Elementary treatise on physics, experimental and applied, for the use of colleges and schools

Google

This is a digital copy of a book that was preserved for generations on library shelves before it was carefully scanned by Google as part of a project
to make the world’s books discoverable online.

It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject
to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country, Public domain books
are our gateways to the past, representing a wealth of history, culture and knowledge that’s often difficult to discover.

Marks, notations and other marginalia present in the original volume will appear in this file - a reminder of this book’s long journey from the
publisher to a library and finally to you.

Usage guidelines

Google is proud to partner with libraries to digitize public domain materials and make them widely accessible. Public domain books belong to the
public and we are merely their custodians. Nevertheless, this work is expensive, so in order to keep providing this resource, we have taken steps to
prevent abuse by commercial parties, including placing technical restrictions on automated querying.

We also ask that you:

+ Make non-commercial use of the files We designed Google Book Search for use by individuals, and we request that you use these files for
personal, non-commercial purposes.

+ Refrain from automated querying Do not send automated queries of any sort to Google’s system: If you are conducting research on machine
translation, optical character recognition or other areas where access to a large amount of text is helpful, please contact us. We encourage the
use of public domain materials for these purposes and may be able to help.

+ Maintain attribution The Google “watermark” you see on each file is essential for informing people about this project and helping them find
additional materials through Google Book Search. Please do not remove it.

+ Keep it legal Whatever your use, remember that you are responsible for ensuring that what you are doing is legal. Do not assume that just
because we believe a book is in the public domain for users in the United States, that the work is also in the public domain for users in other
countries. Whether a book is still in copyright varies from country to country, and we can’t offer guidance on whether any specific use of
any specific book is allowed. Please do not assume that a book’s appearance in Google Book Search means it can be used in any manner
anywhere in the world. Copyright infringement liability can be quite severe.

About Google Book Search

Google’s mission is to organize the world’s information and to make it universally accessible and useful. Google Book Search helps readers
discover the world’s books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web

aifhttp: //books . google. com/|

TRANSLATORS PREFACE
rm
THE FIRST EDITION.

Ts Bléments de Physique of Professor Gaxor, of which the present
Work in = translation, has acquired a high reputation as an Intro-

with which the principal physical laws and phenomena are ex-
Pinel, to its methodical arrangement, and to the excellence of its
Whstrations. In undertaking a translation, I was influenced by the
favourable opinion which a previous use of it in teaching had
wubled me to form.

Tiund that its principal defect consisted in its too close adapta~
tion to the French systems of instruction, and accordingly, my chief
Jabour, beyond that of mere translation, has been expended in making
meh alterations and additions as might render it more useful to the
‘English student.

Thave retained throughout the use of the centigrade thermometer,
Siesta ere cries toe msalie Hinser smesexres on the

ADVERTISEMENT
vo
THE THIRD EDITION.

———

Tur mars> SALE of a large impression of the Second Edition of the
Translation of Gaxor's Pursics is mainly due to its increased adoption
ma text-book in schools and colleges.

The alterations in the present edition comprise 57 pages of

Widitional matter, and 49 new illustrations, In making these
alterations, while the wants of the general reader have been attended
to, the main object of the Editor has been to render the book more
efiul as a text-book for the student of physical science. Accordingly,
4 regards new matter, tho main additions have been in those
mibjects which are calculated to take a permanent place in elementary
instruction. Some parts, too, which the experience of teachers had
indicated as being treated with too great brevity huve been expanded.
This is more especially the case with the Book on Mechanics, for a
tumplete revision of which the Editor is indebted to his colleague,
the Rev. J. F. T'wisnen, of the Staff College.

Saxpnvusr: Joly 1968,

CONTENTS.

oo

BOOK I.

ON MATTER, FORCE AND MOTION.

cur,
L Geremat Notioxs - .

U. Gexenat Proveeries or Boomes .

TL Ox Ponce, Eauremurcy, axp Morion -

BOOK IL.
GRAVITATION AND MOLECULAR ATTRACTION.
TL Graver, Oxerue ov Gnavrrr, tan Batance . .

TL Laws or Farixa Booms, Lerexarry ov Teuussratat GRavrre,
sux Pexprere ;

TL Morecurar Forces

BOOK HL
ON LIQUIDS.
L Hymostarcs .

TL Capritasrrr, » Baoan, Barono, laseoxrnen, aAXD Toenrnt-
0x é ; 5

BOOK IV.
‘ON GASES.

1. Prorzerms or Gases, Armosrmene, Banowerens .
FL Metscxmumxr or tae Exasric Force ov Gases
TL, Paesscxe on Bootes mw Am. Batroows .
Iv. Ayranares Fouxonp ox tux Puovmstiss or Ate

BOOK V,
Acoustics,
mar. rack |
1. Pnopveriox, Proraoatiox, axn Revixcriox or Sox . 161 |
TL, Messemancr or tux Nustwma ov Vumaroxs. .. |. 178
TLL, Tux Parsicar Taxouy or Mus. aa
IV, Visuartoxs or Srumrcuxy Stuvos, axp or Cotuaxs or Am . 190
V. Vimnations or Rops, Prats, ano Mupzases, =. . 208
BOOK VL
ON HEAT. |

T. Pertnoxany Ipmas Tuenwomerers 2 2... OM

Il. Exraxsion or Sours . . . . - . Py - 226
TH. Exraxsiox or Liauins . . . . . « 235
TV. Exraxsion axp Density oF Gasns . . : . 40
V. Cnaxaxs oy Coxprrion. Varouns .
VI. Hyoxowerey . 7 5 + 295
Coxpverreire or Sousa, Liquins, axp Gases + + 808
Rapiarion or Heat. . . . . . . 30
Catonmrerny . . . ‘3 . . . ~ B48
Srmax Exons + . . . ~ 366
Rooncesor Haram Cn. . . . sO. Se

XI. Mectaricat Equivatest oy Hxat + . . - 390

BOOK VIL.

ON LIGHT,

I. Transmission, Vavociry, asp Leruxsrry or Laour . . 395
TL Rerrecrion oy Lior, Munnons < 7 . oe
TL Storm Revracriox, Lenses . . . é . . ~ 426
IV. Disrxnsion axn Acwromatism . Fy . . . . « 446
¥. Orticar Ixsreuments =. Al-, 2° 6" eae
VL Taw Ere Coxswanen as ax Ormeas Ixeravicert - + 498

VIE. Sovnces or Iicnr. Posrnommcerce . . . . . $12
VIE, Dovare Rereaction, Lerenrxmence, Potantsation . 516

+ 738
+ 778

« 782

Reser. =

« 790
. 803

- Se

LIST OF

Pack
Amoumxo powers. 320
Absorption of gasos . 105
— heat by gases 340
——— liquids 883, 884
—— — vupours 836, 342

Busaxoxo weight of substances, 64

Boiling point 267, 268
‘Cosuvsrion, heat of . 384
Conducting powers for heat 306
Conductors of electricity 586
Denarrins of gases . 48

‘of vapours é 293
Diathermanous powor 331
Diffasion of solution . 102
Exposmoric equivalents 101
Electrical series . 690
Electromotive forces . 657
—fiorce of different elements . 783

Expansion, coefficiontsof solids?

——liquids

——pases 45
Eye, dimensions of 496
— refractive indicos of modia of 496
Faoaxo mixtures . 2. 254
Fusing points of bodies . . 249

Grarsamn’s factors 301
Gravity, force of atdifferentlovels 64

TABLES.

Haxpwess, sealo of .

Larext heat, of evaporation.
—— liquefaction

Maoxvnic declination
— intensity.

Rantarma powers
Radiation of powders,
ion of angle of double

Refract

Refractive indices 434,
——of media cfeye 5s
Reflecting powers.

Spxcmc gravity of solids.
—— liquids ioe
— heat of solids and liquids
— — gases :
— inductive

apacities

‘Tecpxuatonns, various remarkable

— af different Intitudes

— thermal springs ge

Tension of aqueous vapour. 263,
— different liquids. s

Usnutarions, longth of

Vexocrrr of sound in rocks
——— gases .
——— liquids
——— woods.

ELEMENTARY TREATISE

PHYSICS.

BOOK I.
ON MATTER, FORCE, AND MOTION.

CHAPTER L
GENERAL NOTIONS.

1 opject of Physios.—The object of Physics is the study of the
Prevented to us bf bodies. It should, however, be added,
What changes in the nature of the body itself, such as the decomposition
if ine body into others, are phenomena whose study forms the more
Immediate object of chemistry.
| 2 peatter.—That which possesses the properties whose existence is
Sees Seb oe eres, wo al! matter or substance,

‘All substances at present known to us may be considered as chemical
Puibinstions of sixty-five elementary or simple substances. This number,
Bowever, may hereafter bo diminished or increased by # more powerful
‘chemical

analysis,
_& Atoms, Molecules.—From various properties of bodies we con-
Wale that the matter of which they are formed is not perfectly
WiBtignons, bat consixts of an aggregate of an immense number of
‘aeeedingly small portions or atoms of matter \e8¢ atoms cannot be
Witeied physically, they are retained side by side, without touching each
‘ther, by means of certain attractions and ropulsions, to which the namie |

malewlar forces ix given.

4 ON MATTER, FORCE, AND 210TI0N. [e-

4, Molecular state of bodies.—With respect to the molecules of
bodies three different states of aggregation present themselves,

First, the solid state, a8. observed in woods, stonos, metals, &c., at the
ordinary temperature. The distinctive character of this state is, that
the relative positions of the molecules of the bodies cannot be changed
without the expenditure of more or less force, Aa a consequence, solid:
bodies tend to retain whatever form may have been given to them by
nature or by art,

Secondly, the liquid state, as observed in water, alcohol, oil, &e. Here
the relative position of the moleculos is no longer permanent, the
molecules glide past each other with the greatest ease, and the body
assumes with readiness the form of any vessel in which it may be
placed.

Thirdly, the gaseous state, ax in air. In gases the mobility of the male=
cules is still groater than in liquids; but the distinctive character af a
gaa is its incessant struggle to occupy a greater volume, or the tendeney
of its molecules to recede from each other.

‘The general term flvid is applied to both liquids and gases,

We shall see in the sequel that the state of a body depends upon the
relations which exist between its molecular attractions and repulsions,
and that for one and the same body these relations vary with the
temperature. On this account most simple bodios, and many compound
ones, may be made to pasa successively through all the three states
Water presents the most familiar example of this,
5, Physical phenomena, laws, and theortes.—Lvery change
which can happen to a body, mere alteration of its chemical constitution
being excepted, may be regarded as a physical phenomenon. The fall of
stone, the vibration of a string, and the sound which accompanies it, the
rippling of the surface of a luke, and the freezing of water, are oxamples
of such phenomena,

A physical /aw ia the constant relation which exists between any phe~
nomenon and its cause. As an example, we havo the phenomenon of.
the diminution of the volume of a gas by the application of pressure;
tho corresponding law hus been determined, and is expressed by saying
that the volume of a gas is inversely proportional to the pressure,

The whole of the laws referring to the same class of phenomena, taken
together, constitute a physical theory. Thus we have the theory of light,
the theory of electricity, and, in more restricted forma, the theory of dew,
and the theory of the mirage. |
6. Phystoal agents.—In our attempts to aseond froma phenomenon —
to its cause, we assume the existence of physical agente, ot natural forces,
necting upon matter; ae examples of euch we have gravitation, heat, oa
amagmetion, and electricity

4 OX MATTER, FORCE, AND MOTION. [s-

Ai its constituents ; for instance, the volume of a mixture of water and sul-
phiuric acid, or of water and alcool, is less than the sum of the volumes
before mixture. In all these cases, howover, the penetration is merely
apparent, and arises from the fact that in every body there are interstices
‘or spaces unoccupied by matter.

9, Bxtension.— Extension or magnitude is the property in virtue of
which every body occupies a limited portion of space.

Many instruments have been invented for measuring linear extension
or lengths with great precision, Two of these, the vernier and micro-
meter screw, on account of their great utility, deserve to be here
mentioned.

10. Vernier.—The vernier forms a necessary part of all instruments
where lengths or angles have to be estimated with precision
its name from its inventor, a French mathematician, who died in 1637,
ani consists essentially of a short graduated scale, ab, which is made to

: = —t

Fig. 1,

slide along a fixed scale, AB, so that the graduations of both may be com
pared with each other, The fixed scale, AB, being divided into equal
parts, the whole length of the vernier, ab, may be taken equal to nine
of thoso parts, and itself divided into ten oqual parts. Each of the
parts of the vernier, ad, will then be less than a part of the scale by one
tenth of the latter,

This granted, in order to measure the length of any object, mu, lot wa”
suppose that the latter, when placed agin the figure, hns a length greater
than four but Jess than five parts of the fixed scale. In order to deter=
mine by what fraction of a part mn excoeds four, one of the ends, a, of —
the vernier, ab, is placed in contact with one extromity of the object, mn, |
and the divigion on the vernier is sought which coincides with a division |
on the scalo, AB. In the figure this coincidence occurs at the eighth
division of the vernier, counting from the extremity, m, and indicates
thut the fraction to be measured is equal to % of a part of the scale,
AB. In fact, each of the parts of the vernior being less than a part of
the scale by j; of the latter, it is clear that on proceeding towards

the left from the point of coincidence, the divisions of the vernicr are

aa] GEXERAL PROPERTIES OF BODIES, 5

eee wen ne abled te ins Fin

that the extremity, m, of the object (that is to say, tho eighth division
ofthe version) is; Lehiod the division marked 4 ox the scale; in otber
words, the Length of ma is equal to 4,5; of the parts into which the scale
AB is divided. Consequently, if the scale AB were divided into inches,
the lemgth of mm would be 4;=4} inches. The divisions on the scale
pemaining the sume, it would be necessary to increase the length of the
vernier in order to measure the length mn more accurately. For instance,
if the length of the vernier were equal to nineteen of the parts on the
‘wale, and this loagth were divided into twenty equal parts, the length
em could be determined to the twentieth of a part on o scale, and so
em. Tn instruments, like the theodolite, intended for measuring anglea,
the seale and vernier have « circular form, and the latter usually carries
8 magnitier, in order to determine with greater precision the coincident
divisions of vernier and scale.

11. Bticrometer screw.—Anothor useful little instrument for mea-
exting small lengths with precision is the micrometer screw. Jt ix used
under various forms, but the principle is tho same in all, and may be
ilustrated by a simple example, Suppose the distance between the
threads of an accurately cut screw to be equal to J, of an inch, and the
‘head of the screw to be « tolerably large circle divided into one hundred
equal parts. If tho screw ix fixed in such u manner that it can only
urn on iis axis, but neither advance nor recede, and if it work in a nut
bald between guides which prevent it from turning, then every turn of
the screw will cause the nut to advance through the tenth part of an inch.
If a fixed pointer be placed before the divided circle at the head of the

snd the Intter turned through #o small an angle that only ono
division of the circle passes under the pointer, the hundredth part of a
turn will have been given to the screw, and the nut thereby caused
fe advance or recede through the hundredth part of the distance between
wo threade—that is to soy, through the ;2y5 part of an inch. Applica-
Hons of this principlo to the measurement of small longths will at once
awl and be readily understood when seen,

‘12, Divistusty—is the property in virtue of which a body may bo
tivided into distinct parts,

‘Wumerous examples may be cited of the extreme divisibility of matter.
Retr agrain of musk will continue for years to fill a room

‘Ite odoriferous particles, and at the end of that time will scarcely

dix composed of rvd, flattened globules floating in a colourless
‘called serum. In man the diameter of one of these globules is
‘the 3,500th part of an inch, and the drop of blood which might
from the point of # needle would contain about a million

6 ON MATTER, FORCE, AND MOTION. fas-

Again, the microscope has disclosed to us the existence of insects
smaller even than those particles of blood; the struggle for existence
reaches even to these little creatures, for they devour still smallor ones,
Tf blood runs in the veins of these devoured ones, how infinitesimal
must be the magnitude of its component globules?

Has then tho divisibility of matter no limit? Although experiment
fails to determino such limit, many facts in chemistry, such as the in
variability in the relative weights of the elements which combine with
each other, would lead ws to believe that a limit does exist. It is om
this account that bodies are conceived to be composed of extremely
minute and indivisible parts called atoms (3).

18. Porosity.—Porosity is the quality in virtue of which interstices:
or pores exist botween the molecules of a body.

Two kinds of pores may be distine
guished: physical pores, whore the
interstices ars so small that the sur-
rounding molecules remain within the
sphere of each other's attracting or
repelling forees; and sensible pores, or
actual cavities across which these mo-
lecular forces cannot act. The cone
tractions and dilatations resulting from
variations of temperature are due to the
existence of physical pores, whilst in the
organic world the sensible pores are the
seat of the phenomena of exhalation and
absorption,

Tn wood, sponge, and » great number
of stones, e.g. pumice stone, the sensible
pores are apparent; physical pores never
are. Yet, since the volume of every
body may be diminished, we conclude
that all possess physical pores.

The existence of sensible pores may
be shown by the following experi-
ment:—A long glass tubo, A (fig. 2), ie
provided with s copper tube, sm, at the
top, and a copper foot made to screw on
to the plate of a machino for exhausting
air. The bottom of the cup consists of

Fig. 2. a thick piceo of loather. After pouring
merenry into the cup s0 a4 entirely to cover the leather, the air-pump is
put io setion, and a partial yacuum produced within the tube, By so

8 ON MATTER, FORCE, AND MOTION.

under ordinary circumstances, In most cases, however, there is a
beyond which, when the pressure is increased, they become liquids.
‘The compressibility of solids is much less than that of gases, and &
found in all degrees. Stuffs, paper, cork, woods, are amongst the most
compressible, Motals are so also to n great extent, as is proved by the
process of coining, in which the metal receives the impression from the
die, There is, in most casos, a limit beyond which, when the pressure is
increased, bodies are fractured or reduced to powder,

‘The compressibility of liquids is so small as to have remained for #
long time undetected : it may, however, bo proved by experiment, as will
be seen in the chapter on Hydrostatics.

17. Btasticity.—Elasticity is the property in virtue of which bodies
resume their original form or volume, when the force which altered
that form or volume ceases to act. Elasticity may be developed in
bodies by pressure, traction, flexion, or torsion. In treating of the
general properties of bodies, the elasticity developed by pressure alone
requires consideration ; the other kinds of elasticity being peculiar to
solid bodies, will be considered amongst their specific properties (arts,
68, 60, 70).

Gases and liquids are perfectly elastic; in other words, they regain
exactly the same volume when the pressure becomes the same, Solid
bodies present different degrees of elasticity, though none present the
property in the same perfection as liquids and gases, and in all it varies
according to the time during which the body has been exposed to
pressure. Caoutchoue, ivory, glass, and marble possess considerable
elasticity ; lead, clay, and fats, scarcely any.

There is a limit to the elasticity of solids, beyond which they either
break or are incapable of regaining their original form and volume,
In sprains, for instance, the elasticity of the tendons has been exceeded.
Tn gases and liquids, on the contrary, no such limit can be reached ; they
always regain thoir original volume,

Tf a ball of ivory, glass, or marble, be allowed to fall upon a slab of
polished marble, which has been previously slightly smeared with oil,
it will rebound and rise to a height nearly equal to that from which it
fell. On afterwards examining the ball a circular blot of oil will be
found upon it, more or less extensive according to the height from which
it fell. From this we conclude that at the moment of the shock the ball
was flattened, and that its rebound was caused by its effort to regain its
original form.

18. Mobility, motion, rest.— Mobility is the property in virtue of
which the position of a body in space may be changed.

Motion and rest may be cithor relative or absolute, By tho relative
motion or rest of & body we mean its change or permanence of position

“GENERAL PROPEETIZS OF poptrs, 9
surrounding bodies; by ita abeolwe motion or rest we
‘or permanence of its position with respect to ideal

reer =
‘Thus a passenger in a railway carriage may be in a state of relative
i the train in which he travels, but he is in a stato

,, but becauso it is acted upon by the force
. A billiard ball gently pushed does not move more and more
Mey ne Snally stop, because it has any profroacn for a tate of ext,

by the friction on the cloth on which
. If all impeding cause were
i, 8 body once in motion would continue to move for ever.

. Application.—Innumerable phenomena may be explained by
the inertia of matter. For instance, before leaping a ditch we run
towards it, in order that the motion of our bodies at tho time of leaping

‘wild itself to the muscular effort then made,
descending carelessly from a carriago in motion, the upper part of
the body retains its motion, whilst the feet are prevented from doing 0
by friction against the ground; the consequence is we fall towards the
moring carriage.

‘The terrible accidents on our rmilways are chiefly due to inertia.
‘When the motion of the engine is suddenly arrested the carriages strive
‘Weontinue the motion they had soquired, and in doing so are shattered
agzinat each other.

CHAPTER IIL
ON FORCE, EQUILIBRIUM, AND MOTION,

2L Measure of Time—To obtain n proper measure of force it is
‘asa preliminary, to define cortain conceptions which are pre-
Yo that mensure; and, in the first place, it is necessary to define
it of time. Whenerer «sew is spoken of without qualification
nd

Ww ON MATTER, FORCE, AND goTION. [22-

it is understood to be & second of mean solar time, Tho exact Yength of
this unit is fixed by the following consideration. The instant whes the
san's centre is on an observer's meridian —in other words, the instant of
the transit of the sun's centre—sdmits of exact determination, and thus
the interval which elapses between two successive transits also admits ef
exact determination, and is called an apporent day. The length of this
interval differs slightly from day to day, and therefore does not serve as
“a convenient measure of time, Its average length is free from this
inconvenience, and therefore serves as the tequired measure, and
called a mewn solar day. The short hand of « common clock would £6
exactly twice round the face in a mean solar day if it wont perfectly:
‘The mean solar day consists of 24 equal parts called ows, these of 60
equal parts callod neiretes, and these of 60 equal parts called seeds
Consequently the second is the 86,400th part of a mesn solar day, and is
the generally received unit of timo,

22, Measure of Space.—Space may be either longth or distance,
‘which is space of one dimension ; area, which is space of two dimensions;
or volume, which is space of three dimensions. In England the standard
of length is the British Imperial Yard, which is the distance between
two points on a certain metal red, kept in the Tower of Londom, when
the temperature of the whole rod is 60° F.=15°5 0. tis, however, usual
to employ as a unit, a foot, which is the third part of a yard. Tn France
the standard of length is the metre; this, too, is practically fixed by the
distance between two marks on a certain standard rod. The relation
between these standards is as follows ;

1 yard = 0914383 motre.
1 metre = 1-093633 yard,

The unit of length having been fixed, the units of area and volume are
connected with it thus:—the unit of area is the area of # aquare, one
side of which is the unit of length. The unit of volume is the volume
of a cube, one edge of which is the unit of length. These units in the
case of English measures aro the square yard (or foot) and the eubie
yant (or foot) respectively ; in the case of Fronch measures, the square
metre and cubic metro respectively.

23, Measure of afass.—Two bodies aro said to have equal
maases when, if placed in « perfect balance in vacuo, they counterpolse
each other. Suppose we take Jumps of any substance, lead, butter,
wood, stone, etc., and suppose that any one of them when placed in
one pon of a balance will exactly counterpoise any other of thom
when placed on the opposite pan—the balance being perfect and the
weighing performed in vacvo; this being the ease, these lumps are
said to have equal massox. That these lumps differ in many respects

12 ON MATTER, FORCE, AND MOTION.

at the temperature 15° C. or 60° F. contain a pound of matter.
quently, if V is the volume of a body in cubic inches, Dits specific
ts mass M in Ibs, avoirdupois will be given by the equation
vo
M Ssr7a74
In this equation D is, properly speaking, the relative density of the
stance at 60°F, when the density of water at 60°F. is takon as the unit

25. Velocity and its measure.—When a material point moves, if
describes a continuous line which may be either straight or curved,
is called its path and sometimes its trajectory. Motion which ts
place along a straight line is called rectilinear motion ; that which t
place along a curved line is called curvilinear motion. The rate of
motion of a point is called ita velocity, Velocity may be either uniform oF
variable; it is uniform when the point describes equal spaces or po
of its path in all equal times; it is variable when the point describes
equal portions of its path in any equal tines.

Uniform velocity is measured by the number of units of space dese
inagiven unit of time. The units commonly employed are feet
seconds, If, for example, a velocity 5 is spoken of without qu .
this means n velocity of 5 feet por second, Consequently, if a body mo
for ¢ seconds with a uniform velocity v, it will describe w foot. A

Variable velocity is measured at any instant by the number of units 5
space it would describe if it continued to move uniformly from
instant for a unit of time. Thus, suppose a body to run down an inclined:
Plano, it is n matter of ordinary observation that it moves moreand mone
quickly during its descent ; suppose that at any point it has a velocity
16, this means that at that point it is moving at the rte of 15 ft p
second, or, in other words, if from that point all increase of velocity
ceased, it would describe 16 ft. in the next second.

28. Force. —When a material point is at rest, it hns no innate power of
changing its state of rest; when it isin motion it has no innate power
of changing its state of uniform motion in « straight line. This property
‘of matter is termed its inertia, Any cause which seta a point in motion,
or which changes the magnitude or direction of its velocity if in motion,
isa force. Gravity, friction, elasticity of springs or gases, electrical or
magnetic attraction or repulsion, ete. are forces. All changes observed in
the motion of bodies can be referred to the action of one or more forces.

27. Aoceterat effect of force.—If we suppose a force to con-
tinue unchanged in magnitude, and to act along the line of motion of a
point, it will communicate in each successive second a constant increase
of velocity, This constant incrense is the accelerative effect of the force.
‘Thus if at any given instant the body has « velocity 10, and if at t

of the first, second, third, etc., second from thet instant its velocity is

stood to moan the force W exorted by gravity on a pound of matter
London. Now, in London, the numerical value of g is 32-1912, so that
W = 1x82-1912;
in other words, when a pound is taken as the unit of force it cont
32-1012 units of force according to the measure given above, It will be
observed that a pound of matter is u completely determinate quantity
matter, irrespective of locality, but gravity exerta on a pound of
‘4 pound (or 321912 units) of force at London and other places in
the snme latitude as London only; this ambiguity in the term
should be carefully noticed by the student; the context in any
will always show in which sense the term is used.
80, Representation of forces.—Draw any straight line AB, and
fix on any point O in it, We may suppose a force to act on the point
, along the line A B, either towards A or B: then
StS XA Vis called the point of application of the force,
Fig. 3. AB its line of action; if it acts towards A, its”
direction is © A, if towards B, its direction is OB.
Tt is raroly necessary to make the distinction between the line of action
and direction of a fores ; it being very convenient to make the convention
that the statement—a force acts on a point O along the line OA—meang
that it acts from O to A. Let us suppose the force which acts on Q along
© A to contain P units of force; from O towards A measure ON con=
taining P units of length, the line ON is said to represent the force. It
will be remarked that the analogy between the line and the foree is very
complete; the line ON is drawn from 0 in a given direction O A, and
contains a given number of units P, just as the force acts on O in the
direction OA, and contains a given number of units P. It is scarcely
necessary to add that if an equal force were to act on O in the opposite
direction, it would be said to act in the direction O B, and would be re-
presented by O M, equal in magnitude to ON.

When we are considering several forces acting along the same line we
may indicate their directions by tho positive and negative signs, Thus
the forces mentioned above would be denoted by the symbols +P and
—P respectively.

81, Forces acting along the same line.—If forces act on the point
© in the direction OA containing P and Q units respectively, thay are
equivalent to a single force R containing 98 many units as P and Q
together, that is,

R=P+Q

Ifthe sign + in the above equation denote algebraical addition, the equation
will continue truc whether one or both of the forces act along OA orO B.
Te is plain that the same rule can be extended to any number of forves,

-33] PARALLELOGRAM OF FORCES, 1b

and if aevéral forces have the mime line of action they ars equivalent to
enn force containing the same number of units ax their algebraical sum.
‘Thus if forces of 3 and 4 units act on O in the direction O A, and a force

wero. In this caso the forces 2 Seepoh epee the point
~~ acai

‘$2, Resultant and components,—In the last article we saw that a
dangle force R could be found equivalent to several others: this is by
‘bo meana peculinr to the case in which all the foros
have the same line of action; in fect, when a ma- |
terial point, A (fig. 4), remains in equilibrium under
‘the action of severnl forces, S, P, Q, it does so be-
rate any one of the forces, as S, is capable of

the combined effects of all the others.

Ifthe force 5, therefore, had its direction reversed,

Fig. 4.

P, Q, act on a point they can only have one result-

single force can be resolved into components in an indefinite
number of ways.

‘Tf a point move from rest under the action of any number of forosa it

Will begin to move in the direction of their resultant.

Parallclogram of forces.—W hen two forces act on a point their

emiltant. iy found by the following theorem, known as tho principle of

of forees:—If two forces act on a point, and if lines be

—oetpapedlg ‘Thus, let P and Q (fig. 5), be two forces acting on the
nlang AP and AQ respectively, and let AB and AC be taken con-

n the theorem states that the resultant, R, of P and Q is repre-

16 ON MATTER, FORCE, AND MOTION.

sented by AD; that is to say, P and Q together are equal to
force R acting along the line AD, and containing as many units of
as AD contains units of length.

Proofs of this theorem are given in treatises on Mechanics; we
here give an account of a direct experimental verification of
truth ; but before doing so we must premise an account of a yery si
experiment.

Lat A (fig. 6), bo a small pulley, and lot it turn on a amooth, hard,
thin axle with little or no friction ; let W be a weight tied to the end of

x
e
7
Fig. 5. Fig. 6.

fine thread which passes over the pulley; let a epring CD be attached by
‘one end to the end © of the thread and by the end D to another piece of
thread, the other ond of which is fastened to a fixed point B; a seale
CE can bo fastened by one end to the point © and pass inside the spring
#0 that the elongation of the spring can be measured, Now it will be
found on trial that with a given weight W the elongation of the apring
will be the same whatevor the angle contained between the parts of the
tring WA and BA, Also it would be found that if the whole were
suspended from a fixed point, instend of passing over the pulley, the
weight would in this case strotch the spring to the same extent as
before. This experiment shows that when care is taken to diminish to
the utmost the friction of the axle of the pulley, and the imperfect flexi-
bility of the thread, the weight of W is transmitted without sensible
diminution to B, and exerts on that point a pull or force along the line
BA virtually equal to W.
‘This being premised, the experimental proof, or illustration of the
parallelogram of forces, is as follows
Suppose H and K (fig, 7), to be two pulleys with axles made aa smooth
and fine aa possible ; let P and Q be two weights suspended from fine and
flexible threads which, after passing over H and K, are fastened at A to
a thint thread AL from which hangs a weight R ; let the three weights
‘came to rest in the positions shown in the figure. Now the point A is
acted on by three forces in equilibrium, viz., P from A to H, Q from A.

-%4] RESULTANT OF FORCES.

toK, and R from A to L, comsoquently, any one of them must be equal
and opposite to the resultant of the other two. Now if we suppose the
‘apparatus te be arranged immediately in front of a large slate, we can
draw lines upon it coinciding with AH, AK, and AL. If now we mea-
sure off along AH the part AB containing as many inches ss P contains
poads, and along AK the part AC containing as many inches as Q con-
tains pounds, and complete the parallelogram ABCD, it will be found
that the diagonal AD is in the same line as AL, and contains es many
inches as R weighs pounds. Consequently, the resultant of P and Q is
npresented by AD. Of course, any other units of length and foree might
lave been employed. Now it will be found that when P, Q, and R
are changed in any way whatever consistent with equilibrium the same
ganstruction can be made,—the point A will have different positions in
the different cases; but when equilibrium is established, and the paral-
Idlogram ABCD is constructed, it will be found that AD is vertical, and
contains as many units of length ns R contains units of force, and conse-
quently itrepresents a force equal and opposite to R, that is, it represents
the resaltant of P and Q.

Fig, 7. Fig. 8.

O. Resultant of any number of forces acting on one plane on

& potne.—Lat the forces, P, Q, R, S (fig. 8), act on the point A, and let
then te represented by the lines AB, AC, AD, AE, a8 shown in the
figure. First, complete the parallelogram AB FC and join AF; this line
the resultant of P and Q. Secondly, complete the parallelo~

gram APGD and join AG; this line representa the resultant of P,Q, R
Tiirdly, complete the parallelogram AGHE and join All; this line re-
presente the resultant of P,Q, R,S. It is manifest that the con-
‘traction

ean be extended to any number of forces. A little considera~
tite will show that the line AH might be determined by the following
Seurtrmetion ;—throwgh B draw BF parallel to, equal to, and towards
‘ee mime partas AC ; through F draw FG parallel, to equal to, and towanis
ths same part ns AD; through G draw OH parallel to, equal to, and to-

=

ww ON MATTER, FORCE, AND MOTION.

wands the samo part as AE; join AH, then AH representa the requ
resultant,

In place of the above construction, the resultant can be
by calculation in the following manner:—Through A draw any
rectangular axes Ax and Ay (fig. 9), and let a, 8,7 be the
made with the axis Ax by the lines representing the pressures,
P, Q, R can be reaalved into P cosa, Q cos 8, R cosy, acting
Ax, and P sine, Q cos, R cosy acting along Ay. Now the
set of forces can be reduced to a single force X by addition, atten!
being paid to the sign of cach component; and in like manner the
foreea can be reduced to a single force Y, that is,
X= Pcosa+ Qeoss+ Reosy +...
Psina + Qsing+Rsiny +...
Since tho addition denotes the algebraical sum of the quantities on
right hand aide of the equations, both sign and magnitude of X and ¥)
Known. Suppose U to denote the required resultant, and @ the
made by the line representing it with the axis Ax;
then Ucos ¢ = X, and U sing = ¥
‘These equations give U?=X* + Y%, which determines the mngni=
tude of the resultant, and then, since both sin ¢
and cos ¢ are known,» is determined without
ambiguity.

Thus let P, Q, and R be forces of 100, 150, and”
120 units, respectively, and suppose xAP, xAQvand
XAR to be anglosof 45°, 120°, and 210° respectively.
Then their components along Ax are 70°7,—15,
—103-9, and their components along Ay are 707,
1209,—60, The sums of these two sets being re
spectively—1082 and 140-6, we have U cos¢ =

Fig. 0.
— 108-2 and Using = 1406,
therefore U? = (108-2)? + (140-8)*

or u
therefore 77-4 cos ¢ = —108%, and 177-4 sin g = 136-7.
If we made use of the former of these equations only, we should
obtain » equal to 232° 25’, or 127° 36’, and the result would be .
oug: in like manner if we determined @ from the second equation only,
we should have ¢ equal to 52° 257, or 127° 35"; but ae we have both equa=_
tions, we know that g equals 127° 35’, and consequently the force U is
completely determined as indicated by the dotted line AU.

36. Conditions of equilibrium of any forces acting in one plane
on a potnt.—If the resultant of the forces is zero, they have no joint
tendency to move the point, and consequently are i equilibrium, This

enables us to deduce the following constractions and
to ascertain whether given forees will keep a point

‘that in the caso represented in fig. 8, Tis the force which
EQS. ~shonben tar plgeernaaen ate ainererd

be proportional to FLA; for then the resultant of the

il ol rst re ae ABEGHA tar ae
‘This construction is plainly equivalent to the following : Let P,

10), be forces acting on the point O, as indicated, their magnitudes
I wa hey es atest Gra eh

draw AB parallel to and towards the same part as 0Q, and take AB

that P:Q:;DA:AB. Thragh B dmw BO panilel to and

i : Rs: AB : BO;

jain CD; through O draw OS parallel to and towards the same part as
the required force § acts along OS, and is in magnitude pro-

Fig, 10, Fig. 11.

Tt &e te bo observed thut this construction can be extended to any
‘Sitmber of forces, and will apply to the case in which these directions
‘we not in one plane, only in this caso the broken line ABCD would not
Me wholly in one plane. The above construction is frequently called the

yyon of Forces.

of three forces acting on a point is, of course, included in the
its is much that we may give a separate statement
2 (6g aye be threo forves in equilibrium on the point O.
BC parallel to and towards the same part OP,
MU cciinaeret oon, and take CA

=Q z: BC : CA; then, on joining AB, the thi R
to aad towards the same part as AB, and must be
sl in maguitude to AF, This construction is frequently called
one Wis orident that while the sides of the ' triangle

20 ON MATTER, FORCE, AND MOTION.

are severally proportional to P, Q, R, the angles A, B, © aro
tary to QOR, ROP, POQ respectively, consequently every trig
cal relation existing between the sides and angles of ABC will
exist between the forces P,Q, R, and the supploments of the
between their directions. Thus in the triangle ABC it ie known
sides are proportional to the sines of the opposite angles; now sino
sines of the angles are equal to the sines of their supplements, we at obi
comelude that when three forces are in equilibrium, each is proportional |
the sine of the angle between the directions of the other two.

We can easily obtain from the equations which determine the
‘of any number of forces (34), oquations which oxpress the
‘equilibrium of any number of forces acting in one plane on @
fact, if U = 0 we must have X = 0 and Y=0
required conditions of equilibrium are these :—
0 = Pcosa+Q cos 8+K cos y+ ...
and 0 = Psina+Q sinf+R siny+ ...
‘The first of these equations shows that no part of the motion of the poin
can take place along Ax, the second that no part can take place along
Ay. In other words, the point cannot move at all, 4

86, Composition and resolution of parallel forces.—The &
the equilibrium of three parallel forces is merely a particular caso of
equilibrinin of three forces acting on a point. In fact let P and @Q
two forces whose directions pass through the points A and B, and
sect in O; let them be balanced by a third force R whos d
produced intersects the line AB in C. Now p
the point © to move along AO, gradually rec
from A, the magnitude and direction of R will o
tinually change, and also the point C will contiz
change its position, but will always lie between A
and B. In the limit P and Q become parallel fore
acting towards the same part balanced by a p
force R acting towards the contrary part through:
point X between A and B. ‘The question is;—Fira
on this limiting case what is the value of R; see
what is the position of X. Now with regard to the
first point it is plain, that if a triangle abe we
drawn a8 in art, 35, the angles a and 6 in the limit
will vanish, and c will become 180°, consequently ad ultimately equ
ae+chor

a
Fig, 12.

R=P+Q
With regard to the second point it is plain that
OC sin POR = OC sin AOO = AC sin CAO,

22 OS MATTER, FORCE, AXD MOTION. [s7-

87. Centre of parallel forees.—On referring to fig 15 and 14, it will

be remarked that if we conceive the points A and B to be fixed in the
diroctions AP and BQ of the forces P and Q, and if we suppose thas
directions to be tamed round A and B, ¢0 as to continue parallel and i
make any given angles with their original directions, then the direction.
of their resultant will continue to pass through C; that point is therefore”
called the centre of the parallel forces P and Q.
It appears from investigation, that whenever a system of parallel
forces reduces to a single resultant, those forces will have a centro; that
if we conceive each of the forces to act at a fixed point, there
will be a point through which the direction of their resultant will past
when the directions of the forces are tarnod through any equal angles
round their points of application in such a manner as to retain the
paralleliam of their directions.

‘The most familiar example of a centre of parallel forces is the case im
which the forces are the weights of the parts of a bod) this case thé
forces all acting towards the same part will have a resultant, viz. their
sum ; and their contre is called the centre of gravity of the body.

38. Moments of forces.—Let P denote any force ucting from B te
P, take A any point, let fall AN a perpendicular from A on BP. The
product of the number of units of force in AP, and the number of waits
of length in AN is called the moment of P with respect to A. Sines the
force P can be represented by a straight line, the
moment of P can be represented by an areas. Tn fact |
if BC is the line ropresenting P, the moment i
properly represented by twice the area of the triamgle

The perpendicular AN is sometimes called
of the pressure, Now if a watch wero
Fig. 16. with its face upward on the paper, the force P would
cause the arm AN to turn round A in the contrary diree=
tion to the hands of the watch. Under these circumstances, it is usual
to consider the moment of P with respect to the point A tobe positive, If
Paeted from C to B, it would turn in the same direction as the hand
of the watch, and now its moment is reckoned negetive.

The following remarkable relation exists between any forees acting

in one plane on a body and their resultant, Take the moments of the
forces und of their resultant with respect to any one point in the
plane, Then the moment of the resultant equals the sum of the moments
Of the soveral forces, regard being had to the aigns of the momenta,
If the point about which the moments are measured be taken in the
direction of the resultant, its moment with respect to that point will be
zero; and consequently the sum of the moments with respect to such
point will be sero,

at

weight. Ina lew of the first kind tho fulcrum is betwoon the po
and resistance, aa in fig. 16, In a lever of the second kind the re ,
between the power and the fulcrum, as in a wheelbarrow or w pair
nutcrackers ; in a over of the third kind the power is bevwoen the fulerar
‘and the resistance, as in a pair of tongs or the treadle of a lathe.
41. Phe Single Pulley.—In tho case of the single fixed pulley,
in fig. 17, it follows at once from (33) that when tho forces P and Q
in equilibrium they will be equal, the axle of the
being supposed perfectly smooth and the thread p
flexible. The same conclusion follows directly from
principle of moments; for the resultant of P and
P @ must pass through C, or otherwise they would cause #
Fig 17. pulley to turn ; now their moments are respectively PX
CM and Q x ON, and since these have opposite signa we have (38)
i] Px€M = QxON,
But CM and CN being equal, this equation shows that P and Q
equal. In the case of the single movenble pulley, shown in fig, 18, 1
have one end of the rope fastened to » point A ina ‘
‘The pulley is consequently supported by two forces, viz. P
and the reaction of the fixed point which is equal to P
these two forces support Q and the weight of the pa
w, In the case represented in the figure, the parts of
rope are parallel, consequently (36)
2P=Qt+w.
‘When several pulleys are united into one machine, thi
2 constitute a system of ‘pulleys; such are—the Block and
Fig. 18. Tackle, the Barton, White’s Pulley, &e.
42, The inclined plane.—A very instructive and useful application of
the resolution of forces is to be found in the case of a body supported om
an inclined plane, Let AB (fig. 19), be the plane, AC its base, and BC ite
height; let a body M considered as a point,
whose mass is M and weight Mg or Q, be sup-
ported on it by a force P acting along MBL
The plane is supposed smooth, and therefore
reacts on M,with a force R at right angles to
Fig. 19. AB. Draw CD at right angles to AB, then
ne the point M is held at rest hy forces P, Q, Ry
whos directions aro severally parallel to the sides of the triangle DBC
which is similar to CBA. Hence 4
BD : DC: CB
AB sin A and ©.

ON MATTER, FORCE, AND MOTION.

0: CA: AB
AB cos A

4a) WEDGES AND SCHEWS. 25

s c rie area ae
e pressure on cos A and its force
dees ne ix My sin A. (2 ar aa patcined bt

"Te areas eet OAts Won. id 12 ft. ‘ively, th:
an a Teapectively, then

sO vial 2 ft. Consequontly, if the weight of Q is 300 ibs it

prodoss on the plane « perpendicular pressure of 288 Ibs., and requires

Gop ite support « force of 216 tbs. acting up the plane,

W. The wedge.—This instrument is nothing but a moveable inclined
jlaoe. It is used in several forms, of which the annexed is, perhaps, the
‘Met for showing the action of the forces called into play. AB is a fixed
thle. ACDE ix w piece whose lateral motion is prevented by a fixed
pide F. ABC ia a wedge whote angle is such that one of its faces ia in
ntact with « face of ACDE as shown in the figure. ABC being forced
arene er oe Teele G esting on AODE: ‘Tho various

in

Sia Be evctin Tol ha euide We

the angles B, D, E, and EAB to

and that P and Q act at right

‘and BC respectively. Moreover,

I iets cocker oreo, S acts

eer nae wine on in equilibria by three forces, T, R,, Q,
‘whose directions are respectively parallel to the side of the triangle

DAO, we bave

R,:Q:: GC :cD
ee und ape are equal, being the mutual actions of the two bodies
AGDE,; therefore, compounding the ratios, we have

P:Q:: DG: DC
wy, by similar triangles,

P:Q:: CB: BA
4 proportion equivalent to the equation

P=QtunA '
4h The screw.—It will be remarked that when the wedge is used
c

26 ‘ON MATTER, FORCE, AND MOTION.

as in the last article, Q cannot be many times greater

that the space through which P can lift Q is limited. The
motely a modification of the wedge by which the limits of ita a]

in both these respects are extended, To explain this it may be ob
that if the thread of s screw were reduced to a line, it would b
curve called the Aeliz, running in whorls round the cylinder; the
between any two consecutive turns measured parallel to the cube
cylinder being constant, and called the pitch of the screw, Now if
(fig. 20) were wrapped round a cylinder, whose dimensions were sud
that the base AB coincided with the circumference of the base of
cylinder, and the height BC with the piteh, the hypothenuse CA co
‘be brought into coincidence with one whorl of the helix. Under
circumstances, the angle BAC (.4) is called the inclination of the dj
and if r denote the radius of the buso of the cylinder, A the pitch of th
sorew, we shall have, since ABtanA equals BC (fig. 20), q

2er tan A= h

Moreover, if ACIDE were wrapped round the inside of a hollow
or nué (fig. 21) of equal radius it
take the form of n helix, or comp
screw cut on the inside of the mut;
if the screw were placed within
nut the two helices would be in.
contact. If now we suppose
power to act at the end of an arm,
shall have transformed the wedge
fig. 20 into a screw, one end of
works on @ fixed table with a mo
able nut. The annexed figure
the arrangement, half the nut b
removed in order to show how
thread of the screw works within
groove of the companion, When
arm is turned in the direction indicated:
by P the point B will pass to BY, bat,
as the nut is kept by the guides G, H
from turning with the screw, it must
now occupy the point © of the com=
panion, and consequently the nut mist
be lifted so that C comes to BY Ifthe
nut were fixed the screw would be
depressed by the same amount, when P acts ns indicated.

If the screw were turned by a force P’ acting tangentially to the base

Fig. 21.

a

28 ON MATTER, FORCE, AND MOTION,

wedge (4). For if the angle of inclination A of the wedge (fig.

‘Tess than the angle of repose, Q will not force the wedge out, even

ceases to act. Now in practice the wedge is commonly driven fo

by a blow ; but, as we shall see in sequel, a blow is a Inrge force

for a short time, consequently a blow will cause « resistance Q, eve

when very great, to yield through @ small space, thus each blow

on the back of BO (fig. 20) will cause the wedge to advance a

and, as Q cannot forve it back, it will stay in the position to whi ‘
been advanced, and consequently by a succession of such blows it can be
caused to advance through any requisite space,

46, Uniformly accelerated rectilinear motion.—Let us suppose:
body containing m units of mass to move from rest under the action of |
force containing F units, the body will move in the line of action of ti
force, and will uequire in each second an additional velocity f given by
the equation é

F=mf
consequently, if v is its velocity at the end of ¢ seconds, we have
o=ft ret)

To determine the space it will describe in ¢ seconds, we may reason a
follows. ‘The velocity at the time ¢ being ft that at a time ¢ -—:
will be f(t +7). If the body moved uniformly during the time +
the former velocity it would describe « spaco # equal to flr, if
the latter velocity a space , equal to f(t + 7)r. Consequently,
era Thy:
therefore, when + is indefinitely small, th ing values of « and 4,
equal, Now since the body's velocity is continually énereasing dui
the time 7, the space actually described
greater than ¢ and Jess than , But si
the limiting values of s ands, are
the limiting value of the space described:
the same as that of « or 4, In other we
if wo suppose the whole timo of the body's
motion to be divided into any number
equal parts, if we determine the velocity of
the body at the beginning of each of these
parts, and if we ascertain the spaces de-—
scribed on the supposition that the body moves uniformly during each pare
tion of time, the limiting value of the sum of these spaces will be the space
actually described by the body. Draw a line AC and at A construct an angle
CAB, whow tangent equals f, divide AC into any number of equal

4a D, BF, ... and draw PD, QE, RP, ... BU at right angles to AC, thes

30 OS MATTER, FRDCE, ASD MOTION. [a

Instead of supposing the body to begin to move from a state of restwy
may suppose it to have an initial velocity V, im the direction of the forte:
In this case equations (1) (2) and (3) can be easily shown to take the
{ntlowing forms respectively :

r=Vif
saVieafe
F=VEYe

If the body move in a direction opposite to that of the force, f mustbr
reckoned negative.

47. Motion on an inctined plane.—Referring to (42), suppose the
force P not to act; then the mass M is acted on by an unbalascat
force Mg sin A, in the direction MA, consequently the accelerating fect
down the plane is gain A, and the motion becomes a particular case uf
that discussed in the last article. If it begin to move from rest, if
will at the end of ¢ seconds acquire a velocity © given by the equation

p=gtsinA
and will describe a length # (ft.) of the plane given by the equation

«= ig@sinA
Also, if v i tho velocity acquired while describing s feet of the plane

2 = Qgesin A
Henes (fig. 19) if « body slides down the plane from B to A the velocity
which it ocquires at A equals 4/2y-ABain A or «/2g-BO. that is to sayy
the velocity which the body has at A does not depend on the angle Ay
faut only on the perpendicular height BC. The same would be traoif
for BA we substituted any smooth curve, and hence we may: state
generally, that when a body moves along any smooth line under the
action of gravity, the change of velocity it experiences in moving from
one point to another is that dee to the vertical height of the former point
above the latter.

48. Composition of velocities.—The rule for the composition of
velocities is the same as that for the composition of forces ; this follows
evidently from the fact that forces aro measured by the momentum they
communicate, and are therefore to one another in the same ratio aa the
velocities they communicate to the same body. Thus (fig. 5, art. 33) if
the point has at any instant a velocity AB, in the direction AP, and
there is communicated to it a velocity AC in the direction AQ it will
move in the direction AR with a yelocity represented by AD. And
conversely the velocity of a body represented by AD ean be resolved into
two component velocities AB and AC, This suggests the method of
dotarmining the motion of a body when acted on by a force in a dinsetiom
éranaverse to the direction of its velocity, namely, suppose the time to he

divided into « great number of intervals, and suppose Toe Welodivy acoualhy

OX MATTER, FORCE, AND MOTION,

body tends at each point to move along tho tangent at that point,
quently a foree must continually act on it towerds the centre to
it from the tangent, and keep it moving in the circle; in the above.
the force contains 430 units, which is nearly 15 Tbs. of foros.

is vertical and radius denoted by r. o
point placed at A, and allowed to slide d
the curve, what velocity will it have acqui
reaching any given point P? Draw the
diameter CD, join CA, OP, and draw the
zontal lines AMB and PNP’, Now aasu
curve to be smooth the velocity i
falling from A to P is that duo to MN the vert
height of A above P (47), if, therefore, #d
the velocity of the point at P we shall have
v= 29. MN
Now by similar triangles DCP, PCN we have
DC : CPs: CP: ON
consequently, if we denote by «the chord CP,

2. NC=#
in like manner if a denote the chord CA,
o®. MC =a?
therfore
2r,.MN
a ae
and ’ y (e-#)

Tt will be remarked that » will have equal values when s has the sam
whether positive or negative, and for any one valuo of s there are twa
equal values of », one positive and one negative. That is to any, since
OP’ is equal to OP, the body will have the same velocity nt P’ that it
has at P, and at any point the body will have the same velocity whe
ther it is going up the curve or down the-curve. Of course it is ime
cluded in this statement that if the body begins to move from A it will
just ascend to a point B on the other side of C, such that A and B arsin
the same horizontal line. It will also be remarked that at C the value
of # is zoro, consoquently, if V is the velocity acquired by the body in
falling from A to C we have

v= aft.

and, on the other hand, if the body begins to move from C with a yelo=
city V it will reach a point A such that the chord AC or ais given by

a ON MATTER, FORCE, AND MOTION.

Asan example of the use of the formula wo may take the follo
—It las been found by careful experiment that 3018983 inches
length of a simple pendulum, whose time of oscillation at Greet
‘one second, the formula at once leads to an accurate determination
accelerating force of gravity; for using feet and seconds as our ur it
have ¢ = 1, r = 326165, and w stands for the known nunmiber 314158,
therefore the formula gives us
g = (314150) x 326165 = 82-1912
‘This is the value employed in (20).

52, Graphic representation of the changes of velocity of an
oscillating body.—T'lie changes, which the velocity of a vibrating body
undergoes may be graphically represented as follows :—Draw a line
indefinite length and mark off ATI to represent the time of one

ae ot
—— e a oo
Fig. 26.

HH to ropresent the time of the second vibration, and so on.
the first vibration the velocity increnses from zero to n maximum at
half vibration, and then decreases during the second half vibration
the maximum to zero, Consequently, if a curved line or are AQH #
drawn, the ordinate QM nt any point Q will represent the velocity
body at the time represented by AM, If a similar curved line or:
TPH’ bo drawn, the ordinate PN of any point P will rprosent the
city at a time denoted by AN. But since the direction of the reeled
the accond oscillation is contrary to that of the velocity in the first 0
lation, the ordinate NP must be drawn in the contrary direction to
of MQ. If, then, the curve be continued succession of equal ar
alternately on opposite sides of AD, the variations of the velocity of |
vibrating body will be completely represented by t
A varying magnitudes of the ordinates of suee
/\ points of the curve,
53, Conteal pendutum.—When a point P
suspended from a point A asa simple pendulum it cas
‘be caused to describe a horizontal circle with a
form velocity V. A point moving in such a
constitutes what is called a conical pendulum, and.
admits of many useful and interesting application
We will, in thix place, ascertain the relation whieh —
exists between the length + of the thread, AP, the
PAN or 0, and the velocity V. Since the point Pmor

Fig.

54] IMPULSIVE PoRcES. 3

redies is PN with a velocity V, n force R must act o it in the direc-
faa PN giren by tho equation (49)

R=M. eR

SS the tension of the thread T along PA,
Sy eysaceperetiardis Bareng consequently their resultant
imust be a force R PN. And therefore these forces will be
panillel to the sides of ian So that (85)

R= My. PX

a Sy = y. PS

van

PN =rainéand AN = VN. tane

Vi = gr sin é tan 6
Ooe conclusion from this may be noticed. With centre A and radius
AP, describe the arc PO. Now when the angle PAC is small, the
fan PS, doos not sensiby difer fo the chor, nor the ening, AN,
the radins, therefore in this case we have

Ve eV = hd. PaO /2

‘On comparing this result with (50) we see that when the angle PAM
‘is small the velocity of P moving in a conical pendulum is the same as P
would have at the lowest point © if it oscillated ase simple pendulum;
Gimmequently, if we conceive the point P to be making small oscilla-
fines about the point A, and denote the velocity at the lowest point by
ere et a teen vbeatin, thore is com-

¢ time, noid gel saab chug velocity, tia t
impulsive foree. Such a foree is called into play wh:

a

ON MATTER, FORCE, AND MOTION.

‘that ft may be of sensible or even considerable magnitude.
contain a pound of matter, and if the force contain ten tha
though ¢ were so short aa to be only the ;J,sth of a second, the
communicated by the force would be one of 10 ft. per second. It |
to be remarked that the body will not sensibly move while this 5
is being communicated ; thus, in the case supposed, the body wo
more through 4/" or the ;13th of a foot while the force acts upam
‘When one body impinges on another it follows from the law
equality of action and reaction (39) that whatever force the first
‘exerts upon the second, the second will exert an equal force upon the:
in the opposite direction ; now forces are proportional to the momenta,
ated in the same time; consequently these forces generate during the:
or any part of the time of impact in the bodies respectively equal 1
with contrary signs; and consequently the sum of the momenta of
two bodies will remain constant during and at the end of the impact, Iti
‘of course understood that if the two bodies mot trary directions
their momenta have opposite signs and the sum is an algebraical sum.
order to test the physical validity of this conclusion, Newton ae
series of experiments which may be briefly described thi T
and B are hung from points C, D, in the same horizontal
% cc py» x in such amanner that their
and B aro in the same horizontal
/ With centre C and radius CA d
a semicircle EAF, and with centre D De
and radius DB describe @ sen
GBH on the wall in front of which te
balls hang. Let A be moved back to
R, and be allowed to deseend to Ay
there impinges on B, both A and B
will now move along the arcs AF and BH respectively, let A and B
come to their highest points at rand k respectively. Now if ¥ denote
the velocity with which A reaches the lowest point, # and # the velocities
with which A and B leave the lowest points after impact, and x the
radius AC, it wppears from 50) that

Vs chd AR /2 >= cht, Ar 1/7, ond w= chd, Be /E
r r

therefore if A and B are the masses of the two balls, the momentum at
the instant before impact was A x chd. AR and the momentam after
impact was A x chd. Ar + B x chd.. Bk, Now when the positions of
the points R, r, and & had been properly corroctod for the resistance of
the aiz, it was found that these two expressions were equal to within
quantities so small that they could be properly referred to errors of —

ie
Fig, 28,

38 OX MATTER, FORCE, AND MOTION.

the direction of the motion, the product of that component and i
of the line is the work done by the foree. If the component sects’
‘opposite direction to the motion, the component may be R

~Tiwill be observed that if tho forces are in equilibrium during the:
0 that the velocity of M is uniform, P equals Q sin A, and
the work done by the power equals that done against the
Also since AB sin A equals BC the work done against the reslatanoe
equals Q x BC. In other words, to raise Q from A to B voquires th
same amount of work as to raise it from © to B.

For strictly scientific purposes a unit of work is taken to be the worl
ae by a unit of force when its point of application moves through ane

the direction of its action. If, as is frequently done, a anit of

eos is defined to be « forve of one pound exerted through ane foot,
attention must be paid to the romark in (20) regarding the meaning of the
term pound when considered as a unit of force, ‘This unit may be oot
veniently distinguished as a ‘foot-pound.’ To raise a pound of matter
through one foot requires more or less than a ‘foot-pound * of
according as the force of gravity ou that pound of matter exceeds or
short of 32-1912 units.
By tho term energy or vis viva is meant a quantity proportional to the
product of the mass of a body m and the square of its velocity vy, itis
most conveniently measured by junv*. If a force containing P unite
acts on a body whose macs is m causing to move from rest over A foot, the
force will do Ph units of work, or if P equals mf it will do mfh units of
work. Now if » is the velocity of the body at the end of the & feet, we
know that v* equals 2h (46). Therefore

Ph or mfh = mo"

‘That is to any, the work done by the force equals the energy of the body.
In the same manner, if the body have an initial velocity V, so that
when the force begins to act, it have already an energy jmV", the work
done by the force will equal the change in the energy of the body oF
4m (t=),

It deserves particular notice that if the point of application of the forse
movés in a direction at right angles to that of tho force, the force does no
work, and thorefore will not communicate energy to the body, nor cane
ita velocity to undergo change. A conspicuous example of this fact is
furnishod by the case of circular motion discussed in (49), Here the
only foree is MV*+-r which acts on the body along the radius, and, there
fore, at right angles to the direction of the motion at each instant, in
consequence it does no work, and the velocity of the body is yniform.

rion LSpah eee

GRAVITATION AND MOLECULAR ATTE

BOOK IL
GRAVITATION AND MOLECULAR ATTBACTION.

CHAPTER L
GRAVITY, CENTRE OF GRAVITY, THE BALANCE

in virtue of which the material particles of all bodies tend
‘approach each other ; it is a mutual action, however, which all be
rest or in motion, exert upon one another, no matter how great or
small the space between them may be, or whether this space be occu
‘or unocenpied by other matter.
A vague hypothesis of the tendency of the matter of the
stare to a common centre was adopted even by Democritus and EF
ler assumed the existence of a mutual attraction between the
the earth, and the other planets. Bacon, Galileo, and Hooke, also
nised the existence of universal attraction. But Newton was the
who established the law and the universality of gravitation.
Since Newton's time the attraction of matter by matter was exp
mentally established by Cavendish. This eminent English ph
succeeded by means of a delicate torsion balance (80) in rendering
the attraction between a large leaden and a small copper ball.

The attraction between any two bodies is the resultant of the
tions of each molecule of the one upon every molecule of the
according to the law of Newton, which may be thus expressed: the at-
traction between tico material particles is directly proportional to the grrodie
of their masses, and inversely proportional to the square of their
asunder. To illustrate this, we may take the case of two spheres
owing to their symmetry, attract each other just as if their masses
concentrated in their centres. If without other alteration the masa of¢
sphere were doubled, trebled, etc., the attraction between them wo
‘be doubled, trebled, etc. If, however, the mass of one sphere being
doubled, that of the other were increased three times, the distance
between their centres remaining the same, the attraction would be in=
creased six times, Lastly, if, without altering their maasos, the distance
between their centres were increased from | to 2, 3, 4, ... . units, the

42 GRAVITATION AND MOLECULAR ATTRACTION. [60-

that the angle between the directions of two plumb-lines, ome ate
ation to the north, and the other to the south of the mountain, wa
Rn ee

dish’s experiments referred to in the last artiele.

60. Centre of gravity, its experimental determination.—Iili
whatever position a body may be turned with respect to the earth, there
is n certain point, invariably situated with respect to the body, through
which the resultant of the attracting forces between the earth and its

Fig. 31.

several molecules always passes. This point is called the centre of gravity:
it may be within or without the body, according to the form of the Tater;
its existence, however, is easily established by the following consideme
tions: Let m, m’, m’, m”’. . . (fig. 80) be molecules of any body. ‘The
earth's attraction upon these molecules will constitute a aystem of parallel
forces, having a common vertical direction, whose resultant, according 1
(36), will be found by secking first the resultant of the forces whieh
act on any two molecules, m, and m’, then that of this resultant, and &
third force acting on m”, and so on until we arrive at the final resultant
W, representing the weight of the body, and applied at a certaim pobnt
G. Ifthe body be now turned into the position shown in fig. 31, the
molecules m, m’, m’. . . will continue to be acted on by the same foros
as before, the resultant of the forees on m and m’ will still pass through
the same point o in the line mm’, the following resultant will again pase
through the same point o in om’, aud so on up to the final resultant Py
which will still pass through the same point G, which is the centre of
gravity.

To tind the centre of gravity of a body is a purely geometrical

61) GRaviPy;:cxerne Or cnavirt, Hr naLance, * 43

4 string in two different positions as shown i
where the directions AB and

one point of the body is fixed,
Seshtis cootss of gra
‘than one point is supported,

ad GRAVITATION AND MOLECULAR Aq

brium if a vertical line through the centre of gravity: tare
point within the polygon formed by joining the ‘support,

‘The leaning tower of Pisa continues to stand becauss the -
drawn through its centre of gravity passes within its base,

Tt is easier to stand on our feet than on stilts, because in the
ense the smallest motion is sufficient to cause the vertical line thr
the centre of gravity of our bodies to pass outside the
which is here reduced to « mere Tine joining the feet of the stilts.
it is impossible to stand on one log if we keep one side of tho foot i
head close to a vertical wall, because the latter prevents us from. is
the body's centre of gravity vertically above the supporting base.

62, Different states of equilibrium.—Althoagh a body
by a fixed point is in equilibrium whenever its centro of gravity ii
vertical lino through that point, the fact that the centre of gravily
incessantly to occupy the lowest possible position leads us to di
between throe states of equilibrium—stable, unatable, neutral,

A body is said to be in stable equilibrium if it tends to return to ite
position after the equilibrium has been slightly disturbed. :
is in this state when its position is such that the slightest alteration:
the same elevates its centre of gravity; for the centre of gravity
descend again when permitted, and aftor a few oscillations the body =
seturn to its original position,
The pendalum of a clock continually oscillates about its position (f

stable equilibrium, and an egg on « level table
in this state when its long axis is horizo
We have another illustration in the toy 7
sented in the adjoining fig. 34. A small figure
cut in ivory is made to stand on one foot at
top of a pedestal by being loaded with two
balls, a, 4, placod sufficiently low to throw
contre of gravity, 9, of the whole compound bo
below the foot of the figure. After being
turbed the little figure oscillates like a pendulum
having its point of suspension at the toe, and its

when after the slightest disturbance it tends 10
depart still more from its original position, A
body is in this stato when its contre of gravity it
vertically above the point of support, or higher)
than it would be in any adjacent position of the
body, An egg standing on its end or a stick balanced upright on the
finger is in this state,

Fig, 34,

GRAVITY, CENTRE OF GRAVITY, THE nALAXcK. 43°

, if im any adjacent position a body still remains in equilibrium,
of equilibrium is said to be neutral, In thia case an alteration

Fig: 36.

position of the body neither raises nor lowers its centre of gravity.
fect sphere resting on « horizontal plane is in this state.
‘35 represents three cones A, B, C, placed respectively in stable,

to produce equilibrium. To teat whether the arms of the
equal, weights are placed in the two scales until the beam
zontal; the contents of the scales being then interchanged,
remain horizontal if its arms are equal, but if not, it will d
side of the longer arm.
fi. The balance ought to be in equilibrium when the scales are
for otherwise unequal weights must be placed in the scales in.
produce equilibrium, It must be borne in mind, however, that
are not necessarily equal, even if the beam remains horis
scales ure empty ; for this result might also be produced by gi to
longer arm the lighter scale,
fii, The beam being horizontal, its contre uf gravity ought to be tn
ame vertical line with the odge of the fulorum, and a. tittle below the lat
for otherwise the beam would not be in stable equilibrium (62).
‘The effect of changing the position of the centre of
shown by means of a beam (fig. 87) whose fulcrum, being the nut
screw, a con be raised or lowered by turning the ecrew-head, &
‘Whon the fulcrum is at the top of the groove e, in which it ali
‘centre of gravity of the beam is below its edge, and the latter
froely about a position of stable equilibrium. By gradually lower
the falcrum ite edge may be made to pass through the centrs of gra

GRAVITY, CENTRE OF GRAVITY, THE BALANCE. 47

longer dseillates, but remains in equilibrium in all positions, When
| fulcrum is lowered still more, the centre of gravity passes above its

Fig. 37.

the beam is in a state of unstable equilibrium, and 1s overturned by
east displacement.
enaitiveness of the baiance.—A balance is said to be seasitiiw
aevory small difference between the weights in the-scales causes &
sl

sptil of the pointer.
“Let A und B (figs. 85 and $9) bo the points from which the seale pans
v ‘uspended, nnd C the axis of suspension of the beam. A, B, and ©

Sempposed to bo in the same straight line, according to
‘weights P and Q to be in the pans

‘the relation
{P-Q). AC=W. GN

Fig. 40.
rendered more sensitive by diminishing friction; to secure this adt
tage the edges from which the beam and scales are suspended are m
as sharp a8 possible, and the supports on which they rest are very bi
And further, the pointer is made long since its elongation renders & gi
etectien more perceptible by increasing the arc which its extremity

nee oe & G

chemical analysis. Its sensitiveness is such that when charged with
a kilogramme (1,000 grms.) in each scale, an excess of a milligrammie

——_~-

liquids fall like solids without r
their molecules. The ster hammer illuste
this; the instrument consists of a thick gla
tube about a foot Song, half filled with

the air having been expelled by eb
previous to closing one extremity
blow-pipe. When such a tube is
inverted the water falls in one undivided
against the other extremity of the tube,
produces a sharp dry sound, resembling |
which accompanies the shock of two soll
bodies.

From Newton's law (68) it follows,
when a body falls to the earth, the force
attraction which causes it to do #0 imen
as the body approaches the earth. Unless
height from which the body falls, he
be very great, this increase will be alto
innppreciable, and the force in question
be considered as constant and econtinons If
the resistance of the air were removed, #

Fig. 41 fore, the motion of all bodies falling to @

earth would be uniformly accelerated, and would obey the laws alt
ined (40).
Like al a machine.—Several instruments have been invented
for illustrating and experimentally verifying the laws of falling bodies
Galilicy, Who Aisedy cot lawsin the burly part of the seventeenth

~~ |

excentric ix so constructed that the little plate falls
the hand of the dial points to zero.

‘The weights M and M’ being equal hold cach other in equilibi
the weight M, however, is made to descend slowly by putting «
Dar or overweight m upon it; and to measure the spaces: i
describes, the rod or scale, a is divided into foot and in
mencing from the plate « Tocomplete the instrament there
number of plates, A, A, C, O% and a number of rings, B, BY
may be fixed by serews at any part of the scale. The plates
the descending weight M, the rings only arrest the bar or o
m, which was the cause of motion, so that after pausing throng
the weight M, in consequence of its inertia, will move on
with the velocity it had acquired on waching the ring. ‘The sive
parts of the apparatus being described, « few words will suffien to
plain the method of experimenting.

Let the hand of the dial be placed behind the zero point, the
D adjusted to support the plate i, an which the weight M) wit
overweight m rests, and the pendulum put in motion. As soon an i
hand of the dial points to zero the plate ¢ will fall, the weights MD
m will descend, and by a little attention and a few trials it
easy to place # plate A so that M may reach it exactly as the d
indicates the expiration of one second. To make a second expert
let the weights M and m, the plate «and the lever D, be placed
first; remove the plate A, and in its place put a ring, B, so as to
the overweight m just when the weight M would have reached’
putting the pendulum in motion again it will he easy, after a fm
trials, to put a plate, ©, #9 that the weight M may fall ape
precisely when the hand of the dial points to two seconds,
the overweight m in this experiment was arrested by the ring Bat
expiration of one second, the space BC was described by M im
second purely in virtue of its own inertia, nnd consequently, by
BE will indicate the velocity of the falling mass at the expiration
‘one second.

Proceeding in the same manner as before, let a third exporiment
made in order to ascertain the point B’ at which the weight M and
arrive after the lapse of two seconds, and, putting a ring av BY, apes
by a fourth experiment the point C’at which M arrives alone, thi
seconde after the descent commenced; B’C’ will then express

velocity acquired after a descent of two seconds, In a similar manneny.
by a fifth and sixth experiment, we may determine the space OB

—_ —

so the pendulum is inverted and d
axis, which, after some trinls, is placed so that the inversion
affect the number of oscillations made in a given time; the 1

becomes applicable to the compound pendulum, whose
vacuo, obey the same laws.

The length of the secouds pendulum —that is to say, of the
which makes one oscillation per second—varies, of course,
tensity of gravity ; at the level of the sea it is, according to Sab

3902074 inches at the Equator (St. Thomas),
3913083 ,, at London, and
3921469, at Spitebergen.
According to tho formula of (51), therefore, the accelerative
gravity nt the above places is obtained by
numbers roducod to fet by the square of 314150, 7
the space described in the first second of its motion by a tole fi
arwo from a state of rest (46)
16-0467 feet at the Equator,
16-0056 ,, at London, and
161264 ,, at Spitzbergen.
From observations of this kind, after applying the necessary correc
and taking into account the effect of rotation (73), the form of the
ean be deduced.

Tl, Verification of the laws of thy jatum.—In onler to
the Jaws of the simple pendulum (51) wo are compelled to employ ae
pound one, whose construction differs as little as possible from thik
the former. For this purpose a small sphere of a very dense sul
such as lend or platinum, is suspended from a. fixed point by
avery fine thread. A pendulum thus formed oscillates almost like
simple pendulum, whose length is equal to the distance of the centre of
the sphere from the point of suspension.

—_~

balance of « watch. The manner of 1

the pendulum is shown in fig. 44. Tho |
dulum rod passing between the prongs of af
@ communicates its motion ta a rod 6,
oscillates on a horizontal axis 0. To this axl
fixed a piece mm called an eseapement or 6
terminated by two projections or pailleta,

work alternately with the teeth of the

wheel R. ‘This wheel being ncted om |
weight tends to move continuously, Tot

in the direction indicated by the “arm

Now if tho pondulum is at rest, the whee
held at rest by the pallet m, and with it ti
whole of the clockwork and the weight
however, the pendulum moves and tales
position shown by the dotted line, am is

the whoo! essapes from the confinement in

it was held by the pallet, the weight dese

and causes the wheel to turn until its motion is arrested by the other:

n; which in consequence of the motion of the pendulum will be

into contact with another tooth of the excapement wheel. In thi

ner the descent of the weight is alternately pormitted and aj

in « word, regulated—by the pendulum. By means of @ proper tral
sf wheelwork the motion of the escapement is communicated to te
hands of the clock ; and consequently their motion, too, is regulated Bp
the pendulum.

. Causes which modify the intensity of terrestrial gravi-
tation.—The intensity of the force of gravity at the earth's surface
modified by two enuses; viz, by the form, and by the rotation of tht
earth.

i. If the earth were a sphere of uniform density the resultant of
the attrnctions which ite parts exert on an external point wonld be
the same as if the whole of its mass were collected nt its centre, anil
therfore the attraction at all points of ita surface would be the samme
In consequence of the flattening of the earth at its poles, this is no longer
exactly, but only very nearly trae; and tho attraction on an external
point is only nearly inversely as the square of its distance from the earth's
contre. Asa further consequence of the flattening at the poles, the

Tig 4

contrary forces, one of which tends to bring them together,

to separate them from each other. Tho fimt force, which is cal
molecular ettruction, varies in one and the same body with the d
only. The secand force, which is due to the action of heat, va
the intensity of this agemt, and with the distance, It ia the
relation between these forces, the °

Molecular attraction is only exerted at infinitely snsall di os
effect is inappreciable when the distance between the nicleotlonil

Gable. The laws which regulate this force are not known.
According to the manner in which it is regarded, molecular
is designated by the terms cohesion, affinity, o adhenion.

ofiron. Cohesion is strongly exerted in solide, lesa strongly it
and scarcely st all in gases. Its intensity decreases as the temy
increases, because then the repulsive force due to heat increases, —
it is that when solid bodies are heated they first liquefy, and
ultimately converted into the ‘gaseous state, provided that heat pi
in them no chemical change.

Cohesion varies not only with the nature of bodies, but also
arrangement of their molecules; for example, the difference |
tempered and untempered steel is due to a difference in the mo
arrangement produced by tempering. It is to the modifications w
this force undergoes that many of the properties of bodies are due,
a4 tenacity, hardness, and ductility.

Tn large masses of liquids, the force of gravity overcomes that of
cohesion. Hence liquids acted upon by the former force have 00 |
special shape; they take that of the vessel in which they are contained. |
But in smaller masses cohesion gets the upper hand, and liquids present

then the spheroidal form. This is seen in the drops of dew on the
loaves of planta; it is also seen when a liquid is placed on « solid which
it does not moisten; as, for example, mercury upon wood. ‘The ex-
periment may also be made with water, by sprinkling upon the surface ol

a GRAVITATION AND.

‘the tenacity of this hollow eytinder is, -
‘ists the internal one in the ratio of Ito.

the stems of corn and other plants, offer greater resistance”
wore solid, the mast remaining the same.
‘Tenacity, like elasticity, is difforeat in different directions in b
example, both the tenncity and the elasticity are
Hirection of the fibres than in a transverse direction. And thir

The following table gives the breaking weight in pounds:
having « sectional area of a square millimeter:

Antimony, cast. . . 147 Copper, annealed.

Bimuth ,.. . . S13) ) yy dmwne

Leu, me » $86 Tron, annealed . .
» drawn - 4 B19 | drawa z

is, - O80 Cast stool, drawn
a! alee e

Ziv, annealed . “s » SIGS Wood in the direction of t
» drown . . . 8

Gold, annealed - 5 9680 Mahogany...
» drawn . . G10 Oxk ——

Silver, annealed : . $608 Keech .
» drawn. ’ . 880 Fir. . . :

Platinum, annealed . 05 “Ash + See

” drawn. . 7700 Box

Tn this table the bodies are supposed to be at fhe extion
rature. At a higher temperature the tenacity rapidly decreases
Seguin, #en,, who has recently made some experiments on this point willl
iron and copper, has obtained the following values for the tenacity
pounds, of millimoter wire at different temperatures:

Tron. . at 10°, 18895 at 370°, LISS; at 500°, 770;
Copper . * ; io 164 ” o.

‘83, Ductitity. mtg is the property in virtue of which # grat
number of bodies change their forms by the action of traction “
pressure.

With certain bodies, such as clay, wax, etc, tho application of « =a
little force is sufficient to produce a change; with others, auch es the
reins and glass, the aid of heat is needed, while with the metals, more

ON Lighins,

‘The ascent of mercary in the capillary tube shows th

vessel A has diminished in volume, and gives the amot
pression, for the capacity of the whole vessel A in: 1
vis ‘the capillary tubo has been provionsly determined,

pressures, p
for this change of capacity, and have found that for a pressure eq
that of the atmosphere, mercury experiences m compression of
parts of its original volume ; water a compression of 0-00005, and él
& compression of 0:000133 parts of its original bulk,

For water and mercury it was also found that wil
decree of volume is proportional to the pressure,
Whatever be the pressure to which a liquid has been subjed
experiment shows that as soon as the pressure is removed the ligt
regains its original volume, from which it is concluded that
perfectly dastic,

89, Rquality of pressures, Pascal's taw.—ly considering |
na perfectly fluid, and assuming them to be uninflaenced by the
of gravity, the following has been establish Tt is often
Pascal's law, for it was first enunciated by that distinguished
trician,

Pressure exerted anywhere npon a mass of liquid is tranamitted
nished in all directions, and acts with the same force on all equal
aad in a direction at right eagles to those snrfaces,
To got a clearer iden of the truth of this principle, let ux ca
vessel of any given form in the sides of which are placed warkous)
drical apertures, all of the same. size, atl
closed by moveable pistons, Let 1s, fartl
imagine this vessel to be filled with liquid
withdrawn from the action of gravity; te)
pistons will, obviously, have no tendeney ts
move. If now upon the piston A (fig.
which has a surface a, » weight of P pow
be placed, it will be pressed inwards, and thi
pressure will be transmitted to the internal
—— faces of each of the pistons, B,C, D, and
Fig. 48. which will ench be forced outwards by &

prossure P, their surfaces being equal to. tbat)
of the first piston. Since each of the pistons undergoes a pressure By
equal to that on A, Jet us suppos: two of the pistons united so as 10)

follows that at every point of the side of any veass] a prisms
erted, at right angles to the side, which we will suppose to be

Bat sinco these prossums increase in propartion to the depth, andl

in proportion to the horizontal extent of the side, their re
only be obtained by calealation, which shows that the total p
any given portion of the side is equal to the weight of o column of
which has this portion of the wide for ite bave, amd shone height |
wertical distance from the centre of gravity of the portion to the mafia
the liquid. If the side of « vessel is a curved surface the same rule
the pressure on the surface, but the total pressure is no longer tt
sultant of the fluid pressures,
The point in the side supposed plane at which the resultant of all i
presture is applied is called the centre of pressre, and is nlways
the centre of gravity of the side. For if the pressures exerted ut di
parts of the plano sido were equal, the point of spplication of thelr
saltant, the centre of pressure, would obviously coincide with the ou
of gravity of the side. But since tho pressures increase with the dept
the centre of pressure is necessarily below the centre of gravity. TW
point ix determined by calculation, which leads to the following results
—i, With a rectangular side whose upper edge is level with the wi
the centre of pressure is at two-thirds of the line which joins the middie
of the horizontal sides moasured from the top, ii. With a telangular
side whose base is horizontal, and coincident with the level of the
the contreof pressure is at the middle of the line which join the y
of the triangle with the middlo of the base, iii, With « triangular
‘whose vertex is level with the water, the centre of pressure is in the
Joining the vertex and the middle of the base, and at throo-fourths of
the distance of the latter from the vertex. |
4. mydrostatic paradox.— We have already seen that the pressure
‘en the bottom of a vessel depends neither on the form of the vessel

iu ON Liquips:

To prove the first condition, let us suppose that mp ix the
all the forces acting upon any molecule m on the surface (fig.
that this surface is inclined in reference to the force mp.

while the second would move the
in the direction mf, which shone
Librium is impossible.

If gravity be the force acting on the liquid, the direction wap is:
hence, if the liquid is contained in a basin or vesel of amall ext
surface ought to be plane and horizontal (50), because then the dit
of gravity is the same in every point. Dut the case is different
liquid surfaces of greater extent, like the ocean, ‘The surfuee Wi
perpendicular to the direction of gravity; but as this changes froai es
point to another, and always tends towards a point near the centre:

allo, and assume a nearly spherical form.

96, Bquiltoriv: © same Hquid in several r
—When several vessels of any given form communicate
each other, there will be equilibrium when the liquid in each

* satisfies the two pi
‘efi vv] ditions (95), and further,
the surfaces of the liquids tw
the vessels are in the same
zontal plane.

In the vessels ABCD (fig.
which communicate with
other, let us consider tr
verse section of the tube sre
liquid can only remy
librium as long as pressures
which this section supports from
min the dirvction of », and from
win the direction of m, are equil
and opposite. Now it has been
already proved that these pret
sures ure respectively equal to the weight of a column of water, whose
‘base is the supposed section, and whose height is the distance from the
contre of gravity of this section to the surface of the liquid. If we coo
eeive then, a horizontal plano, we, drawn through the centre of gravity

Fig. 64

~

76

BC, to pass through B, the column
ee tela a wecery 0: If the heights of
‘then measured, by means of the scales, it will be
of the column of water is about 18) thmes of |
column of merenry, We shall presently seo that t
is about 134 times that of water, from which it fol
aro inversely ax the densities.

Tt may be added that the equilibrium cannot:
sufficient quantity of the heavier liquid for part of it to.
of the tube.

The preceding principle may be deduced by a very
Lot d and d’ bo the densities of water and mercury,
respective heights, and let g be the force of gravity.
will be proportional to the density of the liquid, to
force of gravity; on the whole, therefore, to the
the pressure at C will be proportional to d'A’g.
equilibrium, dig must be equal to @h'g, or di=di,
more than an algebraical expression of the above principle
two products must always be equal, d’ must be as many
than d, 93 4 is leas than A,
In this manner the density of a liquid may be ei t
one of the branches contained water and the other oil, and #

‘were respectively 16 inches for the oil, and 1M inches for the wat
density of water being taken as unity, and that of oil being walled 5,
shall have

Wx 1 = 4X1; whence r= 14 = 0938,

APPLICATIONS OF THE PRECEDING HYDROSTATIC PRINCIPLES

99. Bydraulic press.—The law of the equality of prossume 1
roceived a most important application in the hydraulic pret, &
by which enormous pressures may be produced, Its principle ia das
Pascal, but it was first constructed by Bramah in 17908,

Tt consists of a cylinder, B, with very strong thick sides (fig. 56),
which there is a cast iron ram, P, working water tight in the collar of 4
cylinder. On the ram P there is a cast iron plate on which the substat
to be pressed is placed. Four strong columns serve to support and &)
second plate Q.

By means of a leaden pipe, K, the cylinder B, which is filled w
‘water, communicates with a small force pump, A, which works by ms}
of a lever, M. When the piston of this pump p ascends, & acum
produced and the water rises in the tube a, at the end of which them

] APPLICATIONS OF HiDROSTATIC PRINCIPLES. 7

) to prevent the entrance of foreizn matters. When the piston p
it drives the water into the cylinder by the tube K.

lig. 57 represents a section, on a larger seale, of the system of valyes
femary in working the apparatus, The valve 0, below the piston p,

Fig. 67.
|
(is when the piston rises, nnd closes when it descends, The valve o,

= §. “w

This piece ia bent, so that a se

and is fitted into n groove n made in the
linder, ‘This collar being concave downwards, in proportion
sure increases it fits the more tightly against the ram P on o
‘the neck of the cylinder on the other, and » prevents
water.
‘Tho ‘pressure which ean be obtained by this pros

where the power is applied is five times the distance from the
the piston p, the pressure on p will be five times the power,

a man actson M with a force of sixty pounds, the force tn

Dy the platon p will be 300 pounds, and the force whieh tends to
the piston P will be 30,000 pounds, supposing the section of P isa
hundred times that of p,

‘The hydraulic press is used in all cases in which great pressures
required. It is used in pressing cloth, in extracting the juice of b
root, and in expressing ol] from seeds; it alao serves to test the st
‘of cannon, of steam boilers, and of chain cables. Tho
the tubular bridge which spans the Menai Straits were raised by
of an hydraulic press. ‘The cylinder of this machine, the Inngest whi
haa ever been constructed, was nine foet long and twenty-two inches |
internal diameter; it was capable of raising a weight of two thousm
tons.

100. ‘Water lovel.—The water level is un application of the 60
of equilibrium in communicating vessels. It consists of a metal
bent at both ends, in which are fitted glass tubes D and E (fig. 68),

ik

soems to be, is in fact slightly curved in auch a manner that its axis
are of acirele of very lange radius; it fs filled with spirit with the
of abubble of air, which tends to occupy the highest part. The tube is)
in & brass case, CD (fig. 60), which ie so arranged that when it is
horizontal position the bubble of air is exactly betweon the’

points marked in the case,

‘To take levels with this apparatus, it is fixed on a telescope,
can consequently be placed in a horizontal position,

102. Artesian wells.—All natural collections of water exemplify
tendency of water to find its level. ‘Thus, a group of lakes, sach as
great lakes of North America, may be regarded as a number of

communication, and consequently the waters tend to maintain the
level in all, Thie, too, is the case with the source of a river and the
and as the latter i¢ on the lower level the river continually flows
to the sea along its bed, which ia, in fact, the means of

betweeh the two.

Perhaps the most striking instance of this class of natural phenomeaa
is that of artesian wells, These wells derive their name from the pros
vinoe of Artois, where it has long been customary to dig them, and from
whence their use in other parts of France and Europe was derived. It
seems, however, that at a very remote period wells of the same Kind
were dug in Chinn and Egypt.

To understand the theory of these wells, it must be premised that the
strata composing the earth's crust are of two kinds: the one permeable
to wator, such as sand, gravel, etc.; the others impermeable, auch #4
clay, Let us suppose, then, a geographical basin of greater or leas extent,
in which the two impermenble layers AA, BB (fig. 61), enclose between
them a permeable layer MM, The rain-water falling on the part of this
Inyer which comes to the surface, which is called the outcrop, will filter

as a

Latter, it will sustain prmpeeonenor ss vertically
through the centre of gravity of the displaced liquid, and equal wa
weight of the displaced liquid. If, as almost invariably 4
liquid is of uniform density, the centre of gravity of the displaced ii

example, if asphereiscomposed of a hemisphereof iron and anotheot
its centre of gravity would not coincide with its geometrical centre, I
if it were placed under water, the centre of gravity of the displaced
would bo at the geometrical crntee, that is, will have the same pa

as the centre of gravity of the spbery, if of uniform density.

104, Principle of Archimedes.—Tho proceding principles [wt
that every body immersed in a Liquid is submitted to the aesion af
forces; gravity which tends to lower it, and the buoyancy of the’
which tends to rise it with a force equal to the weight of the’
displaced, The weight of the body is either totally or partially:
come by this buoyancy, from which it is concluded that a body ir
tn a liquid loses a part of its weight equal to the weight of the
liquid,

This principle, which is the basis of the theory of immersed
floating bodies, is called the principle of Archimedes, after the di
It is shown experimentally by moans of the hyvbrostatic balance (fig. 6
This is an ordinary balance, each pan of which is provided with a ho
the beam can be raised by means of a toothed rack, which is worked
a little pinion, ©. A catch, D, holds the rack when it has been
The beam being raised, a hollow copper cylinder, A, is suspended to
of the pans, and below this a solid cylinder, B, whose volume ix 3
equal to the capacity of the first cylinder; lastly, an equipolse is p
If now the hollow cylinder be filled with water
urbed, but if at the same time the beam is lowered:
that the solid eylinder B becomes immersed in a vessel of water p
boneath it, the equilibrium will be restored. By being ims
Wafer, the cylinder B loses ¢ portion of its. weight. equal. to thalgil
water in the cylinder A. Now as the capacity of the cylinder A is

mil act in opposite directions; whence follow the co
brium, name!

i, The floating body mst displace a volume of liguid 1
‘equals that of the body.

ii, The centre of gravity of the floating body must be in the
tical line with that of the Auid displaced

‘Thus in fig. 4, if C tothe contre of. gravity of the body aml
of the displaced fluid, the line GO must be vertical, since w!
the weight of the body and the fluid pressure will act i

directions along the same line, and will be in equilibrium, if equal:
is convenient, with reference to the subject of the present article, to apt
of the line CG produced as the axis of the body.

Next lot it be enquired whether the equilibrium be stable or
Snpposo the body to be tumed through a small angle (fig. 65)
the axis takes a position inclined to the vertical. The centre of gr
of the displaced fluid will no longer be G but some other point G,
since the fluid pressure acts vertically upward through G’, its d
will cut the axis in some point M’, which will generally have dif
positions according as the inclination of the axis to the vertical is gr
orsmaller, If the angle is indefinitely small, M’ will have a

metacentre,

If we suppose M to be above C, an inspection of fig. 66 will show di
when the body bas received an indetinitely small displacement the we

which had entered it, and the
now lighter, risesto the surface,

fishes have nm air-bladder below the
whieh is called the siwinneing Bladder,
fish can compress or dilate this at |
by means of a muscular effort, and p
the same effects a4 thoss just desert
that is, it ean either rise or sink in water,
109, Swimming.—The human bed
"oe lighter, on the whole, than an equal yo
Fig 08, water: it consoquontly floats on the surfaceat
still better in sea water, which is heavier than fresh water. The d
in swimming consists, not #9 much in floating, as in keeping the head abav
water, 0 a8 to breathe freely, In man the head is heavier than the
lower parts, nnd consequently tends to sink, and hence swimming is
art which requires to be learned, With quadrupeds, on the ‘
the hend, being less heavy than the posterior part of the body, ana
above water without any effort, and these animals therefore swift |
naturally. .

SPECIFIC GRAVITY —ItYDROMETERS,

110, Determination of specific gravities,—It has been
explained (24) that the specific gravity of a body, whether solid ee
liquid, is the number which expresses the relation of the weight of
given volume of this body, to the weight of the same volume of distilled
water at a temperature of 4°. In order, therefore, to caloulate the
specific gravity of a body, it is sufficient to deter ‘its weight and that
of an equal volume of water, and then to divide the first weight by the
second ; the quotient is the specitic gravity of the body.

Throe methods are commonly used in determining the specific gravities

ak z|

88 OS LIQuips.

meter is again depressed to the standard o. If, for instance,
necessary to add 65 grains, the weight of the sulphur is |
the difference between 125 und 55 grains, that ia, 70 grains
thus determined the weight of the sulphur in air, it is no
necessary to ascertain the weight of an equal volume of water,
this, the piece of sulphur is placed in the lower pan C at m, aa
sented in the figure, The whole weight is not changed, t
the hydrometer no longer sinks to the standard ; the sulphur, by
sion, has lost « part of its weight equal to that of the water displ
Weights are added to the upper pan until the hydrometer sinks aj
to the standard. This weight, 34-4 grains for example, represents
weight of the volume of water displaced ; that is, of the volume of water
equal to the volume of the sulphur. It ix only necossmry, therefore; |
divide 70 grains, the weight in air, by 34-4 grains, and the quotient 205
is the specific gravity.
Ifthe body in question is lighter than water it tends to rise to #
_ surface, and will not remain on the lower pan C. To obviate this,
small moveable cage of fine wire is adjusted so as to prevent the nse
of the hody. ‘The experiment is in other respects the same,
112. Specific gravity fiask.—When the specific gravity of
substance in a state of powder is required, it ean be found most
‘ently by means of the specific gravity flask, This instrament is a small
flask with a large neck fitted with a carefully ground glass stopper, ‘The
stopper is perforated along its axis, and the bore is continued by meant
of a thin tube which expands into a tube of greater diameter, as shownill
the figure. On the thin tube is a mark a, and at each weighing the
flask is filled with water exactly to the mark. This is done by filling
the flask when wholly under water, and putting in the stopper while itis
immersed. ‘The Haak and the tube are then completely
filled, and the quantity of water in excess is removed
by blotting paper. To find the specific gravity proceed!
as follows. Having weighed the powder, place it inom
of the scale pans, and with it the flask filled exactly to
a and carefully dried, Then balance it by placing small
shot, or sand, in the other pan. Next, remove the tins
and pour the powder into it, and, as before, fill it up
with water to the mark a, On replacing the flask if
the scale pan it will no longer balance the shot, sinee
the powder has displaced a volume of water eqnal te
its own volume, Place weights in the scale pan along
with the flask until they balance the shot. Thess
weights give the weight of the water displaced, Theti
the weight of the powder, and the weight of an equal bulk of water
boing known, its specific gravity is determined as betore.

Fig 70.

» |

» OS LIQUIDS,
‘114. Specie gravity of quids.—i Method of

of the body in these two liquids ir noted.

the weights of equal volumes of water, and of the given
comsequently it ix only necessary to divide the second of
fret to obtain the required specific gravity.

Let P be the weight of the platinum ball in air, P its weight
P” its weight in the given liquid, and let D be the specific
sought. ‘The weight of the water displaced by the
P—P and that of the second liquid is P—P” from which

—P-P" |
pp

ii. Fikrenheit's hyvdrometer.—This instrument (fig. 71)
Nicholson's hydrometer, but is made of glass, so ns to be
Tiquids, At its lower extremity, instead of « pan, it is loeded)
small bulb containing mereury. There is a standard mark on the)

The weight of the instrument is first accurately determined in
then placed in water, and weights added to the seale pan tntil the:

the stem is level with the water, It follows
first principle of the equilibrium of
that the weight of the hydrometer,
the weight in the scale pan, is equal to
of the volume of the displaced water. In
manner, the weight of an equal volume of
liquid is determined, and the specific
by dividing the latter weight by the fornvér, |
Neither Fabrenheit’s nor Nicholson's by
give such accurate results as the hydrostatic
ili. Specific gravity flask —This haa eon
described. In determining the specific gra
liquid, the flask is first weighed empty, and
cessively, full of water, and of the given Ii
Fig. 71 the weight of the flack be subtracted from
- weights thus obtained, the result
weights of equal volumes of the liquid, and of water, from wl
specific gravity is obtained by division,

114, On the observation of temperature in ascertaining
gravities.—As the volume of a bod:
and as this increase varies with different substances, the specific]
of any given body is’not exactly the same at different temperatat

centimeters. Tlence if D is the specific gravity of
weight of the column in grammes, we have P=#rlD, and

Spas
r=/ Sr
If rand Zare in inches and Pin grains, we sball have
and therefore

r= S seem

Tn a similar manner the dinmeter of very fine metallic v
calculated.

117. My¢rometers with variable volume.—The
Nicholson and Fahrenheit are called Aydrometers of comatent
wariable weight, because they are always immersed to the
but carry different weights ‘I'here are also
volume but of constant weight. These instruments, known
different names of avidometer, alcoholometer, lactometer, and
fare not used to dotermine tho specific gravity of the Ii
show whether the acids, alcohols, solutions of sugar, ete,
less concentrated.

N18. Beaumé's hydrometer.—This, which was the first
instruments, may serve as a type of them. It consists of w gl
(fig. 72) londed ut its lower end with mercury, and with a
in the middle, Thestem, the external diameter of which is as
‘as possible, is hollow, and the scale is marked upon it,

The graduation of the instrument differs according as the
which it is to be used, is heavier or lighter than water. In
ease, it is so constructed that it sinks in water nearly to the
the stem, to a point A, which is marked zero, A solution of
parts of anlt in eighty-five parts of water is made, and the it u

oy OS LIQUIDS,

gives the reading of the instrument for each degree af
O up to XP, When the exact analysis of an alcoholic mixture
made, the temperature of the liquid is first determined, anil this
point to which the alcoholometer sinks in it. The number in the!
corresponding to these data indicates the percentage of alcohol.
ita giving the percentage of alcohol, this is often called the «
alcoholometer.

120. Saltmeters.—Swlimeters, or instruments for indicating the pa
centage of salt contained in « solution, are made on the principle of &
centesimal aleoholometer, They are graduated by immersing

pure water, which gives the zero, and then in solutions
ferent percentages, 5, 10, 20, ete., of the salt, and marking om the
the corresponding points. ‘These instruments aré so far objectio
that every salt requires a special instrament. Thus one gradu
common salt would give totally false indications in a solution of

Lactomoters and vinometers are similar instruments, and are
measuring the quantity of water which is introduced into milk
for the purposo of ndulteration. But their use is limited, b
density of these liquids is. very variable, oven when they are part
natural, and an apparent fraud may be really due to a bad mataralg
of wine or milk. Urinometers, which are of extensive use in ii
ure based on the same prineiph
121, Denstmeter.-The densimeter is an apparatus for indicatl
apecitic gravity of a liquid. Gay-Lussne’s densimeter has the
construction as Boaumé's hydrometer, but i
graduated i in a different manner, ‘Rosseau’s den

(fig. 73) is of great use in many selene
title investigations, in determining the
gravity of a «mall quantity of a Liquid. Ht bas
the same form as Beaumé's hydrometer, but on the
apper part of the stem there is a small tube, it
which is placed the substance to be determined.
A mark on the side of the tube indicates s mensane
of a cubic centimeter.

The instrament is so constructed that it sinks
in distilled water to a point, B, just at the bottont
of the stem. It is then filled with distilled water
tw the height measured on the tube, which indi-
= cates a cubic centimeter, and the point to whieh

Fig. 73. it now sinks f# 20°, The intorval between 0 and
20 is divided into 20 equal parts, and this gradua~
tion is continued to the top of the scale. As this is of uniform bore

cach division corresponds to J, gramme or 0-05,

‘When the tubes are mritened by the liquid, its surface sss
form of a omesve hemispberical segment, called the concave
(fie. 76); when the tubes are not moistened, there is a compar!

depressed according as dt dues or dere wat moisen the tube,

IL. Far the same tig™nid the elevation caries inversely as the:
the tube, when the diameter does met exceed two millimeters,

IIL. The elevation varies with the mature of the liquid, aud with)
perature, bet ts tacependent of the nature and thickucss of the tube. —

These laws hold good in vacuo as well as in air.

When liquids are in tubes which they do not moisten, the dep
is in the inverse ratio of the diameter of the tubes; but for tubes
samt diameter the depression depends on the substance of ti
For instance, in an iron tube 1 millimeter in diameter, the dep
mercury is 1226 millimeter; but in « platinum tabeof the same:
tho depression is 0655 millimeter. Moreover the depression d
the height of the convex meniscus of the mereury, and this
for the same tube, according as the meniscus is formed daring am
ing or descending motion of the mercarial column in the tube:
results undergo modification if the mercury is impure.

IM. Ascent and depression between parallel or
surfaces.—When two bodies of any given shape arm dipped
analogous capillary phenomena are produced, provided the Vodien :
sufficicntly near. If, for example, two parallel giass plates are in

iu water, at a very «mall distance from each other, water will rise

the two plates in the inverse ratio of the distance which separates thes
Tho height of tho ascent for any given distance is balf what it be |
in a tube whove diameter is equal to the distance between the plates
If the parnilel plates are immersed in mercury, a corresponding dd
pression ie produced, to the same laws.

If two glass plates AB and AC with their planes vertical and ig
clined to one another at a small angle as represented in tig. 78, hav
their ends dipped into a liquid which wets them, the liquid will ry
between them. The elevation will be greatest at the line of contact 4

ox gui.

which ucts in the diroction mF; and by the attraction
which is exerted in the direetion wm, According to the
sities of these forces, their resultant can take three

i, The resultant is in the direction of the vertical mIt
this case the surfnce m is plane and horizontal ; for, from

os

Fig. 81.

of the equilibrium of liquids, the surface must be
forve which acts upon the molecules,

ii, If the force m incrensos or F diminishes, the resultant R
the angle wmP (fg. 82): in this case the surface takes a
pendicular to mR, and becomes concave. :

iii. If the forco F increases, or w diminishes, the resultant
the direction mR (fig. 88) within the angle PmP, and the
coming perpendicular to this direction is convex.

Fig. 85,

127. Xufuence of the curvature on capillary
‘The elevation or depression of a liquid in « capillary tube
the concavity or convexity of the meniscus. In a concave
abed (fig. 84), the liquid molecules are sustained in equilibrium by)
forces acting on them, and they excreise no downward pressure on the
ferior layers. On the contrary, irtue of the molecular attraction, t
act on the nearest inferior layers, from which it follows that the prom
on any layer, mm, in the interior of the tubo, is less than if thers 9
ne meniscus. The consequence is, that the liquid ought to rise im
tube until the internal pressure on the layer, mu, is equal to the press
op, which acts externally on a point, p, of the same layer,

100 Ox LIQUIDS.

ENDOSMOKE, KPFUSION, ABSORPTION, AND OSTOETION. —

120, Rndosmose and exesmose.—When two different |
are separated by a thin porous partition, either inorganic or
current sets in from each liquid to the other; to these
names endosmose and exosmose are respectively given, ‘These terms,
signify impulee from within, and impulse from withont, were fine inte
by M. Dutrochet, who first drew attention to these phenomens |
may be well illustrated by means of the exdownometer, This cant
a long tube, at the ond of which a membranous bag is firmly
(fig. 86). The bag is then filled with a strong syrup, or some
solution denser than water, such as milk or albumen, and is imme
water. The liquid is found gradually to rise in the tube, tow
which may attain several inches: at the same time, the level
liquid in which the endosmometer is immersed becomes low!
follows, therefore, that some of the external liquid has passed #
the membrane and has mixed with the internal liquid. Thee

liquid moreover is found to |

fl some of the internal liquid,
two currents have been pradi
opposite directions The flow
liquid towards that which it
in volume is endommose, and the

in the opposite direction is «4

Tf water is placed in the bag, a

mersed in syrup, endosmose j

duced from the water towar

syrup, and the liquid in the interit
nishes in volume while the level
exterior is raised.

The height of the ascent in
dosmometer varies with different
Of all vegetable substances, ¢
that which, for the same dens
the greatest power of endosmost
albumen has the highest powe
animal substances. In general,

/ be sald that endosmose take
towards the denser liquid.
Fig. 86. and ether form an exception

they behave like liquids wh
denser than water, With acids, according 2s they are more

investigations

i. When solutions of the same substance, but of different.
taken, the quantities diffused in equal times are pro
strengths of the solutions.

fi, In the caso of solutions containing equal weights of dif
stances, the quantities diffused vary with the mature of the
Saline substances may be divided into a number of eguiclif
the rates of diffusion of each group being connected with the
simple numerical relation.

iii. The quantity diffused varies with the tem] r
the rate of diffusion of hydrochloric acid at 15° C, a8 unity; at 4
is 218.

iv. If two substances which do not combine be mixed in-
they may be partially separated by diffusion, the more

passing out most rapidly. In some eases chemical d
may be offected by diffusion. ‘Thus bisulphate of potassium is de
into free sulphuric acid and sulphate of potassium.
¥. Ifliquids be dilutea substance will diffuse into water, cont
other substance dissolved aa into pure water; but the rate is
reduced if a portion of the diffusing substance be already present.
The following table gives the approximate times of equal

Hydrochloric acid. =. 1:0 Sulphate of mee
Chloride of sodium » 23 Albumen. .
Sirs sl; Caml

It will be seen from the above table that the difference incall
rates of diffusion is very great. ‘Thus sulphate of magnesium, one of
Jonst diffusible saline substances, diffuses seven times as rapidly as alba
and 14 timos as rapidly ns caramel. ‘These last substancos, ike lhydn
silicie acid, starch, dextrino, gum, ete., constitute a class of
which are characterised by their incapacity for taking the
form, and by the mucilaginous character of their hydrates,
gelatine as the type of this class, Graham has proposed to call them :
(oan, glue), in contradistinction to the far more easily
erystalloid substances, “1

Graham has proposed 4 method of separating bodies based on ¢
‘unequal diffusibility, which he calls dialysie. His dialyser consists)
ting of gutta percha over which is stretched while wet a sheet of
ment paper, forming thus a vowel about two inches high
inches in diameter, the bottom of which is of parchment paper, A

a |

104 ON LIQUIDS.

so rapidly that a partial yaewum is produced and mercury
tube toa height of soveral inches (fig. 80). If several wach
filled with different gases, and allowed to diffuse into the air in #.
in the tame time, different quantities of the various gases)
diffuse, and Graham found that the lew:
these diffusions is, that the force of
inversely as the square roots of the densities
‘Thus, if two vessels of equal capacity, eo
oxygen and hydrogen, be separated by a
plug, diffusion takes place, and after the
some time, for every one part of oxygen
passed into the hydrogen, four parts of
have passed into the oxygen. Now the density
hydrogen being 1, that of oxygen is 16, ies
the force of diffusion is inversely as the
roots of these numbers. It is four times a)
in the one which has y4 the density of the ol
132, Bfusion and Transpiration of
Effusion is the term applied to the pheno
of the passage of gases into vacuum, thro
minute aperture not much more or less than 0-013 millimeter ir
meter, in a thin plate of metal or of glass, Within the limite of
perimental errors, the rates of effusion of different gases coincide wifi
those of their diffusion,

Fig, 89.

vacuum, the rate of efflux, or the relocity of transpiration, is inde}
of the rate of diffusion.

i. For the same gas, the rate of transpiration increases, other thinge
equal, directly as the pressure; that is, equal volumes of air of differettl
densities require times inversely proportional to their densities.

ii, With tubes of equal diameters, the volume transpired in equal timmy
is inversely as the length of the tube,

iii. As the temperature rises the transpiration becomes slower.

The rate of transpiration is independent of the material of the taba,
133. Absorption and imbibition.—The words absorption and im”
hibition are used almost promiscuously in physics; they indicate thal
penetration of a liquid or a gas ly: Absorption is used
both for liquids and gases, while imbibition is restricted to liquids.

Tn physiology an important distinction is made between the two words:
absorption means the penetration of a foreign substance into the tissaes
of aliving body, while imbibition refers to penetration into bodies de=
prived of life, whether organie or not,

194, Adsorption of gases.—The surfaces of all solid bodies exert

a al

BOOK Iv,
ON GASES.

CHAPTER L
PROVERTIES OF GASES. ATMOSPHERE NAROMETERS,

f ayatcat properen of gute Cass am bodies whose
are in a constant state of repalsion, in virtue of which they
{es the most perfect mobility, and are continually tending to occupy
fater space. This property of gases is known by the namos expan-
ty; fonsion, or elastic, force, from which they are often called elastic

L

liquids haye several properties in common, and some in
seem to differ are in reality only different degrees of the
ie in both, the particles are empable of moving; in
; in liquids not quite frocly, owing toa certain degre of
are compressible, though in very different degrees; if a
exist under # pressure of one atmosphere, and then
doubled, the water is compressed hy about the sahssa
gua is comprosed by one half. In density there is a great
» which is the type of liquids, ie about 800 times as
of gaseous bodies, while under pressure of one
‘property by which gases are distinguished from liquids
to indefinite expansion,
eeumes the solid, liquid, or gnscous form according to the rela~
the cohesive and repulsive forces exerted between their
Tn Hiquids these forees balance ; in guses repulsion preponde-

G

i
pad zi
E

a

Hi!
teal

oF

‘of pressure and of very low temperatures, the force of
be fo far increased in many gases that they ure converted

‘anil there is every reason for believing that with suilicient
‘end cold they might all be liquefied, On the other hand, heat,
Toros of repulsion, converts liquids, such as water,
Ol, and ether, into the airiform state in which they obey all the

|

108 ON GASES.

laws of gases, This airiform state of liquids is known by
vapour, while gases sure bodies which, under ordinary |
re, remain in tho niriform state.

In describing the properties of gases we shall, for obviouan
exclusive reference to atmospheric air as their type.

188, Expansiditity of gases.—This property of gases, #
dency to assume continually n greater volume, is exhibited by
the following experiment, A bladder closed by a stop-cock an
half full of air is placed under the receiver of the air pump (1
and a vacuum ix produced, on wi
bladder immediately distonda.
arises from the fact that the
of air repel each other and press
the sides of the bladder, Under:
nary conditions this internal pn
counterbalanced by the air im
ceiver, which exerts an equal
trary pressure. But when this
is removed by exhausting the x
the internal pressure becomes
When air is admitted into the reost
the bladder resumes its original &

139, oo .
The compressi ;
dily shown by the pnewinatic a
(fig. 91). ‘This consists of a stout,
tube closed at one end, and p
with a tight-titting solid piston,
the rod of the piston is pressed, it mores down in the tube, and i
becomes compressed into a smaller volume ; but, as soon as the Sores

Fig. 90,

W

Fig. 91.

removed, the air regains its original volume, and the piston rises to it
former position,

—_

must sustain the weight of all the air above it, and
to the air beneath it, and likewise to the curred si

provided the height remains the supe.
For small quantity of gas the pressams due to its weight ane ai

Insiguifcant, and may be nvglected; bat for large q

rotatory motion of the globe, and would remain tixed relatively to
trial objects, but for local circumstances, which produce winds, and
constantly disturbing its equilibriam.

Air was reganied by the ancients as one of the four elements
chemistry, however, has shown that it is mixture of oxygen and
gen gnsea in the proportion of 20-8 volumes of the former to79-2 vol
of the latter. By weight it consists of 23 parts of oxygen to 77
nitrogen.

‘The atmosphere also contains a quantity of aqueous vapour,
varies with the temperature, the season, the locality, and the diree
the winds. It further contains « small quantity of ammoniscal gay,
from % to 6 parts in 10,000 of its volume of carbonic acid.

‘The carbonic acid arises from the respiration of animals, from the |
cesses of combustion, and from the decomposition of onanic substanoea
M. Bousingnult has estimated that in Paris the following quantities of
carbonic acid are produced every 24 hours :

By tho population and by animals. 11,805,000 eubie feet
By processes of combustion. . . 92,101,000,
103,906,000,

Notwithstanding this enormous continual production of carbonic acid
on the surface of the globe, the composition of the atmosphere does mob
vary ; for plants in the process of vegetation decompose the carbonie
acid, assimilating the carbon, and restoring to the atmosphere the oxyget |
which is being continually consumed in the processes of respiration and
combustion.

143, Atmospheric pressure.—If we negloct the perturbations to

ae

112 ON GASES.

the vessel, by working the alr pump, the bladder is depressed by
weight of the atmosphere above it, and finally bursts with a loud
caused by the sudden entrance of the air.

145, Magdepurg hemispheres.—The preceding experiment
serves to illustrate the downward pressure of the atmosphere. By
of the Magdeburg hemispheres (figs. 94 and 96), the invention of

Fig. 4 Fig, 95.
due ta Otto von Guericke, burgomaster of Magdeburg, it can be shown that
the pressure acts in all directions. This apparatus consists of two hollow
brass hemispheres of 4 to 4} inches diameter, the edges of which am
made to fit tightly, and aro woll greased, One of tho hemispheres if
provided with a stopcock, by which it can be screwed on the air pump,
and on the other there is a handle, long as the hemispheres contain
air they can be separated without any difficulty, for the external pwressan®
of the atmosphere is counterbalanced by the elastic force of the air if
the interior, But when tho air in the interior is pumped out by mean
of the air pump, the hemispheres cannot be separated without a power
fol effort; and nx this is the case in whatever position they are held, i
follows that the atmospheric pressure is transmitted in all directions,

DETERMINATION OF THE ATMOSPHERIC PRESSURE, BAROMBTERS.

146. Torricelli’s expertment.—The above experiments demonstrate
the existence of the atmospheric pressure, but they give no indications

4 ON GASES.

peated Torricelli’s experiment at Rouen, in 1640, with other liquide,
took a tube closed at one end, nearly 50 feet long, and having:
with water, placed it vertically in a vessel of water, and found
water stood in the tube at a height of 34 feet; that is, 130 tine
high as mercury. But since mercury is 15°6 times as heavy as wa
the weight of the column of water was exactly equal to that of)
columa of mercury in Torricelli’s experiment, and it was co ”
the same force, the pressure of the atmosphere, which
ported the two liquids, Pascal's other experiments with oil 0d
wine gave similar reaults,

148. Amount of the atmospheric pressure.—Let us assume

the tube in the above experiment is a cylinder, the section of
equal to a square inch, then since the height of the mercurial col
round numbers is 30 inches, the column will contain 30 cubic
and as acubic inch of mereury weighs 34334 grains=0-49 of a p
pressure of such a column on a square inch’ of surface is equal te
pounds, In round numbers the pressure of the atmosphore is taki
15 pounds on the square inch. A surface of «foot aquare contains
square inches, and therofore the pressure upon it is equal to2,160 po
or nearly & ton,
A gas or n liquid which acta in such a manner that a square
surface is exposed to a pressure, 15 pounds, ia called a
atmosphere. If, for instance, the elastic force of the steam of a boilers
great that each square inch of the internal surface is exposed to re
of 90 pounds (=6 x 15) we say it was under a pressure of six i

‘The surface of the body of a man of middle site is about 16 aq
feet; the pressure, therefore, which a man supports on the surface of
body is 37,500 pounds, or upwards of 16 tons Such an enormotid
pressure might seem impossible to be borne ; but it must be remembered
that in all directions there are equal and contrary pressures which
counterbalance one another, It might also be supposed that the effet ot
this force, acting in all directions, would be to press the body together
and crush it. But tho solid parts of the skeleton could resist a far greater
pressure; and ns to the air and liquids contained ivi tho organs and vessel
the air has the came density as the external air, and cannot be farther
compressed by the atmospheric pressure; and from what has been aah
about liquids (88) it is clear that they are virtually incomprossible
When the external pressure is romoved from any part of the body,
either by means of a cupping vessel or by the air pump, the pressure
from within is seen by the distension of the surface.

149. Different kinds of barometers,—Tho instruments used for
measuring the atmospheric pressure are called barometers In ordinary
barometers, the pressure is measured by the height of a column

116 ‘ON GASES.

leather until the mereury, which riscs with it, quite fille the cisters;
barometer may then be inclined, and even inverted, without any:

a bubble of air may enter, or that the shock of the mercury may
the tube,
Fig. 98 represents the arrangement of the barometer, the
which is placed in a brass case, At the top of this case, there are
longitudinal apertures, on opposite sides, so that the level of
mercury, B, is seen. ‘The scale on the case is graduated in mil

An index A moved by the hand, gives, by means of « vernien,
height of the mercury to 4, of a millimeter, At
bottom of the case there is the cistern 4,
mereury, Q.

Fig. 99 shows the details of the cistern on #
soule, It consists of a glass cylinder ef, which allows
moreury to be seen; the bottom of the cylinder ix
with leather, ma, which is raised or lowered by means off
screw, C. This screw works in the bottom of « bam
cylinder, G, which is fastened on the glass cylinder,
the top of the cistern there is a emall ivory pointer @
the point of which exactly corresponds to the gero on te
seale, The upper part of the cistern is closed by buekaliay
d, which is fastened to the barometer tube, K, and toa tube
lure in the brass plate, which covers the cistern, The
atmospheric pressure is transmitted through the pores o
this leather. In using this barometer, tt
morcury is first made level with the point ®
which is effected by turning tho sem O
either in one direction ur the other. In tat
manner the distance of the top, B, of the
column of mercury from the ivory point
gives exactly the height of the barometer,

162, Gay-Lussac’s syphon barometer
—The syphon barometer is « bent glass taba,
one of the branches of which is much longer
than the other, The longer branch, whieh
closed at the top, is filled with mereury as i
* the cistern barometer, while the shorter branch,
which is open, serves as a cistern, ‘The differs
ence between the two levels is the height of |
= : the barometer.

Fig. 98. Fig. 99. Fig, 100 represents the ayphon barometer a
modified by Gay-Lussae. In order to render it more available for
travelling, by preventing the entrance of air, he joined the two branches

118

column by its elastic force. To obtain this result, a small q rr
pure mercury is placed in the tubo amd boiled for some time. Te ist
allowed to cool, and s further quantity, previously warmed, added w
is boiled, and so on, until the tube is quite fall; in this j
moist ure and the air which adhere to the sides of the tube pass
the mercurial vapour.

A barometer is free from air and moisture if, when it is inclin
mercury strikes with a sharp metallic sound against the top of the t#
If there is air or moisture in it, the sound is deadened.

164. Correction for ecapiliarity.—In cistern barometers
always n certain depression of the mereurial column due to &
unless the internal diameter of the tube exceeds 08 inch. To
the correction due to this depression, it is not enough to know
diameter of the tube, we must also know the
the meniscus od (fig, 102), which varies according:
meniscus has been formed during an ascending or @
ascending motion of the mercury in the tube.
quently the height of the meniseus must be d
by bringing the pointer to the level a}, and then tol
level d, when the difference of the readings will givel
height od required. These two terms, namely,
ternal diameter of the tube, and the heightof the
being known, the resulting correction can be
of the following table, which follows the arrangement frequently ad
for a multiplication table :—

Fig. 102.

— ——an

| toternat | Height of Sagitta of Meniscus in inches.
Diameter = S
im ineher) oro | 0-016 | 0-020 | 0-026 ies 0030" | owas

0467 | 020 | 00181 | 0.0555 00077 | 00730 a

0236 | OOTI9 | 0-0176 (00281 0-0204
0-315 | 00000 | 00088 O-0118 0-014 |oo108|
O:204 | 06-0080 | 0-0048 0:0063 0-0078 | 0-0005 | 0-0110 oust
0-472 | 0:0020 | 0:0029 | 0:0036 | 0-0045 | 00063 | 00068 |
0:560 | 0.0010 | 0.0017 o-0024 | 0.0029 | 0.0088 | 0.0080 | 000K

In Gay- etiam barometer the two tubes are made of the same dis-
moter, 80 that the error caused by the depression in the one tubs very
nearly corrects that caused by the depression in the other, As, however
the meniscus in the one tube is formed by a column of mercury with an
ascending motion, while that in the other by a column with a descending
motion, their heights will not be the same, and the reciprocal correction
will not be quite exact.

120

‘maximum at about ten o'clock in the evening, It then again sinks,
roaches a second minimum towards four o'clock in the momiing,
second maximum at ten o'clock.

In the temperate zones there are also daily variations, ut they:
detected with difficulty, since they occur in conjunction with secél
variations,

‘The hours of the maxima and minima appear to be the same 183
climates, whatover be the Intitude; thoy merely vary a little wi
‘seasons,
157. Causes of barometric variations.—It is observed thal’
course of the barometer ig generally in the opposite direction to tis
the thermometer; that is, that when the temperature rises the
falls, and wice versa; which indicates that the barometric
any given place are produced by the expansion or contraction of
and therefore by its change in density. If the temperature wer
throughout the whole extent of the atmosphere, no currents
produced, and, at the same height, the atmospheric pressure wotlll|
everywhere the same, But when any portion of the atmosphere b
warmer than the neighbouring parts, ita specilie gravity is e
and it rises and passes nway through the upper regions of the-ag
whence it follows that the pressure is diminished, and the bare
If any portion of the atmosphere retains its temperatare, whilé
neighbouring parts become cooler, the same effect is produced
this case, too, the density of the first-mentioned portion is Jess
of the others. Hence, also, it usually happens that an extrag
of the barometer at one place fs counterbalanced by an ex/traondiiaal
at another place. With reference to the daily variations, they spat
result from the expansions and contractions which are perio .
duced in the atmosphere by the heat of the sun during the rad t
tho earth,

168, Relation of barometric variations to the state of tf)
weather.—It has been observed that, in our climate, the barometer
fine weather is generally above 30 inches, and is below this point
there is rain, snow, wind, or storm, and also, that for any given sum
of days at which the barometer stands at 30 inches, there are aa
fine a5 rainy days. From this coincidence between the height of
barometer and the state of the weather, the following indications
been marked on the barometer, counting by thirds of an inch abowe aad
below 30 inches :

‘ON GASES.

Height. State of the weather,
81 inches . 7 oe a

30 yy: - . + Sottled weather,
Sh ae - + + Fine weather,

pulley moves round « graduated circle, on which is marked
Sine weather, otc, When the pressure varies the flont sinks or
moves the needle round to the corresponding points on the scale,

The barometers ordinarily met with in houses, and whieh are ed
weather glasses, are of this kind. They are, however, of little use, fo
reasons. The first ie, that they are neither very delicate nor p
their indications, The second, which applies equally to all
is, that those commonly in use in this country are made in Londo
the indications, if they are of any value, are only «0 for a place a]
same level and of the sume climatic conditions as London, Thus®
meter standing at a certain height in London would indicate &
state of weather, but if removed to Shooter's Hill it would stand
inch lower, and would indicate a different state of weather. A¥
pressure differs with the level and with geographical conditicay,
necessary to take these into account if exact data are wanted,

160. Determt
spheric pressure decreases as we ascend, it is obvious that the b
will keep on falling os it is taken tom grenter and greater
fact which suggests a very useful method of determining the d
between the elevations of two stations, such as the base and 41
amountain, The method may be explained as follows.

Tr will be seen in the next chapter that if the temperatune
enclosed portion of air continues constant, its volume will vary im
as the pressure per squareinch, ‘That is to say, if we double the

g we shall halve the yolume. This fact is commonly called B
and Mariotte’s law. But if we halve the volume we man
double the quantity of air in each cubic inch, or double
density of the air, and so on in any proportion. Conaequ
the law is equivalent to this: —That for a constant te
the density of air is proportional to the pressure per square
which it sustains,

Now suppose A and B (fig. 106) to represent two stations,
that it is required to determine the vertical height of B above Ay
it being borne in mind that A and B are not necessarily in the)
same vertical line, ‘Tuke P any point in AB, and Q a point ata)
small distance above P. Suppose the pressure per square inch of
the atmosphere at P to be denoted by p, and at Q let it be
Fig. diminished by » quantity denoted by dp. It is plain that this
105. diminution equals the weight of the column of air between By
and (), whose section is one square inch. But, since the density of the
air is directly proportional to p, the weight of « cubic inch of air will
equal Apg, where & denotes a certain quantity to be determined here
after, and g the accelerating force of gravity(70), Hence, if we denott

ve

£ a

124

X= 2002 in. . * (140500250 cos 24) (14a) ing ;

‘The value of « is 0008865, which is nearly equal to yf5q. If!
stitute the proper values for o+p,, and change the lognrithm into
logarithms, and instead of ¢ use the mean of T and T,, the teny
at the upper and lower stations, it will be found that

t 2

X (in feet) = 60344 (1 4-0-0025 008 29) ( 1+ aon) les ff
which is La Place's barometric formula. In using it, it must be ma
ured that T and T, are the temperatures on the Centigrade

and that H and H, are the heights of the barometer reduced tel
Thus if A is the measured height of the barometer at the lower a

rma ave
H=A(1- gop)

If the height to be measured is not great, one observer is enough,
greater heights the ascent takes some time, and in the interval the:
sure may vary. Consequently in this case there must be two o
‘one at each station, who make simultaneous observations.

Let us take the following example of the above formula:—S
that in latitude 65° N, at the lower of two stations the height
barometer were 30-025 inches, and the temperature of air and a
17°32 C,, while at the upper the height of the barometer was
inches, and the temperature of air and mercury was 10°55 .C,
the height of the upper station above the lower,

(1) Find Af and H,; viz.

1782) _ op,
= 90025(1 — on) = 20045
10°66)
= 58
H, 290(1 ~ ) = 28185
Hence log ;F T= 14768248 1-4500155 = 0-02502088

(2) Find 43) vis, 1065574
(8) Find 14-0:00256 cos 29
Since 000256 cos 130°= —0-00256 cos 50°
—0-001824
therefore 1+0-00256 cos 29—= 0-998355

‘Hence the required height in feet equals
60346 x 0098355 x 1-05574 x 002562688 = 1671
Tt may be easily Fate that if He and H, do not greatly differ!

H,
Napierian logarithm of rear este If for instance H equals 20

a?

126 ON Gases,

Tf mereury be poured into the longer branch until the re
air is reduced to one-third its original volume, it will be found th)
distance between the level of the two tubes is equal to two bara
columns, ‘The pressure is now three atmospheres, while the volt
reduced to one-third. Dulong ani
havo veritied the law for air up to 97)
spheres, by meane of an apparatus ana
to that which has beem described.

The Iaw also holds good in the ¢
pressures of less than one atmosphen
establish this, mercury is poured )
graduated tube uatil it is about two
fall, the rest being air. It ix then in
in a deep trough containing neercur
107), and lowered until the levels |
mercury inside and outside the tubs a
same, and the volume noted. The ¢
then raised, as represonted in the finn
the volume of the alr is doubled.
height of the mercury in the tube,
mercury in the trough, is then fe
exactly half the height of tno
column. Accordingly, for half the
the volume has been doubled.

In the experiment with Mariotte’s
the quantity of air remains the
density must obviously increase as ite
diminishes, and ice vered. The law mi
be enunciated : ‘ For the same t
density of 0 gas is proportional to its

aa Hence, ax water i 770 times as hoary

— under a pressute of 770 atmospher
Fig. 107 would be as dense as water.
Until within the Inst few yoars Boyle's law wng supposed
absolutely true for all gases at all pressures, but Desprets, w|
amined the compressibility of gases, obtained results incompatib|
the law. He took two graduated giass tubes of the same Ieng|
filled ene with air and tho other with the gas to be ¢xamined.
tubes wore placed in the same mercury trough, and the whole a
immersed in a strong glass cylinder filled with water. By me
piston moved by a serew which worked in a cap at the top Po
tho liquid could be subjected to an increasing pressure, and it ef
seen whether the compression of the two guses wos the same

=>

aq ‘POYLE AND MANIOTTE’S LAW. 127

| apparatus resembled that used for examining the compressibility
bait (ti 47). In this manner Despretz found that carbonic acid,
ammonia, and cyanogen, are more compressible
fair: hydrogen, which has the same compressibility as air up to 15
is then less compressible. From these experiments it was

Huded that the law of Boyle and Mariotte was not general, .

Fig. 108.

| sume experiments on tho elastic force of vapours, Dulong and
“hail occasion to test the accuracy of Boyle and Mariotte's law.
adopted was exactly that of Mariotte, but the apparatus had

ie SE

the difference.

By means of the pump, water was injected into A.
being then pressed by the water, rose in the tube GF,
pressed the nir, and in the tube KL, where it rose freely.
then necessary to measure the volume of the air in GF; the
the mercury in KL above the level in GF, together with the p
the atmosphere, was the total pressure to which the gas

These were all the elements necessary for comparing differ
and the corresponding temperatures. The tube GF was kept
the experiment by a stream of cold water.

round pulleys R and R’ and were kept stretched by small buckets,
taining shot. In this manner, each of the thirteen tubes havin
separately counterpoised, the whole column was perfectly free,
standing its weight.

Dulong and Arago investignted the prossure up to 27 atmosp
observed that the volume of air always diminished a little more that
required by Boyle and Mariotte's law. But, as these differences
very amall, they attributed them to errors of observation, and cond
that the law was perfectly exnct, at any mte up to 27 atmospheres,

‘M. Regnault investigated the same subject with an spparatus re
bling that of Dulong and Arago, but in which all the sources of
‘were taken into account, and the observations made with
precision. He experimented with air, nitrogen, carbonic acid, a1
drogen. He found that air does not exactly follow Boyle and
law, but experiences a greater compressibility, which increases ¥

; so that the differonce between the calculated and the o
diminution of volume is greater in proportion as the pressure

a

130 ON GASES.

compressed air is founded on Mariotte’s law ; it consists of in glass
closed at its upper extromity, and filled with dry air Tt is |
cemented in a small fron box containing mercury. bean i
the side (fig. 109), this box is connected with the closed vessel

the gas or vapour whore tension is to be measured.

In the graduution of this manometer, the quantity of air contains
the tube is such, that when the aperture A communicates freely
the atmosphere, the level of the mercury is the same in the
in the tubulure. Consequently, at this lovel the number] ia
on the scale to which the tube is affixed. As the pressure
through the tubulure A increases, the mercury rises in the tube,
weight added to the tension @
compressed air, is equal to the
pressure. It would bee
incorrect to mark two

volume of the air is bea too
its tension is equal to two atmo,
and, together with the
mercury raised in the tube, ix #
fore more than two atmospheres,
position of the number is a little
the middle, at such » height that tt
elastic force of the compressed:
together with the weight of the
in the tube, is equal to two atnibep
‘The exact position of the number
4, ete,,on the manometer scale can Om
be determined by calculation. §
times this manometer is made of
glass tube, as represented in fig. 1
The principle is obviously the rame.

Fig. 109. Fig. 110.
165. aneroid barometer.—This instrument derives its name fon}
circumstance that no liquid is used in its construction (4 without,

ents one of the forms of these instruments, ¢
structod hy Mr. Casella; it consists of a cylindrical metal box, exh
of air, the top of which is made of thin corrugated motal, so
that.it readi ds to alterations in the pressure of the atmosphere,
‘When the pressure increases, the top is pressed inwards ; whon on the
contrary it decreases, the elasticity of ee lid, aided by a spring, onde tH
move it in the opposite direction motions are transmitted
delicate mull ‘ing levers to an ‘anes which moves on a scale, ‘Tht
instrument is graduated empirically by compari
different pressures with those of an ordinary mere

‘be in accordance with the law.

Gaseous mixtures follow
Mariotte's law, like simple
proved for air (161), which is a
| ad nitrogen and oxygen.

107. Mixture of gases and liquids. Absorption.—|
many liquids possess the property of absorbing gases. Under
conditions of pressure and temperature a liquid does not ab
quantities of different gases. At the ordinary temperature
water dissolves ;25; its volume of nitrogen, 544; ita volume of
own volume of carbonic acid, and 430 times its volume of amma

‘The whole subject of gas absorption has been tevin
to whose work* the student is referred for further in!
general laws of gas-absorption are the follawing -—

I. For the same gas, the same liquid, and the same temp
weight of gus absorbed is proportional to the pressure. This may
expressed by saying that at all pressures the volume dissolved is th
cor that the density of the gas absorbed is in a constant relation

of the external gas which is not absorbed. ‘
Accordingly, when the pressure diminishes, the quantity of d

atabgerometio Methods by R. Bunsen, transla by Dr, Rose. Woltoa

bk

14

buoyed up by the weight of the air which it
sphere is much the larger of the two, its woight per as
diminution, and thus, though in reality the henvier body,
Innced by the small leaden weight. It may bo proved by.
same apparntas that this loss ix equal to the weight of the
air. Suppose the volume of the sphere is 10 cubic
weight of this volume of air is 3+1 graing, If now this weight be:
to the leaden weight, it will overbalance the sphere in. nig,
exactly balance it in vacuo.
‘The principle of Archimedes is true for bodies in air; all that hes
said about bodies immersed in liquids applies to them, that is, that
a body is heavier than air it will sink, owing to the exeess of ite
over the buoyancy. If it is as heavy as air, its woight will
counterbalance the buoyancy, and the body will float in the
If the body is lighter than air, the buoyancy of the air will prevail,
the body will ries in the atmosphere until it roaches ® layer of the
density as its own. ‘The forve of the ascent is equal to the excess of
buoyancy over the weight of the body. ‘This is the reason why
vapours, clouds, and air balloons rise in the air.

AIR BALLOONS.

109. Air dalloons.—Air Jalloona are hollow spheres made of
light impermeable material, which, when filled with heated air,
hydrogen gas, or with conl gas, rise in the air in virtue of their nelatite:
lightness
‘They wor invented by the brothers Montgolfier, of Annonay, aud fli
first experiment was made at that place in June 1788, “Thelr balloom
was a sphere of 40 yanis in circumference, and weighed 500 pound
At the lower part there wns an aperture, and a sort of boat was sii
pended, in which firs was lighted to heat the internal air, The balloom
rose to a height of 2,200 yards, and then descended without any asadent
Charles, a professor of physica in Paris, substituted hydrogen for hie
air, Hoe himself ascended in a balloon of this kind in December 178%
The use of hot air balloons was entirely given up in consequence of the
serious accidents to which they were liable,

Since then, the art of ballooning has been greatly extended, and many
ascents have been made, ‘That which Gay-Lussuc mado in 1508 was the
most remarkable for the facts with which it has enriched science, and fer
the height which he attained. 000 feet above the sea level, At thit
height the barometer descended to 126 inches, and the thermometer,
which was 81° C. on tho ground, was 9 degrees below zero.

136
the sir displaced is therefore twice as great; but, since its d

ita density diminishes, and «
arrives at which the) buoyaney
equal to the weight of the
‘The balloon ean now only
horizontal direction, carried
currents of air which prevail i
atmosphere. The aironaut
by the barometer whether he
ascending or descending; amd
the same means he determines t
height which he has reached.
long flag fixed to the boat would
dicate, by the position it takes
above or below, whether the h
is descending or ascending.
When the aéronaut wishes to de
scend, he opens the valve at the to
the balloon by means of the
which allows gus to escape, amd

or to rise again, he
empties out bags of sand, of whic
there is an anplé supply in the ca
The descent is facilitated Thy meas:
of a grappling iron fixed to the
boat. When once this is fixed

any obstac

by pulling the cord.

‘The only practical applicatiant:
which air balloons have hitherto lia
have been in military reconnoitreng
At the battle of Fleurus, in 174,

captive balloon, that is, one held by a cord, was used, in which there ws
an observer who ported the movements of the enemy by means @f
signals. At the battle of vino the movements aud dispositions af
the Austrian troops were watched by a captive balloon, and in the war
in America balloona were frequently used. ‘The whole subjoct of military

138

CHAPTER IV.

APPARATUS FOUNDED ON THE PROPERTIES OF Ati.

172. Air pump.—Tho air pump is an instrument by which «
cnn be produced in a given space, or rather by which ait can be
rarefied, for an absolute vacuum cannot be produced by its means
‘was invented by Otto von Guericke in 1650, a few years after the
tion of the barometer.

Fig. 116.

The air pump, as now usually conatructed, may be deseribed as
in fig. 116, which shows the general arrangement, B is the
which the vacuum is to be produced. It is a bell glass, resting-on #
D of thick glass ground perfectly smooth, Tn the centreof D, nt Oy
is an opening by which a communication i made betweon the in

140

Tt is plain that when the rarefaction has proceede
extent the atmospheric pressure on the top of P will
it will be very nearly balanced by the atmospheric p
of the other piston. Consequently, the experimenter
come only the difference of the two pressures, Itis#
two cylinders are employed.

‘To explain the action of the valves we must go int
general arrangement of the interior of the cylinders is

Fig. 121 shows the sectio

than those of the brass dis

thoroughly saturated witl

air-tight, though with but li

the cylinder. To the centr

fs screwed a piece, B, to!
| is riveted. The piece B
| put the interior of the cy!
nication with the external
nication is closed by a ¥
by a delicate spring 7. ¥
moved downward, the ait
is compressed until it fo
capes, The instant the :
the valve ¢ falls, and is
Fig. 121, spring, and the pressure ¢
which is thereby kept from coming in. The i
tween the cylinder below the piston and the recei
closed by the valve marked o in fig. 118, and ag in fy
passing through the piston is hold by friction, and is
is kept from being lifted through more than a very ati
top of the cylinder, while the piston, in continuing i
slides over 4g. When the piston descends it brings
which at once cuts off the communication between th
receiver.

173, Air pump gauge.—When the pump has beer
the pressure in the receiver is indicated by the differe:
mercury in the two lege of a glass tube bent like a sy
is open and the other closed like the barometer, Ti
which is called the gauge, ia fixed to an upright
under a small bell jar, which communicates with

produced with it in far lees time and in larger apparatus,

176, Sprengel’s air pump.—Sprongel has devised a form;
which depends on the principle of converting best
nto a Torricellian vacuum. Theidea and construction of #
we thus described by the inventor.

If an aperture be made in the top of « berncte
finks and draws in air; if the experiment be so arranged as
tir to enter along with mercury, and that the supply of air
while that of mercury is unlimited, the air will be carried aw:

(barometer; open at both ends, and connected by means of fnd
jabing with a funnel A filled with mercury and supported |
fereury is allowed to fall in this whe at a rate regulated by

46 ON GASES.

following: In acylindor, A, of small diamoter (ig. 12), there is
piston, the rod of which ismoved by the hand. "The cylinder is prot
with a screw which fits into the receiver, K. Fig, 125 shea
arrangement of the valves, which are #0 constructed that the li
yalve, o, opens from the outside, and the lower valve, 4, fr
inside.

fig. 124 Fig. 128,

‘When the piston descends, the valve o closes, and the elasthe fol
the compressed air opens the valve 4, which thus slows the comp
alr to pass into the receiver. When the piston ascends, # clases |
opens, and permite the entrance of fresh air, which in turn becomes
pressed by ihe descent of the piston, and so on.

This apparatus is chiefly used for charging Viqnide with gasea

Beicaigut ofthe atmosphere, and can only be taken fl
effort. And as the olasticity of the fluids contained in the:
‘counterbalanced by the weight of the atmosphere, the palm of
swells, and blood tends to escape from the pores, 1

Fig. 129,

‘By means of the air pump it may be shown that air, by reas
oxygen it contains, is necessary for the support of combustion al
For if we place n lighted taper under the receiver, and begin t
the air, the flame becomes weaker as rarefaction proceeds, and
extinguished, Similarly an animal faints and dies, if a vacuum

do a receiver under which it is placed. Mawalis and birds

‘yp

with unequal legs (fig. 151). It is used in teat

manner: The syphon is filled with some I
onds being closed, the shorter leg is dipped in the liq
in fig. 131; or the shorter leg having heen dipped fin the
is exhausted by applying the mouth at B. A vacuum
duced, the liquid in © rises and fills the tube in © u
atmospheric pressure, It will then run out through the
as the shorter end dips in the liquid.
A syphon of the form represented in fig. 132 is u
presence of the liquid in the mouth would be obj
‘M, is attachod to the longor branch, and itis filled by closing
and sucking stO, An enlargement, M, renders the passage
into the mouth more difficult.

eee

Fig. 131. Fig. 132.

To explain this flow of water from the syphon, let us
and the short leg immersed in the liquid. The pressure then a¢
C, and tending to mise the liquid in the tube, isthe atmospheri¢
minus the height of the column of liquid DC. In like 7
sure on the end of the tube B, is the weight of the
pressure of the column of liquid AB, But as this latter eo
Jonger than CD, the force acting at B is less than the foree acti
and consequently a flow takes place proportional to the di
hetween these two forces. The flow will therefore be mors
ion ns the difference of level between the apertuns By
surface of the liquid in C, is greater. j
182. The intermittent syphon.—In the intermittent yp

_ a

now no more air in the
and the water, forced by be at

not more thin 34 feet above t
level of the water in which tht
tube A dips, for we have
(147) that u column of
this height is equal to the
Fig, 134. sacoetbnaeere

Tn practico the height of the tube A does not often exceed 26 to 38
foot, for, although the atmospheric pressure can support « higher cola
the vacuum produced in the barrel is not perfect, owing to the fact
the piston does not fit exactly on the bottom of the barrel. But
the water has passed the piston, it is the ascending force of the
which raises it, and the height to which it can be brought depends:
foree which moves the piston,

185. Suction and force pump.—The action of this pump, &
which is represented in fig. 185, depends both on exhaustion
pressure. At the base of the barrel, where it is connected with
A, thoro is a valve, 8, which opens upwards, Another valve, O,
in the same direction, closes the aperture of a conduit, which p
# hole, o, near the valve S into a vessel, M, which is called

| vowrs, 183
fr. From this chamber there is another tube, D, up which the
forced.

‘ascent of the piston B, which is solid, the water rises
be A into the barrel. When the piston sinks, the valve S closes,
je water is forced through the valve O into the reservoir M, cnd
(hence into the tube D. The height to which it can be raised in
Ibe depends sololy on the motive force which works the pump.

t Fig. 135.

tule D were x prolongation of the tube Jao, the flow would be
; It would take place when the piston descended, and would
a it ascended. But between these tubes there is an interval,
by means of the sir in the reservoir M, ensures a continuous flow.
forced into the reservoir M divides into two parts, one of
D, preses on the water in the reservoir by its weight,
Jn virtue of this presture, risus in the reservoir above the
of the tube D, compressing the air above. Consequently,
po longer forees the water into M, the airofthe

ae 5

ON GASES.

it, by the pressure it has received, reacts om thi
i tube D, until the piston again descends, 46 that

Fire engine.—The principle of the suction
lied in the case of the hydraulic press, and als
The fire eng
model desert
the barrels
which is to ]
represents a
chine. Ther
tons, P and I
are worked b
in the figure.
I is raised
and water |
When it is
valve d’ close:
through the
larger sir che
the tube at
of the air chamber, while the other fits into the
sido of the roof there is a tubo, 60’, to which the hy
means of the pressure which the compressed. air i
on the water a strong jet is forced through the del
sent in any direction, Both pistons are so fastoned
one is forced down the other rises, consequently
into the air chamber without cessation.

187. Velocity of efflux, Torricelti's ther
an aperture made in the bottom of any vessel, any
particle of Liquid on the surface, without refer
beneath. If this particle fell freely, it would hay
the orifice equal to that of any other body fallij
between el of the liquid and the oritic
falling bodies, is ./2gh, in which g is the acceley
A the height. If the liquid be maintained at
by a stream of water running into the vease!
has escaped, the particles will follow one an
and will issue in the form of a stream.
equally in all dirvctions, a liquid would issu;
with the same velocity, provided the depth
‘The law of the velocity of efflux was diac
be enunciated as follows, The velocity of
Freely feliing body would have on reaching

it,

average, &
of the orifice.
‘This may bo explained by reference to fig. 138,
an orifice in the bottom of a vessel—what is
‘equally true of an orifice in the side of the versel.
abore AB endeavours to pass ont of the vessel, and in #0 de
on those near it, Those that issue near A and B
directions MM and NN; those near the centre of th

the water within the space PQP is
that which does instead of

from the orifice about equal to the diameter of the orifice.
the jet is called the vena contracta. It is found that the
smallest section is about § or 0-62 of that of #1
Accordingly the true value of the efflux por
given approximately by the formula
E=062A,4 39h
or the actual value of E ia about 062 of its
amount,
191. Xefuence of tubes on the quantity of
—The result given in the last article has refi
aperture ina thin wall. If a cylindrical or conical:
tube or ajutage is fitted to the aperture, the amount
efflux is considerably increased, and in some cases falls but little
its theoretical amount,

‘A short cylindrical ajutage, whose length is from two to tl
its diameter, has been found to incroase the efflux per second to
082A,/zph, In this case, the water on entering the ajutage
contracted vein (fig. 139), just as it would do on issuing freely
air; but afterwards it expands, and, in consequence of the ad
the water to the interior surface of the tube, has, on leaving the:

cmepectively « fixed posithc, each dovp |
and Gattened while it is filing (By. 142).
‘there ace smaller ane, ap that the whale jet haa

Fig 141 Fig. 142. Fig. 143.

removed, for it is expended in foreing ont the water; but it re
the other side; and if the vessel were moveable in « horizontal
it would move in a direction opposite that of the issuing jet. 49
Tustrated by the apparatus known as the Ayiruadic tourniguet, oF
sill (fig. 143). It consists of a glass vessel, M, containing w
capable of moving about its vertical axis. At the lower pari
atabe, C, bent horizontally in opposite directions at the two
the vessel were full of water and the tubes closed, the pressun
sides of © would balance each other, being equal and acting in
directions; but, being open, the water runs out, the press
exerted on the open part, but only on the opposite side, as show
figure, A. And this pressure, not being neutralised by an

=

da at

what will be the effect of opening successively one of th
and ¢, supposing, as representod
that the lower extremity of g is b
tubolures 5 and ¢.

i. If the tubulure 6 is open the water
out, and the surface sinks in the tube g uf
it is on the same level as }, when the
stops. This flow arises from the xi
pressure at the point e over that at b,
pressure at ¢ is the same as the pressure 0
atmosphere. But when once the level is
same at band ate, the efflux conses, for
atmospheric prossure on all points of th
horizontal layer, be, is the same (90).

— ii. Ifnow the tubulure } fa closed, a
Fig. 146, opened, no efflux takes place ; om the conte

air enters by the orifice «, and water

the tube g, as high as the layer ad, and then equilibrium ix est

If the orifices ¢ and 4 are closed, and ¢ opened, an efflax
constant velocity takes place, as long ns the level of the water is}
below the open end, /, of the tube, Air enters babble by babbles
and takes the place of the water which has flowed out.

Tn order to show that the efflux at the orifice ¢ is constant, it is)
sary to demonstrate that the pressure on the horizontal layer of is aly
‘equal to that of the atmosphere in addition to the pressure of the el
Al, Now suppose that the level of the water has sunk to the
‘The air which has penctrated into the flask supports @ presiure 6
that of the atmosphere diminished by that of the column of Tigt
or H—pn. In virtue of its elasticity this pressure is transmitted
layer ch, But this layer further supports the weight of
water, pm, so that the pressure at m is really pm-+-H—pn, or
that is to say, H+Al.

Tn the same manner it may be shown that this pressure is the
when the level sinks to d, and so on as long as the level is higher
the aperture Z ‘The pressure on the layer ch is therefore constar
consequently the velocity of the efflux. But when once the level ix be
the point /, the pressure decreases, and with it the velocity.

‘To obtain a constant flow by means of Mariotte’s flask it is filled:
water, and the orifice which is below the tube / is opened. The x
of tho flow is proportional to the square root of the height, Al.

162 ACOUSTICS.

‘means A movement to or fro. The French vibrations are
vibrations, an oscillation or vibration is the movement of the |
molecule in only one direction; dowble ar complete cibration
the oscillation both backwards and forwards. Vibrations are
observed. If a light powder is sprinkled on a body which is in
of yielding a musical sound, a bell jar held horizontally in the
example, a rapid motion is imparted to the powder which
the vibrations of the body: and in the same manner, if
be smartly pulled and let go, its vibrations are apparent to the oy
198, Sound is not propagated in vacuo.—The vibrat
elastic bodies can only produce the tensation of sound in as,
intervention of a medium interposed between the ear and thea
body, and vibriting with it. This medium is usually the air,
gates, vapours, liquids, and solids also transmit sounds.
‘The following experiment shows that the presence of a poi
medium is necessary for the propagation of sound. A small
bell, which is continually struck by a small hammer by means o
work, oran ordinary musical box, is placed under the receiver ol
pump (fig. 145), Aslong as
is full of air at the ordinazy }
the sound is transmitted, but in
tion as the alr is exhausted th
becomes feebler, and is imperce,
a vacuum,
‘To ensure the success of the
ment, the bellwork or musical §
be placed on wadding; for other
vibrations would be transmits
air through the plate of the mar
200. Sound is propagated
elastic bodies,—If, in the ab
periment, after the vacuum &
made, any vapour or gas be a)
the sound of the bell will bt
showing that sound is propag
this medium as in air,
Me Sound is also propagated in
Fig. 16. When two bodies strike galt
other under water, the shock
tinetly heard. And a diver at the bottom of the water cam 7
ound of voices on the bank.
‘The conduetibility of solids is such, that the scratching of a pa
end of a piece of wood is heard at the other ond, The earths

—_

164

H’PQA will reprosent
the line AH" have simultaneously : for instance, at the instant
returned to A, the particle at M will be moving to the right
a velocity represented by QM, the particle at N will be movis
left with a velocity represented by PN, and so on of the other
When an undulatory motion is transmitted through a
motions of any two particles are said to be in the same phase r
icles move with equal velocities in the samo direction; the mnt

‘undulation, their motions are always in opposite phases, but if
tance equals the length of an undulation their motions arvin the:

A little consideration will show that in the condensed save #
densation will be greatest at the middle of the wave, and lil
the expanded seare will be most rarefied at its middle.

It is an easy transition from the theory of the motion of
waves in a cylinder to that of their motions in an uninclosed
It issimply necessary to apply in all directions, to each molecule o
vibrating body, what has beon said about a piston moveable in «

A series of spherical waves alternately condensed and rarefied a
duced around each centre of disturbance. As these waves am 6
within two concentrical spherical surfaces, whose radii gradually
while the length of the undulation remains the same, their mass
with the distance from the centre of disturbance, so that the

of the vibration of the molecules gradually lessens, and the i
of the sound diminishes.

It is theso spherical waves, altornately condensed and expan
which in being propagated transmit sound. If many points wm
turbed at the same time, a system of waves is produced around
point But all these waves are transmitted one through the
without modifying cither their lengths or their velocities. neti
condensed or expanded waves coincide with others of the same natant)
to produce an effect equal to their sum; sometimes they meet and
produce an effect equal to their difference. If the surface of still water
be disturbed at two or more points the coexistence of waves a
sensible to the ¢

202. Causes which influence the intensity of sound,—Many
causes modify the force or the intensity of the sound. These are, the dite
tance of the sonorous body, the amplitude of the vibrations, the density
of the air at the place where the sound is produced, the diretion of
the currents of air, and lastly, the proximity of other sonorous bodies

L The intensity of sound is inversely as the square of the disanch

—_~

turned away, und it becomes gradually weaker when the
moved to a greater distance, showing that the strengthening’
vibration of the air in the cylinder.

The cylinder B is made to vibrate in unison with the bras ve
adjusting it to certain depth, which is effected by making
slide into the other.

Vitruvius states that in the theatres of the ancients
vessels were placed to strengthen the voices of the actors.

Fig, 147.

jecreases in inverse proportion to the equ
the distance does not apply to the case of tubes, especially if they
straight and cylindrical. ‘The sonorous waves in that caso aro not
gated in the form of increasing concentrical spheres, and sound
transmitted to a great distance without any perceptible alteration, ML
found that in one of the Paris water pipes, 1040 yarde long, the voice at
20 little of its intensity, that a conversation could be kept up at the emit
of the tube in a very low tone. The weakening of sound becomes, how
ever, perceptible intubes of large diameter, or whens the sides are rough

rs i

in tabes, that when a well-known air was played on a flute at one dal
‘a tube 1040 yards long, it was heard without niteration at the others
from which he concluded that the velocity of different sounds is the sam
For the sme reason the tune played by a band is heard at a great é
tance without alteration, except in intensity, which could not be
if some sounds travelled more rapidly than others,

‘This cannot, however, be admitted as universally true,
a profound mathematical investigation of the laws of the p ti
sound, has found that the velocity of a sound depends on ite st
and accordingly that a violent sound ought to be propagated with gr
velocity than a gentler one, ‘This conclusion is confirmed by an
tion made by Captain Parry on his Arctic expedition. During artill
practice it was found by persons stationed at a considerable distance’
the guns, that the report of the cannon was heard before the command
fire given by the officer, And more recently, Mallet made a
experiments on the velocity with which sound is propagated in
observing the times which elapsed before blastings made at He
were heard at a distance, He found that the larger the charge of gi
powder, and therefore the louder the report, the more rapid was
transmission. With a charge of 2000 pounds of gunpowder the velod
yas 967 feet in a second, while with a charge of 12,000 it was 1210 fat
in the same time.

MM. Bravais and Martins found, in 1844, that sound travelled
the same velocity from the base to the summit of the Fauthorn, a fn
the eummit to the base.
Mallet has investigated the velocity of the transmission of sound
various rocks, and finds that it is as follows:

Wet sand . : 825 ft. in a second.
Contorted, stratified qharte ‘and ante rock , 1088 "
Discontinuous granite. +» 1306 ns
Solid gmnite . . 2 eee

‘The velocity of sound vi ies in different gnses. Dulong caused ong
pipes to sound by means of different gases, and found that the velocity
sound at zero waa as follows:

Cnfbonicacid . . . . . . 850fh ina necom
ree . 100,
ee Bo sss 0» ty ge

170 MOOUSTICS.

condensation, heat is evolved, and this heat increases the elas

thas the rapidity with which each condensed layer acts on the nex

in the rarefaction of each layer, the same amount of heat diss:

was developed by the condensation, and its elasticity is diminished |
cooling. The effect of this diminished elasticity of the cooled]

the same as if the elasticity of an adjacent wave had been i

the rapidity with which this latter would expand upon the dita
would be greater. Thus while the average temperature of the
unaltered, both the heating which increases the elesticity and the chi
which diminishes it concur in increasing velocity. ;

Knowing the velocity of sound, we can calculate approximately
distance at which it is produced. Light travels with such velocity #
the flash or the smoke accompanying the report of a ens may be
sidered to be seen simultaneously with the explosion, antin
the number of seconds which elapse between pe oat ia
ing the sound, and multiplying this number by 1125, we get the dist
in foot nt which the gun is discharged. In the same way the d
of thunder be estimated,

207. Velocity of sound in liquids and in solids.—The yelocit
sound in water was investigated in 1827 by Colladon and Sturm,
moored two boats at a known distance in the Lake of Genera, Th
supported a bell immersed in water, and & bent lever provided at one
with a hammer which struck the bell, and at the other with a lighted wit
so arranged that it ignited some powder the moment the hammer
the bell, To the second boat was affixed an ear-trumpet, the
which was in water, while the mouth was applied to the ear o
observer, so that he could measure the timo between the flash of
and the arrival of sound by the water. By this method the velocity
found to be 4708 fect in a second at the temperature £41°, or four
a great as in wir,

The.velocity of sound, which is different in different liquids,
calculated by a formula analogous to that given above (206) aa
cable to gases. In this way are obtained the number given im
following table. As in tho case of gases, the velocity varies with ime
temperature, which is therefore appended in each ease:

River water (Seine) . + 19°0,= 4714 fine

, . 501s.

4768
5182
6498

moon ”
Artificial sea-water
Solution of common salt

» » chloride of calcium

Absolute alcobol 3804
Turpentine = 3076 2
Ether . . 380 "

liz ACOUSTICS. [208

angle ACH is called the angle of icidence, and the angle BCH, fie)
by the prolongation of aC, is the angle of reflection.

Fig. 148.

The reflection of sound is subject to the two following laws:

I. The angle of reflection is equat to the angle of incidence.

IL. The incident sonorous ray, and the reflected ray, are in the sam
plane perpendicular to the reflecting surface.

From these laws it follows that the wave which in the figure is p=
pagated in the direction AC, takes the direction CB after reflection #
that an observer placed at B hears, besides the sound proceeding Evil
the point A, ane sound, which appears to come from C,

'The laws of the reflection of sound are the same as those for lel
and radiant heat, and may be demonstrated by similar experiments, O80
of the simplest of these is made with conjugate mirrors (see chagite
on Radiant Heat): if in the focus of one of these mirrors m wateh @
placed, the ear placed in the focus of the second mirror hears the teil
very distinetly, even whien the mirrors are ata distance of 12 or 13 yanli

200. Zehoes and resonances.—An echo is the repetition of a seuph
in the air, caused by its reflection from some obstacle.

Avery sharp quick sound can produce on echo when the reflecting
surface is 55 feet distant, but for articulate sounds at least double tit
distance is necessary, for it may be easily shown that mo one can pronouDel
or hear distinctly more than five syllables in a second. Now, xi ie
velocity of sound at ordinary temperatures may be taken at 1,125 foetill
msecond, in a fifth of that time sound would travel 226 feet. If ite
reflecting surface is 1125 feet distant sound would travel through 20)
fost in going and returving, The time which elapses between the ariel
lated and the reflected sound would, therefure, be a fifth of a second, IDE

ww

from a medium of one density into another it undergoes partial
which, though not strong enough to form an echo, distinctly
the direct sound. This is doubtless the reason, as Humboldt
why sound travels further at night than at day time; even in the
American forests, wher the snimals, which are silent ‘by day, fll |
atmosphere in the night with thousands of confused sounds,

210, Refraction of sound.—It will be shown in the
refraction is the change of direction which light and heat experience
passing from one medium to another.
sonorous waves arorefracted like light and heat. He constructed,
lenses, by filling spherical or lenticular collodion envelopes with
acid. With envelopes of paper or of goldbeater's skin the refraction of
sound is not perceptible.

Sondhatiss cut equal segments out of a large collodion ‘balloon,
fastoned them on the two sides of a sheet iron ring a foot in diameter, a
4 to form a hollow biconvex lens about 4 inches thick in the
‘This was filled with carbonic acid, and a watch was placed in the
of the axis: the point was then sought, on the other side of the lens at
which the sound was most distinctly heard. It was found that when tht
ear was removod from the axis, the sound waa scarcely perceptible; bal
that ata certein paint on the axial line it was very distinctly heant
Consequently the sonorous waves in pussing from the lens had converged
towards the axis, their direction had been changed; in other words, they
had been refracted.

‘Tho refraction of sound may bo easily demonstrated by moans of ont
of the very thin india-rubber balloons, used as children’s toys, inflated By
carbonic acid. Lf the balloon be filled with hydrogen no focus &
detected, it acts like a convex lens, and the divergence of the rays if
increased, instend of their being converged to the war.

Fig. 149.

21L—Speaking trumpet.—Zar trampet.—Thess instruments mf
based both on the reflection of sound, and on its conductibitity
tubes,

The speaking trumpet, as ita name implies, is used to render the ¥oloe
audible at great distances. It consists of a slightly conical tin or bea
tube (fig. 149), very much wider at one ond (which ix called the belf), sad

176 ‘ACOUSTICS. [as-

gives the number of revolutions of the wheel, and consequently ot
number of vibrations in a given time,

When the wheel is moved slowly the separate shocks against the
card are distinctly heard, but if the velocity is gradually increased, the
sound becomes higher and higher. Having obtained the sound
number of vibrations is to he determined, the revolution of the whee
esntinued with the same velocity for a certain number of seconda,
number of turns of the toothed wheel B is then read off on the indie

and this multiplied by the number of teeth in the wheel gives the
number of vibrations, Dividing this by the corresponding umber
seconds, the quotient gives the number of vibrations por second for they
given sound.

Fig. 150,

The ayren ix an apparatus whieh, like Savartia whe)
is used to measure the number of vibrations of » body in a given time
The name ‘syren' was given to it by its inventor, Cagniard Latomy
because it yields sounds under wate

Tt is made entirely of brass, Fig. 151 represents it fixed on tie
table of a bellows, by which a continuous current of air can be see
through it, Figs, 152 and 153 show the internal details, ‘The lowe
part consists of n cylindrical box, O, closed by a fixed plate, B. On thie
plate a vertical rod, T, rests, to which is fixed a disc, A, moving will
the md. In the plate B there are equidistant circular holes, and in tie
disc, A, are an equal number of holes of the same size, and the samt
distance from the centre as those of the plate. These holes are ae
perpendicular to the disc; thsy are all inclined to the same extent in te
same direction in the plate, and arm inclined to the same extent in DF
opposite direction in the disc, so that when they are opposite eaeh othet

v8 ACOUSTICS.

complete revolution, moves one tooth of a second wheel, 6
On the axis of these wheels there are two needles which move
dials represented in fig.151. One of these indices gives the
of turns of the dise A, the other the number of hundreds of
By means of two screws, D and ©, the wheel ¢ can be uncoupled
the endless screw.

Since the sound rises in proportion to the velocity of the dise

wind is forced until the desired sound is produced. The sme
is kept up for a certain time, two minutes for example, and the
of turns read off. This number maltiptied by 18, and divided byl
indicates the number of vibrations in # second.
With the same velocity the syren gives the same sound in sit at
water: tho same is the case with all gnses, and it appears, therefore,
any given sound depends on the number of vibrations, and not on
nature of the sounding body.

The buzzing and humming noise of certain insects is not vocal, but
produced by very rapid flapping of the wings against the uir or the
Tho syren has been ingeniously applied to count the wolocity of|
undulations thus produced, which is effected by bringing it into
with the sound. It has thus been found that the wings of a gmat flip
the rate of 15,000 times in a second,

214. Bellows. —{n ncvustics a bellows is an apparatus by
wind instruments, such as the syren and organ pipes, ane
Between the four legs of a table there is a pair of bellows, S (Gg. 1
which is worked by means of a pedal, P. D is a reservoir of
leather, in which is stored the air forved in by the bellows. If
reservoir is pressed by means of weights on a rod, T, moved by
hand, the air is driven through « pipe, B, into a chest, ©, fixed on
table. In thia chest there are small holes closed by leather valves, whieh
can be opened by pressing on keys in front of the box, The syrem
sounding pipe is placed in one of these holes,

215, Limit of perceptible sounds.—Beforo Savart's rm
physicists assumed that the ear could not perceite a sound when
number of single vibrations was below 82 for deep sounds, or alow
18,000 for ucute sounds, But he showed that these limits wen
too close, and that the faculty of perceiving sounds depends rather 0
their intensity than on their height; so that when extreme sounds

not heard it arises from the fact that they have not been produced witty
sufficient intensity to affect the organ of hearing.

By increasing the diameter of the toothed wheel, and conseqatellt
the umplitude and intensity of the vibrations, Savart pushed the limit
acute sounds to 48,000 single vibrations in a second.

For deep sounds, he substituted for the toothed wheel an imn I

MEASUREMENT OF THE NUMBER OF VIBRATIONS. 179

two fet long, which revolved on « horizontal axis between two
eden plates, about 0:08 of an inch from the bar. As often as the
} a laine a aa peng oraanepeentioniang
(uw the motion beeume accelerated, the sound became continuous,
pave and deafecing, By this means Savart found, that with 14 to
fle vilirations in a second, the ear percelrad distinot but very deep

Fig. 154.

(haprets, however, who bns investigated the aame subject, disputes
{a temuilte a8 to the limits of deep sounds, and holds that no sound
fle that is made by less than 32 single vibrations per second. On
her Band, he holds that acute sounds are audible up to those

landing to 73,700 eingls vibrations per second.
‘Dubamel's graphic method.—When the syren or Savart's
fh teed to dotermine the exact number of vibrations corresponding
sound, it ix necessary to bring the sound which they produce
‘with the given sound, and this cannot be done exactly unless
have a practised car. M. Dubamel’s graphic method is

j

— za

430 AODUETICS.

very simple and exact, and free from this difficulty. It consti
a fine point to the body emitting the sound, and causing it ta
vibrations on a properly prepared surface.
‘The apparatus consists of a wood or metal cylinder A, fig. 165,'

@ vertical axis ©, and turned by # handle. The lower part of the!
# sorew working in # fixed nut, so that according as the handle is
from left to right or from right to left, the cylinder is raised or
Round the cylinder ja rolled a sheet of paper covered with an

Fig. 154.

film of lampblack. On this film the vibrations register themselit
‘This is effected us follows : Suppose the body emitting the note to be
steel rod. It is held firmly at one end, and carries at the other a fit
point which srmzes the surface of the cylinder, If the rod is made)
vibrate and the cylinder is nt rest, the point would describe « short Hin
but if the cylinder is turned the point produces an undulating tee
containing as many undulations as the point has made vibrations, Gat
sequently the number of vibrations enn be counted. It remuins onlpt
determine the time in which the vibrations wero made,

Phere are several ways of doing this. ‘The simplest 18 to compare
curve traced by the vibrating rod with that tmeed by « tuning fe
2), which gives a known number of vibrations per second, for exampll

y222

—_~

182 ACOUSTICS.

number of vibrations per second, and they

and yet the two tonos will have very distinct qualities,

timbre or colour is different. The cause of the peculiar colour of
will be considered Later inthe chapter.

218. Musical intervais.—Let us suppose that a musical tone,
for the sake of future reference we will denote by the letter ©, i
dacod by m vibrations per second, and let ux further suppose that
other musical tone, X, is produced by » vibrations per second, x
greater than m; then the interval from the note © to the note X is
ratio m+ m, the interval between two notes being obtained by
not by swhtraction, Although two or more tones may be #e
musical, it by no means follows that when sounded together they
duce # pleasurable set On the contrary, unless they an:
the result is harsh, and ordinarily the reverse of pleasurable. We
therefore to enquire what notes are fit to be sounded together. New
when musical tones are compared it is found that if they are so
by an interval of 2: 1, 4: 1, ate, thay ao closely resemble one another
that they may for most purposes of music be considered as the same time,
Thus, suppose ¢ to stand for a musical note produced by Qu: vibration
per second, and then © and ¢4o closely resemble one another as to Be
called in music by the same namo. ‘The interval from © to ¢ is called Mi
octave, and ¢ is said to be an octave above C, and conversely O'an e
below c. If we now consider musical sounds that do not differ by ae
octave it is found that if we take three notes X, Y, and Z, resulting

pectively from ¢, g, and r vibrations per second, these three notes Witt
sounded together will be concordant if the ratio of»: g = x equals 4: O20)
Three such notes form a harmonic triad, and if sounded with « fourthy
note, which ix the octave of X, constitute what is called in music a niger
vhord, Any of the notes of a chord may be altered by one or more oetayvet
without changing its distinctive charncter; for instance, O, E, G, sal
aro achord, and C, ¢, e.g, form the same chord.

If, however, the ratio p: g: r equals 10: 12: 16, the three sounds ae
slightly dissonant, but not so much so as to disqualify them. from ped
ducing a pleasurable sensation, at least under certain cireamatances 7
When these three notes and the octave to the lower are sounded together
they constitute what in music is called a minor chord,

219. The musical scale.—The series of sounds which connects ©
given note C with its octave ¢ is called the diatonic scale or gamuih
The notes composing it are denoted by the letters ©, D, E, FG, Ay Te
‘The scale is then continued by taking the octaves of these notes, namely
©, @, ef, 9.4, b, and again the octaves of these last, and #0 on.

‘The notes are also denoted by names, viz. do, re, mi, fa, sof, Ia, si, #6
‘Tho relations existing between the notes are these:—G, E, G, form &

184 acoustics,

220, On semitones and on scales with diferent key
will be een from the Inst article that the term ‘semi toné ‘does:
a constant interval, being in one case equivalent to [f and in
3. It is found convenient for the purposes of music to ints
intermediate to the seven notes of the gamut; this is done by

by the same interval {i stil to be Rational te
Bo, called ‘B fiat ;’ that is B+ Bb =H

easily distinguished by the ear. Thus, reckoning © to equal 1, we’
c ct D> D DE Eb E etc,
1 oH, ty Sees

Hitherto we have made the note € the tonic or key mae. A
of the twenty-one distinct notes above-mentioned, e.g. G, or F, or CF,
may be made the key note, and a scale of notes constructed with rele
ence to it, This will be found to give rise in each case to a series of
some of which are identical with those contained in the series of
C is the key note, but most of them different, And of course the
would be true for the minor scale as well as for the major seals,
indoed for other scales which may be constructed by means of the fund
mental triads.

221. On musical temperament.—The number of notes that
from the construction of the scales described in the Inst article is
mous ;40 much #0 as to prove quite unmanageable in the practice ofin
and particularly for music designed for instruments with fixed n
auch as the pianoforte, Accordingly it becomes practically inypa
reduce the number of notes, which is done by slightly altering their
proportions. This process is called temperament. By Bipeiicn
notes, however. more or lesa dissonance is introduced, and
several different systems of temperament have been devised for render
this dissonance as alight as possible, ‘The aystem usually adopted
least in intention—is called the system of equal temperament. I
consists in the substitution between C and ¢ of eleven notes at”
equal intervals, each interval being, of course, the twelfth root af 2)
or 1.05046, By this means the distinction between the semitones i
abolished, so that, for example, Cz and DP become the same note. ‘The
scale of twelve notes thus formed is called the chromatic scale. Tt ot
course follows that major triads become slightly dissonant. ‘Thus in te
diatonic acale, if we reckon C to be 1, Kis denoted by 125000, and @

—_ = |

lene
La

‘one of the lower notes is sounded on the pis
purpese of experimentally proving the presence
distinct tones, Professor Helmholtz constructed an it
resonance globe, The principle involved in its construction is this:.

Further, other things being the same, the note proper ri
depends on the diameter of the globe and that of the
opening. Consequently, by means of a series of such
whole series of harmonics in a given compound tone can
distinctly audible, and their existence put beyond a doubt.
Helmholtz’s researches show that the different colour or qi
sounds yielded by different musical instruments is due to ¢]
intensities of the harmonica which accompany the primary
sounds, The leading results of these researches into the ©
may be thus stated:

i. Simple tones, as those produced by a tuning fork with
box, and by wide covered pipes, are soft and agreeable without any
neas, but woak, and in the deeper notes dull.

ii, Musical sounds accompanied by a series of harmonics,
sixth, in moderate strength are full and musical, In
simple tones they are grander, richer, and more sonorous.
sounds of open organ pipes, of the pianoforte, ete,

“-

w double vibrations per second, they prod
thus, if © is produced by 128, and D by 144 double

together produce 16 beats
swcertained that the beats produced by two tones:
unless the ratio m:n is less than the ratio 6:5. Hence,
represented by fig. 158, though the alternations of intensity:
would not be audible. Also, if the tones have very d
the intensity of the beat is very much disguised,

It is found that whon beats are fewer than 10 per second.
70 per second they are disagreeable, but not to the extent:
discord. Beats from 10 to 70 per second may be regarded
of all discord in music, the maximum of dissonance being

‘beats per second being 16, On the other hand, ifC, B, and
together there is no dissonance, but if C, EB, G, B are so
the discord is very marked, since © produces o, which is di
B. It will be remarked that C, E, G is a major triad, wh
a minor triad.

A compound musical tone being composed of simple tones
by 1, 2, 3, 4, 5, 6, 7, etc., does not give rise to any simple to
of producing an audible beat up to the seventh, the sixth -
are the first that produce an audible beat. It is for this reason #
is no trace of roughness in a compound tone, unless the seventh
be audible.

beating with its neighbouring tane would produce dissonance,

7, which would beat with 6 aud 8; 9, which would beat
10; ond 11, which would bewt with 10 and 12. It is
stance which renders the major chord of such great it
harmony. If the constituents of the minor chord are
cussed, namely three compound tones whose primaries are proporti
10, 12, 16, it will be found to differ from the major chord in tl
ing principal respects : (a) ‘The primary of the natural tone ti
approximates is very much deeper than that of the
major chord. (8) It introduces the differential tones, 2
form & major chord. Now it has already been ren

for example when C, E, G, A, are sounded together,
of the differential tones forming « major chord shows that
dissonance exists in every minor chord.

CHAPTER IY.

VIBRATIONS OF STRETCHED STRINGS, AND OF COLUMNS OF ATE |
-

297, Vibrations of strings.—By a string is meant the strit
musical instrument, such asa violin, which is stretched by aes
and is commonly of catgut or is ametallic wire, The vibrations
strings experience may be either érmmaversal or longitudinal, but

tically the former are alone important, Transversal vibrations 1

IV, The number of vibrations per eecond of a string ts th
aquare rovt of i's density,
‘These laws are applied in the construction of stringed i

which the length, diameter, tension, and substance of the
chosen, that such and such notes may be elicited from them.
230, Wodes and toops,—Let us suppose the string AD
to begin vibrating, the ends A and D being fixed, and while
20 let a point B be brought to rest by m stop, and let us suppe
be one third part of AD. The part DB must now vibrate
D a8 fixed points in the manner indicated by the continuons
lines; now all parts of the samo string tend to make «
same time; accordingly, the part between A and B will
single vibration, but will divide into two at the point C,
the manner shown in the figure. If BD were one fourth p
the part AB would be subdivided at © and © into three vi

Fig, 161.

portions each equal to BD, ‘The points B, C,C, are called
points; the middle point of the part of the string bets
consecutive nodes is called a loop, or & ventral
remarked that the ratio of By ae must be that of some two
numbers, for example 1 3, 2:3, ote,, otherwise the nodeseas
formed, since the two portions of the string cannot then be
vibrate in the same time, and the vibrations will interfere with
destroy one another.

If now we refer back to fig. 160, the existence of the node
Do easily proved by bending some light pieces of paper, and
them on the string. Say three pieces, one at C and the other:
tively midway betwoen B and C, and between Cand A. ‘The one,
experiences only a yery slight motion, and remains in its place, the

ho

r
Fig. 163, Fig. 162.

1s ACOUSTICS.

| 233. Reed instraments.—In reod instraments = sin
tongue sets the air in vibration, ‘The tongue, which is eith
or of wood, is moved by a current of air. The mouthpieces)
the bassoon, the clarionet, the child’s trumpet, are different
of the reed, which, it may be remarked, is seon in its simples
Jew's harp. Some organ pipes are reed pipes, others are m¢

Fig. 164 represents a model of & reed pipe as commot
lectares, It is fixed on the wind-chest Q of n bellows, a
tions of the reed can be seen through a piece of glass, E, fit
sides, A wooden hor, H, strengthens the sound.

‘ig. 165 shows the reed out of the pipe. It consists of
Ist, a rectangular wooden tube closed below and open above
‘copper plate ce forming on
tube, and in which there
dinal aperture, through wh
from the tube MN to theo
a thin elastic plate ¢ calle
which is fixed at ite up}
which grazes the edge of
dinal aperture, nearly closi
curved wire r, which press
tongue, and can be moved }
It thus regulates the le
tongue, and de
pitch of the no
this wire that 7»
tuned. The re
placed in the piy
acurrent of air
foot P, the ton
pressed, it bends
affords a passage
escapes by the 0
being elastic th
Fig, 164. Fig. 165. Fig. 166, bimrebiks
tions, successive
closes the orifice. In this way sonorous waves result and pi
whose pitch increases with the velocity of the current.

In this reed, the tongue vibrates alternately before an
aperture, merely grazing the edges, as is seen in the bar
sertina, etc. ; such a reed is called 8 freereed. But there a
called Beating reeds, in which the tongue, which is larger thi

end, and a loop in the middle, the column being divided into fa
parte by the alternate loops and nodes. When the seeond

is produced, the column of air will be divided into six s
alternate nodes and loops, and so on. It will be remarked
successive modes of division of the vibrating column are the
compatible with the alternate recurrence at equal intervals of
loops, and with the occurrence of a loop at each end of the pips,

ii. In the case of the closed pipe, there will still be = loop at t
next to the mouthpiece, but there will be « node at the closed
the air in contact with a fixed stop must be at rest. ordi
successive modes of division of the column of air mast be com
with this arrangement, and it will be found that when the i
is produced, there will be merely a loop at one end, and « node
other. When the first harmonic (3) ix produced, the eolumum is divid
into three equal parts by alternate loops and nodes. When the
harmanic (5) is produced, the column is divided into five equal:

40 on.

There are several experiments by which the existence of nodes aa
loops can be shown.

(a) If a fine membrane is stretched over a pasteboard ring, and
*prinkled on it some fine sand, it can be gradually let down ‘—
shown in fig. 169, Now suppose the tube to be producing
note, As the membrane descends it will be set in sie by th
vibrating air. But when it reaches a node it will cease to vibrate, i
there the air is at rest. Consequently the grains of sand, too, will
rest, and their quiescence will indicate the position of the pest On th
other hand, when the membrane reaches a loop, that is, a point
the amplitude of the vibrations of the air attains a maximum, it will B
violently agitated, as will be shown by the agitation of the grains of:
And thus the positions of the loops can be rendered manifest.

(8) Again, suppose ® pipe to be constructed with holes bored in
of its sides, and these covered by little doora which can be
shut, as shown in fig. 167, Let us suppose the little doom to be alt
and the pipe to be caused to produce such a tone that the nodes are =
N and N’ and the loops at V, V’, V”. At the latter points the
ia that of the external air, and consequently if the door at W’ ix o

are alternately condensation and rarefaction taking place If now fl
door wt N’ is opened this alternation of density is no longer
the density at this open paint must be the same as that of the external alt, |

SODES AND LoorS OF AN ORGAN PIPE. 197

oss joe atstalpeaegiaeraeae by the tube is
change of notes changing the fingering of
eee ue expecta a :
, is fige. 168, to be o pipe emitting a certain note, and
plug, fitting the tube, fastened to the end of a long rod
& farced down the tube. Now when the plug is in-

Fig. 165. Fig. 169.

ts position, there will be a node in contact with it.
F Be kn qeradually forced down, the note yildod by tho pipe
hanging. But every time it reaches a position which was
“node before its insertion, the note becomes the same as the
For now the column of air vibrates in exactly

as it did before the plug was put in.

le

space
wood and #0 out by the taba)
of Sep he bre Now

quently there ought to be
density at B; these would cau
brane r to vibrate, and.
flame m, and this is what
If by increasing the force of t
Fig. 170, octave to the primary note
be a loop, and A and C nodes |
the flames at A and C will now be extinguished, as is, in J
the case, But at B, thore being no change of density, the
unmoved, and the flame continues to burn steadily.

By oach and all of these oxperiments it is shown that in|
whether open or closed, there are always @ certain number ¢
midway between any two consecutive nodes there is alwa
ventral point,

236, Zxplanation of the existence of nodes and
mustioal pipe.—The existence of nodes and loops is to be
the co-existence in the same pipe of two equal waves travel
trary directions,

Let A be # point from which » series of waves sets out to
“let tho lengths of these waves, whether of condensation or ra
AC, CD, or DB. And let B be the point from which the ser)
equal waves sets out towards A. It must be borne in mind
case of a wave of condensation originating ut A the particles

am, ’
4

ric oxygen with the issuing jetof hydrogen. 7]
ia called the chemical harmonicon,

and the length of the tube: with
by varying the position of the je
the series of notes in the ratio 1
is obtained,

If, while the tube emits a certa
voice or the syren (213) be gra
to the same height, a+ soon a9
nearly in unison with the hart
flame becomes agitated, jumps u
and is finally steady when the
are in unison, If the tone of
gradually heightened, the pulm
commence; they are the optical
of the beats (224) which occur
unison,

If, while the jot burns in the ¢
duces a note, the position of the ty

altered, a point is reached at which no sound is heard, Ifn
or the syren, or the tuning fork, be pitched at the note pro

eth the fingers or by keys; shen G6 of Ue ticker be GE i
ix produced in the corresponding Inyer of air, which modifies #
bation of nodes and loops in the interior, and thus alters the note.
whistling of a key is similarly produced.
The panda pipe consists of tubes of different sizes
the different notes of the gamut.
In the trampet, the horn, the trombone, cornet-i-piston, and op
the lips form the reed, and vibrate in the mouthpiece. In the
different notes are produced by altering the distance of the lips. Int
trombone, one part of the tube slides within the other, and the per
can alter at will the length of the tube, and thus produce higher or low
notes. In the cornet-d-piston, the tube forms several convolutions ;
placed at different distances can, when played, cut off con
with other parts of the tubo, and thus alter the length of the
column of nir.

CHAPTER V.
VIBRATIONS OF RODS, PLATES, AND MEMBRANES.

240. Vibrations of rods.—Rois and narrow plates of wood, of
and especially of tempered steel, vibrate in virtue of their 4!
strings they have two kinds of vibrations, longitudinal and transverse
The latter are produced by fixing the rods at one end, and passing a baw
over the free part. Longitudinal vibrations are produced
rod at any part, and rubbing it in the direction of its length with w
of cloth sprinkled with resin. But in the latter case the sound is only
produced when the point of the rod at which it has been fixed is some
aliquot part of its length, as a half, a third, or a quarter.

Ti is shown by calculation that the number of transrerse whrations made

and inversely ax the square of their length. The width of the plate |
does not affect the number of vibrations. A wide plate, however,
requires n greater force to sot it in motion thana narrow ona Tt ie
of course, understood that one end of the vibrating plate is bold firmly.
In elastic rods of the same kind the number of longitudinal vibrations @
inversely as their length, whatever be the diameter and form of their treme
werse section.

Fig. 174, Fig. 175.

242. viprations of membranes.—In consequence of their fle
membranes cannot vibrate unless they are stretched, like the skin of
The sound they give is more acute in proportion ay they aro an
more tightly stretched. To obtain vibrating membranes, Savart
goldbeater's-skin on wooden frames.

In the drum, tho skins are strotched on the ends of a eylind:
When one end is struck, it communicates its vibrations to the
colump of air, and the sound is thus considerably strengthened,
cords stretched against the lower skin strike against it when it vil
and produce the sound characteristic of the drum.

Membranes cithor vibrate by direct. percussion, as in the drum, or’
may be set in vibration by the vibrations of the air, as Savart has o

CHAPTER VL
GRAPHICAL METHODS OF STUDYING VIBRATORY

243. ME. Lissajous’ method of making vibrations
—The method of M. Lissajous exhibits the vibratory motion
either directly or by projection on a screen. It has also the
advantage that the vibratory motions of two sounding bodies ma
compared without the mid of the ear, 90 as to obtain the exact re
between them,

This method, which depends on the persistence of visual
on the retina, consists in fixing « small mirror on the vibr
so as to vibrate with it, and impart to a luminous ray a vil
similar to its own. -

Fig. 177.

M. Lissajous uses tuning forks, and fixes to one of peel
metallic mirror, m (fig. 177), and to the other a
is necessary to make the tuning fork vibrate regu! es)
time. At a few yards’ distance from the mirror there is @
surrounded by a dark chimney, in which there is a amall hole, gi

—_=

ACOUSTICS.

bk Rplarremarmper syrencpa cist
motions, Two tuning forks

mn through a lens,
If now the first tuning fork alone vibrates, the image on the st
the aame as in experiment 179; but if they both vibrate, :
are in unison, the elongation increases or dimfnishes according 4
simultaneous motions imparted to the image by the vibrations
mirrors do or do not coincide,

a

Fig. 179.

If the tuning forks pass their posi
and in the same direction, the image attai
image is at its minimum when they pass at the same time but
directions. Between these two extreme cases the amplit
image varies according to the time which elapses between a
instant at which the tuning forks pass through their positi

‘The ratio of this time to the time of a double
called a difference of phase of the vibration.

If the tuning forks are exactly in unison, the luminous
the screen experiences a gradual diminution of length in propa
the amplitude of the vibration diminishes ; but if the itch of onais
little altered, the magnitude of the image varies periodically, and, wi
the beats resulting from the imperfect harmony are distinctly heard,
eye sees the concomitant pulsations of the image,
245. Optical combination of two vibratory motions at
angies to each other,—Tho optical combination of two x
vibratory motions is effected as shown in the figure 180, that
means of two tuning forks, one of which is horizontal and the
vertical, and both provided with mirrors, If the horizontal fork

se

Ses VINRATORY MOVEMENTS. 209

‘alone, « horizontal luminous outline is seen on the screen, while

Iran th efor «ei image. If both tuning
\ribrate simultancously the two motions combine, and the reflected

| describes a more or less complex curve, the form of which depends

Fig. 180.

(@ number of vibrations of the two tuning forks in a given timo.
curve giver valuable means of comparing the number of vibrations

Fig. 181.

luminous image on the screen when the tuning
‘when the number of vibrations is equal.
each curve indicate the differences of phasd

initial form of the curve is determined by the

210
difference of phase. The curve retains exactly the same form when |
tuning forks are in unison, provided that the amplitudes of the
rectangular vibrations decrease in the same ratio. ‘
If the taning forks are not quite in unison, the initial i
phase is not preserved, and the curre passes through all ite variations:

Q 1 3
2
} i z Fa E

Fig, 182.

Fig. 182 represents tho difforent appearances of the luminous ima
whon the difference between the tuning forks is an octave ; that is, whet
the numbers of their vibrations are ns 1:2; and fig. 189 gives tht
aries of curres when the numbers of the vibrations are as $+ 4, |

0 1 3
cs Eu

oe
Fig. 133.

34

th
er
4
pn

It will be soon that the curves are more complex when the ratios |
the numbers of vibrations are less simple, M. Lissijous has examint
those curves theoretically (Annales de Physique et de Chimie, 1857), 80
hhas calculated their general equations.

When these experiments are made with a Dubescq's photo-vlectriel

212 ACOUSTICS,

atretching ring with a moveable piece ¢, which he calls a subdivid
which, being made to touch the membrane first at one point.
another, euables the experimenter to alter the

bones which touch the tympanum,

Tile hele ns soetroetine See a
near the apparatus, the air in tho ellipsoid, the
will vibrate in unison with ft, and it only rensalna to n
surface the vibrations of the style, and to fix them,
there is placed in front of the membrane # copper ¢
round a horizontal axis by means of o handle m. |
axis of the cylinder a sctow is cut which works in a 1
when the handle is turned, the eylinder gradually
direction of its axis, Round the cylinder is wrapped a
covered with @ thin layer of lampblack, The lam
by setting the cylinder in motion, and moving b
flame.

‘The apparatus is used by bringing the prepared
with tho point of the style, and thon setting the
round its axis. So long as no sound is heard the sty!
and merely removes the lampblack along a line which
cylinder, but which becomes straight when the paper
But when a sound is heard, the membrane and the
unison, and the line traced out is no longur straight but
undulation corresponding to a double vibration of the
quently the figures thus obtained faithfully denote the n
tude, and isochronism of the vibrations, The figures are largy
sound is loud, very amall if the sound is very weak; thoy are st
out when the sound is low, squeezed together when it is high.
the sound is clear they are free and regular, feeble and irregular
is confused, It would seem, howevor, that tho figures do not rep
the whole vibration of the membrane, but only the part of it
takes place in a direction parallel to the axis of the cylinder,

Fig. 185 shows tho trace produced when a simple note is sung, #
strengthened by means of its upper octave. The latter note is
sented by the curve of lesser amplitude, Fig. 186 represents the sound
produced jointly hy two pipes whose notes differ by an octave, Fig, 187
in its lower line represents the rolling sound of the letter R wher

aac] ‘THE PHONAUTOORAPH. 213

fonounced with « ring; and fig. 188 on its lower line represents the
produced by s tin plate when struck with the finger.

The upper lines of figs. 187 and 188 are the sume, and represent the

ferfectly isochronous vibrations of a tuning fork placed near the ellipsoid,

Tines were traced by a fine point on one branch of the fork, which

this found to make exactly 600 vibrations per second. In conse

each undulation of the upper line corresponds to the ,}5 part of

wooed. And thas these lines become very exact means of measuring

Fig. 187.

Fig. 188.

fort intervals of time. For example, in fig. 187, cach of the separate
(ecka producing the rolling sound of the letter R corresponds to about
IS doable vibrations of the tuning fork, and consequently lasta about
We or about 4, of a second.

The cures once traced, it remains to fix them on the blackened paper.
For this purpose, M. Scott dipped them first into a bath of pure alcohol ;
fad when they were dry, he then dipped them into a solution of resin—
fr inatanco, anndarack—in alcohol, By this means the lampblack is

Yerfectly fixed.

214

BOOK VIL

ON HEAT

CHAPTER T

THERMOMETERS.

PRELIMINARY IDEAS.

‘7. Heat. Hypothesis as to its nature.—In ontinary language
term Aeat is not only used to express a particular sensation, but
describe that particular state or condition of matter which
this sonsation. Besides producing this sensation, heat acts
bodies; it melts ice, boils water, makes metals red-hot, and so forth,
‘Two theories as to the cause of heat are current at the present times,
these are the theory of emission, and the theory of undulation.

On the first theory, heat ia caused by a subtle imponderable fluid, which
surrounds the molecules of bodies, and which can pass from ane body (6
another, These heat atmospheres, which thus surround the molecules, exert
fa repelling influence on ench other, in consequence of which heat acts ii
opposition to the force of cohesion. The entrance of this substance into omt
bodies produces the sensation of warmth, its egress the sensation of cold.

On the second hypothesis the heat of a body is caused by an oneallating
or vibratory motion of its material particles, and thé hottest bodies are
those in which the vibrations have the greatest velvcity and the ‘
amplitude. Hence on thix view, heat is not a substance, but condition
of matter, and a condition which can be transferred from one body te
another. It is also assumed that ther an imponderuble elastic ether,
which pervades all bodies and infinite space, and is capable of trans
mitting a vibratory motion with great velocity. A rapid vibratory”
motion of this ether produces heat, just ns sound is produced by &
vibratory motion of atmospheric air, and the transference of heat from
one body to another is effected by the intervention of this ether,

‘This hypothesis is now admitted by the most distinguished physiciates
it affords n better explanation of the phenomena of beat than any other
theory, and it reveals an intimate connection between heat and light
In accordance with it, heat is a form of motion ; and it will hereafter be

MEASUREMENT OF TEMPENATURES. THERMOMETRY,

249, Temperatare.—The demperature or hotness of & body
defined as being the greater or less extent to which it tends:
sensible het to other bodies, The temperature of any
varied, by adding to it or withdrawing from it a certain amout
ble heat. The temperature of a body must not be confounded
quantity of heat it possesses; a body may have a high i
yet have a very small quantity of heat, and conversely a low
and yet possess a large amount of heat. Ifa cup of water be
© bucketful, both will indicate the same temperature, yet the:
they possess will be different. This subject of the quantity of
be afterwards more fully explained in the chapter on Specific

temperatures. Owing to the imperfection:

to measure temperatures by the sensations of heat or cold

produce in us, and for this purpose recourse must be had to the

action of heaton bodies, These actions are of various kinds,
ion of bodies has been eclected as the easiest to observe,

also produces electrical phenomena in bodies; and om theese the

bulb and a portion of the stem, :
253, Graduation of the thermometer.—The thermom
filled, it requires to be graduated, that is, to be provided with
which variations of temperature can be referred. And first)
points must be fixed which represent identical tenspertun
always be easily produced,

Experiment has shown that ice always melts at the |
whatever be the degree of heat, and that distilled water undi
pressure, and in a vessel of the same kind, always boils at the
perature. Consequently, for the first fixed point, or zero, tl
ture of melting ice has been taken; and for a second fixe
temperature of boiling water in a metallic vessel under
atmospheric pressure of 760 millimeters.

This interval of temperature, |
range from xoro to the boiling, pol
as the unit for comparing
as a certain length, a foot or @ }
stance, is used asa basis for

264, Determination of the
‘To obtain zero, snow or
a veesel, in the bottom of which is
by which water escapes (fig. 192)
and a part of the stem of the ¢
fare immersed in this for about a qj
hour, and a mark made at the ly
mercury, which represents zero,

The second fixed point is det
means of the apparatus represer
figures 193 and 194, of which fig
sents « vertical section, In botl
letters designate the same parts,
of the apparatus is of copper. A¢
A, open at both ends, is fixed on # cylindrical vessel conta)
@ second tube, B, concentric with the first, and surrou

ON HEAT.

grees and two-thirds. Consequently 1003 would have to
at the point at which the mercury stops,

Gay-Lussac observed that water boils at a somewhat higher
perature in n gloss than in a metal vessel ; and as the boiling po

nocessary to use a metal vessel and distilled water in fixing the b
point. M. Rudberg has, however, shown that these latter prec
euperiluous. The nature of the vessel, and salts diswlved ino
water, influence the temperature of boiling water, but not that
vapour which is formed. That is to say, that if the ten
boiling water from any of the above causes is higher than 100
the temperature of the vapour does not exceed 100, provided the
isnot more than 760 millimetora. Consequently the higher point
be determined in any kind of a vessel, provided the the

quite surrounded by vapour, and does not dip in the water.

Even with distilled water, the bulb of the thermometer must
in the liquid ; for it ia only the upper layer that really has the t
ture of 100 degrees, since the temperature increases from layer
towards the bottom in consequence of the increased pressure.

255, Construction of the scale.—Just as the foot-rule wl
ndopted aa the unit of comparison for length is divided into a number
equal divisions called inches for the purpose of having a smaller unit i
comparison, 80 likewise the unit of comparison of temperatures, the
from zero to the boiling point, muat be divided into a number of
of equal capacity called degrees. ‘There are threo modes in which
done, On the continent, and more especially in France, this
divided into 100 parts, and this division is called the Centigrade or C:
seale; the latter being the name of the inventor, ‘The Centigrade them
mometer is almost exclusively adopted in foreign scientific works, and
its use is gradually extending in this country, it has been and will
adopted in this book,

The degrees aro designated by a small cipher placed a little abore
the right of the number which marks the temperature, and to i
temperatures below zero the minus sign is placed before them,
—15° signifies 15 degrees below zero.

Tn accurate thermometers the scale is marked on the stem itself,
cannot be displaced, and its length remains fixed, as glass haa very
inexpansibility. This is effected by covering the stem with « thits
of wax, and then marking the divisions of the scale, a8 well as thee
sponding numbers with a steel paint, Tho thermometer is then
for about ten minutes to the vapours of hydrofluoric acid, whieh at
the glass whore the wax has been removed. The rest of the waxis tt
removed, and the stem is found to be permanently etched.

222 ON HEAT, ‘[2se-

In like manner we have for converting Réautur's into Faloenheits
Gogroes the formula

formula

structed with the greatest care, are subject to a source of error
must be taken into account; this is, that in course of time the
tonds to riso, the displacement sometimes extending to as much
degrees ; so that when the thermometer is immersed in molting ice Il)
no longer sinks to zero,

This is generally attributed to » diminution of the volume of the
reservoir and also of the stem, occasioned by the pressure of
atmosphere. It ix usual with very delicate thermometers to fill
two or three yeara before they are graduated.

Besides this slow displacement, there are often variations in tt)
position of the zero, when the thermometer has been exposed to bigit
temperatures, caused by the fact that the bulb and stem do not contract
on cooling to their original volume (248), and henes it is necessary 0)
verify tho position of zom when a thormomoter is uscd for delicate
determinations,

Rognault haa found that some mercurial thermometers, which egme!
at 0° and at 100°, diffor between these points, and that these differant
frequently amount to several degrees. Regnault thinks that this is daly
to the unequal expansion of different kinds of glass.

267. Xdmits to the employment of mercurial thermometers—
Of all thermomoters in which liquids are used, the one with mereuty
is the most usefal, because this liquid expands most regularly, and i
easily obtained pure, and because its expansion between —36° and 100)
is regular, that is, proportional to the degroe of heat, Tt aléo has thi
advantage of having » very low specific heat, But for temperatena
below —36° C, the alcohol thermometer must be used, for mancury
solidifies at — 40°C. Above 100 degrees the coefficient of expansion
increases and the indications of the mercurial thermometers are
approximate, the error arising sometimes to soveral degrees,
thermometers also cannot be used for temperatures above 350°, for
is the boiling point of mercary.

268. Alcohol thermometer.—The alcohol thermometer differs from
the mercurial thermometer in being filled with coloured alcohol. Buta®
the expansion of liquids is Joss regular in proportion as they are near the
doiling point, alcohol, which boils at 78° C,, expands very ixregulatlye
Hence, alcohol thermometers are usually graduated by placing thet
in baths at different temperatures together with a standard mercurial

=

224 ON MEAT.

closed the air is passed from one bulb into the other by heati
unequally until the level of the liquid is the same in both brumches
zero is marked at each ead
the liquid column, To gz
the apparatus, one of the bul
is raised to a temperature
higher than the other. ‘The
of the first is
causes the column of Liquid
rise in the other leg. Wh
column is stationary, the: 1
10 is marked on each side at th
level of the liquid, the
between zero and 10 beingd
into 10 equal parts, both
and below zero, on each leg
2604, Matthiessen'’s
ential Thermometer, — Pre
fessor Matthiessen has devised @
form of differential thermometer
which can be used for indicating
the temperature of liquids, and!
which constitutes a valuable addition to our moans of illustrating fr
lecture purposes many important experiments in heat. Its constructed
ia evident from the annexed Siguny
(196). The bulbs are pendent, and)
it can therefore be readily immerse!
ina liquid. In a tube which ean
nects the two limbs thers is a stop
cock, which is very useful me
menns of adjusting the level of the
liquids, a rather troublesome taal
with Leslie's instrament.
261. Breguet’s metallic ther-
mometer.— Breguet invented a”
thermometer founded on the =
equal oxpansion of metals, and ne
markable for its delicacy. Tt consist®
of three strips of platinum, goby
and silver, which are passed throws
Big. 197 ‘a rolling mill so as to form = veEy
thin metallic ribbon. This is then coiled in a spiral form, aa seen in lige
197, and one end being fixed to a support, a light noodle is fixed to tha
other, which is free to move round a graduated scale,

_ >

226 ‘ON HEAT.

alcohol expands, and passing between the sides of the
index, does not displace B. ‘The position of the index gives
the lowest temperature which has been reached: in the figure
9} degrees below zero.

263, Pyrometers,—The name pyrometers is given to inatrm
measuring temperatures so high that mercurial thermometers «
be usod, The older contrivances for this purpose, Wodgewo
Daniell’s (which in principle resembled the apparatus in figs Ll
Brongniart’s, etc., are gone entirely out of use. None of them gi
exact measure of temperature, The arrangements now used
purpose are either based on the expansion of gases and vapo
the electrical properties of bodies, and will be subsequently dese

264, Different remarkable temperatures.—The following
gives somo of the most remarknble points of temperature, Tt 1
observed that it is easier to produce very elovated temperatures:
low dogrees of cold.

Greatest artificial cold produced by a bath of Medphite cy
carbon and Liquid nitrous acid. .

Greatest cold prodused by other and liquit oatbouie weld

Greatest natural cold recorded in Arctic aie expats

Mercury freezes . at

Mixture of snow and ‘alt . . ——

Tce melts . o -¥r ig BEE

Greatest density of vite. oa

Mean temperature of London. . we

Blood heat ok Se

Waterboille . - «© +» © »© «@ &

Mercury boils. . ‘ . . . .

Red heat (just visible) (Daniell) .

Silver melts 3) an

Cast iron melts -

Highest heat of wind furnace ,,

CHAPTER II,
EXPANSION OF SOLIDS,
265, Linear expansion and cubical expansion. Coemi
expansion,—It has boen already explained that in solid bodies the
sion may be according to three dimensions, linvar, superficial, and
The coeficient of linear expansion is the elongation of the unit

228 ON HEAT. [2e7-

and in order to eliminate the effects of friction it rests on two glass”
rollers. Lastly, the telescope bas a cross-wire in the eyepiece, which
when the telescope moves, indicates the depression by a correspouling
number of divisions on & vertical scale, AB, at a distance of 220 yards
‘Tho trough is first filled with ice, and the bar being at zero,
division on the scale AB, corresponding to the wire of the telescope, fs
read off. The ice having been removed, the trough is filled with oil
water, which is heated to a given temperature, The bar thea expand)
and when its temperature bas become stationary, which is determinal!
by means of thermometers, the division of the scale, seen through the
telescope, is read off,

Fig, 200.

From these data the elongation of the bar is determined ; for sine# it
has become longer by a quan’ H, and the optical axis of the
telescope has become lined in the direction GB, the two tris
GHC and ABG, are similar, for they have the sides at right snglet
each to each, 20 that HO — GH

ABT AG”
mother elongation, and AB! a corresponding deviation, them wonld
Ar AG from which it follows that the ratio between the
ptingatiot of the bar and the deflection of the teleseope is constant,
GH

for itis nlways equal to

In the same way, if HC’ wee

A a ineasursment bad shown

that this ratio was Consequently = = shy whence IC =

xB i
that. is, the total elongation of the bar is obtained by dividing the
Jongth on the acale traversed by the cross-wire by 744, Dividing this
elongation by the length of the bar, and then by the temperature of the
hath, the quotient is the dilatation for the unit of length and for a single
dogree—in other words, the coefficient of linoar dilatation,

207. Roy and Ramsden's method.—Lavoisier and Laplace's methed
is founded on an artifice which is frequently adopted in physical deter
minations, and which consists in amplifying by a known amoxil
dimensions which, in themselves, are too small to be easily measunals
Unfortunately this plan is often more fallacious than profitable, for it i

229

EXPANSION OF SOLIDS.

Ent necessary to determine the ratio of the motion measured to that
feo which it depends. In the prosent caso it is necesary to know the
Ieegths of the arms of the level in the apparatus. But this preliminary
Gpeation may introduce error of such importance as partially to
eauterbalance thy advantage of great delicacy. The following method,
, and which wad devised by
Ramsden, depends It measures the elongations
Mirectly, and without auplifying them, but it measures them by means
tifa micrometer, which indicates very small displucements,

‘The apparatus (fig. 201) consists of three parallel motal troughs about
Wfeet long. In the middle one there is a bar of the body whose expan-
Hien is to he determined, and in the two others are cast iron bars of exactly
the samt length as this bar. Rods are fixed vertically on both sides of

big. 201.

Hite the bars, On the rods in the troughs A and B there are rings

[it eroas-wires like those of a telescope. On the rods in the trough C
| amy email telescopes also provided with crost-wires.

The troughs being Alled with ico, and all three bars at zero, the

Points of intermetion of the wires in the disc, and of the wires in the

fire all in a line at each end of the bar, The temperature in

trough & then raised to 100° C, by means of spirit lamps

Deneath the trough ; the bar expands, but as it is in contact with

She eed of m serew, o, fixed on the side, all the clongation tales piace in

230 ON MEAT.
tho direction mm, and as the cross-wire » remsins in position,

to left, the bar is moved in the direction
regains its original position. To effect this,
by # quantity exactly equal to the elongation of
advance of the screw is readily deduced from the number eye
its thread, tho total expansion of tho bar is obtained, which, divided by
the temperature of the bath, and this quotient by the length of the bar
at zero, gives the coeflicient of linoar expansion.

Cogfficients of linear expansion for 1° between O° and 100° C.
White glass ,
Platinum. . .
Untempered steel .
‘Cast iron .
Wrought iron .
Tempered steel .
Gold

From what has te said about the linear atatwkn (265), the co-
efficients of cubical expansion of solids aro obtained by multiplying thos!
of linear expansion by three,

The coefficients of the expansion of the metals vary with their physical
condition, being different for the same motal according as it has been east,
hammered and rolled, hardened or annealed. As 8 general rule, openi-
tions which increase its density incroase also the tate of expansion. But
even for substances in apparently the same conditlon, different observers
havo found very uncqual amounts of expansions; this may arise in the
ease of compound substances such as glass, brass, or steel, from a want of
uniformity in chemical composition, and in simple bodies from slight dife
ferences of physical state,

‘Phe expansion of amorphous solids, and of those which erystalliae i
the regular systom, is the same for all dimensions, unless they are sube
ject to a strain in some particular direction, A fragment of such # sub
stance varies in bulk, but retains the same shape. Crystals not belonge
ing to the regular system exhibit when heated an unequal expansion in
the direction of their different axes, in consequence of which the magni=
tude of their angles, and therefore their form, is altered. In the dimetne
system the expansion is the same in the direction of the two equal axes,
Dut different in the third. In crystals belonging to the hexagonal system
the expansion is the same in the direction of the three secondary axes;
but different from that according to the principal oné. In the trimetrie
system it is different in all three directions,

252 ON BEAT. [270-

Tho density of a body being d at zero, required its density dat P, _
If] be the volume of the body at zero, and D its coellicient of eableal
expansion, the volume at ¢ will bo 1 + Dé, and as the dousity of = body
is in inverse ratio of thejvolume which the body assumes in expanding,
we get the invorse proportion
@@:d=1 : 1404 ;
ae =
a Tee"? “oe

Consequently, when body is heated from 0 to ©, its density, and
therefore its weight for an equal volume, are inversely as the binomial
expression, 1 + Dt.

270, Applications of the expansion of solids.—In the arts we mont
with numerous examples of the influence of expansion. (i.) ‘The hark of
furnaces must not be fitted tightly at their extremities, but must, at leash
be free at one end, otherwise, in expanding, they would split the masonry.
(ii) In making railways a small space is left between the successive mails,
for if they touched, the force of expansion would cause them to cure
or would break the chairs, (ili.) Water pipes are fitted to one another by
moans of teloscopic joints, which allow room for expansion. iv.) If @
glass is heated or cooled too rapidly it cracks; this arises from the fret
that glass being a bad conductor of heat, the sides become unequally
heated, and consequently unequally expanded, which causes # fractiin
have been heated to high temperatures, the force pro
ontraction an cooling is very considerable ; it is equal to
the force which ie needed to compress or expand the material to the
same extent by mechanical means. According to Barlow a bar of tials
Jenble iron a square inch in soction is stretched ;;is5 of its length bya
weight of a ton; the same increase is experienced by about ® C, A
difference of 45° C, between the cold of winter and the heat of summer
is not unfrequently experienced in this count: In that range a wrought
iron bar ten inches long will vary in length by ;}; of an inch, and will exert

ts ends are securely fastened, of fifty tons, It has been cale

d from Joule’s data that the force exerted by heat in expanding &

iron between 0° and 100° during which it increases mbout yy

of its bulk, is equal to 16,000 foot pounds; that is, it could raise a weight
of 7 tons through a height of one foot,

{i.) An application of this contractile force is seen in the mode of
securing the tires on wheela The tire being made red hot, and thus
considerably expanded, is placed on the circumference of the wheel and
then cooled. The tire, when cold, embraces the wheel with such forms
as not only to secure itself on the rim, but also to press home the joints
of the spokes into the folloes and nave. (ji) Anothor interesting appli=
cation was made in the case of a gal! at the Conservatoire des Arts

.

234 ON BEAT. ‘[en-

attained when the sum of the lengths of the steel rods A is to the
of the lengths of the brass rods B in tho inverse ratio of the
expansion of steel and brass, «and 4, thatis,in the proportion A: Bi:
‘Tho elongation of the rod may also be compensated for by means)
ing strips. These consist of two blades of copper and iron
dered together and fixed to the pendulum rod, as represented in fig.
‘The copper blade, which is more expansible, is below the iron. Whea

tte

Fig. 203, Fig. 204. Fig. 205,

the temperatare sinks, the pendulum rod becomes shorter, and the
rises, But at the samo time the compensating stripe become curred)
seen in fig. 204, in consequence of the copper contracting mone
the iron, and two metallic balls at their extremities become lower
they have the proper size in reference to the pendulum ball, the
which tend to approach the centre of suspension compensate those
tend to remove from it, and the centre of oscillation is not displaced,
the temperature rises the pendulum ball descends, but at the same tet
the small balls ascend, as shown in fig, 205, s0 that there is always
compensation.

One of the most simple compensating pendulums is the mercury jem
dulum, invented by an English watchmaker, Graham. The ball of the
pendulum, instead of being solid, consists of a glass cylinder, ae
pure mercury, which is placed in a sort of stirrup, supported by a steel
‘When the temperature rises the rod and stirrup become longer, and
lower the contro of gravity; but at the same time the mereury expand
and, rising in the cylinder, produces an inverse effeet, and as mercury IA)
much more expansible than stecl, a compensation may be effected with:
out making the mercurial veasel of undue dimensions.

The same principle is applied in the compensating balances of chrand=
meters. The motion here is regulated by a balance or wheel, furnished
with a spiral spring, and the time of the chronometer depends on the
force of the spring, the mass of the balance, and on its cireumferente |
Now when the temperature rises the circumference increases, and tht
chronometer goes slower; and to prevent this, part of the mass must be

ya

236 ON HEAT. [a73-

a capillary tube, and kept vertical by an iron support. Each of the tubes
ie surrounded by a metal ease, of which the smaller, D, ix filled with
ice, the other containing oil, can be heated by the furnace, which is rayre
sented in section 0 as to show the case. Mercury is poured into the tube
A and B; it remains at the same level in both as long as thoy arent the
same temperature, but rises in B in proportion as it is heated, and expand

Let A and d be the height and density of the mercury in the leg A, a
the temperate zero, and A’ and @ the samo quantities in the leg B. Fret
the hydrostatical principle previously ated we have had Ad = Wd. Now

from the problem on pago 292, d= ont D being the coeffichunt of abe

solute expansion of mercury; wuteioe this valuo of @ in the equation,
j= Ad, from which we get D= a
er aoa expansion of mercury is obtained from tlt
formula, knowing the heights 4’ and J, and the temperature ¢ of the baile
inwhich the tube B is immersed. In Dulong and Petit's experimiall
this temperature was monsured by a weight thermometer, P (275), tie
mercury of which overflowed into the basin, C. The heights AY anil
wore measured by a cathetometer, K (79).

Fig, 206

Dullong and Petit found by this method that the coeflicient of absolate
expansion of mercury, between 0° and 100°C, is ;Ayy- But they found
that the coefficient increased with the temperature, Between 100° sal
200? it is xcs, and between 200° and 300° it is he. ‘The mame obser
vation has been made in reference to other liquids, showing that their ex
pansion isnot regular, It has been found that this expansion is lees regula
in proportion as liquids are near a change in their state of aggregation, Uae
is, approach their freezing or boiling points. Dulong and Petit found
that the expansion of mercury between —3¢° and 100° is practically quite
uniform,

238 ON BEAT.

pansion of mercury in glass, and that of its apparent expansion.
the coefficient of cubical expansion of glass is
sibx — xara = sefos = 0002584.

Regnault has found that the coefficient of expansion varies with
ent kinds of glass, and further with the form of the envelopes.
ordinary chemical glass tubes, the coefficient is 00000254.

277, Coefficients of expansion of various Mquids.—The
expansion of liquids may be determined by means of the weight themo=
moter, and the absolute expansion is obtained by adding to this coctheient
the expansion of the glass,

Total apparent expansions of liquide betwoen 0 and 100° C,
Mercury . . . . . . 001643 Oil of turpentine .
Distilled water. . . . 00406 Ether. . . .
Water saturated with salt 0-05 Fixed oils.
Sulphuric acid. . . .008 — Nitricacid .
Hydrochloric acid . . .006 Alcohol . . «

‘The coefficient of apparent expansion for 1° ©, is obtained by dividing:
these numbers by 100; but the number thus obtained does not represait
the mean coeflicient of expansion of liquids, for the expansion of thee)
bodies increases gradually from zero. The expansion of mercury is pace
tically constant between — 36° and 100°C, while water contracts fom
zero to 4°, und then expands,

For many physical experiments a knowledge of the exact
of water is of great importance. ‘This physical constant has been dete
mined with great care by Dr, Matthiessen, who has found that between
4° and 32° it may be expressed by the formula
Vt=1—0-00000253 (¢— 4) 00000008889 (¢—4)? +

000000007178 (¢-4)*

and betweon 32° and 100° by
‘Vt=0-999605 +-0:0000054724 ¢+4-0-00000001126 &

Many liquids, with low boiling points, especially condensed gasniy
have very high coefficients of expansion, Thilorier found that liquid
carbonic acid expands four times as much as air, Drion has recently
confirmed this observation, and has obtained analogous results with
éhioride of ethyle, liquid sulphurous acid, and liquid hyponitrows acid,

278, Correction of the barometric height.—It has been
explained under the Barometer (165), that, in onler to make the indics=
tions of this instrument comparable in different places and at different 7
times, they must be reduced to a uniform temperature, which ix that of
melting ice, The correction is made in the following manner :

oe

240

bottom, and the lower thermometer marked 4°, while that of the
one was still at zero. Hope then made the inverse experiment;
filled the vessel with water at 15°, he placed it in a room atj
The Jower thermometer having sunk to 4°, remained 0
some time, while the upper one cooled down until it reached
Both these experiments prove that water is heavier at 4° than at ©
in both cases it sinks to the lower part of the vessel. 7

Hallstrdim made a determination of the maximum density of
in tho following manner. He took a glnss bulb, londed with sand,
weighed it in water of different temperatures. Allowing for the ox
sion of glam, he found that 4-1° was the tempernture at which if I
moat weight, and consequently this was the temperature of tho mas
density of water.

Despretz arrived at the temperature 4° by another method. He
a water thermometer, that is to say, a bulbed tube containing water,
placing itin a bath, the temperature of which was indicated by an onl
mercury thermometer, found that the water contracted to the
extent at 4°, and that this ie therefore the point of greatest density,

‘This phenomenon is of great importance in the economy of antun. Ti
winter the temperature of lakes and rivers falls, from being im oat
with the cold air, and from other causes, suck as radiation. ‘The coder
water sinks to the bottom, and a continual series of currents goe Ga)
until the whole has a temperature of 4°. The cooling on the sutfee
still continues, but the cooled layers being lighter remain on the 0
and ultimately freeze. The ice formed thus protects the water belo,
which remains at a temperature of 4°, even in the most severe winters &
temperature at which fishes and other inhabitants of the waters are not
destroyed.

CHAPTER TV,

EXPANSION AND DENSITY OF GASES.

281], Gay-Lussac’s method.—(Cases are the most expansible bees
and at the same time the most regular in their expansion. The
civnts of expansion, too, of the several gases, differ only by very atl
quantities. The cubical expansion of gases need alone be considered.
Gay-Lussac first determined the coefficient of the expansion of gasea
by moana of the apparatus represented in fig. 200.

Ina rectangular metal bath, about 16 inches long, was fitted am air
thermometer, which consisted of a capillary tube, AB, with a balb, Ay

242 ON HEAT.

obtained by subtracting from its volume ata given temporntdreits)
atzero. Dividing this by the given temperature, and then by t
ber of units contained in the volume at zero, the quotient isthe et
of expansion for a single unit of volume and a single degree; thi
coefficient of expansion. It will be seen, further on, how correct
Pressure and temperature may be introduced.

By thie method Gay-Lusene found that the coefficient of expe
air was 100375 ; and he enunciated the two following laws in 5
to the expansion of gases :

I. AU gates have the same coefficient of expansion as air,

IL. This coefficient is the same whatever be the preeeure sap
the gas.

These simple laws aro not, however, rigorously exaet (283);
‘express the expansion of gases in an approximate manner.

282. Problems on the expansion of gases.—Many of |
blems relative to the expansion of gases are similar to thos
expansion of liquids. With obvious modifications they are sal)
similar manner, In most cases, the pressure of the atmosphens
taken into account in considering the expansion of gusex ‘The
is an example of the manner in which this correction is made:

i. The volume of a gas at @, and under the pressure H, is
will be the volume V of the same gas at zero, and under
preasure 760 millimeters ?

Here there are two corrections to be made; one relative
perature, and the other to the pressure. It is quite ri
is taken first. If « be the coefficient of cubical expansion for
dogroo, by reasoning similar to that in the ease of linear expansit
the volume of the gas at zero, but still under the pressure Hy

This pressure is reduced to the pressure 760, in ae

1+ at
with Boyle and Mariotte’s law (161), by putting
> eres ea
Vxi =a at
~ __ WH
whence Vv =ra0tal)

ii. A volume of gas weighs P’ at ©; what will be its weight
Tat P be the desired weight, « the coefficient of expansion of
@ its density at C, and d its density at zero. As the we
equal volumes are proportional to the densities, we have
If 1 be the volume of « gas at zero, its volume at ¢ will be 1 +
es “4 a |

ag the densities are inversely as the yolumes, 7-1 and }

2

boiling water; the air pump having been detached, the drying
were then disconnected, and the end of the tube bermotically sealed,
the height, H, of the barometer being noted. When the reservoir Byat
cool, it was placed in the apparatus represented in fig. 211. Tt wat
there quite surrounded with ics, and the end of the tube dipped in the
mercury bath, C. After the air in the reservoir B had sunk to zero, thi
point 4 was broken off by means of a forceps; the air in the interiog
became condensed by atmospheric pressure, the mercury rising to &
height oG. In order to mensuro the height of this column, Go, whieh:
will be called 4, & moveable rod, go, was lowered until its point, o,
flush with the surface of the mercury
the bath ; the distance between the pollit
o and the level of tho mercury G, wall
measured by means of the cathetometée:
‘The point b was finally closed with wax ky
means of the spoon a, and the barometrie
pressure noted at this moment, If thir
pressure be H’, the pressure in the reserrait
is H’—A,

The reservoir was now weighed #7
ascertain P, the weight of the mercury
which it contained. It was then come)
pletely filled with mereury at zero, in onder
to have the weight P’ of the mereary im the |
reservoir and in the tube,

If 8 be the coefficient of the cubjeal ex=
pansion of glass, and D the density of
mercury at zero, the coeficient « of the
cubical expansion of air is determined i
the following manner, The volume of the

reservoir and of the tube at zero is $

from the formula P= VD (116); consequently, this volume is

Pe
pil + a) ce ee 8 6

ft the temperature f°, assuming, as is the ease, that the reservoir and
tube expand as if they were solid glass, But from the formula P= WD,
the volume of air in the reservoir at zero, and under the pressure H’ =A,

ones :
is——- At the same pressure, but at ©, its yolame would be

r-P 7
ere +e)5

and, by Boyle and Mariotte's law (161), at the pressure H, under whieh
the tube was sealed, this volume must have been

ON MEAT.

Fig. 211.

246

perature ¢ to which the tube has been raised is readily deduced from
equation (3).

‘Regnault’s researches show that the airand the mereurial
agree up to 260°, but abore that point mercury expands relatively
than air.

Tn cases whero very high tomperatures are to bo measured the

Dulong and Petit’s experiment (273); it was by such an apparntue
Pouillet measured the ature corresponding to the colours
metals take when heated in a fire, and found them to be as follows:

Tncipiontred. . . . . 525°C, Darkorange. . . .
Dullrd . . - . . . 700 Wilts. sane
Cherrymd. . . - . - 900 Dazzling white. .

In the measurement of high temperatures Deville and Troost
used, with advantage, the vapour of iodine instead of air.

285, Density of gases,—The relative dennty of a gas, or its pene:
gravity, is the ratio of the weight of a cortain volume of the gas te
that of the same volume of air; both the gas and the air being at xen)
and at a pressure of 760 millimeters.

In onder, therefore, to find the specific gravity of a gas, it ia

to determine the weight of a cortain volume of this gas, at a pressure:
76) millimeters, and a temperature of zero, and then the weight of the
sume volume of air under the same conditions, For this parpose a large
globe of ubout two gallons capacity is uscd, the neck of which is pro
vided with a stopcock, which can be screwed to the air pump. The
globe is first weighed empty, and then full of air, and afterwards full of
the gas in question. The weights of the gas and of tho air are obtained:
hy subtracting the weight of the exhausted globe from the weight of
the globes filled, respectively, with air and gas, The quotient, obtained
by dividing the latter by the former, gives the specific gravity of the gai.
It is difficult to make these determinations at the same tempuratart aad
pressure, and therefore all the weights are reduced to zero and the normal
pressure of 760 millimeters.
Tho gases are dried by causing them to pass through’ drying tuber
before they enter the globe, and air must also be passed over potash #9
free it from carbonic acid. And as even the bost air pumps never produce
perfect vacuum, it is necessary to exhaust the globe until the mame
moter in each caso marks the same pressure.

The globe having been exhausted, dried air is allowed to enter, amd
the process is repented several times until the globe is perfectly driek
It is then finally exhansted until the residual tension, in millimeters, ise
The weight of the exhausted globe is p. Air, which has been drial

end the weight taken. This gives the weight of the

the capacity of the vessel be measured by means of water, 1

the air which it contains is deduced, for the density of air

760 millimeters pressure, is y}, that of distilled water und

cumstances. The weight of the vessel full of air, less the

contained air, gives the weight of the vessel itself. From these

—the weight of the vessel full of the gas, the weight of the air}

contains, and the weight of the vessel alone—the specific

gas is readily deduced, the necessary corrections being made

ratare and pressure,

Densities of gases at zero and at a pressure of 760 millimeters,
being taken aa unity.

Breer te se 10000 Sulphuretted hydrogen
Hydrogen. . . . . . 0.0003 Hydrochloric acid . -
Marchyas 2. wwe 05500 — Protoxide of nitrogen.
Ammoniacal gas . . . 05367 Carbonic acid . . .
Carbonic oxide. . . . 09670 Cyanogen... .
Nitrogen... + 09714 Sulphurousacid .
Binoxide of vitrogan . 10360 Chlorine. 2. .
Oxygen...» «+ 11057 Hydriodioncid.. . +

Regnault has furnished the following determinations of they
Titre of the moat important gases at 0° C. and 760 mm,
Bret. 1298187 grms, Nitrogen... 1
Oxygen. . . . 1420802 Carbouie acid, . 19
Fiydrogen . . . 0-089578

CHAPTER V.

CHANGRS OF CONDITION.

VAPOURS,

283. Fasion. ts Iaws.—Tho only phenomena of heat
we have hitherto been engaged have been those of expansion.
case of solide it is ensy to see that this expanion is limited, |

=

+~ 886" Sodium . 1 we
+=12% Rose's fusible metal. «

when

temperature at which perceptible softening occurs, and
‘whet the further elevation of temperature
‘but no precise temperatures can be gi

ad

250 ON HEAT,

The variations which take place in the ordinary atmospheric pres
have no perceptible influence on the melting point of aleianrey In
greater variations in pressure have a very appreciable effect.

Thomson found that pressures of 8-1° and 168 atmcapheres lowen
melting point of ice by 0-059 and 0-196" ©. respectively. These
justify the theoretical conclusions of Prof, J, Thomson, acco
which an increase of pressure of m atmospheres lowers the m

of ico by 0.0074 2° 0,

Tn the case of some substances, however, the melting point is h
by pressure, Thus, Hopkins has found that the melting point of wat
which at the ordinary pressure is 64°7°, is 74°7° under a pressure of
and 802° under a pressuro of 793 atmospheres; the melting po
spermaceti is raived 20° by a pressure of 705 atmospheres. Thess n
have been confirmed by Bunsen for lower pressures,

Tn general all those substances which expand on liquefying, such
sulphur, ete., have their melting point raised by increased pressturer
on the contrary, which contract on liguefying, have their melting:
lowered by increased pressure,

280) Alloys. Fiuxes.—Alloys are generally more fusible than 9
of the metals of which they are composed ; for instance, an alloy of
parts of tin and one of lead fuses at 194°. The alloy known as Toe
fusible metal, which consists of 4 parts of bismuth, 1 part of lead,
of tin, melts at 94°, and an alloy of 1 or 2 parts of cadmium with 2
of tin, d parts of lead, and 7 or 8 parts of bismuth, known as Wo
fusible metal, melts between 66° and 71° C. Fusible allays are of exten
uae in soldering and in taking casts.

Mixtures of the fatty acids melt at lower temperatures than the
acids. A mixture of the chlorides of potassium and of sodium fuses:
lower temperature than either of its constituents; the same is the em
with a mixture of the carbonates of potnss and soda, especially wi
they are mixed in the proportion of their chemical equivalents.

An application of this property i ith in the case of flare,
which are much used in metallurgical operations. They consist of
stances which, when added to an ore, partly by their | chemical action,
help the reduction of the substance to the metallic state, and, partly by
presenting a rondily fusible modium, promote the formation of a regulus

200, Latent heat.—Since, during the passage of a body from the slid
to the liquid state, the temperature remains constant until the fusion ie
complete, whatever be the intensity of the source of heat, it must be
concluded that, in changing their condition, bodies absorb « considerable
amount of hent, the only effect of which is to maintain them in the liquid
state. This heat, which is not indicated by the thermometer, is called
Tatent heat, or latent heat by fusion, an exprossion which, though not i

r3 OF 0 251
idons, is convenient from the fact of its
4ig meant by latent heat may be obtained from the

ment. If a pound of water at 80° is mixed with a
water at taro, the temperature of the mixture io 40° But ifs

much heat us will raise a pound of water through
t eee ene eu vai ent ton Senne

fusion, m certain quantity of heat
ateot, aad hence it is that the solution of a substance

and contrary
nm the solid to the liquid condition, which ‘always lowers
Phe second is the chemioal combination of the body
ia lijeld/anil-whioh, na in the case of all chemics] com-
= Cine of temperature. esprmadlniene

Jaws :—

the same pressure, solidifies at a fived temperature,

ra that of furion.
censent to the encl of the solidification, the tempera-
constant.
‘especially some of the futs, present an exception
so far that by repeated fusions they seem to undergo

which alters their melting point.
is consequence of the fact that the latent heat ab-
omies froe at the moment of solidification.

‘a4 aloshol, ether, and hisulphide of carbon, do not
Jowest known teroperature. But M. Despretz, by tho
mixture of liquid protoxide of nitrogen, solid carbonic

such as the pone prisms, erp eas

If the crystals are formed from a body ;
bismuth, the crystallisation is said to take place by the beh —

the crystallisation takes place from the alow olution
asalt, it is said to be hy the moist way, Snow, ice, and ead m
sent examples of crystallisation,

204, Retardation of the point of solidifieation.—The fry
point of pure water can be diminished by several degrees, if the
be previously freed from air by boiling and then kept in a perfect
place. In fact, it may be cooled to —15° O., and even below,
freezing. But when it is slightly agitated, the liquid soon sa
The smaller the quantity of liquid the lower the tem;
it can be cooled, and the greater the mechanical disturbance it:
without freezing. Fournct has observed the frequent occurrence of |
formed of particles of liquid matter suspended in an atmosphere wi
tomperature is 10° or even 16° below zero, “

A very rapid agitation also prevents the formation of ice. The
is the case with all actions which, hindering the molecules in
movements, do not permit them to arrange thomeelves in the i
necessary for the solid state, M. Despretz was able to lower the”
perature of water contained in fine capillary tubes to —20° without #
solidifying. This experiment shows how it is that plants in many:
do not become frozen, as the sap is contained in very fine
vessels, Finally M, Mousson has found that a powerful gm
only retards the freezing of water, but prevents its complete solidificas
tion. In this case the pressure opposes the tendency of eee
expand on freezing, and thus virtually lowers the point of solidific

If water contains salts or other foreign bodies its freezing point
lowered. Sea water freezes at — 26° to —8° O,; the ice which fo
quite pure, and o saturated solution remain. In Finland advantage i
taken of this property to concentrate sea water for the purposr
extracting salt from it. If water contains alcohol, precisely analog
phenomena are observed : the ice formed is pure, and all the aleobol is
contained in the residue,

Dufour has observed some very curious cases of liquids cooled out of
contact with solid bodies. His mode of experimenting was to place the
liquid in another of the same specific gravity but of lower melting pain
and in which it was insoluble. Spheres of water, for inatance,
in a mixture of chloroform and oil, usually solidified between =4° and

a

254

liquid up to the time at which the resistanes was overcome; that
iasued from the shell ina liquid state, but at a temperature
and therefore instantly began to solidify when the preasure wax
‘and thus retained the shape of the orifice whence it issued.

Cast-iron, bismuth, and antimony expand on solidifying Hike
and can thus be used for casting ; but gold, silver, and eopper
hence coins of these metals cannot be cast, but must be stamped with

296. Freexing mixtures.—The absorption of heat in the
Dodies from the solid to the liquid state has been used to produce
cold. This is effected by mixing together bodies which have am
for each other, and of which one at least is solid, such as water
salt, ice ond a salt, or an acid and @ salt, Chemical affinity
the fusion, the portion which melts robs the rest of the mixture of a
quantity of sensible heat, which thus becomes Intent. In many
vory considerable diminution of temperature is produced,

‘The following table gives the names of the substances mixed,
proportions, and the corresponding diminutions of temperature.
Parts Reduction of

Substances, by weight, temperature.
Sulphate of sodium . . . 8
Tiyirectioroadd >... gps 7 + Ha

ON HEAT.

Pounded'icoormow. 2 2 3
Common salt. 2... if: + +1
Sclphats ofiodiun |... 8

licie nits add: | ea Ht
Stlphate of sodium |. 1 | 6

Nibetactemmosium |. st .. #10%to = 28
Dilute nitreadd. . . .. 4

Pheaphéte ofsotium. , . . B |
Dilute nitric acid. ee |

Tf the substances taken be themselves first previously cooled downy
still more considerable diminution of temperature is ocensioned, |

Freezing mixtures are frequently used in chemistry, in physhes, and)
domestic economy, Tho portable ice-making machines which have oop
in uso during the Inst few years, consist of a cylindrical metallie ves
divided into four concentric compartments, In the central one ix plag
the water to be frozen ; in the next there is the freezing mixture, whil
usually consists of sulphate of sodium and hydrochloric acid ; 8 pound
the former and 5 of the latter will make 5 to 6 pounds of ice in am hat
‘The third compartment also contains water, and the outside one contal
some badly conducting substance, such as cotton, to prevent the influen
‘of the external temperature. The best effect is obtained when pret

a til

water be cooled, or if the tube be removed from 1}

which fills the space ab disappears, and the drop of

If, on the contrary, the bath be heated still higher, the

mercury descends below 4, indicating an increased tension.
300, Formation of vapours in a vacuum.—In the p

following experiment, F
tubes, filled with mercury, aro
in the samo trough (fig. 213).
them, A, serves aa a barometer
fow drops of water, alcohol,
are respectively introduced

jon cannot be prod
weight of the liquid, which i
finitely small fraction of the
the displaced mercury, it }
the formation of some
elastic force has depressed the
column, |

‘The experimentalso shows tha
pression is not the anme in all
it is greater in the case of alcol
of water, and greater with et]
with alcohol. We t
the two following laws for the
of vapours. |
I. Ina vacuent ail volatile Wig
instantencously converted into a
IL. Af the same temperature the vapours of different liquide ha
rent elastic forces,

For example, at 20°, the tension of ether vapour is 25 times as
that of aqueous vapour.

‘301. Saturated vapours. Maximum of tension.—When
quantity of a volatile liquid, such as ether, is introduced into a h
tube it is at once completely vaporised, and the mercurial colum
depressed to its full extent, for if some more ether be introd)
depression increases. By continuing the addition of ether, it final

ro 4

When 4 non-saturated vapour ia heated, its volume iner
that of a gas: and the number 000366, which is the

of unsaturated vapours are
those of permanent gases, and
mula: for the
of gases (161 and281) also apply
vapours, But it must not be forg
there is always a limit of m7
ing at which unsaturated vapours
state of saturation, and that they b
maximum of tension and
only be exceeded when the tempe}
while they are in contact with the,
303, Tension of aqueous vap:
zero.—For the sake of measuring
foree of aqueous vapour below
Lussac used two barometer tubes
mercury, and placed in the same
215). The straight tube, A, #
barometer ; the other, B, is bent, #
of the Torricellian vacuum can be,
by a freezing mixture (206). Wi
water is admitted into the bent tut
of the mercury sinks below that }
A, to an extent which varies witl

Fig, 215. perature of the freezing mixture,
At A Sa Hs +e 6 £60 milla
SP rh on oe

» 9 2s

‘3 a eee os

These depressions, which must be due to the tension

Ne d

the atmosphere.
the shove experiment the part B and the part C are not
in the freezing mixture, we shall presently see that when
cating vessels are at different temperatures, the tension
is the same in both, and always corresponds to the lowest

evaporates even below zero follows from the fact, that wet
to the air during frost first
ee ern own that

gee

af
1
FI
re
Be

ty
ff
?
rl
Es

|
i
rt

Pig, 216.

Dulton’s method is wanting in precision, for
-oylinder has not everywhere the same temperature, and
of the aqueous vapour is not indicated.

ami are fitted by caoutchouc collars. The tube containing
with a flask, a, by means of » copper three-way

260 ‘ON Hkat.

tube, 0. ‘The thint limb of this tubs is connected with m drying
D, containing pumice impregnated with sulphuric acid, which is.con
with the air pump. |

When the tlask @ contains some water, a small portion ix distilh
B by gontly heating the flask. Exhausting then by means of}
pump, the water disti]
tinuously from the fa
from the barometric
townrds D, which ox
tho vapours, After
vaporised some quan
water, and it is thougl
the air in the tube
drawn, tho capillay
which connects B wi
thme-way tube is
‘The tube B being thas
it is experimented wit)
Dalton’s method.

‘Tho dram MN bein
with water is. gently
by a spirit lamp, wl
separated from the tul
wooden screen. By me
stiner K all parts of th,
are kept atthe same t
ture, Inthe sideof the)
a glass window through
tho height of the mez
the tubes can be read
means of a cathetomets
the difference in these]
reduced to zero, the

Fig. 217. of vapour is deduct
moans of this apparatus, the olastic force of vapour between O*
has beon determined with accuracy.

305. Tension of aqueous vapour above one hundred deg
Two methods have been employed for determining the tension of |
vapour nt temperatures above 100°, the one by Dulong and A
1890, and the other by Regnault, in 1844.

Fig. 218 represents a vertical section of the apparatus used by
and Arago. It consisted of acopper boiler, k, with very thick ait
of about 20 gallons capacity. Two gun barrels, a, of which onl}

Beeairercapese witkenepeaned alr, m, previously
¥) and fitted into.an iron vessel, od, filled with mercury,
(the beight of the mercury in the vessel, it was connected
(we with a glass tube, #, in which the level was always the
bath. A copper tube, i, connected the upper part of the

Pig. 218,

cal tube, ¢, fitted in the boiler, ‘The tubo «, and the
bath d, were Billed with wator, which was kupt cool by
‘of cold water flowing from a reservoir and circulating
6
was disengaged from the tube ¢, exercised a pres-
of the tube #; this pressure was transmitted to the
‘morenry in tho bath «and the mercury rose in the
‘on the manometer the pressures corresponding to
‘thermometer, Dulong and Arago were able to make a
‘the tension up to 24 atmosphores, and the tension
‘atmospheres was determined by calculation.
v ‘Delow and above one hundred degrees.—
metliod by which the teneion of vapour may be
either below or above 100%, It depends on the

apparat
sealed, and about two-thirds full of water. In the cover |
thermometers, two of which just dip into the water, and

Fig. 219.

almost to the bottom. By means of a tube, AB, the retort Cis
with a glass globe, M, of about 6 gallons capacity, and full of air.
tube AB passes through a metallic cylinder, D, through which # ©
of cold water is constantly flowing from the reservoir, E. ‘Po tl
part of tho globe « tube with two branches is attached, one of
connected with a manometer, O; the othér tube, HH’, which is of les
can be attached either to an exhausting or a condensing air pu -
ing as the air in the globe is to be rarefied or condensed. The
K, in which is the globe, contains water of the temperature of th
rounding air.

a ke

264

Ta the second table the numbers were obtained by direct
up to 24 atmospheres; the others were calculated by the aid of a
of interpolation,
Tension in atmospheres from 100? to 2304P,

ON MEAT,

7 7 ¢ | een :
vil sE| § | 5
sa) 4 § Hy
£2 E 52 43)
ge 2 z i 5
2a 8 |= 2) vere
1 7 | a5 | sire) 38
ly wu 2203 | 3
a || | 7 | 3995 | 20
a | | 18 | soe7 | 3
ja | | a9 | 2268 | a6
122 | 5 20 | atao | a7
6 | 21 | 3309 “7
7

These tables show that the elastic force increases much mors rapidly
than the temperature. The law which regulates this increase is not
accurately known,

determined the elastic force nt various temperatures of « certain namber
of liquids which aro given in the following table :—

Tensions in |

Liquids, | BE itiimeters | Liquids
|_s
50°
Mercury . {| 5
100 )
| 0 Ether ..
Alcohol, 1 50
100, Sulphurous
r -20 acid
Bisulphide 0
of carbon 60
100 $529 | Ammonia {

308. Tension of the vapours of mixed Hquids.—Reguault's oxpe
riments on the tension of the vapour of mixed liquids prove that (i.) when
two liquids exert no solvent action on each other—such ss water antl

temperature ; (ji.) with water and ether, which
rr each other, the tension of the mixture is much less than
‘of the tensions of the separate liquide, being scarcely equal to
ep ete ae aie il emia ald
tand bisulphide of carbon, or water and aleohol, the tension of the
of the mixed liquid is intermediate between the tensions of the

liquids.
tins shoven that the tension of aqueous yapour emitted froma
=x compared with that of pure water, is diminished by an
|S ary acai ye salt dissolved, when
| crystallises without water or yields efflorescent crystals ; when the
or has & powerful attraction for water, the reduction
5 is proportional to the quantity of crystallised salt,

are connected with each other, the elastic force is
to the mean of the two temperatures, as would
~ Thus, if there are two globes, fig. 220, one, A,
zero by means of melting ice, the other, B, con-
*, the tension, as long as the globes are not connected,

- inthe first, and 760 millimeters in the second. But
eted by opening the stopcock, (, the vapour in the
tension, passes into the other globe, and is there
rapour in B can never reach a higher temperature

x

266 ON HEAT.

than that in the globe A. ‘The liquid simply distits from B to
without any increase of tension, 5 y
From this experiment the general principle may be deduced that
teu vessels containing the same liquid, but at dij

been made by Watt in the condenser of the steam engine,

310, Bvaporation. Causes which accelerate it.—Z)
has been already stated (298), is the slow production of vapour
surface of a liquid. It is in consequence of this evaporation #i
clothes dry when exposed to the air, and that open vessels co
water become emptied. The vapours which, rising in the atmo
condense, and becoming clouds fall as rain, are due to the
from the seas, lakes, rivers, and the soil.

Four causes influence the rapidity of the evaporation of a fi
the temperature ‘the quantity of the same vapour in the sa
ing atmosphes the renewal of this atmosphere ; iv. the ext
the surface of evaporation.

Tncrease uf temperature accelerates the evaporation by increasing
elastic forve of the vapoura.

Tn order to understand the
fluence of the second cause, it
bo observed that no
could take place in « space abn
saturated with vapour of the
liquid, and that it would reach Ii
maximum in sir completely fem
from this vapour, It therefore fol
lows that between these two
tremes the rapidity of eva
varies according a8 the su 5
atmosphere is already more oF Te
charged with the same vapour,
The effect of the renewal of #
atmosphere is similarly ex,
® for if the air or gas, which sur
* the liquid, ix not renewed, it
becomes saturated, and
conse.
The influence of the fourth
is self-evident.
811. Laws of ebullition.—Ebuillition, or boiling, is the rapidh,
of elastic bubbles of vapour in the mass of a liquid itself

AE

268 ‘ON HEAT,

Water saturated with common salt. . boils at 106
” ” nitrate of potassium. w 38

» om Shloride of calcium =, 178

Acids in solution present analogous results; but substances
mechanically suspended, such ns earthy matters, bran, wooden
wt¢,, do not affect the boiling point,

Diwolved air exerts a very marked influence on the boiling point a
water, Deluc first observed that water freed from air by ebulli
and placed in a flask with a long neck, could be raised to 112°
boiling, M. Donny found that water deprived of air and sealed up in’
long glass tube may be heated at one end as high as 188° without b
and is then suddenly and violently thrown to the other by a burat of.

When a liquid is suspended in another of tho same specific g
but higher boiling point, it may be raised far beyond its boiling
without the formation of a trace of vapour. Dufour has made a
of valuable experiments on this subject; he used in the case of
aw mixture of oil of cloves and linseed oil, and placed in it b
water, and then gradually heated the oil; in this way ebullition
set in below 110° or 116°, very commonly globules of 10 milli
diameter reached a temporature of 120° or 190°, while very
globules of 1 to 3 millimeters reached the temperature of 174

atmoepherna,
At these high temperatures the contact of a solid body, or the p
tion of gas bubbles in the liquid, occasioned a sudden v;
globule, accompanied by a sound like the hissing of a hot ro in wae
Saturated aqueous solutions of sulphate of copper, chlori
ote., remained liquid at temperature far beyond their boiling ie
immersed in melted stoaric acid. In like manner, globules of chl
(which boils at 61°) suspended in « solution of chloride of zine could
heated to 97° or 98° without boiling.
Tt is a disputed question as to ss is the temperature of the
from boiling saturated saline solutions, It has been stated by R
to be that of puro water boiling under the same pressure ; the most recent
experiments of Magnus seem to show, however, that this is not the
but that the vapour of boiling solutions is hotter than that of pure
water; and that the temperature rises as the solutions become mont
concentrated, and therefore boil at higher temperatures, Ni
apour was always found somewhat cooler than the maas of tie
boiling solution, and the difference was greater at high than at Ww
toumperatures.

270 ‘ON MEAT.

rteam above the surface of the crater; Whe: ara
produced.

Te is in consequence of this diminution of prossure that’

high mountains at lower temperatures, On Mont Blane, fo
‘water boils at 84°, and at Quito at 90°,
Os the more mpid evaporation of water under fe
‘based the use of the air pump in concentrating those solut)
either cannot bear a high degree of heat, or which eas
cheaply evapornted in an exhausted space, Mr. Howat
most important and useful application of this princip
manufacture of sugar. The syrup, in his method, is encloset
tight vessel, which is exhausted by a steam engine. Thee
consequently goeson at a lower temperature, which secures
from injury. The samo plan is adopted in evaporating tl
certain plants used in preparing medicinal extracts.

On the other hand, ebullition is retarded by inereasing tht
under a pressure of two atmospheres, for example, water or

Ve.

315, Franklin's expertment.—The influence of pressur
lition may further be illustrated by means of an exp
Franklin's The apparatus consists of a bulb, a, and
joined by « tube of smaller dimensions (fig. 223), The
drawn out, and thy
filled with water,
then in great p
away by moans )
lamp. When. it
boiled sufficiently
expol all the air
6 is sealed. The
a vacuum in the
Fig. 293. or rather there
sure due to the
aqueous vapour, which at onlinary temperatures is very en
sequently, if the bulb a be placed in the hand the heat is 5
produce a tension which drives the water into the tube 5, a
brisk ebullition,

316. Measurement of heights by the boiling pot
the emnection between the boiling point of water and the pr
heights of mountains may be measured by the thermometer
Dy the barometer, Suppose, for example, it is found that wat
the summit of a mountain “at 90°, and at its base at 98°
temperatures the elastic force or tension of the vapour ix equ

OX MEAT.

272

reproduced in the opposite order. Similar appearanees were
heating ether in a sealed tube at 190°,

Andrews has observed that when liquid carbonic acid was
SL°C. the surface of demarcation between the Hquid and the
faintor, lost its curvature, and gradunlly disuppeared, ‘The spaces

7 id, which, when the pressure was
diminished, or the temperature slightly lowered, exhibited =.
appearance of moving or flickermg stri@ throughout its
Above 30° no apparent liquefaction of carbonic anhydride, ar
into two distinct forms of matter, could be effected, even when the)
of 400 atmospheres was applied, It would thus seem that
for every liquid a temperature at which no amount of pressure i
of rotaining it in the liquid form. It is not
mere pressure, howover intense, should fall to Hquety many of!
which usually exist as gases,

318, Papin's digester.—Papin, a French physician, appears
been the first to study the effects of the production of vapour itt
vessola, The apparatus which bears his name consists of @ elit
iron vessel (fig. 224) provided with w cover, which is firmly fa
down by the acrow B, In order to close the vousel hermutically,
lend is placed between the ed
the cover and the vessel. 4
bottom of a cylindrical cavity,
traveraes the cylinder, 8, al
tubulure, 0, the cover is pert
by a small orifice in which
arod,n. This rod presses a
a lever, A, moveable nt ¢
the proasure may be

means of a weight moveable;
lever. The lever is so wel
that when the tension in t
terior is equal to 6 ntmoapher
example, the valve rises a
vapour escapes. ‘The destruc
the apparatus is thus avoide
the mechanism has hence ry
the namo of aafety valve, ‘T
gester is filled about ‘two
with water, and is heated on
nace, The water may th
raised to a temperature far

ig. 224.
100°, and the tension of the vapour increased to several atmospher

cording to the weight on the lever.

quantity of heat necessary for the evaporation of water
temperature, and is not constant, as Watt had supposed.

sented by the formula
Q = 6065 + 0:05 T

in which Q is the total quantity of heat, and T the
water during evaporation, while the numbers are con
‘The total quantity of heat to evaporate
GOBS +- (0-305 x 100) = O37, at 120" i i O49, a 1E0"
180° it is 661.
$20, Cold due to evaporation. Mercury frozen,—'
the temperature at which a vapour is produced, an absory
always takes place. If, therefore, « liquid evaporates,
yoceive from without a quantity of heat equal to that whi
in produeing the vapour,
sinks, and the cooling is g
tion as the evaporation is
Leslie succeeded in i
means of rapid ears,
receiver of the air pump is
containing strong sulphuric:
it a thin metallic capsule (fig.
ing @ small quantity of
hausting the receiver the
oll (314), and since the '
sorbed ty the sulphuric seid 3 fe
they are formed, a rapid
produced, which quickly ef
of the water.
‘This experiment is best performed by using, instead o

276 ON HEAT.
oacipel bela by evaporation is taeda hot chamstes 87 <

to work an air pump, which produces a rapid
in which is immersed the vessel containing the water to'Be Sout
apparatus is so constructed that the vaporised ether can be cond
» used again,
The cooling effect produced by a wind or draught does not neo
arise from the wind being cooler, for it may, as shown by the
meter, be actually warmer; but arises from the rapid evaporation it cum
from the surface of the skin. We have the feeling of oppression,’
at moderate temperatures, when we are in an atmosphere
moisture in which no evaporation takes place.
8204, Carré’s apparatus for freezing water.—We hay
seen that when any liquid is converted into vapour it absorbs a ¢
able quantity of sensible heat; this furnishes « souree of cold
‘more abundant the more volatile the liquid and the ge
vaporisation.
This property of liquids has been utilised by M. Carré, i
water by the distillation of ammonia. The apparatus eo
drical boiler C (figs. 226, 227) and of a slightly conical ves
is the freezer. These two vessels ary connected by a tubs
n binds them firmly. They are made of strong galvanised plat
resist # pressure of soven atmospheres,
The boiler C, which holds about two gallons, is three parts
strong solution of ammonia, In a tubulure in the upper part of the
boiler some oil is placed, and in this a thermometer ¢ i tenni=
peratures from 100° to 160°. The freezer A consists of two concentric
envelopes, in such @ manner that its centre being hollow, = metal
vessel G, containing the water to be frozen, can be placed in thie
space. Hence only the annular space between the sides of the freezer it
in communication with the boiler by means of the tube m, Im the
upper part of the freezer there is a small tubulure, which can be elased
by a metal stopper, and by which the solution of ammonia is introdweed:
The formation of ice comprehends two distinct operations. Tm the
first, the boiler is placed in « furnace F, and the freezer ina bath of
cold water of about 12°. The boiler being heated to 180° the am=
moniacal gas dissolved in the water of the boiler is disengaged, and,
in virtue of its own pressure, is liquofied in the freezer, along with

a] CARRE'S APPARATUS YOR FREEZING WATER. 277

tenth of its weight of water. This distillation of © towards
Taste about an hour and o quarter, and when it is finished the
operation commences; this consiats in placing the boiler in the

mater bath (fig. 227), and the freezer outside, caro being taken to

it with very dry flannel. The vessel G, about three quarters

. As the boiler cools, the ammonia-

. ear now distils from A towards C, to redissolve in the
remained in the boiler, During this distillation the

ent ix rarefied absorbs a great quantity of heat, which is

from the vessel G and the water it contains, Hence it is

about an hour and a quarter n perfectly
ean be taken from the vessel G.
gives about four pounds of

278 ON MEAT.

LIQUEFACTION OF VAPOURS AND GASES.

821, Liquefaction of vapours.—The liquefaction or
vapours is their passage from the uitiform to the liquid state,
sation may be due to three causes—cooling, compression, or
affinity. For the first two causes the vapours must be sstunsted
while the latter produces the liquefaction of the most rarefied
‘Thus, a lanre number of salts absorb and condense the aqueous
the atmosphere, however small its quantity.

When vapours are condensed, their Intent heat becomes freo, that
affects the thermometer. This is readily seen when a current of
at 100° is passed into a vessel of water at the ordinary te
‘The liquid becomes rapidly heated, and soon reaches 100°, ‘The q
of heat given up in liquefaction is equal to the quantity absorbed in pie)
ducing the vapour.

322. Distitation. stilis,—Disillation is an operation by which ©
volatile liquid may be separated from substances which it holds it

| =

*:

tr

j
i
4
|
U

Fig, 228.

solution, or by which two liquids of different volatilities may! ben]
sepamted. The operation depends on the transformation of Jiquids intl
vapours by the action of heat, and on the condensation of these vapours
by cooling.

‘3234. Apparatus for determining the alcoholic
—One of the forms of this apparatus consists of a glass fy

Fig. 230,

4 tripod, and heated by a spirit lamp (fig. 230), By means
choue tube this is connected with a serpentine placed in a eo
filled with cold water, and below which is a test-glass for
distillate, On this are three divisions, one @, which mea:
tity of wine taken; the two others indicating one-half an
this volume.

The test-glass is filled with the wine up to a, this is
tho flask, which, having been connected with the serp
tillation iscommenced. The liquid which distils over is a |
alcohol and water; for ordinary wines, such a3 clareta and
one-third is distilled over, and for wines richer in spirit, such,
and ports, one-half must be distilled ; experiment has shown
these circumstances all the alcohol passes over in the disti
measure is then filled up with distilled water to a; this gives a m
of alcoho! and water of the same volume as the wine taken, free
solid matters, such as sugar, colouring matter, and acid, but ea
all the alcohol. The specific gravity of this distillate is then |

~

LIQUEFACTION OF Varours 281
of an aleoholometer (119) and the number thus obtained corre-

preparing and collecting them over

reury or water, it occasionally happens that these liquids rush back

itt the generating vessel, and destroy the operation. This arixes from

of atmospheric pressure over the tension in the vessel. Ifa gas,

pas acid, for example, bo generated in the flask m (fig. 231), and

‘into water in the vessel A, as long as

gmt is given off freely, its tension exceeds 4
pressure and the weight Z

by more than the weight of the
‘of water co, the water rises into the
and the operation is spoiled, This accident is prevented by means
safety tubes.
These are tubes which prevent absorption by allowing air to enter in
ortion as the internal tension decreases. The simplest is a tube Co

as the distance or is less than the height DH, air enters by the spe
9, before the water in the bath can rise to A, and no absorptiot tl
place.

Fig. 233 represents another kind of safety tube, Tt has « bulb
taining @ certain quantity of liquid, as does also id, When the
‘of the gas in the retort M exceeds the atmospheric pressure, (he let
tho leg id rises higher than in the bulb, a; if the gas hna the t
one atmosphere, the lnvel is the same in the tubs as in the bulb.
if the tension of the yas is less than the atmospheric pressure the |
sinks in the leg of; and, as care is taken that the height sa is beast
BA, a8 soon ax the air which enters through o reaches the curved part!
raises the coluran f@, and passos into the rutort before the water |
cylinder can reach 6; the tension in the interior is then equal t
exterior, and no absorption takes place,

825. Liquefaction of gases.—We have already seen that
vapour, the temporature of which ix constant, is liquefied by

the pressure, and that, the pressure remaining constant, it is b

the liquid state by diminishing the temperature.

‘Unsaturated vapours behave in all respects like gases. Ard it ix:
to suppose that what are ordinarily called permanent gases are
saturated vapours. For the gaseous form is accidental, and is not
in the nature of the substance. At ordinary temperatures 61
acid is a gas, while in countries near the poles it is a liquids im temp
climates ether is a liquid, at a tropical heat itis gas, And just mle
saturated vapours may be brought to the state of saturation aad
liquefied by suitably diminishing the temperature or inci 7
presmre, 50 by the same means gasos may be liquefied. But ai
are mostly very far removed from this state of saturation greak
and pressure are required. Some of them may indeed be i
cither by cold or by pressure; for the majority, however, both
must be simultaneously employed. Few gases can resist Ol
ined actions, and probably those which havo mot yet Teen Hiqualiedy
hydrogen, oxygen, nitrogen, binoxide of nitrogen, and earbonie oxi
would become so if submitted to a sufficient degree of cold

ure,

Faraday was the first to liquefy some of the gases, THis method
consists in enclosing in a bent glass tube (fig. 234) substance By
whose chemical action the gas to be liquefied is prodveed, and
sealing the shorter leg. In proportion as the gas is dis
Prossure increases, and it ultimately liquefies and collects in the sherter

ree sbSheg ==

284 O8 HEAT.

the liquid, for it is closed below by a valve, S (fig. 285). In
collect some of the liquid ges the reservoir is inverted, and on |
the stopeock r, the liquid escapes by a small tubulurs x.

When carbonic acid has been liquefied, and is allowed to escape
alr, a portion only of the liquid volatilises, in consequence of the la

=a
Fig, 236.

absorbed by this evaporation, the rest is 40 much enoled ns to solidify
white flakes like snow or anhydrous phosphoric acid.

Solid carbonic acid evaporates very slowly. By moans of an
thermometer its tempersture has been found to be about -90% A

by Gay-Lussac, Tt consists of ag]
A (fig. 287), to which two stopeod)
d, are cemented. The lower
vided with a tubulure, which
tube A with a tube B of smaller ¢
A seale between the two tubes 4

measure the heights of the 9
columns in these tubes.

The tube A is filled with mere
the stopcocks 6 and d are closed.
globe, M, fled with dry alr or
gas, is screwed on by means of a
in the place of the funnel a4

placed by the dry air of the
stopcocks are then closed, and as 4
‘the tube expands on leaving the @
pressure on it is leas than that of #
sphere. Mercuryis accordingly po
the tube B until it is at the =
in both tubes. The globe is then removed, and replaced by #{
provided with x stopcock a, of « peculiar construction. It is n
rated, but has a small cavity, as represented inn, on the left of
Some of the liquid to be vaporised is poured into ©, and the heig
mercury, k, having been noted, the stopcock & is opened, and at
that its cavity becomes filled with liquid; being again turned, ¢
enters the space A and vaporises, ‘The liquid is allowed to fall
drop until the air in the tube is saturated, which is the casey
level & of the mercury ceases to sink (301).
As the tension of the vapour produced in the spnce A is adda
of the air already present, the total volume of jas is increased,
easily be restored to its original volume by pouring mercury

the pressure,
‘To solve the first part of the problem we
dry air at 0? and the pressure 700 millimeters

quently V cubic inches of dry sir weigh
O83 we
“a a0
To obtain the weight of the vapour, the weight
dry air at the same temperature and pressure rep ynes
{a to bo multiplied by the relative, density of the
cubic inches of dry airat @, and the pressure F,
Veubie inches of aqueous vapour, whoso density i
(388), weigh

O31 x VF 5
+7 *B* =?
I sum of the weights

oy OS) x V (HF). 081 x VF Y j=
Oe Tw Lata 700* BIO ea)

SPHEROIDAL CONDITION,

‘820, Deidentrost’ henome: ‘Boutigny’s
When liquids are ake upon sscabdeatant mata

have since then beon studied by other physicists, and’
by M. Boutigny, to whom our it ken of
‘he present knowledge

- 62S

paleo Hust

200 ‘ON BEAT,

the same fset. A polisbed silver dish is made red hot, ands
‘of w solution of sulphide of sodium are projected om it
passes into the spheroidal condition, and the
alteration. But if the dish is allowed to cool, the
moistens it, producing a dark spot, due to the formation
silver. In like manner nitric acid assumes the
projected on a hoated silver plate, and does not attack the met
‘&s the plate remains hot,

Similarly liquids may be made to roll upon Liquids, and nd
which vaporise without becoming liquid also assume a conditi
gous to the spheroidal state of liquids when they aro placed on
whose temperature is sufficiently high to vaporise vaporise them rapid
en ie a ee
platinum crucible,

‘The phenomena of tho spheroidal state seem to prove that |
globule rests upon a sort of cushion of its own vapour,
heat radiated from the hot surface against its under side. |
this vapour escapes from under the globule, its place ix
frosh quantity formed in the same way, so that the globule is)
buoyed up by it, and does not come in actual contact with t
surface. When, however, the temperature of the latter fall)
mation of vapour at the under surface becomes less and less
at length it js not sufficient to prevent the globule tomchin
metal or liquid on which it rests, As soon ascontact occurs heal
imparted to the globule, it enters into ebullition, and quickly

‘These oxperiments on the spheroidal state explain the fi
hand may be dipped into melted lead, or even melted irom,
jury. It is necessary that the liquid metal be heated
solidifying point. Usually the natural moistare of the hand it
but-it is better to wipe it with a damp cloth. In
great heat, the hand becomes covered with « layer of :
which prevents the contact of the metal with the band. Ra
alone operates, and this is principally expended in formin
vapour on the surface of the hand, If the hand is immersed
wator, the water adheres to the flesh, and consequently ant
duced,

‘The tales of ordeals by fire during the middle ages, of men)
run barefooted over red-hot iron without being injured, a
true in some cases, and would find a ready explanation in the
phenomena.

oan |

o's method.—The density of « vupour is the relation
‘weight of a given volume of this vapour and of that of the

by the graduation on the jar. Its temperature is

the pressure is indicated by the

the barometer st the time of the

column of mercury in the gas jar. It

the weight of a volume of air equal

‘quotient, obtained by dividing the weight of the vapour
oir, density of the vapour.

ri groins, v its volume in cubic

¢ height of the barometer, and

292 8 HEAT.

A that of the mercury in the gas jar, the preasure on the vapo
HA,

It is required to find the weight p’ of a volume of air x, at
perature ¢, and under « pressure H—A, At zero, under thy
700 millimeters, a cubic inch of air weights O31 grains; con
under the same conditions v cubic inches will weigh O81 © gril
therefore the weight of » cubic inches of air, at © and the pr
millimeters, is es

ey grains (282, prob. ii}.

As the weight of a volume of air is proportional to the pn

above weight may be reduced to the pressure H—A by mult

HA which gives

a1 » (—A)
“Us af) 700°
for the weight p’ of the volume of air v, at the preasure H— A
Consequently, for the desired density we hare
p= aha = pil + ot) 700
~ Ose Ct — A)

331, Dumas’ miteaeine method just described cannot
to liquids whose boiling pointe)
or 160°. In order to raise the
cylinder to this temperature it,
necessary to heat the mercury
degree that the mercurial vapt
be dangerous to the operat,
moreover, the tension of the
vapours in the graduated jar wou
the tension of the vapour of the
so far vitinte the result.

The following method, d
M. Dumas, can be used up
perature at which glass begins
that is, about 400. Ag
is used with the neck draven
fine point (fig. 239). The glo
been dried externally and. in)
weighed, the temperature ¢ and
height A being noted. This w
the weight of the ee G in addition to p, the weight of the
taing, The globe is then gently warmed, and its point in

204

‘The density of aqueous vapour, when a
all temperatures 4, or, more uccurutely,
the same temperature and pressure,

S382. Deville and Troost's method.—Deville and Trost
fied Damas’ method ¢o that it can be used for determining
density of liquids with very high boiling points. The globe ie
an iron cylinder in the vapour of mercury or of sulphur, the temps
of which are constant respectively at 350° and 460°. In other m=
the determination is the same as in Dumas’ method.

For determinations at higher temperatures, Deville and
employed the vapour of zinc, the temperature of which fs
glass vessels are softened by this heat, they use porcelain
finely drawn out necks, which are sealed by means of the
flame.

833, Relation between the volume of a liquid and
vapour.—The density of vapour being known, we ean readily
the ratio between tho volume of a vapour in the satursted
given temperature, and that of its liquid at zero. We may
example, the relation between water at zero and steam at 100%

The ratio between the weights of equal volumes of air
the normal barometric pressure, and of water under the
stances, is as 1: 773. But from what bas been already said
density of air at zero is to its density at 100? as 1 +
ratio between the weights of equal volumes of air at 100? na ¥
O10 romana x10" 773, or 073178 + 778.

Now from the above table the density of steam at 100° Oya)
normal pressure, compared with that of sir under the same eireuma
is as 06225: 1, Honce the ratio between the weights of equal
of steam at 100°, and water at 0°, is

O-73178 x 06: : 778 or 04555 ; 773 or 1 > 1608,

Therefore, as the volumes of bodies are inversely as their dl
one volume of water at zero expands into 1698 volumes of steam)
©, The practical rule that cubic inch of water yields a cubic}
steam, though not quite accurate, expresses the relation in @ com)
form.

~ ON HEAT,

296 ‘ON HEAT.

If fia the actual tension of aqueous vapour in the
saturated vapour at the same temperature, and E the

we have E= f, whence f =P x E.

As a consoquence of this second definition, it is important to
that the temperature having varied, the air may contain the
quantity of vapour and yot not have the same bygrometric state. |
when the temperature rises, the tension of the vapour which th
would contain if saturnted increases more rapidly than the
the vapour actually present in the atmosphere, and hence the
‘between the two forces, that is to say, the hygrometric state,
smaller,

It will presently be explained (313) how the weight of the
present in a given volume of air may be deduced from the
state,

336, Different kinds of hygrometers.—Hyyrometers are
ments for measuring the hygromotric state of the air, There
numerous varieties of them—chemical hygrometers, condensing bygro=
meters and psychrometers.

337, Chemical bygrometor.—The method of the chemical hygr=
meter consists in passing a known valume of air over « stibstance which
readily absorbs moisture—chloride of calcium, for instance. ‘The sib
atance having been weighed before the passage of the alr, and thea
afterwards, the increase in weight represents the amount of
vapour present in the air. By means of the apparatus represinied if
fig. 240, it is possible to examine any given volume. Two brass resem
yoirs A and B, of the same size and construction, act alternately a
aspirators, by being fixed to the same axis, about which they can tir,
They aro connected by a central tubulure, and by means of two teibulares
in the axis the lower reservoir ia always in connection with the alia
sphere, while the upper one, by means of a choutchouc tube, is connected,
with two tubes M and N, filled either with chloride of
with pumice stone impregnated with sulphuric acid. ‘The first
the vapour in the air drawn through, while the other Mf
yapour which might diffuse from the reservoirs to the tube N.

‘The lower reservoir being full of water, and the upper one of ait,
apparatus is inverted so that the Hquid flows slowly from A to By
vacuum being formed in A, air enters by the tubes NM, in the fimt of
which all the vapour is absorbed. When all the water lias ram into IP
it is turned ; the same flow recommences, and the same volume of alr i
drawn through the tube N, Thus, if each reservoir holds a gallo, S08 |
example, and the apparatus bas been turned five times, 5 gallons of air
have traversed the tube N, and have been dried. If then, before the

ae

7 nyGRoweTRY. 207

fiment, the tube with ite contents has been weighed, the incrrase
ives the ee ee ereeimein ellos ae

& sae eer cce SW eee a body gradually cools

moist atmosphere, the layer of air in immediate contact with
tls also, and a point is is ultimately reached at which the vapour
bot ie just sufficient to saturate the air: the least diminution of
feature then causes a precipitation of moisture on the body in the
of dow. When the temperature rises agnin, the dew disappears.

Fig. 240.

me of these two temperatures is taken ox the dew point, and the

of condensating hygrometers is to observe this point. Daniell’s

fgnanlt’s hyzrometers belong to this class.

Daniell’s bysrometer.—This consists of two glass bulbs at the
‘of @ gins tube bent twice (fig. 241), The bulb A is two-
of ether, and a vory delicate thermometer plunges in it; the

foo contains nothing but the vapour of ether, the ether

jbeen before the bulb B was sealed. The bulb B is covered
‘and ether is dropped upon it. The ether in evaporating
bulb, and the vapour contained in it is condensed. The

Taeies being thus diminished, the ether in A forms vapours

o}

= Se

which condense in the other bulb B, In proportion as the liquid
from the lower to the upper bulb, the ether |

ture of this point is noted by mesa
‘the thermometer in the inside

render the deposition of dew more
coptiblo, the bulb A is made of Bleck
glass,

These two points having bem der
termined, their mean is taken =
that of the dew point. ‘The tempera
ture of the sir at the time of the
experiment ia indicated by the there
mometer on the stem. The

f, corresponding to the temperature of the dew point, is
found in the table of tensions (306). This tension is exactly that of)
the vapour present in the air at the time of the experiment, The
tension F of vapour saturated at the temperature of the stmoaphen
is found by means of the samo table; the quotient obtained by
dividing f by F, represents the hygrometric state of the air (5),
For instance, the temperature of the air being 15°, suppose the dew:

is 6°. From the table the corresponding tensions are f= 6634 milli=
moters, and F = 12-699 millimeters, which gives 0-614 for the ratio aff
to F, or the hygrometric state.

‘There are many sources of error in Daniell’s hygrometer. The
principal are: Ist, that us the evaporation in the bulb A only cool
the liquid on the surface, the thermometer dipping om it does met”
exactly give the dew point; 2nd, that the observer standing mear the
instrument modifies the hygrometric state of the surrounding sir, a
well as its temperature; the cold ether vapour too flowing from the
upper bulb may cause innecuracy.

340. Rognauit's hygrometor.—Regnault's hygrometer is free from
the sources of error incidental to the use of Daniell’s. Lt consists of two

ables 175 inch in height, and 0-76 inch in

are fixed two glass tubes, D and E, in each

a thermometer. A bent tube, A, open at both ends, passes
‘the cork of the tube D, and reaches nearly to the bottom of
‘There is a tubalure on the side of D, fitting in a brass
forma a support for the apparatus. The end of this tube
with an aspirator G. The tube E is not connected with
; its thermometer simply indicates the temperature of the

tube 1D) is then half filled with ether, ond the stopcock of the
‘The water contained in it runs out, and just as much

| Fig. 242,

he
fas tiaras the the tube A, babbling through the ether, and causing
‘This evaporation produces a diminution of temperature,

on the silver just as on the bulb in Daniell’s
the thermometer T is then instantly to bo read, and the
|. The dew will soon disappear again,
to be rend; the mean of the two
+: the thermometer ¢ gives the corresponding temperature
thence there are all the elements neosssary for calculating

400 ON HEAT.

As in this instrament, all the ether is at the same teespers
consequence of the agitation, and the temperatures are reed ¢
distance by means of w talescope, the souress of error in Dusiel
meter are avoided.

A much simpler form of the apparatws Pe |
common test tube containing a depth of 14 inch of ether, The
provided with a loosely fitting cork in which is n delicate thert
Sd aatrew teiiabe dipping in the ether, On blowing thro!
ether, by a caoutchouc tube of considerable length, a diminution
perature is caused, and after a little practice the whole proces!
conducted almost as woll as in Regnault’s complete instrumet
temperature of the air is indicated by a free thermometer,

S41, Paychrometer, Wet bulb hygrometer—A moi
covaporates in the air more rapidly in proportion as the air is dele
consequence of this evaporation the temperature of the body sin}
psychrometer or wet bulb hygrometer is based on this principle,

plication of which, to this purpose, was fizst «
by Leslie. The form usually adopted in this et
due to Mason, It consists of two delicate thera
placed on s wooden stand (fig. 243), One of the
covered with muslin, and is kept continually}
being connected with a reservoir of water by mi
string. Unless the air is saturated with moisture
bulb thermometer always indicates a lower
than the other, and the difference between thé int
of the two thermometers is greater in proportion!
can take up more moisture. The tension e of the
vapour in the atmosphere may be calculated |
indications of the thermometer by means of thy
ing empirical formula :
e = ¢—0-00077 (t—t’) A,
in which ¢” is the maximum tension
temperature of the wet bulb thermometer, Ais}
metric height, and ¢ and ¢’ the respective tem!
of the dry and wet bulb thermomoters. If, for
hh =760 millimeters, ¢= 15° ©., “= 10° ©.; sea
the table of tensions (806), ¢' = 9-165, and we
e= 9165 — 000077 x 56x 750 = 627}
his tension corresponds to a dew point of about 4°5° 0. If th
een saturated, the tension would have been 12-609, and the air
fore about half saturated with moisture,
This formula expresses the result with tolerable accuracy,

S02 ON HEAT. |

the property which organic substances have, of elongating when
and of again contracting as they become dry. The most common
the Aatr or Sawaswre's J

It consists of @ hrass frame (fig. 244), on which is fixed
fastened at its upper extremity in a clamp, a, provided with «a
This clamp is moved by a screw 6. Tie lov
of the hair passes round a pulley, 0, and ey
small weight, p. On the pulley there is a
which moves along « graduated scale. Wl]
hair becomes shorter the needle rises, whe
comes longer the weight p makes it sink.

‘The seale is graduated by calling that pal
atwhich the needle would stand if the air we
pletely dry, and 100 the point at which it #
air completely saturated with moisture, 1
tance between these points is divided into 1(
degrees.
Regnault has devoted much stady in (
render the hair bygrometer scientifically us|
without success. And the utmost that can be
for it is that it can be used as a Aygroscope; th
instrument which shows upproximately whe
air is more or Jess moist, without giving any
tion as to the quantity of moisture present.
Fig. 244, lass belong the chimney ornaments, ono of {

common forme of which is that of a small »
female figure, 40 arranged in reference to a little house, with t
that when it is moist the man goes out and the woman goes fm, }
versi when it is fine. They are founded on the property which
strings or pieces of catgut possess, of untwisting when moist
twisting when dry.

‘As theso hygroscopes only change slowly, their indications an
Dehindhand with the state of the weather; nor are they, moreo}
exact.

343. Probiem on hygrometry.—To calculate the weight
yolume of moist air V, the hygrometrie state of which is E, |
perature ¢, and the pressure H, the density of the vapour being |
air.
From the second law of the mixture of gases and vapours, i
seen that the moist air is nothing more than a mixture of Veab
of dry air at ?, under the pressure H minus that of the vapour,
cubic inches of vapour at f° and tho tension given by the hyg
stato; these two values must, therefore, be found separately.

‘ap = lp’ and in the second aP = bp, whence p = P. Req
p by their value deduced from the above quantities, we ha

» [p—9i9) =n[i-
=n Sait ie
ey

whenee

which solves the problem,

| CHAPTER VII,
CONDUCTIVITY OF SOLIDS, LIQUIDS, AND

from 4 fire or other source of eat we experience the
‘The heat is not transmitted by the intervening mir; it

same manner, The heat, which, as in this case, ia t

_

306 ON HEAT.

‘This Jaw, however, only prevails in the case of very good
such as gold, platinum, silver, and copper; it is only
true for iron, zinc, lead, and tin, and does not apply at all to
metallic bodies, such as marble, porcelain, etc.

Taking the conducting power of gold at 1000, Despretz has
structed the following table of conductivities :

Silver... Pe lee Ne
Copper. 807 Marble soe ae
Tron. . 874 Poreelain a
Fine ee: 2 :

Wiedemann and Franz have made some valuable investigations a
the conductivity of heat in metal, By making cavities in the bars, as
in Despretz's method, their form is altered, and the continuity partially
destroyed. Wiedemann and Franz have avoided this source of exrar ip
measuring the temperature of the bars in different places by applying 8 |
them the junction of a thermo-electric couple,

The metallic bars were made as regular as possible: one of the ends |
was heated to 100°, tho rest of the bar being surrounded by air at acom= |
stant temperature, The thermo-electric couple was of small dimensidm
in order not to extract too much heat.

By this method Wiedemann and Franz obtained results which dillie
considerably from those of Despretz. Representing the conduetivity of
silver by 100, they found for the other metals the following numbers:

—~ al

808

Jong test tube is half filled with water and some ioe #0 placed.
that it cannot riss to the surface, By inclining the tabs and heat
surface of the liquid by means of a spirit Inmp, the liquid at
may be mado to boil, while the ice at the bottom remains
made a series of experiments with an apparatus

that which has been described, but he maintained the liquid im
vessel A, at constant temperature, and arrnnged a series of
one below the other in the vessel D, In this manner he found
conductivity of heat in Liquids obeys the same laws as in solids,
much more feoble. For example, the conductivity of water ie.
of copper,

$49. Manner in which Hquids are heated.—When «
Tiquid is hented at the bottom, ascending and descending
produced. It is by these that heat is mainly distributed
liguid, and not by its conductivity. These currents arise front the

Fig. 247, Fig. 248, |

|
pansion of the inferior layers, which, becoming leas denso, rise in the |
Tiquid, and ore replaced by colder and denser layers, They may be madé |
visible by projecting bran or wooden shavings into water, which rise
and descend with the currents. The experiment is arranged as shown itt
fig. 248. The mode in which heat is propagated in liquids and in asst
in said to be by convection.

850. Conductivity of gases.—It is a disputed question whether gases
have a true conductivity; but certainly when they are restrained in thelr |
motion their conductivity is very small. All substances, for instance,
between whose particles nir remains stationary, offer great resistance 12

312 O8 HEAT.
temperature is found to be one-fourth of what it was inthe
case.

The truth of the second law also
from the geometrical principle that
face of a sphere increases as the square
redius, Sappose a hollow sphere, ab
250), of any given rading and a
# heat, O, in its centre ; each unit of
the interior receives a certain
hoat. Now, a sphere, ¢f; of doable the
will present a surface four times as
SRG internal surface contains, therefore, fotit
alec as many units of surface, and as the
of heat emitted is the same, each unit will receive one-fourth the:
The third law is demonstrated by means of the following
devised by Leslie: a flat cylindrical tin canister, mn, is placed
concave mirror (fig. 261). The box tarns on u horizontal axis:
a tubulure at the top, by which it may be fillod with hot water, ama]

Fig. 251.

anterior face is covered with lampblack. Between this box and)
mirror there are two screens with equal apertures, H and K, ao a
allow a pencil of parallel rays to fall on the surface of the mirror

A differential thermometer having been placed in the focus of
mirror the canister is adjusted in the vertical position represented
the dotted lines, and is kept in that position until the thermometer
become stationary, The canieter is then inclined in the position mm

$u4

or heating of a body: The quantity of heat lost or gained by #
second it proportional to the difference betsceen its temperature
the surrounding medium. Dulong and Petit have proved thst
‘not so general ss Newton supposed, and only applies where the:
of temperature do not exceed 15° to 20°. Beyond that, the
heat lost or gained is greater than that required by this law.

‘Two consequences follow from Newton's law :

i, When a body is exposed to a constant source of heat, its
ture does not increase indefinitely, for the quantity which it
the same time is always the same; while that which it loses,
with the excess of the temperature over that of the surrounding’
Consequently a point is reached, at which the quantity of
is equal to that absorbed, and the temperature then remains

fi, Newton's Inw, as applied to the differential
that its indications am proportional to the quantities of beat
receiver. If one of the bulbs of a differential thermometer
of heat from a constant source, the instrument exhibits first ina
temperatures, but afterwards becomes stationary. In this ca
quantity of heat which it receives is equal to that which it emite
the latter is proportional to the excess of the temperature of th
abore that of the surrounding atmosphere, that is, to the num
ogrees indicated by the thermometer; consequently, the tempi
indicated by the differential thermometer is proportional to the qt

of heat it receives.

REFLECTION OP HEAT.

857. Laws of reflection.— When thermal rays fall upon a bol
are, speaking generally, divided ia
D parta, one of which penetrates th
while the other rebounds sa if a
from the surface like an elastic oy
is maid to be reflected
If mn be a plane reflecting |
252), CB an incident ray, BD a tine)
dicular to the surface called the nora
BA the reflected rey; the angle |
called the angle of incidence, and DBA the angie of reffectian
reflection of heat, like that of light, ia governed by the two fa
laws: . |
L. The angle of reflection is equal to the angle of incidence,
Il. Both the incicent and the reflected ray are in the same plame
normal to the reflecting suxface.

Fig. 262.

316 1 ON BEAT.
placed at F, the heat is propagated along the lines FKE,

which is known as the experiment of the conjugate

not only the existence of the foci, but also the laws of
reflectors, M and N (tig. 204), are arranged at a distance of 4:
and so that their axes coincide, In the focus of one of them,
aamall wire basket containing a red-hot iron ball In the
other is placed B, an inflammable body, such aa gun-cotton or
‘The rays emitted from the focus A are first reflected from the
in a direction parallel to the axis (358), and impinging on

murror, N, are reflected so that they coincide in the focus B. n
is 20, is proved by the fact that the gun-cotton in this point tal
which is notrthe case if it is above or below it.

‘Tho experiment also serves to show that light and heat arom
in the same manner. For this purpose a lighted candle ia placed
focus of A, and a ground glass scroen in the focus of B, when #
nous focus is seen on it exactly in the spot where the gun-cotton}
Henes, the luminous and the calorific foci are produced st tht
point, and the reflection takes placo in both cases according to thi

_— d

always kept at 100°, but it is found that the temperature
tho thermometer aries with the nntue ofthe plate, This methed

‘a means of determining, not the absolute reflecting power of #

its power relatively to that of some body taken as a standard
parison. For, from what has been said on the application of Ni
law to the differential thermometer, the temperatures which this
ment indientos are proportional to the quantities of heat
ceives, Hence, if in the above experiment, a plate of glass exn}
temperature to rise 1°, and a plate of lead 6° it follows thi
quantity of heat reflected by the latter is six times ax grent ast
flected by the former. For the heat emitted by the source:

same, the concave reflector receives the same portion, and the
can only arise from the reflecting power of the plates a,

ba d

820 ON HEAT.

experiments, howevor, the relation of the absorbing power ¢
deduced from that of the temperatures indicated by the then
for Newton's law is not exactly applicable in this case, as it)
vails for bodies whose substance does not vary, and here the co
the bulb varied with each observation, But we shall press
(385) how the comparative absorbing powors may be deduced
ratios of the emissive powers,

Twking asa source of heat a canister filled with water
Melloni found by means of the thermo-multiplier the following
absorbing powers :

Lampblack . . . . . .100 Indionink, . . . +
White lead 2 2... £100 Shellac. ©. s+ +
Tsinglss . 5. 1 ss OL © Metals. 2. ws

364, Radiating power.—The radiating or emissive power of
its capability of emitting at the same temperature and with |
extent of surfaco greater or Jess quantities of heat,

‘The apparatus represented in fig. 255 was aleo used by Leslie
mining the radiating power of bodies, For this purpose the ba
thermometer was placed in the focus of the reflector, and
the canister M were formed of different metals, or covered
substances, such as Inmpblack, paper, ete. The cube being:
hot water at 100°, ant all other conditions remaining the
tumed ench face of the cube successively towards the
noted the temperature each time. That face which was
Jampbinck eaused the greatest elevation of temperature, and
faces the least. Applying Newton's law, and rep
emitted by lampblack as 100, Leslie formed the fol
radiating powers:

Lampblack, . . . . . . 100 Isinglams 2. 2

Whito lead... . , .100 Tarnished lead }
Papers... .. . . 98 Mercury a
Scaling wax... . . . 95 Polished load. . . «
Ordinary white glass. . . 90 Polished iron, . . ~
Indinn ink . . . . . . 88 Tin, gold, silver, copper

Tt will bo soon that, in this tablo, the order of the bodies
the reverse of that in the table of reflecting powers.

The radiating powers of several substances were de
Melloni by means of tho thermo-multiplier, and more
Destins and De la Provostaye, who used the same
avoided certain sources of error incidental to previous mi
found in this manner the following numbers compared with
as 100:

the sides of the globe
as cooled to xcro; the

eral which face the right, are coated with
‘of © and A, which faco the loft, are either painted
vrs

2

fllal'with hot water, ite white face radiates
Brad ip bed kee seeks Maan Rae a
circumstances ns ah artis eee e
the two reservoirs are at the same temperaturs. On the
greater emissive power of the black fice of A ix
smaller absorptive power of the white face of ©; while, on a
hand, the feebler radinting power of the white fuce of A iso
if ahem tlorped Biles oacbess in:

‘The experiment say Beil by replacing the two white

radiating powera—As the radiating and absorbing powers are #
any cause which affects the one affocts the other also. And
reflecting power varies in an inverse manner, whatever it st
diminishes the radiating and absorbing powers, and mice eersd,

Tt has been already stated that these different powers
different bodies, and that metals have the greatest reflecting
lampblack the feeblest. In the same body these powers are
‘the dogres of polish, the density, the thickness of the t
the obliquity of the incident or emitted rays, and, lastly, by the ni
of the source of heat.

It has been assumed usually that the reflecting power inen
the polish of the surface, and that the other powers diminish
But Metloni showed that by scratching « polished metallic
reflecting power was sometimes diminished and sometimes
This phenomenon he attributed to the greater or loss density
flecting surface. If tho plate had been originally hammered,

geneity would bo destroyed by this process, the molecules woul
closer together on the surface than in the interior, and the mall
power would be increased. But if the surface is seratched the
and less dense mass becomes exposed, and the reflecting power |
nished. On the contrary, in a plate which has not been hammers’
which is homogencous, the reflecting power is incrensod when the
scratched, because the density at the surface is increased by the

The experiments of Leslie, Rumford, and Melloni farther
the thickness of the radiating substance also modifies its
power. The latter philosopher found that when the faees of
filled with water at a constant temperature were varnished, the
sive power increased with the number of. layers up to 16 layer

that above that point it remained constant, whatever the number,
caleulated that the thickness of the 16 layers was 004 of a milli

— 4

S24 (ON BEAT.

now generally thon of he nature of hat and of the mode
propagated. For additional information the chapter on the
‘Theory of Heat and the book on Light should be read. Aco
what is called the mechanical or dynamical theory of heat, w

is one which takes up and transmits the motion with
even through a bad conductor the propagation of this motion
tively slow ; how then are we to explain the instantaneous
heat experienced when a screen is removed from a fire or whel
ia drifted from the face of the sun? In this case, the heat passes,
body to another without affecting the temperature of the medit
transmits it. In order to explain these phenomena it isi
‘space, the interplanetary spaces a4 well as the interstices in the
crystal or tho heaviest motal, in short, matter of any kind, is p
by a’medium having the properties of a fluid of infinite venuity ¢
ether. The particles of n heated body being in a state of intens
vibration, communicate their motion to the ether around them,)
it into a system of waves which travel through space and pass
body to another with the velocity of light. When the undal
the ether reach a given body, the motion is again delivered 1
particles of that body, which in turn begin to vibrate, that i,
becomes heated. This passage of motion through the hyp
is termed radiation, and a so-called ray af heat is merely the
of the motion of one sories of waves.

Te will facilitate the understanding of this to consider thet
modo in which sound is produced and propagated. A.
one whose entire mass is in a state of vibration; the m¢
the rate of vibration, the more acute the sound; the wis |
of vibration, the deeper the sound. This vibratory motion
cated to the surrounding air, by means of which the peter’
the auditory nerve and there produce the sensation of sound. 1
ball be heated, say to the temperature of boiling water, we cam
‘that it radiates heat, although we cannot see any luminosity, 4
‘temperature be gradually raised wo see it become successively
rod, bright red, and dazzling white. Here it is assumed th}
particular temperature the heated body emits waves of a definit
in other words, its particles vibrate in a certain period. As its,

a 4

0, conses, five
about as distant from R as the latter is from V; that is,

first their

If in the above case prisms of other materials than rock
the position of maximum heat will be found to vary with the
‘the prism, a fact first noticed by Seebeck. Thus with a
water it is in the yellow; with one of crown gluss, in the n

highly important results, “His mode of
‘The electric light was produced between charcoal

Tens, was caused to pass through a narrow slit, and then u
Jens of rock salt; the elices of white light thus obtained being:
hy apriam of the same material. ‘To investigate the thermal

‘was 4 slit that could be narrowed to any extent. The
mounted on a moveable bar connected with a fine screw,
tuming a handle the pile could be pushed forward through

of the electric light, the heating effocted at various:
other was determined by the indications of a very delic

oe

RADIANT HEAT. 827

the case of the solar spectrum, the beating effect gradually in-
from the violet end towards the red, and was greatest in the dark
Be beyond the red. The position of the greatest heat was about as
the limit of the visible red as the Latter was from the green, and
otal extent of their visible spectrum was found to be twice that of

ig- 268,

he increase of tomperature in the dark space is very considerable.
mal intensities are represented by perpendicular lines of propor-
Tength erected at those parts of the spectrum to which they

greatly in eats reach a maximum, and then
iy. If these lines are connected they

Tigh fg. represents
light from the experiments of Miller with a rock salt pris,
lower curve represents the results obtained with the use of a!
prism, which is thus soon to absorb some of the ultra red rad

Prof, Tyndall found that by interposing various substances,
cially water, in certain thicknesses, in the path of the electric Tigh
ultr-red radiation was greatly diminished, the peak was not
Now aqueous vapour would, like water, absorb the Checite Ti A
most probably the reason why the obscure part of the
solar light is not a0 intense as in the case of the electric light, is tt
obscure rays have been partially absorbed by the aqueous vapour
atmosphere. If a solar spectrum could be produced outside the att
aphere, it doubtless would give a spectrum more like that of the:
light, which is uninfluenced by the atmospheric i

This has been remarkably confirmed in other ways. Meloni 0
that the position of the maximum in the solar spectram differs on
ent days; which is probably due to the varying absorption of the ating
sphere, in consequence of its. varying hygrometric state. Receni
Secchi, in Rome, has found the same shifting of the maximum to oe
in the different seasons of the year; for in winter, when thery
moisture in the atmosphore, the maximum is farther from the red ths
in summer, whon the aqueous vapour in the air is most abundant
important observation on the luminous rays has also been made by O
in America, who found that the faint black lines in the solar
attributed to the absorption of light by our atmosphere (see book |
on) are chiefly caused by the presence of aqueous vapour. :|

Luminous and obscure radiation.—It has been stated

as radiation from a luminous object, a gas flame for example,
& composite character; a portion consists of what we term Tight, |
a far greater part consists of heat rays, which are insensible 10
eyes, being unable to affect the optic nerve. When this mixed
falls upon the blackened face of a thermo-electric pile, the whole
is taken to bo absorbed, the light by this act being converted into
and affecting the instrument proportionally with the purely calorific
The total radiation of n luminous source, expressed in units of heat
force, can thus be measured, By introducing into the path of
a body capable of stopping cither the luminous or the obscure
We can ascertain by the comparative action on the pile the
quantities of heat and light radiated from the souree.
to do this by passing a luminous beam through a layer of water
taining alum in solution; a liquid which he found in previous

mn

Luminous, Obscure.

oO 100

es os | 100
7 ee 8 or
. a 26
i ae 54
Pe A a0 20

the ratio of luminous to obscure rays in
Js found to be 10 per cont of the total radiation. By
, the curve shown in fig. 258 was obtained, graphically

ortion of luminous to obscure rays in the electric
ing the areas of the two spaces in the diagram
m in found to be nearly 10 times as large as the

of obscure rays.—We shall find in speaking
um beyond the violet there are rays which

ays, from the philosopher who first discovered
in the book on Optics, Prof, Stokes has succeeded

into rays of lower refrangibility, which then
s0 Prof. Tyndall has recently effected the vorresponding

; ‘
the rays from the points after reflection were concentrated |
about 6 inches distant. On the path of the beam was in

4 piece of charcoal in vacuo was heated to

‘By a proper arrangement of the charcoal points a metal may
to Sitecn ind the light now emitted by the metal yields on.
analysis a brilliant luminous spectrum, which is thus entirely,
from the invisible rays beyond the red.

To the new phenomenon here described, this transmutation of 0
luminous into luminous hoat, Prof. Tyndall has applied the term
reswence.

When tho eye was cautiously placed in the focus, gummed by a)
hole being pierced in a metal screen, so that the converged rays
only enter the pupil and not affect the surrounding part of thea ivy m0
impression of light was produced, and there was scarcely any’

heat. A considerable portion was absorbed by the humours of tal
but yet a powerful beam undoubtedly reached the retina; for, at
Tyndall showed by a separate experiment, about 18 per cont, of
obscure mdiation from the electric light passed through the Irma
of an ox’s eye.

373. Transmutation of thermal rays.—Melloni was the fint)
examined extensively and accurately the absorption of beat by
and liquide The apparatus he employed has already been refer
A figure of it is given in the annexed figure, 260, where AB &
thermo-clectric pile, consisting of a series of slender bars of sutit
and bismuth alternately soldered together. The terminal bars of mille
mony A, and of bismuth B, aro connocted with a galvanoueter Dh bf
means of wires.

The other parts of the apparatus are readily intelligible, There ##
graduated brass support about a yard long, on which are placed
various pieces of apparatus, and which slide, and ean be) fixed

; ais a support for the source of heat, in this cae”
and E are screens, and C is 8 support for the
experimented upon; while m is the pile, and D the galvanometes,

Melloni used in his experiments five different sources of heme

by Melloni. The experiments were made in the following way:

‘of employing a glass vessel to hold the liquids under examinatina,
made use of a little cell whose ends were stoppod by parallel platesof n
salt. The plates were separated by a ring of brass, with an

the top through which the liquid could be poured. As this plate
be changed at will, liquid layers of various thicknosses were easily
tainable, the apparatus being merely screwod together and male
tight by paper washers, The instrument was mounted on a
before an opening in # brass screen placed in front of the pile
source of hoat employed was a spiral of platinum wiro raised to

Fig. 260.

descence by an electric current; the spiral being enclosed in a small)
globe with an aperture in front through which the radiation

changed in its character, a point of essential importance overlooked
Melloni. The following table contains the results of experiments im
with liquids in the various thicknesses indicated, the numbers
the absorption per cont, of the total radiation. The érawsnniasion per’
can be found in each case by subtracting the absorption from 100
a layer of water 0-2 inch thick absorbs 80-7 and transmits 193 per
of the radiation from a red-hot spiral :—

‘These differvat sources of heat correspond to light

Rock aalt is here stated to trananit all kinds of heat with i.
umd to be the only substance which does so. It is analogous t@
glass, which is transparent for light from all sources. Fluor)

78 per cent. of the mys from = Manip, Bet oly Set

ened surface at 100°. A piece of :

400°, and bat 39 of the radiation from the lamp.
contrary, though it cuts off all heat from a source at 100°,

cent. of the heat at 400° to pass, and 2 ely ee
from the spiral, but on account of its

ray ; now as several of th
Tuminous rays, and yet,
Tuminous heat, we have an apparent anomaly ; which, ho
confirmation of the remarkably small proportion which the
of « lamp bear to the obscure,
From these experiments Melloni concluded that as the
the source rose more heat was transmitted, This may be taken:
Jaw, which has been recently confirmed by some refined ©
of Prof. ‘Tyndall. Tho platinum lamp, previously described,
as the source, the temperature of which Prof, Tyndall
to vary from a dark to a brilliant white heat, eee di
‘way the position of the apparatus; the gradations of tempen
obtained by a gradual augmentation of the strength of the electric!
which heated the platinum spiral. Instead of liquids, yap

—~

336 (ON HEAT,

can oven be accurately measured by the aanse means. This has b
by Mr. Barrott, who, in this way, bas made a physical
human breath. In one t the quantity of carbanie ;
tained in breath physically analysed was found to be 4°50 per ceat,
‘the same breath chemically analysed gave 485, difference of or
tenth per cent, |
‘875, Influence of the thickness and nature of screens-
be seen from the table (374) that of every 100 rays rock salt
92, The other 8 may either have been absorbed or reflected.
surface of the plate. According to Melloni, the latter is the east
instead of on one plate, heat be allowed to fall on two or mot
whose total thickness does not exceed that of the one, the qui
heat arrested will be proportional to the number of reflecting
He therefore concluded rock salt to be quite diathermanows,
‘The experiments of MM. Provostaye and Demins, of Mr,
‘Stewart, and those of Prof. Tyndall, show that this conclusio
strictly correct ; rock salt does absorb « very small proportion
rays.
‘The quantity of heat transmitted through rock salt is practh
samo, whether the plate be 1, 2, or 4 millimeters thick, |
other bodies, absorption increases with the thickness,
means in direct proportion. ‘This is seen to be the case in th
absorption by liquids at different thicknesses. The followi
what proportion of 1,000 rays from a Locatelli’s lamp pass
glass plate of the given thickness :
Thickness in millimeters .05 1 2 3 4 6 6 |
Rays tranemitted . . . . 775 788 682 658 634 620 609 ¢

‘Tho absorption takes place in the first layers; the rays wh
passed these possess the property of passing through other Ia
higher degree, so that beyond the first layers the heat
proaches a certain constant value, If a thin glass plate be plaot
another glass plate « centimeter thick, the former dimini
transmission by little more than the reflection from its surfiaes,
a plate of alum were placed behind the gloss plate, the result y
different, for the latter is opaque for much of the heat trans
glass.

Heat, therefore, which has traversed # glass plate traverses
plate of tho same matorial with very slight loss, but is
diminished by a plate of alum, Of 100 rays which had
green glass or tourmaline, only 5 and 7 were respectively transt
the same plate of alum. A plate of blackened rock salt only)
obscure rays, while aluin extinguishes them. Consequently, wl

A, 4

338 OS MEAT.

see aeiatiy ot Set seetaree ‘by the lampblack at 100, the
of the other bodies was us follows :

Hence, white lead absorbs far less of the heat radiated from
descent platinum than lampblack, but it ubsorks the obscure rays |
copper at 100° as completely as Iampblack. Indian ink is the n
this; it absorbs obscure rays less completely than luminous mya,
black absorbed the heat from all sources in equal quantities,
neurly completely. In consequence of this property, all S
which are used for investigating radiant heat are covered with
black, as it is the best known absorbent of heat, The behavi
metals is the reverse of that of lampblack. They reflect the |
different sources in the same degree, Thoy are to heat what

Knoblauch has shown that the different kinds of hest are altered
flection from different surfaces. The heat of an Argand Tasip dil
from white paper passes more easily through calcspar than when
been diffused from black paper,

‘The rays of heat, like tho rays of light, are susceptible of po
and double refraction. These properties will be better und
treating of light.

377, Relation of gases and vapours to radiant ri
time it was believed that gaseous bodies were as permeable to
vacuum; and though subsequently this waa disproved, yet dow
recent period it was thought that whatever absorption such
might exorciso was slight and similar in degree. The whele
has, however, been investigated by Prof. Tyndall in a series of
experiments, which, with regard to the absorption of heat by'g
equal importance to thoso of Leslie, and afterwanis of Mellon, im}
rence to solids and liquids.

‘Tho apparatus used in these experiments is represented, in its
features, in the adjacent figure; the arrangement being looked
above.

Ais a cylinder sbout 4 feet in length and 2} inches i

vine

340
zero, and remained there so long as both sources were equal.

‘ON HEAT.

amount corresponding to the beat cut off by the gas

way, air, hydrogen, and nitrogen, when dried Ny este IT
acid, were found to exert an almost inappreciable effect ; their;
regurds radiant heat being but little different to a vacuum. |
olefiant and other complex gases the ease waa entirely different.
senting by the number 1 the quantity of radiant heat
olefiant gas absorbs 970 times, and ammonincal gas 1196 |
amount. In the following table is given the absorption of obser
by various guses, referred to nir ns unity :

‘Name of gas.
Air . . . . . : * .
Oxygen. ck os
Nitrogen . oe + * " . 3
Hydrogen i me
Chiorime. . . . : OF
Hydrochloric acid

Carbonic acid .

Nitrous oxide .

Manhgee . ws Og
Sulphurousacid 2. 0. sw
Olefiant gas .
Ammonia

ences in absorption are still more strikingly seen. ‘Thus ‘
absorption by 1 inch of dry air to be 1, the absorption by 1 ine,

ant gas is 7950, and by the same amount of sulphurous Tl 00,
S78. Influence of pressure and thickness on the

total absorption is so emall, but in the case of those.
considerable absorptive power it is easily shown. ‘Taking the
sorption by atmospheric air under ordinary pressure at unity, th
bors of clefiant gaa under a pressure of 1, 3,6, 7, and 10 te
mereury are respectively 90, 142, 168, 182, and 193, Thus
of an atmosphere of oleflant gas oxerts 90 times the

entire atmosphere of air, And the absorption, it is seen,

tubs. The absorption was then determined
the vapours into the tube in quantities moasured by the

to the absorption of a whole atmosphere of dry
is thus soon that a quantity of bisulphide of car-
absorbent yet examined, which only exerts a
of morcury, or the sty of an atmosphero, gave

‘an entire atmosphere of air; and j; of an inch
se» much. Comparing air at » preasure of 0:1

2
3

i
=
ii

But
before the other fare of the pile.

tube ©
under a small known Toeamrey air wan aired Nee eee hi
sure inside the tube was the same as that of the atmosphere.
way the entering air by its impact against the tube became heat
ils particles mixing with those of the minute quantity of vapour)
each of them became, so to speak, conted with a layer of the vag
‘The entering air was in this case the source of heat, just as in tl
experiments the Leslie cube was; here, however, one gat
another; the radiation and subsequently the absorption of
could thus be determined.
It was found that vapours differed very

And in all cases those which were the best absorbents were also
radiators, By this method Prof. Tyndall was able to observe a.
radiative power with the more powerful vapours when the
present was immeasurably small.

382. Relation of absorption ts moloonlor sents tae
period it was considered that the absorption of heat was
upon the physical condition of the body examined. This
boliof that it was impossible for sie of such tenuity ns
vapours to absorb any sensible amount of heat; and that the
by bodies when in a liquid state would be unlike the same
solid; moreover, that if all solid bodies were reduced to.an equ
state of division, the present differences in their absorbent and radii
powers would disappear, A few experiments made by
atmospheric air supported the first idea, and a series of
Masson and Courtepée established the belief in the last. But1
seen that Prof. Tyndall's researches have revealed the powerful
of heat by various gases and vapours, and we shall now brielly sho
the researches. of the same philosopher have overthrown the
conclusions, giving us an insight into the cause of the absorption
which before was unattainable,
Afver the examination of the absorption of heat by ¥aj
‘Tyndall tried the same substances in a liquid form. The cond
the experiments were in both cases the samo; the source of hest m
always a spiral of platinum, heated to redness by an electric cum
Imown strength ; and plates of rock salt were invariably.

amount of the vapour of

epics cet crate tree os tosaea, an
of the plates of the rock salt, But the remark-

ul ‘of the absorption by all the other sub-

ost complex constitution were the most powerful

ere a erciral wey to bs transi:
have led Prof. Tyndall to infer that al

chemical constitation ; that is\to' ay, tbat ab

feipacléculér acts independent of the physical con-
d to be contradicted by the experiments of

to which the French experimenters had fallen,
‘that the radiation of powders is similar to that of
aa

radiation in units from some of the

ined by
metal surface of the cube giving a detleetion of 15 unite.

Radiation from poreders.
Rock salt . . . . . . 363 Sulphate of calcium . 1 |
Biniodide of mercury . . 307 Redoxideofirm. . . |
Bolpur. 2. ss ss 406 Hydrated oxide of tine . |
Chloride of lead . . . . 55-4 Black oxide of irom . . |
Carbonate of ealciam =. . 702 Sulphide of iron .

Red oxide of ead . . . 742 Lampblack . . . . =
It will be noticed that these substances are of various
are white, seach as rock salt, chloride of lead, carbonate and
calcium, and hydrated oxide of zine; some are red, such as
morcury and oxide of lead ; whilst others are black, as
and lampblack: we have besides other colours, The colours
have no influence on the radiating power: for example, rock
feeblest radiator, and hydrated oxide of zine one of the most
radiators. The views of Prof. Tyndall therefore, instead of
thrown, were confirmed by these his latest experimenta: \
Nearly # century ago Franklin made experiments on
of cloth, and found their absorption, indicated by their
on which they were placed, to increase with the darkness of
But all tho cloths were equally powerful absorbents of obscure
the effects noticed were only produced by their relative absorp!
light. In fact, the conclusion to be drawn from Franklin's
only holds good for luminous heat, especially sunlight, such as he
‘383, Applicattons.—The property which bodies possess of
ing, emitting, and reflecting heat, meets with numerous
domestic economy and in the arts Leslie stated ina general,
that white bodies reflect heat very well, and absorb very little, 4
the contrary is the case with bleck substances, As we have a¢
principle is not generally trae, as Leslie supposed; for
non-luminous rays white lead has as great an absorbing
lnmpblack (376). Leslie's principle applies to powerful oT
loth, cotton, wool, and other organic substances when expt

yma a

it changes its condition.

Stesatities of eek may be seen 7 So
effects, bat the most convenient is the alteration of it
quantities of heat are usually defined by stating the extent to sia 3
are capable of raising a known weight of a known substance, sic
Water.

‘The unit chosen for comparison, and called the thermal wait,
everywhere the same. In France it is the quantity of heat nee
raise the temperature of ane kilogramme of water
Centigrade; this is called = calorie. In this book we shall

through one degree Centigrade : pyrene yy
mal unit = 0°45 calorie,

886. Specific heat.— When equal weights of two different
at the same temperature placed in similar vessels are subjected.
ame length of time to the heat of the same lamp, or are placed at
same distance in front of the same fi is found that their:
will vary considerably ; the mercury will be much hotter than thew
Bat as from the conditions of the experiment, they bave each
ceiving the same amount of heat, it is clear that the quantity of heat
which is sufficient to raise the temperature of mercury through & .
number of degrees will only raise the temperature of the same ;
of water through a less number of degrees; in other words, that it
quires more hest to raise the temperature of water through ome degre

(ON MEAT.
into ite specific heat, ‘Thin principle ig the basis of

disengaged will be represented by the formula
m (C—) ¢, or m (C—P) 0.

A thorough comprehension of these formule will prevent any:
in the solution of problems on specific heat.

887. Methed of the fusion of iov.—This method of
specific heats is based on the fact that to melt a pound of ice 80
units are necessary, or more exactly 7025. Black's
262) consist of a block of ice in which a cavity ix made, and wl
provided with a cover of ice. The substance whose specific heat i
determined is hented to # certain temperature, and then placed i

cloth which has boen provi
The increase of weight of thiv

been converted into water.

Now, since one pound of ice at it
melting to water at 0° absorbs 80 ther
mul units, P pounds absorbs 80 P usitt

On the other hand, this quantity of heat is equal to the heat given ott
by the body in cooling from © to zero, which is méo, for it may be takes
for granted that in cooling from @ to zero a body gives out as much bet
sit absorbs in being heated from zero to ®, Consequently, from
— — S0P
mtc = 80 P we have ¢= —

It is difficult to obtain blocks of ice as large and pure as thoes ussd A
Black in his experiments, and Lavoisier and Laplace haye replaced the
Dlock of ice by « more complicated apparatus, which is called the et
calorimeter, Fig. 263 gives a perspective view of it, and fig. 204 repre 9
sents asection. It consists of three concentric tin vessels; in the
‘one is placed the body M, whose specific heat is to be determined, while
the two others are filled with pounded ice. The ive im the comspartionat
A is melted by the heated body, while the ico in the compartment cult
off the heating influence of the surroun
cocks E and D give issue to the water which arises from the liquefaction

of the ice.

M(T—0) c= m (¢—2), from which

c= mG

An example will illustento the application of thia formula. A’
iron weighing 60 ounces, and at a temperature of 100° C., is immersed!

180 ounces of water whose temperature is 19°C, After the
have become uniform, that of the cooling water is found to be 22"
What is the specific heat of the iron?

Here the weight of the heated body, M, is 60, the temperature Tit
100°, ¢ is to be determined ; the temperature of mixture, ¢, is 22,
weight of the cooling water is 180, and ita temperature 19°, ‘Therefor

_ 180 (22 —19)_ 9
= (100 = 93) — 7a Oss.
389. Corrections,—The vessel containing the cooling water is usually
i ilver or brass, with thin polished sides, and is ep
ry ly conducting arrangement. It is obvious that this
vessel, which is originally at the temperature of the cooling water, shires
its increase of temperature, and in accurate experiments this most be a=
lowed for. ‘The decrease of temperature of the heated body is equal b
the increase of tomperaturo of the cooling water, and of the vessel in whieh
it is contained. If the weight of this latter be m’, and its specifieheat ¢,
its temperature, like that of the water, is ¢: consequently the proviowt
equation becomes
Me (T—¢) = m (0 —f#) + me’ (0—#),
from which, by obvious transformations,
ox (Mit we OD
M(T—«)

Generally speaking, the value, m’ ¢, is put =m; that ia to say,
the weight of water which would absorb the same quantity of heat asthe
vessel, This is said to be the reduced value in water of the vessel, of tht
water equivalent, The expression accordingly becomes

e= (+4) 0 —t)
M(T—#)

Tn accurate experiments, it is necessary also to allow for the heat sl
sorbed by the glass and mercury of the thermometer, by introducing inte
the equation their values reduced on this principle.

In order to allow for the lose of heat due to radiation, a preliminary ex>

=

thot hae tom
where it is condensed. ‘The third

with double sides, BE, forming. preening
‘Water in order to exclude the heat from AA, and from

‘out of, or into, the chamber K. ‘This yeasel, which.

contains water, in which is immersed a thermometer, t.

mometer at the side, ¢’, gives the temperature of the air,
When the thermometer T shows that the of

in the bath is stationary, the screen A is raised, and the vessel’

just below the central compartment of the water bath. The
thon withdrawn, and the basket ¢ and its contents are lowere
water of the vessel D, the thermometer T remaining fixed i
‘Tho carringo and the vesee] D are then moved out, and the wa
until the thermometer ¢ becomes stationary. The temperaty
indicat 8 ‘This temperature known, the reat of the ca
made in the manner described in art. 389, care being taken tor
necessary corrections. |

In determining the specific heat of substances—phosphorus,
—which could not be heated without causing them to melt,
some change which would interfere with the accuracy of
Regnault adopted an inverse process: he eooled them down t
rature considerably below that of the water in the calorimete
observed the diminution in the temperature of the latter, whi
from immersing the cooled substance in it.

‘To ascertain the specific heat of bodies, such ax
use of water is quite inapplicable, the determination is made
liquid, such ns turpentine or benzole, the specific hent of whie

801. DEethod of cooling.—Equal weights of different bi
specific heats are different, will occupy different times in cool
the samenumber of degrees. Dulong and Petit have
ple in determining the specific heats of bodiesin the following
small polished silver vessol ia filled with the substance in a at
powder, and « thermometer placed in the powder, which ia pn
‘This vesse) is hented to a certain temperature, und is then intr
a copper vessel, in which it fits hermetically. ‘This copper ve

_ J

‘it absorbs or gives out far moro heat than other
double property is applied in the hot water apparatus,
— & most important part in

the specific heats of a number of bodies. The
‘the numbers obtained for the bodies usually mot

Sb8 ON HEAT.

into combination with others to form a compound body, retai
specific heat, so that if p, »’, p”, . .. . represent the atomic
the elements, and P that of the compound ; ¢, ¢, ¢’, - . . - ©, thes
responding specific heats, while n,n’,n”, . . . . are the numbers of

of these simple bodies which make up the molecule of the comy

the relation obtains :

PC =npe + wpe + wpe" +...

M. Werstyn hns found that the results obtained by calealating, a
this hypothesis, the specific heats of the sulphides, iodides and bromides
agree with experimental resulta,
306. Specific heat of gases.—Tho specific heat of a gas
referred either to that of water or to that of air, In the former esse!
represents the quantity of heat necessary to mise a given weight of
gas through one degree, as compared with the heat necessary to
same weight of water one degree. In the latter case it rep
quantity of heat necessary to raiso a given volume of the gua’
degree, compared with the quantity necessary for the same ¥o
treated in the same manner. .
De la Roche and Berard dotermined the specific heats of gases inneh
ence to water by causing known volumes of a given gas ander |
pressure, and at a given temperature, to pass through a spiral gl
placed in water. From the increase in temperature of this
from the other data, the specific heat was determined by = «
analogous to that given under the method of mixtures,
physicists also determined the specific heats of different gases
to that of air, by comparing the quantities of heat which equal
of a given gas, and of air at the same pressure and tempen
parted to equal weights of water. Subsequently to these
De la Rive and Marcet have applied the method of cooling to ti
determination ; and still more recently Regnault bas made =
investigations on the calorific capacities of gases and vapours, in
has adopted, but with material improvements, the method of De!
and Berard. Ho has thus obtained the following results for
hoate of the various gases and vapours, compared first with aa

yn

360 ‘ON HEAT,
It is not possible to determine by dirvet means the specific!
under constant volume with even sn approach to eccuracy

curate is based on the propagation J
latest determination made on this basis gives the number 14]4 fr
value of ra

397. Matent heat of fasion.—Black was the first to obeerve!
ing the passage of a body from the solid to the liquid state, a quaail
heat disappears, as far as thormometric eflycts are concerned, amd whit
accordingly said to become latent.

In one experiment he suspended in « room at the
thin glass flasks, one containing water at 0°, and the other the sa
of ice at 0°. At the end of half an hour the temperature of the
risen 4°, that the ice being unchanged, and it was 10) hours befo
had melted and attained the same temperature. Now the ten
the room remained constant, and it must be concluded that both
received the same amount of heat in the same time. Henee 21
much heat was required to melt the ice and raise it to 4°, as was!
to raise the same woight of water through 4°. So that the total gt
of heat imparted to the ico was 21 x 4 = 84, and as of this aly)
usod in raising the temperature, the remainder, 80, was used in ail
melting the ice.

Ho alao determined this latent heat by immersing 110 parts of its
GF in 135 parts of water at 877° ©. He thus obtained 264 parts:
at 116° C. Taking into account the heat received by the Fessel
the liquid was placed, he obtained the number 79-44 as the latent beat
liquidity of ice.

‘Wo may thus say :

Water at Tce at 0° + Intent beat of liquefaction.

Tho method which Black adopted is essentially that which ix no
for the determination of Jatent heats of liquids; it consists in placing!
substance under examination at a known tempernture in the water
other liquid) of « calorimeter, the temperature of which is
melt the substance if it is aolid, nnd to solidify it if liquid, and
uniformity of temperature is established in the calorimeter this
ture is determined. Thus, to take a simple case, suppose it is
to determine the latent heat of liquidity of ice. Let M be &
weight of ice at zero, and m a weight of water at © sufficient to ih
ice. ‘The ice is immersed in the water, and as soon as it has melted!
final temperature, 6°, is noted. The water, in cooling from © te
parted with a quantity of heat, m(t—0). If x be the latent heat
ice, it absorbs, in liquefying, a quantity of heat, Mir; but, besides @
the water which it forms has risen to the temperature 6°, and to do

om

‘These numbers represent the number of degrees through 4
pound of water would be raised by a pound of tos Waly eee
passing from the liquid to the solid state; or, what is the same thing th
number of pounds of water that would be raised 1° O. by ome of the

in solidifying.

398. Determination of the latent heat of
we have seen, in passing into the state of vapour, absorb a very 60
rable quantity of heat, which is termed latent heat of
determining the heat absorbed in liquids, it is assumed that
liquefying, gives out as much heat os it had absorbed in becoming
verted into vapour.

‘Tho mothod employed is sentially the samo as that for d
the specific ent of gases, Fig. 267 represents the apparatus used By
M. Despretz. The vapour is produced in a retort, C, where its temy
ture is indicated by a thermometer, It passes into worm immened
cold water, where it condenses, imparting its Intent heat to the ca
ing water in the vessel BR The
condensed vapour is collected i

OS HEAT.

Fig. 267.

reduced in water), let be the temperature of the water at the
and 6° its temperature at the end of the experiment.
It is to be observed that, at the commencement of the exp
the condensed vapour passes out at the temperature ©, while at”
conclusion its temperature is #°; we may, however, assume #

mean tompernture during the experiment is oe. The vapour
after its condensation has therefore parted with a quantity of
M (Tt 38) ¢, while tho heat disengaged in liquefaction is
sentod by Mz, The quantity of heat absorbed by the cold watery

Sn i

364 ‘ON MEAT,

which it holds about 50 pounds, and represents,

more than half a gallon. Co es ee eee

which are fitted two sheot iron tubes or reugiles,

of the bulb. Sak ihe aa ee ee
In most cases one mufile and one gl

p i operations. bird
there is also.a mufile, which can be used for determining calorific
by Regnault’s method (300), in which case it is placed beneath th
fig. 264.
‘The tubulure d contains a steel piston; » rod, tumel b
handle m, and which ia provided with a screw thread, transmits #

S66 ON HEAT.

of front p(100—92)=66p, for in this case cis unity. ‘The 208
water in being heated from 14° to 32° absorb 208(32—14) =
‘Therefore

40p + 63p = S744; from which p= 6-158 pounds.

400. Steam
force of aqueous rapour is used as motive farce.
the alternate expansion and condensation of steam imparts to ®
alternating rectilinear motion, which is changed into a circular:
means of Various mechanical arrangements.

Every steam engine consists essentially of two distinct parte:
parntus in which the vapour is produced, and the engine proper
shall first describe the former.

491. steam botier.—The boiler is the apparntus in which
generated, Fig. 270 represents a cylindrieal boiler, such a¢ it

Fig. 270.

monly used in France, and which, in this country, is known as the Fist
boiler. Boilors of this kind are usod for a fixed engine; thos of feovkl
tives and of steam vessels are very different. i
Tt is a long wrought iron cylinder, with hemispherical ends, bees
which there are two smaller cylinders of the same material, and comm)
nicating with the boiler by two tubes. Only one of these oylindars)

368 ON atkaT,

‘We shall first give a general idea of this engine, nnd shall the
cach part separately. On the left of the fig. 271, is the
receives the steam from the boiler. A part of its side is rep
as being left open, and a piston, P, can be seen which ia
nately up and down by the pressure of the steam abore or

The beam transmits its motion to a connecting rod, T, working on # Cea
K, to which it imparts a continuous rotatory motion. The emuk
fixed to a horizontal shaft, which turns with it, and by means of
or endless bands, this shaft sets in motion various machines, suelt
spinning frames, saw mills, lathes, &e,

On the left of the cylinder is a valve chest, where, by a med
which will presently be described, the steam passes alternately

= =

U. Pipe by which the steam from the cylinder passes into the call
after acting on the piston.

Y. Large iron wheel, called the fly whee, which, by ite i
to regulate the motion, expecially when the piston is at the
of its course, and the crank, K, at its dead pointe,

Y. Bent lever which imparts the motion of the eccentric,
slide valve, 6.

Z. Eocentric rod.

@. Aperture which communicates both with the
of the cylinder, according to the position of the slide valve,
steam passes into the condenser through the tube U.

6, Rod transmitting motion to the slide valve, by which stes
nately admitted above and below the piston. This will be
greater detail in the next article.

6, Aperture by which steam reaches the valve chest.

d. Stuffing bor, in which the piston rod works without
the steam,

, Eccentric, fixed to tho horizontal shaft, and rotating in a)
which the rod Z is attached.

1m. Rod which connects the rod of the slide valve 6 to the

Y, and to the eccentric,
Tho lower part of the figure does not exactly represent t
arrangement of the pumps, The drawing has been
more clearly to show how these parts work, and their
each other,

403. Distribution of the steam. Zecentric.—Figure
sents the details of the valve chest or arrangement for the

= = erst
Abe eed 2 eer et & ote Geet

this purpose in Comwall, and also for the supply of water
Thoy are preferred for these purposes from their simplicity, bat
uses they have been superseded by the double action

Fig. 273 represents a section, The beam, BB, is of
wooden segments at each end, to which chains are attached.
those chains is connected with the piston, and the other with

=>

The feeding or supply of water to the boiler is obtained by
of two pumps placed under the frame, and moved by eceentrics.

Explanation of figure 274.

Be Oapie tnbe, late vee see ‘
hy

LOCOMOTIVE EXGINES. 3t5

allie piston, the tod of which is connected with the rod K.
tamwy, by which both steam and smoke escape.

Fig 274.

{ Pred piper, throagh which the water in the tender passes to
he pumps, which are not shown in the drawing,

le &

a6 ON HEAT.

S. Guard for removing obstructions op the rails,

T, T. Springs on which the engine rests.

U, U. Iron rails fixed in chnirs om wooden

Y. Framo of the stuffing box of the cylinder.

X,X. Cylindrical boiler, covered with mahogany stares,

tho bai conductivity, hinder the loss of heat, The level of *
ia just below the tube A. In the water are the tubes aa, thm
the smoke and fame pass into the smoke box-

Y. Smoke box, in which the fire tubes, @, terminate.

Z, Z. Fire box, covered by a dome, into which the steam f
@. Brass tubes, of which there are 125, open at both ends,
minating at one end in the fire box, and at the other in the
‘These tubes transmit to the water the beat of the fire.

34, Toothed segment, placed on the side of the fire box, and in
the arm of the lever B works, When the handle is pushed

nx. Glass tube, showing the height of water in the boiler.

r, r. Guiding rods, for keeping the motion of the pistom ina stenight
¢, &, Blowing-off taps, for use when the pistons are in motion,

v. Rod by which motion is transmitted to these taps

‘The ides of these machines Papin Mh Hero of Alex:
who invented the foun’ ‘hich bears his name, described the
tus, which is known as the reaction machine,
Tt consists of a hollow metallic sphere which rotates on two fi
(fig. 275). At the ends of a diameter are two tubulures, pierced |

made to cause steam
the float board of

‘VARIOUS KINDS OP STEAM ENOINES. 877
ear preer ene is obtained when it acts by expansion

Perec er tease sere A) kn proms caps

thich the tension ‘of the vapour does not much exceed an atmno~

ere re ree tie

—— Se nope meta Low pressure engines
condensing

be where the steam becomes condensed after having acted on the

(on tho other hand, Aigh pressure engines are frequently without a
the locomotive is an example.

communication between the cylinder and boiler remains open

i piston, the steam retains essentially the
| to act without expansion : but if, by

ibe valve, the steam censes to pass into
ches a of its course, then the vapour
to my, in virtue of its elastic foree, which is due to
it still acts on tho pi and causes it to finish
~ Hence a distinction is made between expanding and non-

kofanengine. Morse-power.—The work of an engine
the mean pressure on the piston x area of the piston
‘stroke. In England the unit of work is the foot-pound ;
‘performed in raising a weight of one pound through «

as platinum black, is placed in oxygen, it
its volume, and that the gas is then in
‘the temperature so high, as to give rise to
platinum produces the same effect. A jet of
takes fire.

‘The apparatus known as Didereiner’s Lamp
of finely divided platinum, It consista of two

of these gases cannot, therofare, be a

absorption of
> SA pire
Ttis probable that it is due to that produced by

gus, and the heat due to the imbibition of the liquid
charcoal.

=. |

8100
Carbonic oxide. . . . 2400
The experiments of Dulong, of Despretz, and of Hes
body in burning always produces the samé quantity of
the same degree of oxidation, whether it attains this
reaches it after passing through intermediate stages
‘weight of carbon gives out the same amount of beat in b
to carbonic acid, us if it were first changed into carbonic 03
this burnt into carbonic acid,

HEATING.
417. Disterent kinds of heating, — Heating is the art 9

domestic and industrial purposes the sources of heat which
to us

ee

Kinds of heating, to the

mn fire ; 2nd, heating with an enclosed fire,
ng by hot air; dth, heating by steam; 5th,
hot water,

sd by the difference in temperature of two communi-
may be demonstrated by placing acandle near the
*

278° C. the volume of any gas measured at zero is
manner if the temperature of a given volume at zero
‘through —279° the contraction would be equal to the vol:
the volume would not exist.

At this tempemture the motion of the molecules of the gu
completely cease, and the pressure thereby occasioned. In all
bability, before reaching this temperature, gases would under
change.

This point on the Centigrade scale is called the absolide
perature ; the temperatures reckoned from this point are c
temperatures, They are Breresls obtained by adding 278 to the
ture on the Centigrade scale.

CHAPTER XIL

MECHANICAL EQUIVALEST OF HEAT. —

428, Mechantoa! equivalent of heat—If the various
the production of heat by motion be examined, it will be fi
all cases mechanical force is consumed. Thus, in rubbing
against each other, motion is apparently destroyed by frictio
however, lost, but appears in the form of motion of the
body; the motion of the mass ia transformed into a
molecules,

Agnin, if a body be allowed to fall from » height, it strikes:
ground with a certain velocity. According to older views i
destroyed, vis vive is lost. T' jowever, is not the case
the body appears ns vis viva of its molocules,
In the case, too, of chemical action, the most pro

r= 374 Deducting this from 5:20, the diff

Tepresents the weight of water which would bave been raist
the excess of heat imparted to the air when it could expand fn
this excess has been consumed in the work of raising 2100

ing that now, when the

+ supposil
temperature of the water, it be allowed to act upon a p

that a certain quantity of heat had dissy
mechanical effort of raising the weight.
Joule placed in a calorimeter two equal copper

equilibrium under preasure of 1] atmospheres; but as:

‘ing had done no work, there was no alteration in temy

however, the second reservoir was full of water, the siz
obliged to expel it and thus perform work, and the

owing to an absorption of heat.

For further information the student of this subject is
tmended to road Profesor Tyndall's Heat aa a Mode of Mi
the phenomena of heat are throughout explained in
modern views A condensed, though complete and
of the dynamical theory of heat is mot with
on‘ Heat,’ in Watts’ Dictionary of Chemistry.

398

right line, LD, which cuts the line of centre in B, moves tangentially,
to the two spheres, so a8 to produce a new conical surface, BDO,
be seen that all the space outside this surface ix illuminated, but that

part between the two conical surfaces is neither quite dark nor quite git
So that if scroon, PQ, is placed behind the opaque body, the partion cdit
of thescreen is quite in the shadow, while the space ab receives light
certain parts of the luminous body, and not from others. It is beighter

Fig, 284.

than the true shadow, and not so bright as the rest of the semen, and lt
is accordingly called the penumbra, .
Shadows such as these aro grometrical shadows; phyareal wunulott,
those which are really seen, are by no meansso sharply defined. A cer
tain quantity of light pastes into the shadow, éven whim of
light is n mere point, and conversely the shadow influences the illiaie
ated part. This phenomenon, which will be afterwards described,
known by the namo of diffraction,
434. ages produced by small apertures.—When lumisot
rays, which pass into a dark chamber through @ emall aperture, axe received

upon a scroon, they form images of external objects. ‘These images att
inverted ; their shape is always that of the external objects, and i i=
dependent of the shape of the aperture.

The inversion of the images arises from the fect that the Tusinot®
rays proceeding from external objects, and penetrating’ oto ie

—

400 ON LIGHT. .

and its perception by the eye. And accordingly, this velocity
determined by means of astronomical observations Rimer, a
astronomer, in 1875, first deduced the velocity of light from an
vation of the eclipses of Jupiter's first satellite.

Jupiter is n planet, round which four satellites revolve as the
does round the earth. This first satellite, I (fig, 287), authors
tation—that is, passes into Jupiter's shadow—at equal interval
time, which are 42 h. 28 m. 36.4, While the earth moves in that pat
of its orbit, ab, nearest Jupiter, ite distance from that ‘plnmet d
materially ‘ter, and the intervals between two successive cei
of the satellite are approximately the same; but im proportion
earth moves away in ita revolution round the sun, 5,
between two occultations increases, and when, at the end
the cert ae pala

Fig. 287.

retanintion of 16 m. 36 & is observed between the “J
phenomenon is seen and that at which it is calculated
But when the earth was in the position T, the sun's :
from the satellite E had to traverse the distance ET, while in te)
second position the light had to traverse the distances ET,
distance exceeds the first by the quantity TT, for from the
distance of the natollite E, the rays ET and ET’ may be co
parallel. Consequently, light requires 16 m. 36 «. to travel the di
‘TT’ of the terrestrial orbit, or twice the distance of the earth fom
sun, which gives for its velocity 190,000 miles in « second.
The stars nearest the earth are separated from it by at least 2083
times the distance of the sun. Consequently, the light which ther
requires 3} yeara to reach us, Those stars which are only visible Oy
means of the telescope, are possibly at such a distance that thowpands ol
years would be required for their light to reach our planetary -
They might have been extinguished for years without our knowing tt
436, Foucault's apparatus for determining the valocity
Ught.—Notwithstanding the prodigious velocity of light; M. Fours

—

f VELOCITY OF LIGHT. 401

toeeded in determining it experimentally by the aid of an ingenious
tus, based on the use of the rotating mirror, which has been
d by Mr. Wheatstone in mensuring the velocity of electricity.

be description of this apparatus, a knowledge of the principal
ties of mirrors and of lenses is presupposed. Figure 288
ots the principal parts of M. Foucault's arrangement. The
f shutter, K, of a dark chamber is perforated by a square
fy behind which » platinum wire, o, is stretched vertically, A
of stlar light reflected from the outeide upon a mirror enters
(ik room by the square aperture, meets the platinum wire, and
faveres an achromatic lens, L, with a long focus, placed at a
% fem the platinum wire lesa than double the principal focal

Fig. 288. Fig. 289,

fe The image of the platinum wire, more or less magnified,
thts be formed on the axis of the lens; but the luminous
having traversed the lens, impinges on a plane mirror, m,
E with great velocity; it is reflected from this, and forms in
BB image of the platinum wire, which is displaced with an
F velocity double that of the mirror." This image is reflected
(iieave inirror, M, whose centre of curvature coincides with the
{gotation of the mirror m, and with its centre of figure. The
jprowe this, Jet mm (fig. 289) be the rotating mirror, © a fixed object placed in
Mi forsning ita image at O. When the mirror comes into the position m'n’,
frie fermed at 0”. But the arc O'm equals the aro Om, and the arc Om
he are Om’; hence the arcs O0', re respectively double the arc Om,
[Derefore, by Mabsraction, the arc O’O" is double the mm’, of the angular
|offthe fenage ix doable that of the mirror.

7

In order to demonstrate the first law, Het there be two ei
OD and AB (fig. 200), owe placed at a certain distance from
source L, and the other at double this distance, and Jot « aul
areas of the two screens. If a bo the total quantity of light
emitted by the source in the direction of the cone the
the light on the screen OD, that is, the quantity whick falls en the
of surface, is 7 ,and the intensity on the sereen AT fs &, Xen

triangles ALB and CLD aro similar, the diameter of All i:
that of CD; and as the surfaces of cireles are as tho sy
their diameters, the surface S in four times s, consequently the il)

© is one-fourth of 7.
5 ’

Fig. 290,

‘The same law may also be demonstrated by an experiment wi
apparatus represented in figure 209, It is made by
shadows of an opaque rod cast upon a glass plate, in one case bi
of a single caudle, and in another by that of four candles, pli
double the distance of the first. Tn both cases the shadows ha
same intensity,

Figure 290 shows that it is owing to the divergence of the In)
rays emitted from the same source that the intensity of light ix im
as the square of the distance, Tho illumination of a surface ple
beam of parallel luminous rays i¢ the same at all distances at a)
ina vacuum, for in ait and in other transparent modia the inten
light decreases in consequence of absorption, but far moro slow!
the square of the distance.

The second Inw of intensity corresponds to the law which ®)
found to prevail for heat; it may be theoretically deduced ax

INTENSITY OF LIGHT, 405

B (Gg. be m pencil of parallel rays falling obliquely on a
B, und let om be the normal to this surface. If S is the
the pencil, a the total quantity of light which falls on the
Sand I thet which falla on the unit of surfuce (that is, the
(( lussinstion), we have I = %,. But as $ is only the pro-

‘AB on a plane perpendicular to the pencil, we know from

Fig, 201.
fy that 8 = AB cos «, from which AB= 5, vibe vedas
cos

above equation, gives 1=< cos % ® formula which

cosine, for as « and S are constant quantities,

applies also to rays emitted obliquely by a
the rays are leas intense in proportion as they

to the surfaco which emits them. In this respect they

the third law of the intensity of radiant heat.
photometer i on apparatus for measuring the
ies of light.

Thisconsists of a ground glass acreen, in front of
‘am opaque rod (fig. 202); the lights to be compared—for

_

times as great.
For if é and ¢ are tho intensities
unit of distance, and d and d'

‘on a dark ground. If the greased part and the rest

the intensity of illumination on both sides is the same. But

meter depends on an application of this principle, A cireu

made on & paper acreen by means of « solution of sperm

behind this is placed o light of a certain intensity,
usually a wax eandle of ki

‘The light to be tested is then moved in a right line to su

the greased part and the rest of the screen. By m
of the lights from the screen, their relative illus
deduced from what has been proviously said.

By this kind of determination great accuracy ©
especially when the lights to be compared are of d
for instance, being yellow, and the other of = bluisl:

termins u

phot cipal part of this 3
atoel bead, P (fig. 203), fixed on pags of a disc, which
pinion, o, working in a larger toothed wheel, ‘The whee) ite
drical copper box, which i held in one wank, while the

-

408 Ox LIGHT.

there is also a mirror, M, which can be more or low inelised I
always remains in a plane perpendicular to the plane of the &
circle. Lastly, there is a small polished metallic mirror, m=, place
zontally in the eeatre of the circle.
In making the a
il of solar light, 8, is cout
impinge on the mirror M, #8"
0 inclined that the reflected i
passes through the apertsoeis?

direction mP, which is set
by moving P until am fang of!
aperture is found in its conti
number of degrees compris
are AN is then read off, unl
wise that in AP, these beit
it follows that tho angle of relies
tion, AmP, is equal to the sng il
incidence, AmM,

‘The second law follows fra
arrangement of the
Plane of the rays Mim and mPb
parallel to the plane of the graduated circle, and consequently,

dicular to the mirror m,

Fig: 296.

Second proof. The law of the reflection of light may aleo be dean

~~ e |

REFLECTION OF Licht. 409

} the fellowing experiment, which is susteptible of greater
flea thet just described. In the centre of a graduated circle,
{6}, placed in w vertical position, there is a small telescope
In plane parallel to the limb; at a suitable distance there is
Hofmercury, which forms a perfectly horizontal plane mirror.
bnlir star of the first or second magnitude is viewed through
fein the dimeotion AB, and the telescope is then inclined #0 0s
bemy, AD, coming from the star after being reflected from
temface of the mercury, In thie way the two angles formed
BA and DA, with the horizontal AH, are found to be equal,
itmay easily be shown that the angle of incidence, E'DE, is
‘angle of reflection, EDA. For if DE is the normal to the
(o mercury, it is perpendicular to AII,and AED, ADE are
(ents of the equal angles EAH, DAH; therefore AED, ADE
ut the two rays, AE and DE’, may be considered parallel in
ofthe great distance of the star, and therefore the angles
EA.are equal, for they are alternate angles, and, consequently,
YE’ is equal to the angle EDA,

REVLMOTION OF LIGHT FHOM PLANE SURPACES.

fers. Emages.—dirrore are bodies with polished surfnoes,
by reflection objects presented to them. The place at whic

br iv their émage. According to their shape, mirrors are

(plane, eoneare, conver, spherical, parabolic, conical, Ke.
q

Fig. 297.
by plane mirrors,—Tho determination
of images resolves itself into investigating. the

points, And first, the case of single point, A,
mirror, MN (fig. 2002), will Le conside
from this point on the mirror, is reflected in the dire
Hing the angle of reflection, DBO, equal to the angle of inci-

7

"(REFLECTION OF LiGRT. 41

| by eaying that real images are those
and virtwal images those formed by

formed by glass mirrors.—Metallic mirrors
‘surface only give ane image; it is different

H thniotlier part peares into the Fig. 208,
‘is reflected Bre hearae the layer of metal which covers the
the glass, and reaching the eye in the direction dH, gives
'. This image is distant from the first by double the thickness
t It is more intense, because metal reflects better than glans.
to the other images it will bo remarked, that whenever light
ited from ome medium to another, for instance, from glass to
ene of the rays got through, the remainder are reflected at the
th bounds the two modis. Consequently when the pencil od,
attempts to leave the glass at d, most of the rays com-
Saerieke ats. but oome. ave redontad at d, and continue
§ Those are again reflected by the metallic surface, und
ye of A; after this reflection they come to MN, when
the third image visible, but some are agnin re~
glass, and in » similar manner give rise to a fourth,
by completing the series above deseribed. It is
)above explanation that each image must be much
) preceding it, and consequently not more than a
yisible—ordinarily not more than eight or ten in all.
of images is objectionable in observations, and,
s niirrors are preferable in optical instruments,
from two plane mirrors.—When an object
en two plane mirrors, which form an angle with each
‘or acute, images of the object are formed, the number
ith the inclination of the mirrors. If they are at
‘other, three images are seen, arranged as represented
ys OC and OD from the point O, afters single
2

(ON LiGHT,

reflection, give the one an image O', and the other

the my OA, which has undergone two reflections at A}
third image, 0”, When the angle of the mitrors is 60°,
produced, and seven if it is 46°. ‘The number of images!
Sauet io promiitien w the ge eine ee

Fig. 299, Piers!
ae gular pleces of coloured g
atone end between the ground glass and another gla
looking through the aperture, the other end being held toy
the objects and their images aro seen arranged in beautil
forms ; by turning the tube an endless variety of these aby

446, Irregular reftection.—The reflection from {
polished bodies, the laws of which have just been state
veguiar or specular reflection: but the quantity thus reflec
the incident light, The light incident on an opaque
sepurates into three parts : one is reflected regularly, ano}
that is, in all directions; while a third is extinguished,
the reflecting body. If light falls on @ transparent body
portion is transmitted with regularity.

‘The irregularly reflected light is called scattered light: |
makes bodies visible, The light which is reflected reg
give us the image of the reflecting surfnee, but that of
which the light proceeds, If, for example, a solar beam
a-well-polished mirror in a dark room, the more
reflected the lees visible is the mirror in the different pat
‘Cho oye does not perceive the image of the mirror, but
IE the reflecting power of the mirror be diminished by spt
light powder, the solar image becomes feebler, and the 1
from all parts of the room. Perfectly smooth polished ref
if such there were, would be invisible.

447. Entonaity of reflected light.—The intensity of

Fig. 301.

figure OL it will be seen that when the object, I,
ae from the centre, C, its conjugate focus

de of rellection must be the sume, the ray is reflected
Loner ee object. When the Iuminous
centre, principal focus, the conjugate
other side of the centro, and is further from the

It

re ia, lastly, the case in which the object is placed
focus and the mirror (fig. 202), Any my,
int L., makes with the normal CM, an angle of
uF then FMC; the angle of reflection must be
therefore the reflected ray, ME, diveryos
‘This is also the case with all rays from the

ys do not intersect, and, consequently, form

M6 on Lien.

no conjugate focus; but if they are conceived to be prolonged:
other side of the mirror their prolongations will intersect in the
point, /, on the axis, and the eye experiences the samo impressing:
rays were omitted from the point, 2 Hence « eirtual focns is
analogous to those formed by plane mirrors (443)

In all these cases it is seen that the position of the principal
constant, while that of the conjugate fool and of the bot!

Fig. 302. Big. 303.

The principal and the conjugate fori ere alscaye om the mnme side of
mirror aa the object, while the virtual focuris ahoays om the other
the mirror,

Hitherto the luminous point has always been be
the principal axis itself, and then the focus is ‘this axis,
the case in which the luminous point is sitanto om m rie fe
(tig.
case it will be seen Tt te foous oe ek port
on the secondary axis, und that arconting to the diatanct of the poitt|
the focus may be either principal, conjugate, or virtual,

450, Poet of convex mirrors,—In convex mirrors there are

Fig. 204,

virtual foci. Let SI, TK ... (fig. 904), be rays paraiiel to
principal axis of a convex mirror, These rays, after refieetion, take

diverging directions IM, KH, which, when continued, moet ima point, Fy
which ja the principal virtual focus of the mirror, By means of tit

47

| MEFLECTION OF LIGHT FROM CURVED sURFAckS.
| KF, it may be shown in the same mannor as with cancave
ee middle of the radius of

Incident luminous rays, instead of being parallel to the axis,
from a point, L, situated om the axis at a finite distance, it is nt
x that 8 virtual focus will be formed betwoon the principal focus,

the principal focus.—In the applications
rors it is often necessary to know the radi

to finding the principal focus; for being
radius, it is simply necessary to double the

ii concave mirror, it is exposed to the sun's
ee is fe to them, and then with a small
which the image is pores

eruees i is coered with paper, but two small portions,
distances from the centre of the ‘tran

Fig. 305,

Jarger than the distance HI, is placed bofor:

of solar rays, SH, S'T, parallel to the axis, fall on the

at Hand I, on the parta where the mirror

on the screen two brilliant i images at A and s,

BN nearer to or farther from the mirror, a posi-

the distance Ai is double that of TEL, ‘The distance

to tho mirror, then equals tho principal focal

the are, FLAT, does not sensibly differ from its chord, and

Hor telangles FHT ond Fi aro similar, HUF» but HE is halt

‘therefore also FA is the half of FD, and therefore AD is

DAP, Further, FA is the principal focal distance ; for the rae
| 3

a

413 ON Lint,

SH and S‘T are parallel to the axis: consequently also twice the:
AD equals the radins of curvature of the mirror.
452. Formation of images tn concave mirrors.—HHitherto

owe can conceive a secondary axis drawn through each of its
thus a series of ral or virtual foci could be determined, the

which composes the insage of the object. By the aid of the

which have served for determining the foci, en
position and magnitude of these images in conamicg snd

mirrors,
Rea! image. —We shall first take the ease ieee the minx
cave, and the object AB (fig, 396), is on the other tide of the centre:

Fig. 306,

obtain the image or the focus of any point, A, a
drawn from this point, and then drawing fet
D, the normal to this point OD is taken, antl
€Da, is made equal to the angle of incidences, ADO. ist eset,
the reflected ray cuts the secondary axis, AB, is the conjugate foes,
the point A, because every other ray drawn from this paint
throngh with @, Similarly if n secondary oxis, BE, be drawn
the point B, the rays from this point moot after retlestion in 6, and
the conjugate fccus of B. And as the images of all the polate of
object are formed between a and 4, ab is the complete image of AB,
what has been suid about foci (449) it appears that this mage i
inverted, smaller than the object, awd placed between the contre of
and the principal focus. ‘This image may be soon in two-waya; by
the oye in the continuation of the retlcted rays, and then it & A
image which is seen ; or the rays are collected on a screen, on whidll
image appears to be depicted,
If the luminous or illuminated object ix placed at eb, betwen
principal focus and the centro, its image ia formed at AB Itiy
real but inverted image ; it is greater than the object, and the
the object, ab, is nearer the focus,

le

| REFLECTION OF LIGHT FROM CURVED SURFACES. 41g

\e object is placed in the principal focus iteelf, no image is pro
j for them the rays emitted from each point form after retection
fy pencils respectively parallel to the secondary axis, which ix
(through the point from which they are emitted (449), and hence
¢ foci nor images are formed.

em all pointsof the ubject, AB, are above the principal axis (fig.

Fig. 307,
“the preceding construction, it is readily sven that the
is formed at ab,
cao remaina in which the object is placed
focua and the mirror. Let AB be this object
-rayanfter reflection take the directions DI and KH,
form”: virtual image, @, of the point A, on the

g Fig. 308.

fiery axis, Similarly, an image of B is formed at 4, consequently
fe meen at ab the image of AB. This image is eistual, erect, und

+ than the object.
what bas been stated it in seen that according to the distance of
comeave mirrors produce two kinds of images, or none at all:
fon notices this by placing himself before a concave mirror, At a
in distance he sees an image of himself inverted and smaller: this
(real image: at a less distance the image becomes confused and
when hy is at the focus ; still nearer the image appears erect,

it iss virtual image.

O, Formation of images im convex mirrors.—Let A (fig. ‘J00),

——

420 ON LIGHT.

be au object placed before a mirror at any given distances, AC and
ar secondary axes, and it follows from what bas been slealy
that nll the raya from A are divergunt after reflection, and that ¢
prolengations past through a point @, which is the virtual image al
point A. Similarly the rays from B form o virtual image of iti
point 6, The eye which receives the divergent rays, DE, KA,
mes in ab an image of AB. Hence, whatever the position of a

Fig. 09.

lwfore a convex mirror, fhe image te always virtwal, erect, and euallee
the object,
454. Formulse for spherical mirrors.—The relation betwee

position of an object and that of its image in spherical mirrors may Of
expt imple formaln, In the case of concave mirun We

Fig. 310,

In the trial
‘the angle LM? in two equsl parts,
two segments, LO, oe we to enc
Ce)

ie angle, that is e

of the formula (4) is negative. ‘Therefore, the dist
from the image miust be calculated on the axis n
yp. The image is then virtual, and is on the other side of the
Making negative in the formula (2) it beoomes) — T= af
form it comprohends all cases of virtual. images in coucave mimen
In the case of convex mirrors the insege is always virteal |
snd RB are of the same sign, since the imageand the centre areca!
side of the mirror, while the object being on the opposite aid
contrary sign ; hence in the formula (2) we yet

i =! =? , . r
ae ee |
as the formula for convex mirrors It may also be found directly
seine geometrical considerations as those which have Jed to the |
(2) for concave mirrors,

Tt must be observed that the preceding formule aro pet 1
trus, inasmuch as they depend upon the hypothesis that the li
and /M (fig. 310), are equal to LA and AJ; although this is not}
error diminishes without limit with the angle MCA ; and whonth
does not exceed a fow degrocs, the error is so small that it may,|
tice, be neglected. ‘

158. Calculation of the magnitude of tmages.—DBy mean)
above formule the magnitude of an image may be calculated, #
distance of the object, its magnitude, and the radius of the m|
given, For if BD be the object (fig. 311), bi its image, aa

Fig. 311. }

distance KA, and tho radius AC be known, Ao ean be caleul
means of formula (3) of article 454, Ao known, 0 can be ca
But as the triangles BCD and dCé are similar, thir bases aed
‘are in the proportion bd : BD = Co ; OK, or
Length of the image : length of the object

= distance from image to centre : distance from the object ta)

457, Spherical aberration. Caustics.—In the forgoing t
the foci and images of spherical mirrors, it has already been |

le S|

REFLECTION OF LIGHT FROM CURVED SURFACES, 425

(mllected rays only pass throngh « single point when the aperture
itor dees not exceed 8 or 10 deyrees (449). With a larger
| the mays refleeted near the edges meet the axis nearer the
btm these which are reflected wt a small distance from the neigh-
(tof the centre of the mirror. Hence arises a want of precision
Inares, which is called spherical aberration by retlection, to dise
‘it from the spherical aberration by refraction, which occurs in
of lenses,

reflocted ray euts the one next to it (fig. 312), and their points
tetion form in space a curved surface, which is called the caustic

Fig. 312,

fo. ‘The curve FM represents one of the branches of a section
turince made by the plans of the paper. When the light of a
(reflectod from the inside of a cup or tumbler, a section af the
lirface can be seen by partly filing the cup or tumbler with

\ppiicutions of mirrors.—The applications of plane mirrors
tie economy are well known. Mirrors are also frequently ued
‘al apparatus for sending light in « certain direction, The solar
‘only be sent in a constant direction by making the mirror move-
| must have & motion which compensates for the continual
{the dineetion of the sun's rays produced by the apparent diurnal
@ the sum This result is obtained by means of a clock-work
@ which the mirror is fixed, and which causes it to follow the
ithe sun. This apparatus is called the Aeliastot, The reflection
ie also used to mensure the angles of crystals by means of the
fits known aa reflecting goniometer.

fexpherical mirrors are also often used, They areapplied for
fg mirrors, as in a shaving mirror, They have been employed
fag mirrors, and aro still used in telscopes. ‘They also serve ax
(for conveying light to great distances, by placing a luminous
| thelr principal focus, For this purposc, however, parabolic
Se

(erabolic mirrors.—Parabolic mirrors are concave mirrors,

la.

424 ‘ON LIGHT.

whose surfaes is generated by the revolution of the arc ofa parbal
about its axis, AX (fig. 313).

Tt haa been alrondy stated that in spherical mirrors. the nyt)
to the axis converge only approximately to the principal Get
reciprocally when a source of light in placed in the principal
these mirrors, the refecty
are not exactly paraildl tot}
Parabolic mirrors are ft
this defect; they are mn!
to construet, bat are far bd
rellectors, It ix a wilh
property of a paretols t
wight line FM, draws
focus F, to any point, M
curve, ard the line ML,
to the axie AF, mk

Fig. 318, angles with the tangeat
this point, Consequently,
parallel to the axis after reflection in the focus of the mirtt
reciprocally, when a source of light is placed in the focus,
incident on the mirror, are refleete|
parallel to theaxis, The light thus,
tends to maintain its intensity 6
great distance, for it haa been se
that it is the divergence of the]
rays which principally weakens th
sity of light.

Tt is from this property that }
miryors are used in carriage lamp
the lamps placed in front of ani
railway trains, These reflectors ¥
merly used for lighthouses, but hi
teplaced by lenticular glasses,

When two equal parabolic tm
cut by a plane perpondicular to
passing through the focus, and )

Fig, 314. united at their intersections, ax sho

figure 3l4,s0 that their fool

aystera of reflectors is obtained with which a single Iamp illum)

two directions at once. This arrangement is used im lightit
conse.

bagrenpoeematdocs cede rid the two media,
continues its course in a right line.

Kt ray being represented by SO. (fig. 315), the refrmeted ray
4 in the send medinm ; und of the

refracting Fig. 315.

[ot completely pass into it; ono part is veflocted and seattered,

| penetrates into the medium.

ows that tive direction of refraction depends on the relative
inthe two media, On the undulstory theory the more
medium is that in which the velocity of propagation is

|
Mised media, such as air, liquids, ordinary glass, theluminous
Meiracted; but in certain crystallised bodies, such as Iceland
(&e., the incident ray gives rise to two refracted mys. The
Winom in called doable refraction, and will be discussed in
a book, We shall hers deal exclusively with simple

\of single refraction.—Whon a luminous ry is refracted
inone medium into another of a different refractive power

(laves prevail :—

bthe obliquity of the incident ray, the ratio which the sine of
beara to the sine of the angie of refraction, ia constant. for

varies with different media.

Vident ead the refracted ray fave in the same plane rhich is

tothe mrfare separating the two media,

(ieiowa ns Deseurtes’ laies, and are demonstrated by the

/

—

SINGLE AKFHACTION, 427

tarface of this medium, but they appear to be more distant if
pa bess refracting medium. Let L (fig. $17) be am object im-
amass of water. In passing thence into air the rays LA, LB
the mormal to the point of incidence, and assume the
(GRD. _ . the prolongations of which intersect approximately
pee on the perpendicular 1/K, The eye receiving
dhject Lat L/, The greater the obliquity of the rays
Ae object appear,

the same “ar a that a stick plunged cei ‘into water

( (6p S19, she Emer pt appearing raed

(317, Fig. 318 Fig. 319.

} an effect of refraction stars are visible to us oven when they
(he horizon. Tor as the layers of the atmosphere are dénser
@m a4 they ure nearer the earth, and as the refractive,power of
bases with density (473), it follows that on entering the
{ the luminous’ rays become bent, as seen in the tig. 319,
curve before reaching the eye, so that we seo the star at S’
tangent of this curve instead of at S. In our climate the
fe refraction does not raise the stars when on the horizon more
(degree.

tal reSection. Critical angie.—When a luminous ray
ene mediam into another which is les refracting, as from
nit, it lens been seen that the angle of incidence is greater than
of refraction. Hence, when light is propagated in a muss of
(8 to O (fig. $20), thore is always a value of the anglo of in-
YB, such that the angle of refraction, AOR, is a right angle,
fe the refracted my emerges parallel to the surface of the

SOB, in called the critical angle, because for any grenter
the incident ray cannot emerge, but undergoes an internal
fPhich is called total refection, because the incident light is
fected. From water to air the critical angle is 48° 35’; from
yah 48.

= ==

ON LOHT,

428

‘The oceurrenes of this) intornal refeetion may to ce
following experiment. An object, A, is placed aghe
with water (fig. 321); the surface of the liquid ia

Fig. 320,

Fig, 321,

shown in the figure, and an image of the object A is seen st
by the rays reflected at m, in the ordinary manner of a mirror
405, Mirage.—The mirage is an optical illusion
images of distant objects are seen as if below the ground or
atmosphere. This phenomenon is of most frequent occurrence Bi)
climates, and more especially on the sandy plains of Egypt. The

there has often ¢ d
and the surrounding villages, The phenomenon has long been i

Fig, 322,

but Monge, who accompanied Napoleon's expedition to Bgypty
first to give an explanation of it,

It is a phenomenon of refraction, which results from the
density of the different layers of the air when they are expandel

=~

430 ‘ON Licnt.

batisen topline: ties Sag ee ‘The’
these two faces is the erige of the pris, and
Eatery Every section

the pristn, and the right line, BO, is called the pears
have reference to the triangle ABO, and not to the prism.

Fig. 824.

468. Path of rays in sing ae the Laws of refracti
known, the passage of rays in ® prism is readily determined.
« luminous point (fig. 226) in the same plane as the
ABC, of a prism, and let OD bo an incident ray, "This ray is
at D, and approaches the normal, because it passes into « niore
rofmcting medium. At K it experiences a mcond refraction, butd
deviates from the normal, for it passes into air, which is leas rah
than glass. The lizht is thus refmcted twice in the same direct!
that the ray is deflected towards the base, and consequently the eye!
receives the emergent ray, KH, sees the object O at O”; that is,
seen through a prisin appear deflected towards its newunit. ‘The angle)
which the incident and emergent rays form with éach other, expres
deviation of light causod by the prism, and is callod the ragleaf dee
Besides this, objects seen through a prism appear in all the ¢
of the rainbow; this phenomenon will be described under the m

ys refracted at the first face of a prism may emerge fro
second, it in necessary that the refractive angle of the priam be les
twice the critical angle of the substance of whieh the prisen ia

For if LI (fig. $26) be the ray incident on the first face, IE the pee |
ray, PI and PE the normals, the ray TK cap only embnge fror
second face when the incident angle, LEP, is less than the critical
(464), But as the incident angle LIN increases, the angle BE

a iil

‘TRANSMISSION OF LIGHT. 431

fhile TEP diminishes. Hence, according as the direction of
‘tends to become parallel with the face AB, does this ray tend
the second free,

tenew parallel to AB, the angle r is then equal to the critical

Fig. 326.

face,
‘ ternal reilection, and would emerge at a third face,
‘would BRAUN ciate she: cane, with rays whose incident
‘than BIN, because we have already seen that # continually
Thus in the ease in whick the refmeting angle of a prism is
is greater, no Iuminons ray could pass through the faces of

fitieal angle of glnss is 41° 48’, twice this angle is less than
cordingly, objects cannot be seen through a glass prism whose
(ingle is & right angle. As the critical angle of water is 48°35’,
(pew through & hollow rectangular prism formed of three glass
(Glled with water.
A to be greater than and less than 2%, then of rays in-
some within the angle NIB will emerge from AC, others
berge, nor will any emerge that are incident within the angle
we suppose A to have any magnitude less than /, all rays in-
(within the angle NIB will emerge from AC, as also will
jee incident within the angle NIA.
Bimum deviation.—When a pencil of solar light passes
@ aperture, A, in the side of a dark chamber (fig, 327), the
irojected in a straight line, AO, on a distant screen. But if
be interposed between the Aperture and the sereen, the
towards the base of the prism, and the image is pro-
}, nt seme distance from the point ©, If the prism be turned,
(imetdemt angle decreases, the luminous disc approaches the
P to meeriain position, E, from which it reverta to its original
fen when the prism is rotated in the snme direction. Hence

=

ON LIGHT,

there is a deviation, EBC, less than any other, It may by
mathomatically that this minimum deviation takes place whan the
of incidence and of emergence are equal.

The angle of minimum doyiation msay be calculated when thei
angle and the refracting angle of the prism are known. Fer,
deviation is least, as the angle of emergence r/ is equal to the?
angle f (fig. 326), 7 must =a, But it has bien shown abore (400)
r+ ?j comequently, |

AS? 2 9 he
If the minimum angle of deviation CDL be ealled a, this angled)
exterior to the triangle DIE, we readily obtain the equatiaa

4

whence

which gives the angle d, when ¢and A ar known.
From the formulm (1) and (2) a third may be obtained, which #

Fig. 927.

to calculate the index of refraction of # prism, when its refracting)
and the minimum deviation are known. The index of refractiog,

the ratio of the sines of the angles of Incidence and refractions |
»="""; replacing @andr from their values in the mbore eqat
ain
(1) and (2), we get
+ (A+d)
sin( AF)
= i
A
sin

471. Measurement of the index of refraction in solid&;
means of the preceding formula (3) the refractive index of « solid mi
caleulated when the angles A and d nown.

In onder to determine the angle A, the eubstance is cut in the}
of « triangular prism, and the angle menstuvd by means of a gooldll
(458).

TRANSMISSION OF LIGHT. 483

a ‘the following manner: a my LI emitted

9G) is rocefved on the prism, which is tured
the minimum deviation EDL’. By means of a
tated circle, the angle EDL is read off, which the
kee with tho ray DL’, coming directly from the
je angle of minimum deviation, assuming that the

Fig. 328

‘the two rays, LI and L’D, are approximately
mn only need to be substituted in the equation

‘Under many circumstances it cannot
the refractive index of a mere drop of
. use may be made of a method due to
on the determination of the critical angle of

ef the index of refraction of liquids.—M.
on's mothod to determining the refractive index

(prism, PQ 829)
reg fe,

the cavity O having
‘r values are introduced into the formula (3), which

dt of the index of refraction of gases.—A

tue founded on that of Newton has been devited by

‘The apparatus which they use consists of a glass

led at its two extromitios, and closed by glaas plates,

fof 143%, ‘This tube is connected with » belljar, H,
v

I Lenses. 437

the principal tocas coincides vory clossly with the centre of
ture.

Fig, 332,

\skall now consider the case in which the luminous object is out-
be principal focus, but. so near that all incident rays form a diver-
ined, as shown in fig. 833. ‘The luminous point being at L, by
bing the path of a diverging ray, LB, with that of a ray, SB,

Fig. 333.

WW tothe axis, the former is found to make with the normal an
[Lis greater than the angle SBn; consequently, after traversing
tot, tht ray cuts the axis at » point, , which is more distant than
Hintipal focus, F. As all rays from the point L intersect approxi-

Fig. 334,

Ipin the same point J, this latter is the conjuyate focus of the point
Histerm bas the same meaning hero as in the cases of mirrors, and

expresses the relation existing between the two points L mil
is of such a nature, that if the Iuminous point is moved to (ht
passes to Le .

Acconting a4 the object comes near the lenses, the converges
emengent raya decreases, and the focus! becomes more distil;
object L coincides with the principal focus, the emergent mart
other side are parallel to the axis, and there ix no fhous, or, wht
same thing, it is infinitely distant, As tho refracted rays are
this caso, the intensity of lizht only decreases slowly, and a simple
can illuminate great distances, Tt ix morvly necessary to place it ft
focus of a double concave lens, as shown in fig. 334.

Virtwal foci. A double convex lens has a virtual focus,
luminous object is placed between the lens and the
shown in fig. $35. In this case the incident rays make with the

Fig. 335.

greater angles than those made by the mys FI from the principal
hence, when the former rays emerge, they move farther from the
than the latter, and form a diverging pencil, HK, GM, Thee
cannot produce a real focus, but their prolongations intsrmat in
point Zon the axis, and this point is the virtual foeus of the
(43).

176. Foet in double concave lenses.—In double concave
there are only virtual foci, whatever the distance of the object

-

Fig. 336.

Fig. 337.
ST bo any pencil of rays parallel to the axis (ig. 386), any mp,
refracted at the point of incidence, 1, a4. eygroaches the normal)

440 ON LiiT,

refracted ray, KA, A’K’, is propagated in « mediom with
Hence, a ray which reaches A at much ‘an inclination, that after.
tion it takes the direction AA‘, will emerge parallel to its first,
(468) ; the point O, at which the right lin line cuts the mxis, is |
the optical centre. The position of this point may he
the case in which the curvature of the two faces is the
the usual condition, by observing that the triangles COA an
equal, and therefore that OC=O0' which gives the pointO,
vatures are unequal, the triangles COA and C/OA’rre similar,
CO or C’O may be found, and therefore also the point O,

In double concave or concavo-convex lomses, the optical e
be determined by the same construction. In lenses with a
this point is at the intersection of the axis by the curved fhe,

Fig. 399

Every right line, PI” (fig. 340), which p
centre without pasting through the centres of ¢
axis. From the property of the optical centre, overy
represents a luminous rectilinear ray passing eae ‘this
from the slight thickness of the Tenses, it may be
passing through the optical centre are in a right line,
amall deviation may be neglected which rays pele
a medium with parallel faces (fig. $23).

So long as the secondary axes only make a small angle with @
cipal axis, all that has hitherto been said about the principal axis iss
cable to them; that is, that rays emitted from a point, P (fig, 340
secondary axis, PP’, nearly converge to a certain point of this axt
and according ns the distance from the point P to the Lens in gr
Jess than tho principal focal distance, the focus thus formed will he ex
Jugate or virtual. ‘This principle is the foundation of what follows a!
the formation of images.

479. Formation of images in double convex tonses.—In Ie
as well as in mirrors the image of an object is the collection of thet
of its several points; hence the images furnished by lenses aren
virtual in the same case as the foci, and their construction resalyes

determining m series of points, as was the case with mirrors

fmage. Let AB (6g. 341) be placed beyond the principal focus,
axis, Aa, be drawn from the outaide point, A, any ray,

‘this point, will be twice refracted at C and D, and both times
direction, approaching the secondary axis, which it cuts at a.
‘Wliat Ihas been said in the last paragraph, the other rays from the
interpect in the point «, which is accordingly the con-

‘the point A. If the secondary axis be drawn from the
swill be seen, in like manner, that the rays from this point
‘the point 4, nnd as the points between A and B have their
— of AB will be formed at

‘it may be received on a white screen, on

Fig. 341.
were the luminous or illuminated object which

for the theory of optical instruments follow from this: that,
‘an object, even a very large one, is at @ sufficient distance from a
comever lens, the real and inverted image which is obtained of it is very
itis near tha principal focus, but somewhat farther from the lens
iss 2nd, if @ very small object be placed near the principal focss,
Befare it, the image which is formed is at a great didance, it ts
-, and that in proportion as the object is nearer the principal
- Tnall eases the object and the image have the same proportion as
Fdistances from the lens,
bese two principles are oxperimentally confirmed by receiving on a
‘the image of a lighted candle, placed succe ious
from a double convex lens.
Firtuat image. There is another caso in w
Wz), is placed between the lens and its princ’
otis, Oc, be drawn from the poi:

point at n less distance then the principal focal distance (475),
ray, continued in an opposite direction, will cut the axis Oa, in the)
a, which is the virtual focus of the point A. ‘Tracing the sscosdiey
of the point B, it will be found, in the same manner, that the ¥2

Fig. 342,
focus of this point is formod at d ‘There is, therefore,

e magnifying power ia greater in propertio
vex, and the object netrer the principal focus

how the magnifying power may be caleulated by ms
relating to lenses (502). Double convex lenses,

480, mation of images in double concave
concave lenses, like convex mirrors, only give virtaal |
the distance of the object.

Let AB (fig. 343) be an object placed in front of rach

In like manner, drawing&
dary axis from the point 5,8
rays from this point form pencil of divergent rays, the direchast

which, prolonged, intersect in 6, Hence the eye sees at ob a virtual
image of AB, which is alcays erect and smaller than the object.

Fig. 343.

point. This is virtually the case with «
een tee

to the and definition of

be placed exactly in the focus of a

ee ere aoe repel

if the image is sharp at the edges,

t the contre. 3 This defect is very objectionable,
used for photography. It is partially obviated
Jenses dinphragma provided with a central aperture

Fig. 344.
near the centre, but cuts off those which
by combining two lenses of suitable curvature,

may be destroyed,
relating to lenses.—In al] lenses, the relation
of the image and object, the radii of curvature, and
index, may be expressed by a formula, In the case of a
Tet P be « luminous point, situate on the axis fig.
it ray, LE its direction within the lens, EP’ the
that P is the conjugate focus of P. Further, let OT
to the points of incidence and emergence, and
ee eee 3 7A =, NIP =i,

DISPERSION OF LiGhT. 445
voneare lenses, p’ and f retain the same sign, but that of p

CHAPTER IV.
DISPERTON AXD ACHROMATISM.

Decomposition of white light. Solar spectram.—The phe-
jon-of refraction is by no means so simplo as we have hitherto as-
[phon waite light, or that which reaches us from the sun, passes
ine medium into another, w is decomposed into several kinds of lights,
jomenon to which the name dispersion is given.

(der to show that white light is decomposed by refraction, a pencil
flight, SA (fig. 34), is nllawed to pass through a small aperture

‘window shutter of a dark chamber. This pencil tends to form a

Fig. 345.

(mm colourless image of the sun at K; but if a flint glass prism
fel horizontally be interposed in its passage, the beam, on emerg-
tm the priew, becomes refracted towards its base, and produces

screen a vertical band, coloured in all the tints of the
QS, which ix called the solar spectrum. In this spectrum there
reality, an infinity of different tints, which imperceptibly merge
ech other, but it is customary to distinguish seven principal colours.

|

DISPERSION OF LIGHT. 7

& refraction will be olverved, but the light remains unchanged;
4G, the imnge received on the scrven H is violet if the violet pencil
allowed to pass, blue if the blue pencil, and so on. Hence the
sof the spectrum are simple; that is, they cannot further be de-
by the prian,

the colours of the spectrum aro unequally rofrangible ; that
o different refractive indices. The elongated shape of the
would be sufficient to prove the unequal refrangibility of the
4 for it is clear that the violet, which is most deflected
‘the base of prism, is aleo most refrangible, and that red
iiedi is lenst doflected is least rofrangible. But the unequal refrangi-
: may be shown by numerous experiments, of

Eb the two following may be adduced :
of coloured paper, one red and the other violet,
Sther on a sheet of black paper. On looking
, they are seen to be unequally displaced, the
it than the violet; hence the red rays are less

viol

am# conclusion may be drawn from Newton's experiment
Prinma. Onn prism, A (fig. 347), in a horizontal position,

‘of white light, S, is recived, which, if it had merely traversed
prim, A, would form the spectrum, rv, ona distant screen. But if
pram, B, be placed in a vertical position behind the first, in
‘that the refracted pencil passes through it, the spectrum

d towards the base of the vertical prism, but instead

deflected in a direction parallel to itself, as would be the case

DISPERSION OF LIGHT. 449

te. When the mirrors are moved so that the separate
iporposed a single image is obtained, which is white,

Fig. 352.
Newton's diso it may bo shown that the seven
form white, This isa cardboard disc of about

Fig. 354,

(ater, the centre and the edges are covered with black
(the space between there are pasted strips of papers of

G

Fig. 356.
light on a dark ground. The acale is then placed at m in|
focus of the lons e, consequently, when the scale is lighted by

Fig. 367.

second face, and leave the prism by
right angles to that face. By this
direction parallel to its axis,

‘hich experiment is to be made ;
theso salts the motala modify the transmitted

is a jet of ordinary gas. The apparatus
burner.

orifice to admit air to support the combustion of

‘orifice can be more or lees closed by a small diaphragm
aregulator. If we allow a moderate amount of air to entor,
Dib irises Sarne, and tho Tinea ozs checnsed. But if
current of air enters, the carbon is rapidly oxidisod,
‘brightness, and burns with a pale blue light, but with
In this state it no longer yields a spectrum. If, how-

eye.
it ‘many of these more refrangible rays, which is
to the same extent with quartz, When prisms

om is closed by means of a piece of blue glass,
| to fall upon a piece of canary glass, it instantly

Yalu gives yellow and violet light from about
folet; an alcoholic solution of chlorophylle gives

produces more highly refrangible rays.
very remarkable spectrum. With quartz
@ spectrum six or eight times as long as the
x2

ee

ACHROMATISN. 461

lain this result, lot two prisms, BFC and CDF, be joined and
‘direction, as shown in fig. 360, ad ee peers

Prisms aro of the same material, but that the

ofthe setonds CDF ss than the refracting angle of

‘will produce the same effect ass single prism,

that white light which traverm it will not only be

Af, on the contrary, the first prism, BCF,

Fig. 360;

sa a fea is fread
imi
fanneuteg angle (483), it follows that shih end

ra iron, neutood sii aae erage
parallel, and therfore give white light, _Nevortholess,
wmgles BOF and CFD, which is suitable for the en

| wera suitably combi

not destroyed ecard tie us thie diepersion ; that
t of a body varied in the same
not the case. Consequently the

of two lenses of unequally dispersive ma-
is a diverging concavo-convex (fig. 359) :
8 is double convex, and one of its faces may
‘the concave face of the first. As with prisms,
‘necessary to obtain perfect achromatiam ; but for
are suflicient, their curvature being such as to

CHAPTER Y.

OPTICAL INSTREMENTR,

498. The different kinds of optical instruments.—Ily
optical instrument ia meant any combination of lenses, or of
mirrors, Optical inatruments may be divided into three classes:
ing to the ends they are intended to answer, vin z—i.
which aro designed to obtain n magnified image of any abject
dimensions too small to admit of ita being seen distinctly
naked Telescopes, by which very distant objects,
celestial or terrestrial, may be observed. iii. Tnsiriments
to project on sereen ified or diminished image of any
which can thereby be either depicted or renderod visible toa
spectators ; such as the camera fucida, the camera obscura,
apparatus, the magic lantern, the solar microscope, the,
zcope, ete. The two former classes yield virtual images, the let,
the exception of the camera Iweida, yield real images.

MICROSCOPES,

499, The simple microscope.—The simple microscope OF
giazs is merely a convex lens of short focal length, by means off
look at objects placed between the lens and its principal focus
(fig. 861) be the object to be observed placed between the Hens

Fig. 381.

principal focus, F. Draw the secondary axes AO and BO, and alg
A and B rays parallel to the axis of the lens FO, Now these aa
passing out of the lens, tend to pass through the second priicipal!
F’, consequently they are divergent with reference to the secondary)

depending on data that art of easier
Tn fig, 368 let AB be the object, and A‘B’ its image
distance of most distinct vision, Leta’ be the p
AB’. Then, since the eye is very near the glass, the:
ee or A that in Are But sinee the
are similar, A’B’: AB =DO: CO. Now DO

Fig. 368,

distinct vision, and CO is very nearly equal to FO, the focal lent
lena, Therefore the magnification equals the ratio of the di
most distinct vision to the focal length of the lens. Henos wi)
that the magnification is greater:—Ist, as the foral length of tht}
smaller, in other words, as the lens is more convergent;
observer's distance of most distinct vision is greater,

By changing the lens the magnification can be incressed bi
within certain limits if we wish to obtain a distinet image B
asimple microscope distinct maguification may he obtained
diameters.

The magnification we have now considered is fimear m
Superficial magnification equals the square of the Hnear mig
for instance, the former will be 1600 when the latter isd
503. Compound mtcrescope.—The compound micro
simplest form consists of two condensing lenses ; one with n 4hot!
called the object glass or objective, because itis turned towants Bt
the other is less condensing, and is called the eygoiece or pet
it is close to the observer's eye,

Fig. 367 represents the path of the luminous rays, and the
of the image in the simplest form of a compound miarsscopt

467

focus of the object glass, M, but

a real image, ab, inverted and somewhat

the other side of the object glass (479). Now
; ss, Mand N, is such that the postion of the
N, and its focus, F. From this it

a3 a simple microscope, and
, 2%, is seen, which is virtual,
rh erect as
Tt may thus be anid,
Lt ote than a simple micro-
but to its image already magnified by

A (a) principle of the com-
eet expla the Lace
principal ac:

microscope known as

“am arrangement by which the tube could be

‘ot horizontally, and Chevallier was the first to

he ‘use of achromatic lenses, The figure shows

‘& Horizontal position, which ia loss fatiguing for the

be placed vertically. This is effected by removing

utting the long tube A, which contains the eyepiece,

‘object glass, E. Tho microscope may also be

lined position by removing @ pin, m, which fixes the

‘part; the whole system then moves on a hinge,
‘microscope on a cylindrical column,

@ rod, parallel to this column, is the age, B. This

‘by a pinion working in a rack by means of «

object, o, to be observed is placed on the stage be~

of glass, ©. The diffused light of the atmosphere is

object by means of & concave glass reflector, M;

Fig. 368,
Fig. 369 shows the position of the glasses, and the
rays in the microscope. The object glass, R, may be
two, or three lenses ; in this case there aro three, whose _
distances are 8 to 10 millimeters. The eyepiece is formed
convex lensos, m und x, ‘The path of the rays is ensily:
luminous rays, after being reflected from the mirror,
wands the object, o, and aro thence directed towards the
Having traversed it, thoy fall on a glass prism, p, on whose
they experience total retlection (464). The luminous rays
the tube AB, and, falling on the lens m, form at be a real

interm De Be isis vi spk eh
eed abt fall on the eyeglass, m. It

‘the internal reflection, which might

the inside of the tube is blackened.
tmieroscope varies according as the object is
An the former case the object is illuminated as
@ reflector placed below the stage. In the second
called the bull's eye is used; it is placed on the

4 the rays S on the object.

numerous eyepleces and object glasses, by
‘varivty of magnifying power is obtained. A
‘sales obtained by removing one or two of the

the essential features of the microscope; it is
: of forms, which differ mainly in the construction

nent of the lenses, and in the illumination.
the student is referred to special works on the

of two lenses the Jens to which the eye is
» the one towards the object glass is called

Spani. ‘The relation between the focal lengths of the lenses is na
‘The focal length of the fieldglass is three times that of the
and the distance between their centres is half the sum of the
_ It easily follows from this that the image of the point @
of the fieldiens, be formed at D, which

a]
== Eas

Fig. 371.
; if we suppose the fieldlens removed, the pencil of
eee & focus at A, and none of them would

now within the field of view. It is in this
on of an eyepiece for a single eyelens enlanres

Micrometer.—The magnifying power of
is the ratio of the magnitude of the image to the

478

‘TELESCOFES.

ig the leavenly bodies; like the microscope, it consiats of a
(eing eyupiece and object glass, The object glass, M (fig. 373),
(Between the eyepiece, N, and its principal focus an inverted

fie faeecty betas fhe laps, once cc

Fig. 873.

the heavenly body, aud this eyepiece, which acts as a magni-
then gives a virtual and highly magnified image, a’#/, of the
‘The astronomical tolescope appears, therefore, analogous to
but the two instraments differ in this respect: that in
the object being very near tho objective, the image is
miuch beyond the principal focus, and is greatly magnified, so
the object glass and the eyepiece magnify; while in the
Sanikeal telescope, the heavenly body being at great distance, the
gaya am parallel, and the image formed in the principal focus of
glass is much smaller than the object. There is, therefore, no
exeept by the eyepiece, and this ought, therefore, to be of

focal length,
BTA shows an astronomical telescope mounted on ita stand. Above
inemall toloscope, which is called the finder, ‘Telescopes with a
power are not convenient for finding « star, as they
sauall field of view : the position of the star is, accordingly,
by the finder, which has a much largor field of view, that
(a far greater extent of the heavens: it is then viewed by means

(note, p. 485) equals 47 (Ag. 378), hati, it equal
therefore is approximately equal to o F being the focus of

glass, M, and being supposed very nearly to coincide with
(aH Of the eyeploos, N; it may, therefore, be concluded that the

a

Fig. 874,

When the telescope is used to make an accurate ob
stars, for example, their zenith distance, c
over the meridian, a cros wire is added.
two very fing metallic wires or spider 1]

Fig, 875.

of the telescope, which thus becomes the line of sight

500, Terrestrial telescope.—The ferreatrial
astronomical telescope in producing images
‘This is offected by means of two pee Sees glasses,
placed between the object glass, M, and the eyepites, Re
being supposed to be at AB, at a greater distance than can
the drawing, an inverted and much amaller image is fors
other side of the object glass, But the second lens, Py
distance that its principal focus coincides with the
which it follows that the luminous rays which pass
example, after traversing the lena, P, take a direction

TELESOOPES, 475

filary axis, 40 (475). Similarly the rays passing by @ take a
(tim parallel to the axis, a0. After crossing on H, these various
|Qavere = third lens, Q, whoee principal focus coincides with the
$H. The pencil, BSH, converges towards 3’, on a secondary nxis,

1
fer ata’, an erect image of the object, AB, is produced at al.

image is viewed, as in the astronomical telescope, through a
(sing eyepiece, R, so placed that it acts as a magnifying ginss,
| its distance from the image, a’d/, is less than the principal focal

hee; henes, there is formed, at a)’, a virtual imngo of a’¥, erect,
{uch magnified. Tho lonscs P and Q, which only serve to rectify
peeition of the image, are fixed in « brass tube, at a constant
fies, which is oqual to tho sum of their principal focal distances,
Object glass, M, moves in a tube, and can be moved to or from the
P, no that the image, ab, is always formed in the focus of the lens
be the distance of the object. The distance of the lens R
he varied so that the image a’ may be formed at the distance
vision.
pstresient may aleo be used as an astronomical telescope by
Pa different eyepiece; this must have a much greater magnifying
(than in the former cases,
(Hie terrestrial telescops the magnifying power is the same as in
Wetonomical telescope, provided always that the correcting glasses,
/, have the same convexity.
Galilean telescope.—Tho Galiloan telescope is the simplest
Telescopes, for iv only consists of two lenses, namely, an
M, end a diverging or double concave eyepiece, Ri (fig.
‘god it gives at once an erect image. Opera glasses are constructed
ple.

‘Ohject be represented by the right line, AB, « real but inverted
(eataller image would be formed at da; but in traversing the eyepiece,

Taye omitted from the points A and B are refracted, and diverge
[tbe seconiiary axes, 14’ and a0’, which correspond to the points 6 and
heimage. Hence, those rays produced backward meet their axes in

—— =]

476 ON LIGHT.

a’ and 8°; the eye which receives thom snes accordingly us
magnified image in «8, which eppors nearer because it ia tal

dal igre than the angle, AO'B, under which the

Er rrhswaapialviog porren kei! to the ratio of the angle a\t!
angle AO'B, and is usually from 2 to 4- -
The distance of the eyepicce RY from the image wy r

Fig. 377.

equal to the principal focal distance of this —— + it follows,
fore, that the distance between the two lenses is the difference
their respective focal distances: honce, Galileo's telesoape is wry
It hus the advantage of showing objects ix
m, and, further, as it has only two lenses, it absorbs very
in consequence, however, of the divergence of the emenguit
it has only a small field of view, and in using it the eye must be
very near the eyepiece, The eyepiece can be moved to or fru!
object glass, so that the image a’b’ is alws
distinct vision.

The opera glass is usually double, so as to produce an image in
eye, by which greater brightness is attained.

The time ut which telescopes were invented is not knows, Saat
tribute their invention to Roger Bacon in the 13th century; othe
J. B. Porta at the end of the 16th; others agsin to « Dutchman, J
Metius, who, in 1609, accidentally found that by combining two
one concave and the other convex, distant objects appeared meant
much largus

Galileo's was tho first telescope directed towards the beayens, By
means Galileo discovered the mountains of the moon, Jupiter's
and the spota on the sun,

511, Reflecting telescopes.—Tho telescopes previously:
refracting or dioptric tolescopes. It is, however, only in recent ti
it has been possible to construct achromatic lenses of Snipe
this, a concave metallic mirror was used instead of the
Telescopes of this kind are called reflenting or caloptrse telestopet |
principal forms are those devised by Gregory, Newton,
Cassegrain.

J tig a8

i i alad equatorinlly mounted. Fig. 879 gives a longitudinal
‘of @ long brass tube closed at one end by a concave
‘which is perforated in the centre by » round aperture

i As the contre of curvature of the large
| its focus at ab, rays, such as SA, emitted from a

P, which predentin,
wi
sen i hat iene
As the objects viewed are not
mirror,

face marin calcd by lela se Ta
from the larger one; this is effected by means of » milled |
883), which vurns a rod, and this by a screw moves a piece to
mirror is fixed,

constructed mirrors of 8, 12, 13 inches, and at
had completed one of 32 inches diameter.

Fig. 381 represents a Newtonian teleseope mounted om
stand, and fig. 350 gives a horizontal section of it, This
how the luminous rays reflected from the parabolic mirror,
rectangular prism, mn, which replaces the inclined plane
the old form of Newtonian telescope. After undergoing &

| fix the telescope in declination, there is a brass plate, E,
Fi ‘It is provided with a clamp, in which the limb O

a copper plate, on which there is also the small prism
section fig. 380, To bring the image to tho right

480 ‘OS LIGHT,

place, this plate may be moved by means of n rack and mil
‘The handle, n, serves to clamp or molamp the screw, V, The

Pig. 381.

was one taken from n telescope, the mirror of which ix only he
in diameter, and which gives a magnifying power of 150 to 200.

514. The Werschetian telescope.—Sir W, Herachel’s teew
which, until recently, was the most calsureted tuitrument’ of

482 os LiGET,

slide in and out. ‘The luminous raya, R, pass oy
sod foem an image on the oppealte aida, (/" ici

‘more or less about an axis parallel to

n the face, AB, is turned towards the object,

r ‘perpendicular on this face, pass into the

Tufraction, and are totally reflected from

is perpendicular to BC, and nl. to AB, the angle

dle B, that is, it will contain 674°, and this being

al angle of glass (404), the ray Ln will undergo

aro again totally reflected from 0, and emerge

direction almost perpendicular to the face DA,

3 Esper emeese Lay hangs ish a
Yr

483 os ErontT,

object L. Af the outlines of the image are ct
is obtained ; vs meee

divergence. In this case, however, it
near the edge of the prism, so that the ett
into two parts, one of which sees the image, and the

Amici’s camera lucida, represented in fig, Sst
‘Wollaston, inasmuch as it allows the eye to change its
siderable extent, without ceasing to ec the image and the
same time. It consists of a rectangular glass prism, ABC,
its perpendicular faces turned towards the object to be
the other is at right angles to an inclined plate of glass, tom
LI, proceeding from the object, and entering the prism, ar
flected from its buse at D, and emerge in the direction KH.
then partially reflected from the glass plate mus at H, and form)
image of the object, L, which is seen by the eye in the direction
eye, at the same time, sees through the glass the point of a penel
to the paper, and thus the outline of the picture may be traced
exactness,

Fig, 388.

Fig. 389,

517, Magic tantern.—This is an mpparatus by which a
image of small objects may be projected on a white serve |
room. It consists of a tin plate box, in which there is a lamp
the focus of a concave mirror, A (ig. 888). ‘Whe rellected raya

4 . |

ay a ouBla conver Ving, O ata die
than its focal distance, and, consequently,
Passi fecagub i Agere co toa pane i pote’

in such a manner that both pil

ly seen to change into the other.
| ‘power of the magic lantern is obtained by dividing
Gf the lens © froea tha image by ite distance from the object.
is 100 or 1000 times farther from the lens than the object,
dll be 100 or 1000 times as large. Hence a lens with a very
mm produce a very large image, provided the screen is suffi-

f microscope.—The solar microscope fs in reality a magic
inated by thu solar raya; it serves to produce highly mag-

Fig. 300.

my emall objects. It is worked in a dark room; fig.
‘fitted in the shutter of a room, and fig. 391 gives the

Ion a plane mirror, M, placed outside the room, and
# « condensing Jens, /, and from thence to » second

‘PHOTOELECTRIC MIcRoscort. 487
curious phenomens, such, for instance, as the circulation

Fig. 392.

ith which it can be procured at any time of the day, ia far
[ethesolarlight. ‘The photoeleetric microscope alone will be
fre: the electric light will be considered under the head of

Topresents the arrangement dovised by M. Duboseq. A

(cope, ABD, identical with that already described, is fixed on
(of s bres box. In the interior are two charcoal points which

b

Fig. 393,
(Mia igs eal dase

il cas bo seen at a distance of above 4
a

| PHOTOGRAPHY. 491

hind operation comiists in exposing the sensitive plate to the
‘Tight, placing it in that position in the camera where the image
fed with greatest delicacy, For photograpbie purposes a camera
if pecaliar construction is used. The brass tube A (fig. 395)
(mn achromatic condensing lens, which can be moved by means
(work motion, to which is fitted a milled head, D. At the op-
fof the box is a ground glass plate, E, which slides in a groove,
|

\

| Fig. 395.
{fhe cass containing the plate also fits. The camera being
(a proper position before the object, the sliding part of the box
fd until the imge i produced on the glass with the utmost
§ this is when the ginss slide is exactly in the focus, The final
made by means of the milled head, D.
‘i# then replaced by the case containing the sensitive
lide which protects it ie raised ; and the plate exposed for a
‘duration of which varies in different cases, and can only be
‘by great practice. The plate is then removed toa dark room,
perceptible to the eye, but those parts on which the light
lave acquired the property of condensing mercury: the plate
i a box and exposed to the action of mercurial vapour at

‘is deposited on the parts affected in the form of globules
‘to the naked eye. Tho shadows, or those parts on which
bat ot acted, remain covered with the layer of iodide of silver.
with hyposulphite of sodium, which dissolves
Without affecting the rest af the plate. The plate is next
of chloride of gold in byposulphite of sodium, which
‘silver, While some gold combines with the mercury and silver
attacked, and grast)y increases the intensity of the lustre.

Sl

4

piceoen aoa
in the anterior than in the posterior part.

”

Pig 297.

into the sclerotica,

anatomists have considered them as one and the same, and
guished them by calling the cornea the ¢rangparent, and the
opaque commen,

Solerotica. The sclerotica, é, or sclerotic cont, is a membriit
together with the cornea, envelopes all parts of the eye. In fr
is an almost circular aperture into which the cores fites
perfornted so ns to give passage to the optic nerve,

Iris, The iris, d, is on annular, opaque diaphragm, placed |
the cornea and the crystalline lens, It constitutes the
the eye, and is perfornted by un aperture called. the pupil, |
is circular. In some animals, especially those talon to
felis, it is narrow and elongated in a vertical direction i
it is elongated in a transverse direction, It is « on
‘ond its diameter varies in the same individual between O12
an inch; but these limite may be exceeded. The luminous
the eye through the pupil. The pupil enlarges in darkness,
under the influence of a bright light. These alternations of
and enlargement take place with extreme rapidity; they are veryl

Thickness of the crystalline ; 2

Distance from the pupil to the cornea

Length of the axis of the eyo. o 04
‘The curvature of the cormea, according to ices is
ellipsoid of revolution round its major axis, and the :
crystalline that of an ellipsoid of revolution round its minor
628. Path of rays in the eye.—From what has bee
the structure of the eye, it may be compared to a cam
of which the pupil is the aperture, the crystalline is the
and the rotina is the screen on which the image fs formed.
effect is the sume as when the image of an object placed |
double convex lens is formed in its conjugate focus. Let
‘be an object placed before the eye, and let ws consider tf
from any point of the object A. Of all these rays
directed towards the pupil are the only ones which p

—_=

el be Ua image of the point A.
like manner an image of it at

to show that the images

inverted, the eye of an albino or any
taken ; this has the advantage that, as the

if light can traverse it without loss, ‘This

d at its posterior part of the cellular tissue surrounding it,
in the shutter of a dark room; by means of a lens it

‘that inverted images of external objects are depicted on the

; (SRI I ae Mee lle
ologista, and many theories have been proposed to

| RSE ao anon etapa ters Some
it is {femeped and by @ regular education of the

‘penses, such as that of touch, Miller, Volkmann,
aa ee everything inverted, and not simply
tt can appear inverted, because terma of
‘Tt must, however, be admitted that none of

is, optic angle, visual angle.—The principal optic aris
of its figure; that is to say, the straight lit ree

‘is symmotrical, In a well-shaped eye it is the

3 the centre of the pupil and of the erystal-
ee. ‘The lines Aa, Bb, which aro almost

eye sees objects most distinctly in

He

F

between the figures, and they are steadily looked at, x by
and / simultaneously by the left, for a few seeands, there will
aingle picture having the untistakeable sppearance of
without « card interposed, the eye, by a i
taught so to combine the two as to form

pictures will in that case be seen, the central b

outside ones plain. Fig, 402 will explain thia. Let r
corresponding points, say the points marked by a.

draven above; R and L the positions of the right

Fig. 402.

i ‘the apparent position of the pictures, so that they
} men in the same direction, and their combination by the
rendered ey and almost inevitable. If ab ab (lig. 408)
‘mirrors inclined to one another at an angle of 00°, the two
y, would both be seen by the eyes situated at R and L in the
(marked by the dotted arrow. If, instend of the arrows, we now

eeeeaeteeanadl

4 <s

val

4 pair of dissimilar pictures as we have spoken of above,
solid object, it is evident that, if the margins of the pictures

points of the pictures will not. The eyes,
effort, soon bring such points into coincidences

follow almost the opposite direction: there is first a
white ground, which gradually changes into bine, is
green and yellow, and ultimately cannot be di

may thus appear much larger than they really aro; also

of the moon when two or three days old, the brightly

[crescent seeming to extend beyond the darker portion of the
‘it in its grasp.

J who has investigated this subject, finds that irradiation differs

in different people, and even in the sam ee differs on

and, of course, obliquely, and the eye be then dine
defined line of light, such as a slit in the shutter of a

a strip of white paper on 8 black ground, this line of light
@ complete spectrum.

Miiller concluded from theso exporimenta that the
achromatic as long as the image is received at tho fox
when it is accommodated to the distance of the object.
apparent achromatism cannot be exactly stated. It has.

very little refracted, from which it follows that the chromate
is impereeptible (496).

As to the spherical aberration, we have already seen
rected by the iris (525), ‘The iris is in point of fact a
arrests the marginal mys, and only allows those to pass
the axis,

543, Short sight and long sight: myopy and y
most usval affections of the eye are myopy and preebytiam,
and long sight. Short sight is the habitual accommodation o

distance less than that of ordinary vision, so that persons:

only see very near objects distinctly. ‘The usual cau
too great convexity of the cornea or of the

en too convergent, the focus, in place of forming on the
in front, 0 that the image is indistinct, It may be remedied
diverging glasses, which in making the rays deviate from
axis throw the focus farther back, and cause the image to
the rotina.

‘The habitual contemplation of small objects ag when children

the imago approsches tho retina, and when they are
is exactly formed upon it, so that the object is

cted by means of converging lenses, These glasses
before their entrance into the eye, and, therefore,
is properly chosen, the image will be formed

jeukh
tases. Spectacies.—The glasses commonly used by
ghted persons are known under the goneral name of eye-
voles, Generally speaking, numbers are engraved on these
xpress their focal length in inches.
eter ee Sod eho perace cag to we may be
‘The formula

| 7 a
Perea ot ditinct viiea in cbdtzary case (boas
Ifiee sete ot atnne rion fo te venen afecod by

is obtained from the equation 1 —1 by sub-
poe fF

In this case the formula (6) of article 482 is used,

, because the image seen by spectacles being on the

ct in reference to the lens, the sign of p’ ought to

of p, as in the case of virtual images from the

ans, fi calelated bythe formula 2 — I= —?
‘concave lonses, and which, replacing p’ by d, gives

f= —F See e as eet

®

entirely super
is much more distinct than the other. ng
co-operation of two unequal eyes, but it may

is complete, while in others some colours can be very i

Persons affected in this manner can distinguish the outlines
without difficulty, and they can also discriminate b I
ehade, but they are unable to distinguish the different tints

D'Hombres-Pirmas cites an primaire ith
matopsy, who had painted in a room a landseape of which
trees, houses, and men were all painted blue, and ig:
had not given each its proper colour, he replied that
assimilate the colour of his drawing to that of perp
was red.

Achromatopey is also sometimes called Davfamirn, because D
has carefully described it, was so affected.

547. Ophthatmoscope.—This instrument, as its name
designed for the examination of the oy, and was invented in 1851
Helmholz. It consists:—1. Of a concave spherical reflector of
motal, M (figs. 406, 407), in the middle of which ia a small hole a
a aixth of an inch in diamotor. Tho focal length of the
8 to 10 inches. 2. Of a converging achromatic lens, o,
front of the eye of the patient. 3. Of several lenses, some ‘
others divergent, any one of which can be fixed in a framo behis
mirror so as to correct any given imy
the mirror is of silyered glass, it is not
the centro; it is sufficient that the silvering at the centre ba

‘To make use of the ophthalmoscope, the patient is placed in «
room, and a lamp furnished with a screen put beside him, B.

the patient, and with his
in front of the eye. By this ar~

Fig. 406.
the back of the eye is lighted up, and its structure can be

ig. 407 shows how the image of the back of the eye is produced,
ch the observer A sees on looking through the hole in the reflector.
‘be the part of the retina on which the light is concentrated,
Of raye proceeding from ab would form an inverted and aitial
of eb at eV. These pencils, however, on leaving the eye, pass
dh the lens o, und thus the image a6” is in fact formed, inverted,

et, and in 8 position fit for vision.

— +e
o——,
2’

Fig. 407.

great quantity of light concentrated by the ophthalmoscope is
rr painfully the eye of the patient, The: therefore in-
batween the Inmp and the reflector coloured glasses, to cut off
ays, viz., the red, yollow, and violet rays, The glasses
‘employed are stained green or cobalt-blue.

insolation or exposure to the sun. A large number
after having boon exposed to the action of solar light, or of
of the atmosphere, emit in darkness a phosphorescence,

‘by insolation.—This was first observed in
us (sulphide of barium), but M. Ed. Bocquerel
it Tae great number of substances. The sulphides

of ftuorspar; then

concretions, ipatite, heavy spar, dried

c and dried chloride of calcium, cyanide of calcium, a

‘of strontium or barium compounds, magnesium and its car-

Besides theso a large number of organic substances

‘by insolation; for instance, dry paper, sillk,
Siiaets Gas the Sect ect

it spectral rays are not equally well

! wabstances phosphorescent, The eee seus takes

‘violet rays, or oven a little beyond ; while the light emitted

bodies generally corresponds to rays of a smaller re-

‘than those of the light received by them, and giving rise to

“whieh phosphorescent bodies assume is very variable, and

‘body it changes with the manner in which it is pro-

‘Ta strontium compounds green and blue tints predominate; and
eens we Salpiee of besten

‘varies also in different bodies. In the

ieee strontium phosphorescence lusts as much as

SATE cilia wligtaneaa It does not exceed fav seovada Ge

of a second.

Insta a few minutes or even a few seconds, it is simply

them to solar or diffused light for a short time, and then
darkness: their luminosity is very apparent, especially
vioualy been taken to close the eyes for a few instants. Bat in|

Fig. 408,

af bodies whose phosphorescence lasts only a very short
method is inadequate. M. Becquerel has invented a very i
paratns, the phosphoroscope, by which bodies can be viewed
after being exposed to light: the interval which separates the
and observation can be made as small as possible, and
great precision,

the open parts of the one cor~
» The two screens, aa al
to the axis, which by means
d by a handle, can be made to tum with apy

‘the phosphorescence of any body by means of
‘is placed on a stirrup interposed between the
_ The light cannot pass at the same time through
rt ‘of the sides A and B, because one of the closed
MM, or of the screen PP, is always between them.
hen « body, a, is illuminated by light from the other side of
fatus, it could not be seen by an observer looking at the aperture
@ it would be masked by the screen PP. Accordingly, when an
taw the body a, it would not be illuminated, as the light would
opted hry the closed parts of the screen MM. The body @ would
ly appear and disappear; it would disappear during tho time of
and appear when it was no longor so, The time
lapses between the appearance and disappearance depends on,
ity of rotation of the scrocns. Suppose, for instance, that they
) turns in a second ; as one revolution of the screens is effected
‘a second, there would be four appearances and four disnppear-
ting that time, Hence the length of time clapsing betwoon the
Usenination and of observation would be } of y}5 of a second
{of & second.
{ations with the phosphoroscope are made in # dark chamber,
(rer being on that side on which is the wheelwork. A ray of
‘electric light is allowed to fall upon the substance a, and
ms being made to rotate more or less rapidly, the body «
mani by transparence in « continuous manner, when the in-
insolation and observation is less than the duration of the
‘of the body. By exporimonts of this kind, Booquerol
| that eatstances which usually are not phosphoreecent become
; such, for instance, is Iceland spar. Uranium
the most brilliant appearance in this apparatus; they
‘bright luminosity whon the observer can see them 0-008 or

CHAPTER VIIL
DOUBLE REPRACTION. INTERFERENCE, POLARISATION.

due to undulations propagated through the air,
undulations cause the drum of the ear to vibrate and p
tion of sound. In the former case the undulations caus
retina to vibrate and produce the sensation of light. The
differ in this, that in the case of sound there is i it
the existence and vibration of the medium (air) which
undulation, whereas in the case of light the existence of
and its vibrations aro asmmed, because that

explains in the most complete manner a long series of
nomena. There is, however, no independent evidence
the luminiferous ether.

The analogy between the phenomena of sound and light is:
thus, the intensity of a sound is groater as the amplitude of
of each particle of tho air is greater, and the intensity of fi
as the amplitude of the vibration of each particle of the etl
Again, a sound is more acute as the length of each und
the sound is leas, or, which comes to the same
number of vibrations per minute is greater. atten
of light is different according to the length of the u
the light; a red light is due to a comy Jong u
‘corresponds to a deep sound, while a violet light is due to @
lation, and corresponds to an acute sound.

Although the length of the undulations cannot be o d
yet they can be inferred from certain phenomena with great
‘The following table gives the longth of the undulations
to the light at the principal dark lines of the spectrum. ‘The
given in decimals of an inch.

B
o
D
E
Fr
G

Bes 8 Sita van very seco, within which the

ur of a body is due to the power it has of extinguishing cer-
reflecting others; and the body appears of the colour
the coexistence of the reflected vibrations. A body appears

| it reflects all different vibrations in the
it in the spectram : it appears black when it reflects light
mall quantities ns not to affect the eye. A red body is one
Siipcoperty of reflecting. in predominant strength those
produce the sensation of red. This is seen in the fact
paper is held against the daylight, and the re-

spectrum of a brighter red, and

paper held in the blue part appears of a brighter blue;

: in the violet or blue part uppoars almost black.

‘the red paper can only reflect red rays, while it extinguishes

‘and as the blue of the spectrum is almost froo from red, 80
| that the paper appeara black.

ory theory likewise explains the colours of transparent

vibrating motion on reaching a body sets itin vibration. So

jof the luminiferous ether are communicated to the ether

‘petting itin motion produce light of different colours, When

transmitted through any body, it is eaid to be transparent

‘according to the different degrees of strength with which

ssion is effected. In the opposite case it is said to be opaque,

extinguished, nd thn palocr da doa thie org
‘produce blue light.
The

phenomenon by
that of the mode of propagation of a plane wave.
Sein Bi sy ot P

will mutually destroy each other

quently the wave advances ax H its

being the successive positions of the if

in the medium with a yelocity e, it will a space
Now let us conceive a plane wave moving through

at an angle I, the plane surface of an ordinary | eting 1

suppose its velocity of propagation in vacuo to be », and in

to bev. Itis obvious, that, if the wave begins to enter at a

of the wave will advance within the rt

parts being inclined to each other ata certain angle, and nt;

time, ¢, the perpendicular distances of these two parts &

and vt, Consequently, if R is the angle which the par

within the medium makes with the surface of the m

have
Sin I; sin R =: ot; wt
But a succession of parallel plane waves
parallel rays at right angles to the waves; |
any one of these rays, I and Rare the angles of i

‘DOUBLE REFRACTION. 519

, if is the refmotive index of the substance, » + v’ equals x,
t " Now, under all circumstances, » is greater than 1,

eis greater than v’, a result which coincides with that ob-
experiment (496).

DOUBLE REFRACTION,
Doubdle refraction.—It has bein already stated (460) that a
t of erystals possess the property of double refraction, in
thich a single incident ray in passing through any one of them
into two, ot undergoes bifurcation. Whence it follows that,
pobject is seen through one of these crystals, it appears double,
the existence of double refraction in Iceland spar was first
h rtholin in 1689, but the law of double refraction was firat
ud exactly by Huyghens in his treatise on light written in 1678,
im 1690.
Maly which possess this peculiarity are said to bo double refracting,
d to a greater or leas extent in all crystals which do not belong
c tema. Bodies which crystallise in this system, and those,
‘glass, are destitute of crystallisation, have no double refrac~
‘property can, however, be imparted to them when they are
or when they are cooled quickly after having been
[it which state glass is said to be unannealed. Of all substances,
lich possesses it most remarkably is Iceland spar or carbonate of
. In many substances the power of double refriction can hardly
p exist diroctly by the bifurcation of an incident ray; but ite
shown indirectly by their being able to ‘depolarise’ light.
ined double refraction by assuming that the etber in
bodies is not equally elastic in all directions; from
that the vibrations in certain directions at right angles
‘are transmitted with unequal velocities ; these directions
on the constitution of the crystal, This hypothesis is
‘by the property which glass acquires of becoming double
oedteal anunneéaled and by pressure.
erystals.—In all double refracting crystals thero is
and in some a second direction possessing the following
‘When point is looked at through the crystal in this parti-
It does not appear double. The lines fixing these directions
‘exes; und sometimes, though not very properly, axes of
A.exystal is callod wniaxial when it has one optic axis,
r when there is one direction within the crystal along which
without bifurcation, When a crystal has two

Hepapeser
pres

angles to the face and parallel to the optic axis, Tr fee
pose the edges of the rhombohedron to be
abed contains the optic axis (ab), pe
and chbg; consequently it is parallel to the principal pa
‘of either of those two faces, For this reason acbd is oftett
sieied plane with respect to those faces.

and oxtraordinary ray.—Of the two mys
an Noli ray is divided on entering s uniaxial crystal,
the ordinary and the other the extraordinary tay. The
follows the laws of single refraction, that is with respect to
sine of the angle of incidence bears & constant ratio to the:
angle of refraction, and the plane of incidonce coincides with the j
refraction. Except in particular positions the extraordinary ry
neither of these laws. The images corresponding to the ordi
extraordinary rays are called the ordinary and «
respectively.

If transparent specimen of Icoland spar be placed over a dots
‘on a sheet of white paper, the two images will be seen. One:
the ordinary image, will seem slightly nearor to the eye than i
the extraordinary image, Suppose the [spectator to view 0
direction at right angles to the paper, then, if the crystal, with)
still on the paper, be turned round, the ervinary image will
fixed, and the extraordinary image will describe a cirele
line joining them being always in the direction of the sho
of the face of the crystal, supposing its edges to be of equal

the ordinary ray always obeys

‘incidence,
refiaction (461). ‘The refractive index for

of Lo ae pectiegeredpatdepeeigetgaatony

‘tan important difference between the velocity of the ray and
‘of the corresponding plane wave. If the velocities of the
to the ordinary and extraordinary rays are
‘the difference between the squares of these velocities is
to the square of tho sine of tho angle between the axis

‘and the normal to that plane ware which corresponds
wy ray. The normal and the my do not generally

gave a very remarkable geometrical construction, by means
Pree of ths tatnstel saya canbe determined when ths
a oes Bac rtiay fo he
‘This construction was not generally accepted by
tnd subsequently Malus showed its truth by
Eememenrecoents,
) and negative uniaxial orystal.—The term extra-
index has been dofined in the last article. For the

the fundamental form of Sa Soe ee
‘Th Hane rn Ue the ane different in

‘Whon a ray of light enters « biaxial crystal, and passes in att
not coinciding with an optic axis, it undergoes b 0
however, neither ray conforms to the laws of single refrac
are extraordinary rays. To this general statement the
tion must be made. Ina el
line one ray follows the laws of ordinary refraction,
right angles to the supplementary line the other ray fi
ordinary refraction.

INTERFERENCE AND DIFFRACTION. —

559. Interference of ligbt,—The name interference
mutual action which two luminous raya exert upon
are emitted from two neighbouring sources, and

ae

ISTERFERENCE OF L{GitT. 523

Je. This action may be obeerved by moans of the following
[fn the shutter of a dark room two very small apertures
the same diameter, at a very slight distance from each
spertures are closed by pieces of coloured glass, red for
hich two pencils of homogeneous light are obtained, These
tm two divergent luminous cones, which meet at a certain
fare received on a white screen a little beyond the place at
tet, and im the segment common to the two dises which form
ten some very woll defined alternations of red and black
|. If ane of the two apertures be closed, the fringes dis~
we replaced by an almost uniform red tint. From the fact
fringes di: when one of the beams is intercepted, it
bist they arise from the interference of the two pencils which
Peete eacie hy Gotmsia bet wun modified by
Qaldi had drawn from it the conclusion that light added to
—— The full importance of this principle remained

|, until these inquiries were resumed by
Gooch of whom the lattor, by modification of Grimaldi's
tndered it an experimentum crucis of the truth of the un-
thesis,

Fig, 411.

‘s experiment diffraction (500) takes place; for the luminous
be edge of the aperture. In Fresnel’s experiment the two
fe without the possibility of diffraction.
tairrora M and N (fig. 411), of metal are arranged close to
‘as to form a very obtuse angle, MON, which must indeed
less than 180°.
red light, which passes into the dark chamber, is brought to
of w lens, L, in front of the mirrors, and falls partly on
on the other, After reflection, the luminous raya which

2]

any point
Geypose the distances of A Featitea E98 ieaee ieee
‘Thon the undulations which reach A will always be in #

would reach A in exactly opposite phases. Oo
Iocity would be communicated at any instant apart
one undulation, an exactly equal and ite veloc
nicated by the other undulation, and n
‘at rest, or there would be darkness at that point;
duced in a manner precisely resembling the

DIFFRACTION OF LIGHT, 525

t) will be equally true of points on the screen which is
je at right angles to the plane of the paper. Between the
k lines the intensity of the ight will vary, increasing gra-
larkness to its greatest intensity, and then decreasing to the
ne, and 20 on.
fred light any other coloured light were used, for example
exactly similar phenomenon would be produced, but the
iiark line to another would be different. If white light
iorseste colour tends to produce a different set of dark
Bee Rateg pecspnet on cach other, and not coin-
itk lines due to one colour are illuminated by other colours,
Of dark lines a succession of coloured bands is produced.
f coloured bands produced by white light is much amaller
her of dark lines produced by a homogeneous light ; since
tance from the middle band the various colours are com-
d, and a uniform white light produced.
(etion and fringes.—Diffraction is a modification which
bs when it passes the edge of a body, or when it traverses
tre; a modification in virtue of which the luminous rays
me bent, and to penotrate into the shadow,
fmenon may be observed in the following manner: A beam
| is allowed to pass through a very small aperture in the

Fig. 412.

fk room, where it is received on a condensing lens, L (fig.
hort focal length. A red glass is placed in the aperture
(low red Hight to pass. An opaque screen, ¢, with @ sharp
[behind the lens beyond its focus, and intercepts one por-
fmltous cone, while the other is projected on the screen b,

afront view. The following phenomena are now
|the geometrical shadow, the limit of which is represented
|, mfaint Hight is ween, which gradually fades in proportion
} from the limits of the shadow. In that part of the screen
above the line ab, might be expected to be uniformly illu-
fies of alternate dark and light bands or fringes are seen
te line of shadow, which gradually become more indistinct
[Pdisappear. The limits between the light and dark fringes

k ES

GRATINGS. 527

tree of light, and the colours of striated surfaces, such as
vpearl, are due to @ similar cause,

tole of these phenomena are in exact accordance with the
| theory, but the explanation is in many cases difficult,

@ of gratings is more simple and important than the others,
bre shall be considered in detail.

jes of fine equidistant lines ruled on glass, or a series of fine
t wires, be placed before the eye or before a telescope, and m
nt or line of light be viewed through the grating thus formed,
each side of the bright point or line # series of equidistant

Fig, 413.

413 let O represent the Iuminous point, AB the grating, and
screen.

ive of the effect on the screen of the light transmitted
grating in the following manner. The ethor in the trans-
of the grating becomes simultaneously disturbed and
by the light from O. The disturbance of each point
Becomes the origin of a spherical wave, as in art, 652,
Produced at any point of the screen Is the sum of the effects
action of the waves thus proceeding from all the transparent
Now, st the point o, which is equidistant from all parts of
‘all thiso waves will arrive in the same phaso, and will,
each other, and give « bright point.
‘paints, pp, on each side of o, whore distance from successive
the grating diffur by one wave length, or any whole number

bodies, solids, liquids, or gases, when in suff
coloured with very bright tints, especially by reflection.

tug cad
soap bubbles, wishing to investignte the relation between
of the thin plate, the colour of the rings, and their ex

them by means of a layer of air interposed
and the other convex, and with a very fears
being cleaned and exposed in ontinary ligh
aa to reflect light, there is seen at the !
surrounded by six or seven coloured rings, the tints of wi
gradually less strong. If the glasses
the centre of the rings is white, and euch of
complementary of that of the rings by reflection.

With homogeneous light, red for example, the
black and red ; the diameters of rings
is more refrangible, but with white light the rings are of
colours of the spectrom, ‘which writes from the fiset that,

length of the lens ia from three to four the rings
the naked eye; but if the length eae ae rings
at with a lens.

his
Fig. 414.

pro-
5 2... He found that for the first bright ring the

‘of an inch, when the light used was the brightost
etrum, thatis the part on the confines of the orange and yel-
+ further found that for rings of the same order the dia~
| ae light producing it is leas.

equal intensity, viz, the ordinary ray,
rays are found to possess other pecu-
saying that they are polarised, namely, the
and the extraordinary ray in » plane st
‘The phenomena which are thus dosig-
ss follows :—Suppoee a ray of light which,
AA

:

rZy
e
fe
tf

BFSE

FL

F
EB
aD

ee
i
Ee

a maximum value when the surfaces are

serve to describe the phenomenon of pola~
8 the principles are concerned ; the sppa~

M
7

i
Fs
E
y
TH
:
4
i
Zz

see
hell
He
Ai:
Hi

are aeration’
this angle is 54°35’, and if in the preceding experiment
inclined at any other angle than thia, the light
eater tien: this would be shown
reflected from the upper surface in all positions,

kind of volesnic glass which is often used in these
is reflected from the surface of water, from slate roof,

532 O§ LIGHT,

ae ee a
ence to the polarising angle.

The polarising angle of a substance 1 that angle of
reflected,

Fig. 416.

ttom.—When an unpolarieed Tuminous ray’ falle upon's gia
Placed at the polarising angle, one part is reflected ; the othe
in passing through the glass, becomes refracted, and
Light is now found to be partially polarised, If the

is

plete. A bundle of ‘such plates, for which the best i
glass used for covering microscopic objects, fitted in w tube
polarising angle, is frequently used for examining or jocduchg,
light.

If a ray of light fall at any angle on a transparent mediam,
holds good with a slight modification. In fact, part of the lightis
and part refracted, and both are found to be
quantities in each being polarised, end their planes of polarisation
right angles to cach other. tis, of course, to be understood that]
larised portion of the reflected light is polarised in the plane of
which is likewise the plane of refraction,

568, Polarising instruments.—Livery instrament for
the properties of polarised light consists easentially of two
polarising the light, the other for ascertaining or exhibiting
light having undergone polarisation, The former part is
riser, the lattor the analyser. Thus in art. 664 the erystal
first refraction is tho polariser, that producing the second
analyser. In art, 565, the mirror at which the first is
is the polariser, that at which the second reflection takes
analyser. Some of the most convenient means of producing
will now be described, and it will be remarked that any iz

__:

POLARISATION OF LIGHT. 533

as & polariser can also be used as an analyser, Tho experi-
§ therefore considerable Liberty of selection.

weremberg'’s apparatas.—The most simple, but complete,
} for polarising light is that invented by M. Norrembeng.
¢ used for repesting most of the experiments on polarised

Fig. 417,
(of two brasss rods and d (fig. 417), which support an un-
,m, of ordinary glass, moveable about a horizontal axis. ~
circle indicates the angle of inclination of the mirror.
ie feet of the two columns thoro isa silvered glass, p, which
horizontal. At the upper end of the columns there is a gra-
#, in which a circular disc, 0, rotates. This disc, in which
‘eperturs, supports a mirror of black glass, m, whieh is
‘Yertical at the polarising angle. An annular diso, h, can be
heights on the columns by meahs of a screw. A second.

crystal, that is, a plano at right
refmtcting edge, coincides with the

obtuse angles, as shown in fig. 421, that is along the
10). The two halves are then again joined in the

method of observing these new phonomona is by means of the
‘This is a small instrument consisting of two tourma~

to the axis, each of them being fitted in a copper disc

, which aro perforated in the centre, and blackened, are

Fig. 422.

o rings of silvered copper, which is coiled, as shown in the

form a spring, and press together the tourmalines. The

mm with the disc, and may be go arranged that their axes
2 .

licular or paralle!
tal to be experimented upon being fixed in the centre of a cork
‘between the two tourmalines, and the pincette is held be-
o as to view diffused light, The tourmaline farthest from the
polariser, and the other as analyser. If the crystal thus viewed
and cut perpendicularly to the axis, and « homogeneous light,
ig looked nt, a series of alternately dark and red rings
another simple colour, similar rings aro obtained, but
decreases with the rofrangibility of the colour, On the
, the diamotors of the rings diminish when the thickness of the
aves, and beyond « certain thickness no more rings are pro-
f ‘of illuminating the rings by homogeneous light, white
ings of the different colours produced have not the
sy are partially superposed, and produce yery brilliant

crystal has no influence on the rings, but this is

‘the relative position of the two tourmalines. For

ing on Iceland spar cut perpendicular to the axis,

tora in thickness, when the axis of the tour-

ular, a beautiful series of rings is seon brilliantly

by a black cross, as shown in fig, 423. If the
are parallel, the rings have tints comy

is the eve of the observer. Henes it follows that

the film which the rays traverse increases with their
for rays of the same obliquity this thickness is the
result different derrees of retardation of the ordinary with

Ma
WF

Fig. 428. Fig. 424. Fig. 426,.
extraondinary ray at different points of the plate, and eo

mualines, and are due to so absorption of the polarised lightin thes#d
tions. When the tourmalines are parallel the vibrations sre tram
and hence the white cross,

Analogous effocts are produced with all acrid for insta
tourmaline, emerald, sapphire, beryl, mica, pyromorphite,
nide of potassium,

Ah

Fig. 426. Fig. 427.

579. sie ts iansetecyoeaie ty coloare
are also produced, but their form is more complicated. “The ct

a

sky is incident, and the analysor is a Nicoi’s

glass plates traversed by polarised light are viewed, figa.
represent the appearances presented successively, when #)
of compressed glass ia turned in its own plane; figs. 431

Fig. 429. Fig. 430, Fig.
o¢@
oa

o°¢
0 ®

Fig. 432. Fig. 433. Fig. 44,
present the nppearances produced by « circular plate under |
circumstances ; nnd fig. 433, that produced when one rectanguls
superposed on another, This figure also varies when the system
is turned.

Compressed and curved glasses present phenomena of the =
which also vary under the same conditions,

ELLIPTICAL, CIRCULAR, AND ROTATORY POLARISATION,
581, Desnition of elliptical and circular polarisation,

cases bitherto considered the particles of ether composing a poll

vibrate in parallel straight linos; to distinguish this case

are now to consider such light is frequently called plane

Lt sometimes happens that the particles of ether describe ali

their positions of rest, the planes of the ellipses being

direction of the ray. If the axes of these ellipses are equal and

the ray is said to be elliptically polarised, In this case the particlt

y—

ees, the particle will describe an ellipse. Now as the
‘produce separately two simple vibrations in direo-

to exch other, we may state the result arrived at

‘two rectilinear vibrations are superinduced on the

in directions at right angles to each other, then:—I. If

the same or opposite phases, they make the point describe a
tion in a direction inclined at a certain angle to eithor of

ons. 2. But if their phases differ by 00° or a quarter

lon, the particle will describ «circle, provided the vibrations ame
‘Under other circumstances the particle will describe an ellipse.
p this to the case of polarised light, Suppose two rays of
io perpendicular planes to coincide, each would separately

il icular directions, Con=

the vibrations are in the same or opposite phases, the

the two rays is plane-polarised, 2. If the rays are

snd their phases differ by 90°, the resulting light is

3. Under other circumstances the light is ellipti-

‘reference is made to arta. 576 and 676, it will be seen
‘by 0 and E aro superiuaposed in the manner above
the light which leaves the depolarising plate is

Tf, however, the principal plano of the depolarising

‘circular polarised light, If analysed by Iceland spar,

disappears, but they undergo changes in intensity.
‘also be polarised clliptically in Fresnel’s rhomb, If the angle
of primitive polarisation and of incidence be any other

le emergent ray is ellipticully polarised.

st polarisation.—Rock crystal or quartz possesses a
which was long regarded as peculiar to itself among
it has been since found to be shared by tartaric acids
with some other crystalline bodies, This property
polarisation, and may be described ns follows :—Let a
us light (for examplo, red light) be polarised, and lot the
yy & Nicol-prism, be turned till the light does not pass
"Take a thin section of a quartz crystal cut at right angles
place it between the polariser and the analyser with its
esto the may. ‘The light will now pass through the
rt menon is not the same as that previously described
Ble ickiccyeial is turned round its axis, no effect is pro-
pallet eaey the ray is found to be plane polarised,

Jn some specimens of quarts the plane of polarisation is

be right hand, in others to the loft hand. Spocimens of the

said to be right-handed, those of the latter kind left~

difference to a difference in crystallographic

possessed by rock crystal of turning the plane

‘8 cortain angle was thoroughly investigated by

otherresults arrived at this:—For a given colour tho

fick the plano of polarisation is turned is proportional to
quartz,

explanation of rotatory polarisation.—Tho cx-

described in the Inst article is ax follows: —

d Light passes along the axis of the quarts eryatal,

rays of circularly polarised light of equal intensity,

depend :
volocity is lenst, that is to say, it will depend om:
of quartz. In this manner the phonomens of rot
completely accounted for. ;
tion is different with differeat colours; its maguit

case of red light plato 1 millimotre in thickness

17°, while @ plate of the same thickness will e
capt Teco with ee eee
analysing Nicol, be a greater or lees quantity of each ool
SE ieee ee
right, the colours will =
feoovn, at ss the colar to via
Jeft-handed crystal in the reverse order, Obyiou:

to the loft, tho rovorse of thos reaults will take place.
nee

Fig, 436.

‘This will be understood from what bas bees
rotation for different colours, Quarts rotates the
for red 17° for cach millimetre, and for violet .
difference of these two angles, when the polarised
traversed the quartz plate emerges, the various
contains are polarised in different planes, ©
thus tranamitted by the quartz pasd through a di

_

of calorntion may be well seen by means of Nor-

(fig. 417), A quartz plato, « cut at right angles to

‘in m curk disc, is placed on the screen, ¢; the mirror n
‘then ro inclined that a ry of polarised light passes

3 the latter is viewed through « refracting prism, g ;

is turned, the complementary images furnished by the

Olarised light through the quartz are seen.

‘power of liquids.—Biot has found that « groat num-

Ieactcal eosin For instance, unerystallisable grape-
the plane of polarization to the left, while cane-sugar de-
ithe right, although the chemical composition of the two
samme.
‘power of liquids is far lons than that of quartz, In concen-
of eane-sugar, which possesses the rotatory power in the
free, the power is Jy that of quartz, so that it is necessary to
_ of liquids of considerable length, 8 inches for

the apparatus devised by Biot for measuring the

‘of liquids. On a copper groove, g, fixed to a support, r, is
Bi este Yoga wel contained the liquid experi-

Peete ace satis ‘At m iss mirror of black glass,
the ig angle to the axis of the tubes bd and a, so that
‘by the mirror m, in the direction bda, is polarised. In
graduated circle 4, inside the tube a, and at right angles
isa double refracting achromatic prism, which can be

| toa limb, ¢, on which is a vernier, to indicate the number

‘through. Lastly, from the position of the mirror m,

polarisation, Sod, of the reflected ray is vertical, and the zero
on the circle, 4, ie on this plane.

the tube din the groove g, the extraordinary image fur-

+ doable refracting prism disappears whenever the limb ¢ cor

‘of the graduation, because then the double refracting

tion takes place to the right; and if with the same solution
different lengths are taken, the rotation {s Soest fo ee
to the length, in conformity with art. 585: further, with |
tube, but with solutions of various strength, the rotation énen

the quantity of sugar dissolved, so that the

solution may be made by means of its angle of deviation,
Tn this experiment homogencous light must be used;

various tints of the spectra have different rotatory:

is decomposed in traversing an active liquid, and the

- od

SOLKIL'S SACCHARIMETER. 451

Fe ar ay seein of the dontle re.
it simply changes tho tint. The transition tint (590)
be observed. To avoid this inconvenience, a piece of red
in the tube between the eye and the double refracting

Gh only allows red light to pass, The extraordinary image

(im that case, whenever the principal section of the prism

ith the plane of polarisation of the red ray.

ef's sacoharimeter.—M. Soleil has constructed an apparatus,

(the rotatory power of liquids, for analysing saccharine sub-

which the name eacoharimeter is applied.

(88 represents the eaccharimeter fixed horizontally on its foot,
B gives @ longitudinal section with the modifications which
introduced by M. Duboseq,

(ciple of this instrument is not the amplitude of the rotation
(of polarisation as in Biot’s apparatus, but that of compen-
ft in to my, a second active substance ia used acting in the
rection to that analysed, and whose thickness can be altered
jontrary actions of the two substances completely neutralise
| Instead of measpring the deviation of the plane of polarisa~

be

- other to the left. Hence, looked at through a double
m, they present exactly the same tint.
fersed the quartz, g, the polarised ray passes into the liquid
eee nrtiale'a ainphe plats of gotrts of azy thick
| of which will bo seen presently. ‘The compensator »,
ye the rotation of the column of liquid m, consists of
ee ee eee eee
‘the plate & These two quartz plates, a section of
Boa fig. 412, are obtained by cutting obliquely a
) > heh Gaeta apesearhe aren drag
then these two priams, as shown in the figure,
‘is obtainod with parallel faces, which can be varied at will.
fbgeietnysach pele toa sl, woo to move itn shar
disturbing the li ‘This motion is effected by
(uble rackwork and pinion motion turned by a milled head,

move in the direction indicated by the arrows
yar that the sum of their thicknesses increases, and that
the plates are moved in the contrary direction. A

{mice follow the plates in their motion, and measure the
the compensator. This scale, represented with its vernier
‘has two divisions with a common sero, ano from left to
(handed liquids, and another from right to left for left-
l

to that of the compensator, the effect is zero. But
‘plates of the compensator in one or the other direction,
or the quartz, é, preponderntes, and there is a

is a double refracting prism, ¢ (fig. 439), serv
‘to observe the polarised ray which has traversed the

prism ¢, wo will noglect fora moment the crystals
‘the right of the drawing. If at first the zero of the
with that of the scale, and iftho liquid in the tube
> of the compensator, and of the plate #, neutralise
(the liquid having no action, the two halves of the plate
‘the prism c, give exactly the same tint as has been
Bat if the tube filled with ive liquid be replaced
‘solution of sugar, the rotatory power of this solution is
‘of the hulves (a or 6) of the plate g (viz. that half
‘the plane of polarisation in the same direction as the

mB

making tho analysis of raw sugar, a normal
sugar is taken, dissolved in water, and the solution made

obtained. This number being 4, for ample, ft is
amount of erystallisable sugar in the solution ix 42
the solution of sugar-candy contained, and, therefore,
or 6918 grains. This result is only valid when the si
with uncrystallisable sugar or some other left-handed
case the erystallisable sugar, which is right-handed, mas
of hydrochloric acid, converted into uncrystallisable sugar,
handed ; and a new determination is made which tog
gives the quantity of crystallisable sugar.

‘The arrangement of crystals and ees eae ap
prism ¢, forme what M. Soleil calls the producer of

most delicate tint, that by which ony aes
tion of the two halves of the rotation plate ean! "
same for ali eyes ; for most people it bs of avidlet Bus
blossom, and it is important either to produce this
equally sensible to the eye of the observer. This is

in front of the prism c, ak fir a quarts glate, 0, out

feerertie ott asst sree arouses is the most
$ of diabetic urine—In the disease diabetes, the urine

| estimate the quantity of this sugar, the urine is first
ing itwith acetate of lend and filtering; the tube is filled
liquid thus obtained; and the milled head, 6, turned,
f the double rotating plate the sume tint is obtained as
bsition of the urine, Experiment has shown that 100
tharimetric seale represent the displacement which the
tors must have when there are 225° grains of sugar in a

ivision of the scale represents 2-266 of sugar, Ac-

hin the quantity of sugar in a given urine, the number

vernier st the moment at which the primitive tint
iplied by 2-256,

of heat.—The rays of heat, like those of light,

hy reflection and by refraction. The experiments

difficult of execution ; they were first made by Malus

; after the death of Malus they were continued by

when the plane of reflection of the second mirror was
‘that of the first. As this phenomenon is the same as
‘light under the same circumstances, Berard concluded
polarised in being reflected.
of heat may be shown by concentrating the
of @ hellostat on a prism of Iceland spar, and
pencil by means of a thermopile, which must
edge. Tn this caso also there is an ordinary and an
‘which follow the same laws as those of light. In the
heat is not doubly refracted, A Nicol’s priam
ion of heat as well as for that of light: «
traverse the second Nicol if the plane of ita princi
eed

f the ray. The:
sd by means of pla
ion is virtually ¢
the priams must t

wence, of rays (
ts of Knoblauch
timent with a rh
total internal 1
ight

BOOK VIII.
ON MAGNETISM.

CHAPTER L
PROPERTIOS OF MAGNETS,

and artificial magnets.—Mugnels are substances
the property of attracting iron, and the term smagne-
applied to the caus of this attraction, and to the resulting

ena,
|property was known to the ancients; it oxists in the highest
‘an ore of iron which is known in chemistry as tho magnetic
from. Its composition is represented by tho formula Fo, O,.
fametic oxide of iron, or loadstone, as it is called, was first found
hesia, in Axia Minor, and has derived its name from this eircum+
| Te is very abundant in nature; it is met with in the older
fal formations, expecially in Sweden and Norway, where it is
an iron ore, and furnishes the best quality of iron.
fi & bar or noodle of steel is rubbed with s magnet, it acquires
je properties, Such bars are called artificial magnets; they are
Qwerfal than natural magnets, and as they are also more con-
| they will be exclusively referred to in describing the phenomena
fetiem ; the best modes of preparing them will be explained in a
lent article.
Poles and neutral line.—When a simall particle of soft iron is
led by & thread, and a magnet ix approached to it, the iron is
towards the magnet, and some force is required for its removal.
be of the attraction varies in different parts of the magnet: it is
ft at the two ends, and is totally wanting in the middle,
also be seen very clearly when a magnetic bar is
ron filings; these become arranged round the ends of the bar
tufts, which decrease towards the middle of the bar, where
mone. That part of the surface of the bar where there is no
foree it called the newiral line; and the points near the

north pole, and the other end is the south pole. TI
needle pointing to the north is also sometimes called
the needle, a

594 Beutaal action of two potes.—The ty

i

Fig. 444.
poles of the Jatter are also

repulsion, in the other attraction. Henee thé foll
enunciated :

Poles of the same name repel, and poles of
another.

‘PROPERTIES OF MAGNETS. 561

‘in all parts of the bar, and not simply accumulated at
evident from the i

=e also that the magnetic fluids aro not noutralised,
ray for if they had bees neutralised, they would not
@ two particles. ‘This pro~
sal tisme, of becoming latent
ly neutralised, is illustrated in the experiment

magnets and a key described in article 594,
7 induction.— When « magnetic substance is placed in
ith « magnet, the two fluids of the former become separated; and
contact remains, it is a complete magnet, having its two
yneutral line. For instance, if o small cylinder of soft iron,
1), be placed in contact with ono of the poles of a magnet, the
‘en in turn support # second cylinder ; this in turn a third, and
‘Many as seven or cight, according to the power of the magnet.

Fig. 447.
these little cylinders is a magnet; it it be the north pole of the
'which the cylinders are attached, the part a will have south,

h tis ; } will in like manner develope in the nearest end
linder south magnetism, and so on. But these cylinders

\ ‘#0 long as the influence of a magnetised bar continues,

pt cylinder be removed from the magnet, the other cylinders

yp, and retain no trace of magnotism. The separation of

‘only momentary, which proves that the magnet yields

iron. Nickel also bocomes magnetised under the influence

and

lents the variations which it has undergone:—

ip

md that the greatest declination waa attained in \8\A,
the neodlo has gradually tended towards the enst.

rei Thus in the island of Rewak it never oxceeds
a

“GS. Accidental variations or perturbations,—The declination is
disturbed in its daily variations by many causes, such a8
the aurora borealix, and volcanic eruptions, The effect of
‘surora fn felt at great distances. Auroras which are only visible in
Berth of Europe act on the needle even in these latitudes, where
variations of 20’ have been observed. In polar regions the
frequently oscillates several degrees; its irregularity on the day
the aurora borealis is a presage of the occurrence of this phe-

Fig. 450,

Another remaricable phenomenon is the miscellaneous occurrence of
\lmiignetic perturbations in very distant countries. Thus Sabine mentions
WW tiagnetic disturbance which was felt simultaneously at Toronto, the
t huaieciet and Van Diemon’s Land. Such simultaneous perturbations
feceived the name of magnetic storms.
‘BOT. Dectination compass.—The declination compass is an \watra~

“MAGNETIC INCLINATION.

outside the box. The bottom of the cylin-
x, O, in which is the needle, is of glass, and gives passage to
‘by which the mica plate, ¢, is illuminated. ‘The compass ia

Fig, 451. Fig, 462,
Med by a second gins#, m, and on a pivot, #, in its centre can be fixed
A, when the bearing of the Jand is to be taken,

‘the inventor of the compass nor the exact time of its invention
yen. Guyot de Provins, a Fronch poot of the twelfth century, first
ons the use of tho magnet in navigation, though it is probable thut
Chi long before this had used it, The ancient navigators who
inted with the compass, had only the sun or pole star a4 4
d were accordingly compelled to keep constantly in sight of land

tar of steering in a wrong direction when the sky was clouded.
Magnetic equator.—It might be supposed from
ly direction which the magnetic needle takes that the force
it is situated in a point of the horizon: this is not the case,
needle be so arranged that it can move freely in a vertical
# horizontal axis, it will be seen that, although the centre of
af the needle coincides with the centre of suspension, the north
are dips downwards. In the other hemisphere the

‘is inclined downwards.

which the magnetic needle makes with the horizon, when
dl plane, in which it moves, coincides with the magnetic
sis called the inclination or dip of tho noodle. In any other

the magnetic meridian, the inclination increases, and i WF

'

(the inclination, the magnetic meridian must first be deter-
tis effected by turning the plate A on the circle m, until the
ical, which is the case when it is in a plane ut right angles

Fig. 463.
meridian (610). Lr prelates
| the vertical circle, M, is brought into the magnetic
be angle, dea, which the magnetic needle makes with the
in the angle of inclination.

Deep of are, which net be allowed. fer —
‘of the needle may not coincide with its axis of figure:
is corrected by # method of reversion analogous to

|described (608). 2. The centre of gravity of the needle

‘with the axis of suspension, and then the angle, dea, is
0 ‘secording as the centre of gravity is below or

a

the preceding equation
or M and M’, and we have—
M cosi _n™
M cos?
increases with the latitude, Humboldt found a
intensity on the magnetic equator in Northern Peru.
f taken as the unit to which magnetic intensities.
‘are referred, as in the following tables—

| + + + 1800 43:52 1348
| » + +» 1829 5251 1366
“oo” 1828 59-66 1410
amo 7940 1607

ries in the same place with the time of day ; it attains
n4and 5 in the afternoon, and is at its minimum
the

ill vary with the dimensions and force of the needle, with the di-
and nature of the wire, and with the intensity of the enrth’s
m in that place. Accordingly, the piece E is turned until ab
in angle with the magnetic meridian. Coulomb found in
nis that E had to be turned 25° in order to move the needle
‘that is, the earth's magnetism was equal to a torsion of the
onding to 35°. As the force of torsion is proportional to the
torsion, when the needle is deflected from the meridian by
ogres, the directive action of the earth's magnetiem is equal
« times 35°,
‘setion of the earth's magnetism having been determined, the
is placed in the case so that similar poles are opposite each
Ts one experiment Coulomb found that the pole a was ropelled
38, Now the force which tended to bring the needle into the
ridian was represented by 24°+ 24 x 35=864, of which the
dus to the tarsion of the wire, and 24 x 35° was the equi-
a ‘of the directive force of the earth's magnetiam. As
was in equilibrium, it is clear that the repulsive force which.

‘MAGNETISATION. 579

of soft iron be twisted while held vertically, or, better, in the
» dip, it acquires a feeble magnetism.
mugnetising

They become magnotised with their north pole
if placed over the pale of a powerful magnet. The

a feeble coercive force; hence a feeble magnetic
y is found to be possessed by tho tools in a smith’s shop.
m, ‘too, has usually a great coercive force, and can be permanently

7, too, of wrought iron oe produce there

a Fig. 461.

ative Foree,—Tho portative force is the weight which a
tupport, and numerous experiments have been made upon
| He found that the portative force of a anturated harse-shoo

oh, by ly detaching the keeper, has become constant,
‘the formula

f Pool ph

# the portative force of the magnot, p its own weight, and

|t, which varies with the nature of the steel and the mode of

Tt follows from this thut a magnet which weighs 1000

pports 25 times as much as one weighing 8 ounces or hy

‘bars would support as much as one which is as
immaterial whether the section of

‘tho same stato of saturation, for their coercive force
hammering imparts to iron or steel a considerable co-
jo most powerfal of these influences ia the operation
. Coulomb found that a stecl bar tempered ot duit

“MAGNETISATION. 583

hary temperatures are unmagnotic, would become so if exposed to

| degree of cold.

{us found that a steel bar eould be powerfully magnetized by heat-

to redness, and allowing it to cool between the opposite poles of

atangnets. A steel har heated to rednoss, and then hardened
cooling in the vertical position, retains the magnetism imparted
the inductive action of the earth.

‘Distribution of free magnetinm.—Coulomb investigated the
force in different parts of the magnet by the following method.
a large magnet in u vertical position in the magnetic meridian ;
took a small magnetic needle suspended by a thread without

sand, having ascertained the number of its oscillations under the
ig ‘the earth’s magnetism alone, he presented it to different parts

‘The oscillations were fewer as the needle was nearer the

‘te bar, and when they had reached that position, their number

sams as under the influence of the earth's magnetism alono, He
‘with saturnted bars of more than 7 inches in length the distri-

|. an age ipatpeenglerspepete yon
from the ends of the magnet, and whose ordinates were the

(Crangnetism at these poii

hh magnets of the above dimensions the poles are at the same

‘from the end; Coulomb found the distance to be 16 inches in a

pehes long. The same physicist found that, with shorter bars, the

of the poles from the end is } of the length; thus with a bar of
alert
‘Tesults presume that the other dimensions of the bar are very

‘compared with its length, that it has a regular shapo, and is

When these conditions are not fulfilled, the

of tho poles can only bo dotormined by direct trials with a

peedle, With lozenge-shaped magnets the poles are nearer the

found thnt these lozenge-shaped bars have a greater directive
rectangular bars of the samo weight, thickness, and hardness.

‘of mercury in a dry glass be connected with a gold-leaf

fe particalasiy metals; Ao not seem cepsble of receiving
yent, When « rod of metal is held in the hand, and
‘or flannel, no electrical effects are produced in it; and

r distinctions no longer obtain in any absolute sense; it
‘be seen that, under appropriate conditions, all bodies may

by friction (051),
‘to the cause of the production of electricity by friction
Woollaston attributed it to oxidation ; but Wilson
shown that electrical phenomena may be produced in
; fe oa chico sny, Yo Sarelope tai

See scases. Wren plan vol, rod, rubbed
: ee corns tee part only will be elece

ae

MEASUNEMENT OF ELECTRICAL FORCES, 598

onstant at « distance, at which the force of repulsion is equal to
Of torsion. In» special experiment Coulomb found the angle of
(tween the two to be 36°; and as the force of torsion is propor-
‘the angle of torsion, this angle represents the repulsive force
fw and wm. In order to reduce tho angle to 18° it was
the dise through 127°. The wire was twisted 126° in the
(of the arrow at its upper extremity, and 18° in the opposite
at its lower oxtremity, and hence thero was a total torsion of
moving the micrometer in the same direction, until the
eviation was 8}°, 567° of torsion were necessary, Honce the
(sion was 575}°. Without sensible error these angles of devin~
‘taken at 96°, 18°, and 9, and on comparing them with the
angles of torsion 36°, 144°, and 576°, we see that while the

hy
1:4: 16;

| for = distance } us great, the angle of torsion is twice as
Eeaak for w distance +/av' great the repulsive force is 16 times

with this apparatus, the air must be thoroughly dry,
Mies fc uk penatte en of elatetaty, This is effectod
‘in it « small dish containing chloride of calcium.
by which tho law of attraction is proved are made in
manner, but the two balls are charged with opposite
‘A certain quantity of electricity is imparted to the moveable
of an insulated pin, and the micrometer moved until there
rangle below. A change of electricity of the opposite kind ia
to the fixed ball. The two balls tend to move together,
by the torsion of the wire, and the moveable ball
(a distance, at which thoro is equilibrium between the force of
which draws the balls together, and that of torsion, which
“wparate them. The micrometer screw is then removed to
Gistance, by which more torsion and a greater angle between
tells are produced. And it is from the relation which exists
angle of deflection on the one hand, and the angle which
foree of torsion on the other, that the law of attraction has

this second law let a charge be imparted to m; n being in
‘it becomes charged and is repelled to a certain distance.
‘deflection being noted, let the ball m be touched by an in-
‘ball of exactly the same size and kind; in this

its charge is romoved, and the angle of deflection will now be

bo

Fig. 468.

istribution of electricity on the surface may also be shown
‘of the following apparatus. It consists of a metallic cy-
Gu insulated supports, on which is fixed a long strip of tin

ved

d (fig. 470) it is found that the electrical
t different points of the surface, In virtue

touching i
ail then ringing this into the. torsion balance. By thie
that the greatest deflection was produced when the
td been in contact with the point a, and the least by con-
fo ale Laplace has found by calculation that the
} pointis proportional to the square of the thickness of the
re

i Fig. 470.

bo

| tthe electro density or electric thickness is the torm used
of fluid found at any moment on a given surface.

& the surface a Gin egep iormappertitael ieee

ae electricity is equally distributed, its elec

equal to & 2
fae
‘hy quantitative experiments, that in an ellipsoid the
at the equator of the ellipsoid, is to that at the
ratio ns the length of the minor to the major axis.
e +, terminated by two hemispheres, the density of
‘at the ends is greater than in the middle, In one case,
densities was found to be as 2351. On a circular
sreatest at the edges,
F of points.—On a sphere, the clectric density is every-
he further a body is removed from the shape of « sphere,
its ncoumulation, A pointed rod may be regurded.

| ‘DISTRIBUTION OF ELECTRICITY. 599
|

direetly proportional to the tension:’ a law analogous to
cooling (850)

(rimented with moist air. In perfectly dry gases, Mat-
id the los: of electricity in accordance with Coulomb's
that within certain limits of tension, the loss was indo-
yuantity of electricity, and proportional to the time; in
{im equal times there was an equal loss of electricity.

tension the loss of negative electricity is greater than
j in dry gases, under a constant pressure, the loss in-
temperature; and, lastly, that in dry gnacs the loss is
he nature of the eloctrified body ; that is, it is the same
ynduetor or not.

not only that supports never insulate completely, but
cause of an abundant loss of electricity in bodies strongly
‘loss diminishes gradually, and is constant when the ten-
(nay be neglected by giving to the supports an adequate
peording to Coulomb, must be proportional to the square
ension of the charged body, Brown shellac is the best
is a hygroscopic substance, and must be dried with grent
fovered with a thin layer of shellac varnish, as has already

{ electricity in vacuo.—Inasmuch ae electricity is
surface of bodies by the pressure of the insulating nt
‘the preasure diminishes, tho loss of eloctricity increases,
‘which resistance i zero, all electricity eseapes. This is

of the mathematical thoory of electricity, which
‘equilibrium of electricity on the surface of bodies But
this, Hawksbee, Gray, Snow Harris, and Bocquerel have
table electrical tensions may be rotained in vacuo. Bec-
hat in a vacuum of # millimeter a body retained a feeble
days, And it is probablo, that if an electrified body
Eyaecuum, it would retain an electrical charge, provided
tly removed from any body which could exert upon it an
(645).

ELECTRICAL INDUCTION. 601

ttrected, showing that it is charged with positive electricity,

(a glass rod, excited by friction with silk, and therefore
ive electricity, be approached to the end nearest the conductor,
{um will be attracted ; while if brought near the other end, the
ling pendalum will be repelled. If the influence of the charged
be suppressed, either by removing it, or placing it in commu-
ith the ground, the separated clectricities will recombine, and
{uma exhibit no divergence. The cause of thie phenomenon is
t decomposition of the neutral fluid of the cylinder, by the froe
of the conductor, the opposite or negative electricity
Bact that end of the cylinder nearest the conductor, while
is repelled to the other end. Between these two
Befeceice apse dontsute af free electricity, This is seen by
the cylinders a series of pairs of pith bulls suspended by
divergence is greatest at each extremity, and there is a point
ds no divergence at all, which is called the nenral point,
although equal in quantity, are not distributed over the
manner; the attraction which accumulates the
ity at tho one end is, in consequence of the greater near-
the repulsion which drives the positive electricity to
‘and hence the neutral line is nearer one end than the other.
‘the gradual loss of positive electricity is leas than that of ne-
(tricity, the cylinder, if the experiment be sufficiently pro-
Mremain charged with positive electricity. Tho distribution
ity is also dependent on the form and size of the cylinder, and
‘of the charged conductor. If the cylinder be placed in
‘with the ground, by metallic contact with the posterior ex-
the charged conductor be still placed near the anterior ex~
Will exert its inductive action as before, But it
the conductor alone which is influenced. It is a con-
of the conductor itself, the metallic wire, and the whole
‘neutral line will recede indefinitely, and since the conductor
infinite, the quantity of neutral fluid decomposed will be in~
when the posterior extremity is placed in contact with
the pendalum atthe anterior extremity diverges more widely.
od be now removed, neither the quantity nor the dia-
‘be altered ; and if the conductor be removed, or be dis-
‘charge of negative electricity will be left on the cylinder. It
charged with electricity, the opposite of that of the

of connecting tho posterior extremity of the cylinder with
may other part had been so connected, the general result
heen the same All the parts of the cylinder would tbe

pp

‘This is analogous to what is met with in
stantancously evokes magnetism in a piece of soft iron,
temporary, and depends on the continued action of the m
magnetises stecl with fur greater difficulty, but this
permanent.

640, Limit to the action of induction—Tho |
which an electrified body exerts on an adjacent body in d
neutral fluid is limited. On the surface of the api
we have considered in the proceding paragraph, let |
small quantity of neutral fluid (tig. 472). The positive

tracting its negative fluid towards A, and repelling ite
B; but in the degree in which the extremity becomes

ty,
are developed at A
direction to the original force. For the forces f and”
towards B the negative fluid of , and towards A its po
as the inducing foree F which is exerted at m ia
forces f and f” aro increasing, a time arrives at which
balanced by The forces f wi f'. Nii Aecompasition of th
then ceases; the inducing action has wishoe Wala,

604 FRICTIONAL ELECTRICITY.

the most formidable of which is the action which «

exert on others at a distance even in vacuo; unless, ir it

that even in the most perfect vacuum Obtain suticiet
molecules remainto produce the polarisation. In

Matteucci has recently made on the propagation of electricity |
dete hes strived at socdhiis Se aera

capacity.—Faraday names
sctich bolic: tees af aooniting 1k ainnon RE
tive power. All insulating bodies do not possess it in the samo
To determine and compare the inductive power Faraday usde
paratus represented in fig, 473, and of which fig. 474
ry

Al

Pig. 474.
‘brass sphere made up of two b
each other, like the Magdebang
spherical envelope, there is &
metallic rod, terminating in
envelope, PQ, by a thick layer

EF.

|

ELECTRICAL INDUCTION. 605

envelopes: connected with thy ground, and the
Metron eee aon ectrcity

Jund to be 125°, showing that the electricity had become equally
lated om the two spheres, as might have been anticipated, since
feces of apparatus were quite equal and each contained air in the

on.
experiment having been mado, the space mn in the second appar-

Hit jing boon ciaryed, the tenaion of the free electricity

fax measured. Let it be taken at 200°, the number observed by
feeetieectalcam. ‘When the knob B of the first apparatus was
[ted with the knob B’ of the second, the tension was not found to
Pas would be expected, The apparatus containing air exhibited
ber of 114°, and that with shellac of 113°. Hence the former had
Eeaibed aetsind 124°, while tho latter ought to have éx-

lhad been dissimulated by the shellac, Of the total quantity of
fity, the shellec had taken 176°, amd the air 114°; hence the
t inductive capacity of air is to that of shellac as 114 : 176, or as
6. ‘That is, the inductive power of shellac is more than half as
(gain as air,
(paring together other substances by this general method, but
‘the details, Faraday and Harris have obtained tho following
errs covey of dielectrics, aa they are called

ae em Wax. . 186
res) Gk we 1-90
ee 176 Gh. sk. 2.00
avai ws 180 Sulphor .. 1... 224

*
the following simple experiment the influonce of the dielectric
}shown. At a fixed distance above a gold leaf electroscope, let
ttriffed sphere be placed by which a cortain divergence of the
lis produced. If now, the charges remaining the same, a disc of
FF or of shellac be interposed, the divergence increases, showing.

ELECTRICAL INDUCTION. 607

The moval ley in a concer ands tif If the electricity

the moyeablo body is different from that of the fixed body, there
ys attraction, but if they are of the same kind there is at first
and afterwards attraction. This anomaly may be thus explained :

its charge of electricity, the moveable body contains natural fluid.
decomposed by the induction of the positive @uid on M, and

ly, the hemisphere 6 obtains an additional supply of positive

ity, while @ becomes charged with negative electricity. ‘There in
“attraction and repulsion as in the foregoing case. The force of
x ‘ie at first greater, because the quantity of positive fluid on N is
than that of negative fluid; but the distance ac diminishing, the
fores increases more rapidly than the repulsive force, and
y exceeds it.
Hii, The moveable body ts a bad conductor. If N is charged, repulsion or
takes place, according as the electricity is of the same

kind to that of the fixed body. If it is in the natural state,
‘powerful and permanent source of electricity can more or less de-

jose the natural fluid even in bad conductors, the body M will
‘the natural fluid of N, and attraction will take place as in the

. Gold leaf electroscepe.—The name clectrosoope is given to
for detecting the presence, and determining the kind, of

glass shade (fig. 476)
on « motallic foot

d. A metal rod ter~
at ite upper extromity
& knob, and holding at its
end two narrow strips of
‘eat, fits in the tubulure of
be shade, the neck of which is
with an insulating varnish.
Air in the interior is dried by quicklime, or by chloride of calcium

Fea

‘Besides this, there is a wooden disc, A (fig. 478), of =
‘somewhat less than that of the cake, lined on its under surface
oil, and provided with an insulating glass handle, ‘The mode of
‘thin apparatus is as followe: All the parts of the apparatus
well warmed, the cake, which is placed in the form, or rests
fic surface, is briskly flapped with a catskin, by which it

Fig. 477. Fig. 478.

charged with negative electricity. The cover is then placed on
‘From the nonconductivity of the resin, the negative electricity
gake does not pass off to the cover. On the contrary, it acts by
‘on the neutral fluid of the cover and decomposes it, attracting
fluid to the under surface, and repelling the negative fluid to

¢ If the upper surface be now touched with the finger, the
electricity passes off, and the cover remains charged with positive
held, however, by the negative electricity of the cake; the
do not unite in consequence of the nonconductivity of the

‘If now the cover be raised by its insulating handle, the charge
eset over the surface, and, if a conductor be brought near it, a

Tieealers which tho. cake reets plays an important part in
of the electrophorus, ns it increases the quantity of elec~
makes it more permanent. For the negative electricity of the

pos

BLEOTRICAL MACHINE. 61

action of the machine is founded on the excitation of electricity
and on the action of induction. By friction with the rubbers,
glass becomes positively, and the rubbers negatively electrified. If
‘the rubbers were insulated, they would receive a certain charge of
electricity which it would be impossible to exceed, for the

of the opposed electricities to reunite would be equal to the

‘of the friction to decompose the neutral fluid. But the rubbers
with the ground by means of # chain, and, consequently,

Fig. 47

fast ax the negative electricity is generated, it passes off. Tho postive

‘of the glnss acts then by induction on the conductor, attract-

‘the negative fluid. The conductors thus lose their nogative elec-

and remain charged with positive fluid. The plate accordingly

‘up nothing to the conductors; in fact, it only abstracts from them
negative fluid.

BLECTRICAL MACHINE. 613
On bringing the finger near the uprights, a sharper spark than
ne is obtained.

‘Maximum oftension. Quadrant electrometer.—It is im-
o exceed « certain limit of electrical tension with the machine,
precautions are taken, or however rapidly the plate is turnod.
is attained when the loss of electricity equals its production.
depends on threo causes: i. The loas by the atmosphere, and
re it, contains; this is proportional to the tension. ii. The

‘Tho recombination of the electricities of the

a glass,
first two causes have been already mentioned. With reference to
* it must be noticed that the electrical tension increases with
lity of the rotation, until it reaches a point at which it over-
Tesistance presented by the nonconductivity of the glass. At
portion of the two electricities separated on the rubbers and
recombines, and the tension remains constant, It is, there-
es tely independent of the rapidity of rotation.
pe electric tension is measured by the quadrant ar Henley's electro-
hich is attached to the conductor. This
electric pendulum, consisting of a
rod, @, to which is attached an ivory or
scale, ¢ (fig. 480), In the contre of this
il whalebone index, moveable on an axis,
: ina pithball, a, Being attached
conductor, the index rises aa the machine
ceasing to rise when the limit is
‘When the rotation is discontinued the
lex falls rapidly if the air is moist, but in dry
‘only falls slowly, showing, therefore, that
loss of electricity in the latter case is less than

Fig. 480.

ge cylinders of copper, or tin, or wood coated
tinfoil, which aro insulatod by placing them on glass supports, or
them by silk threads. They are then placed in connection
conductors, C. The surface on which the electricity is accu-
being thus greater, the tension does not increase; but with an
wal — quantity reread is proportional to the surface, In
machine is , much more erful luminous
obtained, i”

f electrical machine.—The construction of the cylinder

a» ordinarily used in England, is due to Nairne,
for obtaining either kind of electricity. Tn Nain

hydroclectrical
eloctricity is produced by the disengagement of aqueous vapoat t
narrow oritices’ The discovery of the machine was
accident. A workman having accidentally held one hand in ®
steam, which was issuing from an orifice in a steam boiler at high
sure, while his other hand grasped the safety valve, was
experiencing a smart shock, Sir W. Armstrong (then Mr.
of Newcastle), whose attention was dmwn to this phenomenon,
tained that the vapour was charged with positive electricity,
repeating the experiment with an insulated locomotive, found |
boiler was negatively charged. Armstrong believed that tl
was duo to a sudden expansion of the vapour; Famday, who afte
examined the question, ascertained its true cause, which will
understood after describing a machine which An
reproducing the phenomenon.

Tt consists of a boiler of ‘eronghtieon plate (ig, 489), with a

=

RUECTRICAL MACHINE. 615

‘insulated on four logs. It is about 6 feet long by 2 feet in
and is provided at the side with a gauge to show the height
be water in the boiler. A small manometer, not represented in the
i the stopeock, which is opened

Above this is the box, B, in

pour is disengaged. On these

jets of « peculiar construction, which will be understood from
of one of them, M, represented on a larger scale, They are
Bed with hard wood in manner reprosented by the diagram. The
B contains cold water. Thus, the vapour, before escaping, under-

Fig. 482.

‘partial condensation, and becomes charged with vesicles of water;

Phcemary condition, for Faraday found that no electricity is produced
the vapour is quite dry.

Whe development of electricity in the machine was at first attributed

1 the condensation of the vapour, but Faraday found that it is solely

lies to the friction of the globules of water against the jot. For if the

Vile cylinders which line tho jots are changed, the kind of electricity is

thteo ‘yiare euch machines Ravi Insie r
‘The form roprosented in fig. 483 was,

Tt consists of two cirexlor platen of thin,

i. Tie ipepapnae
the armefures. The two plates, the armatures, and their

of the tongues.
of the plate Bat the height of the armatures are two brass

C inclined towards each other. The plate is turned by

‘a winch M, and a series of pulloys, which transmit its motion to

xis, ne 8 to 18 teas tag

n should take place in the direction indicated by

ds the points of the cardboard tongues n n’.

ne thé armatures p p’ must be first primed; that is,

and the other negatively electrified. This is

of @ sheet of ebonite, which is excited by striking it

, better, with catekin; the two knobs rr’ having been con-

plate is brought near one of them p, for instance,

d. The ebonite is charged with negative elec-

ting inductively on the armature p decomposes its

and the negative electricity repelled ia discharged by tho

moveable plate, the armature remaining charged with

sctricity. After balf a turn the negative electricity of the plate

front of the window F” acta in the same way on the

eemeeseng Ht with positive oleciticity by taking from it a

quantity of negative clectricity by the tongue n’. After

4 the two armatures being thus electrified, one positively

| other negutively, the inducing plate of ebonite ia removed,

; rr’ separated aa represented in the figure. On continuing

plate uninterrupted, a torrent of sparks strikes across from
to the other.

‘details being known the following is the explanation of the

‘Znsulating stool.—One of the most curious pheno-
the electrical machine, is the spark drawn from the

| fingor is presented to it. The positive electricity of
‘scting inductively on the noutral fluid of the body, de-
ve and attracting the negative fluid.

Ss alaeticitine is sufficiently great to

of the air, they recombine with a smart crack

_ Tho spark is instantaneous, and is accompanied by a sharp
° ‘more especially with a powerful machine. Its shape

us the form of a sinuous curve with branches (fig. 485). Ifthe
‘very powerful, the spark takes a zigzag shape (fig. 486),
‘appearances are secon in the lightning discharge.
‘be taken from the human body by the aid of the taswlating
‘simply a low stool with stout glass legs. The person stand~
touches the prime conductor, and as the human body is
‘electrical fluid is distributed over ita surface as over an
‘metallic conductor, Tho hair diverges in consequence
peculiar sensation is felt on the face, and if another person,
the ground, prosents hie hand to any part of the body, a smart
produced.

th pricking sensation is

insulated stool, the striker becomes positively, and the person
gatively olectrified.

Fig. 490,

shows this experiment, The same effect is
on the conductor, and bringing near it a
(fig. 491), The current arises, in this case, from the contrary fi
escapes by the point under the influence of the machine.

The electrical orrery and the electrical inelined plane are
these pieces of apparatua,

CHAPTER IV.
CONDENSATION OF XLRCTRICITY,

663. Condensers. Theory of condensers.—A condenser is mn
ratus for condensing # large quantity of electricity om a 6
small surface. The form may vary considerably, but in all
essentially of two insulated conductors, separated by a nono
the working depends on the action of induction.

Epinus’s condenser consists of two ciroular brass plates, A sad 2
492), with a chet of glass, O, between them. ‘The plates,
with a pithball pendulum, are mounted on tasalating glee legs

CONDENSATION OF ELECTRICITY. 623

ed along s support, and fixed in any position, When electricity
accumulated, the plates are placed in contact with the glass, and

z

Fig. 492.
(eof them, B for instance, is connected with the electrical machine,
{other placed in connection with the ground, as shown in fig. 493.
(plaining the action of the condenser, it will be convenient in each

Pig, 493.

fall that side of the metal plate nearest the glass, the anterior,
ther the posterior side. And first let A be at sucha distance

le :

CONDENSATION OF ELECTRICITY. 625

-sccumulation is only on its anterior face, while on the

tension is simply equal to that of the machine at the

connections are interrupted. Ee ence
Aromains vertical. But if the two plates are removed the
tims diverge (fig. 492), which is owing to the ‘circumstance
| plates no longer act on each other, the positive fluid is
] 7 stele lela erie polis

AL

W discharge and instantaneous discharge.— While the
1B are in contact with the glass (fig. 493), and the connec-
ipted, the condenser may be discharged, that is, restored to
(tate, in two ways; either by a slow or by an instantaneous

Ps toy poatctos a iveye, won it aoe te
the pendulum a sinks while } rises, and #0 on
alternately the two plates. The discharge only

; in very dry air it may require several hours, If the

no electricity would be removed, for allit has

by that of the plate B. To remove the total quantity of
the method of alternate contacts, an infinite number of

would theoretically be required, ns will be seen from the

ital quantity of positive electricity on B be taken = 1} by

quantity be called m; m being a fraction in all cases
eee aes oi the distance ofthe plates and the

he dielectric. Now the m of negative electricity on A
{um on the positive on B, retaina there mxm=m* of posi-
ity, and owe ‘the free electricity on B, that which
pendolum 4 diverge, is 1—m*, and if B be touched this
temoved. The m of negative on A now retains, on B, m* of
# binds in turn m times its own quantity, that is, m? of negative
be free negative electricity which now makes the pendulum
| represented by m—m?=m(1—m*), If A be now touched
¥ is removed, the pendulum a sinks and 6 rises, for B has
fet of free electricity, which it is roadily soen is represented
W). By pursuing this reasoning it will be seen that the
| ER

b

i
i
3
F
i
%
i

Fulminating pace.—This is a simple form of the condenser,
gore suitable for giving strong shocks and sparks. It consiate

' Fig. 496.
plate fixed in s wooden frame (fig. 496); on each side of the
‘of tin foil are fastenod opposite each other, leaving = space
the edge and the frame. It is well to cover this part of
‘with an insulating layer of shellac varnish. One of the theets
eee

— &

r, otherwise a smart shock will be felt. To

is placed on nn insulated plate, and first the

eae ace suos cis: with ho bond
conductor. A slight spark is seen at each discharge,

sents @ very pretty experiment for illustrating the slow

Fig. 498.
Fig. 499.

The tod terminates in # small bell, d, and the outside coating

ip demonstrate that in the Leyden jar, the opposite eloctri-
tributed on the coatings merely, but reside principally ou
sideo of the glass. It consiste of a somewhat conical glass
moveable contings of xinc or tin, C and D, Those separate
‘in the other, as shown in figure A, form a complete

naually four, six, or nine. ‘The larger nd

longer is the time required to charge the batten

ar cacnanee tobe discharged, the coatings br
‘When is Hi vo ted |

Sf the ‘dischuning: 103; fhe ules uultagr aeons "

Fig. 501.

care’ is required, for with lange batteries serious accidents may 0
resulting even in death,

673, The universal discharger.—This is un” almost indi
Apparatus in experiments with the electric battery. On a woodia
(fig. 502) are two glass legs, each provided with universal j
which movenble brass rods mre fitted. Between these legs is #'
ivory tuble, on which is placed the object undor experiment.
metal knobs being directed towards the object, ane of them fi
with the external coating of the battery, and the moment i
is made between the other and the internal coating by means of the
discharging rod, a violent sbocke passes through the object on the’

O74, Charging by cascade.—A sories of Lorden jars aro
separately on insulating supports. ‘The knob of the first ix in coat
with the prime conductor of the machine, and its outer coating
Imob of the second, the outer coating of the second, with the
third, and so on; the outer coating of the lat i
ground. Tho inner coating of the first receives a change of
tricity from the machine, and the ecrresponding positive
free by induction on its outer comting instead of passing to the

argo to the inner coating of the second, which, acting
ant ‘is charge in the third jar, and #0 on, to tho last,
Patitive eloctricity devoloped by induction on the outer cont~
to the ground. The jars may be discharged either singly, by
§ the inner and outer coatings of each jar, or simultaneously by
fg the inner coating of the first with the outer of the Inst. In
the quantity of electricity nocessary to charge one jar is avail-
{hanging a series of jars.

| Fig. 502.

im the preceding explanation, it is clear that with a series of
jure charged by casende, if we call the charge of positive
Which the inside of the first jar receives 1, it will develope by
the outside a quantity m (m<1) of negative electricity, and
mm of positive electricity which will pass into the inside

Jor; this in turn vill develope mxm=m? of negative elec=
Outside of that jar, and the same quantity m* of positive
(pass into the inside of the third jar and ao forth. ‘Thusik

eed

‘Thus, if there be six jars and m= 0-9, the quantity:
Seroisers by tho unit charge is 469,

ional to the slectzlc deniity of tlatsgotat ob ae
Pri the dacharge takes place ; | thd the dy of

electricity im a jar. Tensei eee z
evor, by means of the striking distance, can only take pl

tho caida of tie jor; ered adee eonennia eee

measured distance from the knob of the inser coating, Fig.
sents the opermtion of moasuring the charge of a jar by ed
paratus, The jar 4, whose charge is to be measured, is} a
lated stool with ita outer coating im metallic conection with :

takes place the same quantity of positive electricity will
from the machine into the battery ; and hence ita charge ix
to the number of discharges of the electrometer.
unit jar (Gig. 604) is an application of the same principle, and is
Gent for measuring quantities of electricity. It consists of a
phial 4 inches in length, and ? of an inch in diameter, coated
jan inch from the end, #0 as

Fig. 604.

on the distance of the two balla m and n, a discharge

tnd marks a cortain quantity of electricity received as a charge by
pry in terms of the small jar.

eharge.—Harris, by means of experiments

| nit jar suitably modified, and Riess, by analogous arrangements,

t independent researches, that for small distances the strik~

{noe isdireetly proportional to the quantity of electricity, and in-

to the extent of coated surface ; in other words, it

{tlonal to the electric density. Thus, taking the surface of one jar

20 =O
|

q z= 162.

also found that when a battery or a jar is discharged in the
istance, a charge still remains, for when the contings are brought
pimilar discharge may be taken, and so on. The amount of this
charge when the discharge takes place at the greatest striking

100.9 =I.

sera hnvenel be Veh Mt easteaanaTe indi
tromoope (51). The rod to which the gold Jenves are

in m disc instead of ina knob, and there is another dise
provided with an insulating glaw handle. ‘Te dises are 0
layer of insulating shellac varnia (fig. 106).

varnish cn the noutral fluid of the
ctricity, but repelling that of like

for thess knobs being excited by induction from the gold leaves

degree of delicacy is attained by replacing the rods by
dry piles, one of which presents its positive and the
pole. Instead of two gold leaves there ia only one;
trace of electricity causes it to oscillate either to ane side or to
‘and at the same time shows the kind of electricity.
electrometer.—Sir William Thomson has derived
delicate form of electrometer, by which quantitative measure-
ff the amount of electrical charge may bo made. The principle
[instrument may be understood from the following description
bdel of it constructed for lecture purposas by Messra. Elliott, by
drawing has been kindly furnished.
fiat aluminium needle B, balanced by a counterpoise, is sus-
yn platinum wir from a support connected with the inner

it is directly over the
the two rings. Sup-
B not charged with

"will tum slightly towards

bik whether the electricity of
tion is positive or nega

‘the Leyden jar be charged,
negative electricity, the needle
an equal charge,or, as is now

iy will be ot the same
Tt will now be more strongly

| than before, if the charge of D be positive, and would be more
repelled, if the charge of D wore negative, If 1) loses its

r, and is therefore in the same condition as the earth, B returns
position between the two rings, One object of connecting
with m Leyden jar is to provide a considerable supply of
for the needle, so that the small leakage which occurs may

Fig. 407.

os

THE ELECTRIC DISCHARGE. 639
j poe eee pene

“ange Leyden jar and batteries the ie ated very
are Priestley killed rats with batteries of 7 square feet conted
, und cats with a battery of about 44 square yards conting.

‘sparks are taken from a machino, or when a Leyden jar is
‘The better the conductors on which the electricities are

d the more brilliant is the spark ; its colour yaries nat only

\ture of the bodies, but also with the nature of the surrounding
with the pressure. The spark between two charcoal points
between two balls of silvered copper it is green, between knobs
ior ivory it is crimeoe. In atmospheric air at the ordinary pres-
e ¢ spark is white and brilliant; in rarefied air it is reddish ;
it is violet. In oxygen, as in air, the spark is white; in

‘ia reddiah ; and green in the vapour of mercury ; in carbonic

o green, while in nitrogen it is blue or purple, and sccom-

‘a pecaliar sound. Generally speaking, the higher the tension
is tho lustre of the spark. It is asserted by Fusinieri that in
#park there is always a transfor of material particles in a
‘extreme tenuity, in which case the modifications in colour must

‘assume may bo classed under two heads—the spark
brush. The brush forma when the electricity leaves the con-
in a continuous flow; the spark, when the discharge is discon-
‘The formation of one or the other of these depends on the
conductor, and on the nature of the conductor in its vicinity;
‘alterations in the position of the surrounding conductors

gaag-shape with diverging branches. Its length depends on
| at the part of the conductor from which it is taken; and to
Tosgest sparka the electricity must be of as high tension as
but not so high es to discharge spontancously. With long
‘the luminonity is different in different parts of the spark.
‘brash derives its name from the radiating divergent arrangement
ht, und presents the appearance of a luminous cone, whose apex
the condector. Its size and colour differ with the nature end

640

form of the conductor; it is accompanied b

different from the charp. creck nf ith agar et sinosi

than that of the spark, fe wits Solera N

the former is only visible in » darkened rooms.

Ip tclnad hy taclan oh the sondaebsew eee a Ee

with a powerful machine, by placing a small bullet on the;

The brush from a negative conductor ia less than from a
ductor; the cause of this hs
been very satisfactorily made out, be
ates probably in the fact which F
observed, that negative electricity i
into the air at a somewhat lower tendon
positive electricity; so that =
charged knob sooner attains that
which spontaneous discharge takes place,
does « positively charged one, and
discharges the electricity st smaller
and in less quantities.

When electricity in virtue of ite high
sion ixsues from a conductor, no. other
being noar, the discharge takes place
noise, and at the places at which it «ppd
thore inn pale blue luminosity, called |
electrical glow, or on points, & star-like etl
of light. It is seem in the dark by phaci,
point on the conductor of the machine, —

633, Blectrio egs.—The intluence of |
pressure of the air, or rathor of its panel

5 ductivity, on the electric light, may he still
Fig. 608. by means of the electric egg. This oma

of an ellipsoidal glnss vessel (tig. 608), with anetallic caps wt wack 4

The lower cap is provided with « stopeock, so that it can be ana: |

an air pump, and also into a heavy metallic foot. The

rod moves up and down in a leather staffing box ; the lower ome is &

tothe cap. A vacuum haying been made, the stopeock is tured #

the vessel screwed into its foot; the upper part is then connected =i
powerful electrical machine, and the lower one with the

working the machine, the globe becomes filled with a feeble violet i

continuous from one end to the other, and resulting from the

tion of the positive fluid of the upper cap with the negative of the

If the air be gradually allowed to enter by opening the stopeo)

tension increases with the resistance, and the light whieh appear

and brilliant i only seen ns wn ordinary weyers,

re ail

of the electric light are obtained. by means of
hich will be noticed under Dynamical Electricity.
tube, square, and pottie.—The fuminous tube
tube about a yard long, round which are arranged

Fig, 609.
form a series of lozenge-shaped pices of tin-foil, botween
‘very short intervals, There is a brass cap which hooks at

which the spiral terminates. If one end be presented to a
action, while tho other is held in the hand, sparks appear
ly at each interval, and produce a brilliant luminous

1 pane (8g, 510) is constructed on the eame principle, and
of m aquare pf erdinary glass, on which is fastened & narrow

f times.
yorticn

THe REECYMIC DISCHARGE. 643

‘motal foot. A quantity of liquid sufficient to cover the

in the vessel. The outer coating of the jar having been

lected with the foot by means of a chain, the spark which passes
two knobs are brought near each other, inflames the liquid.
‘ether the experiment succeeds very well, but alcohol requires to be

also be ignited by means of the electric spark. A person
insulated stool places one band on the conductor of

is then worked, while he presents the other tothe jot of gas

from ® metallic bumor. ‘The spark which passes ignites the gas.
battery is discharged through an iron or steel wire it becomes
‘and even made incandescent or meltod, if the discharge is very
ferful. The laws of this heating effect have been investigated inde-

Sway it hies been found that the increase in temperature ia the wire
to tho electric density multiplied by the quantity of
sand since the electric density is equal to the quantity of
, usually measured by the number of discharges of the unit
, divided by the surface, the heating effect is proportional to the
lire of the numbor of discharges divided by the surface; that is, hak
ee er eo ene
rise of temperature is inversely as the Pata
pee diameter, ooh, compared with a given wire a8 unity, the
temperature in a wire of double or treble the diameter would be yy
pees (¥ ‘but as the masses of these wires are four and nine times
the heat produced would be respectively } and } as grent as in a
unit thickness,
en electric discharge is sent through gunpowder placed on the
discharger, it is not ignited, but is projected in all
But if s wet string be interposed in the circuit « spark

periences in traversing @ semi-conductor, such asa
; for the heating effect is proportional to the duration of the

fs charge ix passed through sugar, heavy spar, fluorspar, and
they afterwards become phosphorescent in the dark.
#tc,, may be made luminous in the dark in this way.

1x battery is discharged through a gold loaf, preasnd hetwoen two.

di

produced by frictional eloctricity is more difficult.

plished by making uso of a galvanometer (712) co
turns of fine silk-covered wire, which is further inealated

coated with shellac varnish, and by separating the layers by mea
oiled silk. When the prime conductor of « machine im ac
connected with one end of the galvanometer wire, and the other’
Wee atin of the needle is produced.

Fig. 513.

lacerations, fractures, and sudden expansions which ensue whet
ful discharge is passed through a badly conducting substance
perforated, wood and stones are fractured, and gases and liquids:

knob of the inner coating is brought
the discharge passex between the two con-
| ibe The experiment only succeeds with «
the glass is very thin; otherwise a battery must be

on and sudden expansion which the discharge produces
trated by means of Kinnersley's thermometer. ‘hia
gl tabs (fig. 514), which fit into metallic caps, and
-with each other, At the top of the large aha eae
in « knob, and moving in « stuffing-box, and at the bottom

Fig. 614.

the two knobs, the water is driven out of the larger tube and

)
bh
ye level of the lower knob, When the electric shock passes
w slight extent in the small one, The level is immediately
:

FRICTIONAL ELECTRICITY.

48

arranged as to be easily removed for the purposes of unulgemen,
Fastened to a knob on the base of the spparatus and prapcite!
the plates is a pointed brass rod, which acts ash olntet
electricity, The condenser or Leyden jar arrangeaust ¥
case, part of which hat been removed to show the

consists of India-rubber cloth, coated on wacls sidu with tial
formed into a roll for the purpose of greater compactowa By

Fig. 617.

a metal button the knob is in contact with one tinfoil coating whith!
receives the electricity of the machine, and corresponds to the i
coating of the Leyden jar, Another button connected with the

tinfoil conting, rests on u brass band at the bass of the
is in metallic contact with the cushions, tho knob d, and the
knob in which slides a rod at the front of the apparatus, Thee
in connection with the earth. The knob ¢ is in metallic connection”
a disc g provided with @ light arm. By means of s flexible chain
connected with a tfigget on The tle ofthe eppanttus not

_ ail

649

that when the trigger is depreased, the arm, and therewith the
ee ee ete eetog of tho coud
after a certain number of turns a spark
eee eseasiss ate rads and te wishing distant

‘of the working condition of the instrument.
used is known aa Abel's electrical fuse, and has the following
(Rica, The ends of two fine copper wires, fig. 518, are imbedded
eld gutta percha rod, parallel to each other, but at a distance
1Smim, At the lower end of the gutta percha a small eap of
Petetoll ce Sa fastened; In which placed a mall quantity of the

Fig. 518. Fig, 519.

which consists of an intimate mixture of subsulphide

of copper, and chlorate of potassium, The paper

od down #0 that the exposed ends of the wires are preserved in
with the powder.

‘the actual fuse; for service the capped end of the fuse is placed

fn the rounded head of a wooden cylinder, so as to project

eee ot Seclinia. ‘This cavity is filled with meal

is well rammed down, so that the fuse is firmly imbedded.

em eens ci unde whole 8 SAY

rr

a

it thi the handle is tumed the. m
Sn ie ge tho triggor is depressed,

*Whea a nusber of dhange arp to be red ep

sag ciroit; ears ba site Wa insulation is
600, Duration of the electric spark. Velocity

Whentstone ‘has measured the ‘duration of the ole
velocity of electricity, by means of the 0
for this purpose. At some distance from this instrument, ¥
mado to rotate with a mensured velocity, erin
that the spark of ita discharge ia reflected from the
tho laws of reflection (Note, p. 401) the image of the
describes an are of double the number of degrees which |
scribes, in the time in which the mirror passes from the
the image is visible, to that in which it ceases to be so. —
‘of the image were absolutely instantaneous the are would
‘a mere point. Knowing the number of turns which th
s second, and measuring, by means of a divided circle,
grees occupied by the image, the duration of the spark
mined. In one experiment Wheatstone found that this
Now, in the time in which the mirror traverses {360° the
720°; but in the experiment the mirror made SOO turns ia |
therefore the image traversed 676,000? in this time; and
24°, the image must hove lar\ed Ye noe expreimed z

‘VELOCITY OF ELECTRICETY. 651
Peeters he Pevintanecns, bet hana curtain dem
the velocity of electricity, Wheatstone constracted an

| the prineiple of which will be understood from figure 520;
fed metal knobs were arranged in m horizontal line om a piece
talled the spark Loard; of these the knob 1 was connected with
while 6 could be connected with tho inner coating of a
jar; tho knob 1 was s tenth of an inch distant

® quarter of a mile of wire
first and second spark, and
(he same distance between the se-

Fig. 620,

atk board was arranged at a distance of 10 feet from the rotating
id at the eame height, both being horizontal; and the oleerrer
en on the mirror. Thus the sparks were visible when the
ade an angle of 45° with the horizon,
(tho mirror were at rest or had only a small velocity, the images
fee sparks would be seen as three dots ; , but when the mirror
tain velocity these dota appeared as lines, which were longer aa
fon was moro rapid. ‘The greatest length observed was 24°,
{th 600 revolutions in a second, corresponds to a duration of
& second. With a slow rotation the lines present the appear-
=; they are quite parallel, and the ends in the same line.
[greater velocity, and when the rotation took place from left to
athe SS _ " and when it turned from
the sppearance — because the imago of the centre
fend ‘after the Iateral ones. Wheatstone found that this
jent amounted to half a degree before or behind the others.

Jeorresponds to a duration of 5 a4 ap oF srafano of
— ss being a quarter of a mile, gives

vera

1a excond, which ingest
> two outer sparks sper
& follows that the cleetis
coatings of the Leyim
tbe made to the chapter @

CHAPTER 1
VOLTAIC PILE. ITS MODIFICATIONS.

. Galvant’s experiment and theory.—The fundamental experi-

lod to the discovery of dynamical electricity is due to Galvani,

‘of anatomy in Bologna. Occupied with investigations on the

of electricity on the nervous excitability of animals, and especi-

Fig. 521.

the frog, bo observed that when the lumbar nerves of dead frog
with the crural muscles by a metallic circuit, the latter

briskly contracted.

this celebrated experiment, the legs of a recently killed frog,

cribed, consisting
in which they are placed, constitutes a simple voltaic element or
ag tho metals are not in contact, the couple is said to be
dl when connected it is closed.
he production of a voltaic current it is not nocessary that one of
bo unaffected by the liquid, but merely that the chemical
‘one be greater than upon the other, For then, in
ith what bas been before stated (604), the two metals may
give rise to two separate currents, of which the one
metal most attacked ix tho stronger, and the current
between

He

to realiso, we must assumo that no electrical effects would. be
‘The metal which is most attacked is called the positive or

pa PE

=

the
tid through the connecting wire from the negative to the positive

of the direction of the current the positive current is
; to avoid confusion, the existence of the current
bppasite direction, the negative current, is tacitly ignored.
(mere immersion of two different metals in a liquid is not alone
Wit to produce a current, there must be chemical action, When a
and « gold plate are connected with # delicate galvanometer
2 ‘in pure nitric acid no current is produced ; but on adding a
fic acid a strong current is excited, which proceeds in
from the gold to the platinum, because the gold is attacked by
‘acid, while the platinum is loss 60, if at all.
‘voltaic current is produced whenever two metals are placed in
contact in a liquid which acts more powerfully upon one than
‘other, there is great choice in the mode of producing such
Tn reference to their electrical deportmont, the metals have
‘in what is called an electromotive series, in which the most
aro at one end, and the most electronegative nt the other.

Big. a8.

parte: La vessel, F (fig. 528), either of stoneware or of
(tontaining dilute sulphuric acid; 2. a hollow cylinder, Z, of amal-
ied zine ; 3. a porous vessel, V, in which is ordisary nitric acid; 4.

Pig. 629.

‘of carbon, C, prepared in the above mann: In the vossel F
is first placed, and in it the carbon as seen in P, To the carbon
a a binding serew, #, to which a copper wire is attached ; forming
polo. ‘The zine ia provided with « similar binding screw, m,
wine, which is thua the negative pole.

si)

AMALGAMATED ZINC, 669

as being the tendency of the electricity accumulated at the ex-
@ to free itself, and to overcome the obstacles offered to its
| It is proportional to the electromotive force ; thus the tension
\e-carbon battery is greater than that of a zinc-copper battery.

| the number of couples, while the quantity, other things being
| proportional to their surface. The larger this surface the greater
hantity of electricity which flows in the circuit. This quantity
teases with the conductivity of the liquid interposed between the
} the tension on the contrary is independent of the nature of this

p& in the case of avery considerable number of couples, the tension
axtremities of the battery is always far woaker than in electrical
‘For neither of the extremities gives a spark or attracts light
ind it is only by moans of a condensing gold-leaf electroscope, and
tmely careful insulation, that the tension can be observed. For
pose ono of the plates of the electroscope is connected with one
he pile and the other with the other end or with the ground. The
fas is then charged, and on breaking the communications the gold
liverge. A Leyden jar may even be charged when the interior
(ds connected with one end of the pile, and the external coating
@ other; but this charge is far fecbler than that furnished by the
machine.

a

Amalgamated zinc.—Deo la Rivo observed that perfectly pure
L zinc was not attacked by dilute sulphuric acid, but became £0
famered in that liquid in contact with a plate of copper or of
mm. Ordinary commercial sinc, on the contrary, is rapidly disolved
te seid. This, doubtless, arises from the impurity of the zinc,
always contains traces either of iron or lead. Being electro~
fe towards zine they tend to produce local electrical currents, which
ate the chemical action without increasing the quantity of elec~
In the connecting wire.

} When amalgamated, acquires the properties of perfectly pure zinc,
‘unaltered by dilute acid, so long as it is not in contact with a cop-
[platinum plate immersed in the same liquid. To amalgamate a
(ate, it ix first immorsed in diluto sulphuric or hydrochloric acid so
Hitain as clean surface, and then a drop of mercury is placed on the
tad spread over it with a brush. Tho amalgamation takes place
listely, and the plate has the brilliant aspect of mercury.

t plates may also be amalgumated by dipping them in a solution of
(ry prepared by dissolving at a gentle heat one pound of mercury in

be

magnetic pheno-
hhere, and the description of the reet postponed

on electro-magnetism.
experiment.—Oersted published in 1819 a dis-
magnetism and electricity in a most intimate
became in the hands of Ampére and of Faraday, the source
‘of physics. The fact discovered by Oersted is the
which « fixed current exerts at a distance on a magnetic

Merete » copper win So mupended osioatlly fx
u of the magnetic -

more nearly at right Fig. 533.

magnetic meridian in proportion as the current is more intense,

te to the direction in which the poles are deflected, there

fees, which may, however, be referred to a single Principle.

er assumption (604) that in the connecting wire the |
from the negative to the positive plate, the preceding

(resents the following four cases :—

tarrent passes above the needle, and goes from south to

1 pole of the magnet is deflected towards the west; this
is in the above figure,

Fig. 634,
existence, direction, and intensity of currents may be d io
was invented by Schweigger in Germany @ short time after 0
discovery.

euspended by a filament of silk
of the magnotic meridian by a copper wire forming
round the needle in the direction of

would have his left always

horizon, and consequently, that the

would tend to turn the north pole in the same
that the actions of the four \runches of the circuit

directive action of the earth continually tends to keep the

magnetic meridian, and thus opposes the action of the
latter

a3
F

4
E
abe

Epes
F

Hit

it

F

StF
EF
i

t
f

it
i

‘wire, coverod with silk so aa to insulate thecoils, Above
is» horizontal circle, the zero of which corresponds to the
‘parallel to the direction of the wire ; there are two graduations,
oe

MARINE GALVANOMETER. 675

§ directions, no doflection is produced; where the needle is de-
bae of the currenta differs from the other. Hence the apparatus
to mecertain a difference in intensity of two currents and to this
(tamee owes its name,

Str W. Thomson's marine galvanometer.—In laying sub-
fables the want was felt of a galvanometer sufficiently sensitive
(nsulation, which at the same time was not affected by the pitch~
‘rolling of the ship. For this purpose, Sir W. Thomson inventod
fe galvanometer. Fig. 537 is from a drawing of this instrument
furnished by Messrs. Elliotts, by whom it is made. B represents
f many thousand turns of the finest copper wire, carefully insu-

a

=

v

Fig. 587.

froughont, terminating in the binding screws EE, In the contro
boil ix « slide, which carries the magnet, the arrangementof which
(ented on a larger scale in D, The magnet itself is made of a piece
|wateh spring about { of an inch in length, and does not weigh
fan grain; it is attached to @ small and very slightly concave
lof very thin silverod glass, A single fibre of silk ix stretched
he alide, and thé mirror and magnet are attached to it in suck a
| that the fibre exactly pnass through the centre of gravity in
beition. As the mirror and magnet weigh only a few grains, they
heir position respecting tho instrument, however the ship may
(droll. ‘The slide fits in a groove in the coil, and the whole ix
{within « wrought-iron case with on aperture in front, and a
(efron lid on the top. Tho object of this is to counteract the in-
lof the terrestrial magnetism when the ship changes its course,

oad

One's LAW.

677
s emall resistance, the tangent compass is well adapted
’ low tension, but in which a agar eae of

=i

Fig. 638,

ty is set in motion. For currents which can overcome great

8 law,—For a knowledge of the conditions which mgulate
f the voltaic current, science is indebted to the late Professor

Fei W ot Grei Gbdioed Brom Cuvoretical donsldereSinns; wat by

‘or cause by which electricity is setin motion in the voltaic
d the electromotive force. ‘The quantity of electricity which
‘time flows through a section of the circuit is called the

onx’s LAW. 679
resistance, and the formula bocomen PE. If the. resistance
Seterpate, very exal which is the case, for instance, when

is, a battery consisting of several elements produces in this case no
effect than a singlo element. If, however, the external resist-
r is very great, which is the case where the current has to pass
pough « long thin wire, or through a liquid, the intensity is within
Timits very nearly proportional to the number of elements,
SDtas ice dieters ba mele we tise as largo, there is no
‘in tho electromotive force, for this depends on the nature of the
pand of the liquid, but the resistance is m times as nmall, for the

an increase in the size pf the plate, or, what is the same thing,

in the internal resistance, does not increase the intensity to an

te extent; for ultimately the resistance of the element R

in comparison with tho resistance r, and the intensity always
to the value I =

in a thermoelectric pile, which consists of very short metallic con-
the internal resistance FR ia very small. It may hence be
and Ohm's formula becomes
i=";
3

the intensity is invorsely as the length of the connecting wire,
y law enables us to arrange a battery so as to obtain the
mt effect in any given case. For instance, with a battery of six
dlements there are the following four ways of arranging them :
‘Baingle series (fig.639), in which the zinc Z of one element is
with the copper © of tho second; the zinc of this with the
of the third, and 40 on; 2.. Arranged in # system of three double
its, exch element being formed by joining two of the former
$40); 3. Tn ao system of two elements, each of which consists of
Pe Baten ccna elements joined, 20 us to form one of triple the
(fig. 641); 4, Lastly, of ono large element, all the zincs and all
ee a ese large salto ix times the

‘With o series of twelve elomonts there may be six different coniiss
tions, and so on for a langer number. |

Now let us supposo that in the particular case of a battery of sit
} resistance R of each element is 3 and the ext
. Then in the first case where there are six

= 6E =
-aR+r Oxs+
jited so aa to form three inane COC

tive force E in each clement; there weal
jement, but this would only be half a ¢

‘onm’s LAW. 681

is change would lessen the intensity.
th the same clements tho resistance in the connecting wire
ly r= 2, we should have the values in the two eases respec-

[= OxE _0F
~6xs+2 90°

7—_ 38 _ GE _ bE
ae wee oan

jeult in this case is, therefore, more favourable. If the resist-
‘ere 9 the intensity would be the same in both cases. Hence,
(ng the size of tho plates or the arrangement favourable or
fable results are obtained according to the relation between R

| given combination the maximum effect is obtained when the
istance in the elements is equal to the resistance of the inter-
luppose that in a given case » elements are arranged so as to form
tof s couples, each consisting of ¢ colls, n= a. Denoting the re-

bf @ single element by r, the total resistance is @ Now accond-
f abore law the maximum effect is obtained when 7 = J, where

resistance of the interpolar, But t=5 hence fst or

given case we have 8 eloments, each offering a resistance 15 and
with the resistance 40, we get s= 4:3. But this is an im-
it, for it is not a whole number, and the nearest whole
be taken, This is 4, and it will be found on making a cal-
to that above, that when arranged so ns to form 4
each of double surface, the greatest effect is obtained.

683

‘THERMAL
of a powerful current, rabbits which have been suffocated
hour have been restored to life; the head of » man who had

actions were imperfectly reproduced, but ceased immediately

current.
‘Thermal effects.—When a voltaic current is passed through #
wire the same effects aro produced as by the discharge of an
battery; the wire becomes heated and even incandescent if it
labart and thin, With a powerful battery all metala are melted,
and platinum, the most infusible of metals, Carbon is the
which hitherto has not been fused by it. M. Desprets, how-
composed of G00 Bunsen's elements joined in six

tttery of 90 to 40 Bansen’s elements is sufficient to melt and vola~
fae wires of lead, tin, zinc, copper, gold, silver, iron, and even pla~
with differently coloured sparks. Iron and platinum bum with a
kite light 5 lead with « purple light; the light of tin and of
§ Blaish whi 3 the light of sinc is a mixture of white and gold ;
(Copper and silver give a green light.
‘thermal effects of the voltaic current are used in fring mines for
and for blasting operntions. ‘The following arrango-
Boadapeed in the English service, Fig. 643 represents a small
& box provided with a lid. Two moderately stout eopper wires,
ed by being covered with gutta percha, uro deprived of this coating
beds, which are then passed through and through the box in the
Fmpresented in the figure, ‘The distance between them is j of an
nd a very fine platinum wire (ono woighing 102 grains to the yard
fogulation size) is soldered across. Tho object of arranging the wires
(manner is that they shall not be in contact, and tho strain which
fert may be spent on the box and not on the platinum wire joining
which, being very thin, would be broken by a very alight pull.
ix is them Billed with fine-grained powder, and the lid tied down.
fires of the fuse are then carefully joined to the long conducting
lead tothe battary; these should be of copper, and ns thick aa
0:48 to offer very little resistance: No. 16 gauge copper wire
(table size. The fure ix then introduced into the charge to be fired:
+ for « submarine explosion the powder is contained in a canister,
ek of which, after the introduction of the fuse, is carefully faa

t

‘the current passes through « chain of platinum and aves -winktd

the becomes moro heated than the silver from its
resistance ; and with a suitable current the platinum may become
while thesilvor remains dark, This experiment was devised
‘Children. If a long thin platinum wire be raised to dull redness
‘pasting a voltaiccurrent through it, and if part of it be cooled down
fice, the resistance of the cooled part is diminished, the intensity of
current increases, and the rest of the wire becomes brighter than

Ff, on the contrary, a part of the fesbly incandescent wire be heated
‘& spirit lamp, tho resistance of the heated part incronses, the intensity
current diminishes, and the wire ceases to be incandescent in the

The conling b ‘by the surrounding medium exercises an important in-
on the phenomenon of ignition. A round wire is more heated
the same current than the same wire which has been beaten out
for the latter with the samo section offers a greater surface to the
medium to the others. For the same reason, when a wire is
in a glass tube on which two bras caps are fitted air-tight, and
‘wire ix raised to dull incandescence by the passage of current, the
is more vivid when the air has been pumped out of the
because it now simply loses heat by radiation, and not by commu-
to the surrounding medium.

ly, @ current which will melt a wire in air will only raise it
qedness in ether, and in ofl or in water will not heat it to red-
‘at all, for the liquids conduct heat away more readily than air does,
the above laws it follows that the heating effect is the same in
‘whatever be its length, provided the current is constant ; but it
be remembered that by increasing the length of the wire wo
the resistance, and consequently diminish the intensity of the
; further, in a long wire there in a greater surface, and hence
Beat is lost by radiation and by conduction.

thermal effect depends more on the size than on the number of
plates of n battery, for the resistance in the connecting wires is small.
wire may be melted by a single Wollaston’s element, the zinc
is 8 inches by 6, Hare's battery (607) has received its name
on account of its greater heating effect produced by the great

of its plates,
When any circuit is closed « definite amount of heat is produced
the entire circuit; and the amount of heat produced in
particular part of the circuit is greater, the greater the proportion

ns] LUMINOUS EFFROTS. 087

pari in also perceived on breaking contact, These luminous
‘are obtained when the battery is sufficiently powerful, by bringing
‘two electrodos very noarly in contact ; a succession of bright sparks
across the interval, which follow each other with such
‘as to produce a continuous light. With eight or ten of Grove's
brilliant luminous sparks are obtained by connecting one
of the battery with a file, and moving its point along the tecth
file connected with the other terminal,
‘most beautiful effect of the electric light is obtained when with
terminals of the battery two pencils of charcoal are connected in
manner represented in fig. 544. The charcoal & is fixed, while the
a can be raised or lowered by means of a rack and pinion
The two charcoals being placed in comtact the current
and their ends soon become incandescent. If they are then
to a distance of ubout the tenth of an inch, according to the
of the current, a luminous arc extends between the two
which has an exceedingly brilliant lustre, and is called the

ere.
‘The length of this arc varies with the force of the current. In air it
‘exceed 2 inches with a battery of 600 elements, arranged in aix series
100 cach, provided the positive pole is uppermost, as represented in
figure; if it is undermost, the arc is about one-third shorter. In
‘the distance of the charcoal may be greater than in air; in fact,
‘the electricity meets with no resistance, it springs between the two
even before they arc in contact, The voltaic arc can also be
im Liquids, but it is then much shorter, and its brilliancy is

‘The voltaic are bas the property that it is attracted when a mag~
is prosented to it; a consequence of the action of magnets on

Some physicists have considered the voltaic are as formed of a very
succession of bright sparks. Its colour and shape depend on the
‘of the conductors between which it is formed, and hence it ix
that it is due to the incandescent particles of the conduetor,
are volatilised and transported in the direction of the current,
ut is, Son the positive to the negative pole. The morn easily the

by the current, the greater is the distance
eeiiede on be placed. Charcoal, which is a very
substance, is one of the bodies which gives the langest lumi-

first made the exporimont of the electrie light, in 1801, by means

of 2000 plates, each 4 inches square, He used charcoal points
light wood charcoal which had been heated to rednem, end im

i

‘THE ELRCTRIC LIGHT.

the negative charcoal first becomes luminous, but the light of
charesal is the brightest; as it also wears away the most
‘it ought to be rather the larger.
of the electric light.—When the electric light
‘used for illumination, it must be as continuous as other modes
For this purpose, not only must the current be constant,
distance of the charcoals must not alter, which necessitates the
some arrangement for bringing them nearer together in proportion
wear away. One of the best modes of effecting this is by an
invented by M. Duboscq.
‘this regulator the two charcoals are moveable, but with unequal
‘which aro virtually proportional to their waste. ‘The motion
a by a drum placed on the axis, zy (fig. 646). This turns
of the arrows two wheols,a and 4, the diametors of
1: 2,and which respectively tranamit their motion to
C’ and C. C lowers the positive charcoal, p, by
rod sliding in the tube, H, while the other C’ raises the
8, twice as rapidly. By means ofthe milled head,
can be wound up, and at the same time the positive char-
by the hand; the milled head, x, movea the negative
by the hand, and independently of the first. For this
‘axis,.ry, consists of two parts pressing against each other
force, so that holding the milled head, x, between the fingers,
'y, may be moved, aud by holding the latter the former can be
But the friction is sufficient when the drum worke to move
wheels @ and 4 and the two rack-works.
‘two charcoals being placed in contact, the current of a powerful
of 40 to ( elements reaches the apparatus by means of the
B and E’. The current rising in H, desconds by tho positive
then by the negative charcoal, and reaches the apparatus, but
into the rackwork, ©, or into the part on the right of
NN; these pieces being insulated by ivory discs placed at their
‘part The current ultimately reaches the bobbin B, which forma
of the regulator, and passes into the wire, E’, Inside the
is a bar of soft iron, which is magnetised as long as the current
in the bobbin, and demaguotised when it dooa not pass, and this
magnet is the regulator. For this purpose it acts attractively
‘armature of soft iron, A, open in the centre s0 as to allow the
O to pass, and fixed at the end of a lever, which works on
nim, and transmits a slight oscillation to a rod, d, which, by
® catch, ¥, seizes the whoel s, as is seen on a larger scale in
647. By an endless screw, and a series of toothed wheels, the
transmitted to the dram, and the rackwork being fixed, the same

=]

ay ‘THE ELECTRIC LIOHT. 691
jons remain fixed. As their distance only varies within very narrow

illuminates the
represented fig. 302, by ‘which all the optical experiments may
for which solar light was formerly necessary.
Properties and intensity of the electric tight—Tho electric
has similar chemical properties to solar light; it effects the
‘of chlorine and hydrogen, acts chemically on chloride of
lied to photography gives fine impressions, remarkable
the warmth of the tones however, inapplicable for taking
‘as it fatigues the sight too greatly,
through a prism, the electric light, like the sun, is deeomposed
gives a epoctram. Wollaston, and more especially Fraunhofer, ha
that the spectrum of the electric light differs from that of other
and of the sunlight by the prosence of several very bright lines,
‘of which moro especially is of almost dazzling brilliancy as compared
‘the rest of the spectrum. Wheatstone has found that by using
of different metals, the spectrum and the lines are modified.
to Desprets the bright lines are fixed, and independent of the
of the current.
has recently studied the electric light in great detail, and has
ited upon the light of the electric machine, that of the voltaic
and that of Ruhmkorff's coil. He haa found the same colours in
‘electric spectrum as in the solar spectrum, but traversed by very
luminous bands of the same shade a% that of the colour in
they occur. The number and position of these bands do not
‘on tho intensity of the light, but, as we have seen, upon the
between which the voltaic arc is formed.
carbon the lines are remarkable for their number and brilliancy ;
txine the spectrum is characterised by very marked apple-green
silver produces a very intense green; with lead a violet tint pre-
and so on with other metals,
in experimenting with 48 couples, and removing the charcoals
of a quarter of an inch, has found that the intensity of the
light is equal to that of 572 candles,
and Foucault have compared the chemical effects of the solar
electric lights, by investigating their action on iodized silver
Representing tho intensity of the sunlight st mid-day at 1000,
found that that of 46 Bunsen’s clementa was 236, while
elements was only 233. It follows that the intensity does
to any matorial extent with the number of the couples; but
shows that it increases considerably with their suriuce. Yor

i.

CHEMICAL EFFECTS. 693

:)
}. Chemical offects.—These are among the most important of
he actions, either of the simple or compound circuit. The first

wires with the binding screws. Tho vessel is filled with water
some sulphuric acid has been added to increase its conductivity,
fe water is a very imperfect conductor; two glass tubes filled with
‘ate inverted over tho electrodes, and on interposing the apparatus
circuit of = battery u decomposition is rapidly set up, and gas
‘tise from the surface of ench pole. ‘The volume of gas liberated
j@ negative pole is about double that at the positive, and on exa-
the former gus is found to be hydrogen and the latter gas
‘This experiment accordingly gives at onco the qualitative and
ivo analysis of water.

Blectrotysis.—To those substances which, like water, are re-
into their elements by the voltaic current, the term electrolyte has
“applied by Faraday, to whom the principal discoveries in this
and the nomenclature are due. Electrolysis is the decomposition
‘voltaic battery; the positive electrode is called the anode, and
electrode tho Aathode, The products of decomposition are
fone, that which appears at the kathode, and anione, that which
‘at the anode,

bmeans of the battery, the compound nature of several substances
had previously been considered as elements has been determined.
‘of s battery of 250 couples, Davy, shortly after the discovery
decomposition of water, succeeded in decomposing the alkalies
and soda, and proved that they were the oxides of the hitherto
metals potassium and sodium. The decomposition of potass
demonstrated with the aid of the battory of 4 to 6 clements in the
manner: s small cavity is made in a piece of solid caustic
which is moistened, and a drop of mercury placed in it, Tho

aa

CHEMICAL EFFECTS. 695

filled with a saturated solution of « salt, say sulphate
coloured with tincture of violets. The platinum electrodes of

of four Bunsen’s clements are then placed in the two legs of
“After x few minutes the liquid in the positive leg, A, becomes
‘and that in the negative leg, B, of a green colour, showing that
tins been resolved into acid which has passed to the positive, and
which has gone to the negutive for these are the

B which a free seid and a free baso respectively produce on tincture

ition of sulphate of copper, free acid and oxygen gas appear at
electrode, and metallic copper is deposited at the negative

in Tn like manner, with nitrate of silver, metallic silver is de-
[on the negative, while free acid and oxygen appear at the positive

tion of salts was formerly explained by aaying that the

and potass, K,0. This view regarded salts composed of three
‘# different in their constitution from binary or haloid salts,
lectrolytic department has led to a mode of regarding the
of salts, which brings all classes of them under one
In malphate of potassium, for instance, the eloctropositive
is je while the clectronegative element is a complex of
quad oxygen, which ix regarded as a single group, SO,, and to
am axy-selphion may bo assignod. ‘The formula of sulphate
would thus be K,SO, and its decomposition would be quite
to that of chloride of potassium, KCl, chloride of lead, PbCI,,
KL. The electronegative group SO, corresponds to
or iodine. In the decomposition of sulphate of potassium the
jam liberated at the nogative pole decomposes water, forming
sand liberating hydrogen. In like manner the electronegative
S0,, which cannot exist in the free stato, decomposes into

which is liberated, and into anhydrous sulphuric acid, SO,,
diately combines with water to form ordinary sulphuric acid
Tn fact, where the action of the batter trong these gasos are
at the corresponding poles; in other cases they combine in the
reproducing water. The constitution of sulphate of copper,
Relic alieate of silver, AgNO, and thelr decomposition, will be

r pod from these examples,

effected by the current.—In chemical decom-

‘but 4 passage of the one to the positive, and of the other to the
electrode. This phenomenon has been demonstrated by Davy

ELECTROLYSIS. O97

a with the positive pole arranges itself as shown in fig. 652,
the oxygen is attracted and the hydrogen repelled. The oxygen
molecule is then given off at tho positive electrode, the liberated

n immediately unites with the oxygen of the molecule 6,
en of this with the oxygen of the molecule ¢, and so on,
‘negative electrode, where the last atoms of hydrogen become free
pear on the poles, The same theory applies to the metallic
to tho acids and salts, and explains why in the experiment
ned in the preceding paragraph, the syrup of violets in the veawsel
neither red nor green. ‘The reason why, in the fundamental

it, the hydrogen is given off at the negative pole when the
closed will be readily understood from a consideration of thin

‘Zaws of clectrolysis.—The laws of electrolysis were discovered
iy ; the most important of them are as follows:
eomnot take place unless the electrolyte is a conductor,
<9 io8 is not decomposed by the battery, becai a bad conductor.
bodies, such as oxide of lead, chloride of silver, ete, are only
frolysed in « fused state, that is, when they can conduct the current.
b The electrolytic action of the current is the same in all ite parts,

containing respectively fused oxide
im the samo voltaic current, which must be sufficiently
fil, these substances will be decomposed © electro-negative
‘will be separated at tho positiv iti
poles. The quantities of substances liberated are a
relation. Thus for every 18 Leth of water sp seat the

¢ weights) of the bodies,
il further be found, that in each of the

OROVE’S GAS BATTERY. 699

‘Of this is to produce two different electromotors, which produce a
‘oppesed in direction to the original one, and which, therefore,
‘it,

batteries may be constructed of pieces of metal of the

elf produce all the offocts of a voltaic battery. Such batteries are
batteries. Their action depends on an alteration of the
‘the metal produced hy the electric current; the constituents of
‘with which the cloth is moistened having become accumulated
opposite members of the cirenit.
y pile which has become inactive may be used as a secondary
‘When a current is passed through it, in a direction contrary to
the active battery yinlds, it then rogains its activity.
gas dattery.—On the property which metals have, of
F gases on their surfaces, Grove has constructed his gas battery.
form It consists of two glass tubes, in each of which is
Platinum electrode, provided on the outside with binding screws.
to expose a greater surface theso electrodes are covered with
divided platinum. One of the tubes is partially filled with hydro-
‘the other partially with oxygen, and they are inverted over
iphurie ucid, so that half the platinum is in the liquid and half
On connecting the electrodes with a galvanomoter the existence
is indicated, whose direction in the connecting wire i
um in oxygen to that in hydrogon; so that the latter is posi
former. As the current passes through water this is decom-
axygen ix separted at the positive plate, and hydrogen at the
‘These gases unite with the gases condensed on their surface, so
of gas in the tubes gradually diminishes, but in the matio
volume of oxygen to two volumes of hydrogen. ‘These elements
i formed nto a battery by joining the dissimilar plates with one
trjust ax they are joined in un ordinary battery. Ono element of
ia sufficient to decompose jodide of potassium, and four will
water.
state of iron.— With polarisation is probably connected
ble chemical phenomenon, which many metals exhibit,
p lly irom. When this is placed in contact with platinum
tmmersed in concentrated nitric acid it is unnttacked, while the
we would bo dissolved by the acid. This condition of iron is called
ive state, and upon it depends the possibility of the zinc-iron bat-

1 ELECTROMETALLURGY. TOL

ELECTROMETALLURGY.

(k mlectrometatiargy.—The decomposition of salts by the battery
& most important application in electrometallurgy, or the art of
metals from their solutions by thie slow action of a galvanic
‘This art was discovered independently by Spencer in England,
Jacoti in Petersburgh.
Onier to reproduce a medal or any other object by this process a
east must firet be made, on which is deposited the metallic layer,
teproduces the metal in relicf. If the medal is of metal the
way to form the cast is to use Arcet’s fusible alloy, which con-
5 parts of lead, 8 of bismuth, and 3 of tin. Some of tho melted
Polited into a shallow box, and just as it begins to solidify the
Js placed horizontally on it ina fixed position. When the alloy has
Be cool a slight shock is sufficient to detach the medal. A copper
‘then bound round the edge of the mould, by which it ean be con-
‘with the negative electrode of the battery, and then the edge and
surface are covered with a thin non-conducting layer of wax,
thw deposit is only formed on the mould itself.
fonder to take a copper cust, « bath ia filled with saturated solution
of copper, and two copper rods, B and D, stretched actos
862): one connected with the negative and the other with the
pole of « Daniell’s or Grove’s element. From the rod connecteit

nogative pole is suapended the mould m, and from the other n
ef copper, ©. Tho current boing thus closed, the sulphate of
ix decomposed, acid ii is liberated at the positive pole, while copper
[posited at the negatiro pole on the mould suspended from the rod,
which indeed several moulds may be attached. At the expiration
Bhours the mould is covered with a non-adherent, solid, vexioting

. Electrodynamics.—(! electrodynamics
laws of electricity in a state of motion, or the action of electric cu)
upon each other and upon magnets, while electrostatics deals »
laws of eloctricity in a state of rest,

fundamental experiment (711). All the even”
complicated, follow from two elipla ews ieee
geometry from the axioms, These laws aro— |
1, Two currents which are paral, and i he ame iin,
ane anuther,

TL, Two currents parallel, but in contrary directions, |
onvther,
Ta crler to decacnstsate tbeee Jews fen insole ean
traverses must consist of two parts, one fixed and the other mo}
ay shown in fig. 653. The fixed part consists of two brass
fastened on @ wooden base. The positive elecizode of a Bunsen’s
of four or five elements being connected with the foot of the col

the left of the figure, the current rises in this column, reaches
A, and then passes into a mereury cup, B. Here commences the nu
part of the cireait, which consute of « eopper wire; ome end reste

iat ail

TPES we Uw thas pa to wn ited pO) tran
current rises in the column on the right, which is connected
PseHRCt sigalivaitectiods of the battery:

the arrangement of the arrows it will be seen that the current

jaonstrate the first law the moveable

fig. 653 is removed and replaced by

resented in Gg. 554. The current is

(the same direction both in the

‘and in the moveable part ; and when ig. 664.

is removed out of the plane of the columns, #9 long as the
james it tends to return to it, proving that there fa attreotion
‘the two parts,

taws of angular currents.—I. J'wo rectilinear currents, the
1 of whick form an angle with each other, attract one another when

roach or recede from the apex of the angle.

. Piel One Well ohe mppironchen und the otter recedes from

‘the angie.
two laws may be demonstrated Sra atasaee aa
devised by M. De Ja Rive, which is » modification of

708 CAL
its axis, undil the plane of the axis and of
to PQ; the vertical current

from which the current PQ comes (Gig. 580), or on the sid

ee
a

2________#

ae

2
Fig. 562, Fig. 563.
vertical currents is asconding and the other descending (Bg. 642), be
that if they are both ascending or both desconding (fg. 643) thay ate
directed.

741, Action of am infinite rectilinear current on a

current in the same plane us the supports, the move~
passes into that plane. It is best to use the circuit in fig. 671,
‘fs astatic, while that of fig. 664 is not,

Fig. 664, Fig. 566.
‘has been said about the rectangular current in fig, 562 applies
( te the circular current of fig. 666, and is demonstrated by the
is experiments,

ROTATION OF CURRENTS BY CURRENTS,
2, Rotation of uw finite horizontal current by an infinite
rectangular current.— The attractions and repulsions

ich angular currents exert on one another may rendily be transformed
} = continuous circular motion. Let OA (fig. 566) be o current

z
Fig. 566, Fig. 607.

about the point O in a horizontal plane, and let PQ be a fixed
‘current also horizontal. As these two currents flow in the
° the arrows, it follows that in the position OA, the moveable
is attracted by the current PQ, for they are in the same direc-
Having reached the position OA’ the moveable current is attracted

Lo ROTATION OF MAGNETS BY CURRENTS. 71

column, it passes by the wires 65 into the copper ring, into the
water, and into the sides of the vessel, whence it rotarns to
battery by the strip D. The current being thus closed, the circuit
the ring tend to turn in «direction contrary to that of the fixed
a motion due to the action of the circular current on the current
ithe vertical branches 63; for, ax follows from the two laws of angular
the branch 4 on tho right is attracted by the portion A of the
eurrent, and the branch 6 on the left is attracted in the contrary
by the opposite part, and these two motions coincide to give
: ® continuous rotatory motion in the eamedirection. The uetion
‘the circular current on the horizontal part of the circuit 45 would
tead to turn it in the same direction; but from its distance
evidently be neglected.

Rotation of magnets by currents-—Faraday has proved that
impart the same rotatory motion to magnets which they do to
‘This may be shown by means of the apparatus represented in

60. Tt consists of n large glass vessel, almost filled with mercury.
the centre of this is immersed & magnet about 8 inches in length,
Projects a little nbove the surface of the mercury ; it is loaded at
Jowerend with a platinum cylinder, represented at ab on the right of
‘Epparatus. At the top of the magnet is n small copper cup contain-
‘mercury ; the current passes into this cup by the rod C, As soon as
eurrent ascending in the column A, passes into the magnet, thence
the mercury, and emerges by the column D, the magnet begins to
round its own axis with a velocity depending on its magnetic
p And on the intensity of the current.
This rotatory motion is readily intelligible on Ampirv's theory of
which will be subsequently explained (755), according te

+ Iattor is directed and sets across the direction
may be illustrated by the apparatus represented in

Dass oe, snn in ae attached to m
‘it can float freely on water. The plates arv connected with
cusps on the cork float; and with these can be connected

[ler ox rectangular wires, colls, ce scloncids; and they are than

by a current, and can be subjected to the action either of magnets

ents,
tation ofcurrents by magnets.—Not merely can currents be

eee ayer Set ba dato rotate nab en Sa
experiment, devised by Farnday,
‘On « base with levelling screws,
g 08 an ivory support, is a eoppor
‘Tt is surmounted in part of its
{m magnetised bundle AB and at
® mercury cup, A copper circuit
leed on a steel point, rests in the
he other ends of the circuit, which

reaches the mercury by the steel
(ence it passes by the framework,
Mf copper, to the battery by the
trew a If now the mnagnetised
| raised, the circuit EF rotates
ne direction or the other accord~
| pole by which it is influenced.
‘on is due to currents assumed to
found magnets, currents which
\ vertical branches EF, in the
Ned mrad current on the Fig. $72.

emer eceeues bundle may be replaced by « solenoid

ie sz

\RTH ON MORIZONTAL CURRENTS. m5
of the vessel a, and so back to the battery

ting thus ele, the mire moves rund the column
‘the cast of it, when it descends, as is tho case in the
bedpeviey:

aanereirst peopenteae riers

the rod will not move, for as each wire tends

ca the eat of the olunn b, two equal and contrary
i, which counterbalance one another.

‘Vertical axts,—Tho action of the earth on horizontal currents
but gives them @ continuous rotatory motion from the east
the horizontal current moves away from the atts of rotation,

the west to the east when it is directed towards this axis.

‘Fig. 574.
he illustrated by moana af the aj ropresented in
hich only differs from that of fig. 674 in having but one
‘current ascending by the column a, traverses tho two
and descends by tho wires bb, from which it regains the pile.
iit ecb then begirs a continuous rotation, either from the
wost, or from the west to the east, according as in the wires
it gous from the centre, as ia the case in the figure; or
ss it goes towards it, which is the caso when the current
‘the wire m instead of by n. But we have seen (747) that the
the earth on the vortical wires 5b in destroyed ; hence the
is that produced by the action on the hoiizontal branches ce.
action of the terrestrial current on horizontal currents is a
nee of the rotation of a finite horizontal by an infinite horizon-
it (742),
Directive action of the earth on closed currents move-
a ‘@ vertical axts.—If the current on which the earth acts

SE Eee eae ee

to turn them in opposite directions, and

condition is fulfilled in the eireults faded

‘Hence theso currents aro called astatic currents.

SOLENOIDS.
750, Structure of a solenotd.—A solenoid is a aystem

and parallel cireular currents formed of the sane

copper wire, and coiled in the form of

in fig. 677. A solenoid, howover,

A
= i

Li

Fig. 977.

action of a solenoid im a Longhvuilinmah Sheen AR)

SOLENOIDS.

of fixed rectilinear currents on finite, rectangular, or circular cur-
1), applies evidently to each of the circuits of « solenoid, and

‘to the fixed current; and further, in the lower part of ench of

the current is in the same direction as in the rectilinear

‘of passing « rectilinear current below the solenoid, it is

cally on the side, an attraction or repulsion will take place,

in the vertical wire, and in the nearest part of the solenoid,
are in the same or in a contrary direction.

action of the earth on solenoids,—If a solenoid be

din two eups, A and B, Ampére’s stand (fig. 556), not in the

magnetic meridian, and a current be passed through the

begin to move, and will finally set in such a

action of the earth on solenoids is accordingly a consequence
which it exerts on circular currents, In this experiment the

‘THEORY OF MAGNETISM. ny

ic in steel, opposes this motion, and tends to keep them

in ik hey bppen tobe. When the magnetic sab-

these molecular currents, under the influence of

étractions, occupy such positions that their totel action on

mal substance is null, Magnetisntion consists in giving to these

‘currents a parallel direction, and the stronger the magnotising

more perfect the parallelism. The limit of magnetisation is
‘when the currents are completely parallel,

‘resultant of the actions of all the molecular currents is equivalent

a ‘of a single current which traverses the outside of a magnet.

with the surface; there the molecular currents at ab are not neutra-

dy other currents, and as the points abe are infinitely near, they form

iseries of elements in the samo direction situated in planes perpendicular
ti the axis of the magnet, and which constitute « true solenoid.

| The direction of these currents in magnots can be ascertained by can-

dering the suspended solenoid, fig. 579. If we suppose it traversed by

it, and in equilibrium in the magnetic moridian, it will sot in such

om that in the lower half of each coil the current flows from eat

‘We may then establish the following rule, At the north pole of

the direction of the Amperian currents is opposite that 7

st
trial magnotic effects, the existence of electrical currents is assumed
contisually circulate round our globe from east to west, perpen-
to the magnetic meridian.
‘resultant of their action ie a single current traversing the magne-
ar from east to west. These currents are supposed to be thermo-
c currents dua to the variations of temperature caused by the
influence of the sun on the different parts of the globe from

‘currents direct magnetic needles, and impart « naturel magnelh-

‘ELECTROMAGNETS, ‘Te

jn the case in a left-handed spiral. But whatevor tho direction
Felling: ths: polarity ia: easily found by the following rule: Jf a
Dementia fe csarromt look at he axis of the apivad, the ‘north pele

Fig. 682.

tom Ads left, If the wire be not coiled regularly, but its direction
feed, at each change of direction a consequent point is formed in

ieee cE tha tbe on. wh wh alice eile in act-whhat
& Wood and glass are of no effect, but « thick cylinder of copper
destroy the action of the current. ‘The same is the ease
)e, silver, and tin.

however, to magnetise a steel bar by means of electricity, it
placed. in tube, at shown in figs 581 and 583, It
3 to coil round it a
(ire covered with silk, in
lo insulate the circuits
(eanother, The action of
rent ix thus multiplied,
beble current is sufficient
(tee « powerful charge of

foen.

Blectromagnets.— Liic-
fete are bars of soft iron
under the influence of a
current, become magnets ;
§ magnetism is only tem-
for the coercive force of
yeott iron is null, and the
fagnetic fluids neutralise
her as soon as the current
jo pase through the wire.
fever, the iron is not quite
(t retains more or less
€ magnetism, The elec-
pets have the horse-shoe
(6 shown in fig. 583, and
ir wire, covered with silk
on, is rolled several times Fig. 58:

them on the ea rhed va og be roa tna eevaone eee

Ir

ELECTROMAGNETS. 72

is attract each other with # force proportional to the square of the
of both currents.
(sii) For powerful currents tho length of the branches of an electro~
net is without influence on the weight which it can support.
regands the quality of the iron used for the electromagnet it must
and be made as soft as possible by being reheated and cooled a

at many times; it is polished by means of a file no as to avoid twist-
‘Af this is not the case the bar retains, even after the passage of the
‘a quantity of magnetism which is called the remanent magnetiom,
of soft iron wires loses its magnetism more rapidly than a
bar of the same size.
mann has proved that an analogy exists between the phenomena
tim and those of torsion which extends even into details.

tions during the twisting of a wire increase the torsion, just as they
the magnetism of a wire while under the influence of the eur-
‘The permanent torsion of iron wires is diminished by their magne-
on, a5 the permanent magnetism of steel bars is by their torsion; a
d wire loses some of its torsion by heat, as a magnet loses some of

Ve shull presently see the numerous applications which have been

‘of electromagnets in electric telegraphs, in electro-magnetic motors,

etric clocks, and in the study of diamagnetic phenomena,
0. Vibratory motion and sounds produced by currents.—
wrod of soft iron is magnetised by a strong electric current, it
a very distinct sound, which, however, is only produced at the
of closing or opening the current. This phenomenon, which
first obeerved by Page in America, and by Delezenne in France, has
particularly investigated by De la Rive, who has attributed it to
motion of the molecules of iron in consequence of a rapid
suneeesion of magnetisations and demagnetisations.

When the current is broken and closed at very short intervals, De In
fas observed, that whatever be the shape or magnitude of the iron
‘two sounds may always be distinguished: one, which is musical,

mesponds to that which the rod would give by vibrating transversely ;

other, which consists in a series of harsh eounds, corresponding to the

tions of the current, is compared by Dela Rive to the noise of
falling on a metal roof, The most marked cound, says ho, is that
A by stretching on a sounding board pieces of soft iron wire, well
Jed, from 1 to 2mm. in diameter, and 1 to 2 yards long. These
being placed in the axis of one or more bobbins traversed by
ful currents, send forth a number of sounds, which produce a
g effect, and much resemble that of a number of church bells

m2

is

La aperaiery ee, |
respond at a distance by means of the ¢|

was unknown, Ampére proposed to corres
needles, shave. which a current was sent,
being used as letters were required. In 1)
Lele pepsin cpepye ick ||
mitted by a wire acted on & magnetised by
under its influence were obrerved by # tel
thus vending signals from the Observatory
Géttingen, a distance of a mile and = quar!
honour of having first demonstrated expe)
electrical communication at a considerable ¢
in Munich, and Wheatstone in London, con
several wires each acted on a single needle:
being produced by an electromagnetic mach
constant battery.

Every electric telegraph consists .essent
circuit consisting of a metallic connection,
electromotor for producing the current ; 2,
the signals from the one station; and, 3, at

ELECTRIC TELEGRAPH. 725

other station. The manner in which these objects, more espe-
the Inst two, are effected can be greatly varied, and we shall limit
hes to a description of the three principal methods,

electromotor generally used in England is a modification of
(tom's battery. It consists of a trough divided into compartments,
bof which is an amalgamated zine plate and a copper plate; these

Fig. 584.

fare usually about 44 inches in height by 3} in breadth. The
(rtments are filled with sand, which is moistened with dilute sul-
tacid. This battery is inexpensive and ensily worked, only requiring
fime to time the addition of a little acid; but it has very low
b-motive force and considerable resistance, and when it has been at
for some time, the effects of polarisation begin to be perceived,
@ telegraphs of the South Eustern Railway, the platinised graphite
battery invented by Mr. C. V. Walker is used with success. In
©; Daniell's battery is used for telegraphic purposes,

}eonnection between two stations is made by means of galvanised

Fig. 688.

of laying the cable, and to enable it to ret
currents,

At the station which sonds the despateh, t
positive pole of a battery, the current a
station, and if there were a second return lin
opposite direction to return to the negativi
made the very important discavery that the)
retarn conductor, thereby saving the expel)
this purpose the end of the conductor at the
Pole of the battery at the other are connect

an] ELECTRIC TELEGRAPH. 727

(Re Bich are runk to some depth in the ground. The action is then the
me if the earth acted asa return wire, |

Ul. Wheatstone’s and Cooke's single needle tolegraph.—This

essentially of « vertical multiplier with an astatic needle, the
— of which is seen in fig. 585, while fig. 584 gives a
i view of tho case in which the apparatus is placed, A (fig. 686) is
bobbin consisting of about 400 feet of fine copper wire, wound in a
[Hitkare in two connected coils. Instead of an astatic noodle, Mr. Walker
Hoar found it advantageous to use a aingle needle formed of several piecos
sof very thin steel strongly magnetised ; it works within the bobbin, and
@ licht index joined to it by a horizontal axis indicates the motion of the

mitedle on the dial.

The signs are msde by transmitting the current in different directions
Phrough the multiplier, by which the needle is deflected vither to the
Hight of left, necording to the will of the operator. The instrument by
Which this is effected is a commutator or key,

G; its construction is
|

shown in fig. 585, while fig, 596 shows on a large acals how two stations
ar connected. It consists of a cylinder of boxwood with « handle, which
Projects in front of the case (fig. 584). On its circumference parallel to
the axis are seven brass strips (fig. 586), the spaces between which are
fosilated by ivory; these strips are connected at the end by metallic

telographs.—Of |
589 represent # lecture-model of one |
and which well serres: psa

parts ; the mandpulator for
(tig. 580) for receiving them. The |
battery, Q, and the two ay
metallic wires, one of which, AOD (fig. 38),
the arrival station, and the other, HKLI (fig
the departure, In practica, the lattor is rj
Ench apparatus is furnished with a dial wi)
alphabot, on which a needle moves. The ne¢
is moved by hand, that of the arrival by elect
The path of the current and its effects”
battory it passes through a copper wire, A (fi
N, which presses against a metal wheel, R,4

730

Monse’s TELKGRAPH. 731

he despatch moves the needle P to the letter 8, where it stops fora very
hort time; x the needle at Brighton accurately reproduces the motion
the London needle it stops at the same letter, and the person who
berives the despatch notes this letter. The one at London always con-
{euimg to turn in the same direction, stops at the letter I, the second
leedle immediately stops at the same letter ; and continuing in the same
Qanner with the letters G, N, A, L, all the word is soon transmitted to
Grightom. The attention of the observer at the arrival station is attracted
by means of an electric alarum, Fach station further must be provided
the two apparatus (figs. 689 and 689), without which it would be
ible to answer.

signals there is no means of remedying them, These incon-

Wraitnees are mot met with in the case of the writing telegraphs, in which

foo on a strip of paper at the time at which
tramemicted.

| Of the aumerows printing and writing telegraphs which have been
levised, that of Mr. Morse, first brought into use in North America, is
fect known. It has been almost universally adopted on the continent.
fa this instrament there are three distinct parts: the indicator, the
frmeneatoator, and the relay; figs 590, 501, and 392 represent these

Pparatas, F

Indicator. We will first deseribe the indicator (fig. 590), leaving out

. G and T, placed on the

ofthe figure. The current which enters the indicator by the wire,

}, passes into an electromagnet, E, which, when the current is closed,

(tracts an aroiature of soft iron, A, fixed at the end of a ‘herisontal lever
howeable about an axis, x; when the current is open
‘t By means ‘of two screws, m and v, th

| which writes the signals, For this purpose « lo é

taper, pp, rolled round a drum, R, passes between two ‘copper rollers
Fith « rough surfaco, w, and taming in contrary din

fireetion of the arrows, the band of paper Tana rolled on a second

Fig. 600. —

points may be produced at another station
give a definite meaning to these combinatio
A line and @ point (—.) represe
A line and three points (—. , .)
Three points (...)
A line and two points (—..)_
Tn this manner words and phrases can be §
leave a space betwoen each letter.
Communicator or hey. This consists ofa
acts as support for & metallic lever ab (fig.
on « horizontal axis. The extremity @ of
upwards by a epring beneath, so that it 7
finger on the key B that the leversinks and
‘the base there are three binding screws; on
which comes from the positive pole of the’
with L, the wire of the line; and the th
passes to the indicator, for of course two
euch provided with an indicator and comm

‘MORSE’S TELEGRATK. 733

details known, there are two cases to be considered: 1. The

a4 or is arranged so ax to receive a despatch from a-distant
5 the exrenityb then depres trp in he drwing,

that the current which arrives by the wire of the line L, and ascends

En the metallic piece m, redescenda in the wire A, which leads it to the

Fig. 591.

post at which the apparatus is placed. 2. A despatel:
i in this case the key B is prossed so that the lever
ituct with the button x. The current of the local battery,
by the wire 2, ascending then in the lever, redescends by
the wire L, which conducts it to the post to which the
‘addressod, According to the length of time during which B

® presed, a dot or a line is produced in the receiver to which the

Relay, In describing the receiver we have assumed that the current
the line coming by the wire C (fig. 590) entered directly into the

etromagnet, and worked the armature A, producing a despatch ; but
a the current has travorsed a distance of a few miles ite intensity has

:

At each oscillation the top of the lever
and at this moment the current of the:
binding screw, c, ascends the column m, pas
by the rod 0, which transmits it to the sci
electromagnet of the indicator, whence it
return to the local battery from which it sta)
of the line is open, the electromagnet of thi
lover p, drawn by a spring r, leaves the but)
ing, and the local current no longer passes
to the indicator exactly the same phases of }
thoso effected by the manipulator in the post

‘With a general battery of 26 Daniell's 4)
enough at upwards of 90 miles from its sta
For « longer distance # new current must

agraph on the change of current (p. 7

Working of the three apparatus, The thre
apparntus being thus known, the:

The current of the line coming by the
piece T intended to serve as lightning conduc
of atmospheric electricity in time of storm, |
charged with so much electricity as to gi
apparatus consists of two copper dises, d and
the sides opposite each other, but not touchij
with the earth by a metallic plate at the bc)
this lightning conductor, while the disc f i
coming by the line L enters the Hightniny
screw fixed at the lower part of the stand

commutator, n, which conducts it to a butt,
‘ disc f by a metallic plate at the back of t
tricity, acting inductively on the dise 4, om

which conducts it to 2 small galvanometer, G, serving to

by the deflection of the needle whether the current passes.
this galvanometer the current proceeds to a communicator (fig.
which it enters at L, whence it emerges at A to go to the relay
502). Entering this at Lit works tho clectromagnot, and establiahes

show current outers at P (6g. 590), reaches
it to the column H, and thence only proceeas further when the
A sinks. A small contact placed ease tae Daren touches then

ph.—If a strip of paper be
d in an aqueous solution of ferrocyanide of potawium and con
with the negative pole of a batt
ed with s steel pointer connected with th

‘the dots and marks
a heel and a

Fig. 693.

An electromagnet, 13, attracts an arn
on a pivot, a, The armature P transmits its
4, which, by means of a ratchet, », turns t]
pinion D, turns the wheel C, which by as
moves the hands. The small one marks |
minutes; but as the Intter doos not move my
from second to second, it follows that it may
seconds. Pat,

At is obvious that the regularity a
on the regularity of the oscillations of the
the oscillations of the current before
are regulated by standard clock, which its
lated by a seconds pendulum. At each oscil
current is open and closed, and thua the armi

To illustrate the use of these electrical
railway from London to Birmingham each 1

q
|

ves] ELECTRICAL CLOCKS. * 737

fd that from the London station a conducting wire pastes to all the
locks on the line as far as Birmingham. When the current passes in
bis wire all the clocks will simultaneously indicate the same hour, the
Qme minute, and the same second; for electricity travels at the rate of
190,000 miles jn 8 second, so that it takes an inappreciable time
go from London to Birmingham.

| 705, mectromagnetic machines.—Numorous attempts have been
ete to apply electromagnetism as a motive force in machines. - Fig.
represents a machino of this kind constructed by M, Froment. It

Fig. 595,

(inaists of four powerful electromagnets, ABCD fixed on an iron
Gfume, X. Between these electromagnets is a aystem of two iron wheols
on the aame horizontal axis, with eight soft iron armatures, My,

bo their circumference,
The current arrives at K, ascends in the wire E, and reaches a metallic

emerging from the
megati of the battery by the wine
papi ah tet armatures M1

767, Zaduction by currents.—We ha
under the name fduction is meant the ac
exert at a distance on bodies in the matun
only bad to deal with electrostatical indueti
dynamical electricity produces analogous eff

Farnday discovered this class of phenom¢
name of cwrents of induction or induced exert
developed in metallic conductors under th
ductors traversed by electric currents, or h
magnets, oreven by the magnetic action of
which give: rise %9 Uwe Toe nes gave’ inate

‘The inductive action of currents at the moment of opening or closing
‘be shown by means of a bobbin with two wires. This consists (fig.
of m cylinder of wood or of cardboard, on which a quantity of «ilk-

No. 16 copper wire is coiled ; on this is coiled # considerably

Fig. 596.

length of fine copper wire, about No, 35, also insulated by being
with silk. This latter coil, which is called the secondary coil, is
hy its ends with two binding screws, a, 6, from which wires
to sgalvanometer, whilo the thicker wire, the primary coil, is con-
by its extremities with two binding serews, ¢ and d. One of
d, being connected with one pole of a battery, when a wire from
other pole is connected with c, the current passes in the primary coil,
Mid in this alone. The following phenomena are then observed :—
E At the moment at which the thick wire is traversed by the current
fie galvanometer by the deflection of the needle indicates the existence
the secondary coil of a current inverse to that in the primary coil, that
in the contrary direction; this is only instantaneous, for the needle
ly reverts to zero, and remains so long as the inducing current
through od,
Hi At the moment at which the current is opened, that is, when the
cd ceasos to bo traversod by a current, there is again produced in
‘wire ab an induced current instantaneous like the first, but direct,
tat is, in the rame direction as the inducing current.
703. Production of induced currents by continuous ones,—
currents are also produced when a primary coil traversed by
is approached to or removed from a secondary one: this may
shown by the following apparatus, fig. 697, in which B is a hollow
‘cotsiating of a great length of fine wire, and Aa coil consisting of
Phorter and thicker wire, and of such dimonsions that it can he placed
the secondary coll. The coll A being traversed by a curront, if it is

is rapidly withdrawn, the galvanometer
by Diabad borict Ute rapic
primary coil this is done slowly, the gal
current, and which is the feebler the slo
If instead of varying the distance of 4
bo varied, that is either increased by br
into the circuit, or diminished by inera
current is luced in the secondary wit
of the eb current increases and dir
709. Conditions of induction. %
ments which have been desoribed in the
ing principles may be deduced.
1. The distance remaining the same ¢
does not induce any current in an adjiacers
IL A current at the moment of being
ductor an inverse current.
TIT, A current at the moment it ceases

INDUCTION. 7H

'. A current which is removed or whose intensity diminishes, gives rise
induced current.
A current which is approached or whose intensity increnses gives rise
m inverse induced current.
On the induction produced between a closed circuit and a current
Retivity whon their relative distance varies, Lenz has based the
law, which is known us Lenz's law :
the relative position of two conductors A and B be changed, of which
(lie tramersed by a current, a current is induced in B in such a direction,

‘Thus, for instance, in V, when « current is approached to a conductor, an
eurrent is produced ; but two conductors traversed by currents in
directions, repel one another according to the received law of elec-
Inversely when a current is moved away from n conductor

ced either by the
: current,

[et, and can either bo approached or
fo6 of the plate A are coiled abo
» The two end:

At the moment at which the current
distance of the two conductors is vi
served as in the caso of the apparatay
771. Induction by magnets.—I)
a current magnetises a steel bar; in
induced currents in metallic circuits,
of a coil with a single wire of 200
extremities of the wire being connect
in fig. 690, a strongly magnetised br
and the following phenomena are obs
i, At the moment at which the r
moter indicates in the wire the exit
which is opposed to that which eirew
the latter as a solenoid on Ampére's |
ii, When the bar is withdrawn,
which has returned to zero, indicates
‘The inductive action of magnote mi

Tig. 609,
same inductive effects are produced in the wires of an electro~
ifs strong magnet be made to rotate rapidly in frovt of the
remities of the wire in such a manner that its poles act successively
it ‘on the two branches of the electromagnet : or also by form-
‘two coils round a horse-shoe magnet, and passing a plate of soft
on rapidly in front of the poles of the magnet; the soft iron becoming
tie reacts by influence on the magnet, and induced currents are
in the wire alternately in different directions.

inditctive action of magnets king confirmation of Ampéro’s
of magnetism. For as in this theory all magnets are solenoids,
periments which have been mentioned may be explained by

ductive action of currents which traverse the surface of magnet
uction of magnets is in short on induction of currents. And it is
mreful exercise to see how on this view the inductive action uf magnets

‘under Fenz’s law (709),

\ ‘Inductive action of magnets on bodies in motion.—Arago
as the firet to observe, in 1824, that the number of oscillations whicl
r d needle makes in w given time, under the influence of the
magnetinm, is very much lessened by the proximity of certain

!

Fig. 600,

axis. On this axis is a sheave, B, round »
also round the sheave A. By tur
tlisc M may be rotated with great rapidity
plate, on which is a small pivot
dise be now moved with 4 slow and unifor,
flected in the direction of the motion, and
the direction of the magnetic meridian, a¢
rotation of the disc. But if this velocity
mately deflected more than 90°; it is the
entire revolution, and follows the motion ol

Babbage and Herschel modified Arg
horse-shoe magnet placed vertically to rot,
pended on silk threads without torsion; ¢
direction as the magnets,

‘The effect decreases with the distance of
nature. The maximum effect is produce
gloss, water, ctc., it disappears, Babbage 4
rupresenting this action on copper at 100, |
as follows: zinc 95, tin 46, lead 25, antin
the effect ia enfedbled if the disc presents bi

INDUCTION, 75

ity in the direction of the radii; but the same physicists have ob-
|, that it virtually rognins the same intensity if these breaks have

soldered with any metal.
farnday made an experiment the reverse of Arago’s first observation ;
the presence of a metal at rest stops the oscillations of a magnetic
the neighbourhood of magnet at rest ought to stop the motion
Shoe mass of metal. Faraday suspended a cube of copper to a
thread, which was placed between the poles of « powerful
fromagnet. When the thread was left to itself it began to spin
with speet velocity, but stopped the moment a powerful current
the electromagnet.

oy was the first to give an explanation of all these phenomena
m by rotation. They depend on the circumstance that a
‘of a solenoid can induce currents in a solid mass of metal.
ithe abore case the magnet induces currents in the disc, when the
is rotated; and conyersoly when the magnet is rotated while the
is primarily at rest, Now these induced currents by their electro~
nic action tend to destroy the motion which gave rise to them;
oy are simple illustrations of Lenz’s law ; they act just in the sane way
frietion would do.
% For instance, let AB (fig. 601) be a needle oscillating over a copper
‘and suppore that in one of its oscillations it :
in the direction of the arrows from N to M. z( |

hing the point M, for instance, it developes
a current in the opposite direction, and which
fore repela it; in moving away from N it pro-
currents which are of the same kind, and

ch therefore attract, and both these actions
ar in bringing it to rest. Fig. 601.
Sapposs the metallic mass turns from N to-

M, und that the magnet is fixed: the magnet will repel by induc~

‘points such as N which ure approaching A, and will attract M which

g away; hence the motion of the metal stops, as in Faraday’s

‘fin Arugo’s experiment the disc is moving from N to M; N ap-
ghee A and repels it while M moving away attracts it ; hence it moves
‘The same direction as the disc,
(Hf this explanation is trae all circumstances which favour induction
Gnerease the dynamic reaction, and those which diminish the former
‘iilso lomen the latter, We know that induction is greater in good
etors, and that it does not take place in insulating substances ; but
Tiave seen that the needle is moved with a force which is lem, the
the conducting powers of the disc, and it is not moved when the diss,
RK

Fig. 602.

for showing the exiatence of terrestrial indi
wooden ring, RS, about two feet in diame!
which it can be turned by means of « hay
fixed in a frame, PQ, moveable about a hor
to these two axes the inclination towanls
and therefore of the axis 0a, is indicated on
, gives the angular displacement of the rin
which is coiled a lange quantity of insulated
the wire terminate in ® compadelor mallogs)

Fig. 603.
since in the part CA the current goes fy
entire cireuit in the direction AFBDG, |
current. This current, which thus appi
is the extra current.

775, Mxtra current on opening a:
spiral act inductively on each other, no)
closing the current. Here in accordance
tion, each spire acting on each succeed
‘opposite direction to its own, that is an
the extra current on closing or the fnvera
direction to tho principal one, diminis)
suppresses the spark on closing,

When, however, the current ix open
tively on each succeeding one, producing
‘as its own, and which therefore greatly
principal current, This is the extra ev
current,

To observe the direct extra current, t
is to be traced may be introduced into #]
any suitable manner with the binding ect
galvanometer,

It can thus be shown that the direct e
bright eparks, decomposes water, melts
steel needles. Aria has found that th

INDUCED CURRENTS. 749

“shout 0-72 of the principal current. The shock produced by the
z may be tried by attaching the ends of the wire to two files, which
held in the bands, On moving the point of one file over the teeth of
ue other a series of shocks is obtained, due to the alternate oponing and
‘ g of the current.
) The above effects acquire greater intensity when a bar of soft iron is
duced into the bobbin, or, what is the same thing, when tho current
pasted through the bobbin of an electromagnet ; and still more is this
ease if the core, instead of being massive, consists of a bundle of
ght wires. Faraday explains this strengthening action of soft iron
: If inside the spiral there is an iron bar, when on opening the
it the principal current disappears, the magnetism which it evokes
‘the bar disappears too; but the disappearance of this magnetiam acts
the disappearance of the electrical current, the disappearing mag-
ns induces # current in the same direction as the disappearing prin
earrent, the effect of which is thus heightened.
in the experiments just described the effects of the two extra currents
way those of the principal current. Edlund has devised an
‘arrangement of apparatua by which the action of tho principal
‘on the measuring instruments can be completely avoided, so that
‘that of the extra current remains, In this way he has arrived at
following laws:
The intensity of the currents used being the same, the extra-currents
‘on opening and closing have the same electromotive force,
The electromotive force of the extra-current is proportional to the
By of the primary current,
6, Induced currents of different orders.—Spite of their instan-
character, induced currents can themselves, by their action on
1 cireuits, give rise to new induced currents, these again to others,
pd wo on, producing induced currents of different orders.
P These currents, discovered by Henry, may be obtained by causing to
each other a series of bobbins, ench formed of copper wire covered
th silk, and coiled spirally in one plane, like that represented in the
‘A, in fig. 597. The currents thus produced are alternately in op-
. Sosa and their intensity decreases in proportion as they are of

Se ies ef induced currents.—Notwithetanding their
neous character, it appears from the preceding experiments
induced currents have all the properties of ordinary currents.
produce violent physiological, luminous, calorific, and chemical
fhets, and finally give rise to new induced currents. They also deflect
Magnetic needle, and magnetise steel bars when they are passed
gh # copper wire coiled in « helix round the bars,

i. The inte of tered erent pi
os hint proportional tthe pra

or _ waives aad deceloped |
tricity is the same whatever be the mature, #4
sina |

‘This latter law ix in dianccord with the
the induction of statical electricity (648).

APPARATUS FOUNDED 0}

779, Magnete-electrical apparatus
neto-electrical induction, several attempt
uninterrupted series of sparks by means
this purposs wore dovised by Pixli and
Saxton, Ettingshausen, and Clarke. Fig.
by Clarke. It consists of a powerful hor:
fixed against # vertical wooden support. |
bobbins BB’, moveable round @ horizontal a

APPARATUS FOUNDED ON INDUCTION. 751

of soft iron joined at ono end by s plate of soft iron, V,
at the other by a similar plate of bras. These two plates are fixed
copper axis, terminated at oneend by a commutator, gi, and at

bys pulley, which is moved by an endless band passing round
wheel, which is tarned by a handle.

Exch bobbin consists of about 1,500 turns of very fine copper wire
Worered with silk. Ono end of the wire of the bobbin B is connected on
The axis of rotation with one of the wires of the bobbin B’, and the two
ends terminate in a copper ferrule or washer, g, which is fixed to
axis, but is insulated by a cylindrical envelope of ivory, In order
in each wire the induced current may be in the same direction, it
om the two bobbins in different directions, that is, one is
the othor left-handed.
When iow the electromagnet turns, its two branches become alter-
tely magnotised in contrary directions under the influence of the
agnet A, and in each wire an induced current is produced, the direction
Ywhich changes at each half turn.

are seon here as thoy are in fig. GO4; and bh
end, which grazes the magnet, that the Ampé
the hands of n watch, These currents act i
bobbin, producing a current in the same di)
away from the pole a, its soft iron i+ dema
currents cease (769). Tho intensity of the j
decreases, until the right line joining the
perpendicular to that which joins the pole!
is now no magnetism in the bar, but quickly
soft iron is then magnetized in the opposite d

APPARATUS FOUNDED ON INDUCTION. 758

(th pole (fig. 07). The Ampérian currents are then in the direction of
barrow a’; and as they are commencing, they develops in the wire of the
an inven current (769), which is in the same direction os that
Feloped in the first quarter of the revolution. Moreover, this second
frent adds itself to the first, for while the bobbin moves away from a,
fepproaches 5, Hence during the lower half revolution from a to 8,
b wire was eucosssively traversed by two induced currents in the
he direction, and if the rotatory motion is sufficiently rapid, we
fit admit during this half revolution, the existence of a single current
the wire.
Phe same reasoning applied to the figures 608 and 600 will show that
fing the upper half revolution the wire of the bobbin B is still
Fersed by a single current, but in the opposite direction to that of the
ver half revolution. What has been said about the bobbin B applies
Hloualy to the bobbin B’; yet as one of these is right-handed and the
jer left-handed, during each upper or lower half revolution
fremts are constantly in the same direction in the two bobbing. At
th successive half revolution, they both change, but are in the same
tion as rogurds each other.

| Fig. 610.

780. Commmutator.—The object of this apparatus (fig. 610) of which
/ GLI isn section, is to bring the two alternative currents always in

Fig. 611,
toon that the two ends of the wire of th
direction, terminate in the metallic axis |
ferrule o’ ; while the two other ends, both in
to the ferrule g and therefore to the half fen:
are constantly poles of alternating currents
bobbins, and an these are alternately in o
0 and o’ are alternately positive and negatiy
which the half ferrule o’ is positive, the ew
follows the plate m, arrives at » by the join
ia closed by contact with the ieee es 4
rotation o takes the place of o’, the eurren|
for aa it is then reversed in the bobbins, o
nogative, and #0 forth as long as the bobbin

With the two éprings } and ¢ alone, the
two pieces o and o’ could not unite; this is

ring, @ (fig. 604), and of two appendices, |
ta she fame These two pisces are insula
ivory cylinder, but communicate respectiy
As often as the plate @ touches one of thes
the spring , and the current is closed, for it
reaches the spring ¢ by the platen. On
spring a does not touch one of thes»

For physiological affects the use of the |
intensity of the «hocks, For this purpose ty
with handles p and p’, are fixed at # and m.
hands so long as the plate ¢ does not touch

renewed at cach semi-revolution of the electromagnet, and its
‘increases with the velocity of the rotation. The muscles con-
such force that they do not obey the will, and the two hands
‘be detached. With a well-constructed apparatus of large
sions & continuance of the shock is unendurable; the person re~
‘it is prostrated, rolls on the ground, and is soon completely at tho
‘of the operator.

t -effects of voltaic currents may be produced by the induced
puts of Clarke's machine, Figure 605 shows how the apparatus is

Fig. 613.

armaged for the decomposition of water. The spring is sup-
‘the current being closed by the two wires which represent the,

2 Figures 612 and 613 represent the arrangement of the
and the commutator in each case. The first representa the in-
of ether, and the second the incandescence of a metallic wire,
* the current from the plate, o, to the plate ¢, always passes
same direction.

ixif’'s and Saxton’s electromagnetic machine differs from Clarke's in
ng the electromagnet fixed while the magnet rotates

n has recently devised a compendious form of the magneto~
trical machine, for the purpose of using the induced spark in firing
bs (689).

We

Fig, 614,
The first machine of this kind was inven

1850; this has been greatly improved by
applied it to electrical illumination,

‘Hotel des Invalides, in Paria, where it was constructed, One of
‘machines was exhibited in the International Exhibition of 1862. It
of # cast-iron frame, 54 feet in height, on the circumference of

of SwWhich (enn kupport from 190 to 190 pounds, aro so arranged, that.
re considered either parallel to the axis of the frame, or in «
dicular to this axis, opposite poles always face one another.

the outside batteries consist of three magnetined plates,

midile ones have six plates, because they act by both

iron axis going from one end to the other of the frame,

fare fixed, each corresponding to the intervals botwoen

‘Dasterien of two vertical series. There are 16 bobbins on

ence of each of these, that is, as many as there are magnetic

th vertical series of magnets. These bobbins, represented in

from those of Clarke's apparatus in having, instead of a

12 wires ench, Li} yards in length, by which the resistance

‘The coils of these bobbins ure insulated by means of

ved in oil of turpentine, These are not rolled upon

‘of iron, but on two iron tubes, slit longitudinally, so as to

gnotisation and demagnotisation more rapid when the

front of the poles of the magnet, Further, the dises of

rminate the bobbins are divided in the direction of the

to prevent the formation of induced currents in these

jour wheels being respectively provided with 16 bobbins

ogether G4 bobbins arranged in 16 horizontal series of

ED ca the left of the frame. The length of the wire on

i 12 times 114 yards, or 138 yards; the total length in
saratus is 64 times 138 yards, or 8832 yarda.

are coiled on all the bobbins in the same direction, and not

‘wheel, but on all four, all wires are connected with one

of this purpose the bobbins are joined, as shown in figure

first whool the twelve wires of the first bobbin, x, are ean-

‘4 piece of mahogany fixed on the front face of the wheel, with

‘copper, mm, connected by a wire, O, with the centre of the axis,

supports the wheels, At the other end, on the other face of the

the same wires are soldered to a plate indicated by a dotted line

connects them with the bobbin v; from this they are connected

the bobbin s by n plate i, and 20 on, for the bobbins f, w... up to

last, v, Tho wires of this bobbin terminate in « plate, n, which

sees the first wheel, and is soldered to the wires of the first bobbin

Fig. 615,

manner that all the ends of the same name |
ring. Each of these rings is then a pole,
used where a high degree of tension is not n
From these explanations it will be easy t
which electricity is produced and propaga
endless band receiving its motion from # |
fixed at the end of the axis which «
bobbins, and moves the whole system with 4
rience has shown that to obtain the greate
suitable velocity is 235 revolutions in a mini
‘we at first conaider a single bobbin, the tub
coiled, in passing in front of the poles of ¢
two ends an opposite induction, the effect
change from one pole to another. As thest
pass successively in front of sixtoen poles al
they are magnetised eight times in one dire
opposite direction. In the same time ther
bobbin eight direct induced currents, and cig
in all, sixteen currents in each revolution. —
in a minute, the number of currents in the #
alternately in opposite directions. The sat
with each of the 64 bobbins; but as
direction, and are connected with each oth
and there is the same number of currents, bt

MAGNETO-ELECTRICAL MACHINE. 759

To utilise these currents in producing an intense electric light, the
are made aa shown in figure 617, On the posterior side

last bobbin, x’, of the fourth wheel terminates by a wire, G, on the

MN, which supports the wheels: the current is thus conducted to

‘exis, and thence over all the machine, so that it can be taken from
desired point. In the front the first bobbin, 2, of the first wheel
by the wire O, not with the axis itself, but with «steel
* ¢, fitted in the axis, from which, however, it is insulated by an

collar. The screw ¢, to which the wire O is attached, is likewise
by apiece of ivory. From the cylinder ¢ the current passes to

fixed metallic picco, K, from which it passes to the wire H, which

it to the binding screw, a, of fig. G14. The binding screw &
janicates with the framework, and therefore with the wire of the
bobbin, x’ (fig. 617). From the two binding screws, a and 6, the
is conducted by means of two copper wires to two charcoals,
‘distance of which is regulated by means of an apparatus analogous in
ple to that already described (720).
Th this machine the currents are not rectified so as to be in the same
hence each carbon is alternately positive and negative, and im
they sre consumed with equal rapidity, Experiment bas shown
‘when. these currents are applied to produce the electric light, it is
wot necemary they should be in the same direction; but when they are
‘be used for cloctrometallurgy or for magnetising they must be rectified,
is effected by means of a suitable commutator.
‘Tho light produced by the magnoto-clectrical machine is very intense ;
‘a machine of four wheels the light obtained is equal to that of 160
Inmps. A machine ‘of six wheels gives a light equal to 200
lamps.
Serrin has constructed a new regulator for this light, which, like the
‘ones, brings the charcoals together in proportion as they become
; and further removes them when they are in contact. It contains

Fig. 618.
* A deop groove is cuton tho outer length «
which is coiled the insulated wireas in a
of the cylindor brass discs E and D are sect
commutator C, consisting of two pieces of st
and connected respectively with the two en
disc is a pulley round which passes a cord, 4
rapidly on the two pivots, (
‘When a voltaic current circulates int
segments, A and B, are immediately magn
and the other with the opposite. On tl
passing « voltaic current through the win
itself be made to rotate rapidly between the
miasses asthe segments A and Bbecomealten
netised, their induction produces in the wi
nately positive’and negative, as in Clarke'aa
currents are collected in a commutator whic
all the positive currents on one spring and
these springs become electrodes, from ono
starts and from the other negative. If thet
conductor, the same effects are obtained |
battery are united.
Siemens hns constructed magneto-olects
armature is utilised, Wea Uae great whew

} WILD'S MAGNETO-ELECTRICAL MACHINE. 761

magnets may be used instead of one large ane, As, weight for

weight, the latter possess greater magnetic force than the former, they
fan be made more economically. And as the armature ix always

| Fig. 619.

Sty near the magnets it receives greater momentum, and is more rapidly

banged.

783. wita's magneto-electrical machine.—Mr, Wild has recently

Fig, 20.
by a brass plate O. These three pieces are |

and negative currents to two binding terew
is represented on a largor scale in figure 621.
by which the armature can be turned at thi
‘The wire on the armature is 20 long.
‘Below the support for the magnets and |
eloctromagnets BB. Each consists of 9
36 inches in length by 26 in breadth and 1
coiled about 1600 feet of insulated copa
electromagnota are joined at one end, 80}
3200 foot. One of the other ends is conn
aand the other with &. At the top the
transverse plate of iron so as to form a sing
At the bottom of the electromagnets
separated by a brass plato O, and in the
channel in which works a Siemens’ armat
however, is above a yard in length, nearly
wire is 100 feet long. "Toe ends exe comune

the armature, m, bec
cinder thetiatiuence oftiiavela

more intense induced current is
directed, adjusted or not, according to the v

Fig. 622.

In 8 machine which Mr. Ladd exhibite
1867 the plates AA wore ‘only 24 inches
dinmeter, With these small dimensions th
Bunsen’s cells. It can work the electric |
& platinum wire a metre in length and 05 9
Tho above form of the machine is worke
devised a more compact form, which may
represented in fig. 622. ‘The two armatures
in one, and tho coils are wound on it at ri
shown in the figure. The current from ¢
18 inches of platinum wire 0-01 in, thickne
containing 3 inches on the secondary wire 2

~785) RUAMKOREF'S COIL. 765

Both Ladd’s and Wild's machines are liable to the objection of re-
Wiring to be rotated at a rate which cannot be kept up during
Méveral hours. The armatures become heated by the repeated develop-
“ment of induction currents, the magnetism is weakened, and therewith
the intensity of the current. Before they can be applied industrially,
Whelr velocity must be reduced, either by multiplying the number of

Siemens’ armatures or modifying their arrangement,

I
|

mem” ¢

Fig, 628.

|) Tiany cnse they furnish a remarkable instance of the transformation
of mechanical force into electricity, light and heat (247, 291)

785. Xnductorium. Rubmkorff's coil.—These are arrangements for
producing induced currents, in which » current is induced by the action
@ an electric current, whose circuit is alternately opened and closed in
Tapid succession. These instruments, known as inductoriums or induction
‘His, present considerable variety in their construction, but all consist
eentinlly of a hollow cylinder, in which is « bar of soft iron, or bundle
of iron wires, with two helices coiled round it, one connected with the
Poles of a battery, the current of which is alternately opened and closed
yn self-ncting arrangement, and the other serving for the development
Mithe induced current. By means of these apparatus, with a current of
three or four Grovo's cells, physical, chemical, and physiological effects are

Produced equal to and superior to those obtainable with electrical ma-
Ghines and even the most powerful Leyden batteries.

‘Of all the forms those constructed by Ruhmkorff are the most powerful.
Big. 624 ix « representation of one, the coil of which is about 14 inches
Tn length. The primary or inducing wire is of copper, and is about 2mm,

Fig. 624.
wires are not merely insulated by being in.)
‘but each individual coil is separated from |
shellac. The length of the secondary wi
Ruhmkorff's largest sizes it is as much as
lengths the wire is thinner, about jm. ‘TH
‘the tension of the induced electricity.

‘The following is the working of the ap}
by the wire P at a binding serew, «, passe
©, to be afterwards described (fig. 025), th
it enters the primary wire, where it acts
wire; having traversed the primary wit

|
the secondary wire (767), when it opens or

CONDENSER. 167

This is offccted by means of tho oscillating
0 (fig. 625). In the centre of the bobbin is a bundle of soft iron

s, farming together a cylinder a little larger than the bobbin, and
{ ee ict cen A. When the current pastes in the
wire, this hammer o is attracted ; but, immediately, there being
ontact between o and A, the current is broken, the magnetivation
sean eemenes Ollsy the current egaln peteing, the same series
recommences, #0 that the hammer oscillates with great

ion as the current passes thus inter-

aa this is perfectly insulated, the current acquires such an

as to produce very powerful effects. au has increased this

ity by @ condenser in the induced circuit. As con-
‘by Ruhmkorf, for his largest apparatus, this consists of 150 sheets

oil about 18 inches square, so that the total surface is about 75 square
‘These sheets being joined are coiled on two sides of a sheet of
‘silk, which insulates them, forming thus two armatures, they are

B coiled several times round each other, s0 that the whole can be

One of these arma~

Tt is this extra current which
break between the hammer s
this spark rapidly alters
they are of platinum. By in
circuit, the extra current, instead ¢

Hig, 688. exactly {
rent reaches ee
is only turned through 90
ancien

‘The two wires from the bobbin ats
of the secondary wire. They are con
so that the induced current can be set
large coils the hammer cannot be use
heated as to melt, But M. Foucaul
interrupter which is free from this in
‘tant improvement.

‘787. Effects produced by Ruhnu
tension which the electricity of ini
Jong been known, and many lumin
obtained by their means, But it is
Ruhmkorff has introduced into his
utilise all the tension of induced eurre
possess the properties of statical as wi)

Induced currents are produced in th
of contact. But these currents are
tension. The direct current or that
but more tension; that of closing:
Hence if the two ends P and P of th
connectod, as there are two equal and
the wire the two currents neutralise
placed in the circuit, onlya very feebl
tion of the direct current. This is not)
P of the wire are separated. As the
to the passage of the currents, that w
direct one, passes in excess, and the}

‘effects of Ruhmkorff’s coil are very powerful ; in fact,
‘are so violent that many experimenters have eon suddealy
by them. A mbbit may be killed with two of peas ad
& somewhat larger number of couples would kill a man.
a effects are also cnsily observed ; it is simply necessary to
very fine iron wire between the two ends P and P’ of the
fire; this iron wire is immediately melted, and burns with a
tht. A curious phenomenon may here be observed, namely,
‘each of the wires P and P’ terminates in a very fine iron wire,
two are brought near each other, the wire corresponding to
alone melts, indicating that the tension is greater at
at the positive pole,
effects are very varied, inasmuch as the apparatus pro-
dynamical electricity and electricity ina high stato of tension,
to the shape and distance of the platinum electrodes
in water, and to the degroo of acidulation af tho water, cither
{ effects may be produced in water without decomposition, or
may be decomposed and the mixed gases separnted nt the two
or the decomposition may take place and the mixed gases separate
at w single pole orat both poles.
umay also be decomposed or combined by the continued action of
rk from the coil. Beequerel and Frémy
d that if the current of a Ruhmkorff's
d through « hermetically scaled tube
as shown in fig. 627, nitrogen and
ibine to form nitrous acid.
effects of Ruhmkorft's coil are
parkable, and vary according as they
lace in nir, in vacuo, or in very rarofiod

cab ‘The experiment is made by a

‘the two wires of tho coil P and P’ with the two rods of the

ne (fig. 508) used for producing in vacuo the Inminous offects
machine. A vacuum haying beon produced up to 1 or 2

beastiful luminous trail is produced from one knob to the
h is virtually constant, and has the same intendty wa Yow
Lb

Fig. 62

‘The positive pole of the current sht
light is of a fiery red, while that of the
colour; moreover, the latter extends ali
rod, which is not the case with the posi

‘The coil aleo produces mechanical ¢
largest apparatus glass plates two ing
This result, however, is not obtained b
successive changes,

‘The oxperiment is arranged as shaw
the induced current correspond to thet
of a copper wire, the pole @ is conm
apparatus for piercing glass like that
the pole is attached to the upper condi
ingulated in a large glass tube 7, filled y
in a state of fusion, Between the #
perfomted, V. When this presenta too
lest the spark pass in the coil itself,
which separates the wire, und then the
two wires ¢ and ¢ connect the poles
whose distance from ench other can b
cannot penctrate through the omit
injured.

‘The coil can also be used Pe A

REFECTS OF NUIDIKORF’S Cor.

-787)

iving «parks of 6 to 8 inches, and using 6 Bunsen’ elements with
Targe surface, Rubkorif changed large batteries uf 6 jars each, having
About 3 square yards of coated surface.

‘Tho experiment with a single Leyden jar (fig, 629) is made as follows,
MPhe armatures of the latter aro in connection with the poles of the coil by

Fig. 629.

wires dand ¢, and these same poles are alo connected by means of
wires e and ¢, with the two horizontal rods of universal dischargor
1502), ‘The jar is thon being constantly charged by the wires i and d,

Fig. 630.

times in one direction and sometimes in another, and as constantly
by the wires ¢ and c; the discharge from m to n taking place
a spark two or three inches in length, very luminous, and producing ®
un?

wire d, and the internal coating
the wire c. The rods mand » are not, 1

Fig. 631, Fig, 634
phido of carbon, ete., be introduced into th
aspect of the light is totally modified. It
alternately bright and dark zones, forming }

the two poles (Kg, B82).
Tn this experiment NV ictlows trom fe)

bs] GEISSLER'S TUBES, 773

(uetion, that the light is not continuous, but consists of a series of dis-
(rges which are nearer each other in proportion as the hammer a (fig.
5) oscillates more tpidly. The zones appear to possess a rapid
atory and undulitory motion. M. Quet considers this aa an optical
(sion; for if the hammer is slowly moved by the hand, the zones
very distinct and fixed.
The light of the positive pole is most frequently red, and that of the
mative pole violet. The tint varies, however, with the vapour or gas
the globe,
ML Despretz has observed that the phenomena obtained by Ruhmkorif
‘by Quet, with » continuous current, aro also reproduced with an
continuous current, with this important difference, that the
tinuous current requires a cousiderable number of couples, while the
Continuous current of the coil only requires a single element. It is
tarkable that the luminous effects of this coil are very little increased
an inerease in the number of elements,
780. Getester's tubes.—The briiliancy and beauty of the stratifi-
fon of the electric light are most remarkable when the discharge of
| Rubmkorif’s coil takes place in glass tubes containing @ highly rare-
Lyapour or gas. These phenomena, which have been investigated by
(son, Grove, Gassiot, Pliicker, etc., are produced by means of sealed
constructed by Geissler, of Bonn. These tubes are filled with
gases or vapours, and are then exhausted, so that the prossure
te not exceed half s millimeter. At the ends of the tubes two platinum
Jare soldered into the glass.
the two platinum wires are connected with the ends of a Ruhm-
WS coil, maguiticent lustrous strim, separated by dark bands, are
Auced all through the tube. These strim vary in shape, colour, and

Fig. 684.

tre with the degree of the vacuum, the nature of the gas or ¥apour,
[ the dimensions of the tube. The phenomenon has occasionally a
Lmore brilliant aspect from the fluorescence which the electric dis-
nge excites in the glass,

Fig. 634 represents the strim given by hydrogen under half @ willi-

4

774 DYNAMICAL ELROTI

moter of preasure; in the bulbs the light)
it is red.

Fig. 636 ahowe the strim in carbonic ac!
Meter prossure; the colour ie greenish, an)
form aa in hydrogen. In nitrogen the ligt

Fig. 636.
Pliicker has found that the light in Ge
on the substance of the electrodes, but sin
‘or vapour in the tube. He has found that
gen, nitrogen, carbonic oxide, ete, give d
by a prism. The discharge of
abighly rarefied gas would not pass throo
presence of « ponderable substance is absol
of oloctricity.
By the aid of « powerful magnet Plise
nétiem on the electric discharge in a Geit
with the ordinary voltaic are, and obtained

different action ¢
on tho two extra

charge.

| ‘The light of |
cently applied t
cnpillary tube is
vided with plati:
in the middle,
Fig 636. ‘ouch, and their

ahown wate

highly rarefied gas, Whe those yoetioatt

jee, & light is produced ata, bright enough to illuminate any
the body into which the tube is introduced.

ation of induced currents by magnots.—Do la Rive
ly dovised an experiment which shows in a most ingenious
| light in Geiasler’s tobes in accordance

Fig. 687.

ee a fow drops of liquid into the globe. At the other end #
@ cemented, through which passes a rod of soft iron about ¢
1 in diameter, the top of which ia about the centre of the
xcept at the two ends, this bar ia entirely covered with a vory
lating layer of shellac, then with # glass tube also cvated with
nd finally with another glass tube uniformly coated with &

of rotatory motion from east to west, by
‘Tho rotation of the luminous are in the ab
be referred to the rotation of currents by m

Geissler has constructed a very useful fi
which is exhausted ones for all. Apart ft
was originally devised it is a very conve
tigating tho action of magnets on currents,

701. Meat developed by the inductic
‘Ddodies in motion.—Wo have already soon
that a rotating copper dise acta at a distane
municating to it a rotatory motion, Wes
of copper, rotating with great velocity, iss
ence of the poles of two strong

mangnets (74
to prevent the rotation of thendedle or of th
force must be consumed in overcoming the
the inductive netion of ne rasgw, Boers

ea] HEAT DEVELOPED BY INDUCTION. V7

funsformation of mechanical work into heat, which has occupied physi-
fist in the last few yeors (425), it has been attempted to ascertain what
jmasitity of heat is developed by the action of induced currents under
influence of powerful magnets. Joule, with a view of determining
mechanical equivalent of heat, coiled a quantity of copper wire
band a cylinder of soft iron, and having enclosed the whole in a glaas
lube fall of water, he imparted to the system a mpid rotation between
he branches of an electromagnet, A thermometer placed in the liquid
frved to measure the quantity of heat produced by the induced currents
ft the soft iron and the wire round it.
Foucwult bas recently made a remarkable experiment by means of the
(pparatns Topresented in fig. 638, It consists of a powerful electromagnet

Fig, 638,

horizontally on a table. Two pieces of soft iron, A and B, are in
with the poles of the magnet, and becoming magnetic by induc-
they concentrate their magnetic inductive action on tho two faces of
Metallic disc, 1; this disc, which is of copper, is 3 inches in diameter,
A quarter of an inch thick, partly projects between the pioces A and
tod can be moved by means of a handle and a series of toothed wheels
‘a velocity of 150 to 200 turns in a second.
So long as the current does not pass through the wire of the electro~
|, very little resistance is experienced in turning the handle, and
‘once it has begun to rotate rapidly, and is left to itself, the rotation
ines in virtue of the nequired velocity. But if the current passes,
Ris disc and other pieces stop almost instantaneously , and \€ the andts
Lad

eo,

the tempernture of the diso rose from
formed by throo of Teansen’s elements;
that the rotation could not long be contin)

CHAPTER \
OPTICAL EFFECTS OF POWERFUL ™A

792, Optical effects of powerful mu
1845, that a powerfal exer
stances, such that if a polarised ray trav
tho line of the magnetic poles, the plane o
to the right or to the left, according tothe

Figure 639 represents Farnday's appart
korff. It consists of two very powerful 4
‘on two iron supports, OO', which can be |
current from a battery of 10 of 11 Bunsor
A to the commutator Wh, Toe vation MA,

OPTICAL EFFECTS OF MAGNETISM.

@ wire g, descends in the wire #, passes again to the commutator, and
terges at B. ‘The two cylinders of soft iron, which are in the axis of

Ye to pass. At 6 and a there are two Nicol’s prisms, the first serving
By means of a limb this latter

ependicalar to each other, the prism a completely extinguishes the
tht transmitted through the priam 6. If atc, on the axis of the two
ils, plate be placed with parallel faces, cither of ordinary or flint

light is still extinguished #0 Tong as the current docs not pass but
hhen the communications are established, the

}the direction of the current, the light passes through the different
ts of the spectrum, ns is the case with plates of quartz cut perpendicu-
(ly to the axis (585). Becquerel has shown that « large number of
‘can also rotate the plane of polarisation under the influence
magnets. Taraday assumes that in these experiments the

ra the plane of polarisation is due to nn action of the magnets

‘the luminous rays, while Biot and Bocquere] ascribe the phenomena
action of magnets on the transparent ies submitted to their

Diamagnetism,—Coulomb observed, in 1 that magnets act
a Pent in a more or less marked | action was at first

This ee on
the bodies wore trausy ae

Fig. 640, Fig. 641,

‘as solutions of iron or cobalt, the tubes set
wator, alcohol, ether, essence of tarpeatii
the tubes set equatorially.

Very remarkable changes take place in 4
diamagnotic substances when they are aus)
substance is indifferent in an equally sti
equatorially in a stronger magnetic substar
which ia loss strongly magnetic ; it sets axi

A diamagnetic substance surrounded b
substance scts equatorially, According to
times magnetic and sometimes
tions glass tubes are used for
must first bo determined, and then taken i}

‘The action of powerful magnets on liq
the following experiment devised by Pi
placed. on a watch gow ‘between The xe

DIAMAGNET ISH, 781

When the current passes, the solution forms one or
enlargements, as represented in A and B (fig, 642) ; those continue
Tong as the current passes, and are produced to different extents with
-magnetic liquids. Diamagnetic liquids present the opposite effects, ax
observed, with mercury, in noting its curvature on a piece of

‘amalgamated silver placed between the two poles.
iii, Diamagnetieom of gases. Bancalari observed that the flame of a
placed between the two poles in Faraday’s apparatus waa strongly
(fig. 640). All flames present the same phonomenon to a diffe-
extent. M, Quet has obtained very intense repulsive effects by
in the sme manner with the electric light obtained with

carbon cones in the figure 544, page 686,

Whe magnetic deportment of gases may be exhibited for lecture
by inflating soap bubbles with them between the poles of the
and projecting on them either the lime or the electric

| Faraday has experimented on the magnetic or diamagnetic nature of
He allowed gas mixed with « small quantity of a visible gas or
‘80 as to render it perceptible, to ascend between the two poles
‘magnet, and obterved their detlections from the vertical line in the
“OF equatorial direction; in this way he found thet oxygen was
nitrogen more, and hydrogen most diamagnetic, With iodine
produced by placing a little iodine on a hot plate between the
Wo poles, the repulsion is strongly marked. Becquerel, who has made
(portant researches on magnetism, has found that oxygen is most
frongly magnetic of all gases, and that a cubic yard of this gas con-
fused would act on a magnetic needle like 5°5 grains of iron, Faraday
found that oxygeo, although magnetic under ordinary circumstances,
diamagnetic when the temperature is much raisod, and that the
‘or diamagnetiam of s substance depends on the medium in
it is placed. A substance, for instance, which is magnetic in
may become diamagnetic in air.
An the crystallised bodies which do not belong to the regular system,
directions in which the magnetism or diamagnetism of a body is
easily excited, are generally related to the crystallographic axis of
substance. Tho optic axis of the uniaxial cryatals sets either axially
‘equatorinlly when a crystal is suspended betwoon tho poles of an
jet. Faraday has assumed from this the existence of a mage
ine foree, but it appears probable from Knoblauch’s re-
that the action arises from an unequal density in different
inasmuch as unequal pressure in different directions produces
‘same reault.
dy, Detonation produced by the rapture of a current under the influcwos,

Fig. 643,

of which aro bent and soldered to a
interior of the cireuit is a magnetic

the apparatus is placed in the magnetic m
ings gently heated, as shown in the figur
manner which indicates the passage of &
from the heated to the cool junction it
eating the juncthon wy, W coded wg hes

on series.—If small bars of two different metals
‘dered together at one end while the froo ends are connected with

res of a galvanometer, and if now the point of junction of the two
(s be hented, a current is produced, the direction of which is indi-
‘by the deflection of the needle of the galvanometer, Moreover,
tensity of the current calculated from the deflection of the gal-
eter is proportional to the electromotive force of the thermoclement,
in this way with different metals, they may be formed

that each motal gives rise to positive electricity when neso-

one of the following, and negative electricity with one

that precodo; that is, that in heating the soldering, the positive
from the positive to the negative metal across the soldering,

if the soldering represented the liquid in o hydroelectrical
tat; hence out of the element, in the connecting wire in the gal-
beter for instance, the current goes from the negative to the positive

couple bismuth-antimony heated at the junction would corre=

‘a couple zinc-copper, immersed in sulphuric acid. The following

ist drawn up from Dr. Matthiessen's researches, which alao gives
imtive numerical values for the electromotive force.

th » #25 Silver 10
ee POR te vs Ses, are ts « 02

65 Cadmium... . . + 08

5 OI east ost 3 eo ae

8 Thos el oe we esate, 52

aA 108 Ted phosphorus * 96

ee Pe siany: . eee 98

| cc grad! Tellurium 1798
un ees -- . OF Solontum 2. . : . .= 2000

foun of the numbers in this ligt js that, taking the electromo-
jree of the copper-silver couple as unity, the electromotive force of
tir of metals is expressed by the difference of the numbers whero
gre aro tho mame and by the sum where the signs are different

electromotive force of « bismuth-nickel couple would be 25-5

" &

antimony is the nogative a
‘muth the positive metal but the negative,
ae

ee
dered together to the ends of an ant
mr |
observed in the galvanometer, Ifsimilarl,
‘sntimony and tin soldered together, be ct
galvanometer, and if the junction copper:
mony, be heated to 60°, while the junctio:
‘the deflection is the same as in the previo!
tive force produced by heating the two_
antimony, | is bc! to the een ine |

P Trcgueral 0d with a eres
‘tion was heated to a given SRE
intensity of the current was
tion. If the two junctions are at any |
the current is proportional to the differem
places, provided that this does not exceed
‘The direction of the current frequent},
ture of the couple is raised beyond @ cer)
and iron circuit the current goes from cop
part, provided the temperature does not ¢
perature the current changes its direction,
As compared with ordinary bydroelec
force ef thermocurrents is very small; tl
bismuth-copper element with u difforence
of their junctions is according to Wheat
mann 5}, that of Daniell’s element: the
argentan couple with 10 to 16° difference of
is shy that of a Danicll’s accarding to Ke
796. Causes of thermoelectric ©
‘currents cannot be attributed to contact, 1
cults formed of nsingls metal, Nor do tl
for Beequerel thie found Yaak Yaey ste tot

) COUPLES AND BATTERIES, 735

same physicist ascribes them to the unequal propagation of
parts of the circuit, He found that when all the
homogeneous, no current is produced on heating,
‘heat is equally propagated in all directions, This is the case if
‘of the galvanometer ure connected by a second copper wire. But
of this is destroyed by coiling it in a spiral, or by knot~
indicates by its doflection a current going from the
to that in which the homogeneity has been destroyed. If
fof tho galvanomoter wires be coiled in spiral, and one end is
touched with the other, the current goes from the heated to
end,
has found that the thermoelectro-motive foree is influenced
: ‘ion ; for instancé, if the cloavage of bismuth is parallel
ee se ee ed
‘reverse is the case with antimony, Thermoelectric element
it either twa pleoes of Hanuth or twol gloas' af
if in the one the principal cleavage is parallel to the place
bt, and in the other ix at right angle’. Hence the position of
the thermoelectric series is influenced by their crystalline
\-
[hermociectrio couples and batteries.—From what has
Wit will be understood that a
lectric couple consiste of two me-
bred together, the two ends of
in bejoined: by aconductor. Fig.
tsents a biemuth-copper couple ;
tepresents a couple used by M.
_ Ttconsists of » bar of bismuth
joat right angles, at the ends of
je soldered two copper strips, ¢, = _~
orminate in two binding serews
material,

thon the second coppor of this Fig. 644,

math of the third, and so on, this arrangement constitutes »

| ae ae gare ar does eesti
in ice, and the even ones in water, which is kept at

Topi’

petric

's thermoelectric battery.—Nobili devised a form of
battery in which there are « large number of elements in

i
Fig. 646, Fig. 647,

that only the solderings appear at tl
copper binding scrows, m and m, in
nicate in the Interior, one with th
positive pole, and the other with 4
negative pole. These binding een
mities of a galvanometer wire when

that artificial sulphuret of copper he
positive, and that a couple of this si
motive force nearly ten times as

couple in fig. 644, Native sulphuret,
tive. As the artificial sulphuret onl:

ay ‘BECQUEREL'S THERMOELECTRIC BATTERY. 787

|at very high temperatures The metal joined with it is German
(© (90 of copper and 10 of nickel). Fig. 648 represents the arrange-
fof @ battery of 50 couples arranged in two series of 25. Fig. 650
fom # larger scale the view of « single couple, and fig. 649 that of

Fig. 648,

‘in two series of 3. The sulphurotis cut in the form of roctan-
‘prisms, 10 centimeters in length, by 14mm. in breadth, and 12mm,
. In front is a plate of German silver m, intended to protect the

from roasting when it is placed in gas flame. Below there

of German silver MM, which is bent several times so as to be
to the sulphuret of the next, and so on. The couples, thus

Fig. 649. Fig. 650.

in two series of 25, are fixed to n wooden frame supported by
‘Hrass columns A B, on which it can be more or less raised. Below
‘couples there ie a brass trough, through which water is constantly
fing; arriving by the tube 6 and emerging by the stopcock r. The

end of the frame are two binding 4
‘ment of the couples in different wa
‘Tho resistance of sulphuret of e

Let therm |

freoly decomposes water,
qual to about 8 oF 9 of ths coup
800. Mellont's:

rule, about a yard long, is fixed
parts composing the apparatus ans
fixed by means of binding scrows.. |
or other source of best, F and E
bodies experimented on, snd m \a;

PROPERTIES AXD USES OF THERMORLECTRIC CURRENTS. 789

‘is « galvanometer, D; this has only a comparatively few turns

‘thick (1 mm.) copper wire ; for the electromotive force of the

socurrents is «mall, and as the internal resistance is small too, for

consists of metal, it is clear that no great resistance can be intro~

Anto the circuit if the current is not to be completely stopped.

ivanon are called thermomultipliers, The delicacy of this

is so great that the heat of the hand is enough at a distance
from the pile to detlect the neodle of the galvanometer.

‘it for measuring temperature, the relation of the deflection of

and ther -“ore of the intensity of the current, to the difference

tures of the two ends, must be determined. That known,

ture of the ends not exposed to the source of heat being

Sate aiecred deflection gives the temperature of the other, and

‘the intensity of the source of heat.

Propertics and uses of thermoelectric currents.—Thermo-

ite are well adapted to produce constant currents; for their junc-

by means of melting ice and boiling water, can easily be kept at

ment of his Inw, They can produce all the actions af the
Battery in kind, though in less degree. By means of # thermo-

‘which differed in temperature by about 10° to
the presence of free positive and negative electricity
the open pile respectively. He found that the density of the
was nearly proportional to the number of elements, and
the electromotive force of a single clement under the above
tances was about i, that of a i
nt of their fovblo tension, thermoelectric pi

to their very important use in mensurit
ture, thermopiles have been constructed for the
E high temperatures. Pouillet’s electropyrome

ths barrel. ‘Tho pletinuo wire and ¢
Rlvanometer, and the other end where

through. When the ive curn
Mam the ae ites ah Soha

stem sinks; but if it passes

‘that whenover the effects of heat
whenever the effeets ordinarily pn
duced, cold is the result.

DETERMINATION OF EL

803. Rheostate.—The rheostat
resistance of any given circuit can
opening the circuit. As invented b
parallel cylinders, ono, A, of brass, |
the latter there is # meee |
ting, to which is fixed thi
which is about 40 yards py is

| + RREOSTATE. 791

to iby ool, “aid after a ‘great number of turns ou
ylinder, @ extremity ¢ Two bin
», connected with the is eee
, communicate by two
= ome with the cylin-
ithe other with the ring a.
em acurrent enters at o, it
( traverses that portion of
Gre rolled on the cylinder
ere the windings are insn-
[by the grooves; passing
+ to the cylinder A, which
metal, and in contact with
wiry, the current pastes
Iy to m and n. Hence, if
fogth of the current is to be
ised, the handle, d, must be
1 from right to left. If, on
Ontrary, it is to be diminished, the handle is to be fixed on the

Fig. 653.
fe, and turing then from left to right, the wire is coiled on the

at tH
Pla

angle of deflection, I the intensity of th
action of the earth. If the direction and i
represented by ak, it may be replaced by
fig. 654, Now, as the first has no direct
component ac must alone counterpoise thy
in the triangle, ack, aexak. com. cak, frot
angle cak is the completement of the angle,
lastly, I=T sin. d, which was to be proved

805, Determination of the reststancs
Yength.—If in the circuit of a constant +
interposed, a certain Aeection of the nm
then, different lengths of copyer ine

DRITISH ASSOCIATION UNIT OF ELECTRICAL Resistance, 793!

‘corresponding deflections will in each case be
produced. Let us suppose, that in a particular case the tangent of
angle of deflection (714) observed with the clement and tangent
alone was 1-88, and that when 5, 40, 70, and 100 yards of copper
‘were successively placed in the circuit, the tangents of the corre-
deflections were 0-849, 0-172, 0-105, and 0-074. Now, in this
iment, the total resistance consists of two components; the re-
ce offered by the clement and the tangent compass, and the resistance
by the wire in cach case. The formor resistance may be supposed
‘equal to the resistance of x yards of copper wire of the same dia-

‘as that used, and then we have the following relations.

Tangent of angle of deflection,

x. oe a a i ae 2
a a ee
On he a cere elt.

BBR sc te OO La
PE a Ww «CHO
If the intensities of the currents are inversely as the resistances, that
‘the lengths of the circuits, the proportion must prevail,
rie +5 = 0849: 1886;
Combining, in like manner, the other obser
owe get a series of numbers, the mean of which is 408, That ia,
‘resistance offered by the element and galranometer is equal to the
tance of 408 yards of such copper wire, and this is said to be the re
Fength of the element and galvanometer in terms of the copper wire.
It is of great scientific and practical importance to have a unit or
sudard of comparison of resistances, and numerous such have been pro-
|. Jacobi proposed the resistance of a meter of a special copper wire
fimeter in diameter. Copper is however ill adapted for the purpose,
is difficult to obtain pure. Matthiesson has proposed an alloy of
iid and silver, containing two parts of gold and one of silver; its con-
g power is very little affected by impurities in the motals, by an-
or by moderate changes of temperature.
unit is a metor of pure mercury, having a section of a square

ili wedi Ya eocks used. ta telegraphic work, is « standard mile
qpecial coppor wire yi of an inch in diameter, Matthiessen has
di instead of this a mile of pure annealed copper wiro ¥, in. in

6, British Association unit of electrical resistance.—The great
ce, both theoretically and practically, of having sows waiorsi
uM

794 DYNAMICAL ELECTE
standard for the comparison of electrical
engaged the attention of a committee of 1
includes the principal electricians in this
resulted in the adoption of a standard whit
men of science both in this and other coun
of this unit, which it is proposed to call th
kindly furnished by the secretary to t]
Jenkin.

It represents a convenient multiple «
of electrical resistance. The word ‘abs
imply accuracy of construction, but is
measurement of electrical resistance is n
definite relation to the fundamental units
instead of being a mere comparison with t
piece of metal arbitrarily chosen as the w
foot and a cubic foot may be called abso
city, an acre and a gallon arbitrary unit:

It seems strange at first that the unit
measured by reference to time, mass, and
the specific qualities of any material ; but
phenomena is derived from an observatio:
need, therefore, feel no surprise at learnit
measured in purely mechanical units. TE
force, and resistance, quantity, and capa
more than one way. The electro-magn’
determined by the following consideratic
by a current of strength C, and length
being the magnetic strength of that po

current, it is found by experiment tha

fe
«Lm
in unit length of circuit exerts unit force
we get k= 1, and the equation for © bec

where « is some constant. Nowi

and C may be measured by the express

Again, for the resistance we get
r= WH

OF
where W is the wotk Aone in the tine

BRITISH ASSXCTATION UNIT OF ELECTRICAL RESISTANCE.

‘of tho resistance. Now, tho first equation allows us to measure

nt it terms of n fores f, two lengths K and L, and a magnitude m,
again depends on measurements of force and length only, so that
there have «current measured in mechanical units in virtue of a ma-

" tical relation between the phenomena produced by the current and
lis mechanical unite It follows from the equation that the unit current
il be that of which each unit length exerts a unit force on a unit pole
Unit distance. The second equation, like the firat, is deduced from ob-
ration, the resistance of acireuit is found to be proportional tothe work
by a current in that circuit, and inversely proportional to the square
‘the current and to the time during which it acte; any two circuits

which pA Es equal have equal resistances ; if this quantity for circuit

Ge double what it ia for cirouit B, thon tho resistance of circuit A is double
of cireuit B. Therefore, we have exactly the same ground for saying

al measures the resistance of the circuit that we have for saying a*

the contents of a square with sides equal to a, In equation 2,
the work, is essentially a mechanical measurement, for, though
observed in the form of heat, it is by Joule’s equivalent
to the mochanical unit of energy or work.

erOhm'slawO =~. ee ee ees BD

‘Mensures electromotive force in terms of C and r, and Faraday's
expressed by equation
Q=Ct

Qis the quantity of electricity conveyod by the current C in the
4 shows how quantity is measured in the same mathematical series.
ough nothing can be simpler than the mathematical conceptions
involved, the pructical measurement of resistance, or any other of
shore magnitudes by direct reference to force, work, time, ete., in~
mzuch labour, #0 that for each kind of measurement it is necessary
use to construct a standard which affords the desired men-

by direct and simple comparison with the thing measured. Thus,
chman to measure wine does not work out the cubic contents of a
but measures the number of litres by reference to a standard litre,
lich is a simple decimal submultiple of the cubic metre. In like
0 tical measurements of resistance are made by comparison
the Ohm or BA unit prepared to represent o simple decimal
ple (ten million times) the absolute electromagnetic unit; the
: the gramme, and the second of time were taken as fundamental
Ets by the committee, and one which is approximately equal to 10? metre

uu?

AM, & «', @ of, would offer ¢

Now the sections of the wires are directly
,
and hence we have}? 6 or ¥ =1
:
yards of copper wire 4 mm. in thickness
sistance ns 12 yards of copper wire 1 mm.
‘How thick mast an iron wire be which
the same reeistance a8 a copper wire 2°50
Here the length being the same, the ex
since the sections are as the squares of t]
conductivity of copper is unity, and that)
2:5" = dx 0138 or d? = 626+0-138 =)
is, any length of a copper wire 25mm, in
an iron wire of the same length, provided
808. Determination of electrical
methods of determining the electrical ¢
essentially in determining whet length of

body will offer the same tedshance wh eV

1 ices introduced at a, 6, c, «, and which we will designate by
Ietters, bear « certain relation, no current will pass in the galvano-

Fig. 655.

Suppose first of all that the resistances are all equal in every ree
et; the current arriving at A would divide, one part would traverse

ALDGB, and 2s both these are equal and opposite in direction no
‘would be produced on the galvanometer; but if the resistances «
Mi bare different, the tensions at B and D will be different, and necom
ly a current will traverse the galvanometer either from B to D or

ides may be made equal, and then # current ceases to pass. We can
Bal express one resistance, 4, in terms of c.

Théatstone’s bridge, however, is more general. It can be shown that
passes, provided that the four resistances bear to each other
b=d:c. So that if c, for instance, is the resistance to be
termined, by varying the others in » suitable manner the proportion
B always be obtained. In practice two of them are generally fixed

798 DYNAMICAL ELECT!

resistances of known amount, and the th
possible, it is most convenient to take a =

The following is a method of determinii
element. A circuit is formed consisting «
a galvanometer, and the intensity I is nc
second element is then joined with the fin
the size, and therefore half the resistance,
of the rheostate wire, the intensity is bro
Then if E is the electromotive force, and
r, the resistance of the galvanometer and
the intensity 1 in both cases is

Ri. e.
R+r_— §R+r

809, Electrical conductivity.—We
aspects, and consider them ss endowed wi
allowing electricity to traverse them, a pr
tivity: or we may consider conductors int:
an obstacle to the passage of electricity, th
overcome. A good conductor offers a fee
dnetor a great resistance. Conductivity ¢
each other.

The conductivity of metals has been i
by methods analogous in general to that d
graph, and very different results have beer
from the different degrees of purity of th
their molecular condition has also great
the difference in conductivity between hi
wire to amount to 8%, for copper 2:2, a
following are results of a series of carefu
on the electrical conductivity of metals +
a standard,

Silver . . . 1000 Im
Copper . . . 999 Tit
Gold. . . . 800 Le
Aluminium . . 660 Ge
Sodium . . B74 An
Zinc. . . . 200 Me
Cadmium 7 Bis
Potassium =... 208) Gn
Platinum. 180

The conductivity of metals is diminishes
The law of this diminution is expressed by
f=. (1 —at4

| ELECTRICAL CONDUCTIVITY. 799

ve wt and ° are the conductivities at ¢ and O° respectively, and « and
constants, which are probably the same for all pure metala For
metals investigated by Mathiesen he found that the conductivity
‘pressed by the formula

K,=K. (1-0-0037647¢4-0:000008346).
iquids are infinitely worse conductors than metala. The conductivity
-eolation of one part of chloride of sodium in 100 parts of water is
zaoa that of copper. In general, acids have the highest, and solations
ikalis and neutral ealts the feeblest conductivity. Yet, in snimtions,
conductivity does not increase in direct proportion to the quantity of
dissolved.
‘he following is « list of the conductivizy of a few liquids as crempared
h that of pure silver.

Puesilver . . . . . . 1O0/00S0000
Nitrate of copper, saturated solution “ea
Sulphate of copper ditto Z 542
Chloride of sodium ditto f sla
Sulphate of zine ditto S77
Sulphuric acid, 1-10 ep.gr. 2. “ar

mn om V2dteper . -. 2%

» » WOpgr . 75
Nitric acid, commercial. ws me
Distilled water» ww we G1

Liquids and fused conductors increase inexerluetivity by an iseraw -f

aperature, This increase is expreaw-d by the frxiia

me = «(1+ af),
athe values of K are considerable, Thus. for a seturevd wintin +!
Iphate of copper, it is 0-0286.
By most physicists the conductivity of liquide Lae tern reverted as 0
rely electrolytic conductivity that is due tw chewina demmupnition
‘t Faraday, in stating his law of electruistic deompmition, band wa:
anced that it was subject to certain restrictive it came uy wich
ids could conduct electricity without being domupeet. Fowmsv.t
‘recently shown by delicate experiments, that liguide isso poovls
‘ductivity, a physical conductivity analoguu: W tus f metal ‘Dou
however, much less than the electrolytic caductivny, but wey bev
istinct influence on the chemical effects f curreute aud on Kerwhuy's
rn
310. Determination ef electromotive teres, wWheetetons’,
vthoa.—In the circuit of the element whee eltzvantive funup i

800

DYNAMICAL ELECT

be determined, a tangent compass and ar
being so arranged that the intensity I of t)
fot example, the galvanometer indicates 4
of the rheostate wire by the leugth J, adir
40°) is obtained.

A second standard element is then subs
by arranging the rheostate, the intensit;
equal to I, and then, by the addition of /
=i

Then if E and E, are the two electro
resistances when they have the intensity
added ; we have

Trial element.

from which we have

Hence the electromotive forces of the e
as the lengths of the wire interposed.

Another method is described by Wiei
connected in the same circuit with at
apparatus for measuring intensity, first in
rents go in the same direction, and, sec
Then if the electromotive forces are E and
the other resistances in the circuits r, wh
elements are in the same direction, and L.
opposite directions, then :

t= Et!
“=R+R
oEEN

R+R
Ed, - 1
L+1

S11. Siemens’ electrical resistance
a Wheatstone’s bridge arrangement, afte
been established, the temperature of one
increased, the above ratio will no longer pn
haye been altered by the temperature, |

and cep: 4

whence

the altered on ae te gre eyuienienes, On this

; ‘imei tue Genyeracany uf Effowty
places He piece a call of deren seaitamee i the particular

ty oe a haere remands
x ‘Wins woth che plaww af cere, where 2 farms pest

agitate
hee been of ememtun eecvanr in watching the tempers
large coils of telecragit vir, wick, mownd ewes i the bald of
fare very Linhie to benume heated. it migict ale be used fur the
and cosvesiant checrtucim of unGerpround apd eubenazine
Ta coal of platipuan wire wom aiintiouied for the copper,

Derived currents —in fg “i she curmat Sus « Baneen's
traverses the wire rypmm ; jet or take the cue in which any two

Vig. 056.

points g and # from which the second eondector starte and ends arm
the points of derivation, the wire gpa and the wire gin ure derived
E The currents which traverse these wires are called the derived or
eurrente; the current which travelled the circuit rypnon before it

‘When the two wires are of the same 1
the current would divide unoqually, and
‘each wire would be proportional to its set
into two branches, the quantity of water
proportional to its dimensions. Hence
ductors joined would be the same as thi
length, the section of which would be the

Tf the two conductors gpm and g rm ar
and section, they could always be replay
kind and longth, with such sections tl

aame effect as a single wire, which had’
section would be the sum of the section
divides at the junction into two parts pr
inversely as the resistances of the two wir

‘Suppose, for instance, g pn is an iron wit
‘square in section, and g.rm @ copper wire.

‘The first might be replaced by a copper
section would be $ x } (taking the cond
that of iron) or J; square mm. The seco
‘ copper wire a metre in length with ae
two wires would present the same resistat
length, and with a section of y + $= 2f

‘The principal current would divide al
which would be as 2:

S13. Peemter sever women eee eT ee
thet emimai secrecy ne eer ce anieer C Swecamo meweer cre
logists ead eves | Sc Su ean me
Ween made mcs anteen eeu or ue Emma, Lam
Marien. ma Scar

By meew if De Serum, Fs cere. = o> eee
Tike those of

tach caymie me fe oar 1 6 oe Sate
tained a éedercun wo tum J i."
bo the head uf ae mnma

Mateeeci :oranet mawetas 2fice we Soom nie of ae Saye of
Frogs. Fee cme purser ue tuut Sie mas ae are lm er, DU
without remowng aie Jumosr Jers amit et TPL Shem er Ava Zh
pther, oo tas sara aeret metet eM Se LUE act af Ge AT
Sem cat corns re mean if an pete le
faizee of ewe tether 2 1

ls the

be aways

Observed s current, wes tis was chueed, trom ibe isietiar of thy

‘Manscle to the surface. M. Maczrucc! calls this cussemt the seaweiur

Owrent, to distinguish it from ti+ proper current of the thy. In thew

@mimals be always met with bota carenta, while in other animals he
nothing more than the maseular current.

‘WM. Dubois-Reymond has recently published researches on the muavular
Puztents in man. Owing to the great resistance of the human bauly, it
Was necessary to use in these researches a galvanometer with 4,000)
Windings, M. Duboie-Reymond observed that when the two sila uf
She galvanometer were connected with two symmotric parte of the huly
—for instance, with the two hands or the two feot—the qalyanometer
Fevo at first very irregular indications; but soon a ourrent waa produend,

investignte
in South America, and in England by Fan
of examining live specimens.

‘The shock which they give serves both
defence, It is purely voluntary, and beeo
and as these animals lose their yi

soon exhausts them materially.
‘The shock is very violent. According

the gymnotus gives is equal to that of a
coating of 25 square feet, which explains b
give way under the repeated attacks of thy

Numerous experiments show that thea
electricity. For if, touching with one har
Delly is touched with the other, or with
folt in the wrists and arms; whilo no 5
touched with an insulating body. Furthe
‘with one end of « galvanometer wire and
each discharge the noedle is deflected, but
which shows that there is an instantaneou
direction of the needle shows that the cur
belly of the fiah, Lastly, if the current o
a helix, in the centre of which is » small
netised by the passage of the discharge.

By means of the galvanometor, Matteuc(
ing facts:

1, When » torpedo is lively, it eam git
body; but as its vitality
shock are nearer the organ which is the
electricity.

2.. Any point of the back is always posi!
responding. point of the belly.
» 3 Of any two points at different distan

to the apex of the prism, appears like the

phe ‘These priema, perpendicular to their summits,
d by disphragms, forming a series of small cells which are
& liquid consisting essentially of 9 parts of water to 1 of

it with a pin, contractions were observed in the frog.

i investigated farther the influence of the brain on thedis-
For this purpose he laid bare the brain of a living

‘that the first three lobes could be irritated without the
oduced, and that when they were removed the animal atill pos
the faculty of giving shock. The fourth lube, on the contrary,
be irritated without an immodiate production of the disehange :
removed, all disengagement of electricity disappeared, even if
lobes remained untouched. Hence it would appear that the
Fy source of the electricity elaborated is the fourth lobe, whence
tmnsmitted by means of the nerves to the two organs described
pve, which act as multipliers In the silurus the head appears also
. of the electricity; but in the gymnotus it is found in the

from this considerable disengagement of electricity in the
‘certain fish, physicists have inquired whether a similar elaboration
ity does not take place in other animals; not perhaps in suf-
quantity to produce shocks like those of the Leyden jar, but
ly #0 to effect slow actions, and to serve for the essential fane-

of lifo, like the secretions, digestion, etc.
15. Application of electricity to medicine.—The first appli-
of electricity to medicine date from the discovery of the Leyden
*. + Nollet and Boze appear to have been the first who thought of the
lication, and soon the spark and electrical frictions became # universal

effects of the latter differ, according
second order are used.

Th fact, since induction currents, +
feeble chemical action, it follows tt
they do not produce the chemical ef
and hence do not tend to produce the
electrifying the muscles of the face
ferred, for Dr. Duchenne has found th
the retina, while the currents of th
organ, and may affect it
‘There is a difforenco in the action of
for while the primary induced curren
has little action on the cutaneous
current, on the contrary, iucroases |
point, that its use ought to be prow
irritable,

Hence electrical currents should »
out a thorough knowledge of their v1
used with great prudence, for their ¢
accidents. Matteucci, in his lectures)
bodies, expresses himselfas follows :!
muzt always be used. This preeaut)
portant, as I did not think it so befo
with almost tetanic convulsions unde
& single clement. Tako care not t
especially if tho current is energetic,
rupted current than a continuous one
20 or 30 shocks at most, let the patic

Of the numerous apparatus whic
interrupted currents to therapeutics, t
by Dr. Duchenne, one of which gives
other an induced current either of th

DUCHENNE'S ELECTROVOLTAIG APPARATUS. 207

third, invented by M. Pulvermacher, gives the ordinary current of

@ battery, but interrupted, and of great tension.
Duchenne's eloctrovoltate apparatus.—This consists of a
. ‘with twowires, analogous to that already described in speaking

Anduction currents, and inclosed in m brass case, V (fig. 657). This
fs fixed on a wooden box in which are two drawers, one of these
ining a compass, which acts as galyanometer, and mensures the
tensity of the induced current by the deflection of the needle, and the
‘a zinc-carbon clement arranged s0 as to occupy ns little space aa
le. The zinc cloment, Z, has itself the shape of a small drawor,
which is a solution of common sult and a rectangular plate of well
gascoke. In the centre of the carbon is a small cavity, O, in
hich a small quantity of nitric acid is placed, which is absorbed. A
plate, L, communicates with the zine, and another, N, with the
hou, When the drawers aro closod, thoir electrodes, L, and N, are
ively in contact with the lower ends of two binding screws, E and
[hich are connected by means of copper wires, EF and OB, with two
Mtallic plates, H and G, the first of which is moveable, When it ix
the current is closed; if raiscd, as shown in the figure, the cur~

it is open.
As the induced current is only formed when the inducing current
‘or finishes, it is necessary that the latter be frequently intor-
Wted. In Duchenne’s apparatus this may be effected either rapidly or
Wwly. For rapid intermittences, the current passes into a piece of aoft
fb, A, which oscillates very rapidly under the influence of a bundle of

magnotised when
A, breaks and makes
induced current. .

‘For slow interruptions, the oscil Inti)
Nidan chat

elastic plate, K, and by the metallic

aro in metallic connection with I 9
the current is broken whenever the
‘tooth; and ax there are four teeth,
‘one revolution, and hence, by a mora
ot oernpons, wat een ea
at J

handles Re to the atthe
action of the current.

‘There is in the apparatus a reguiial
surrounding the bobbin, and whieh,
‘be drawn out like a drawer. The ;
the regulator is withdrawn so as tole
least when it is covered. The action
referred to the induction currents prov

817. Duchenne’s clectromagne
ductive action of a powerful magnet |
in the ease of Clarke's apparatus (779)
two branches united “posteriorly by az
the other ends there is a soft iron arm

axis, to which the motion is transmit)
‘Vaucanson's chain, A, and « handle, 3

‘On the two brinches of the mag
coiled, intended to experience the in
this first wire a second, EE, is coiled 4
the second order,

When € is made to rotate, it beeor
front of the magnots KK, and exercise
first wire an induced current of the #
this developes in the wire EE an in

‘These currents may be soparately re
Pieces, P or Q, each of which is doub)
system is seen in the figare. The et
coppor wires to handles, YY, which a

-s18] PULVERMACHER'S GALYANIC CIAL, 809

can thus be spplied to various parts of the body. The interruptions
for the formation of induced currents aro produced by a com-
, B, analogous to that in Clarke’s apparatus, and by means of a

of pieces, S, I, D, and F, the dotails of which need not be given,

Fig. 658.

| Tho intensity of the shocks may bo regulated by moans of a scrow, N,
alters the distance of the piece C from the magnet, The principal
consists of two copper cylinders, which surround the bobbins,
can be withdrawn to any extent by means of a slide, to which they
fixed. The shocks aro least: violent when tho cylinder cover the

fins completely, and most so when the bobbina are entirely un-
, effects duo to the induced currents developed in the mass of the

era.

TPhe medical action of these currents has been found to be most
ious in cases of paralysis.

S15, putvermacher's galvanic chain.—Pulvermacher has devised
remarkable for its great tension and the facility with which it
od. Figuro 659 gives a view of this battery, which has a general
iblance to the Voltaic pile (697) ; fig. G60 gives a few of its details,
At consists of a series of small wooden cylinders, on which are coiled,
by side, but without touching, a zinc wire and a copper wire, At
of its ends (fig. 660) the zine wire, ab, of the cylinder M is joined
the copper wire of tho cylinder N, by means of two small copper
fixed in the wood ; the zine of the cylinder N is joined in the same

Fig. 656

some liquid, and thus play the part o

fils, and the chemo atten whi

acid produces a current which is mor

are more numerous, With a chain
are obtained, 2

‘To break the current, which is ne:
Pal

ae B (fig.

Fig. 660. to that

nately 1

of tho battery and the side # of t

oscillations, and hence the numba

certain limita by means of « amall

clockwork motion is wound up by
handle for the armature.

ELEMENTARY OUTLINES

METROROLOGY,

iogy—The phenomena which aro produced in the
is that part of physics

Rasp hitetita apuicar ‘ibteory, ‘compiling (ety GSS
snow, and hail; and /uminous meteors, as lightning, the rain-

sight di
7 aaa points 0
X pte rhumbs on w circle in the form o

have been calculated on the supposi
‘to the square of the velocity of the
by the formula 4

tho equilibrium in some part of th
resulting from a difference in temy
‘Thus, if the temperature of a certai
ths higher rion of the top
which blow from hot to cold countr
librium is destroyed at the surface of
on the colder adjacent parts is greate
and hence a current will be produa
difference between these pressures ;
duced, an upper one setting outward)
‘one setting inwards towards it.
822. Regular, periodical, and
the more or less constant directions
claased ns regular, periodical, and va
L Regular winds are those whic
virtually constant direction. These
trade winds, are uninterruptedly obx
regions, blowing from the north-eas
here, and from the south-cas
sphere, They prevail on the ty
of latitude, and they blow in the sar
of the sun, that is, from east to west,
The air above the equator bein,
passes round from east to west, and i
from the north or south, ‘The diree!
hy this fact; that the velocity whicl
rotation of the earth, namely, the vi
«the point from which it started, is le
the earth at the point at which it b

WINDs. $13

‘in reference to the equator, the constant direction which cor-
the trade winds.

Periodical winds are those which blow regularly in the same direction

is given to winds which blow for «ix months in one direc-

six months in another. They are principally observed in
and in the Arabian Gulf, in the Bay of Bengal and in the

in feels dry, the respiration is accelerated, and a burning thirst is

wind is known under the name of sirocco in Italy and Algiers,
it blows from the great desert of Sahara. In Egypt, where it
from the end of April to June, it is called Aemain, The natives
ea, in order to protect themselves from the effects of the too

perspiration occasioned by this wind, cover themselves with fatty

Tend and sea breeze is & wind which blows on the sea coast during
from the sen towards the land, and during the night from the
Wh to the sea, For during the day the land becomes more heated than
sea, in consequence of its lower specific heat and greater conduc~
y, and hence as the superincumbent air becomes more heated than
pon the sea, it ascends and is replaced by a current of colder and
air flowing from the sea towards the land. During the night
Tand cools more rapidly than tho sea, and hence the samo pheno-
ion is produced in a contrary direction. The sea breeze commences
‘sunrise, increases to three o'clock in the afternoon, docroases towanls
and is changed into a land breeze after sunset. These winds
perceived at a slight distance from the shores. They are regular
tropics, but loss so in our climates; and traces of thom are seen
as the coasts of Greenland. The proximity of mountains also

Is rise to periodical daily breezes.
Pariable winds are those which blow sometimes in one direction
Fsomotimes in another, alternately, without being subject to any law.
Iatitudes the direction of the winds is very variable; towards
this irregularity incroases, and under the arctic zone the winds
blow from several points of the horizon at once. On the
d, in approaching the torrid zone, they become more regular,
est wind prevails in the north of France, in England, and in

‘the atmosphere, they constitute foye
more than the appearance seen over |

A chief cause of fogs consists in t
perature than the air. The vapour
become visible. In all cases, howe!
point of saturation before condensati)
produced when u current of hot an
Jower temperature than its own, for |
it is saturated, the excess of vapour p

The distinction between mists anc
of kind. A fog is a very thick mist.

825, Clouds.—Clouds are massor
drops or vesicles of extreme minut
only differ in occupying the highe
always result from the condensati
éarth, According to their appean
Howard into four principal kinda:
and the cirrus. These four kinds are
signated respectively by one, two, thr

‘The cirrus consist of small whitish

ouds. 815
and occupy the highest regions of the atmosphere.

they occupy, it in more than probable that cirrus clouds consist of
particles; and hence it is that haloes, coronm, and other optical
wances, produced by refraction and reflection from ics crystals,
w almost always in these cloods and their derivatives. Their
prance often precedes a change of weather.

@nmules are rounded spherical forms which look like mountains
one on the other. They are more frequest in summer than in

, and after being formed in the morning, they generally disappear
ds evening. If, on the contrary, bi become more numerous,

Fig. 661.
Mi especially if surmounted by cirrus clouds, rain or storms may be

tue clouds consist of very large and continuous horizontal sheets,
chiefly form at sunset, and disappear at sunrise, They ato fre-
in autumn and unusual in spring time, and are lower than the

nimbus, or rain clouds, which are sometimes classed as one of
Peemestsl varieties, are properly = combination of the three pre~
gkinds, ‘They affect no particular form, and are solely distinguished

a

Sorge Te Se Sopuse ttl
pathos on hl

pope eho
‘Th foes unt polative potion eta
‘the idea of shoals of fish. The ten
settle down into the wimbus, and fir
The height of clouds varies grea
1,600 yards in winter, and from {
they often exist at greater heights)
at a height of 7,650 yards, obser
appeared still to be at a consider!

like soap bubbles filled with air, w
air; so that these vesicles float in

fogs consist of extromely minute d
in the atmosphere by the ascension
light powders are raised by the win
‘to descend, but this absence of dow
fact, clouds do usually fall slowly,
dissipated on coming in contact wit,
at the same time the upper part is;
tion of new vapours; so that from
retain the same height.

826. Formation of clouds.—M
tion of clouds. £ The low tempé
atmosphere, For owing to the sol
disengaged from the earth and fi

RATS. SIF

force and lower density rise in the atmosphers; mecting there
‘colder and colder layers of aiz, they sink to the point of
and then condensing in infinitely email droplets, they give rise

i, The hot avd moist currents of air rising during the day undergo
ily feebler pressure, and thus is produced an expansion which is a
‘of intense cold, and produces a condensation of vapour, Hence it
a igh mountains, stopping the aerial currents, ahd forcing them to
p, are an abundant source of rain,
A hot, moist current of air mixing with a colder current, undergoes
brings about « condensation of the vapour. Thus the
moist winds of the south and south-west, mixing with the colder
of our latitude, give rain. The winds of the north and north-east
bd aleo, in mixing with our atmosphere, to condense the vapours; but
winds, owing to their low temperature, are very dry, the mixture
attains saturation, and generally gives no rain.
formation of clouds is thus explained by Hutton. The tension
Aqteous vapour, and therewith the quantity present in a given space
saturated, diminishes according to a geometric progression, while
falls in arithmetical progression, and therefore the elas-

fm air, saturated with aqueous vapour, meet a current of cold air
‘saturated, the sir acquires the mean temperature of the » two, but

Be fe Wie'ty ‘tho constant condensation of aqueous vapour
Individanl vapour vesicles become larger and heavi

Fig. 862.

Tt bas been noticed that the qua
gauge is greater as this instrumen’

colder than the layers of air which t
these layers, and, therefore, constant
rain falla on the surface of the grou
has been objected that the exceas ¢
over that at a certain height, is si:
arise from condensation, even during
from the clouds to the earth. The d
to purely local causes, and it is now
from eddies produced in the air abo)
perceptible as it is higher above the,
drops which would otherwise fall int
quantity of water which it reosives.

In any case it is clear that if rain
from their temperature condense vap
the contrary, they traverse dry air, 1
rain falls than at « certain height; |
did not reach the earth.

Many local circumstances may affe
in different countries ; but, other thi
hot climates, for there the vaporiaati
decreases, in fact, from the equator 1

WATERSPOUTS. 819

at Bordeaux it is 258; at Madeira it is 27-7; at Havannah it is
fd at St. Domingo it is 107-6. The quantity varies with the
+ im Paris, in winter, it is 42 inches; in spring 60; in summer
(in autumn 4:8 inches,
ach of rain an a square ynrd of surface exprossos a fall of 46°74
or 4°67 gallons. On an acre it corresponds to 22,622 gallona, or
$5 toma, 100 tons per inch per acre is a ready way of remem~
this,

‘Waterspouts.—These are masses of vapour suspended in the
ayers of the atmosphere which they traverse, and endowed with
ory motion rapid enough to uproot trees, upset houses, and break
(troy everything with which they come in contact.
© meteors, which aro generally accompanied by hail and rain,
mit lightning and thunder, producing the sound of carriages

Pig. 664,

over a stony road. Many of them have no gyratory motion,
mut a quarter of those observed are produced in a calm atmo-

they take place on the sea they present a curious phenomenon.

ter is disturbed, and rises in the form of « cone, while the clouda
reseed in the form of an inverted cone; the two cones then unite

eo

probably causes the descent of a very fing
rain has mora the characteristics of fall
time after sunsct, when the sky is clea)
deen attributed to the cold, resulting fro
is not the air, however, but the aqueous
own mdiation chills itself, so that it cond
The absorbent power of aqueous rapou
‘Whenever the air is dry, terrestrial radin
cause intense cold. ‘Thus in the central y
tralia, the daily range of the thermomete
of tho last continent a difference in temp
‘hag been recorded 24 hours. In Ir
to the copious radiation, ice has been fo
Aqueows YRpour abeorbs most largely is of

DEW. HOAR #KOST, S24

| Of low temperature: it is to a large extent transparent to the heat emitted
from the aun, whilst it is almost opaque to the heat radiated from the
Consequently, the solar rays penetrate our atmosphere with a loss,
estimated by Pouillet, of only 26 per cent., when directed vertically

Q ds, but after warming the earth they ‘cannot retraverse the

esphere. Through thus preventing the escape of terrestrin] heat, the

jas vapour in the air moderates the extreme chilling which is due
the unchecked radiation from the earth, and raises the temperature of
region over which it is sprond. Tyndall has thus described ‘the

n of this substance :—* Aqueous vapour is a blanket more necessary
the vegetable life of England, than clothing is to man. Remove for
Single summer night the aqueous vapour from the air which over
this country, and every plant capable of being destroyed by a
temperature would perish. ‘The warmth of our fields and gandons

pour itself unrequited into space, and the sun would rise upon an

md held fast in the iron grip of frost.’ .
890, Dew. moar frost.—Dew is merely aqueous vapour which has
on bodies during the night in the form of minute globules.
is occasionod by the chilling which bodies near the surface of the

experience in consequence of nocturnal rdiation. Their tem-
r having then sunk several degrees below that of the air, it
ntly happens, especially in hot seasons, that this temperature is
below that nt which the atmosphere is saturated. ‘The layer of air which

Immediately in contact with the chilled bodies, and which virtually
the snine temperature, then deposits a portion of the vapour which it
} just as when a bottle of cold water is brought into a warm
it becomes covered with moisture, owing to the condensation of
iqueous vapour upon it,

‘Acconling to this theory, which was first propounded by Dr. Wells,
eanses which promote the cooling of bodies increase the quantity af
» Thess causes are the emissive power of bodies, the state of the
;, and the agitation of the air. Bodies which have a great midiating
niore readily become cool, and therefore ought, to condense
¥spour. In fact, there is generally no deposit of dew on metals,
shose radiating power is very small, espocially when they are polished ;

file the ground, sand, glass, and plants, which have a great radiating
, become abundantly covered with dow.

The state of the sky also exercises great influence on the formation

dew. If tho sky in clondloss, the planctary spnces send to the earth an

preciable quantity of heat, while the earth radiates very considorably,
therefore becoming very much chilled, there is an abundant deposit
dew. But if there aro clouds, as their temperature is far higher

than that of the planetary spaces, they radiate in turn towards the earth,

ant

832. atatt.— Heil is » mass of compi
sizes, which fall in the atmosphere. ]
cipally during spring and summer, and al
it raroly falls at night, The fall of bail 4
noire,

‘Hail is genorally the procursor of ston
and follows them more rarely still Hail
to that of on ogg or an orange, ‘The for
boon altogether satisfactorily accounted
great size. On Volta’s theory the bail
by two clouds charged with opposite elec

‘thus attracted, it is much moro probable that the two clouds would

tually attracted, and would unite.

533. ree. Rogelation,—Ice is nothing more than an aggrogate of
‘crystals, such as are shown in fig. 605, The transparency of ice is due
close contact of these crystals, which causes the individual particles

into an unbroken mass, and renders the substance optically, a8

a mechanically, continuous, When large masses of ice slowly
‘away, © crystalline form in sometimes seen by the gradual disin-
into rude hexagonal prisms: a similar structure is frequently
let with, but in greater perfection, in the ice caves or glaciers of cold

striking experiment of Tyndall hax, however, more clearly revealed

6 Leautiful structure of ice. When a piece of ice is cut parallel to its
of freezing, and the radiation from any luminous source, as the sun,

ing fire, a gas or oil flame, is permitted to pass through it, the dis

‘of the substance proceeds in a remarkable way, By observing

(@ plate of ice through a lens, numerous small crystals will be son
tho interior of the block; as tho heat continues these eryatals
and finally assume the shape of six-rayed stars of exquisite beauty.
jis ico kind of negative crystallisation, tho crystals produced boing
of water, and owe their formation to the molecular disturbance

by the absorption of heat from the source. Nothing is easior than
luce this phenomenon, if care be taken in cutting the ion, The
of freezing can be found by noting the direction of the bubbles in
which are either sparsely arranged in strim at right angles to the

. or thickly collected in beds parallel to the surface of the water.

lessened by the act, By placing the
‘original position shall be regained, the
surfaces again become bounded by lee ¢
excessively thin, the force of cohesion is)
quence of this is, the liquid particles pa)
the block is reunited by revelation... Not
together, but regelation also takes place
conducting aolid body,-ns flannel or say
‘that just given has been applied here,
ice on one side. It must be remarked
philosophers dissent from the

Whatever may be the true cause of n
that this interesting observation of Farad
nomena, For example, the formation
regelation of the snow granules compos
take place at temperatures below 0° ©,, for
it is only possible tomake a coherent no

‘The snow bridges, also, which span
elsewhere, and over which men can wal
to the regelation of gradually accumulati

834. Gtactors.—Tyndall has nppliod 4
the explanation of still grander phenom
of glaciers, of which the following is a
regions what is termed the saa fine)
snow, for above this the heat of summy
snow. By the heat of the sun and the
suelted from the surface, the lower port!

snow into a coberent mnsa.
pdr i increasing epee the intermingled air which renders snow ape
nes ejected and transparent; ice then results, Its own gravity, and
Richaenre from behind, urges downwards the glacier, which hea thus
formed. In its descent from the mountain the glacier behaves in

respects like a river, passing through narrow gorges with comparative ~

ity, und then spreading out and moving slowly as its bed widens.
sr, jtist a8 the central portions of @ river move faster than the
80 Professor Forbes has ascertained, that the centre of a glacier
quicker than its margin, and from the eame reason (the difference
im the friction encountered) the surface moves more rapidly than the
To explain these facta, Forbos assumed ice to be a viscous body
ble of flexion, and flowing like lava, but as ice has not the pro~
of w viscous substance, the now generally accepted explanation of
motion is that supplied by the theory of regelation. Acoording
§ this theory, the brittle ice of the glacier is crushed and broken into
its passage throogh narrow channels, such as that of Trélaporte on Mout
and then as it emerges from the gorge which confined it, becomes
Feunited by virtwe of regelation ; in this instance forming the well known
Mer de Glace. By numerous experiments, ‘Tyndall has established that
tion is adequato to furnish this explanation, and with complete
has artificially imitated, on «small scale, the moulding of glaciers
the crushing and subsequent regelation of ice.

LUMINOUS METEORS,

B35. Atmospheric electricity, Franklin's experiment.—The
frequent luminous phenomena, and the most remarkable for their
ets, are thoes produced by the free electricity in the atmosphere, The
physicists who observed the electric spark compared it to the gleam
Tightning, and its crnckling to the sound of thunder, But Franklin,
Hy tho aid of powerful electrical batteries, first established a complete
pallel between lightning and electricity, and he indicated, in a memoir
blished in 1749, the oxperimentsznecessary to attract electricity from
clouds by menns of pointed: rods. The experiment waa tried by
band in France; and Franklin, pending the erection of a pointed
‘on & spiro in Philadelphia, had the happy idea of flying a kite,
with « motallic point, which could reach the higher regions
if th atmosphere. In June, 1762, during stormy wonther, he flew
ay in # field nonr Philadelphia, The kite was flown with

finary pack-thread, at the end of which Franklin attached a key, ind
axa

SEE

s
i

i

Fig. 666.
end of which was attached to a ring, whil

Fa

827

ATMOSPHERIC ELECTRICITY.

leaves, the electrical condition of the air at the height which
had attained could be determined. M. Bocquercl, in oxperi-
made on the St. Bernard, improved Saussuro's apparatus by sub-
ing for the knob an arrow, which was projected into the atmo-
by means of a bow. A gilt silk thrend, 88 yards long, was fixed
th one end to the arrow, while the other was attached to the stem
“OF an oloctroscope. Pelticr used n gold-loaf electroscope, at the top of
“which was « somewhat large copper globe. Provided with this in-
. the observer stations himself in a commanding position, it
Ts then quite sufficient to raise the electroscope even a foot or #0 to
“obtain signs of clectricit
‘To observe the electricity of clouds, where the tension is very con~
rable, use is made of a long bar terminated in a point. This bar,
is insalated with caro, is fixed to the summit of a building, and
ower end is connected with an electrometer, or even an electric
nes (fig. 487, page 620), which announces the presence of thunder
As, however, the bar can then give dangerous shocks, a mo-
ball must be placed near it, which is well connected with the
ind, and which is nearer the bar than the observer himself; 90
if'm discharge should ensue, it will strike the ball and not the ob-
}- Professor Richmann, of St, Petersburg, was killed in an ex-
Petiment of this kind, by a discharge which struck him on the forehead,
| Sometimes also kites are uscd, provided with a point, and connected
eee Bue cord with an electrometer. Captive balloons are also
ly used.
Enigos collector of atmospheric electricity consists of a fishing rod
‘an insulating handle which projects from an upper window, At
fummit is a bit of lighted amadou held in a metallic forceps, the
of which, being an excellent conductor, conveys the electricity of
air down a wire attached to the rod. A sponge moistened with
ol, and set on fire, is also an excellent conductor.
_ 857. Ordinary electricity of the atmosphere.—By means of the
it apparatus which have boen described, it has been found that the
nes of electricity in the atmosphere is not confined to stormy weather,
that tho atmosphere always contains froo electricity, sometimes posi-
‘and sometimes negative. When the sky is cloudless the electricity
always positive, but it varics in intensity with tho height of the locality,
with the time of day. ‘The intensity is greatest in the highest and
isolated places, No trace of positive electricity is found in houses,

or under trees; in towns positive electricity is most perceptible in
‘open spaces, on quays, or on bridges. In all cases, positive electricity
fenly found st n certain height above the ground. On flat land, it only

to the vegetation of plants, or to the evay
have courpared the earth to a vast yoltai
electrical apparatus. Many of these cau
ducing the phenomena,

‘Volta first showed that the evaporatio
Pouillet and others have subsequently #
duced by the evaporation of distilled wat
diwolved, even in atnall quantity, the rap
is negatively electrified. The reverse ia
acid, Hence it haa been assumed that a
surface of the earth and on the sea aly
vapours disengaged ought to be positivel
trified.

‘The development of electricity by oF
heating strongly @ platinum dish, adding
and placing it on the upper plate of the o
paire 630), taking care to connect the lows
the water of the capsule is evaporated, 1
is broken, and the upper plate raised. |
the water contained salts, but remain qui

ATMOSPHERIC RLECTINCITY. 829

from this experiment, Pouillet has ascribed the development
‘by evaporation to the separation of particles of water from

nces dissolved ; but Reich and Riess have shown that the eloc-
‘disengaged during evaporation could be attributed to the friction
of water carried away in the current of vapour exercise

Gbinks it-no longer allowable to. ascribe the atmospheric electricity
‘ebanges that take place during the tranquil evaporation of sea

support of the hypothesis which considers the earth us an immense
of voltaic electricity due to chemical, actions, Beequerel has re«
numerous experiments to show that when land and

bomie ia contact, electricity is always produced: the land taking a
je excess of positive or negative electricity, and the water a
on ‘excess of the opposite electricity, according to the nature
je salts or other compounds which the water held dissolved, This
general fact which, according to M. Becquerel, is liable to no

juerel experimented with an ordinary multiplier, the wire of which

onnected with two platinum plates immersed in the pieces of ground,

water whose electrical condition ho wished to investigate. He

found that when two moist pieces of ground are connected, that
contained the strongest solution took an excess of

We found that in the neighbourhood of a river, even at some

sce, the land and objects placed on the surface possessed an excess

ative electricity, while the water and the aquatic plants which

ram on the surface wore charged with positive olectricity, But accord

‘to the nature of the substances dissolved in the water, different effects

produced. As from Becquerel’s experiments, the waters aro some-

‘positive and sometimes negative, and the earth in a contrary con-

it follows that water in evaporating must constantly send into the

Io Ap excess of positive or negative olectricity, while the earth,

‘Yapours disengaged om its surface, allows an excess of the contrary

tity to escape. Now this excess of electricity ought necessarily to

Nn nee the distribution of the electricity in the atmosphere, and may

wo to oxplnin how it is that thé clouds are sometimes positively and

negatively electrified.

9. Blectricity of clouds.—In general the clouds aro all electrified,

positively and sometimes negativel; only diffe in their

or les tension. The formation of positive clouds is usually

isengaged from the ground, and ean-

‘Several kinds of lightning-flashes may 1
flashes, which move with extreme veloci!
with sharp outlines, and which entirely 1

-
probable of the many hypotheses which |
for its origin, is that which supposes it t
flashes, which strike across the clouds at st
thunder cannot reach the ear of the obs
lightning flashes which appearin the form
are sometimes visible for as much as tens
to the earth with euch slowness that th
often rebound on reaching the ground ;
explode with « noise like that of the repo.

‘The duration of the light of the first thy
thousandth of a second, as has been deter
moans of « rotating wheel, which was tur
wer invisible : on illuminating it by the H
80 short that whatever the relocity of rot
quite stationary; that is, its displacement
time the lightning exists,

841. Thunder.—Tho thunder is the }
lightoing in stormy weather. The lightni

‘but an interval of several seconds is always observed
these two phenomens, which arises from the fact that sound
‘travols at the rate of about 1100 feet in a second (205), while the
of light is almost instantaneous, Henco an observer will only
the noise of thunder five or six seconds, for instance, after the
gt, according as the distance of the thunder-cloud is five or six
1100 feet. The noise of thunder arises from the disturbance which
{he electric discharge produces in tho air, and which may be witnessed
Kinnerley's thermometer. Near the place where the lightning
Hikes, the sound ia dry und of short duration. Ata gronter distance a
rive of reports are heard in rapid succession. At a still greater distance
noise, feeble at the commencement, changes into a prolonged rolling
nd of varying intensity. Some attribute the noise of the rolling
thunder to the reflection of sound from the ground and from
clouds. Others have considered the lightning not as a single
ge, but as a series of discharges, each of which gives rise to «
cular sound. Bat na those partial discharges procoed from points at
rent distances, and from zones of unequal density, it follows not
‘that they reach the ear of the observer successively, but that they
g sounds of uncqual density, which occasion the duration and
lity of the rolling. The phenomenon has finally been ascribed to
tigzage of lightning themselves, assuming that the air at each
it angle is at its greatest compression, which would produce the
intensity of the sound.
B42. Effects of tightaing.—Tho lightning discharge is the electric
ge which strikes between a thunder-cloud and the ground. The
» by tho induction from the electricity of the cloud, becomes
d with contrary electricity, and when the tendency of the two
icities to combine exceeds the resistance of the air, the spark
which is ofton expreased by saying that a thunder-bolt has fallen.
dhtning in general strikes from above, but ascending lightning ix nlso
times observed ; probably this is the case when the clouds being
atively the earth ix positively electrified, for all oxperiments show
‘at the ordinary pressure the positive fluid passes through the
papbere more easily than negative electricit,
From tho first law of electric attraction, the discharge ought to fall
on the nearest and best-conducting objects, and, in fact, trees,
ed buildings, metals, are more particularly struck by tho discharge.
iJ oe it is impradent to stand under trees in stormy weather, especially
they are good conductors, such as oaks and elms. But the danger is
Waid not to be the same undor resinous trees, such as pines, for they
jact less well.
The effects of lightning are very varied, and of the samo kind as thoes

eS

ATMOSPHERIC ELECTRICITY.

invented by Franklin in 1756.

‘There are two principal parts ina lig!
the conductor, - Tho rod is a pointed bar
roof of the edifice-to be protected ; it is {
its basal section is about 2 or 3 inches i
« bur of iron which descends from the bo

ATMOSPHERIC ELECTRICITY. 833

penotrates to some distance, As, in consequence of their

, fron bars cannot always be well adapted to the exterior of

m, they are best formed of wire cords, such as are used for

and for suspension bridges. In a report made by the Academy
‘Sciences on the construction of lightning conductors, the use of
instead of iron wire in these conductors is recommended, inas~

& copper is « better conductor than iron. The metallic section of
Sema ought ‘to be-about } » square inch, und the individual wires
fi to 0-06 inch in diameter; they ought to be twisted in three strands,
ke an ordinary cord. The point of the lightning conductor ought to be
topper instead of platinum, for the sake of better conductivity.
conductor is usually led into a well, and to connect it better with
goil it ends in two or three ramifications. If there is no well in
neighbourhood, a hole is dug in the soil toa depth of 6 or 7 yards,
‘the foot of the conductor having been introduced, the hole is filled
swood-ashes, which conduct very well and preserve the metal from

action of a lightning conductor depending on induction and the
m of points (641),{Franklin, as soon as he had established the
ry of lightning and eloctricity, assumed that lightning conductors
drew electricity from the clouds; the converse is the case. When
-cloud positively electrified, for instance, rises in the atmosphere,
inductively on the earth, repels the positive and attracts the
legative fluid, which accumulates in bodies placed on the surface of the
|, the more abundantly as these bodies are at a greater height. ‘The
dion is then greatest on the highest bodies, which are therefore most
d to the electric discharge ; but if these bodies are provided with
points, like the rods of conductors, the negative fluid, withdrawn
the soil by the influence of the cloud, flows into the atmosphere,
neutralises the positive fluid of the cloud. Hence, not only does a
ightming conductor tend to prevent the accumulation of electricity on
j@ surface of the earth, but it also tends to restore the clouds to their
state, both which concur in preventing lightning discharges. The
ent of electricity i is, however, sometimes so abundant, that
Setting cootuctor. fs insdequate to dlacharge the ground, and the
ag strikes ; but the conductor receives the discharge, in canse-

Z ee of its greater conductivity, and the edifice is preserved.
| Experiment has shown that, approximately, a lightning conductor
0 a circular space around it, the radius of which is double its
ght. Thus, a building, 64 yords in length, would be preserved by two

B yards in height, at a distance of 92 yards,

) A canductor, to be efficient, ought to satisfy the following conditions +
#L the rod ought to be so large as not to be melted if the discharge pasyes;

the clouds opposite the sun when they a)
of seven concentric ares, presenting suc!
spectram. Sometimes only a single b
usually two; a lower oue, the colours of
external or secondary one, which is paler,
colours is reversed. In the interior ral
colour; in the other rainbow the violet i
‘aro seen; theoretically a greater number
80 feeble that they are not perceptible.
The phenomenon of the rainbow is p
the white light of the sun when it passe:
tion from their inside face. In fact, the
im dew-drops and in jets of water; in sl
into drops of water under a certain anglt
The appearance and the extent of the
of the observer, and on the height of th
only some of the rays refracted by the 1
concavity to the eye of the spectator, an
menon. ‘Those which do so.am called ef
‘To explain this let » (fig. 667) be a dh
ray Sa penctratea. At the point of inci
flected from the surface of the liquid; an
and traverses the drop im the direction al
‘emerges from the rain-drop, the other ps
surface, and tends to emerge atg. At t
tially reflected, the remainder emerges:
with the incident ray Sa, an angle calle
such rays ns yO, proceeding from the sid
duce on the retina the sensation of colo
ciently intense,
[t con be shown muthomatically thal

impinge on the same drop, and only undergo a reflection in the
the angle of deviation incroasos from the ray 8"n, for which itis

‘up to a certain limit, beyond which it decreases, and that near this
rays passing parallel into a drop of rain, also emerge parallel, From
& beam of light is produced sufficiently intense to impress

‘retina; those ure the rays which emerge parallel and are efficient.

‘RAINBOW.

| As the different colours which composo white light are unequally re=
the maximum angle of deviation is not the same for all. For
‘gays the angle of deviation corresponding to tho active rays is 42° 2,
for violet raya it is 40° 17’. Hence, for all drops placed #0 that rays
from the sun to the drop make, with those proceeding from
rop to the eye, an angle of 42° 2, this organ will receive the sen-
of red light ; this will be the case with all drops situated on the
ference of the base of a cone, the summit of which is the spectator's
¢ the axis of this cone is parallel to the sun’s rays, and the angle
by tho two opposed gonorating lines is 84° 4, ‘This explains the
ion of the red band in the rainbow ; the angle of the cone in the
‘of the violet band is 80° 34’,
“The cones corresponding to each band have a common axis called the
axis. As this right line is parallel to the rays of the sun, it follows
‘when this axis is on the horizon, the visual axis is iteelf horizontal,
the rainbow appears as a semicircle. If the sun rises, the visual axis
wad with it the rainbow. Lastly, when the sun is at a height of
2 ', the arc disappears entirely below the horizon, Henco, the phe-
‘of the rainbow never takos place excopt in the morning and

3
(Whaat has been said refers to the interior are. The secondary bow is

light became more regular, nd for

with its concave side turned towar

‘the magnetic meridian. -
Blackish raya. soon separated the

rays always showed the brightest li
arc, The length of the rays was
verged towards the same point of t
gation of the north end of the dipy
prolonged as far as their point o
fragment of an ithmense cupola.
‘The arc continued to rise in anu
Sometimes one of its feet or even b
more distinct and more numerous;
long band of rays convoluted in’ ¥
called the boreal crown. ‘The lus}
tensity, and attained that of stars o
with rapidity, the curves formed ay
(fig. 668), the base waa red, the 1
tained ith bright yollow colour, Las
disappeared : everything became fe
» A-French scientific commission

AURORA BOREALIS,
in 200 days; it appears that at the poles, nights withou
borealis are quite exceptional, so that it may be assumed
‘take place very night, though with varying intensity. They

Fig. 668.

at aconsiderable distanée from the poles, and over an imm
Sometimes the same aurora borealis has been seen at the &
A at Moscow, Warsaw, Rome, and Cadiz,
Numerous hypotheses have been devised to account for the aw
The constant direction of their are as regards the magr
dian, and their action on the magnetic needle (606), show that t
it to be attributed to electric currents in the higher regions of
here. This hypothesis is confirmed by the circumstance obser
and other countries on August 29 and September 1, 1859,
Tvilliant aurore boreales acted powerfully on the wims of theelac
ph; the alnrums were for a long time violently rung, and )
ins were frequently interrupted by tho spontaneous abnormal worl
apparatus,
g¢ to M. De ln Rive the aurorm boreales are due to ele
mmges which take place in polar regions between the positive ¢
by of the atmosphere and the negative electricity of the terres!
+ eleétricities which themselves are separated by the action of
Principally on the equatorial regions,
occurrence of irregular currents of electricity which mani
by abnormal disturbances of telegraphic communication

a

ee of product
which ‘the part
the page rere Rubmkorif's 9
current is derived from that of the
‘be correct the energy of the aurora
the sun; but until we know more¢
terrestrial magnetism these ideas a1

of the day and of the night, whi
maximum and minimum thermome
tected from the solar rays, mised
objects which might influence then

‘The temperature of a month is |
temperature of the year is the m

469° F, The temperatures in all ¢
of the ground,

S48, Causes which modify t
principal causes which modify thet
a place, its height, the direction of

Tnflucice of the latitude, The it
greater or less obliquity of the se
absorbed is greater the nearer thet
the heat absorbed decreases from
are then more oblique. This loss

as] CLIMATOLOGY. 839

and nrctic zones, partially compensated by the length of the days.
the equator, where the length of the days is constant, the tem-
is almost invariable; in the latitude of London, and in more
r ply countries, where the days are very unequal, the temperature

greatly; but in summer it sometimes rises almost as high as under

pinfluence on the temperature than its latitude. In the temperate
ya diminution of 1° C. corresponds in the mean to an ascent of 180

cooling on ascending in the atmosphere has been observed in
pon ascents, and a proof of it is seen in the perpetual snows which
F the highest mountains, It is caused by the greater rarofaction of
e which necessarily diminishes its absorbing power, besides which
jair is at a greater distance from the ground, which heats it by contact,
n there is the great dinthermanous powor of dry air.
| The law of the diminution of temperature corresponding to a grenter
it in the atmosphere has not been made out, in consequence of the
us perturbing causes which modify it, such as the prevalent winds,
hygrometric state, the time of day, &c. ‘The difforence between the
ture af two places at unequal heights is not proportional to the
mee of level, but for moderate heights an approximation to the Inw
be made, As the mean of n series of very careful observations made
fs. Walsh during balloon ascents, a diminution of 1°.0, corresponded
in increase in height of 232 yards.
tion of wuuls, As winds share the temperature of the countries
Which they have traversed, their direction exercises great influence on
ae sir in any place. In Paris the hottest winds aro the south, then
the south-east, the south-west, the west, the east, the north-west,
< and, lastly, the north-east, which is the coldest. The character
the wind changes with the seasons; the east wind, which is cold in
Aeater, is hot in summer.
‘Proximity of the seas, Tho neighbourhood of the sea tends to raire
temperature of the air, and to render it uniform, The average
miure of the sea in equatorial and polar countries is always higher
that of the atmosphere. With reference to the uniformity of
fomperature, it has been found thet in temperate rogiona, that is
26° to 50° of latitude, the difference between the maximum and
imum temperature of a day does not excoed, on the sea, 2° to 3°;
‘upon the continent this amounts to 12° to 16°. In islands the
lity of temperature is very perceptible, even during the greatest

great body of water, tnking its origin
the Guif of Mexico, from whenes it d
southern shores of North America it m

fin oan S-whh be Sin tag
ita influence is dae the milder climate o

Togions
‘open sea; and thus the harbour of Ham
‘Besides its influence in thus moderating
teapitant help tonerigaiore
800. Xsothermal lines.—When on a
perature is known to be the same are j
eens first noticed, and which be
of a place only varied with
tachi with the Istitude, isothermal It
equator; but as the temperature is in
ieee by the height, the isotherma
curved. On the sea, however, they are
is. made betwoen isothermal lines, tsoth
where the mean general, the mem sum
perature are respectively, constant, A
comprised between two isothermal lin
isogeothermic lines where the mean temp
851, Ciimate.—By the climate of a
of the mcteceslogball conditions to whic
annual temperature,summorand winter t
within which these are comprised. Som
of climates, according to their mean annu
29° 1 wo 26° C.; 0 warm climate from 2
20° to 15°; a temperate climate from 16
10° to 5°; a very cold climate from 5° to}
the temperature is below zero,
Those climates, again, are classed aa4

the mean and summer and winter temperature doos not

'; Variable climates, where the difference amounts to from
jand extrome climates, where the difference is greater than
(climates of Paris and London are variable ; those of Pekin and
fare extreme, Island climates are generally little variable, as
tature of the sen is constant; and hence the distinction between

és climates. Marine climates are characterised by the fact that
nee between the temperature of summer and winter is always
fn the case of continental climates. But the temperature is by
‘the only character which influences climates; there are, in
the humidity of the air, the quantity and frequency of the rains,
pens the direction and intensity of the winds, and the
the.

\stribution of temperature on the surface of the globe.
erature of the air on the surface of the globe decreases from
(eto the poles; but it is subject to perturbing causes so nume-
© purely local, that its decrease cannot be expressed by any law.
jorto not been possible to do more than obtain by numerous
na the mean temperature of each place, or the maximum and
‘temperatures. The following table gives m general idea of the
in of heat in the northern hemisphere,

Mean temperatures at different latitudes.
sinda 2 2» SIO. Paris, ws 2
ite. 2... 280 London. «
ia. . 261 Brussels. . .
Pp . 46 Strasburg . .
i «+ 81 Geneva, - 6.
» » 24 Boston. . . «

: 1107 © Moscow...
~ . 166 St. Petersburg .
[441 StGothard .

. 187 Greenland . .
Pes 17 Melville Island 187
re mean cacipassbaiea The highest temperature which has
fred on tho surface of the globe is 47-4° at Esne, in Egypt, and
tis —667 st Fort Reliance, in North America; which gives
te of 104-2° between the extreme temperatures observed on the
the globe.
thest temperature observed at Paria was 84° on July 8, 1798
fwest —23'5° on December 26, 1798. Tho highest observed at
b was 36° OC. in 1808, and the lowest —20° ©, in 1838,

oo

Kale view iies ve
which the isothermal lines

have shown that in this hemisphe:

in Asia to the north of Gulf Taymour, a
of Barrow’s Straits, about 15° from the

853.
temperature of the sea is generally the wt

regions the sea is always warmer than th

‘The temperature of the sea under the
to 27° at the curface ; it diminishes as thy
rato as well 1s in tropical regions the t
depths is botwoon 25° and $4°. This +
is caused by submarine currents which o¢
seas towards the equator,

‘The variations in the temperature of la
surface, which becomes frozen in winto)
25° in summer, The temperature of t
virtually 4°, which is that of the maxim

which arise from rain water
crust of the globe to a greater or less de
the temperature of torrestrial Inyo
Hence when they reach the surface thi
depth which they have attained. If
invariable temperature, the springs b
this country, for this is the temperature
annual temperature, If the springs are
ture is raised in summer and cooled in wii
they traverse in passing from the invaria
they come from below the layer of inv
perature may considerably exceed the me
they are then called thermal springs. TI
peratare of some of them,
Wildbad
Vichy sc, 3 Te
Baths 5 a SR

Great Geyser, in Iceland, at adepth of 6 ft 124
Yrom their high temperature they have the property of dissolving

mineral substances which they traverse in this passage, and hence
mineral waters. The temperature of mincral waters is not modified
by the sbundance of rain or of dryness; but it is by earth~
after whieh they have sometimes been found to rise and at others

t

‘Distribution of land and water.—The distribution of water
surface of the earth exercises great influence on climate. The
covered by water is considerably greater than that of dry land, and
listribution is unequal in the two hemispheres, The entire surface
globe cecupies about 200 millions of square miles, nearly 3 of
is covered by water ; that is, that tho surface of the water is nearly
reece 9 Sat of the Lend, The surface of the sea in the
hemisphere is to that in the northern in about tho ratio of 13

The depth of the sea is very variable, the lead generally reaches the
om at a depth of 300 to 450 yards; in the open soa it is often 1900

da, and instances are known in which a bottom has not been reached

a of 4500. Hence the total mass of the water does not exceed
tof liquid layer surrounding the earth, which would be about 1100
deep.

Plants, 106; in snimals, 106; of
gases by liquids, 132; of heat by
gases, 339; by vapours, 336; heat
produced by, 381

Balloons, 134; chamber, 152
air pump, 138, condensing, 146
Bianchi's, 142, Sprengol's, 143,
ange, 140, uses of, 147

“Alcohol thermometer, 222

value of wines, 280
omoter, 92; Gay Lussac's, 93;
cea 9

INDEX.

4 —
ABE ART
BERRATION, chromatic, 460; | Amalgam, 612
4h sphorical, 422 Amalgamated zinc, 669
beolute expansion of mercary, 235 | Amici's microscope, 407; camera
° power, 319 Incida, 484
104; of gases, 104; in | Ampére’s memoria technica, 672;

stand, 706; theory of magnotiem, 718
Amplitude of vibration, 33
Anslectrics, 685, 605
Analogous pole, SUI
Analyser, 532
Analysis, spectral, 653 ; of solar light,

326
Anemomotor, 811
Ancroid barometer, 130
Angle of deviation, 430; of polarisa-

tion, 631 ; of rpose, 23
Animals, absorption in, 105
Annealing, 63
Annual variations, 566
Anode, 603
Antilogous pole, $92
Aqueous humour, 496
Aqueous vapour, its influence on eli-

mate, 822
Anigo’s experiment, 146
Arbor Dianw, 700 ; Saturni, 700
Are of vibration, 33; voltaic, 687
Aschimede prineiple, 82; applied to

gases, 133
Area, unit of, 10
Armatures, 680, 628; Siemons’, 760
Arms of levers, 23

sation, 234; hydrostatical, 82, 87;
knife edge of, 46; physical and
chemical, 48; torsion, 62, 674, 692
Balloons, 134; constraction and man-

Barker's mill, 158

Baromoters, 114; aneroid, 130; Bun-
ten's, 117; cistern, 115; corrections
in, 118, 119; determination of
heights by, 122; Fortin's, 116; Gay
Lussac’s, 116; height of, 116; pre-
cautions with, 117; whoel, 121; va
riations of, 119

Barometric formula, Laplace's, 124;
height of corrected for heut, 238

Baroseope, 138

Battory, Bunson's, 664; Callan’s, 666;

Daniell’s 663; electric, 631; gra-

vity, 667; Leyden, charged by: coil,

eet eet enor

CS eh eet etek at hh a bat at

INDEX. S47

ice, 350 ; Favre and Silber-
ns, 363.

metry, 348

Iueida, 483; Amici's obscura,
491

ty, specific inductive, 604

irity, 95; attraction and repul-
| produced by, 97; difficulty of
ry of, 99

tty phenomena, 95-99; tubes,
‘ascent and dopression in, 96

# mode of freezing, 275, 276

‘ey
© of a Leydon jar, penetration of,
(g measurement of, 634 ; Jaws of,

ical discharge, 646, of voltaic

‘monicon, 200 ; hygrometer, 296

‘ni's experiments, 204

{s, major and minor, 182; tones
ainant and subdominant, 183;
‘sical constitution of, 189

Choroid, 196

Chromatic scale, 184; aberration, 460

Circular polarisation, 644

Cirrocumulus, 816

Cirrostratus, 816

Cirrus, 814

Cistern barometer, 115

Clarke's machine, 770

Cleavage, electricity produced by, 590

Climate, 840; influence of aqueous
‘vapour on, 821

Climatology, 838-843

Clocks, 46 ; electrical, 736

Clouds, 814 ; formation of, 817 ; vlectri~
city of, 829

Coatings, 328; Leyden jar with move-
able, 629

Cooreive force, 562

Coefficients of linear expansion, 227,
230

Cohesion, 68

Coil, Ruhmbkorif’s,766; effects produced
by, 768-775

Cold, apparent roflection of, 317 ; pro-
duced by evaporation, 274; sources,
of, 389, 390

Colladon and Sturm’s experiments, 170

Collimation, 674

Collision of bodies, 37

Colloids, 102

Coloration produced by rotatory pola
risation, 338

Colour, 3 ; of hoat, $37

Colour disease, 610

Colours,simple,446;complementary,430

Combustion, 383

Common reservoir, 687

Communicator, 729

Commutator, 727, 753, 767

Compensation pendulum, 233; strips,
234, balance, 234

Component forces, 15

Composition of velocities, 30

Compressed glass, colours produced by,
543

Compressibility, 3, 7; of gnses, 1255
of liquids, 66

Condenser, 618, 622; limits to charge
‘of, 617; of Ruhmkorif's coil, 767;
Liebig’s,

586 ; lightning,

Conductivity of bodies for heat, 305;
of liquids and of gases, 307; for
electricity, 878; 790; 798

Conductors, equivalent, 796

‘Congelation, 261

Conjugate mirrors, 316

Conservation of vis riva, 89

Constant currents, 662-668

‘Contractile force, 232

‘Convection, 308

Conyox meniscus, 96 ; mirrors, 416

Cooling, method of, 364; Newton's
Inw of, 313

‘Cornea, 494

Corpuscular theory, 395

Cosine, law of the, 313, 405

Couple, 21; voltaic, 650; thermoeloc-
tric, 785,; terrestrial magnetic, 664

Couronne des taxses, 661

Critical anglo, 627

Crystals, expansion of, 230; doubly
refracting, 519; uniaxial, 619

Crystal, homihedmal, 501

Crystalline, 495

Crystallisation, 262

Orystalloids, 102

‘Cubo, Leslie's, 318

‘Cumulostratus, $15

Cumulus, 817

Currents, action on currents, T04~
711; action of magnots, 711; action of

earth on,714~716; action onsolenoids,

717; derived, 801; detection and mea~ |

re

afenueee

SESEPY 22. FFE. PPE PPE. PP Eer eS

ant,
fe refraction, $19—522

‘action steam engine, 367—372
bet, Wollaston’s, 463

ig snd Arago’s experiments on
tes Jaw, 127; mothod of detor-
aaa of aqueous vapour,

me ‘znd Potit's determination of

olute expansion of mercury, 237
ig ‘aap ‘Petit’s mothod of cooling,
‘i law, 367

es its action on currents, 714

—716; action on solenoids, 717 ;
flattening of by rotation, 59; poles
of the, 58; magnetisation bys 578

Ear trumpet, 174

Earnshaw on velocity of sound, 168

Ebullition, 255 ; laws of, 266

Eccentric, 370

Fehelon lenses, 481

Echoes, 172; monosyllabic, teisylabie,
multiple, 173

Efflux, velocity of, 164; quantity of,
166 ; influence of tubes on, 166

Effusion of gasos, 104

Elasticity, 3, 8; of traction, 60, mo-

dulus of, 61, of torsion, 62, of flexure,

63

Elastic force, 107 ; of vapours, 355

Electricity, e atmosphoric, 827-836 ;
current, 656; communication of,
606 ; dynamical, 6535; di
of in chemical actions, 655; dis-
tribution of, 594; loss of, 643;
produced by induction, 600 ; velocity.
of, 650; theories of, 688

Electric batteries, 631; charge, 635;
chimes, 620; clocks, 736; density,
597 ; egy, 690; fish, $04; light, 686
—602, stratification of the, 772;
machines, 608—619; pendulum,
584; wheel, 621

Electrified Dodies, motion of, 606

Electromagnets, 721
Electromagnetic machine,

Equilibrium, 9; of forces, 18; of float-
‘ing bodies, 84; of heary bodies, 43;
of a liquid, 73—76 ; mobile, of tem-
perature, 313; noutral, 46; stable,
44; unstable, 44

Equivalent endosmotic, 101

Escapement, 66; wheel,'56

Ether, $24; luminiferous, 395

theory of, 313

Exhaustion, produced by aitpump,
144; by Sprengel’s pump, 145

Exosmose, 100

Expansibility of gases, 107

Expansion, apparent and real, 235;
apparent of mercury, 237 ;of liquids,

288; of solids, 216; of liquids, 216;

of gnacs, 216, 240, 242, 243; linear

and cubical, coefficients of, 226;

Measurement of linear, 227, 228;

of crystals, 230; applications of,

232; force of, 232, 253.

Fo oF REET oe

Zoa"d

~

BESPEL Yee Fee

ces, 2; impolsive, 35; molecular,
neon? polygon of, 19;

‘Formula for
‘tec, 124; for sound, 169 ; for sphori~
_calmirrors, 420—422; forlenses, 663

comet: 116

Fountain in vacuo, 147; at Giggles-
“wick, 161; intermittent, 159; Hero's,
149

Frank! fomag sake 825;
See

Abel's, 649; Chatham, 683
point, 249
lows of, 284; vitreous, 249;
“Tatent heat of, 360

(AALLERIES, whisporing, 173
Gallon, 91

Sir W. Thomson's, 675

Salranomcoe ut
paaarn ssonepeea’ ef of by ty liquid, 182;
application of Archimedes’ principle

to, 133; compressibility of, 108,
125; conductivity of, 308 ; diamag-
notism of, 781; density of, 246;
expansion of, 108, 240—246 ; endos-
mose of, 10% effusion and transfor-
mation, 104; Inwa of mixture of,
182; and vapours, mixtures of, 286 ;
protlems in, 287; Liquefaction of,
282—286 ; physical properties of,
107; pressure exerted by, 109; ma
diation of, 348; specific heat of, 308

Gascous state, 2

Gauge, air pump, 140; rain, 819

Gay Lussac’s aleoholometer, 98 ; baro-
meter, 116; determination and ex

Goissl ‘tabes, 145, 773

Glass, expansion of, 238; magnifying,
462 ; object, 346 ; opers, 475

Glasses, periscopic, 600 ; weather, 122

Glaciers, 823

Glaisher's balloon ascents, 195; factors,
301

Glow, electrical, 640

Goniometers, 423

Gramme, 11, 91

Graphic method, Duhamel’s, 179

Gratings, 626

Gravosande's ring, 216

Gravitation, 2, 40; terrestrial, 41;
gravity, 12, 40; accelerative effect
of, 13; centre of, 43 ; specific, 12

Gruvity battery, 668

Gridiron pondulam, 233

Grimaldi’s experiment, 523

Grotthi became ine 696;

Grove's gas battery, 699

Gulf stream, 842

sources of, 378—294 ; spetific, 348.
‘ing, 384—389

Heights of places, dotorminetion of,
by tarometer, 1225 hy Boiling

point, 26

Height of adress 115; variation
in, 119

‘Hoar frost, 821

Howard's nomenclature of clouds, 818

Heliostate, 423

Helix, 26, 720

Hemispheres, Magdeburg, 112

Hemihodeal crystal, 591

Henloy’s electromoter, 613

Henry's experiment, 749

Herapath’s aalt, 535

Hero's fountain, 149

Herschelian rays, $26

Holmes’ magnetoelectrical machine 750

Holta's olectrical machine, 616

Homogeneous light, 450

Hope's experiments, 239

Horizontalline, 42

Horse power, 377

Hotness, 216

Hour, 10

‘Humour, aqueous, 495

Hydraulics, 60

Hydraulic press, 76; friction, 167 ;
tourniquet, 168

Hydrodynamics, 66

Hydrocleetrical machine, 613

i

ri

e 4

pu’
a

L
i

1 680

27, 732, 753, 767 ;—note, 184
amalgam, $12

tame, 11, 91

of, 523; oxy!
ti 529; sources

Long-sight, 499, 608

‘Loops and nodes, 192

Loss of electricity, 698; of weight in
air, correction for, 303

Loudness of a musical tomo, 182

760; pencil, 396; ray, 396; tube,
square, and bottle, 641

‘ACHINE, 391; Attwood’s, 50;
electrical, 608—619; ron Etmer's,
697 ; electromagnetic, 738

Mackarel sky, 816

Magdeburg hemispheres, 112

Magnetic battery, 580; couple, 564;
declination, 565 ; dip, 689; effoete
of electrical dischange, 644; equator,
669; induction, 561; inclination,

oPPEREPEREERREER. PES. PPB. co ceeccPeu Bebo

re a®

rectilinear, 28 ;

ly accelerated
nity of, 13; pexcemtiteay 88
2 instrumonta, 193

ple ase images formed
mirrors, 41
plier, 672

Norremberg’s apparates,
Notes in music, 182, 184
Not of a screw, 26

Organ pipes, 193, 194
Orrery, electrical, 622

Percussion, heat duo to, 879

Periscopic glasses, 509

Persistance of impression on the retina,
505

Perturbation, magnetic, 665, 687

Phenakistoscope, 508

Phenomenon, 2

Phinl of four elements, 75

Phonautograph, 211-218

Phosphorogenic rays, 452

Phosphorescence, 512-516

Phosphotoseope, 614

Photoslectric microscope, 487

Photographs on paper, 492; on albu~
monised paper and glass, 493

Photography, 490-493

Photomoters, 405407

Physics, object of, 1

Physiological effects. of the electric

Begnault’s determination of density of
gases, 247; of specific heat, 353;
of essing of aqecous vapour, 258,
261; 288

Resonance, 172 ; box, 185; globe, 180
Rest,

8
Resultant of forces, 15, 17
Retina, 495; persistance of improsica
6

Salts, decomposition of, 695

Saturntiony degree of, 295; magnetic,
679

‘Savart’s toothed wheel, 175

Scale of hariness, 65

Seales in music, 183; chromatic, 184;
of a thermometer, 220; conversion
‘of, into one another, 221

Scattered light, 412

Schehallion experiment, 41

Scleroties, 494

Scott's phonautograph, 211

Screw, 5, 25

‘Second of time, 9, 10

Seconds pendulum, 64

Secondary batteries, 699; currents,
662; coil, 739

Secular maguotic variations, 565

Segments, ventral and nodal, 157, 192

Seguer's water-wheel, 169

Somi-conductors, 686

measure af, 9, 10
a 619, 639; duration

hg trumpet, 174; tubes, 167

5 gravity, 11, 86; flask, 88, 90;
90, 91

t heat, 348, 369; determination

Fatus, 362 ; of solids and liquids,
of gases, 356

{inductive eapacity, 604

os, 509

(analysis, 463

cops, 153, 456

(im, 325; solar, 446

ir reflection, 412

alabernition, 422, 443;mirrora,
focus of, 413

[dal form of liquids, 58; stute,

engines, 366; boiler, 367;
le section, or Watt's, 967; va-
‘Kinds of, 377; work of, 377;

eulating, 619
k, doubly-oxhausting, 142;
Lassac's, 286

386
sation of electric light, 772

pump, 151; and force pump, 162
alysis of, 468; constitution of,

ita, 574
ing, 86; bladder of fishes, 86

THE
‘Symmer's theory of elasticity, 688
Syphon barometer, 117

Syphon, 150; intermittent, 150

‘AM motal, 66
‘Tangent compass, or boussolo, 676
‘Tolegraph, electric, 724, 736
Tolescopes, 462; astronomical, 473;
torrestrial, 474; Galilean, 475; re-
fleeting Gregorian, 477; Newtonian,
478; Herschelinn, 480
Tomperature, 216; correction for, in
baromoter, 119; of a body, determined
by specific heat, 356
Temperature, influence of, on specific
ean, B10; how modi-
tribution of, 843; of

226 ; influence on expansion, 231

Tempering, 63, 65

Tonacity, 3, 63

‘Tension, 107; maximum of electrical
machine, 613; maximum of vapours.
266; of aqueous vapour at various
toriiporatures, 268—204; of vapours
of different liquids, 264; of mixed
Liquidsin two communicating vessel,
266

‘Terrestrial currents, 719; heat, 381;
telescope, 474

‘Thaumstrope, 606

Theodolite, 4

Theory, 2

‘Thermal analysis, 326; unit, 343

Thermocrosis, 337

Thermoelectric, currents, 784 789;
pile, 323

‘Thermoclectricity, 782

‘Thermoelomont, 783

‘Thormomultiplier, 789

‘Thernfomoters, 216; division of tabes
in, 217; Alling, 217; graduating in,
218; determination of fixed points

576, 592; force of, 62

‘Tourmaline, 334; pincette, 64

Tourniquet, bydraulic, 158

‘Triad, harmonic, 182

‘Triangle of forces, 19

‘Transition tint, 551, 652

‘Transparency, 3,396

Transpiration of gases, 104,

‘Translucent bodies, 396

‘Transmission of heat, 306; of light, 424

‘Transmission of sound, 166

‘Trompet apeaking, ear, 174

‘Tubes, Geissler's, 773 ; luminous, 641
safety, 381; speaking, 207

‘Tuning fork, 185

‘Turbine, 759

‘Tyndall's researches, 326 ¢ $07,

TJSASSEALED glass, colours pro-
duced by, 543

Undershot whoels, 159

Undulation, length of, 163

Uniaxial crystals, 519, 621; positive
and negative, 621

Unit of length, area und volume, 10;

ent, 348

in, 94
distance of distinct, 464, 499;
's theary of, 500; binoculsr,

& are, 627; couple, 656; pile,
0

condensing electroscope, 636

(, 7; unit of, 10, 11 ; determina-

of, 83; change of on solidifiea-

263

's electrical machino, 648

maximum density of, 239;
tx, $19; wheels, 169

208

‘Wells, Artesian, 80

‘Wells's theory of dew, 821

Wet bulb hygromoter, 300

Wheststone’s photometer, 407; und
Cooke's telegraph, 727

Wheel barometer, 121

Wheels friction, 61; eseapament, 56;
water, 169

Whirl, electrical, 621

Whispering galleries, 173

White's pulley, 24

White light, decomposition of, 446;
recompesition of, 448

Wild's mnguetoelectrical machine,
761

Windchest, 194; instruments, 193,

Winds, 811
‘Wines, alcoholic value of, 280

| Wellaston's battery, 661 ; doublet, 463

‘ood, conductivity of, 307
‘Work, 37 ; of an engine, 877; rate of,
378

i yn British, 10, 91

AMBONT'S pile, 670
Zev, absolute, 390 displacement
of, 222

Errc

Page 161, bottom line, after Germany
and fro.

» 164, line 24, ford + (1 +a re
» 484, line 5 from bottom, for refl