions.
Thus, the chloride of lead is an electrolyte, and when electrolyzed evolves the
two ions, chlorine and lead the former being an anion, and the latter a
cation.
These terms, being once well defined, will, I hope, in their use enable me to
avoid much periphrasis and ambiguity of expression. I do not mean to press
them into service more frequently than will be required, for I am fully aware
that names are one thing and science another.
It will be well understood that I am giving no opinion respecting the nature
of the electric current now, beyond what I have done on a former occasion ;
and that though I speak of the current as proceeding from the parts which
are positive to those which are negative, it is merely in accordance with the
conventional, though in some degree tacit, agreement entered into by scientific
men, that they may have a constant, certain, and definite means of referring
to the direction of the forces of that current.
* fi\KTpov and A.vo> solvo. N. Electrolyte ; V. Electrolyze.
t aviov (hat ti<hich noes up. (Neuter participle.) % KO.TLOV that -which goes down.
Since this paper was read, I have changed some of the terms which were first proposed, that I might
employ only such as were at the same time simple in their nature, clear in their reference, and free
from hypothesis.
ELECTRICITY.
ON A NEW MEASURER OF VOLTA-ELECTRICITY.
I have already said, when engaged in reducing common and voltaic elec-
tricity to one standard of measurement, and again when introducing my theory
of electro-chemical decomposition, that the chemical decomposing action of
a current is constant for a constant quantity of electricity, notwithstanding the
greatest variations in its sources, in its intensity, in the size of the electrodes
used, in the nature of the conductors (or non-conductors) through which it is
passed, or in other circumstances. The conclusive proofs of the truth of these
statements shall be given almost immediately.
I endeavoured upon this law to construct an instrument which should
measure out the electricity passing through it, and which, being interposed in
16
FIG. 291.
the course of the current used in any particular experiment, should serve, at
pleasure, either as a comparative standard of effect or as a positive measurer
of this subtile agent.
There is no substance better fitted, under ordinary circumstances, to be the
indicating body in such an instrument than water ; for it is decomposed with
facility when rendered a better conductor by the addition of acids or salts; its
elements may in numerous cases be obtained and collected without any
embarrassment from secondary action, and, being gaseous, they are in the
best physical condition for separation and measurement. Water, therefore,
acidulated by sulphuric acid, is the substance I shall generally refer to,
although it may become expedient in peculiar cases or forms of experiment
to use other bodies.
The first precaution needful in the construction of the instrument was to
avoid the recombination of the evolved gases, an effect which the positive
electrode had been found so capable of producing. For this purpose various
forms of decomposing apparatus were used. The first consisted of straight
tubes, each containing a plate and wire of platina soldered together by gold,
and fixed hermetically in the glass at the closed extremity of the tube (Fig.
291). The tubes were about 8 in. long, 07 of an inch in diameter, and
graduated. The platina plates were about an inch long, as wide as the tubes
FARADAY'S RESEARCHES.
would permit, and adjusted as near to the mouths of the tubes as was con-
sistent with the safe collection of the gases evolved. In certain cases, where
it was required to evolve the elements upon as small a surface as possible, the
metallic extremity, instead of being a plate, consisted of the wire bent into the
form of a ring. When these tubes were used as measurers, they were filled
with the dilute sulphuric acid, and inverted in a basin of the same liquid,
being placed in an inclined position (Fig. 293 a), with their mouths near to
each other, that as little decomposing matter should intervene as possible; and,
also, in such a direction that the platina plates should be in vertical planes.
Another form of apparatus was that delineated (Fig. 292). The tube is bent
in the middle ; one end is closed ; in that end is fixed a wire and plate, ^, pro-
ceeding so far downwards, that, when in the position figured, it shall be as
near to the angle as possible, consistently with the collection, at the closed
extremity of the tube, of all the gas evolved against it. The plane of this plate
is also perpendicular. The other metallic termination, , is introduced at the
time decomposition is to be effected, being brought as near the angle as
possible, without causing any gas to pass from it towards the closed end of
the instrument. The gas evolved against it is allowed to escape.
The third form of apparatus contains both electrodes in the same tube ; the
transmission, therefore, of the electricity, and the consequent decomposition,
is far more rapid than in the separate tubes. The resulting gas is the sum
of the portions evolved at the two electrodes, and the instrument is better
FlG. 294.
FIG. 293.
adapted than either of the former as a measurer of the quantity of voltaic
electricity transmitted in ordinary cases. It consists of a straight tube (Fig. 293)
closed at the upper extremity, and graduated, through the sides of which pass
the platina wires (being fused into the glass), which are connected with two
plates within. The tube is fitted by grinding into one mouth of a double-
necked bottle. If the latter be one-half or two-thirds full of the dilute
sulphuric acid, it will, upon inclination of the whole, flow into the tube and
fill it. When an electric current is passed through the instrument, the gases
evolved against the plates collect in the upper portion of the tube, and are not
subject to the recombining power of the platina.
3 20
ELECTRICITY.
Another form of the instrument is given at Fig. 294.
A fifth form is delineated (Fig. 295 b\ This I have found exceedingly useful
in experiments continued in succession for days together, and where large
quantities of indicating gas were to be collected. It is fixed on a weighted
foot, and has the form of a small retort containing the two electrodes : the neck
is narrow, and sufficiently long to deliver gas issuing from it into a jar placed
in a small pneumatic trough. The electrode chamber, sealed hermetically
at the part held in the stand, is 5 in. in length and o'6 of an inch in diameter ;
the neck about 9 in. in length, and o'4 of an inch in diameter internally. The
figure will fully indicate the construction.
FIG. 295.
It can hardly be requisite to remark, that in the arrangement of any of
these forms of apparatus, they, and the wires connecting them with the
substance, which is collaterally subjected to the action of the same electric
current, should be so far insulated as to ensure a certainty that all the elec-
tricity which passes through the one shall also be transmitted through the
other.
The equivalent numbers do not profess to be exact, and are taken almost
entirely from the chemical results of other philosophers in whom I could
repose more confidence, as to these points, than in myself.
357
22
24
&
%
36
16
Oxygen ....
TABLE C
Ani
8
)F IONS,
ons.
Phosphoric acid
Carbonic acid
Boracic acid .
Acetic acid
Tartaric acid .
Citric acid
Oxalic acid .
Sulphur (?) .
Selenium (?) .
Sulpho-cyanogen
Chlorine
Iodine ....
35'5
126
Bromine
Fluorine
Cyanogen
Sulphuric acid
Selenic acid .
Nitric acid
Chloric acid .
78-3
. 187
. 26
. 40
. 64
54
75'5
FARADAY'S RESEARCHES.
321
Hydrogen
Potassium
Sodium .
Lithium .
Barium .
Stiontium
Calcium
Magnesium .
Manganese .
Zinc
Tin
Lead .
Iron
Copper .
Cadmium
Cerium .
Cobalt .
Nickel .
Antimony
Bismuth
Cat
I
. 39'2
23-3
10
. 687
. 43-8
. 20-5
127
. 277
32-5
. 57-9
. 103-5
. 28
. . . 31-6
. . . 55-8
. 46
. 29-5
. 29-5
. 64-6?
. 71
ons.
Mercury
Silver ....
Platina ....
. 200
. 1 08
q8'6 ?
Gold ....
(?)
Ammonia
Potassa ....
Soda ....
Lithia ....
17
. 47-2
3i'3
18
Baryta ....
Strontia .
Lime ....
. 767
. 51-8
28'?
Magnesia
Alumina
Protoxides generally.
Quinia ....
Cinchona
Morphia
Vegeto-alkalies generally.
. 20-7
- (?)
. 171*6
. 1 60
. 290
Now it is wonderful to observe how small a quantity of a compound body
is decomposed by a certain portion of electricity. Let us, for instance, con-
sider this and a few other points in relation to water. One grain of water,
acidulated to facilitate conduction, will require an electric current to be con-
tinued for three minutes and three-quarters of time to effect its decomposition,
which current must be powerful enough to retain a platina wire, i-io4th of an
inch in thickness*, red hot, in the air during the whole time ; and if interrupted
anywhere by charcoal points, will produce a very brilliant and constant star
of light. If attention be paid to the instantaneous discharge of electricity of
tension, as illustrated in the beautiful experiments of Mr. Wheatstonef, and
to what I have said elsewhere on the relation of common and voltaic electri-
city, it will not be too much to say, that this necessary quantity of electricity
is equal to a very powerful flash of lightning. Yet we have it under perfect
command ; can evolve, direct, and employ it at pleasure ; and when it has
performed its full work of electrolyzation, it has only separated the elements
of a single grain of water.
I showed in a former series of these Researches on the relation by measure
of common and voltaic electricity, that two wires, one of platina and one of
zinc, each i-i8th of an inch in diameter, placed 5-i6ths of an inch apart,
* I have not stated the length of wire used, because I find by experiment, as would be expected in
theory, that it is indifferent. The same quantity of electricity which, passed in a given time, can heat
an inch of platina wire of a certain diameter red hot, can also heat a hundred, a thousand, or any length
of the same wire to the same degree, provided the cooling circumstances are the same for every part in
both cases. This I have proved by the volta-electrometer. I found that, whether half-an-inch or 8 in
were retained at one constant temperature of dull redness, equal quantities of water were decomposed
in equal times in both cases. When the half-inch was used, only the centre portion of wire was
ignited. A fine wire may even be used as a rough but ready regulator of a voltaic current ; for if it be
made part of the circuit, and the larger wires communicating with it be shifted nearer to or further
apart, so as to keep the portion of wire in the circuit sensibly at the same temperature, the current
passing through it will be nearly uniform.
t "Literary Gazette," 1833, March i andSj "Philosophical Magazine," 1833, p. 2045 " L'Institute,"
i&33 p. 261.
21
322 ELECTRICITY.
and immersed to the depth of 5-8ths of an inch in acid, consisting of one drop
of oil of vitriol and 4 oz. of distilled water, at a temperature of about 60 Fahr.,
and connected at the other extremities by a copper wire 18 ft. long and i-i8th
of an inch in thickness, yielded as much electricity in little more than three
seconds of time as a Leyden battery charged by thirty turns of a very large
and powerful plate electric machine in full action. This quantity, though
sufficient if passed at once through the head of a rat or a cat to have killed it,
as by a flash of lightning, was evolved by the mutual action of so small a
portion of the zinc wire and water in contact with it, that the loss of weight
sustained by either would be inappreciable by our most delicate instruments ;
and as to the water which could be decomposed by that current, it must have
been insensible in quantity, for no trace of hydrogen appeared upon the
surface of the platina during those three seconds.
What an enormous quantity of electricity, therefore, is required for the
decomposition of a single grain of water ! We have already seen that it must
be in quantity sufficient to sustain a platina wire i-io4th of an inch in thick-
ness, red hot, in contact with the air for three minutes and three-quarters, a
quantity which is almost infinitely greater than that which could be evolved
by the little standard voltaic arrangement to which I have just referred. I
have endeavoured to make a comparison by the loss of weight of such a wire
in a given time in such an acid, according to a principle and experiment to
be almost immediately described ; but the proportion is so high, that I am
almost afraid to mention it. It would appear that 800,000 such charges of
the Leyden battery as I have referred to above would be necessary to supply
electricity sufficient to decompose a single grain of water ; or, if I am right,
to equal the quantity of electricity which is naturally associated with the
elements of that grain of water, endowing them with their mutual chemical
affinity.
In further proof of this high electric condition of the particles of matter, and
the identity, as to quantity, of that belonging to them with that necessary for
their separation, I will describe an experiment of great simplicity, but extreme
beauty, when viewed in relation to the evolution of an electric current and its
decomposing powers.
A dilute sulphuric acid, made by adding about one part by measure of oil
of vitriol to thirty parts of water, will act energetically upon a piece of plate
zinc in its ordinary and simple state ; but, as Mr. Sturgeon has shown*, not
at all, or scarcely so, if the surface of the metal has in the first instance been
amalgamated ; yet the amalgamated zinc will act powerfully with platina as
an electromotor, hydrogen being evolved on the surface of the latter metal, as
the zinc is oxidized and dissolved. The amalgamation is best effected by
sprinkling a few drops of mercury upon the surface of the zinc, the latter
being moistened with the dilute acid, and rubbing with the fingers so as to
extend the liquid metal over the whole of the surface. Any mercury in excess,
forming liquid drops upon the zinc, should be wiped offf.
Two plates of zinc thus amalgamated were dried and accurately weighed :
one, which we will call A, weighed 163*1 grains; the other, to be called B,
weighed 148*3 grains. They were about 5 in. long, and 0*4 of an inch wide.
* "Recent Experimental Researches," &c , 1830, p. 74, &c.
t The experiment may be made with pure zinc, which, as chemists well know, is but slightly acted
upon by dilute sulphuric acid, in comparison with ordinary zinc, which during the action is subject to
an infinity of voltaic actioni. See De la Rive on this subject, " Bibliothfeque Universelle," 1830, p. 39 1 -
FARADAY'S RESEARCHES. 323
An earthenware pneumatic trough was rilled with dilute sulphuric acid, of the
strength just described, and a gas jar, also filled with the acid, inverted in it*.
A plate of platina of nearly the same length, but about three times as wide as
the zinc plates, was put up into this jar. The zinc plate A was also intro-
duced into the jar, and brought in contact with the platina, and at the same
moment the plate B was put into the acid of the trough, but out of contact
with other metallic matter.
Strong action immediately occurred in the jar upon the contact of the zinc
and platina plates. Hydrogen gas rose from the platina, and was collected
in the jar ; but no hydrogen or other gas rose from either zinc plate. In about
ten or twelve minutes, sufficient hydrogen having been collected, the experi-
ment was stopped: during its progress a few small bubbles had appeared
upon plate B, but none upon plate A. The plates were washed in distilled
water, dried, and reweighed. Plate B weighed 148*3 grains, as before, having
lost nothing by the direct chemical action of the acid. Plate A weighed 1 54/65
grains, 8*45 grains of it having been oxidized and dissolved during the experi-
ment.
The hydrogen gas was next transferred to a water-trough and measured ; it
amounted to 12*5 cubic inches, the temperature being 52, and the barometer
29*2 inches. This quantity, corrected for temperature, pressure, and moisture,
becomes 12*15453 cubic inches of dry hydrogen at mean temperature and
pressure, which, increased by one-half for the oxygen that must have gone to
the anode, i.e. to the zinc, gives 18*232 cubic inches as the quantity of oxygen
and hydrogen evolved from the water decomposed by the electric current.
According to the estimate of the weight of the mixed gas before adopted, this
volume is equal to 2*3535544 grains, which therefore is the weight of water
decomposed ; and this quantity is to 8*45, the quantity of zinc oxidized, as 9 is
to 32*31. Now taking 9 as the equivalent number of water, the number 32*5
is given as the equivalent number of zinc a coincidence sufficiently near to
show, what indeed could not but happen, that for an equivalent of zinc oxidized
an equivalent of water must be decomposedf.
But let us observe how the water is decomposed. It is electrolyzed, i.e., is
decomposed voltaically, and not in the ordinary manner (as to appearance) of
chemical decompositions : for the oxygen appears at the anode and the hydro-
gen at the cathode of the decomposing body, and these were in many parts of
the experiment above an inch asunder. Again, the ordinary chemical affinity
was not enough under the circumstances to effect the decomposition of the
water, as was abundantly proved by the inaction on plate B; the voltaic
current was essential. And to prevent any idea that the chemical affinity was
almost sufficient to decompose the water, and that a smaller current of elec-
tricity might, under the circumstances, cause the hydrogen to pass to the
cathode, I need only refer to the results which I have given to show that the
chemical action at the electrodes has not the slightest influence over the
quantities of water or other substances decomposed between them, but that
they are entirely dependent upon the quantity of electricity which passes.
What, then, follows as a necessary consequence of the whole experiment ?
Why, this that the chemical action upon 32*31 parts, or one equivalent of
* The acid was left during a night with a small piece of unamalgamated zinc in it, for the purpose of
evolving such air as might be inclined to separate, and bringing the whole into a constant state,
t The experiment was repeated several times with the same results.
21 2
3 2 4
ELECTRICITY.
zinc, in this simple voltaic circle, was able to evolve such quantity of electricity
in the form of a current as, passing through water, should decompose 9 parts,
or one equivalent of that substance ; and, considering the definite relations of
electricity as developed in the preceding parts of the present paper, the results
prove that the quantity of electricity which, being naturally associated with
the particles of matter, gives them their combining power, is able, when thrown
into a current, to separate those particles from their state of combination ; or,
in other words, that the electricity which decomposes, and that which is evolved
by the decomposition of, a certain quantity of matter are alike.
But admitting that chemical action is the source of electricity, what an
infinitely small fraction of that which is active do we obtain and employ in
our voltaic batteries! Zinc and platina wires, i-i8th of an inch in diameter
and about half an inch long, dipped into dilute sulphuric acid, so weak that it
is not sensibly sour to the tongue, or scarcely to our most delicate test-papers,
will evolve more electricity in i-2oth of a minute than any man would willingly
allow to pass through his body at once. The chemical action of a grain of
water upon four grains of zinc can evolve electricity equal in quantity to that
of a powerful thunder-storm. Nor is it merely true that the quantity is active ;
it can be directed and made to perform its full equivalent duty. Is there not,
then, great reason to hope and believe that, by a closer experimental investi-
gation of the principles which govern the development and action of this
subtile agent, we shall be able to increase the power of our batteries, or invent
new instruments which shall a thousandfold surpass in energy those which we
at present possess ?
After Faraday had invented his first apparatus or volta-measurer, other
and more convenient contrivances were made by himself and others.
FlG. 296. Large Voltameter for measuring the quantity of Mixed Gases
obtainable from small or large batteries.
The apparatus made by Elliott Brothers consists of a large pair of platina-
plate electrodes, doubly folded and approximated one to the other, so as to
present the largest amount of surface for the liberation of the mixed gases,
oxygen and hydrogen ; the plates are contained in a glass bell jar, surmounted
by a bent conducting tube to convey the gases to the graduated cylindrical
glass jar, which is provided with a stop-cock.
GROVE'S GAS BATTERY.
325
Faraday's voltameter experiment has been reversed in the most philosophi-
cal manner by Professor Grove, who partly rilled fifty tubes alternately with
oxygen and hydrogen. The tubes each contained a plate of platinum roughened
with a deposit of black platinum powder upon them, and, when connected
as in Fig. 297, they produced a current which afforded all the ordinary elec-
trical results. The tubes are partly filled with, and stand in glass jars con-
taining, dilute sulphuric acid, specific gravity 1*200, and, when placed on a
.stool supported on glass legs, give a shock which can be felt by five persons,
n n
FIG. 297. Grove's Gas Battery.
FIG. 2
affected the electroscope and the galvanometer needle, gave an electric spark,
and decomposed water, iodide of potassium, &c. This beautiful experiment
proved that just as the current of electricity decomposed water into oxygen and
hydrogen (Fig. 298), so the same gases properly disposed, as in Fig. 299, would
reunite, and, in the act of reunion, evolve a current of electricity that would
again repeat itself in the production of all those phenomena already detailed
FIG. 299. Glass Cellwith cardboard Diaphragm for Chemical Decomposition.
When the poles of the battery, usually platinum plates, are immersed in a
glass cell, divided in the centre with a slip of card, and filled with a solution
of iodide of potassium and starch, electrolytic decomposition occurs; the
iodine is liberated on one side, and, combining with the starch, produces a
purple colour, whilst the other side remains colourless because the alkali is
there liberated and has no action upon the starch. If, however, a little tinc-
ture of turmeric is carefully dropped in and mixed, it turns a reddish brown,
326
ELECTRICITY.
and thus indicates the presence of the potassium oxidized and changed to
potash in the presence of water.
A number of amusing chemical decompositions may be performed with the
same arrangement. A very good one is that in which a solution of common
salt and indigo is used. The liberation of chlorine at one pole causes the
half of the contents of the trough to be bleached, the other remaining a
blue colour.
When iodide of potassium or chloride of sodium yield their elements to
the power of the circulating electricity, such and other similar cases are called
"primary results." But when metals are reduced, as already mentioned, the
nascent hydrogen and the oxide of the metal are deposited together, and
the former, reacting on the latter, deoxidizes the oxide, and reduces it to the
metallic state. It is thus we have the art of electrotyping, already alluded
to at page 313, and such a decomposition would be called a "secondary result. '
FIG. 300.
The above are two arrangements of a Daniell's single cell and trough, con-
taining the solution of sulphate of copper and the various seals, casts, &c.,
which are to be copied by the deposit of metallic copper upon them. In one,
the Daniell's cell is connected with a trough ; in the other, the cell itself, zinc,
acid, and the medals to be copied in the sulphate of copper, perform the same
functions.
OHM'S LAW.
In the admirable and exhaustive work on the electric telegraph, Mr. Robert
Sabine says :
" Until the end of the first quarter of the present century, physicists were
still in darkness as to the mode and laws of the propagation of the galvanic
current. The immense velocity with which the galvanic impulse is transmitted
led to the seeking an analogy between it and light, and on this wrong scent
much time and labour was lost ; when Ohm, a German physicist, conceived
the happy idea that a juster analogy was to be found in the propagation of heat,
and proceeded to apply to galvanic electricity the formulae of Fourrier and
Poisson. He expressed the intensity of an electric current as directly pro-
portional to the electro-motive force, and inversely to the resistance of the
circuit. Algebraically, if E is the electro-motive force, R the resistance, and
I the intensity,
OHM'S LAW. 327
" Of these magnitudes, R is made up of two resistances that interior and that
exterior of the element. The internal resistance, or resistance of the element, is
again the sum of the several resistances due to the passage of the current from
one plate to the liquid, to its passage through the liquid, and from its passage
from the liquid to the other plate. We will call this resistance of the element r.
The remaining component, the external resistance, is that due to the passage
of the current through the interiors of the plates, the wire connecting them,
and through whatever conductor may be otherwise inserted between them.
Let this be p. Substituting these values for R in (l.
I--*- .... (II.
H-P
" The truth of this equation may be proved experimentally as follows :
" Evidence of the direct proportion of the intensity to the electro-motive
force is obtained by comparing the known function of the deflections of a mag-
netic needle of a galvanometer, due to a current in a circuit, in which r and p,
the circuit resistances, remain constant, while the numbers of pairs are changed.
The resistance r of a pair of plates of equal surface at the same distance
diminishes as their surface is increased, and vice versa; but the resistance of
more pairs joined up in series increases proportionally to the number. There-
fore we take a single pair of plates of known surface, and connect them in the
circuit of a galvanometer, and of a length of wire, determined by a rhecord
or other adjustible resistance, and note the deflection <. Then we double the
electro-motive force E by inserting in the place of these two pairs of plates of
each, double the surface of the former, by which the resistance r remains un-
changed. The wire p remains also the same; but we have another deflec-
tion, <i. For the intensity I with the single pair we have the expression
and with the second reading by the two pairs
f , _ s 2 E
2) . . I 1 = F(
F being the function, sine, tangent, or whatever it may be, which connects
degrees of arc with those of force. From these two equations it follows, and
will also be found, that
1 = 2 I
" The, same method of experimental proof may be extended to n elements,
connected in series, by increasing the surfaces n times. The remaining relation
expressed by Ohm's law, that of current and resistance,- is proved experimen-
tally by obtaining a deflection fa, with a certain inserted resistance p, and
electromotive force E, and then doubling the length of the wire p, diminishing
the size of the plates to half, and doubling their distance from each other, by
which the total resistance of the circuit is doubled, while the electro-motive
force remains the same, and the needle is deflected a smaller angle fa.
Expressed algebraically, the first observation gives
328 ELECTRICITY.
and the second
2) . . Il=FW , ?)= -JL_
from which it follows that
FW) = 2F(#)
1 = 2 Ii
which will be verified by reducing the reflections to degrees of force. A law,
upon which the truth of these results depends, has yet to be proved. It is that
the resistance is reciprocal, and the intensity thereof directly proportional to
the surface of the plate and to the section of the conductor. If the plates be
first immersed a known fraction of their surface in the solution, and after-
wards other fractions and completely, and at the same time the sectional area
of the conductor be similarly increased, by taking thicker wire or two or more
wires of the same length and diameter parallel to each other, the intensity, as
indicated by the functions of the galvanometer, will be found to increase,
other things being equal, as the section of the conductor and surface of the
exciting plates increase. The application of Ohm's law in the solution of
different problems which the electrician finds it necessary to answer is very
extended ; it forms, in fact, the basis upon which all exact inquiry in elec-
trical science is built up. We will see now, as an instance, what it affords us
when we combine elements together in different ways.
" When the poles of a pair of plates are joined together, the intensity, I, of
rp
the current passing in every section of the current is 1= r . There are two
principal ways in which a number of galvanic pairs maybe connected together.
i st. They may be connected in series for intensity, so as to add their electro-
motive forces and resistances together; and (2nd) they may be connected
parallel to each other for quantity, as it is called, so that the electro-motive
force of the combination remains the same; but the surfaces of the plates are
increased, and hence the resistance in the same measure diminished. First,
let n elements be connected thus : the negative pole of the first element is
joined to the positive pole of the second, the negative pole of the second to
the positive pole of the third, and so on, up to the ;/th element. We have then
what is vulgarly called an ' intensity battery,' and the intensity of each indi-
vidual element of the series, if they are of the same size and kind, will be
E E
I i j-, - ^ = ; - . . . (ill.
9+r+(n 1} r g+n r
and that of the whole battery
E
! n - ...... (iv.
When the resistance, n r, of the battery is so small in comparison with p, that
we can, without appreciable error, neglect it, the intensity of the whole battery
becomes
n E
'"=T
That is to say, that when the resistance of the battery is very small in com-
parison with the resistance of the circuit exterior to the battery, the strength
THE RHEOSTAT OF WHEATSTONE. 329
of the current is increased in direct proportion to the number of elements
added to it. Dividing both numerator and denominator of the above fraction
(iv.) by the number of elements, n, we get
which becomes, if we set p=o,
In=? (VI.
affording us light upon another relation of the galvanic current, viz., that when
the resistance exterior to the battery is so small that it may be neglected, the
current of a number of elements will do more work than that of a single pair.
" The first of these laws applies to a battery used for working a long line of
telegraph, whose resistance with the coils of the apparatus is very great in
comparison to that of the element, and where it is evident that a large battery
is necessary.
" The second law applies to a local circuit, where the resistance of the cir-
cuit is small, and a few elements do as well as a great number."
THE RHEOSTAT OF WHEATSTONE.
The Rheostat, or Current Regulator. We print from the memoirs of Sir
Charles Wheatstone, in the Transactions of the Royal Society, "An account
of several new instruments and processes for determining the Constants of a
Voltaic Circle." This most distinguished philosopher says :
u The instruments and processes I am about to describe being all founded
on the principles established by Ohm in his theory of the voltaic circuit, and
this beautiful and comprehensive theory being not yet generally understood
and admitted, even by many persons engaged in original research, I could
scarcely hope to make my descriptions and explanations understood without
prefacing them with a short account of the principal results which have been
deduced from it.
" It will soon be perceived how the clear ideas of electro-motive forces and
resistances, substituted for the vague notions of intensity and quantity, which
have been so long prevalent, enable us to give satisfactory explanations of
most important phenomena, the laws of which have hitherto been involved in
obscurity and doubt. Viewing the laws of the electric circuit from the point
at which the labours of Ohm has placed us, there is scarcely any branch of
experimental science in which so many and such various phenomena are
expressed by formulae of such simplicity and generality. In most of the
physical sciences, the facts of observation and experiment have kept pace
with theoretical generalization. In this science alone they had gone on accu-
mulating in prolific abundance without any successful attempt having been
made to reduce them to mathematical expression. But this is now happily
effected ; and what has hitherto been mere matter of speculative conjecture is
removed into the domain of positive philosophy.
" By electro-motive force is meant the cause which, in a closed circuit,
originates an electric current, or, in an unclosed one, gives rise to an electro-
scopic tension. By resistance, is signified the obstacle opposed to the passage
330 ELECTRICITY.
of the electric current by the bodies through which it has to pass ; it is the
inverse of what is usually called their conducting power. When the activity
of any portion of the circuit is increased or diminished, either by a change in
the electro-motive force or in the resistance of that portion, the activity of all
the other parts of the circuit increases or decreases in a corresponding degree ;
so that the same quantity of electricity always passes in the same instant of
time through every transverse section of the circuit. The force of the current
is directly proportional to the sum of the electro-motive forces which are
active in the circuit, and inversely proportional to the total resistance of all
its parts, or, in other words, the force of the current is equal to the sum of the
electro-motive forces divided by the sum of the resistances.
********
" It is seldom that any real advance is made in a scientific theory without
a corresponding change in its terminology being required. Now that it is
proved, beyond doubt, that the various sources of continued electric action
differ from each other only in the amount of their electro-motive forces,
modified by the resistances of the circuit of which they form a part, it
becomes of importance, in order to give precision to our statements, and to
avoid circumlocutions otherwise inevitable, to adopt general terms to express
the source of a current, without reference to the peculiar mode of its produc-
tion. I shall, therefore, employ the word ' rheometer ' to denote any apparatus
which originates an electric current, whether it be a voltaic current or a voltaic
battery, a thermo-electric battery, or any other source whatever of an electric
current. When speaking of a single element, I shall term it a rheomotive
element ; and what is usually called a voltaic or thermo-electric pile or battery
I shall term a rheomotive series. I shall still use the ordinary expressions
when I have to refer to the specific sources of the productions of electric
currents ; but when I employ the general terms, they must be understood to
apply to all these sources indifferently. The want of a general term to desig-
nate an instrument to measure the force of an electric current, without refer-
ence to its particular construction, has been long felt. I shall use the word
rheometer for this purpose, continuing occasionally to employ galvanometer,
voltometer, &c., to distinguish the particular instruments to which these
names have been applied though, perhaps, the terms magnetic, chemical,
calorific, &c., rheometer would be more appropriate.
" This may be the proper place to explain a few other terms, which I have
frequent occasion to use, though not in the course of the present communica-
tion. By rheotome is meant an instrument which periodically interrupts a
current; and by rheotrope, an instrument which alternately inverts it. A
rheoscope is an instrument for ascertaining merely the existence of an electric
current. The word rheostat will be explained hereafter.
" I have not introduced these terms (which will be found greatly convenient,
and will enable us to state general propositions much more clearly) without
good authority. The word ' rheophore ' was employed by Ampere to designate
the connecting wire of a voltaic apparatus as being the carrier or transmitter
of the current ; and the word ' rheometer,' first proposed by Peclet as a
synonym for galvanometer, has been generally adopted by the French
writers on physics.
" The method of obtaining the constants of a rheophoric circuit, adopted by
Fechner, Lenz, Pouillet, &c., in their experimental verifications of Ohm's
theory, is essentially the following : The resistance of a circuit is determined
THE RHEOSTAT OF WHEATSTONE.
331
FIG. 301.
by observing the force of the current first, without any extra-interposed resist-
ance in the circuit; and afterwards, when a known resistance is added.
The principle of this method is extremely simple; but the difficulty of
determining immediately the force of a current by means of a galvanometer
is an obstacle to its general employment. Fechner measured the force of the
current by the number of oscillations of the needle when placed at right
angles to the coils a very tedious operation ; and others have employed the
deviation of the needle, the corresponding degrees of force having been
previously determined by some peculiar process, or inferred from some rule
depending on the particular construction of the instrument. Another impedi-
ment to the use of the galvanometer, to measure the force of a current, arises
from the changes in the magnetic intensity of the needle, which frequently
occur, especially when it has been acted upon by too strong a current.
" The principle of my method is that of employing variable, instead of con-
stant, resistances, bringing thereby the currents in the circuits compared to
equality, and inferring from the amount of the resistance measured out
between two deviations of the needle the electro-motive forces and resistances
of the circuit according to the particular conditions of the experiment. This
method requires no knowledge of the forces corresponding to different devia-
tions of the needle. To apply this principle, it is requisite to have a means
of varying the interposed resistance, so that it may be gradually changed
within any required limits. I have contrived two instruments for effecting
this purpose one intended for circuits in which the resistance is considerable,
the other for circuits where the resistance is small.
" The first instrument is represented in Fig. 301 A : g is a cylinder of wood,
and h is a cylinder of brass, both of the same diameter, and having their axes
parallel to each other ; on the wood cylinder a spiral groove is cut, and at one
of the extremities a brass ring is fixed, to which is attached one of the ends of
a long wire of very small diameter, which, when coiled round the wood
cylinder, fills the entire groove, and is fixed to its other end to the remote
extremity of the brass cylinder. Two springs,/ and k, pressing one against
the brass ring on the wood cylinder and the other against the extremity of
332 ELECTRICITY.
the brass cylinder h, are connected with two binding screws for the purpose
of receiving the wires of the circuit. The movable handle m is for turning
the cylinders on their axes. When it is placed on the cylinder ft, and is turned
to the right, the wire is uncoiled from the wood cylinder and coiled on the
brass cylinder ; but when it is applied to the cylinder g ? and is turned to the
left, the reverse is effected. The coils on the wood cylinder being insulated
and kept separate from each other by the groove, the current passes through
the entire length of wire coiled upon that cylinder; but, the coils in the brass
cylinder not being insulated, the current passes immediately from the point of
the wire which is in contact with the cylinder to the spring kj the effective
part of the length of the wire is, therefore, the variable portion that is on the
wood cylinder.
" In the instrument I usually employ, the cylinders are 6 in. in length and
l^ in. in diameter; the threads of the screw are forty to the inch; and the
wire is of brass, i-iooth of an inch in diameter. I employ a very thin wire
and a badly conducting metal in order that I may introduce a greater resist-
ance into the circuit. A scale is placed to measure the number of coils
unwound ; and the fractions of a coil are determined by an index which is
fixed to the axis of one of the cylinders, and points to the divisions of a
graduated circle.
"As the principal use of this instrument is to adjust or regulate the circuit,
so that any constant degree of force may be obtained, I have called it a
rheostat. Fig. 301 shows the arrangement of the circuit when prepared for
an experiment ; B is a delicate galvanometer with an astatic needle, furnished
with a microscope for reading off the divisions of the circle, which greatly
facilitates the observations ; C is the rheomotor.
" The rheostat which I employ for circuits in which the resistance is com-
paratively small is represented at Fig. 302. a is a cylinder of well-seasoned
wood, on the surface of which a spiral groove is cut. A thick copper wire is
wound round the cylinder, occupying the groove, forming, as it were, the thread
of the screw. Immediately above the cylinder, and parallel with its axis, is
a triangular metal bar, ^, carrying a rider or slide, c ; to this rider a spring, </,
is fixed, which constantly presses against the spiral wire, yielding to any
slight inequality. One end of the spiral wire is attached to a brass ring, e,
against which a spring,/ presses, which is connected by means of a binding-
screw to one end of the circuit : the other end of the circuit is held by the
binding-screw which is in nutation with the triangular metal bar. On turn-
ing the handle ^, the cylinder is caused to move on its axis in either direction ;
and the rider <:, guided by the wire, moves along the bar, advancing or re-
ceding according as the cylinder is moved right or left. The rider coming in
contact with a different point of the spiral wire, a different resistance is intro-
duced into the circuit, consisting of that portion of the wire only which is
included between the rider and the end of the wire connected with the spring/
"The cylinder of the instrument I have constructed is io| in. in length
and 3^ in. in diameter : the wire is of copper, i-i6th of an inch thick ; and
it makes 108 coils round the cylinder. The dimension of the instrument, and
the thickness, length, and material of the wire, may be varied according to
the limits of the variable resistance required to be introduced into the circuit,
and the degree of accuracy with which those changes are required to be
measured.
" Fig. 302 represents the arrangement of a thermo-electric circuit in which
THE RHEOSTAT OF WHEATSTONE.
333
FIG. 302.
this instrument is interposed. C is the thermo-electric element ; B, the gal-
vanometer, which in this case must not have numerous coils of fine wire, as
in the preceding arrangement for this would introduce too great a resistance
in the circuit but must consist of a single thick plate or wire, making a single
convolution ; or, which I think is preferable, the method of diverting a portion
of the current from the wire of a delicate galvanometer described may be
adopted. Any rheometer in which the resistance is small may be employed
in conjunction with this form of the rheostat, instead of a thermo-electro ele-
ment described. The rheostat, especially under the form last described, may
be usefully employed as a regulator of a voltaic current, in order to maintain
for any required length of time precisely the same degree of force, or to change
it in any desired proportion. Interposed in the circuit of an electro-magnetic
engine, however, the rheometer may vary in its energy ; the same velocity may
be constantly restored by turning the cylinder of the regulator to the left or to
the right, according as the velocity increases or decreases; or any different
velocity, within given limits, may be obtained by adjusting the rheostat accord-
ingly.
*** * * * * * * *
"It is of the highest importance to have a correct standard of resistance,
and one that can easily be reproduced for the purpose of comparison. A
copper wire of a given length and diameter might be employed ; but, as very
small differences of diameter are attended with considerable differences in the
resistances of wires, it is more convenient to assume for the unit of resistance
a wire of a given length and weight, which allows small differences to be very
accurately determined.
" It is frequently required to measure resistances much greater than can be
effected by means of the rheostat, though the reduced length of its wire is
334 ELECTRICITY.
considerable. I may wish to know, for instance, the resistance of the wire of
the electro-magnets of my telegraphic apparatus, which is sometimes many
hundred yards in length, or that afforded by an extensive telegraphic line, or
the resistance of a certain extent of an imperfectly conducting liquid. In all
these cases, and a variety of others, I employ another instrument, which enables
me to interpose in the circuit resistances to any amount, and yet to obtain by
the compound use of the rheostat, which serves, in its fine adjustment, any
required degree of accuracy. This instrument is represented at Fig. 301. It
consists of six coils of fine silk-covered copper wire, about the i-2ooth part of
an inch in diameter : two of these coils are 50 ft. in length ; the others are re-
spectively 100, 200, 400, and 800 ft. in length. The two ends of each coil are
attached to short thick wires, fixed to the upper faces of the cylinders, which
serve to combine all the coils in one continued length. The two wires, a, b,
form the extremities of the coils by which they are united to the circuit. On
the upper face of each cylinder is a double brass spring, movable round a
centre, so that its ends may rest at pleasure either on the ends of the thick
connecting wires, or may be removed from them and rest only on the wood.
In the latter condition the current of the circuit must pass through the coil ;
but in the former position the current passes through the spring, and removes
the resistance of the coil from the circuit. When all the springs rest on the
wires, the resistance of the whole series of coils is removed ; but, by turning
the springs so as to introduce different coils into the current, any multiple
of 50 feet up to 1600 may be brought into it.
" As the measurement of these long lengths of wire cannot be accurately
depended upon, it is advisable to ascertain the number of units of resistance
in each coil, which, with the aid of the rheostat, may be easily effected. I
find the resistance of the entire 1,600 feet to be equivalent to 218,880 units of
resistance, or feet of the standard wire. I occasionally employ an auxiliary
series of coils, combined in the same way as the preceding, consisting of six
coils of the same wire, each 500 yards in length. The reduced length of this
series is above 233 miles of the standard wire. By combining it with the pre-
ceding, I am able to measure resistances equal to 274! miles.
*********
" The rheostat affords a most ready means of ascertaining the sum of the
electro-motive forces active in a voltaic circuit, without requiring for this pur-
pose the aid of a rheometer graduated to indicate proportional forces, or
having recourse to the tedious process of counting the oscillations of a needle,
employed by Fechner in his investigations. To save time and tro.ible in this
operation will be of great importance to the future progress of electro-chemistry,
on account of the great number of experiments of this kind which yet remain
to be made, and also from the fluctuations in the electro-motive forces of many
circuits, from chemical and other actions, which render observations requiring
considerable time valueless.
" The principle of my process is as follows : In two circuits producing
equal rheometric effects, the sum of the electro-motive forces divided by the
V 11 P*
resistances is a constant quantity, i.e., - = ; if E and R be proportionately
R % R
increased or diminished, F will obviously remain unchanged. Knowing, there-
fore, the proportion of the
we are able immediately
fore, the proportion of the resistances in two circuits producing the same effect,
.re able immediately to infer that of the electro-motive forces.
CALORIFIC EFFECTS. 335
" But as it is difficult in many cases to determine the total resistance, con-
sisting of the partial resistance of the rheometer itself, the galvanometer, the
rheostat, &c., I have recourse to the following simple process : Increasing
the resistance of the first circuit by a known quantity, r, the expression becomes
. In order that the effect in the second circuit shall be rendered equal
-
to this, it is evident that the added resistance must be multiplied by the same
factor as that by which the electro-motive forces and original resistances are
p ^/ "P
multiplied; for = - - . The relations of the lengths of the added
R r n R-j-;z r
resistances r and n r, which are known, immediately give those of the elec-
tro-motive forces. Experimentally, I proceed thus : I interpose the rheostat
and the galvanometer in the circuit, and then add by means of the former,
assisted if necessary by the resistance coils, a sufficient resistance to bring
the needle exactly 45. I then ascertain the length of wire uncoiled from the
brass cylinder of the regulator necessary to reduce the deviation of the needle
to 40. The number of turns is the measure of the electro-motive force, the
number corresponding to that of a standard element having been previously
determined."
The description of Sir Charles Wheatstone's differential-resistance measurer
will be found in the article on the Telegraph, under the name of " Wheat-
stone's British Association Bridge."
THE CALORIFIC EFFECTS OF THE VOLTAIC CURRENT.
When the poles of the battery, or rather the terminal wires, are connected
with the arms of the universal discharger, to which crayon-holders, containing
hard gas-retort carbon, have been attached, no effect is observed until the
carbons are brought in contact, because the intensity of the voltaic current is
FlG. 303. The Charcoal Points.
not sufficient to polarize the intervening air and cause a disruptive discharge ;
but, once brought in contact, a brilliant spark or intense light is perceptible ;
and then the carbons may be more or less separated without interrupting the
current; and, with very powerful batteries, the distance between the two
carbons* may be increased to several inches.
By throwing a picture of the charcoal poles on the disc, it is seen that a
luminous arc extends between the two poles, and there is a constant transfer-
ence of heated particles going on between the two carbons. It is this passage
336
ELECTRICITY.
of divided carbon which serves as a conductor to the current, and preserves
its continuity.
De la Rue states that " The length of the luminous arc consequently bears
a close relation to the facility with which the material of the poles admits of
division?
Scientific men have always agreed that the transference of particles took
place in the same direction as the current, viz., from the positive element to
the negative; and the explanation is made clearer by the discovery of Neeflf,
that the positive is more strongly heated than the negative pole.
Van Breda has shown that incandescent and fused particles are not only
propelled from both poles towards one another, but in every direction.
Maas, of Namur, affirms that the transference does not always take place
from the positive to the negative pole, but is determined by the density of
the charcoal. He states that he succeeded in reversing the direction of the
particles by connecting a very hard, fine-grained bit of carbon with the posi-
tive, and a coarse, soft piece of charcoal with the negative ; and he then found
that the incandescent particles moved from the negative to the positive pole.
FIG. 304.
The negative cylinder, when examined, appeared slightly excavated ; the
positive one, slightly obtuse. Amidst this conflicting evidence, the writer states
from experience that the wasting of the charcoal points is always unequal, and,
provided they are of the same quality, the transference takes place regularly
from the positive cowards the negative. By arranging a number of crayon-
holders, containing various metals, such as zinc, copper, lead (Fig. 304),
according to the method proposed by Mr. De la Rue, a number of beautiful
colours may be obtained by the intense heating and partial combustion of
the metals, copper throwing out a green light, zinc a bluish white, with the
formation of a large quantity of smoke oxide of zinc.
If the charcoal points are brought together in a void space, or vacuum, the
light is very peculiar: it appears softer, though still very brilliant, and pre-
sents a marked difference to the same light observed in air ; the carbons
appear to last much longer, and the writer has often thought that when the
electric light is required to be very continuous, as in the Duboscq lantern, it
would always be better, if possible, to produce the light in small glass chambers
from which the air had been removed.
It has been seen that the resistance of platinum wire to the passage of the
electric current is at least nine and a half times greater than that of silver.
When a wire resists the passage of the current, viz., motion, heat is the
CALORIFIC EFFECTS. 337
FlG. 305. Electric Light in vacuo.
product, which may be so increased by the reduction of the thickness of the
metal, that it becomes intensely hot.
A platinum wire of moderate thickness stretched between two upright
pillars, of course metallic, becomes ignited throughout its entire length, if
connected with a battery of sufficient power. The writer has had 18 feet of
wire incandescent whilst using very powerful Grove's batteries. For experi-
ments on the small scale, it is well to protect the platinum from the cooling
action of currents of air, and by this means a much greater length of platinum
9 9
FIG. 306. Glass Tube containing a Platinum Wire y
Attached to one end and connected with a sliding brass rod, which being drawn out lengthens or
shortens the platinum wire.
wire can be ignited. The light emitted is peculiar, and the wire appears four
or five times thicker than it really is, by irradiation. In a vacuum, a wire of
platinum may be ignited which would remain cold and dark in the air. A
platinum wire which is thoroughly ignited in air remains perfectly cool if sur-
rounded with hydrogen gas. This fact is easily shown by using two bell-jars of
the same size, one full of air and the other filled, by displacement, with hydrogen
from an india-rubber bag. As the jars are alternately placed over the plati-
num wire, the latter becomes incandescent in the air. but cool in the hydrogen
(Fig. 307). This fact was discovered by Professor Grove, but is not yet clearly
explained. Magnus ascribes it to conduction ; and theoretically this idea seems
more consistent with the statement that hydrogen is really a metal. Tyndall
ascribes it to the convective mobility of the gas ; its particles are supposed to
be more quickly set in motion than air, and hence carry off more heat.
The dynamical effect of electricity, and its power of producing motion, is
well shown in the movements of the magnetic needle belonging to the galva-
22
338 ELECTRICITY.
nometer, and will be more fully described in the article upon Electro-Mag-
netism ; and having now discussed the various effects producible from the
voltaic current, viz., chemical, calorific, magnetic, and dynamical, we may
FIG. 307.
A, glass rod, insulating the top pole, B, from c, between which the coil of platinum, p, is placed.
conclude this chapter with the description of a few practical applications of
the principles already described.
First, the use for surgical purposes, by Mr. Sylvan De Wilde, C.E., of the
Electrical Probe and Forceps.
Blood, bone, and animal matter generally are practically non-conductors
of electricity of low tension.
Soft iron, around which an electric current is made to circulate, becomes
instantly magnetic. The magnetism ceases the instant the current is broken
constituting what is called an electro- magnet.
Advantage is taken of these properties to detect the existence and assist in
the extraction of bullets from wounds.
The apparatus consists principally of four parts: I, the battery; 2, the
alarum ; 3, the probe ; and 4, the forceps, contained, with their accessories,
in a box 1 1 in. X 3^ in. X 2| in., and is complete in itself, requiring no external
supplies for about three years.
The Battery. Electricity is developed in a vulcanite cell (on the left of the
bell) by zinc and carbon, in a solution of sulphate of mercury. The pieces of
zinc and carbon drop into slots in the interior ends of the cell, where they
impinge upon platinum springs, which, being riveted to conductors on the
exterior of the cell, form positive and negative poles.
The zinc and carbon are interchangeable, it being immaterial as to which
way the current travels through the apparatus.
The poles of the battery come into contact with the conductors of the
ELECTRICAL PROBE AND FORCEPS. 339
alarum at the top of the partition on the left of the bell, and it is necessary
to see that the metallic surfaces are clean, as a thin film of dirt or oxide will
prevent the passage of the current.
The Alarum. This consists of brass strips or conductors (mounted on an
insulating bed of vulcanite) which proceed from the poles of the battery to the
binding-screws or terminals, which stand on the partition to the right of the
bell. One of these conductors, on its road, passes in the form of silk-covered
wire many times round pieces of soft iron, forming together an electro-magnet.
It is on the immediate right of the bell. The instrument so far may be tested
by making communication between the two terminals or binding-screws with
a knife-blade or any metallic conductor. This completes the circuit : the
current passes, the iron is magnetised, the keeper (to which is attached the
hammer) attracted, and a stroke given on the bell. We may thus know that
the battery is in action, and all metallic contacts clean and of sufficient pres-
sure, before attaching the wires of the probe and forceps to the terminals.
The alarum is covered with glass, to keep out dirt, which is considered a
greater evil than the partial muffling of the sound. Should a violent jerk affect
the adjustment of the hammer upon the bell, the glass can be removed by
taking the screws out of the beading. A piece of steel which will serve this
purpose will be found in the receptacle on the left of the battery. The bell
must be turned upon its axis (which is excentric) until a position is found
which gives a clear ring.
The Probe consists of two pointed steel wires firmly fixed in an ivory handle,
and projecting about 4 in. from it. These wires are insulated from each
other by a strip of vulcanite lying between them. Between the points and
the handle, the wires have a slender vulcanite tube passed over them, which
is screwed into a short length of German silver tube, upon which is mounted
a small shield. The tubes, thus screwed together, are free to slide to and fro
about a quarter of an inch, being pushed forward by pressure of the second
finger upon the shield, and thrown back by a spiral spring which obtains
purchase upon two screws inserted in the ivory handle through slots in the
German silver tube.
PROBE
2 **" _^^^^ ^
ZINC
CARBON
^
FIG. 308.
The Forceps consists of two tempered steel limbs, having curved and bow
handles like scissors. One of these limbs has riveted upon it a slip of ivory,
which, combined with ivory bushing of the pivot and the small piece of ivory
between the bows, completely insulates it from the other, in all positions.
To the bows are screwed and soldered very pliable plated wires, 2 ft. long,
covered with silk, which are coupled and soldered to similar wires proceeding
from the two steel wires of the probe.
22 2
340
ELECTRICITY.
It will be seen from the above sketch that each instrument has the electrical
current " laid on," but broken at A and B (the points of the probe and of the
forceps). Let each instrument be tested immediately before use by touching
and seizing a bullet, which will supply a bridge for the current to pass at A
and B, and instantly cause the bell to ring. It is needless to say that the
instruments cannot be used quite simultaneously.
The box in the right-hand compartment contains about forty charges of the
battery, each charge lasting several weeks. Shaking the battery-cell about
will often revive an apparently dead solution. It ends by gradually getting
too feeble to attract the keeper. It should then be thrown away, the cell
washed out, and fresh charged when required.
Method of using. The probe being handled as a pen, the shield is pushed
forward by the second finger ; this has the effect of covering the points of the
wires with the vulcanite tube. The instrument is now inserted, and the
wound explored until the supposed bullet is felt. The tube is then allowed
to retreat, by the withdrawal of the second finger, and the substance is
examined with the points. As it is necessary that both points should touch
the metal at once, it will follow that, as the probings are carried on, quarter
and half revolutions of the probe on its axis should be made, by rolling it
between the fingers, as we might otherwise touch with one point only, and
obtain no signal, as for instance on the back edge of a bullet at A.
FlG. 309.
If the points become entangled in fibre, c., they can always be released
by sliding forward the tube for a moment. The advantage of points is their
ability to obtain good contact through pus or fibre which may overlie the
bullet.
The lodgment being ascertained, the forceps are brought into use ; and
these equally give a ring upon the bell when the bullet is seized, the falling
back of the bell-hammer showing if contact is lost. The curved points will
seize the bullet in any position, generally allowing it to revolve to that of
least resistance. As for instance, should seizure be made at right angles to
the bullet, it would revolve between the points, as shown by the sketch at
c and D.
In many cases the forceps could be at once used, without the intervention
of the probe.
Should the plaited wires, by repeated twisting, become broken, they may
THE ELECTRIC TORPEDO. 341
be resoldered by any tinman or the regimental smith, or, in emergency, tied
on or fastened in any way so that fair metallic contact is made.
The probe was invented by Mr. De Wilde to solve the difficulty presented
in the case of General Garibaldi. It is made by Mr. Apps, and was submitted
to the Director-General of the Army Medical Department in December, 1 866.
and, shortly after, approved in a report by Professor Longmore, of Netley
Hospital, and by the Directors-General of the Navy and of the Indian Board.
THE ELECTRIC TORPEDO.
Among the many important applications of electricity the electric torpedo
occupies a very prominent position, as a means of defence against the approach
of an enemy both by land and by sea.
By sea this defence against attack is carried out by sunken torpedoes or
mines, containing charges of gun-cotton or gunpowder proportionate to the
depth of water, and which are planted in the harbour or channels to be pro-
tected, in such positions that the enemy's ships in their approach must pass
over them, and, upon the ignition of the torpedo, be destroyed by its explosive
force. ,
On 'land, the torpedo assumes the form of a hidden mine, any number of
which may be grouped around the city or place to be protected from the
approach of the enemy. These mines are large pits in the form of inverted
truncated cones, into the apex of which the torpedo is placed, the rest being
filled with one or two hundred tons of paving-stones and broken granite, and
the mine concealed from observation; but, when it is fired, it will deal death
and destruction to all'around.
The ignition of these torpedoes by land and by sea is effected by means of the
electric spark, and, by the arrangement of Mr. Nathaniel J. Holmes, is entirely
under the will and control of the operator, and may be employed with the
greatest safety, while at the same time it is certainly one of the most deadly
and destructive engines of warfare which the mind of man has devised.
Of the horrors of the torpedo the American struggle between North and
South affords example, and the following narrative of the blowing up of a large
five-gun vessel, with a crew of a hundred and twenty men, by means of one of
these torpedoes, will suffice.
The explosion took place on a clear afternoon, and was witnessed by many
persons. The boilers, engines, and smoke-stack went up 20 or 30 feet, the
boilers bursting at the same time, and the hull of the vessel was reduced to
fragments, while the bodies of the crew were projected high in the air, and in
many cases their garments were, by the force of the explosion, rent from their
bodies, and heads, arms, and limbs were scattered in all directions. Not one
of that crewx:ame down alive. This vessel was destroyed in the James River,
and stopped the advance of the Federal fleet for a week. Again, on the I5th
of December, 1864, while in Mobile Bay. the gun-boat "Narcissus" was blown
up by a torpedo, and crew and vessel annihilated in a moment. Admiral
Farragut's ship, the " Richmond," which happened to be within 50 feet of the
torpedo, was also damaged, and several of the men on board frightfully scalded
and mutilated. The Americans may with justice remark, these torpedoes are
infernal things.
The torpedo was first introduced into naval warfare by the Russians during
the advance of the British fleet into the Baltic, at the time of the Crimean
war, in 1854.
342 ELECTRICITY.
These torpedoes, constructed by Professor Jacobi, were small iron tanks,
filled with gunpowder and a charge of chlorate of potash and sugar, and fired
by percussion, that is, coming into contact, as they floated down the stream,
with the hull of any vessel they chanced to meet. An iron rod, projecting,
would strike the ship, and by trie force of the shock be driven back and break
a vessel containing sulphuric acid within the charge, and so explode the mine.
The nature of the construction and ignition of these torpedoes rendered
them as likely to be injurious to friend as well as foe, and in time the percus-
sion arrangement corroded, and, when required for service, they were often
found useless ; and no improved method of construction being then known,
the torpedo was abandoned until the year 1860, when the Austrian government
took up the subject in connection with the defence of Venice, by the closing of
the harbour and three important channels by electric torpedoes.
To Baron Ebner is due the merit of these investigations into the proper
construction and discharge of torpedoes for naval defences, and he also carried
out a series of experiments regarding the destructive effects of certain charges
of gun-cotton and gunpowder under ascertained conditions, but which were
interrupted, after the armistice, after the placing of five torpedoes. In these
experiments nothing really practical was developed by which the torpedo could
be introduced generally into naval and military tactics as an auxiliary to rifled
guns and ironclads ; and to Mr. N. J. Holmes and Commander Maury, the
deep-sea hydrographer, belong the merit of reducing the whole practically into
a system.
In the month of August, 1863, during the meeting of the British Association,
at Newcastle-on-Tyne, Mr. Holmes carried out the novel idea of firing an
i8-pounder cannon at Newcastle each day at i p.m., Greenwich time; the
electric spark to discharge the gun being flashed through the wire from the
Royal Observatory, Calton Hill, Edinburgh, a distance of 120 miles. By
the able assistance of Professor Piazzi Smythe, the Astronomer Royal for
Scotland, the necessary connections at the observatory with the clock
were made; and each day the clock transmitted the current to the gun
at Newcastle, which was discharged at I p.m., Greenwich mean time,
to the gratification of the Association. This time-gun has since been
permanently established, and is daily fired, enabling those at Newcastle,
when they hear the report of the discharge of the gun, to set their watches
and clocks according to correct Greenwich time. The means used for this
experiment were, first, the spark developed from Sir C. Wheatstone's magneto-
exploder ; and, secondly, the excellent chemical fuse of Professor Abel, of the
Royal Laboratory, Woolwich, and which, being inserted into the touch-hole,
in its ignition fired the gun.
Commander Maury, who witnessed this time- gun experiment, was struck
with the importance of the application for the ignition of torpedoes as a means
of assisting the defences of the Confederate armies in the struggle then going
forward. Commander Maury and Mr. Holmes commenced a series of experi-
ments, and the result of their labours may be described as follows : Before
torpedoes can be safely relied upon for defence, the power of ascertaining if
they are in perfect order is necessary, and that the enemy have not destroyed
the submarine connections between the shore and the mine. This is accom-
plished by a peculiar arrangement of parts within the fuse, charge, and mine,
whereby the testing spark shall pass harmlessly through the mine without
exploding it, and telegraphing through the entire series of mines without risk
THE ELECTRIC TORPEDO. 343
or danger. The importance of testing the wires by sending an intensity cur-
rent from Sir Charles Wheatstone's magneto-electric telegraph through the
torpedo is easily explained. According to the old plan, a large charge of
gunpowder was placed under the water, and the two conducting wires con-
nected with it brought to the shore ; and on a vessel passing over it, directly the
terminals were attached to the voltaic battery the explosion took place. If tor-
pedoes arranged in this simple way were placed under the water, and the enemy
became aware of their existence, they would soon go to work to destroy them,
by sending out at night, so as to be favoured by darkness, small boats, with
men provided with the proper tackle, such as ropes and grapnels. On drag-
ging the wires to the surface of the water and simply cutting them, not one
of the torpedoes could then be fired, while those who had charge of them
would not be aware of the circumstance, and expect to be able to fire them at
any moment ; on the contrary, the enemy's ships would be able to pass harm-
lessly over the torpedoes. Mr. Holmes has made a great improvement, by
placing a coil of fine platinum wire in the circuit near to the fuse, so that the
intensity current from a magneto-exploder or telegraph will not ignite the
charge when it is allowed to pass through it. The men having charge of these
torpedoes stationed probably three or four miles away, and the wires being
connected with the magneto-telegraph, they could easily send or receive mes-
sages by currents of electricity passing through the torpedoes from one instru-
ment to the other, and would thus afford satisfactory evidence of all being in
a proper condition to fire. On the other hand, if they found suddenly the
telegraph refused to work, and it was not in their power to send a message,
then they would very likely say, " Depend upon it, our enemies have been at
work and cut the wires. It is useless to waste our time ; we had better employ
some other means of defence to repel their ships, if they attempt to enter our
port."
FIG. 310.
a, The Polytechnic Torpedo, constructed by the writer; and, b, Mr. Holmes's Torpedo made of iron
boiler-plate.
We will now imagine that the wires have been satisfactorily tested, and that
the moment has arrived for igniting the torpedoes. It may, however, be
naturally asked, if intensity electricity will not fire them, how can they be ex-
ploded ? We must here recollect, there are two qualities of electricity intensity
and quantity. The former is used only for testing the wires ; the latter, quan-
tity or accumulated, is used by Mr. Holmes for igniting torpedoes constructed
344
ELECTRICITY.
FIG. 311.
according to his plan. The Messrs. Elliott, of Charing Cross, have arranged
a very convenient and portable frictional-plate electrical machine, with con-
denser or small Leyden jar combined, and, being made of ebonite, it is always
in good working order. The wires being detached from the telegraph and
connected with this machine, which becomes charged by about thirty revolu-
tions, on pulling a little trigger at the side, the contact is made with the torpedo
and the Leyden jar, and the quantity-spark is discharged in the centre of the
fuse, and the explosion instantly takes place.
These torpedoes in practice are usually arranged in groups of three, and
consist of vessels similar to steam-engine boilers, made of thick plates of
iron riveted together, each one being charged with 500 Ibs. of gunpowder.
It can easily be imagined that one of these, placed 16 ft. under a ship and
fired, would be quite sufficient to blow it into the air.
We give a drawing of the miniature torpedo experiment performed so fre-
quently at the Royal Polytechnic, Mr. J. L. King, an old and much-respected
pupil of the writer, now lecturer at the Institution, superintending the arrange-
ments. A copper cylinder (Fig. 310, a\ containing a few grains of gunpowder,
and covered with bladder, is sunk in the centre of the great tank, to a depth
of about 2 ft., and when the spark is passed it explodes, bio wing a model ship high
into the air, to the great delight of all small warriors of the rising generation.
THE ELECTRIC LAMP.
The young people who may read this book will, no doubt, be glad to hear
that they can now experiment with the electric light at a very moderate cost,
with a new and beautiful apparatus constructed by Mr. John Browning, of
in Minories.
THE ELECTRIC LAMP.
345
The apparatus, Fig. 312, is most simple and effective, and, with a small
Grove battery and a moderate-sized lamp, an electric light may be procured
at about the cost of two or three pounds. There is no clockwork ; and the
regulator is simply an electro-magnet which cannot very easily be put out of
order. The writer can strongly recommend this apparatus to those who want
a cheap and good lamp. Of course, when an electric lamp i| required to be
constantly used, and is subjected to much wear and tear, more substantial
arrangements are required, as in Serrin's lamp.
FlG. 312. Ar. Browning's new and
cheap Electric Lamp.
FlG. 313. Serrin's Automatic
Regulator of the Electric Light.
In the following description of Serrin's apparatus, Figs. 313 and 314
represent a vertical section and plan (at the height of the electro-magnet) of
apparatus ; it consists,
First, of a motor or driving power A, A', A", forming at the same time the
motor and holder of the positive electrode, as a motor; it is furnished with a
toothed rack, and acts by weight on the moving part of the clockwork. The
tube G, G', serves as a guide, which carries a binding-screw stud E, which
serves to receive the wire from the positive pole of the battery.
ELECTRICITY.
Secondly, of a wheel train, composed of four movable parts, B, F, H, I, the
first mover of a single piece forming at the same time a toothed wheel, K, and
pullev, M. The diameter of these last are as two to one, so as to correspond
Hit:
FIG. 314.
and be in proportion, as near as may be, to the different consumption of the
positive and negative carbon points, and thus maintain the point of light
stationary. To effect this, the wheel K, in turning under the action of the
motor, A, A', A", allows the upper carbon point to descend, whilst the pulley M,
by the same movement, winds up the chain N, O, P, and effecting at the same
time the ascension of the lower carbon point. The toothed wheel K, in combina-
tion with the other movable parts, serves to regulate the approach of the carbon
points by the aid of the flyer R, R', which carries the last wheel. This carries
besides a detent C, c', which acts to prevent or permit the approach of the
charcoal points.
Thirdly, of an electro-magnet, Q, wound round with an isolated conducting
wire, communicating by one of its extremities to the binding-stud S, and by
the other to small chains U, P.
Fourthly, the combination of parts, which I will term the oscillating arrange-
ment, and which forms the particular feature of this improved regulator.
a, b, d, oscillating support, properly so called, which is held by two arms of
equal length, a e, d f, jointed, at e f, to the plates or case of the train; the
joints, e f and a d, permit of a vertical to-and-fro movement of about half
an inch, which movement is limited by a stop, ^, oscillating between two
screws, //, /.
The pulley O, which is mounted on the oscillating support and disposed to
receive the chain N, O, P. I call this chain and pulley O the parts of ascension.
The pawl or catch ;;/, also fixed to the oscillating support, engages with or
disengages from the teeth of the detent C, c', according as it is to be raised or
lowered : thus, when the stop g is in contact with the screw fr, the gearing is
free ; if, on the contrary, it separates itself a fraction of an inch, the pawl m
engages with and stops the train, a, b, n, ;/', collars formed on the oscillating
support ; they are bushed with ivory, so as to isolate the lower carbon-holder,
and serve at the same time to guide the latter in its raising movement.
The armature V, fixed to the oscillating support, is formed of a horizontal
plate and two vertical plates of soft iron ; the horizontal part covers the
electro-magnet Q, and the other two are placed at opposite ends of the soft
iron of the electro-magnet, but without touching it. This armature is disposed
in such way that its central horizontal line is higher than the usual lines of
the electro-magnet, thereby assisting the action of the magnet.
The appendix z", P, fixed to the base of the carbon-holder /', q, serves to join
it with the chain N, O, P, electrically isolated at P.
The object of the small chains U, P, is for the passage of the electric cur-
THE ELECTRIC LAMP. 347
rent, while at the same time they serve as a variable counterbalance by raising
the extremity P, and thereby compensating for the loss of weight that the
lower carbon incurs by being used in combustion. The fixed support U of
the chains is electrically isolated. The friction surface s, v, is also isolated at
?y, the helix v, r, serving also to conduct a part of the electric current passing
from the lower carbon to the wire of the bobbin.
The suspension spring has for its object to sustain the oscillating frame
and effect of the weight ; it can be lengthened or shortened at will by the aid
of the button p and the pulley //.
Action of the Regulator. The drawing represents the apparatus in action,
and the carbon points about half used. The carbon points are placed in their
supports A", ^, while the apparatus has no communication with a battery;
the electro-magnet being inactive, the spring / should be lengthened in such
way that the stop, g, presses lightly on the screw h, or in this case the detent
C', c", being in gear with the hook m, is raised to its highest elevation. During
this time the gearing is free, and the charcoal points approach and come in
contact. By connecting the negative pole of the pile or battery to the bind-
ing stud, S, and the positive pole to the screw E, the electricity enters the
apparatus by the latter, passes through the motor, carbon-holder, and the
carbon itself by reason of the contact which exists between the two carbons ; it
will continue its course by the lower carbon, the support 2, g, the small chains
P, U, also the friction surface, j, 77, and helix z/, r, to the conductor wound on
the electro-magnet, leaving by the binding-screw stud, S, to re-enter the battery,
and so on. At the moment of connecting the second pole of the battery, the
electro-magnet becomes active, and the armature, V, is attracted and drawn
down, thus lowering the oscillating frame or system ; the hook then catches
the wheel train. By this action the train is stopped, and the upper carbon
remains stationary, while the lower one being unable to rise, their separation
is maintained, and the voltaic arc formed. The arc being formed, the carbon
points are consumed by transfer and combustion; therefore the interval
between them increases, while the attractive force of the bobbin becomes
gradually weaker, and the oscillating system raises, its motion being complete
when the screw-stud, -, touches the screw ^, the train liberated at the same time
and acts on the ascending parts, which thus effect the simultaneous approach
of the two carbons ; this approach is effected in relation to the unequal wear
of the carbons so as to maintain the point of light stationary. After the
approach of the carbons the electricity passes more easily, causing the arma-
ture to be attracted more strongly, and to overcome the resistance of the
suspending spring so as to draw down the oscillating frame, when the hook
gears with the train and prevents the carbons approaching until the wear
again produces another approach, followed by another stop, and so on. Thus,
by the alternate opening and closing of the circuit, the movements described
can be reproduced and continued at pleasure, and so forming and determin-
ing the voltaic arc by its self-action. After the preceding, it will be seen that
it is the oscillating system or frame which determines all the actions of the
regulator, and separation and adjustment of the carbons. The seven principal
pieces of which it is composed produce the different effects under the action
of electro-magnetic apparatus in communication with the voltaic arc.
The recapitulation of the seven principal parts which compose the oscilla-
ting system or frame are the following: First, the suspension spring t;
secondly, the stop^y thirdly, the electro-magnet Q and V; fourthly, the carbon-
348
ELECTRICITY.
FIG. 315. Illumination of the Ball and Cross and Dome of St. PauFs
by the Electric Light.
holder z, q; fifthly, the ascending part O and N, O, P ; sixthly, the hook of
detent m; seventhly, the compensating chain u, P.
With a number of Serrin's lamps, Captain Bolton and the writer illumi-
nated Trafalgar Square and St. Paul's Cathedral, on the occasion of the
festivities connected with the nuptials of H.R.H. the Prince of Wales.
The night was unfortunately too foggy to enable even the strongest lights to
pierce the smoke-contaminated atmosphere of London, so that the imagination
(unless, like the writer, the spectator was close to the soot-begrimed dome of
St. Paul's) had to suppose what might have been the effect if the air had been
free from the pea-soup mixture of aqueous vapour and smoke.
FIG. 316. The Shepherd discovering the Magnetic Stone on Mount Ida
with the Iron of his Crook.
MAGNETISM.
The magnetic or black oxide of iron, Fe 3 O 4 , sometimes called the lead-
stone or loadstone, is estimated as one of the most valuable ores of iron,
because it enjoys the property, when freely suspended, of pointing to the
north ; and it does this by virtue of an inherent property which belongs to it,
called magnetism.
The loadstone occurs native, and crystallizes in cubes, and is said to have
been discovered by a shepherd on Mount Ida, who first noticed that the iron
of his crook was attracted by it. (Fig. 316.)
The magnet was not only called magnes, but "lapis Heracleus," from
Heraclea, a city of Magnesia, a part of ancient Lydia, in Greece. It is also
called lapis ncqtticus, because of its use in navigation ; and siderites because
the mineral attracts iron, which the Greeks called o-iSepos.
" The earliest mention in English records of the primitive mariner's Com-
pass is that by Alexander Neckham, who describes the same in his ' Treatise
on Things pertaining to Ships.' Neckham was born at St. Albans in 1157.
A translation of his works, from the Latin, was published in 1866. In the
reign of Edward III., the magnet was known by the name of the sail-stone
or adamant, and the compass was called the sailing-needle or dial, though it
is long after this period before we find the word compass. A ship, called the
' Plenty/ sailed from Hull in 1338, and we find that she was steered by the
sailings-stone. In 1345, another entry occurs of one of the king's ships, called
350
MAGNETISM.
the * George/ bringing over sixteen horologies from Sluys, in Normandy, and
that money had been paid at the same place for
twelve stones, called adamants or sail-stones, for
repairing divers instruments pertaining to a ship."
Fine large pieces of loadstone are usually mounted
in handsome brass or silver boxes, and were highly
prized in the reign of King Charles II., when the
Royal Society of England began to exert itself in
the acquisition of scientific knowledge.
When examined with a magnetic needle, the
mineral is found to have two points where the mag-
netic virtue exists in the greatest intensity : these are
called poles, and are connected with the pieces of
^ soft iron which protrude from the case containing
. 3 I 7- the loadstone; they take off the friction and wear
A mounted Loadstone. and tear of the mineralj whilst all cutting of the
stone, in order to obtain a hollow space between the
two poles, as in an ordinary horse-shoe magnet, is avoided. The magnetism
from the loadstone is easily conferred upon and retained by hardened steel.
FIG. 3.18. Two Bars of Steel,
Each marked N and s at their opposite extremities, and connected by pieces of soft iron, called
" feeders."
It is only necessary to rub the steel or drag the loadstone round in one
direction, taking care to put the pole N of the latter on the end of the steel
bar marked S. An assemblage of steel plates in the form of an
elongated horse-shoe, when carefully magnetised and fixed to-
gether, constitutes a kind of magnetic battery having greatly
increased powers. (Fig. 319.)
This would be called a compound horse-shoe magazine
or battery, composed of an odd number of horse-shoe bars of
different lengths. The union of unequal bars produces a step-
like arrangement at the poles, the largest bar being in the
centre, with the pair of bars next largest on each side, and so
on progressively. This peculiar arrangement, with all other
magnetic instruments, may be obtained from Elliott, Charing
Cross, and possesses several advantages, especially when used
to confer magnetism on other pieces of steel.
The magnets (Fig. 320) bearing the name of Scoresby are
composed of many magnetized, laminated-steel plates, com-
bined together so as to act uniformly as one bar, by which
means a powerful magnetic arrangement is obtained. A piece
FIG. 319.
FIG. 320. Scoresb^ s Magnets.
THE MAGNETIC NEEDLE.
35 r
of steel, usually called a needle, when carefully balanced and suspended on a
sharp point with a central hard metal cap, and then magnetized, is called a
magnetic steel needle.
FIG. 321.
It is extremely useful for showing the influence of the magnetism of the
earth as regards the horizontal-directive force, and is absolutely necessary in
showing a repetition of the facts already explained in the article on " Static
Electricity " (page 212), viz., that just as similar electricities repel, and opposite
ones attract, so a north pole of a magnet repels the north pole of the magnetic
needle, and the south behaves in a like manner with the south pole of the
needle. Dissimilar magnetisms attract, therefore, the north pole of a bar
magnet; one of those, shown at Fig. 318, will attract the south pole of the
needle, and vice versa.
At Elliott's may be obtained magnetic needles suspended in a beautiful
manner, so that the needle moves either in a horizontal or in a vertical plane.
When the needle moves in the horizontal plane, it is an ordinary mariner's
compass; but when it is free to move in a perpendicular plane, it however
carefully balanced before magnetizing dips downwards, and points to the
earth like a finger-post, directing the eyes of the student to the terrestrial
power of magnetism which causes the " dip."
FIG. 322. Needle suspended, and dipping towards the Earth.
The direction of the horizontal magnetized needle not only varies daily,
called " diurnal variations," but it has changed during various periods of years.
The magnetic needle does not point due north and south, but in a plane or
352 MAGNETISM.
direction peculiar to itself, called the magnetic meridian, to distinguish it from
the true or terrestrial meridian. Magnetic meridian lines are planes passing-
through the centre of the earth in the direction of the magnetic needle. The
terrestrial meridian is the plane passing through the same place on the axis of
the earth.
The angle made by these two planes is called the declination #/ the needle.
It is determined by measuring the angle which the direction of the needle
makes with the meridian line. The declination was eastward at the .begin-
ning of the iyth century ; it was zero, or o, in 1660, i.e., the needle pointed due
north and south. The declination now changed to the westward, and had
increased to 24 30' in the year 1818, since which period it has steadily retro-
graded, and about ten years ago had reached 21 48' in London.
It would appear from the observations set on foot many years ago by
General Sabine, that the sun and moon are magnetic, and do affect the needle
in its diurnal movements.
FIG. 323.
The marine compass only differs from the ordinary one in being suspended
in such a manner that the motion of the vessel shall not disturb its horizontal
position. The marine azimuth compass (Fig. 323) is a more elaborate mari-
ner's compass, having within the circumference of the inner box sights for
determining the angular distances of objects from the magnetic meridian,
and being hung in detached gimbals.
The dipping needle or inclination compass is also found to vary as the dip
increases, as might be expected, the nearer we approach to the north pole. At
a point in 70 5' of north latitude and 96 46' west longitude on the west
coast of Boothia Felix, a place was discovered by Captain Parry (the north
magnetic pole) where the dipping needle became vertical, and the horizontal
compass ceased to move right or left, or traverse. Captain James Ross dis-
covered the other end of the great terrestrial magnetic power, the south
magnetic pole, to be about latitude 73 south and longitude 130 east.
The student may realise such movements of the dipping needle by laying
one of the bar magnets (Fig. 318) in the centre of a sheet of cardboard on
which a circle has been described.
On moving the dipping needle round the circle, it will be found to take the
vertical position at the poles A A, whilst it becomes horizontal at the equatorial
position B B, i.e. midway between the north and south pole.
The inclination or dip varies like the horizontal declination. At London, it
was 70 27' in 1720, 69 2' in 1833, and 68 51' in 1849; at tne present time it
is about 68 30'.
INDUCED MAGNETISM.
355
The earth being virtually an enormous magnet, whose north pole is in the
southern hemisphere, and vice -versa, must affect all ferruginous matter on
the earth by induction.
FIG. 324.
It was stated, in the article on Electricity, that the term induction would
have to be used again ; and the student is reminded that this is defined to be
the magnetic influence set up by the mere neighbourhood or proximity of a
body the earth, or the loadstone, or a magnetized steel bar having or
possessing the magnetic virtue or force.
By placing variously shaped pieces of soft iron near a powerful magnet,
they are supported or attracted so long as the magnet is kept sufficiently near
them ; but, as the distance is increased, they drop off one by one.
FIG. 325. Variously shaped pieces of soft Iron for showing Induced
Magnetism.
The magnetic power so quickly conferred on soft iron is as rapidly lost
when it is removed from the disturbing cause, reminding one of conductors
of electricity, which cannot maintain polarity ; whereas steel, which acquires
magnetic power more slowly, retains it with a tighter grasp, and, like non-
conductors of electricity, glass, wax, &c., can maintain magnetic polarity.
23
354
MAGNETISM.
On the supposition that all terrestrial magnetism has an electrical origin,
and is produced by currents of electric force which circulate around the globe y
a very pretty piece of apparatus is constructed, consisting of a distribution of
wires, covered with silk, over a terrestrial globe in parallel lines of latitude.
FlG. 326. Model made by Elliott,
Showing that electrical currents circulating around a globe produce magnetic currents.
The dipping needle and horizontal needle held in different positions on the
surface of the globe, whilst the wires are connected with the voltaic battery,
give the student a very good notion of the natural directive power of the
magnetism that exists over the surface of the earth on which we live, and also:
illustrates again the "inductive" power of magnetic force.
The force which rules the position of the magnetic needle is neither attrac-
tive nor repulsive, but simply directive. A magnetic needle floating on a cork
neither advances nor moves backward ; it simply takes a position nearly north
and south, and places itself in the magnetic meridian.
The engraving, Fig. 328, is a correct copy of the photographic curves of
the self-registering " Declination Magnet ograph," as used at the Magnetic
Observatory at Stonyhurst College, near Blackburn.
This is one of a series of magnetic instruments which are self-registering
night and day ; and it is interesting to notice in the photographic curves the
amount of disturbance shown between the 8th and loth of August, 1868. The
instruments are under the charge of the Rev. S. G. Perry, who has most kindly
furnished the following drawing and description of the Magnetic Observatory
at Stonyhurst:
" An idea of the disposition of the instruments may be formed from the
drawing (Fig. 327), and a very brief description will make it still more clear.
" The instruments record the oscillations of three magnets suspended under
the glass shades ; and we thus get completely all the changes, both as to direc-
tion and intensity, in the earth's magnetism. The magnet which is to the
right in the sketch is suspended by a silk thread in the magnetic meridian,
and, by the aid of a mirror attached to it, describes on a cylinder, which is put in
motion by the clock on the centre pier, all the variations in the magnetic de-
clination. The other two magnets give the two components of the total
magnetic force of the earth. That which records the variations of the vertical
A MAGNETIC OBSERVATORY.
355
FIG. 327. The Magnetic Observatory at Stonyhurst College.
component rests, under the shade near the doorway, on two agate edges ;
whilst the horizontal-component magnet is suspended by a double steel thread,
under the shade to the left of the picture, and is held nearly at right angles to
the magnetic meridian by the torsion of the thread.
" Under the clock-box, which stands in the centre, are the three cylinders
covered with sensitive paper. To each magnet is attached a semicircular
mirror, which sends the rays from a jet of gas to one of the cylinders in the
clock-box, and thus describes, by a curved line, all the oscillations of the
magnet. A second semicircular mirror is fastened to the pier on which the
instrument stands, and, describing always a straight line on the cylinder which
is opposite to it, gives the zero line for the curve.
" These magnetographs were constructed by Mr. Adie, and are similar in
nearly every respect to those made for the Kew Observatory, under the direc-
tion of Mr. Welch.
" The magnetic room is built underground to prevent sudden changes of
23 a
356
MAGNETISM.
FIG. 328.
temperature, and we have been so for-
tunate that the daily range is scarcely
over 0-2."
It is curious that every kind of vibra-
tion assists the magnetization of iron
or steel by terrestrial magnetism. If
half-a-dozen iron wires, 12 or 15 in. in
length, are twisted strongly together
whilst held in the- direction of a dip-
ping needle, viz. 68, they become very
magnetic, and, having now distinct
poles, will affect a magnetic needle like
a steel-bar magnet. Iron columns,
guns, the plating of ships of war, car-
goes of iron or steel, all acquire mag-
netic power; and, until this fact was
understood and provided for, many
disastrous shipwrecks were caused by
the compass pointing in the wrong di-
rection, and thus conducting the un-
fortunate ship to the rocks, instead of
keeping her in mid-ocean. Mr. Barlow
has devised certain means by which the
compasses of ships may be corrected,
and the influence of local magnetic
attraction, due to the guns, or shot, or
other iron or steel cargo, neutralized, so
that the " directive " force of terrestrial
magnetism alone shall guide the ship
over the pathless ocean. A late and
lamented friend of the writer (Mr. Evan
Hopkins) tried a vast number of expe-
riments, and wrote an interesting pam-
phlet on terrestrial magnetism, with re-
ference to the compasses of iron ships,
their deviation and remedies.
It is impossible in our limits to do
justice to the arguments brought for-
ward and discussed by Mr. Hopkins;
but the remarks made at the termina-
tion of the debate at the Royal United
Service Institution on his paper will
give the reader some notion of the
opinions entertained by the meeting
on the method of destroying the po-
larity of iron ships, as proposed by Mr.
Hopkins.
"Sir FREDERICK NICOLSON: The
subject has been treated in an eminent-
ly practical way. In the abstract of
Mr. Hopkins's papers, I find that there
HOPKINS' S EXPERIMENTS. 357
is one statement which appears to me the most important, that is, Mr. Hopkins
says he is prepared to destroy the polarity of any given ship in ten minutes.
The only question I wish to ask, because the gist of the paper lies in that
assertion, is whether Mr. Hopkins has performed that operation upon any ship.
" Mr. HOPKINS: No, not in any ship as yet; but I have made experiments
on long bars and plates of iron, and I am quite satisfied that I can produce
the same results on the iron plates of a ship. In reply to the observations
which have been made I will not detain you long, because I do not think the
remarks made require lengthy replies. First, with regard to Sir Edward
Belcher's remarks, he said that I stated that there was no magnetic pole. I
did not state that there was no magnetic pole ; on the contrary, I have endea-
voured to explain that the entire areas bounded by the antarctic and the arctic
circles are the great magnetic poles of the earth, towards which all the magne-
tic meridians converge. I do not mean to say for one moment but that a
dipping needle at the north latitude of 70 approached nearly 90, observed
by Sir James Ross, and probably over a great number of square miles in that
region ; but I have seen dipping needles approaching 90 near the equator.
There are many places in the islands of Scotland, also in Norway, Sweden,
and Russia, where the dipping needle will not only approach 90, but remain
at 90. Therefore I repeat that the dip of the dipping needle does not neces-
sarily depend on the action of the terrestrial pole, but on local attraction.
Besides, neither experiments, analogy, nor observations on the magnetic
meridians support the notion of the magnetic pole being merely a mathema-
tical point near Boothia Gulf. We have only to prolong the observed magnetic
meridians to the circle of 70 of latitude to show the fallacy of the Boothian
pole. We must be guided by the meridians of the needles to determine the
position of the active polar areas. Go to Norway; go to Sweden; where do
the needles point? Do they point to Boothia Felix? No; they do not.
They point towards the arctic region, and not to any special point. With
regard to the other point that Sir Edward stated with reference to the com-
pass, I do not believe there is a possibility for the compass to point correctly
unless it be left entirely under the control of the great terrestrial force : any
interference, whether by magnets or electric appliances, can only increase the
confusion and danger, and therefore the compass should not be tampered
with, but left to act freely and under the sole influence of terrestrial magnetism.
With regard to what Captain Selwyn stated about the steering co'mpass. He
said, ' Never mind that ; I believe you do not care much for the steering com-
pass ; you go by the standard compass.' Well, there is now always a difference
between the standard and the steering compasses. We know that in iron
ships that difference constantly varies. You do not know what the variation
is that is constantly going on. Were you certain of the exact amount of
variation, it would be like the watch and chronometer spoken of by Captain
Selwyn ; but you cannot compare the case of your watch and chronometer
with those of the standard and steering compasses when you have an iron
vessel, and where you have a perpetual change going on in the action of the
polarity of the iron vessel. With regard to the reflector, I see Captain Selwyn
apprehends difficulty. I see none, and the appliance is already appreciated
by several experienced captains. I do not think there would be much diffi-
culty in seeing a compass, with a good strong light, with a 1 2-inch card at a
distance of even 30 feet. However, I leave that to others. There is one thing
Sir Edward Belcher mentioned with regard to the needles. I am perfectly
358 MAGNETISM.
familiar with all the needles they use in high latitudes. They are utterly
worthless in directive power. As to the dipping needles, they have no direc-
tive power whatever, and, as justly observed by Captain Fishbourne, have no
lateral directive power at all, and cannot therefore serve as guides to deter-
mine questions connected with meridian lines. The curved magnetic needle
will act where neither the straight nor the dipping needles can be rendered
serviceable in high latitudes. It only remains for me, in conclusion, to thank
you for the patience and kindness with which you have listened to the obser-
vations I have made.
" The CHAIRMAN : I am sure there will be but one opinion among you as
to a vote of thanks to Mr. Hopkins for the very interesting paper he has read.
He has brought forward some of the old ideas relating to magnetism, which
many here were not acquainted with, and he has given us some new ideas. I
must say that his idea with respect to the bent needle is one which I think is
deserving of a trial. I must also say I should like to see that dissipation of
the polarity of a ship tried, although I am afraid that the soft iron of the ship
would become magnetised by some other extraneous cause at present un-
known. I really believe this, although we are very thankful to him for what
he has told us, that we shall still find it positively necessary to have recourse
to observation. I hope what you have heard to-night will strengthen your
confidence in the compass as a means of steering. There is another remark
about the pole. As I have passed within 70 miles of it, and the dip was 89
47", I must say that I can only look upon the pole as capable of being defined,
not perhaps exactly as a point, but very nearly as a point, because as I passed
up, I changed from 89 47" north dip to 89' 46" south dip. With respect to
the deviation of the compass, it has been an old thing with us who have been
in high latitudes. We know perfectly well that we suffer the same incon-
venience which is experienced now in iron ships. In Behring's Straits, in
going about there, the deviation of the ship amounted to six points of the com-
pass ; and I can say, which I have no doubt Captain Maguire will corroborate
me in, that we should have had the greatest difficulty in the world to take our
ships up into the position we did, if it was not for the admirable charts of
Admiral Bechey, and in which expedition Sir Edward Belcher served. There
is only one other point. I will say that I have listened to this paper with a
great deal of gratification and pleasure, because, during the course of my
service in the Arctic regions, it so happened that for two years I was not able
to use a compass at all ; therefore, I am able to appreciate anything that will
increase the value of it."
The sequel is soon told, for Mr. Hopkins caught a violent cold whilst
engaged in attempting to depolarise one of the iron-clads ; and, although he
partially recovered, his system received a shock which ended in death. His
kind and enthusiastic spirit was spared the disheartening report of the non-
success of his method, subsequently brought before the Royal Society.
Mr. Barlow corrects the local magnetic power of the iron of the ship by
placing a piece of soft iron in a particular position, so as to compensate for
the derangement of the compass produced by the anchors, chains, guns, &c.,
of the vessel.
Amongst the latest practical applications of magnetism to useful purposes
is that of Mr. Saxby, who proposes to test the iron of guns by magnetic power.
Mr. Paget, C.E., in a very able paper in " The Engineer," thus reports on the
process or method of Mr. Saxby for testing iron*
SAXBY'S EXPERIMENTS. 359
" It is well known to engineers that it is a most difficult and often impos-
sible thing to find out the existence of a false weld in a forging, or of a blow-
hole or honeycomb in an iron or steel casting. The only safe way of doing
this is by carefully measuring the elongation of the piece under a given load,
as with a false weld all the work is thrown on the diminished area at the
defective weld, and the thicker parts are scarcely extended by the force which
is perhaps rupturing the bar at the flawed spot. It need scarcely be said that
> there are many important cases where this process, or the equivalent but
dangerous one of trying the effects of an impulsive force, could neither be
mechanically nor commercially practicable. Every one knows that a simple
method by which internal flaws and solutions of continuity in constructive
details could be easily detected would be of enormous value to the world.
Such a method has undoubtedly been discovered by Mr. S. M. Saxby, R.N.,
who has very judiciously been allowed by the Admiralty, during the course of
this year, to experiment with it in the royal dockyards. Though compara-
tively new, and not yet completely worked out, the process will possibly have
a yet more extended application than finding out only mechanical flaws in
iron, and possibly in cast iron and steel.
" The principle upon which Mr. Saxby's method is founded is so simple
that it certainly seems strange that it had previously escaped notice. It has
been known for nearly a century and a half that when a bar or any mass of
soft iron is placed in the position of the dipping needle, it is at once sensibly
magnetic, the lower extremity being a north pole in our latitudes, and the
upper extremity a south pole. In the southern hemisphere the poles are of
course reversed. The same action, only weakened, takes place in a bar hang-
ing in a vertical or any other position ; only the effect is weaker the more the
position of the longitudinal axis of, for instance, a long bar departs from that
of the magnetic dipping needle.
iv
A * *
a
FIG. 329.
" When, therefore, as in Fig. 329, a small compass needle is slowly passed
in front of a bar of very good iron, placed in an east and west direction, the
needle will not be disturbed from its proper direction, which is of course at
right angles to this, or north and south.
" All this refers to regularly homogeneous bars of best quality to bars
-without any mechanical solutions of continuity. With internal flaws or inter-
ruptions of continuity the bar is no longer regularly magnetic. It has long
been known that a good compass needle, or a good permanent magnet, must
be homogeneous and without flaws in order to take and retain its maximum
amount of magnetism. In a word, any mechanical solution of continuity is
accompanied with a polar solution of continuity, and the given bar or mass
with flaws whether permanently magnetized or temporarily so by the indue-
360
MAGNETISM.
J
-AROUND 12 INS. LONG
FIG. 330.
tive action of the earth is no longer one regular magnet, but several different
magnets, with the different magnetisms separated from each other. The
delicately-poised magnet of a compass can thus be made to tell the presence
of such solutions of continuity. The above drawing (Fig. 330), showing the
actual results of the test with a f- in. bar, 12 in. long, will illustrate the manner
in which the compass magnet is affected by the presence of cracks, of solutions
of continuity, in the bar, which is supposed to be lying in the equatorial
magnetic plane, or east and west.
" By the enlightened permission of the Admiralty Board, Mr. Saxby, as
stated, has already been allowed to test his method in various ways in the
royal dockyards of Sheerness and Chatham, and we will describe some of the
practical results of these experiments. Amongst these were a number of very
remarkable trials conducted in the presence of the master smiths, the foremen
of the testing-houses, and several of the chief engineers of the royal navy.
I.M.SQR.
1 IN.SQR.
BROKE AT 24 TON 3
'8 TONS
ANNEAL
ED D \D
7JTONS
!
o E:
Si TONS
O OFF SAME BAR. AS D.
NOT ANNEALED
ROUND
J^" ROUND
AROUND
FIG. 331.
Mr. Saxby, for instance, was requested to find out the weakest spots in a
number of bars, and to tie a string or make a chalk mark on each spot.
Immediately afterwards all these bars were put into the testing machine and
broken. Their history is given above, in the annexed cuts (Fig. 331), the
prediction having in every case been verified. The bars are shown by lines
to scale, and a scroll is placed where the weakest part was found out by the
needle. The vertical dotted lines indicate the spots where the several bars
broke.
" The smiths of the royal dockyards seem to have properly tried Mr. Saxby's
powers in almost every possible way, and most ingenious devices were some-
SAXXY'S EXPERIMENTS. 361
times resorted to for the purpose. As examples out of many, in the centre of
a bar (Fig. 332) of I in. square forged iron was welded a piece of unmagnetized
steel about 5 in. long. The needle detected a fault at about the centre of the
piece of steel.
LENGTH
< -5.INS ->
FIG. 332.
" Now Mr. Saxby's method can detect the presence, and negatively of course
the absence, of small or large solutions of continuity. It can detect false
welds, smaller flaws caused by bad workmanship or wear, and, we believe,
what is commonly termed ' crystallization,' which will, probably, at once be
generally acknowledged to consist in a disruption or parting of the facets of
the amorphously arranged crystals of which iron is built up. It can, of course,
only detect the results of the chemical constitution of iron, as evidenced in
the less perfect cohesion of the crystals when alloyed, in relatively consider-
able quantities, with foreign bodies. There is little doubt that the magnetic
method is a test of the homogeneous character of the iron and of its freedom
from fissures and cracks, and so far it undoubtedly forms a test of quality. It
will appear scarcely credible that a common pocket-compass needle should
be able almost like the divining rod said to be used for finding out springs
of water to discover important defects in large iron bars. A mere statement
of the fact does sound almost incredible until the simple means actually
employed are explained."
Amongst the influences which open the pores of the steel, as it were, to
receive a full charge of magnetic force is that of heat, and it is found that
when steel is made red hot, and allowed to cool in the direction of the mag-
netic dip, it acquires more quickly and largely the magnetic charge.
It was contended by Mrs. Somerville that unmagnetized needles were
magnetized if exposed to the violet ray of the spectrum ; but Riess and Moser
have shown that these effects only take place when the needle is perpendicular
to the magnetic meridian, facilitated by the heating of the needle, first by ex-
posure to the violet rays, and secondly and more especially by the subsequent
cooling.
A powerful steel magnet, heated to a white heat, loses its magnetic power.
Red-hot iron is no longer rendered magnetic by induction.
Nickel, raised to the temperature of boiling oil, loses its magnetic virtue.
It ought to be mentioned here, that certain metals, nickel and cobalt, have
distinct magnetic powers ; and Sir Charles Wheatstone has given a very in-
genious and elegant method of detecting minute quantities of magnetic force.
He says
" If a short sewing needle, A (Fig. 333), the eye end being broken off, rest
upon its point on the polar surface of a powerful bar magnet, it will generally
take a position inclined to the surface ; but a locality may generally be found
in which the needle will stand nearly vertical ; this point may be ascertained
by placing a piece of unglazed paper, D, between the needle and the magnet,
and moving it about until the vertical position of the needle is obtained.
" If we elevate the paper and needle above the magnet to the greatest
362
MAGNETISM.
distance at which the needle will remain vertical, it becomes to the last degree
sensitive of magnetic force ; so that by bringing specimens of nickel or cobalt,
which have the least magnetic power, or any impure metal, such as a specimen
of metallic manganese, which Faraday thought he had proved (when entirely
free from iron) does not indicate the slightest magnetic power,* or rhodium,
iridium, or hammered brass, if the latter metals contain any iron, they will
affect Wheatstone's test needle, but not otherwise."
FIG. 333.
There are other influences that may affect the magnetic needle. When a
plate of copper is rotated quickly (say 800 revolutions per minute) beneath
a suspended magnet, the latter also is thrown into rapid rotation.
FIG. 334.
It might be thought that this was brought about by the motion of the air;
but the same effect occurs even when the copper plate rotates in a vacuum,
or is wholly screened by glass from the magnetic needle.
* See Dia- magnetism, for further information.
ROTATION EXPERIMENTS.
363
The apparatus (Fig. 334) exhibits this curious property of metallic plates in
motion, and is usually made by Elliott with a variety of metallic plates, all
of which, when spun round rapidly, first cause the magnetic needle to deviate
from its natural position, and then finally 'to assume rotation.
When the experiment is reversed, and a compound bent magnet is caused
to revolve with great velocity about its axis of symmetry, and below the metallic
plate, which is carefully suspended, then the latter commences revolving in
.the same direction.
FIG. 335.
All these experiments have arisen from the original one performed by Arago,
who first tried the effect of a ring of copper upon the oscillation of a delicate
magnetic needle which it enclosed. In free space the magnet performed 420
oscillations before it reached an arc of 10, whereas, when surrounded with
a copper ring, they were reduced to fourteen oscillations ; under the same
circumstances in a ring of wood, the oscillations were reduced from 420 to
.about 300.
DIA-MAGNETISM.
In the preceding chapter, it has been
pointed out that the loadstone, iron and
steel, cobalt and 'nickel, possess ordi-
nary magnetic powers, and can attract
or repel a magnetic needle. We have
now in the beautiful experiments first
made by Faraday to consider the mag-
netic powers of other substances, and
shall discover that a vast number of
bodies are affected by magnetism when
produced by and circulating from pole
to pole of a very powerful electro-mag-
net, such as that depicted at Fig. 336.
The dia-magnetic apparatus is spe-
cially designed to illustrate Faraday's
celebrated experiments on the dia-mag-
netism or para-magnetism of bodies,
and the effect on light in the rotation
of the plane of the polarized ray, c.
Besides these very extensive and
varied applications, the actual lifting
power of the electro-magnet is easily
found by turning the poles downwards,
when they face the armature connected
with the compound-lever arrangement.
The power obtained with a single cell
of Bunsen's, of very small size, will lift
5 cwt., and with twenty Grove's cells
this magnificent apparatus will lift 3
tons. It was exhibited before the
Royal Society, April, 1868.
In the experiments, which will pre-
sently be detailed, there are certain po-
sitions constantly referred to, z>., the
positions which various bodies may
assume between the poles of the electro-
magnet (Fig. 336). Thus the space
between the two poles is called the mag-
netic field, and a straight line drawn
from pole to pole, like the poles of the
earth, is called the axial line, similar to
the imaginary line around which the
earth rotates, called its axis. Any body
subjected to the action of the magnetic
current is said to place itself axially
when it takes the above direction. If,
, however, the body under experiment
I- IG. 336. takes a position at right angles to this
Apparatus made by Mr. Apps, direction^ it is said to point equatorially.
Which maybe used either for dia-magnetic expe- fl-,,,* ; Trio- 0^7 fh P nnlfs are renro-
rimentsor to show the enormous weight which "=,*" -i *& jj/, LIIC puics CMC lt F 1< -
can be supported by a powerful electro-magnet. SCnted by pieces of soft iron bevelled
DIA-MA GNETISM.
3 6 5
FIG. 337.
off to a rough point ; and if a rod of iron is suspended between them and the
electro-magnet connected with the battery, the rod takes up an axial position,
whilst a similar rod of bismuth, also suspended by a filament of silk, places
itself at right angles to that position, as is shown at Fig. 338.
EQUATOR] A UV
ISHdUTH
FIG. 338.
In all these experiments the poles of the magnet, with their soft-iron arma-
tures, are surrounded with a glass box, like the lantern of a balance, to prevent
the action, of currents of air. Faraday discovered that when the crystals or
solutions of salts of metals that are magnetic, such as ferrous sulphate,
are placed in a glass tube which is not magnetic, they do, as a general rule,
place themselves axially. Cobaltous and nickelous sulphate behave in the
same manner ; and this axial position is always maintained, provided the metal
enter into the basyl of the salt.
FIG. 339. The Cube of Bismuth taking the Equatorial position.
When a single pole of the electro-magnet is used, repulsion takes place with
very many bodies, and, of course, if the substance is repelled by both poles
when placed in the magnetic field, it will take a place at right angles to the
magnetic current, or the equatorial position.
3 66
MAGNETISM.
Phosphorus, bismuth, and antimony the first a non-conductor of elec-
tricity, and the second and third metals therefore conductors are each and
all repelled from a single pole, or place themselves in the equatorial position
between the two poles.
It is most amusing to twirl a suspended halfpenny between the poles of the
electro-magnet (Fig. 340). Of course this may be done as often or as long
as the experimenter pleases ; but if, whilst the coin is rotating, the electro-
magnet is connected with the battery, the halfpenny stops dead, and instantly
places itself in the equatorial position.
FIG. 340. The Halfpenny twirled, then stopped by the magnetic force.
The preceding experiments show that those bodies which are not magnetic
will exhibit dia-magnetic properties, i.e., they are substances through which
the lines of magnetic force (represented by the beautiful curves assumed by
iron filings when sprinkled on a sheet of cardboard held over the poles of a
powerful magnet or, still better, an electro-magnet) pass without affecting them
like iron, cobalt, or nickel.
This mode of experimenting is more delicate as a test for magnetism than
the use of the needle, already alluded to at page 362, Fig. 333.
And it was by taking solutions of pure salts of manganese and chromium,
and placing them in the magnetic field, that they were discovered to be mag-
netic, whilst as metals it was so difficult, if not almost impossible, to obtain
them in the pure state and free from iron. (Fig. 341.)
FIG. 341.
Faraday, always so exact and orderly in his classification and nomenclature,
proposes to include all the phenomena under one general title, viz., that of
magnetism, and to subdivide this into para-magnetic and dia-magnetic phe-
nomena. A very long list, originating with Faraday, has therefore been framed
n this principle.
DIA-MA GNETISM.
367
Fara-Magnetic, usually
called Magnetic.
Dia-Magnetic.
Fara-Magnetic, usually
called Magnetic.
Dia-Magnetic.
Axial.
Equatorial.
Axial.
Equatorial.
Manganese
Lead
Sulphate of zinc
Litharge
Nickel
Cadmium
Shellac
Phosphorus
Cobalt
Sodium
All sorts of iron, \
Common salt
Iron
Mercury
where the latter >
Nitre
.Titanium
Zinc
is basic )
Sulphur
Palladium
Tin
Vermilion
Resin
Cerium
Bismuth
Tourmaline
Spermaceti
Chromium
Antimony
Charcoal
Iceland spar
Platinum
Arsenic
Oxygen, which ^
Tartaric acid
Osmium
Silver
stands alone as f
Citric acid
Paper
Gold
a para-magne- f
Water
Sealing-wax
Copper
tic gas )
Alcohol
Berlin porcelain
Tungsten
Salts of chromium
Ether
China ink
Uranium
Salts of manga- )
Sugar
Plumbago
Rhodium
nese )
Starch
Peroxide of iron
Iridium
Oxide of titanium
Gum arabic
Fluor spar
Alum
Oxide of chro- )
Wood
Asbetos
Glass
mium J
&c., &c.
Silkworm gut
Rock crystal
Chromic acid.
Red lead
The mineral acids
Nitrogen.
Nitrogen is like a vacuum it is neither para-magnetic nor dia-magnetic ; it
is, in strict reason, like space, with reference to these experiments ; it is a zero,
or a starting-point.
The magnetic or dia-magnetic property of a body, curious to say, varies
according to the medium in which it is placed : thus, a glass rod, suspended
horizontally in water, which we find, with glass, in the dia-magnetic column,
points axially, like any ordinary magnetic body ; but if the same glass rod is
suspended in a solution of ferrous sulphate, a magnetic body, it points equa-
torially.
The magnetic-field test discovers whether a metallic salt has the metal in
the basyl, the basic, or electro-positive state ; or whether the metal is simply*
a part or constituent of the acid or electro-negative compound. Iron is basyl
in ferrous sulphate, and sets axially, and is para-magnetic ; but in potassic
ferrocyanide it forms part of the ferrocyanic acid, and therefore the crystal
sets equatorially, and is dia-magfnetic.*
The reader will find all the apparent exceptions and peculiarities attending
their structure in Tyndall and Knoblauch's paper (Phil. Mag., 1850, vol. xxxvi.,
p. 178, and xxxvii., p. i). The same gentlemen have discovered that dia-
magnetic repulsion is as the square of the intensity of the current ; and Reich,
Weber, and Tyndall seemed to have proved that which foiled Faraday, viz.,
that bodies under dia-magnetic influence exhibit polar characters. The polarity
is the reverse of all other polarities, electrical or magnetic : the feeble polarity
of a dia-magnetic substance is the same as the pole of the magnet in its neigh-
* The same test will discover, for Instance, m a roll of paper, whether it contains iron or not: if
i' contains the metal, or is coloured blue with cobalt, it will set axially, because iron and cobalt ate
magnetic; 01, to use Faraday's phraseology, para-magnetic.
3 68
MAGNETISM.
bourhood ; whereas we have learnt that north induces south magnetism in a
piece of iron, and vitreous electricity induces negative in the body to which
it is approached.
The dia-magnetism of gases was first shown by Father Bancalari, of Genoa,
who discovered that flame, such as the flame of a candle, was influenced by
the poles of a powerful electro-magnet.
FIG. 342. Effect of the Poles on Flame.
Faraday tried Bancalari's experiment, and found that when the axial line of
the magnet was horizontal, and the flame of a taper held near it, and on one
side or the other, with about one-third of the flame rising above the level
of the upper surface of the poles, the flame seemed to be repelled away from
the axial line, moving equatorially until it took an inclined position, as if a
gentle wind was acting upon it, and causing its deflection from the perpendi-
cular line.
It was the flame experiments which led to the discovery of the magnetic
property of oxygen, and of the dia-magnetic properties of atmospheric air,
nitrogen, hydrogen, coal gas, olefiant gas, &c.
Faraday showed that soap-bubbles, filled with various gases and blown
from the end of a capillary tube, were either attracted or repelled according
as the gas was magnetic or dia-magnetic.
FlG. 343. Melting Ftisible Metal between the Poles of the great
Electro-Magnet.
One of the most curious experiments which may be performed with the dia-
magnetic apparatus is that of overcoming the equatorial or para-magnetic
force by physical power. The twirled penny-piece brought to rest between
the poles, if forcibly turned round, will by the motion generate heat, and may-
be made very hot.
DIA-MA GNETISM.
3 6 9
If a brass tube, containing some solid fusible metal, composed of two parts by
weight of bismuth, one of lead, and one of tin, with a few drops of mercury, is
rotated very fast by a whirling-table wheel between the poles of the powerful
magnet, no effect is produced until contact is made with the battery, and then
the rotation or motion is speedily converted into heat, and the fusible metal is
melted as if it had been held over the fire. Here again is a perfect conservation
of force. The heat which melted the alloy is the exact equivalent of the
chemical power of the battery used, although it acts by an intermediate force,
viz., magnetism ; but the chemical action produced the electricity, the current
electricity produced the magnetism, and, the magnetic force which tends to
keep the bismuth in the alloy in the equatorial position being overcome and
resisted by physical force, the muscles of the arm acting on the whirling table
eliminate heat.
Faraday thought he had proved, by using heavy glass and permitting a
ray of polarized light to pass through it, that the ray was affected by the
powerful magnetic force eliminated from the great electro-magnet. Faraday's
glass consists of a mixture of silicate and borate of lead, and is much denser
than ordinary glass. If a ray of polarized light is allowed to pass through it,
and is then examined in the ordinary manner with an analyzing plate or a
bundle of plates of glass, or by a tourmaline or a Nicol's prism, the light, of
course, disappears, as already explained in the article on Light, when the plane
of reflection from the analyzing plate is at right angles to the plane of polari-
zation. (Fig. 344.)
FIG. 344.
If now the battery is connected with the electro-magnet, between the poles
of which the bar or cube of Faraday's dense glass is placed, the light re-appears
instantly, again disappearing when contact is broken with the battery.
Matteuchi found that the effect was increased by increasing the temperature
of the cube of heavy glass to 600 Fahrenheit ; and he also ascertained that
by subjecting the heavy glass to pressure he could change the direction of the
ray of polarized light, as Faraday had done. So that, in fact, Faraday was
wrong ; the magnetic force did not act upon the ray of polarized light, but on
the molecules or particles of the glass, which were under a strain during the
time they were subjected to the action of the powerful electro-magnetic force..
24
370
MAGNETISM.
FIG. 345. Apps's half-horse power Electro-Magnetic Engine.
ELECTRO-MAGNETISM, MAGNETO-ELECTRICITY,
THERMO-ELECTRICITY.
In 1820, CErsted, a Danish scientific man, discovered the connection between
electricity and magnetism. It was not found where philosophers sought for it.
They thought to imitate Nature ; and as some steel knives were found to be
powerfully magnetic after a discharge of lightning had passed through a box
containing them, they subjected other pieces of steel to the discharge of
powerful Leyden 'batteries without producing the effect they expected.
CErsted found that the electricity must be in motion, or in a dynamical state,
such as it would be in when evolved from the voltaic battery.
Static electricity will, under certain arrangements to be hereafter described,
magnetize steel ; but the mere fact of allowing a wire charged with statical
electricity (the force from the electrical machine) to approach a magnetic
needle does not affect the needle like the same wire conveying a current from
a single voltaic circuit or, still better, a battery.
M. Ampere, who took up the subject directly after CErsted had published
his discoveries, laid the foundation of the science of electro-dynamics. He
discovered that every part of the whole circuit the wires, the terminals or
poles, the battery, in fact, all parts exercised a magnetic power upon the mag-
netic needle. He also proved that the force was in an eminent degree one of
circulation. Ampere made himself fully understood by asking his readers to
conceive a man lying down in the circuit, so that the wire lies along his face
ELECTRO-MAGNETISM.
37*
and body. We are now to suppose that the current enters the wire at his feet
and goes out at his head, and that his upturned %ce and eyes are directed to a
magnetic needle suspended parallel with and over the wire conveying the
electric current, so that the north pole of the needle points to his face.
Directly the current passes, the needle is deflected to his left hand ; and by
reversing the direction of the current, and causing it to flow into the wire at
his head and out from his feet, the needle will now move to his right hand.
FlG. 346. Wire conveying a Current of Electricity affecting the Magnetic
Needle.
Thus every possible variation may be imagined as long as we maintain the
same relative positions of the wire and the human body ; and it was further
ascertained that the intensity of the electro-magnetic force is in the inverse ratio
to the simple distance of the magnetic needle from the current; or, in other
words, that the elementary action of a simple section of the current upon the
needle is in the inverse ratio to the square of the distance.
If a single wire can affect a magnetic needle, it is evident that by doubling
and trebling the wire, or winding it round in a helix, the effect must be
enormously increased, provided the coils of wire do not touch each other, or
are covered with some non-conducting material, such as silk or cotton ; hence
it is that coils of wire are constructed so that a piece of soft iron placed
FlG. 347.
inside the core becomes a most powerful magnet directly contact is made
with the battery. When the immense power of the electro-magnet was ascer-
24 2
372
MAGNETISM.
tained, great anticipations were formed of the application of this force as a
motive power. It is not surprising that this should have been the first con-
clusion. Thus the great electro-magnet, made by Mr. Apps, that heads the
chapter on Dia-magnetism, will lift five hundredweight with a single quarter-
pint Grove's cell, and three tons with twenty cells. This conveniently arranged
magnet, after being used for dia-magnetic experiments, may be employed for
showing the attractive force of the great electro-magnet. It is attached to a
lever, which turns it over ; and, when suspended with the poles downwards,
it is connected with a compound-lever arrangement, on the same principle
as railway weighing-machines, and the weights used are one quarter, one half,
and one hundredweight.
The writer well remembers the late Prince Consort, on the occasion of one
of his private visits to the Polytechnic, putting a question to him as to the rate
at which the electro-magnetic power increased or decreased with the distance
from the great electro-magnet belonging to the Polytechnic. The attractive force
diminishes enormously. Thus, in a paper read by Mr. Robert Hunt before the
Institution of Civil Engineers, the following instructive diagram was exhibited:
J- of an inch = 36 ib.
" " 4 "
47
50 .
contact=
220
FIG. 348.
It is shown that, whilst contact gave a power of 220 Ib., at a distance of
of an inch the attractive force diminished to 36 Ib.
FlG. 349. A Dextrorsat and a Sinistrorsal Helix.
"When a wire, traversed by an electric current, is held in iron filings, they
ELECTRO-MAGNETISM.
373
adhere to it as long as the current passes. If the wire is coiled upwards round
a glass tube from left to right, it is called a dextrorsal helix; and if coiled
downwards, and in the same direction, it is termed a sinistrorsal helix.
A piece of steel placed inside such a helix, conveying the voltaic current, is
soon magnetised. If the same coil is used to convey the charge from a Leyden
battery of 6 ft. surface, a piece of steel is instantly magnetized. Electricians
had missed this form of the experiment until (Ersted's discovery.
If a bar magnet be held so that it is horizontal, and the north pole directed to
the vertical portion of the rectangular wire, so supported that whilst conveying
the electric current it moves freely round in a circle (Fig. 350), it will be found
t t I t t T- I/
FIG, 350. The Rectangular
Wire freely suspended on a
vertical Standard.
V
FIG. 351.
that, if the upright portion of the wire is conveying the current from below
upwards, it is repelled, but attracted if the south pole is substituted ; and thus,
by the dexterous substitution of one pole for another in presenting the bar
magnet to the rectangular wire, it may be caused to rotate.
Polarity is shown by the sides of the wire, whereas in steel magnets it is
discoverable at the ends.
The same attraction and repulsion occurs if another electrified wire is
brought towards the suspended rectangular wire whilst conveying the electrical
current.
Fig. 35 1 is a good illustration of the direction of the current circulating
around each section of a magnet everywhere in the same direction, viz., from
top to bottom in the face that is turned towards the moving wire, and from
bottom to top in that which is opposite to it. The sum of these directions
amounts to a current.
A similar result is obtained when a horizontal wire is directed to a magnet
suspended vertically. The magnetic currents circulating around the magnet are
again shown by arrows. A magnet may, therefore, says De la Rive, be con-
sidered as formed by an association of electric currents, all circulating in the
same direction around its surface, and all situated in planes parallel to each
other, and perpendicular to the axis of the magnet. It is this hypothesis <. f
Ampere of the constitution of magnets, shown in Figs. 35 1 and 352, and which
374
MAGNETISM.
FIG. 353.
FlG. 352. Magnet suspended in a perpendicular
line, the Current flowing horizontally.
explains GErsted's original experiment, and also all those that relate to the
deviation. In order to confirm the hypothesis to which he had been led, of
the nature of magnetism, Ampere endeavoured to arrange electric currents in
the same manner as he had conceived they were naturally arranged in a
magnet.
FlG. 354. Magnet revolving around Wire conveying the Current.
Thus a flat spiral coil of wire (Fig. 35 3), nicely supported and resting on points,
and perfectly mobile, takes a position perpendicular to the magnetic meridian.
By reversing the experiment, and causing the wire to be fixed, and the magnet
ELECTRO-MAGNETISM.
375
to revolve around it (Fig. 354), further proof was obtained by Faraday of the
mutual relations between magnets and wires conveying the voltaic current.
In this case we have the revolution of one pole of a magnet about a verti-
cal wire transmitting a rectilinear current. The direction of rotation is
reversed each time the direction of the current is reversed.
Or the experiment may be again modified and reversed by supporting (as
with the apparatus made so nicely by Messrs. Elliott) two helices or coils of
copper which are made to convey the voltaic current, and rotate in opposite
directions around the pc.'es of the horse-shoe magnet, as shown in Fig. 355.
FlG. 355. Contrary Rotation of two helical FlG. 356.
Coiled Wires around the Poles of a Magnet.
This apparatus is usually called Ritchie's spirals. De la Rive says Ampere
succeeded in overcoming all objections to his theory, and established it on
such a solid basis that it is at the present time generally admitted. He set
out from the principle that the electric currents to which, according to his
view, magnets owe their properties are molecular, that is, that they circulate
around each particle. These electric currents pre-exist in all magnetic bodies
even although they have not been magnetized, only they are arranged in an
irregular manner, so that they neutralize each other. Magnetization is the
operation by which a common direction is impressed upon them ; whence
it follows that the series of the exterior portion of the molecular currents,
which are all moving in the same direction, constitutes a finished current
around the magnet, whilst the interior portions are neutralized by the exterior
ones, moving in the contrary direction, of the following molecular stratum.
Fig. 356 represents the lection of a cylinder magnet and the magnet itself.
The direction impressed upon the currents by magnetization is maintained in
bodies that are endued will coercitive force, and ceases in others, such as
soft iron, as soon as the force that determined it ceases, because then all the
molecular currents, being free to obey their mutual action, take the relative
position that produces equilibrium, or the neutralization of every exterior effect.
376
MAGNETISM.
To Faraday is due the credit of realising the idea that the mutual reaction
of magnets or wires conveying electrical currents, and vice versa, should pro-
duce rotation ; and he was the first to cause a wire conveying a current to
revolve around a magnet, and the latter to rotate about a wire through which
the voltaic current is passing.
These original and philosophical experiments have been extended to larger
apparatus, and various attempts have been made to use the electro-magnetic
rotation successfully: Dal Negro, 1832; Professor Botto and Professor Jacobi
in 1835 ; Mr. Thomas Davenport, of the United States, in 1837 ; and Mr. Taylor
in 1839.
Davidson, in 1837, placed an electro-magnetic locomotive on the Edinburgh
and Glasgow Railway. The carriage was 16 ft. long and 6 ft. broad, and weighed
about 5 tons, with all the arrangements ; but, when put in motion, a speed of
only 4 miles per hour could be obtained.
Professor Page constructed an electro-magnetic engine which created much
interest at the time, and he calculated that the consumption of 3 Ib. of zinc per
diem was equal to one horse power. Page's engine was followed by those of
Talbot and Wheatstone.
Mr. Hjorth exhibited in London an engine which found many admirers.
The attractive force of Hjorth's machine is thus given by Mr. Hart, from
whose valuable paper the above historical details are taken :
72 Ib,
80
124
140
1 60
FIG. 357. Hjorttts Principle.
but, like the rest, it was abandoned.
Dr. Botto states that 45 Ib. of zinc consumed in a Grove's battery are suffi-
cient to work a one-horse power electro-magnetic engine for twenty-four hours.
Mr. J. P. Joule calculates that the same result would have been obtained
by the consumption of 75 Ib. of zinc in a Daniell's battery. Mr. Joule and
Dr. Scoresby thus sum up a series of experimental results : " Upon the whole,
ELECTRO-MAGNETISM. 377
we feel ourselves justified in fixing the maximum available duty of an electro-
magnetic engine, worked by a Daniell's battery, at 8c Ib. raised one foot high
for each grain of zinc consumed. This is about one-half of the theoretical
maximum duty. In the Cornish engines doing the best duty, one grain of coal
raised 143 Ib. one foot high. Zinc i. .vorth about ^35 per ton, and engine coal
is worth less than i per ton, delivered in London. Comment upon this is
unnecessary.
The fact is, an electro-magnetic engine is a very pretty toy, and can be
used, like Mr. Apps's half-horse power engine (Fig. 345, p. 370), to turn a
small lathe, or propel a small boat, or turn whirling tables or other apparatus
on the lecture-table, z>., where the cost of zinc and acids from the battery is
of no consequence. Mr. Apps furnishes the following particulars of the above
named electro-magnetic engine:
" Weight 80 Ib. When driven to 400 revolutions per minute by 20 cells
Grove (platina 6 in. X 3 in.), a half-horse power is obtained. It will drive with
equal facility in either direction, or, on reversing the current by the double
commutator, the magnetic power produced is opposite to the momentum
previously acquired (acting like a friction-brake) ; the direction of rotation is
reversed, and in about three seconds the former rate of speed is acquired.
" A very important point is gained in this machine. The current being
gradually broken, the spark usually produced at the breaking of the contact is
avoided. Besides this great advantage, the residual magnetism is destroyed,
which alone in the old machines diminished their power by at least one
quarter. The machine is well adapted to drive a lathe or the screw propeller
of a small boat."
378
MAGNETISM.
MAGNETO-ELECTRICITY.
INDUCTION BY CURRENT ELECTRICITY.
It has been noticed that a current of electricity elicits magnetism, and
therefore it is not surprising that the effect should be reversible ; but, simple as
this may appear in theory, it was a long time before Faraday succeeded in
overcoming the difficulties he encountered, and was enabled to relate his suc-
cess in the " Philosophical Magazine, 1^32, page 125.
The extremities of a helix or large hollow bobbin of wire were connected
with the galvanometer needle, care being taken that the galvanometer should
not be near enough to be affected by the magnet which Faraday used.
FlG. 358. Faraday's first Experiment.
The movement of the bar magnet across the coils produced a current
which affected the needle, and still better when, as in Fig. 358, the magnet
was intruded into the axis or hollow of the bobbin or helix. Not only is the
needle deflected when the magnet is insulated, but it is also moved in an
opposite direction when the magnet is removed.
When two concentric helices, of course of insulated or covered wire, are
arranged, the inner one being of thicker wire than the outer, and wound round
an axis or core of soft iron, a very powerful secondary current is obtained in
the outer coil when the inner core is magnetized. Such currents are called
induced currents, and are always more powerful when soft iron forms the axis
or core, because the iron, in acquiring or losing magnetism, produces a
secondary current which occurs in the same direction as that induced by the
inner coil or helix.
Here, then, is a distinct excitation or elimination of electricity by magnetism
alone, and is called magnetic electric induction to distinguish it from volta-
electric induction, also investigated by Faraday, and brought before the Royal
MAGNETO-ELECTRICITY. 379
Society in 1831. In the latter experiments, two great coils of wires were
wound together, metallic contact, of course, being prevented. One coil was
connected with the galvanometer, and the other with the voltaic battery.
The induced electricity in the second coil was suddenly produced like a wave,
presenting a marked difference to the magneto-electric induction, which was
much slower in its production. Here, then, are two modes of induction:
1. VOLTA-ELECTRIC INDUCTION;
2. MAGNETO-ELECTRIC INDUCTION.
The magneto-electric induction has been applied to the production of cur-
rents of electricity by Pixii the first in Paris, 1832, followed by Saxton and
E. M. Clarke.
Such instruments, in which a powerful compound-mag-
net, having rotating in front of its poles an armature or
bobbin of fine wire (which may be varied to produce
either quantity or intensity effects), elicits a current that
can be made to illustrate physiological, mechanical, che-
mical, and ordinary electrical effects, are so fully described
in every book on electricity that the writer prefers to pass
to newer and more perfect arrangements.
Magneto-electricity was applied and exhibited by Mr.
Holmes in the Great Exhibition of 1862, and obtained
from a machine of novel construction. At the same Ex-
hibition, and also in Paris, 1867, the writer saw Nollet's FlG. 359.
machine as improved by Mr. van Malderen, who took
great pains to show the writer the construction of his magneto- electric ma-
chine for light-giving purposes ; and it was understood that, at a cost of ^300,
one of these machines, turned by a steam-engine, might supply the Polytechnic
with the electric light at any time it was set in motion. The current passed
to a Serrin's lamp, and certainly produced a most brilliant light.
In the article on the Telegraph, it will be noticed that Sir Charles Wheat-
stone uses a magneto-electrical machine of improved construction, instead of
the voltaic battery. Wheatstone's exploder for military purposes generates
its electricity in the same manner. There are many other modifications of
induced currents, such as the experiments of Faraday, u On the Induction of
a Current on itself," read before the Royal Society, 1835; and Dr. Henry's
(College of New Jersey, Princeton; experiments (described in 1833) with flat
coils of insulated copper ribbon and helices of fine covered copper wire, by
which induced currents of the third, fourth, and fifth order could be obtained,
by alternately arranging the insulated copper ribbons and the helices of fine
wire.
In the "Proceedings of the Royal Society," No. 90, 1867, Sir Charles
Whcatstone describes a most interesting series of experiments " On the Aug-
mentation of the Power of a Magnet by the reaction thereon of Currents
induced by the Magnet itself," as follows :
" The magneto-electric machines which have been hitherto described are
actuated either by a permanent magnet or by an electro-magnet deriving its
power from a rheomotor placed in the circuit of its coil. In the present note,
I intend to show that an electro-magnet, if it possess at the commencement
the slightest polarity, may become a powerful magnet by the gradually aug-
menting currents which itself originates.
380 MAGNETISM.
" The following is a description of the form and dimensions of the electro-
magnet I have employed. The construction, it will be seen, is the same as
that of the electro-magnetic part of Mr. Wilde's machine.
" The core of the electro-magnet is formed of a plate of soft iron, 1 5 in. in
length and \ an inch in breadth, bent at the middle of its length into a horse-
shoe form. Round it is coiled, in the direction of its breadth, 640 ft. of insu-
lated copper wire T ^ of an inch in diameter. The armature, which is according
to Siemens's ingenious construction, consists of a rotating cylinder of soft iron,
8^ in. in length, grooved at two opposite sides so as to allow the wire to be
coiled upon it longitudinally ; the length of the wire thus coiled is 80 ft., and
its diameter is the same as that of the electro-magnet coil.
" When this electro-magnet is excited by any rheomotor the current from
which is in a constant direction, during the rotation of the armature, currents
are generated in its cell during each semi-revolution, which are alternately in
opposite directions ; these alternate currents may be transmitted unchanged
to another part of the circuit, or by means of a rheotrope be converted to the
same direction.
" If now, while the circuit of the armature remains completed, the rheomo-
tor be removed from the electro-magnet, on causing the armature to revolve,
however rapidly, it will be found by the interposition of a galvanometer, or
any other test, that but very slight effects take place. Though these effects
become stronger in proportion to the residual magnetism left in the electro-
magnet from the previous action of a current, they never attain any consider-
able amount.
" But if the wires of the two circuits be so joined as to form a single circuit,
in which the currents generated by the armature, after being changed to the
same direction, act so as to increase the existing polarity of the electro-
magnet, very different results will be obtained. The force required to move
the machine will be far greater, showing a great increase of magnetic power
in the horse-shoe ; and the existence of an energetic current in the wire is
shown by its action on a galvanometer, by its heating 4 in. of platinum wire
0067 in diameter, by its making a powerful electro-magnet, by its decompo-
sing water, and by other tests.
" The explanation of these effects is as follows : The electro-magnet always
retains a slight residual magnetism, and is therefore in the condition of a weak
permanent magnet ; the motion of the armature occasions feeble currents in
alternate directions in the coils tnereof, which, after being reduced to the
same direction, pass into the coil of the electro-magnet in such manner as to
increase the magnetism of the iron core ; the magnet, having thus received an
accession of strength, produces in its turn more energetic currents in the coil
of the armature; and these alternate actions continue until a maximum is
attained, depending on the rapidity of the motion and the capacity of the
ehctro-magnet.
" If the two coils be connected in such manner that the rectified current from
the coil of the armature passes into the coil of the electro-magnet in the
direction which would impart a contrary magnetism to the iron core, no cur-
rent is produced, and consequently there is no augmentation of magnetism.
" It is easy to prove that the residual magnetism of the electro-magnet is
the determining cause of these powerful effects. For this purpose it is suffi-
cient to pass a current from a voltaic battery, a magneto-electric machine, or
any other rheomotor, into the coil of the electro-magnet in either direction,
WHEATSTON&S EXPERIMENTS. 381
and it will invariably be found that the direction of the current, however
powerful it may eventually become, is in accordance with the polarity of the
magnetism impressed on the iron core.
" If, instead of the currents in the coil of the rotating armature being re-
duced to the same uniform direction, they retain their alternations, no effects,
or at most very small differential ones, are produced, as no accumulation of
magnetism then takes place.
" I will now call attention to the fact that stronger effects are produced at
the first moment of completing the combined circuit than afterwards. The
machine having been put in motion, at the first moment of completing the
circuit 4 in. of platina wire were made red hot ; but immediately afterwards
the glow disappeared, and only about one inch of the wire could be perma-
nently kept at a red heat. This diminution of effect was accompanied by a
great increase of the resistance of the machine. The cause of the momentary
strong effect was, that the machine from its acquired momentum continued
its motion for a few seconds, though it required a stronger force than could
be applied to maintain that motion. Each time the circuit is broken and re-
completed, the same effect recurs.
" On bringing the primary coil of an inductorium (RuhmkorfFs coil) into
the circuit formed by connecting the coils of the electro-magnet and rotating
armature, no spark occurs in the secondary coil. On account of the great
resistance of the circuit, which now also includes the primary coil of the in-
ductorium, the current is not in sufficient quantity to produce any noticeable
inductive effect.
" A very remarkable increase of all the effects, accompanied by a diminu-
tion in the resistance of the machine, is observed when a cross wire is placed
so as to divert a great portion of the current from the electro-magnet. The
four inches of platinum wire, instead of flashing into redness and then dis-
appearing, remains permanently ignited. The inductorium, which before gave
no spark, now gave one a quarter of an inch in length ; water was more abun-
dantly decomposed; and all the other effects were similarly increased.
" I account for this augmentation of the effects in the following way :
" Though so much of the current is diverted from the electro-magnet by
the cross wire, the magnetic effect still continues to accumulate, though not
to so high a degree; but the current generated by the armature, passing
through the short circuit formed by the armature branch and cross wire,
experiences a far less resistance than if it had passed through the armature
and electric-magnet branches; and though the electromotive force is less, the
resistance having been rendered less in a much greater proportion, the result-
ant effect is greater.
" I must observe that a certain amount of resistance in the cross wire is
necessary to produce the maximum effect. If the resistance be too small, the
electro-magnet does not acquire sufficient magnetism ; and if it be too great,
though the magnetism becomes stronger, the increase of resistance more than
counterbalances its effect.
" But the effects already described are far inferior to those obtained by
causing them to take place in the cross wire itself. With the same applica-
tion of force, 7 in. of platinum wire were made red hot, and sparks were
elicited in the inductorium 2| in. in length.
" The force of two men was employed in these, as well as in the other
experiments. When the interrupter of the primary coil was fixed, the machine
382 MAGNETISM.
was much easier to move than when it acted. For when the interrupter acted,
at each moment of interruption the cross wire being, as it were, removed, the
whole of the current passed through the electro-magnet, and consequently a
greater amount of magnetic energy was excited, while in the intervals during
which the cross wire was complete the current passed mainly through the
primary coil.
" The effects are much less influenced by a resistance in the electro-magnet
branch than in either of the other branches.
" To reduce the length of the spark in the inductorium (the primary coil of
which was placed in the cross wire) to f of an inch, it required the resistance
f 5i m - f the fine platinum wire in the cross wire, 5 in. in the armature
branch, and 4 ft. in the electro-magnet branch.
" When there was no extra-resistance in either of the branches, the length of
the cross wire being only about a few feet, the intensity of the current in the
electro-magnet branch, compared with that in the cross wire, was as I : 60 ;
and when the resistance of the primary coil of the inductorium was interposed
in the cross wire, the relative intensities were as i -.42.
"In conclusion, I will mention that there is an evident analogy between the
augmentation of the power of a weak magnet by means of an inductive action
produced by itself, and that accumulation of power shown in the static elec-
tric machines of Holtz and others, which have recently excited considerable
attention, in which a very small quantity of electricity directly excited is, by a
series of inductive actions, augmented so as to equal, and even exceed, the
effects of the most powerful machines of the ordinary construction."
Mr. Wilde's machine has been fully described in all the illustrated scientific
papers, such as "The Engineer" and "The Mechanic's Magazine." The
writer, therefore, proposes to give drawings of Mr. Ladd's improved magneto-
electric machine, which he thus describes in the " Transactions of the Royal
Society," No. 91, 1867:
" In June, 1864, 1 received from Mr. Wilde a small magneto-electric machine,
consisting of a Siemens's armature and si magnets. This I endeavoured to
improve upon, my object being to get a cheap machine for blasting with Abel's
fusees. This was done by making one of circular magnets, and a Siemens ; s
armature revolving directly between the poles, the armature forming part of
the circle; with this I could send a very considerable power into an electro-
magnet, &c. It was then suggested to me, by my assistant, that if the arma-
ture had two wires instead of one, the current from one being sent through a
wire surrounding the magnets, their power would be augmented, and a con-
siderable current might be obtained from the other wire available for external
work ; or there might be two armatures one to exalt the power of the magnets,
and the other made available for blasting or other purposes. Want of time
prevented me carrying this out until now; but since the interesting papers of
C. W. Siemens, F.R.S., and Professor Wheatstone, F.R.S., were read last
month, I have carried out the idea as follows: Two bars of soft iron, mea-
suring 7\ in.X2^ in.X| in., are each wound, round the centre portions, with
about thirty yards of No. 10 copper wire ; and shoes of soft iron are so attached
at each end, that when the bars are placed one above the other there will be a
space left between the opposite shoes, in which a Siemens's armature can rotate:
on each of the armatures is wound about ten yards of No. 14 copper wire,
cotton-covered. The current generated in one of the armatures is always in
connexion with the electro-magnets ; and the current from the second arma-
MA GNETO-ELECTRICITY.
,83
ture, being perfectly free, can be used for any purpose for which it may be
required. The machine is altogether rudely constructed, and is only intended
to illustrate the principle ; but with this small machine three inches of platinum
wire 'oi can be made incandescent."
Mr. Ladd now calls his improved machine, which it is hoped may be per-
manently erected some day at the Polytechnic as a convenient source of elec-
tricity for all purposes, the " Dynamo-Magnetic Machine " (Fig. 360).
FIG. 360.
This machine was awarded a silver medal at the Paris Exhibition, 1867.
Another form of the apparatus (Fig. 361), also constructed by Mr. Ladd, is
that in which the two armatures are combined in one, and the coils are wound
at right angles to each other.
The results obtained are simply regulated by the amount of mechanical
force used to rotate the armatures ; and thus indirectly coal, used as a means
of exciting electricity, is made to generate steam, which produces force in
the steam engine, and this ultimately turns the dynamo-magnetic machine ;
and thus indirectly coal generates an electric current, by which the electric
light is obtained.
A convenient little magneto-electrical machine is made by Mr. Browning,
lor the purpose of giving shocks and for medical use. (Fig. 362.)
384
MAGNETISM.
FIG. 361.
Directions for using the Instrument. Take the hollow conductors A B off
from the large studs on which they are placed ; uncoil their metallic cords
which are wound upon them, and insert the pins which are attached to the
ends of these cords into the small holes which will be found in two upright
brass studs at the back of the stand of the machine, marked c D in the dia-
gram ; then upon holding the hollow conductors, one in each hand, and turn-
ing the handle of the machine quickly, a strong electrical current will be felt.
A horizontal stud in front of the machine, projecting beyond the frame, serves
to move an iron feeder before the ends of the large circular magnet. By shift-
ing this feeder, the strength of the current given out by the machine can be
regulated within any desirable limit. When the feeder is lifted up in front of
the magnet, the current will be very feeble ; when it is withdrawn quite below
the magnet, it will be very intense.
Two brass springs project from the brass studs C D ; these springs should
rest on the edge of a small wheel of ebonite and brass, known as a commu-
tator. It sometimes happens that, from rough usage in carriage, these springs
are bent, so that they no longer touch the edge of the wheel ; in this case the
current becomes greatly weakened, or altogether ceases ; but the machine can
be easily set right by carefully bending down the springs so that they again
rest upon the edge of the wheel.
We now come to the last of the induction machines, sometimes called the
induction coil, the inductorium, &c. In 1851, M. Ruhmkorff, a most clever
instrument maker in Paris, made a coil which produced in the scientific world
MA GNETO-ELECTRICITY.
385
FlG. 362. Browning's Magneto-Electrical Machine.
of Paris and London a profound sensation of surprise and delight at the beau-
tiful light-effects obtainable.
Mr. H carder, of Plymouth, and Mr. Bentley subsequently made coils of
great power ; but to Mr. Ladd is due the merit of constructing a serviceable
apparatus which would always produce the most reliable results. A very
large coil, having a secondary coil of seven miles of wire, has long been used
at the Polytechnic. It consists of the usual primary coil, wound round a
faggot of iron wires ; around this is the secondary coil, of the required number
of miles in length. The condenser, composed of alternate sheets of tinfoil
and well dried and varnished paper, is placed under the coil, and, by making
and breaking contact with the primary by a convenient " contact-breaker," an
enormous current is induced in the secondary, which produces the most bril-
liant results.
FlG.
. P linker's Tube.
A Leyden jar or Leyden plate may be incessantly charged and discharged
with a continuous roar. Paper is immediately set on fire when held between
the poles. Tubes of glass are filled with various gases or liquids, or rather
not rilled according to the ordinary acceptation of the term, because they are
'vac^^a, the last gas which has been permitted to enter the tube alone repre-
senting the attenuated atmosphere through which the electric current passes.
The reader is referred to Dr. Noad's little book, entitled " The Inductorium,"
and published by Churchill for Mr. Ladd, for all the minute details connected
with the primary coil, the secondary, the condenser, and the thousand-and-
25
3 86
MAGNETISM.
one experiments which, like the "Arabian Nights' Entertainments," crowd
upon the student, but which may all be performed with the apparatus described.
Amongst the most interesting experiments, that of Pliicker deserves especial
notice.
" Two aluminium rings are hermetically sealed into a glass tube, 4 or 5 in.
long and about \\ in. in diameter; the air in the tube is then exhausted as
perfectly as possible. On passing the discharge from the induction coil between
the two rings, the tube becomes filled with a beautiful pale blue light.
FlG. 364. P tucker's Tube with Aluminium Wires.
" If the small ring be made negative, and the tube placed between the poles
of an electro-magnet, the moment the latter is excited the light arranges itself
in the form of a broad arc between the rings.
FlG. 365. Gassiofs Cascade,
The current passing into and out of a glass vessel placed in a vacuum.
" On rendering the electro-magnet passive, the arc disappears, the light in
the tube re-assuming its different character ; but, on re-exciting the magnet,
MAGNETO-ELECTRICITY.
387
the arc re-appears. If, instead of two rings, the terminals in the tube are two
aluminium wires, as shown in Fig. 364, the long wire being made positive and
the short wire negative, the arc produced is very broad and brilliant."
It must be apparent from the preceding figures that the stratification notice-
able in all experiments of this type is a special object of interest, to which M.
Gassiot, the generous and large-hearted friend of science, has paid particular
attention.
Speaking of Geissler's (of Bonn) tubes, one of the prettiest arrangements
the writer has seen is that of Mr. Apps, and shown in the next figure.
FlG. 366. Front View of Geissler's Tubes, arranged on a disc of blackened
Mahogany.
The back view exhibits the use ot the electro-magnetic engine for rotating
or reversing the disc. (Fig. 367.)
The electro-magnetic engine, in a convenient and handsome form, well
adapted to rotate the vacuum tubes, is attached to the black polished disc, and
arranged so as to turn in either direction : the speed can be easily regulated.
The discharge from the coil passes through the entire series of tubes.
Amongst the remarkable effects produced by the induction coil, there are
none more interesting than the generation of ozone by the " ozone tube," which
is thus described by Dr. Noad, and made by Mr. Ladd. (Fig. 368.)
It consists of a glass tube, about the size of an ordinary test tube, coated
with tinfoil or, still better, silvered, and enclosed in an outer tube lined out-
side with tinfoil. The two tubes are sealed together at the neck of the outer
25 2
3 88
MAGNETISM.
FIG. 367.
one, and so adjusted that
the space between them
shall be as narrow as pos-
sible.
At the projecting end of
the inner tube is a brass
button, which is connected
by a spring with one of the
binding-screws on the frame
of the apparatus, which
screw is to be connected
with one of the terminals of
the secondary coil of an in-
ductorium, and the other
with another binding-screw
in metallic communication
with the coating of the ex-
terior tube.
The apparatus is, in fact,
a sort of slit Leyden jar;
and air or oxygen, admitted
through the lateral tube, be-
comes during its passage
through the apparatus pow-
erfully ozonized.
The air may be driven
through by means of a bladder or india-rubber bag, or drawn through with an
aspirator.
FIG. 368. The Ozone-Tube.
Mr. Edward Beanes, who has already done so much in improving certain
processes required in the manufacture of sugar, has patented the application
of apparatus for generating ozone and bleaching syrup, and, although there
appears to be some difficulty in obtaining enough ozone for this purpose, the
experiments hitherto tried are very promising.
The writer abstains from saying anything about a new gigantic coil, building
for the Polytechnic by Mr. Apps. Like David with his armour, he has not
proved it : had he done so, this article would have contained an account of
the Mammoth Induction Coil.
THERMO-ELECTRICITY.
389
THERMO-ELECTRICITY.
Electricity produces magnetism, heat, light, mechanical and chemical effects.
It is not opposed to the harmony of created forces that heat should produce
electricity.
FlG. 369. Marcus's Thermo-Electric
Battery, made by Mr. Ladd.
FIG. 370.
The above battery (Fig. 369) consists of thirty-six elements ; the negative
bars, which are 6 in. long, being composed of 12 parts of antimony, 5 of zinc,
and i of bismuth; and the positive bars, which are 7 in. long, of copper 10 parts,
zinc 6 parts, and nickel 6 parts. The bars are ranged on a frame in the slanting
position shown in the figure, and were facetiously referred to by a writer in
" Punch " as a " chestnut roaster," the positive bar of the first pair being metal-
lically connected with the negative of the second, and the two extreme bars
connected with binding-screws which form the terminals of the battery. The
upper ends of the bars are heated by a series of Bunsen's burners, the flames of
which can be easily regulated.
This battery at the Polytechnic, under the charge of Mr. J. L. King, decom-
posed water, of course very feebly ; it gave small sparks between iron points
without the assistance of a coil, and enabled an electro-magnet to support a
considerable weight, and, when connected with an induction coil, gave sparks
which were very marked in their character and length.
We have now to ask how this apparatus, in which heat takes the place of
friction, chemical action, or magnetism, elicits electric force.
Seebeck's apparatus, a rectangular figure, made of bismuth and antimony,
with an astatic magnetic needle supported inside, well exhibits the thermo-
electric action ; and, directly one of the angles is gently heated by a spirit
flame, the needle, like that of the galvanometer with the voltaic circuit, is
deflected. (Fig. 370.)
Pouillet's thermo-electric apparatus (made by Elliott), and already figured
in Wheatstone's paper on the Rheostat (p. 333), consisting of a short cylindrical
bar of bismuth, bent twice at right angles, with soldered copper wires attached
to the ends, communicating with an ingenious contrivance on the stand for
39
MAGNETISM.
FIG. 371. Pouillet Thermo-Electric Circle.
completing the electric circuit in any direction, is another and most perfect
arrangement for showing currents of electricity obtainable by the exciter,
"heat." (Fig. 371.)
On the second or third page of this work, in the article on Light, Melloni's
small and compact composite " thermo-electric pile " is specially alluded to.
When the writer was a student, thirty years ago, he well remembers trying
experiments with this beautiful contrivance for showing minute disturbances
of heat ; and, at that time, it had the reputation of being delicate enough to
show the heat of the body of a " fly or a blue-bottle." Exaggeration apart, its
FlG. $j2s~-Mellon?s Thermo-Electric Pile or Battery.
power to show the slightest heat-wave disturbance has never been equalled by
any other apparatus. It consists of a series of pairs of very slender bars of
antimony and bismuth soldered alternately together, and arranged parallel
side by side, so that all the soldered pairs are at one end, and all the solders
not pairs at the other. This apparatus, mounted in a brass tube and placed
on a stand, is now the special attendant at all lectures in which the dynamical
theory of heat is taught. (Fig. 372.)
The late Mr. Francis Watkins, the predecessor of the Messrs. Elliott, paid
particular attention to this subject, and constructed a " Thermo-Electric Com-
binator." Eighteen pairs of bismuth and antimony, united alternately by
THERMO-ELECTRICITY.
FIG. 373. Van der Voorfs Thenno-Electric Battery.
solder top and bottom, and fixed in a mahogany box by plaster of paris,
leave the two extremities to be acted upon, the one by heat and heated iron
or boiling oil, and the other by cold some ice or a freezing mixture. All the
common effects of an electric current, such as the spark, &c., can be shown
with this contrivance.
Thus the corelation of forces is complete, and Light, Heat, Electricity, and
Magnetism resolve themselves into each other, and represent probably a
series of waves, every one of which is different from the other in the phases
of its vibrations and resultant form.
392
MAGNETISM.
FlG. 374. Portrait and Autograph of Sir Charles Wheatstone.
WHEATSTONE'S TELEGRAPHS.
The limits of this article will not permit of any lengthened history of all
the clever inventions either proposed or carried out by the various scientific
men who have contributed to our knowledge of the science of telegraphy.
Whatever amount of Credit may be accorded to others, there can be but
one opinion respecting the merits of a living philosopher, whose portrait graces
the head of this chapter. Foreigners are usually very frank and honest in
their expression of the amount of merit due to their contemporaries in other
WHEATSTONE' S TELEGRAPHS. 393
countries. The jury of the French Exhibition of 1855 thus report upon
Wheatstone :
" La transmission de 1'electricite entre les pays separes par la mer n'a pu
s'effectuer qu'au moyen de cables particuliers unissant entre elles les stations
telegraphiques. Mais combien de travaux n'a-t-il pas fallu pour atteindre ce
but ; et meme maintenant que la question est resolue, on ne peut sans admiration
penser que la transmission des depeches telegraphiques est aussi facile a 1'aide
des cables sous-marins qu'au moyen des fils isoles et tendus dans Fair.- Cest
par 1'emploi de ces cables que 1'on a pu mettre en relation telegraphique la
France et 1'Angleterre, la Crimee et les provinces Danubiennes, les pays enfin
dans lesquels ces principes ont &6 appliques, et peut-etre bientot 1'Europe et
1'Amerique. Le Jury a vote une mention tres-honorable pour M. Wheatstone
(Royaume Uni), membre du Jury de la IX e classe, pour avoir con^u 1'idee pre-
miere et pour avoir propose, en 1840, un moyen de resoudre la question; il
accorde la meme distinction a M. Brett (Royaume Uni), sous la direction
duquel a ete place un conducteur au travers de la Manche, entre Douvres et
Calais, et qui a montre ainsi que le succes etait possible. Le Jury decerne
egalement une mention tres-honorable a M. Crampton (Royaume Uni),
membre du Jury de la V e classe, auquel revient 1'honneur d'avoir realise cette
immense application, en unissant definitivement, en 1851, par un cable sous-
marin, la France et 1'Angleterre."
Another very distinguished foreigner, A. De la Rive, thus speaks of Wheat-
stone in his "Treatise on Electricity:"
"The philosopher who was the first to contribute by his labours, as inge-
nious as they were persevering, in giving to electric telegraphy the practical
character that it now possesses is, without any doubt, Mr. Wheatstone. This
illustrious philosopher was led to this beautiful result by the researches that
he had made in 1834 upon the velocity of electricity researches in which he
had employed insulated wires of several miles in length, and which had
demonstrated to him the possibility of making voltaic and magneto-electric
currents to pass through circuits of this length."
The following is the order of the inventions made by Sir Charles Wheat-
stone :
The 5 -needle telegraph, 1837.
The alphabet-dial telegraph, 1840.
The type-printing telegraph, 1841.
The new magnetic alphabetic-dial telegraph, 1858-60.
The fast-speed automatic telegraph, 1858 1867.
Sir Charles Wheatstone, in addition to the other honours he has lately re-
ceived, has just been elected to replace Faraday as one of the twelve corre-
sponding members of the " Societa Italiana delle Scienze, detta dei XL.," and
has also received their first gold medal, instituted during the present year by
the late Minister of Public Instruction, Signer Matteucci, to honour the most
important discoveries in physical science.
The president, in his address, says :
" I will not here pass in review the various memoirs in physics which you
have published in the ' Philosophical Transactions/ since all carry the impres-
sion of the inventive genius which ever distinguishes all that you have done.
I cannot, however, refrain from calling to mind that to you we owe the dis-
covery of the method, as ingenious as it is original, for measuring the velocity
of electric currents and the duration of the spark.
394 MAGNETISM.
" The applications of the principle of the rotating mirror are so important
and so various that this discovery must be considered as one of those which
have most contributed in these latter times to the progress of experimental
physics.
" Not less ingenious was the invention of the stereoscope and of the modes
by which binocular vision is effected, which enable us to obtain the percep-
tion of relief from the simultaneous observation of two plane images.
" Also the memoir on the measure of electric currents, and on all the ques-
tions which relate thereto and to the laws of Ohm, has powerfully contributed
to spread among physicists the knowledge of those facts and the mode of
measuring them with an accuracy and simplicity which before we did not
possess.
"All physicists know how many researches have since been undertaken
with your rheostat (see p. 333) and with the so-called Wheatstone's bridge,
and how usefully these instruments have been applied to the measure of elec-
tric currents, of the resistance of circuits, and of electro- motive forces.
" And here it would be impossible to leave out of view that to you we prin-
cipally owe the practical invention and the true realization of the electric
telegraph.
" Finally, I would call to mind your recent researches on the augmentation
of the force of a magnet by the reaction which its own induced currents
exert upon it.
" All these great acquisitions, procured by you, to physical science render
you well worthy of this distinction from the Italian Society of Sciences.
" Preserve yourself in health and activity, and your country and all your
admirers and friends are certain to find, in the discoveries still to be added
while you continue to work, some compensation for that immense and irrepa-
rable loss which natural philosophy has received by the death of Faraday."
In addition to the memoirs by Sir Charles Wheatstone, alluded to by Signer
Matteucci, the following may be specially noticed :
" On the Acoustic Figures of Vibrating Surfaces," published in the " Philo-
sophical Transactions " for 1 832. In this memoir, which gained for Sir Charles
his admission into the Royal Society, the author gave for the first time the
laws of formation of the varied and beautiful figures discovered by Chladni.
Attention has recently been revived to this subject by Konig and others on
the Continent.
"On the Transmission of Sound through Solid Conductors" ("Journal of
the Royal Institution," 1828). This memoir describes the means discovered
by the author of transmitting musical pertormances to distant places.
"On the Prismatic Analysis of Electric Light" (British Association, 1832).
By these experiments Sir Charles proved for the first time that the spectrum
of the electric spark from different metals presented each a definite series of
lines differing in colour and position from each other, and that these appear-
ances afforded the means of distinguishing the smallest fragment of one metal
from that of another. This investigation was one of the earliest starting-
points of an entire new branch of physical science, in which there are now
many distinguished workers.
"On the Polar Clock" (British Association, 1849). This is an optical
instrument which indicates the time by means of the changes in the plane of
polarization of the blue light of the sky in the direction of the pole. It is
founded on the discoveries of Arago and Quetelet ; and Arago states that
WHEATSTONE' S TELEGRAPHS. 395
" 1'honneur de la construction de 1'horloge polaire, je la reconnais avec em-
pressement et sans reserve, revient exclusivement a M. Wheatstone."
It would carry us beyond our limits to enumerate the various inventions
relating to the electric telegraph and other applications of electricity which
have emanated from Sir Charles. We will mention two only.
We owe to him, in addition to his former inventions relating to the electric
telegraph, the alphabetical-dial telegraph, .working without any clockwork
power, and in which a magneto-electric machine supplies the place of a voltaic
battery. These instruments were first introduced on the Paris and Versailles
Railway in 1846, and, with the improvements which the inventor has since
made, have been employed to a great extent throughout the kingdom by the
Universal Private Telegraph Company in furnishing telegraphic communica-
tion between public offices and private establishments, to which purposes,
from their facility of manipulation and constancy of action, they are admir-
ably adapted.
A more recent invention is his fast-speed telegraph, in which the messages,
previously prepared on strips of paper by manipulations as easy as those for
sending an ordinary message, are, by passing through a very small machine
constructed on somewhat the principle of a Jacquard loom, made to print the
messages at the remote station in the ordinary telegraphic characters, with a
rapidity and distinctness unattainable by the hand of an operator. The inven-
tion of these instruments dates from 1858; but they have only, with recent
improvements, been practically introduced, by the Electric Telegraph Com-
pany, during the last year. Since June last these instruments have been in
constant action for the ordinary business of the establishment between London
and Newcastle, printing from sixty to a hundred and ten words per minute.
The result has been so successful that the company have just resolved to adopt
them on other leading lines of communication.
In the report of the Paris Exhibition of 1855, honourable mention was
awarded to Sir Charles, he being hors de concours, for having been the first to
conceive the idea, and for having proposed, in 1840, a means of resolving the
question, of a submarine telegraph between Dover and Calais.
It may be mentioned in reference to an eminent philosopher, Sir David
Brewster (whose loss we have had to deplore), that one of the last acts of his
life was to nominate Wheatstone for election as an honorary member of the
Royal Society of Edinburgh, thus falsifying the couplet of Dryden, who says,
"Forgiveness to the injured does belong;
But they ne'er pardon who have done the wrong."
In 1868 Wheatstone received the honour of knighthood at the hands of
his gracious sovereign, and this same year of grace the Royal Society have
awarded to him their highest distinction, viz., the Copley medal.
"This is the state of man: to-day he puts forth
The tender leaves of hope; to-morrow, blossom*
And bears his blushing honours thick upon him."
In concluding this brief notice of the labori'ous and useful life of Wheat-
stone, we may, in common with all his friends and admirers, be permitted to
hope that he may pass the evening of his days in peace and in the enjoyment
of health, and that he will give to the world, in the calmness of matured age, a
monograph of the " Labours of his Life."
In every book devoted to the consideration of electric telegraph instruments
396 MAGNETISM.
we find illustrations and descriptions of Cooke and Wheatstone's earlier inven-
tions of the single and double needle telegraph. We will, therefore, commence
at the year 1840, when he constructed the alphabet-dial telegraph, which the
writer has always found to be one of the best forms for teaching and demon-
strating the broad principles upon which motion is developed by a current
thrown alternately from one electro-magnet to another. Such is the con-
struction of the telegraph, the dial of which is shown at Fig. 375.
U ^ E5==^E3
FlG. 375- Wheatstone" s first Alphabet- FIG. 376. Wheatstone's
Dial Telegraph (1840). Communicator (1840).
It consists of a circular dial, on which the letters of the alphabet are painted
in black letters on a white ground. The mechanism is very simple. Two electro-
magnets, with feeders and long arms, strike alternately the pallets ; these take
up at each blow one tooth of a wheel or escapement, and every time a tooth
is taken up the hand on the dial moves forward one letter. To make the letters
on the dial coincide with the letters of the sender of the message, another
instrument is required, called the " communicator." (Fig. 376.)
This consists of a wheel, upon the circumference of which are thirty alter-
nations of brass and ivory corresponding to the letters of the alphabet, c.,
with which also this instrument is provided. There are two springs, one on
each side, which communicate alternately with the communicator and through
that to the battery and wires of the dial telegraph. When the communicator
is turned round one letter, the hand or the dial moves one letter; and, if the
instruments are very carefully made, they answer remarkably well.
Wheatstone, however, found that they sometimes missed a tooth in the
escapement, and, of course, one letter being gone, the message afterwards
might be very chaotic, particularly when a number of words in rapid succes-
sion had to be forwarded. This system was, however, at the time adopted on
some of the continental lines.
Passing by the type-printing telegraph of 1841, we now come to the new
magnetic alphabetic-dial telegraph of 1858 and 1860.
The reader will be able to understand the construction better by reading
and examining the annexed description and diagrams than if a minute descrip-
tion of the above instrument (Fig. 377) were given at once. It is, perhaps,
unnecessary to remark that the"se instruments are in daily use by the Universal
Private Telegraph Company.
Instructions for connecting-up the Instruments. The instruments (commu-
nicator, indicator, and alarum) at each station should first be placed in short
circuit in the following manner (Fig. 378) :
Place short wires upon the two upper terminals, a b, at the back of the indi-
WHEATSTONE'S TELEGRAPHS.
397
FlG. 377-~ Wheatstonts- new Magnetic Alphabetic-Dial Telegraph.
cator, and connect them with c and d respectively, the switch, *, being turned
to point to the letter T Telegraph. The handle, z, of the communicator is
then to be turned steadily at a rate of about a hundred and twenty revolu-
tions per minute, and the index or pointer passed from + to + on the dial by
Indicator.
Communicator.
FIG. 378.
398
MAGNETISM.
depressing the finger-key opposite the full stop (.) and the key opposite the
+ immediately afterwards. If the index of both communicator and indicator
correspond, the connections will be right ; but should the hand of the indi-
cator be either in advance or behind the + one space, the connecting wires
must be reversed.
a being now joined up to d, and b to <r, the instruments will be found to cor-
respond in the revolution of their pointers round the dials. The line wire may
now be connected to the instruments by removing one of the short wires at
each station, and substituting the line wire and earth wire, as shown at a b
and c d. The same signal of passing the pointer from -f- to -f- is now to be
sent from station to station, and if the index at the other station falls either
one in advance or behind, the position of the line and earth wires at one sta-
tion only must be reversed.
The hand of the indicator may be reset by gently moving the small button
under the face backward and forward between the thumb and finger.
When more than two stations require to be connected up in the same circuit,
the above rules are to be observed with reference to the signals from -f- to +
at each successive station, the connections appearing thus (Fig. 380)
LINE WIRE
EARTH
FIG. 380.
EARTH
WffEATSTONE'S TELEGRAPHS.
399
When several stations are in the same circuit, it will often be found conve-
nient to introduce the switch, enabling the operator to send up and down the
line in either direction, without interrupting the communication of those sta-
tions situated in an opposite direction to that in which he is speaking. The
DOWN LINE
UP LINE
EARTH
THROUGH CIRCUIT
FIG. 381.
manner of connection will be seen by reference to the drawing. This arrange-
ment will enable several stations to communicate with each other at the same
time.
a b c d- e / ^ h
For instance, while a is speaking to b, c can talk to d, e with f, and so on.
This system requires that each station has its own signal or preface for calling
attention, and that when no station is called either up or down the line, the
handle of the switch remains on the through circuit, as shown in the digram.
The switch is generally adapted to the peculiar requirements of the line.
When alarums or bells are used to call attention, they must be placed in
circuit by connecting their binding-screws to the two lower binding-screws at
the back of the indicator. The alarum may be placed at any distance from
the instrument, in the most convenient position for calling attention. The
switch, x, of the indicator should point to A, alarum, when no messages are
being sent, but be turned to T when operations begin.
Instructions for working the Telegraphs. The following summary of rules
for working the telegraph may be advantageously introduced here :
1. The handle in front of the instrument (Fig. 377), which causes the arma-
ture of the magnet to rotate, must be kept in continuous motion by one hand,
while the fingers of the other are employed to manipulate the stops or keys.
Care must be taken not to intermit the motion until the end of the message.
2. A key need not be continuously pressed down ; it will suffice merely to
touch it ; but another key must not be pressed down until the index or pointer
has arrived at the letter previously indicated.
3. The same key cannot be pressed twice down in succession ; to repeat a
letter it is necessary to touch the preceding key, and, without waiting for the
arrival of the index, to touch again the proper key.
4. Before commencing to send a message, the index of all the instruments
must point to +. To bring the telegraph to this position when out, the small
400 MAGNETISM.
pin or button on the face of the telegraph must be meved alternately back-
wards and forwards between the finger and thumb until the index stands at +.
5. If by inadvertence the index of the communicator has been left at a
letter, it must be brought to the cross before the telegraph is adjusted.
6. The pointer of the alarum must invariably, when the instrument is not
in use, be turned to the letter A.
7. To call attention for the purpose of sending a message, first turn your
own alarum off, then rotate the handle of the communicator and let the
needle pass from -\- to -j-. This will ring the bell at the other end. Wait
an interval of time sufficient to allow of reply. If no reply, continue to call
in the same manner.
8. Receiver will notify his attention by repeating the signal.
9. The receiver will then turn off his alarum, by passing the pointer from
letter A to T.
10. A short time must be allowed the receiver before sending, to enable
him to put his indicator in accord with his transmitter, if it be wrong.
11. At the end of each word the needle to be brought to the -f-.
12. Should the receiver not understand, he will send the letter R for repeat,
prior to giving -f . The sender will then repeat the last word.
13. Every initial letter or part of a word used for abbreviation must be
followed by the full stop, and the full stop must be given at the end of each
sentence.
14. At the end of message, needle to be turned from + to + twice.
15. Receiver to repeat this double revolution.
16. If by accident the needle of the indicator becomes misplaced, so as to
render a message unintelligible, the receiver must break in by pressing down
several keys in succession. The sender will immediately stay sending. Both
receiver and sender will then set needles at +, and receiver will give repeat, R.
17. To signify figures, use the semicolon, and then the +, before and after
them.
Instructions for keeping the Instruments in order. When the telegraph is
in operation, the handle of the communicator should be turned at a uniform
rate of 120 revolutions per minute, and the finger-keys should not be depressed
when the handle is at rest.
The working parts and bearings of the communicator will require occasion-
ally to be oiled with good watch-oil, procured from any respectable watch-
maker. If the oil is good, and the telegraph moderately used, the instrument
will work eight or ten months without touching ; but, when in constant use, it
is desirable to apply a little oil regularly every two months. Access for this
purpose may be obtained to the interior of the communicator by unscrewing
the bottom of the communicator. The various parts to be oiled are shown in
the annexed diagram at a, 6, c, d; and by dipping the point of a penknife
into the oil, it may be neatly applied in small quantities where desired.
If the centre, <, has become worn by constant revolution, and causing the
armature, *, to touch the iron prolongations of the magnet, the handle will
work stiffly or stop altogether. This may be remedied by tightening slightly
the screw, g (Fig. 382), with a pair of small pliers, or other means sufficient
to free the armature from contact with the poles of the magnet.
After long use, the watch-chain, which runs round the rollers on the lower
plate, for the purpose of mechanically raising each key, after it has been
depressed by the hand, may become too slack ; this is remedied by slightly
WHEATSTON&S TELEGRAPHS.
401
FIG. 382.
tightening the screw, A. attached to a lever carrying an extra roller, care
being taken to leave sufficient slack in the chain to allow of one key always
remaining depressed, as shown at B (Fig. 383).
FIG. 383.
If it becomes necessary to take the communicator to pieces (this operation,
had always better be performed by a clock or watch maker, or other experi-
enced person), the bottom of the case must be taken off first, ana the little
ivory number-plate in front of the instrument
pushed out from the inside. This will enable the
position of the wheel and pinion to be marked
through the hole of the number-plate, by making
a scratch (Fig. 384), as at .r, across both, care being
taken in putting together that the marked parts
of the wheels are placed as before. The magnet
may then be taken out, having previously un-
screwed the wires leading from the coils. The
brass casing which covers the upper portion of
the mechanism is now to be unscrewed, and the
ring with the glass, which is only sprung on, re-
moved ; then the dial card and plate. Unscrew
the four pillars below, and, after the whole frame has been taken off the
wooden case, all may be taken to pieces. It will be necessary to mark the
26
FIG. 384.
402
MAGNETISM.
position of the two wheels, h and z, by a scratch across both, before taking
that portion asunder. Oil must be put to the teeth of the wheel k, and also
to , m, o, and p (Fig. 385.)
FIG. 385.
The operation of putting together is as follows : First put the centre arbor
and all upon it in the frame, and secure the same by the four pillar screws.
Then place the finger-keys, the dial-plate, the springs for the keys, the dial,
the index, and the glass together, and fix the whole on the wooden case.
Lastly, place the magnet irl its proper position, and, when all is ascertained
to be correct, screw on the brass casing and the wooden bottom of the in-
strument.
The indicator and alarum may be taken to pieces, when necessary, and put
together again, by marking the proper position of the several parts. In the
indicator, pivots only require to be oiled, and that in very small quantities.
The indicator, when good oil has been used, will work without attention for
two or three years.
FlG. 386. WheatstonJs Bell in box, and ready for Militarv or other
Service.
Professor Wheatstone's instruments have been adopted by the army authori-
ties, and are made, as in Fig. 377, p. 397, very portable and wholly independent
WHEATSTON&S TELEGRAPHS.
403
of all battery power, the trouble of putting batteries together, the supply of
acids, breakage, and all the trouble that would be multiplied tenfold in the
hurry of the battle-field. These instruments, as already described, work by g
current developed by magnetism and by the use of steel magnets ; they are
made very strong and substantial, and are well calculated to bear the wear
and tear of military operations conducted in the field.
The bell is rung, as nearly all other electric bells are rung, by clockwork
wound up, but stopped by a " detainer." Directly the detent is removed by
the current, the bell rings.
The same instruments, connected with enlarged dials, are used on board
the iron-clads. We show an enlarged dial, and can easily understand how
quickly the commander's orders could be conveyed to the engine-room.
FIG. 387. Wheatstone's enlarged Dials, such as are used in the Engine-
rooms of Ships of War.
The dials, of course, would have special orders printed on them, being those
given constantly in the navigation of these immense vessels.
In a very short time, similar dials will be placed in the various rooms occu-
pied by members in the House of Commons, and the dials will show what
business is in progress and what has been done. The business to be trans-
acted, being printed in a circular form, is laid upon the dial, and the hand
points to that in progress, whilst all behind it is over.
The steering of the iron-clads is also to be conducted with the assistance of
similar dials.
One of the most useful of Sir Charles Wheatstone's elegant and beautiful
inventions is the instrument he has supplied to the editor of " The Times"
newspaper to record the number of copies printed and printing. The editor
26 2
404 MAGNETISM.
reads in his own room the progress of that great undertaking, the daily print-
ing of " The Times."
FiG. 388. Wheatstone's Recording Instrument for Newspaper Offices or
Public Buildings.
This instrument will record from ten thousand to one million copies. The
same contrivance the writer hopes to be able to adopt at the Polytechnic, so
that, without moving from his office, he will be able to know the number of
persons in the building.
These instruments culminate to their highest degree of perfection in the
inventions of 1858 and 1867, viz., Wheatstone's Fast-speed Automatic Tele-
graph, of which the inventor gives the following particulars :
" My invention consists of a new combination of mechanism for the purpose
of transmitting through a telegraphic circuit messages previously prepared,
and causing them to be recorded or printed at a distant station. Long strips
or ribbons of paper are perforated, by a machine constructed for the purpose,
with apertures grouped to represent the letters of the alphabet and other signs.
A strip thus prepared is placed in an instrument, associated with a rheomotor
(or source of electric power), which on being set in motion moves it along,
and causes it to act on two pins in such manner that, when one of them is
elevated, the current is transmitted to the telegraphic circuit in one direction,
and when the other is elevated, it is transmitted in the opposite direction ; the
elevations and depressions of the pins are governed by the apertures and
intervening intervals. These currents, following each other indifferently in the
two opposite directions, act upon a printing or writing instrument at a distant
station, in such manner as to produce corresponding marks on a ribbon of
paper moved by appropriate mechanism.
" I will proceed to describe more particularly the several parts of this tele-
graphic system, observing, however, that each part has its independent origi-
nality, and may be associated with other apparatus already known.
" The first improvement consists of an instrument for perforating the slips
WHEATSTON&S TELEGRAPHS. 405
of paper with the apertures in the order required to form the message. The
slip of paper passes through a guiding groove, at the bottom of which an
opening is made sufficiency large to admit of the to-and-fro motion of the
upper end of a frame containing three punches, the extremities of which are
in the same transverse line. Each of these punches is capable of being sepa-
rately elevated by an appropriate finger-key. By the pressure of either finger-
key, besides the elevation of its correspond^ i punch in order to perforate the
paper, two different movements are successively effected first, the raising of
a clip, which holds the paper firmly in its place, and, secondly, the advancing
motion of the frame containing the three punches, by which the punch which
is raised carries the ribbon of paper forward the proper distance during the
reaction of the key consequent on the removal of the pressure ; the clip first
fastens the paper, and then the frame falls back to its normal position. The
two external keys and punches are employed to make the holes which, grouped
together, represent letters and other characters, and the middle punch to make
holes which mark the intervals between the letters. The perforations in the
slip of paper appear thus :
\
j
o o OOP
FIG. 389.
" The second improvement consists of an apparatus which may be called
the transmitter, the object of which is to receive the slips of paper prepared
by the previously described instrument or perforator, and to transmit the cur-
rents produced by a voltaic battery or other rheomotor in the order and direc-
tion corresponding to holes perforated in the slip ; this it effects by mechanism
somewhat similar to that by which the perforator performs its functions. An
eccentric produces and regulates the occurrence of three distinct movements:
1st, the to-and-fro motion of a small frame, which contains a groove fitted to
receive a slip of paper, and to carry it forward by its advancing motion ; 2nd,
the elevation and depression of a spring clip, which holds the slip of paper
firmly during the receding motion, but allows it to move freely during the
advancing motion ; 3rd, the simultaneous elevation of three wires placed
parallel to each other, resting at one of their ends on the axis of the excentric,
and their free ends entering corresponding holes in the grooved frame ; these
three wires are not fixed to the axis of the excentric, but each of them rests
against it by the upward action of a spring, so that when a light pressure is
exerted on the free ends of either of them, it is capable of being separately
depressed. When the slip of paper is not inserted, and the excentric is in
action, a pin attached to each of the external wires passes, during each ad-
vancing and receding motion of the frame, from contact with one spring into
contact with another spring, and an arrangement is adopted, by means of
insulations and contacts properly applied, by which, while one of the wires is
depressed and the other remains elevated, the current passes from the voltaic
battery to the telegraphic circuit in one direction, and passes in the other
direction when the wire before elevated is depressed, and vice versd; but while
both wires are simultaneously elevated or depressed, the passage of the cur-
rent is interrupted. When the prepared slip of paper is inserted in the groove,
and moved onwards, whenever the end of one of the wires enters an aperture
406 MAGNETISM.
in its corresponding row, the current passes in one direction, and when the
end of the other wire enters an aperture of the other row, it passes in the
other direction ; by this means the currents are made to succeed each other
automatically in the proper order and direction to give the requisite variety of
signals. The middle wire only acts as a guide to the paper during the cessa-
tion of the currents.
" The wheel which drives the excentric may be turned by hand or by the
application of any motive power. Instead of a voltaic battery, a magneto-
electric or an electro-magnetic machine may be employed as the source of
electric power. In this case the transmitter and the magneto-electric or
electro-magnetic machine form a single apparatus moved by the same power,
and they are so adapted to each other, that the shocks or currents are pro-
duced at the moments the pins of the transmitter enter the apertures of the
perforated paper.
" The transmitters just mentioned require only a single wire of communica-
tion, and currents in both directions are available for printing the signals ; but
in some cases it may be advantageous to employ two telegraphic wires, and to
use the inversions of current to bring back the pens or markers without the
aid of reacting springs. In this case the only modification of the apparatus
required is in the disposition of the insulations and contacts necessary to trans-
mit in their proper order the currents from the rheomotor into the two wires.
" The third improvement is in the recording or printing apparatus, which
prints or impresses legible marks on a strip of paper, corresponding in their
arrangement with the apertures in the perforated paper. The pens or styles
are depressed and elevated by their connection with the moving parts of the
electro-magnets ; they are entirely independent of each other in their action,
and are so arranged that, when the current passes through the coils of the
.electro-magnets in one direction, one of the pens is depressed, and when it
passes in the contrary direction the other pen is depressed ; when the currents
cease, light springs restore the pens to their usual elevated positions. The
mode of supplying the pens with ink is as follows : A reservoir, about an
eighth of an inch deep, and of any convenient length and breadth, is made in
a piece of metal, the interior of which may be gilt, in order to avoid the corro-
sive action of the ink placed in it. At the bottom of this reservoir are two
holes, sufficiently small to prevent by capillary attraction the ink from flowing
th.'ough them. The ends of the pens are placed immediately above these
small apertures, which they enter when the electro-magnets act upon them,
carrying with them a sufficient charge of ink to make a legible mark on the
strip of paper passing beneath them. The motion of the paper ribbon is pro-
duced and regulated by apparatus similar to those employed in other register
or printing telegraphs.
" Instead of reacting springs for restoring the position of the pens, the
attractive or repelling force of small permanent magnets may be employed.
All the essential parts of my new recording or printing telegraph are included
in the previously mentioned three improvements. The following improve-
ments are either auxiliary or substitutions for parts already mentioned.
" The fourth improvement is an instrument which I call a translator ; its
object is to translate the telegraphic signs, consisting of successions of points
or marks, adopted in this system, into the ordinary alphabetic characters. In
the system I have adopted, limiting the number of points in succession to four,
thirty distinct characters are represented.
WHEATSTON&S TELEGRAPHS. 407
" The instrument presents externally nine finger-stops, eight of which are
arranged in two parallel rows, four in each, and the remaining one is placed
separately.
" The principal part of the mechanism within is a wheel, on the circum-
ference of which thirty types are placed at equal distances, representing the
letters of the alphabet and other characters ; other mechanism is so disposed
and connected thereto, that when the keys of the upper row are respectively
depressed, the wheel is caused to advance i, 2, 4, or 8 steps or letters, and
when those of the lower row are in like manner depressed, the wheel advances
respectively 2, 4, 8, or 16 steps. By this disposition, when the stops are touched
successively in the order in which the points are printed on the paper touch-
ing the first stop for one point, the first and second for two points, &c., and
selecting the stops of the upper or lower row, according as the point is in the
upper or lower row of the printed ribbon the type wheel will be brought into
the proper position for placing the letter corresponding to the succession of
points over a ribbon of paper. The ninth stop, when it is pressed down, acts
to impress the type on the paper, to cause the advance of the paper, in order
to bring a fresh place beneath the type-wheel, and subsequently to restore
the type-wheel to its initial position.
" The fifth improvement is a modification of the electro-magnets of the
instrument of the third improvement, which enables the pens to go back to
their normal positions when the currents in the telegraphic circuit cease, with-
out the aid of reacting springs or permanent magnets. An extra coil of wire
is wound round each of the electro-magnetic bars, which act on one side of
each of the double magnetic needles appropriated to the two pens. These
coils are entirely insulated from those connected with the telegraphic circuit,
and form together a short local circuit, in which a feeble voltaic current con-
tinually circulates, in consequence of the interposition of a small rheomotor;
by this current the needles are held, when no current exists in the telegraphic
circuit, constantly attracted towards these electro-magnets. When, however,
the current transmitted through the telegraphic circuit acts on the coils, besides
its direct action to cause the deflection of one of the double needles and the
detention of the other, it neutralizes the current of the local battery in that
electro-magnet where its effect for the time would be disadvantageous.
"The sixth improvement consists in the application of ribbons of paper
prepared by the perforator, and passed through the transmitter as heretofore
described, to produce the successive motions of a magnetic needle or needles
corresponding to the signals required, whether separately employed for this
purpose or in conjunction with the printing apparatus already mentioned."
Even these beautiful instruments were not considered perfect by the inde-
fatigable inventor, and we again find him, after a most severe illness, recording,
in 1867, further great improvements in the mechanism of all their parts.
IMPROVEMENTS IN ELECTRIC TELEGRAPHS, AND IN APPARATUS
CONNECTED THEREWITH.
"My present invention (1867) consists in certain improvements in the various
instruments constituting the electric telegraph system described in the speci-
fication of the patent grantod to me on the second day of June, A.D. 1858,
No. 1239.
"This system comprises three distinct apparatuses: first, a perforating
MAGNETISM.
machine for preparing the messages to be sent on the strips of paper or
other suitable material ;
" Second, a transmitter, or apparatus for receiving the strips of paper so
prepared, and for transmitting the currents produced by a voltaic battery,
magneto-electric machine, or other rheomotor, in the order corresponding to
the holes perforated in the strip, the direction and sequence of these currents
being governed by pins, or other suitable apparatus, disposed so as to enter
the perforations, and operating in a manner analogous to that in the mechanism
of a Jacquard loom, and the strip being advanced intermittingly by the action
of pins or other apparatus appropriated for that purpose ;
" And, third, of a recording or printing apparatus adapted to print or impress
marks on a strip of paper, such marks corresponding in their arrangement
with the currents transmitted to the telegraphic line and with the apertures
in the perforated paper.
" Having separately described each system of recording telegraphs, with the
improvements which form the objects of the present specification, 1 proceed
to designate those points which I specially claim as new.
" First, the modification of the perforator for the dot-printing telegraph,
which enables it to prepare the strips of paper with an uninterrupted series
of central apertures ; this modification, described as the first improvement,
consists of the mechanism being so arranged that when either of the keys
corresponding with the outer apertures is depressed, besides acting on its own
punch, it carries with it the punch which corresponds with the central apertures,
while the latter is alone acted upon by means of another key causing the per-
foration only of a single aperture at a time.
v " Second, the modification of the perforator, described as the fourth improve-
ment, having five punches, and the mechanism so arranged that, when the first
key is pressed, three of the punches in the order described are simultaneously
acted upon ; when a second key is depressed, four of the punches are in like
manner simultaneously acted upon ; and when a third key is depressed, the
single punch only of the central line is acted upon. I claim also, in connec-
tion with this arrangement, the mechanism by which when either the first or
third keys are pressed down the paper advances only a single space, and when
the second key is depressed it advances two spaces ; but be it understood that
I do not claim the advance of the paper by unequal spaces, unless in connec-
tion with the arrangement of the punches described.
" Third, the additions of extra keys to the preceding modification of the
perforator, with additional punches, described in the fifth improvement, which
are so arranged that each additional key when depressed, while it punches
simultaneously all the required apertures, shall advance the paper at once
three, four, or more steps, so that all the perforations may be simultaneously
made which are necessary to cause lines of the various required lengths to be
marked or printed by the receiving instrument.
" Fourth, the modification of the transmitter, described as the second im-
provement, whether actuated by a magneto-electric machine or by a voltaic
battery, in which the central needle alone has a to-and-fro motion for the
purpose of propelling forward the strip of paper by means of the central
apertures alone, and not also by means of the external apertures and outer
pins, as described in the second improvement of the specification of my patent,
No. 1239 ( A - D - 1858).
" Fifth, the modification of the transmitter, described as the sixth improve-
WHEATSTQNE'S TELEGRAPHS.
409
ment, which is adapted to send into the telegraphic circuit short currents at
various intervals and alternately in opposite directions, so as to determine the
occurrence of printed lines and intervals of various lengths in the receiving
instrument : in this modification one current-governing needle has a to-and-
fro motion simultaneously with the central needle, while the other has no such
motion, the latter acting only while the paper is at rest, and the former while
it is in motion.
" Sixth, the modification of the transmitter, described as the eighth improve-
ment, which is suited to send into the telegraphic circuit currents of various
lengths in one direction only in a different way to that described as the seventh
improvement in my patent, No. 2462 (A.D. 1860). The characteristics of this
new method are, first, that lines of any lengths can be produced, instead of
lines of two different lengths only; second, that the short lines occupy a
shorter space on the paper than the long lines do ; and, third, that strips of
paper prepared by the perforators of the third and fourth improvements may
be employed to regulate the motions of the needles in order to produce the
required effects.
" Seventh, the modification of the dot-printing receiving instrument, de-
scribed as the third improvement, in which the pens or markers are acted
upon by one set of electro-magnets and magnetic bars, instead of by two
sets, as described in the specification of my patent, No. 1259 (A.D. 1858).
" Eighth, that modification of the printing apparatus of the receiving instru-
ments of the second and third systems described as the eighth improvement,
by means of which lines of various lengths are printed with great rapidity,
certainty, and distinctness. The characteristic distinction of this mode of
printing is, that the inking-disc and tracing-disc are both independently kept
in motion by the maintaining power, and are not in actual contact with each
other, and that the ink is retained on the circumference of the inking-disc by
capillary attraction."
We now give the description of the three instruments :
I. The perforator.
II. The transmitter. .
III. The recorder.
FIG. 390. The Perforator (1867).
" The present improvement provides for the continuity of the middle perfora-
tions of the paper strip. The punching-plate carries three punches (Fig. 391),
4io
MAGNETISM.
placed transversely to the path of the paper through the machine. Three lever
finger-keys act upon the punches in such a manner that whenever either of the
outer keys is depressed, it acts upon the punch which belongs to it, and at the
* o
/0 Oa,
e O 06
FIG. 391.
same time carries with it the middle punch by means of a collar which is fixed
thereto, and simultaneously perforates the two apertures ; but the depression
of the middle key acts upon the middle punch alone, and perforates a middle
aperture only, which is equivalent to a space in the receiving instrument.
On the removal of pressure from any finger-key, the corresponding punch or
punches is or are restored to its or their normal positions by means of a re-
acting spring or springs. A lever and link arrangement, moved by either of
the three keys, draws back the paper-moving lever during the depression of a
key ; the release of a key permits a reacting spring to force the paper-mov-
ing lever forwards and to advance the paper one step, the said lever having
a rough end next to the paper strip for that purpose: this mechanism propels
the paper quite independently of the middle row of holes.
" Fig. 392 is a perspective view of a transmitter arranged to work with two
line wires ; in this instrument, besides the necessary change in the insulations
and contacts, the mechanical arrangements, are slightly varied, the construc-
tion shown being more convenient when two line wires are employed than
that first described, a is a permanent magnet, and b is an armature mounted
on an axis c, so as in revolving to pass in front of the poles of the magnet.
On the axis c there is a toothed wheel, d, which drives the pinion e on the
vertical axis/, so that this axis makes twice as many revolutions as the axis c ;
at the upper end of the axis/ is a cam, g, arranged to act on the pin //, which
is mounted on a rocking-frame similar to the rocking-frame of the transmitter
already described. The pin h is kept in contact with its cam g by a spring i.
The form of the cam is such that the forward motion of the frame is gradual,
but its return motion takes place as rapidly as the spring i will react. ._/ is
another cam on the axis/; it comes in contact with a projection on the lever
k just as the return motion of the rocking-frame is going to take place, and
so Causes this lever to draw down the three needles carried by this frame. At
the; same time the tail of the lever k presses on the end of another lever /,
which is fixed to the spring-clip ;, and so causes the clip, by turning slightly
on its axis, to nip the paper under it. It will be seen that the two outside needles
carried by the rocking-frame have projections from their lower ends, and when
they are allowed to rise by the perforated paper, as before explained, their ends
come in contact with the springs n and 0, which are insulated from the rest of
the instrument, and are in communication with the two line wires. On the
WHEATSTQNE'S TELEGRAPHS.
411
FIG. 392. The Transmitter (1858).
axis c a metal disc is mounted ; it is made in two parts, p and ^, which are
insulated from each other and from the axis, r and s are two springs, which
press on the periphery of the disc as it revolves ; the spring r is in metallic
communication with the working parts of the instrument, and th^ spring s is
insulated from these parts, but is put into metallic connection with the earth.
When one of the needles of the rocking-frame comes into contact with its
corresponding spring, n or 0, it brings the line wire in connection with the
spring into metallic communication with the working parts of the instrument,
and any currents or shocks transmitted to these flow into the line wire. From
the construction of the apparatus, the contact between the needles of the
rocking-frame and their corresponding springs when established lasts during
half a revolution of the axis c, and in this period two currents in opposite
directions are transmitted into the line wire. The first current acts to bring
one of the pens or markers of the receiving instrument into contact with the
surface to be marked, and the second current to bring this pen or marker to
its original position. It is evident that, if necessary, the instrument above
described may be worked with one line wire only, without any change being
made in the instrument ; all that is necessary is that, in perforating the strip
412
MAGNETISM.
for the message, only one of the outside finger-keys of the perforator should
be employed (the alphabet or signs employed being modified accordingly;.
Or the perforating instrument and the transmitting instrument may both be
modified if desired, so as to be suitable only for working with one line wire,
by constructing the perforator with two in place of three finger-keys and
punches, and the transmitter with two in place of three needles.
FIG. 393. The Recording or Printing Instrument (1858).
Another improvement is in the recording or printing apparatus ; but as
the chief parts of this instrument have already been described with sufficient
minuteness, it is only necessary to refer our readers to page 406 for the details
of the beautiful mechanism which regulates the marking of the slips of paper
and the supply of ink to the dotting apparatus.
The improved instruments are now working between London and New-
castle, Edinburgh, Manchester, and Glasgow; and they can send and print
messages from seventy to one hundred and twenty words per minute, accord-
ing to their exigences. They are also used in connection with the submarine
cable extending from Newcastle to Denmark.
WHEATSTONE'S TELEGRAPHS.
SIR CHARLES WHEATSTONE'S LAST AND MOST COMPLETE
TELEGRAPHIC APPARATUS,
AND OTHER BEAUTIFUL APPLICATIONS OF ELECTRICITY THE CHRONO-
SCOPE AND TELEGRAPH THERMOMETER FOR GREAT ALTITUDES.
J I
X .
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H 1
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5i S ^ T3
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rapid working ; therefore, Sir Charles br
inking Morse. But words could be trans
would print ; therefore, it remained for Sii
FlG. B The Line-printing Transmitter.
which he accomplished, and it is now known by the name of the "line-printer,"
printing the dot and dash alphabet (No. 3, Fig. A), such as is used by all tele-
graph companies, printing 600 letters per minute; the dot and line printing
differing especially in this respect the line currents always being inverted
alternately; in the dot, three or four currents in the same direction sometimes
follow each other.
THE LINE TRANSMITTER WITH MAINTAINING POWER (Fig. B), is a modi-
fication of the transmitter described as the sixth improvement for receiving the
strip prepared by either of the perforators described as the fifth improvement,
and transmitting voltaic currents along the telegraphic conductor to the receiv-
ing instrument at the distant station, in accordance with the arrangement of the
WHEATSTON&S TELEGRAPHS. 415
perforations in the paper strip (motion being produced by a weight) ; the pro-
pulsion of the paper strip and the makings of the contacts with the batteries
are accomplished by the same power; and, by means of levers, beam,. eccen-
tric, and springs, the upper ends of two vertically moving pins, being alternately
pressed against the paper, are free to enter the perforations, if any present
themselves; or, being prevented from entering the paper by the absence
of apertures, they regulate the succession, frequency, and direction of the
electric currents -sent into the telegraphic circuit.
The action of the pins in conjunction w'ltu trie paper strip is as follows: the
FIG. Q.Line Printer or Receiver.
only means of propulsion of the paper is by the pins of a star-wheel entering
the middle perforations, and by its rotation moving the paper forward, the
strip being held down by a broad-toothed wheel pressing it against the paper-
ledge, the vertically moving pins entering the notches in the before-mentioned
wheel, pass through an aperture in the paper, and are carried forward by it,
thus not interfering with the duration of contact at the lower end of the pins ;
the reacting springs restore them to their normal position on their downward
movement, effected by the levers to which they are attached receiving an
up-and-down motion from an oscillating beam, connected with an eccentric
driven by the maintaining power ; and, on the arrival of an outer aperture
on one side of the middle line of holes, the pin of that side will enter and
transmit a current in one direction ; and on the presentation of an aperture on
the opposite side, the pin will also enter and transmit a current in an opposite
direction, the apertures in the paper regulating the frequency, direction, and
duration of the current sent into the telegraph line.
In the Line Printer or Receiver (Fig. C), the magnetic armatures are placed
in a vertical position ; the central axis is prolonged so as to carry the cross-
piece, through an aperture in the extremity of which a horizontal rod passes ;
on this is mounted at one extremity the small, light tracing-disc, whilst the
opposite end, which is loosely centred, so as to be capable of a slight lateral
movement, carries a small toothed wheel ; this wheel, gearing with the main-
416
MAGNETISM.
taining power of the instrument, imparts a rotatory motion to the tracer, at
the same time that the axis is capable of receiving a to-and-fro motion in a
horizontal plane from the movement of the armatures and arm.
In the same vertical plane, and immediately beneath the tracing-disc, is an
inking-disc, caused to rotate, by appropriate gearing, with the maintaining
power of the apparatus : this disc revolves in a reservoir containing ink or
other suitable marking fluid. The periphery of the disc is slightly hollowed,
and the edge of the tracing-disc just enters this hollow without contact or
friction with the inking-disc ; during the revolution of the disc, capillary at-
traction keeps the hollow full of ink, and a constant and uniform quantity will
be supplied to the tracing-disc.
FlG. D. The various parts of Apparatus used with Wheatstone''s
Chronoscope.
The paper intended to receive the marks is drawn forward at suitable speed
over a roller in close proximity to one edge of the tracing-disc. It will be
understood that a series of instantaneous alternate currents passing through
the electro-magnet causes a to-and-fro motion of the tracing-disc, a current in
one direction pressing the tracing-disc against the paper, where it will remain,
by reason of the residual magnetism of the electro-magnets retaining the
armatures in that position, until a current in the opposite direction withdraws
the tracer from the paper. By this arrangement lines of more than two lengths
can be printed with perfect accuracy in connection with the perforator with five
keys described as the fifth improvement. Another remarkable instrument is
WHEATSTONE'S CHRONOSCOPE. The various parts of this arrangement
are shown at Fig. D, and employed to ascertain the velocity of projectiles.
They will be readily understood when we describe the ball-holder and target
used in the falling bodies experiments. A and B are enlarged parts of screens \
WHEATSTON&S TELEGRAPHS. 417
C is the ball-holder closed to receive the ball, each side being insulated. The
electric circuit is not complete ; but, at the moment of the release of the ball,
the two sides will meet and complete the circuit, which, traversing in one
direction, will start the chronoscope : this will continue running until the ball
strikes the target, when it will reverse the current and stop it. The method of
reversing is readily understood by E and D, Fig. D. Two springs are fixed to the
target, which is hinged at one end, the other end falling when the ball strikes
it. The springs slide over the reversing-piece, consisting of two poles of the
battery, which are bridged over at the back, as indicated by the dotted lin.es, E.
FIG. E. The Ckr0,n&sc&pe in Elevation.
Fig. E represents the chronoscope as arranged for indicating^aiitomatically
the time occupied by falling bodies. A is a column, upon which the ball-holder
slides, the target being placed at the base; B is the chronoscopy consisting of
clockwork mechanism, with two dials, one divided into hundredtjis, and the
other into thousandths, of a second, with hands like a watch, motion being com-
municated toitbyaweight passing overapulley, which is regulated byanescape-
ment with a musical spring, tuned to a thousandth part of a second, caused to
sound by the pressure of air from the bellows, C. The clockwork is in two dis-
tinct parts, the driving and the dial parts ; they are made to gear by sensitive
magnetic needles and an electro-magnet. One pole of the battery is connected
with the ball-holder, the other with the target,* two wires from the target
connect' it with the chronoscope, one wire connecting the ball-holder with the
target. The poles of the battery are so arranged that on the release of the ball
the electric circuit is completed, and the dials are brought into gear with the
driving part ; the current is reversed the instant the ball strikes the target, and
27
4i8
MAGNETISM.
FlG. F. Wheatstone's Projectile Arrangement.
The targets, B and c, connected with the battery, D, and Wheatstone's chronoscope, arranged to
receive and indicate the velocity of the shot from the Armstrong gun, A.
the dials are disengaged, enabling the operator to read off the time by the
hands, without the tedious calculation necessary by other means generally
employed. The almost inexhaustible inventive faculty of Wheatstone, ever
devising new or improving older inventions, is again displayed in his New
Telegraph Thermometer (Fig. G).
This instrument was invented by Wheatstone to supply a scienific want,
viz., the means of ascertaining, day or night, without making tedious ascents,
the temperature of any lofty summit such as that of Mont Blanc.
The cut (Fig. G) represents the general internal arrangement of the instru-
ments requisite to ascertain the temperature at a distant point, two insulated
wires connect them, the earth being used to form the third conductor.
' The apparatus includes the thermometric arrangement, and also an electro-
magnetic contrivance for converting the vibrations of magnetic needles between
electro-magnets into a circular motion, for the purpose of altering the electric
conduction from one circuit to another.
In order to indicate the temperature measured by the instrument above
mentioned, there is an electro-magnetic arrangement, and also a permanent
compound magnet with fixed coils, having an armature opposite to its poles,
capable of being rotated by a handle, to produce a series of alternately inverted
currents.
Fig. G, p. 419, represents the internal construction of both instruments ; the
dotted and other lines represent the wires necessary to conduct the electric
currents. In the knob A, which is attached to the glass covering the dial, is
contained a metallic thermometer, having a hand or pointer attached to its 
