-co
-00

"to

-CO

(/ Vz

xfc

SCIENTIFIC MEMOIES

BY

JOHN WILLIAM DKAPER, M.D., LL.D.

SCIENTIFIC MEMOIRS

BEING

EXPERIMENTAL COiNTRIBUTIONS

TO

A KNOWLEDGE OF RADIANT ENERGY

BY

JOHN WILLIAM DRAPER, M.D., LL.D.

PRESIDENT <JF THE FACULTY OF SCIENCE IN THE UNIVERSITY OF NEW YORK

AUTHOR OF "A TREATISE ON HUMAN PHYSIOLOGY" "HISTORY OF THE

INTELLECTUAL DEVELOPMENT OF EUROPE " " HISTORY OF

THE AMERICAN CIVIL WAR " ETC.

NEW YORK

HARPER & BROTHERS, PUBLISHERS
FRANKLIN SQUARE

1878

Entered according to Act of Congress, in the year 1878, by

HARPER & BROTHERS,
In the Office of the Librarian of Congress, at Washington.

PREFACE.

DURING the past forty years I have devoted much time to the experi-
mental investigation of scientific topics, and have published the results
in various journals, pamphlets, and the transactions of learned societies.
They have been largely disseminated in European languages, and many
of the conclusions they have presented have been admitted into the
accepted body of scientific knowledge.

It has therefore become desirable for me to collect these scattered
memoirs and essays together, and, since they are too voluminous to be
published in full, to offer an abridgment or condensation of those that
are of less interest. I propose in this book to include only such as are
connected with the effects of Radiations or of Radiant energy, these
having been distinguished by the American Academy of Science, as mani-
tV-ti-d by its award to me of the Rumford medal for discoveries in light
and heat. A statement of the action of the Academy is annexed.

Besides these, I have several other memoirs on chemical, electrical,
and physiological topics, some of them hitherto unpublished. These, for
the present, I must reserve.

Among many other subjects treated of in these pages, the reader will
find an investigation of the temperature at which bodies become red-hot,
the nature of tin- light, they emit at different degrees, the connection be-
tween their condition as to vibration and their heat. It is shown that
ignited solids yield a spectrum that is continuous, not interrupted. This
has become one of the fundamental facts in astronomical spectroscopy.
At the time of the publication of this Memoir, no one in America had
given attention to the spectroscope, and, except Fraunhofer, few in Eu-
rope. I showed that the fixed lines might be photographed, doubled
their number, and found other hew ones at the red end of the spectrum.
The facts thus discovered I applied in an investigation of the nature of
flame and the condition of the sun's surface. I showed that under cer-
tain circumstances rays antagonize each other in their chemical effect,
and that the diffraction spectrum has great advantages over the prismat-
ic, which is necessarily di>t<>rted. I attempted to ascertain the distribu-
tion of heat in the diffraction spectrum, and pointed out that great ad-

x PREFACE.

vantages arise if wave-lengths are used in the description of photographic
phenomena. I published steel engravings of that spectrum so arranged.
I made an investigation of phosphorescence, and obtained phosphores-
cent pictures of the moon. Up to this time it had been supposed that
the great natural phenomenon of the decomposition of carbonic acid
by plants was accomplished by the violet rays of light, but by perform-
ing that decomposition in the spectrum itself, I showed that it is effected
by the yellow. Under very favorable circumstances, I examined the ex-
periments said to prove that light can produce magnetism, and found
that they had led to an incorrect conclusion. The first photographic
portrait from the life was made by me : the process by which it was ob-
tained is herein described. I also obtained the first photograph of the
moon. I made many experiments on and discovered the true explana-
tion of the crystallization of camphor towards the light. When Da-
guerre's process was published, I gave it a critical examination, and
described the analogies existing between the phenomena of the chemical
radiations and those of heat. For the purpose of obtaining more accu-
rate results in these various inquiries, I invented the chlor - hydrogen
photometer, and examined the modifications that chlorine undergoes in
its allotropic states. Since in such researches more delicate thermom-
eters are re-quired than our ordinary ones, I entered on an investigation
of the electro-motive power of heat, and described improved forms of
electric thermometers. In these memoirs will be found a description of
the method made use of for obtaining photographs of microscopic ob-
jects, together with specimens of the results. In a physiological digres-
sion respecting interstitial movements of substances, I examined the pas-
sage of gases through thin films such as soap-bubbles, and the force with
which these movements are accomplished, applying the facts so gathered
to an explanation of the circulation of the sap in plants, and of the blood
in animals. Returning to an inquiry as to the distribution of heat and of
chemical force in the spectrum, I was led to conclude, in opposition to
the current opinion, that all the colored spaces are equally warm ; and
that, so far from one portion — the violet — being distinguished by pro-
ducing chemical effects, every ray can accomplish special changes. This
series of experiments on radiations is concluded in this volume by an
examination of the chemical action of burning-lenses and mirrors.

I have endeavored to reproduce these memoirs as they were original-
ly published. When considerations of conciseness have obliged me to
be contented with an abstract, it has always been so stated, and the
place where the original may be found has been given. Sometimes, the
circumstances seeming to call for it, additional matter has been intro-

duced; but this has a I \va\- been formally indicated under the title of

. or included in parentheses. An instance of the former oci-m-, ..n

I"), of the latter on p.-i-v :;n. Wood-cuts and their explanation, -,-|-

dom occurring in the original publications, have been introduced. Thev

ha\e for the nio-t ]>art been obtained from some popular articles pub-

li>hed by me in Unr/Hr's M<I,/,I._

Ksc,-pt in a feu instances, I have adhered, in these memoir-, to the chem-
ical nomenclature in use at the time they were written. Though often
very weighty reasons may be given that the designations under whieh
>ubstances pass might be made more in accordance with their constitu-
tion or properties, and therefore more correct, yet such are the con-
fusions, the inconveniences, the difficulties attending an introduction of
new names, that sweeping changes of nomenclature should never be in-
troduced until they have become absolutely indispensable. In Memoir
X., \\ Inch treats of the action of the leaves of plants under the influence
of yellow light, I have preferred to retain the term carbonic acid instead
of any of its more recent synonyms.

Her.- and there the reader will detect statements that seem to be con-
tradictory. On examination, however, he will find that this arises from
changes which the general progress of science had made necessary. A-
an illustration of what I mean, I may refer to what is given as regards
wave-lengths (from Mosotti) on page 112. These numbers do not agree
with the more exact ones of Angstrom, page 120. It is better in such
cases to let the original statements stand.

The pages offered in this volume, though not very numerous, repre-
sent a very large amount of work, the occupation of many years. Ex-
perimental investigation, to borrow a phrase employed by Kepler respect-
ing tin- testing of h\ pot hoes, is "a very great thief of time." Some-
time- it costs many days to determine a fact that can be stated in a line.
The things related in these memoirs have consumed much more than
forty years.

Such a publication, therefore, assumes the character of an autobio^-
raphv, since it is essentially a daily narrative of the occupations of its
author. To a reader imbued with the true spirit of philosophy, even
the shortcomings easily detectable in it are not without a charm. From
the better hori/.oii he has gained he watches his author, who, like a
pioneer, is doubtfully finding his way, here travelling on a track that
leads to nothing, then retracing his footsteps, and again, undeterred,
making attempts until success crowns bis exertions. To explore tin-
path to truth implies many wanderings, many inquiries many mis-
take-.

Perhaps, then, since this book is a sort of autobiography, its reader

xii PKEFACE.

will bear with me if I try to make it more complete by here referring to
other scientific or historical works in which I have been engaged.

In early life I had felt a strong desire to devote myself to the experi-
mental study of nature ; and, happening to see a glass containing some
camphor, portions of which had been caused to condense in very beauti-
ful crystals on the illuminated side, I was induced to read everything I
could obtain respecting the chemical and mechanical influences of light,
adhesion, and capillary attraction. Experiments I soon made in con-
nection with these topics are described in these memoirs. Some of
them I used in a Thesis for the degree of Doctor of Medicine in the
University of Pennsylvania. My thoughts were thus directed to physi-
ological studies, and I published papers on these topics in the Ameri-
can Journal of Medical Sciences. The favorable impression they made
caused me to be appointed, in 1836, Professor of Chemistry and Physi-
ology in Hampden Sidney College, Virginia, an appointment which en-
abled me to. convert experimental investigation, thus far only an amuse-
ment, into the appropriate occupation of my life.

Several of the memoirs contained in this volume were composed at
that time. To them I was indebted, without any application on my part,
for an appointment to the Professorship of Chemistry and Physiology
in the University of New York. Soon afterwards I published a work
on the Forces that Produce the Organization of Plants. The lectures on
Physiology I gave at that time I improved from year to year, and at
length published them as a treatise on Human Physiology. It was very
favorably received by the medical profession.

Among new experiments and explorations on physiological subjects
contained in that book may be mentioned the selecting action of mem-
branes ; cause of the coagulation of blood ; theory of the circulation of
the blood ; explanation of the flow of sap ; endosmosis through thin films ;
measure of the force of endosmosis; respiration of fishes; action of the
organic muscle fibres of the lungs ; allotropism of living systems ; new
observations on the action of the skin ; functions of nerve vesicles and
their electrical analogies ; function of the sympathetic nerve ; explanation
of certain parts of the auditory apparatus, particularly of the cochlea
and semicircular canals ; the theory of vision ; the theory of muscular
contraction.

From the study of individual man it is but a step to the consideration
of him in his social relation, and this, accordingly, had been done in the
second part of my work on Physiology. But the subject being too ex-
tensive to be dealt with satisfactorily in that manner, I published the

PREFACE. x\\\

materials that I ha<l collected in a separate book, under the title of " A
History «>f the Intellectual Development «.f Kurope." The object of
this was mainly to point out that the intellectual progress of nations
proceeds in the srnie coiir>e as the intellectual development of the indi-
vidual ; that the movement of both is not fortuitous, but under the
dominion of law ; that tin- stages of personal development are paralleled
l>y the >tages of social development, and, indeed, as pahi-ontology has
proved, by the evolution of all animated nature; that there is an asei-nt
of man through well-marked epochs from the most barbarous to the mo-t
highly civilized condition. This book was translated into many lan-
guages: in some of them several editions of it were issue. 1. 1'ortions
of it relating to Mohammedan science were translated into Arabic.

About this time, circumstances led me to deliver before the New York
Historical Society a course of lectures on American topics, considered
from a similar point of view. These were enlarged, and published un-
der the title of "Thoughts on the Civil Policy of America.'' This, like
the preceding, was extensively translated and circulated. The train of
investigation on which I had thus entered led me to a far more serious
undertaking — a " History of the American Civil War," which had just
then closed. To this, moreover, I was incited by the earnest request of
some who had been chief actors in the events, and who very effectively
aided me. I had the inestimable advantage of enjoying the friendship
of many whose names have now become illustrious in connection with
those times. The Secretary of War gave me access to the public docu-
ments on both sides, and to him I was indebted for guidance in the de-
scription of many of the most important incidents and the course of na-
tional policy. To generals who had commanded in great battles and
conducted great campaigns I owed information which they alone could
impart, and, in like manner, most valuable assistance was given me in
special cases by persons eminent in military and civil life. It is often
said that the history of any very great social event cannot be correctly
written by a contemporaneous author, and that we must wait until
passions have subsided and interests ceased for a narrative of the truth.
But this is not so. More depends on the impartiality of the writer than
on the deadening lapse of time. The best history will always be written
by one who has had the best opportunity of getting at the facts, who
has had the privilege of the friendship or personal acquaintance of those
who have been conspicuous in the events.

No one can consider the intellectual development of Europe without
contemplating the forces that have brought that continent to its present
social condition — forces that have never ceased to be in active opposition.
Under the title of a " History of the Conflict of Religion and Science,"

xjv PREFACE.

I endeavored to describe their warfare. This work has passed through
a great many editions in America and England. It has been translated
into French, Spanish, German, Russian, Italian, Polish, Servian, etc. It
finds very many readers in Eastern Europe. And of some of these
translations several editions have been issued.

When I thus look back on the objects that have occupied my atten-
tion, I recognize how they have been interconnected, each preparing the
way for its successor. Is it not true that for every person the course of
life is along the line of least resistance, and that in this the movement
of humanity is like the movement of material bodies ?

To my American reader I need say nothing for the purpose of secur-
ing his kind appreciation of this work. I know that he, recognizing the
difficulties encountered in such a long series of experiments, will exten-
uate its imperfections, and regard it as a contribution from this side of
the Atlantic to the common fund of human knowledge, and especially
to one of its most important departments, at a period when the main
subject on which it treats had scarcely attracted scientific attention.

NEW YOKK, January, 1878.

CONTENTS.

MEMOIR I.

KXAMINATION OF THE RADIATIONS OF RED-HOT BODIES. THE PRO-
DUCTION OF LIGHT BY HEAT.

Ascertainment of the temperature at which bodies become self-luminous ;
it is 977° Fahr. — Proof that all solids begin to shine at the same de-
gree.— The spectrum of incandescent solids has no fixed lines. — The
reference spectrum. — Colors of light emitted as the heat increases are
in the order of the spectrum. — Frequency of vibration increases with
the temperature. — Intensity of the light emitted. — Intensity of the heat
radiated Page 23

MEMOIR II.

SPECTRUM ANALYSIS OF FLAMES. PRODUCTION OF LIGHT BY CIII-M-

ICAL ACTION.

Spectrum analysis of a candle flame. — Examination of various other
tlm/icx. — Sjxrfr/iin iiiidl i/sis of the light of a, burning solid. — Flames
consist of a succession of shells. — Spectrum of cyanogen. — Combustion
of flames in oxygen. — Effect of air in the interior of a flame. — The
blowpipe cone. — General result, the more energetic the chemical action
the higher the refrangibility of the resulting light; the vibrations
incr^se in frequency as the chemical action is more violent. — Fraun-
hofer's flxed lines 52

MEMOIR III.

ON INVISIBLE FIXED LINES IN THE SUN'S SPECTRUM DETECTED BY

VIIOTOGRAPHY.

The photography of Fraunhofer's flxed lines. — The lines in the rot </">
to absorption by the eartJi's atmosphere. — Nomenclature of the Fr<»'i>-
hof<r fines. — Discovery of tin- im'ixililr //.m/ liinx. — Original mnp of
(linn. — The new ultra spectral red lines a,/S, y. — Rediscovered by FOII-
ciinltuml Fizeau. — M. BecquereFs discovery. — Experiment sin 1834.. 74

CONTENTS.

MEMOIK IV.

ON THE NATUKE OF FLAME AND ON THE CONDITION OF THE SUN5S

SURFACE.

Dove's experiments on electric light. — Dark lines replaced by bright ones.
— Electric spark between metallic surfaces. — The lines depend on the
chemical nature of the substance from which the light issues. — They
may be used for determining the physical condition of the sun and
stars. — Three hypotheses of the condition of the sun's surface exam-
ined .............................................. Page 81

MEMOIR V.

ON THE NEGATIVE OK PEOTECTING KAYS OF THE SUN.

Original discovery of protecting radiations. — Case of a daguerreotype
plate. — Spectrum -photographs made in Virginia. — Protecting action
of the less refrangible rays. — Protecting action of the extreme violet. —
Variations of the protecting action. — Spectrum of darkness and spec-
trum of daylight. — Interference of rays of different colors. — Action of
waves of red, 'yellow, and violet light loith wave-lengths 2, l£, 1. — Use
of the diffraction spectrum ................................. 87

MEMOIR VI.

ON THE DIFFRACTION SPECTKUM.

Mode of obtaining the diffraction spectrum. — The yellow is in its mid-
dle ; it is a centre of chemical action. — It is also the hottest ray. —
Diffraction spectra by reflection. — Cold lines. — Dilatation of the more
refrangible rays in the prismatic spectrum, compression of the less re-
frangible. — Action of the diffraction spectrum on salts of silver. — Use
of wave-lengths for spectrum division ........................ 97

MEMOIR VII.

STUDIES IN THE DIFFKACTION SPECTKUM.

Elementary description of the diffraction spectrum. — Young^s discovery
of interference. — FresneVs discovery of transverse vibrations. — Gratings
and the spectra they yield. — Gratings on reflecting surfaces. — Optical
action of a grating. — Its spectra of different orders. — Interpretation
of wave-lengths by the mind. — Mental appreciation of multiple wave-
lengths. — Refutation of the principle that to every color there belongs
an invariable wave-length. — Increase in the range of perception in the

CONTESTS.

eye. — Extension to photographic impressions. — Encroachment on f//<
first dark space. — Photographs of the diffraction spectrum. — Proposal
to use wave-lengths for spectrum divisions. — Replacement of imaginary
i/njfvnderable principles by wave-lengths Page 115

MEMOIR VIII.

ON THE PHOSPHORESCENCE OF BODIES.

Earl n observations on phosphorescence. — The diamond. — Duration of
shadows. — Lemery's theory. — Du Fay's theory. — Qualities of dia-
mond and fluor-spar. — The volume of a phosphorescent body does not
clniix/e during its glow. — A structural change accompanies the phos-
phorescence of bodies ; there is a minute disengagement of heat. — Phos-
phorescence is not communicable. — Absolute quantity of light emit-
ted 133

MEMOIR IX.

ON THE EFFECTS OF HEAT ON PHOSPHORESCENCE.

Experiment of Albertus Magnus. — Degree of phosphorescence at different
tem])eratures. — The quantity of light a substance can retain is inversely
as its temperature ; the quantity it can receive is directly as the inten-
sity and quantity of light to which it has been exposed. — Phosphores-
cent images of the moon. — Action of ether waves. — Effects of cohesion.
— Reason that oases, liquids, and metals are non-phosphorescent. . 159

MEMOIR X. •

ON THE DECOMPOSITION OF CARBONIC- ACID GAS BY PLANTS IN THE
PRISMATIC SPECTRUM.

The decomposition of carbonic acid by light formerly attributed to the
violet ray. — It can be successfully accomplished in the prismatic spec-
trum.— It takes place not in the violet but in the yellow ray. — Decom-
position by yellow absorbent media. — Analysis of the gas evolved ; it
always contains nitrogen. — Decomposition of alkaline carbonates and
bicarbonates. — Analysis of the gas evolved in different rays 167

MEMOIR XL

OF THE FORCE INCLUDED IN PLANTS.

Growth of a seed in darkness and in light. — Action of plants and ani-
mals respectively on the atmosphere. — Examination of Rumford's Ex-
periments.— Dark heat rays cannot decompose carbonic acid. — Germi-

B

xviii CONTENTS.

nation in colored rays. — Greening of leaves takes place in the yellow
and adjacent rays. — The essential condition of all chemical changes
by radiation is absorption. — Plants absorb force from the sun ; it is
associated with their combustible parts, and is disengaged by oxida-
tion Page 177

MEMOIR XII.

EXPERIMENTS TO DETERMINE WHETHER LIGHT PRODUCES ANY MAG-
NETIC EFFECTS.

Mrs. Somerville's experiments. — Christie's experiment. — Their results
cannot be substantiated. — The violet ray has no effect. — Blue glasses
and blue ribbons also ineffective , 191

MEMOIR XIII.

AN ACCOUNT OF SOME EXPERIMENTS ON THE LIGHT OF THE SUN,
MADE IN THE SOUTH OF VIRGINIA.

Absorption of luminous radiations. — The reference spectrum. — Absorp-
tion of heat radiations ; the apparatus employed. — Absorption of chem-
ical radiations. — Screen of bromide of silver. — Coloration of chloride
and bromide of silver by radiations that have passed through corre-
spondingly colored solutions. — Early application of photography to
the investigation of physical problems. — Crystallization of camphor to-
wards the light. — The side of vessels towards the sky is the colder. .197

MEMOIR XIV.

EXAMINATION OF THE PROCESS OF DAGUERREOTYPE. — NOTE ON
LUNAR PHOTOGRAPHY.

Description of the process. — Cause of the deposition of mercury and pro-
duction of the light parts of the picture. — Polishing of the plate. — The
operation of iodizing. — Effect of keeping the iodide. — The achromatic
lens. — Reduction of focal length in the non-achromatic. — The develop-
ment.— fixing by hyposulphite and galvanism. — Necessity of heating
the plates. — Lunar impressions. — Artificial light. — Note on lunar
photography. — The first photographs of the moon 206

MEMOIR XV.

ON THE TAKING OF PORTRAITS FROM LIFE BY PHOTOGRAPHY.

History of the invention. — First attempts by whitening the face. — Use of
refiecting mirrors. — Use of a blue-colored trough. — Kind of camera

CONTENTS. xjx

necessary. — The neat or support. — The background. — Appropriate
dresses. — Ladies' dresses. — Arrangement of the shadow. — Reflecting
camera Page 215

MEMOIR XVI.

ON THE CHEMICAL CONDITION OF A DAGUERREOTYPE SURFACE.

Mercury exists all over a daguerreotype surface. — There is no superpo-
mtion of the parts. — The shadows have metallic mercury; the lights
silver amalgam. — No iodine is ever evolved from the plate. — Action of
a solution of gum and one of gelatine in tearing off the films. — The
starch experiments. — The etching of daguerreotypes 222

MEMOIR XVII.

ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE CHEMICAL
RAYS AND THOSE OF RADIANT HEAT.

The chemical rays are absorbed. — Photographic effects are transient. —
The chemical rays are not conducted ; they become latent. — Optical
qualities control chemical action. — The active rays are absorbed and
the complementary reflected. — Relation of optical conditions and chem-
ical affinities 230

MEMOIR XVIII.

DESCRIPTION OF THE CHLOR-HYDROGEN PHOTOMETER.

Properties of a mixture of chlorine and hydrogen. — It is acted upon by
lamplight, an electric spark at a distance, etc. — The gases unite in pro-
portion to the amount of light. — Mode of measuring out known quan-
tifies of radiations. — The maximum action is in the indigo space. —
Construction of the instrument. — The gases are evolved by electricity
and combined by light. — Theoretical conditions of equilibrium. — Pre-
liminary adjustment. — Method of continuous observation. — Method of
interrupted observation. — Remarkable contraction and expansion . . 245

MEMOIR XIX.

ON MODIFIED CHLORINE.

Description of the experiment. — The change in the chlorine is not tran-
sient. — There are two stages in the phenomenon. — Rays are absorbed
in producing this change. — The indigo ray is absorbed. — The action is
positive from end to end of the spectrum. — The indigo ray forms hy-
drochloric acid and also produces the preliminary modification. —

xx CONTENTS.

Change in other elementary bodies. — Verification of these results with
the chlor-hydrogen photometer Page 271

MEMOIR XX.

ON THE ALLOTROPISM OF CHLORINE AS CONNECTED WITH THE
THEORY OF SUBSTITUTIONS.

Chlorine exists in two states, active and passive. — Decomposition of water
by it in the sunlight. — Facts connected with this decomposition. — The
relations of chlorine and hydrogen. — The allotropism of chlorine. —
Connection of these facts with the theory of substitutions 284

MEMOIE XXL

ON THE INFLUENCE OF LIGHT UPON CHLORINE, AND SOME REMARKS

ON ALCHEMY.

Modification of chlorine by the sun-rays. — The modification is not tran-
sient.— Alchemical attempts to modify metals. — Exposure of silver
chloride to a burning-lens. — The resulting silver is not acted upon by
nitric acid 312

MEMOIR XXII.

ON THE ACTION OF GLASS AND QUARTZ ON THE RADIATIONS THAT
PRODUCE PHOSPHORESCENCE.

Phosphorescence by rays from a Leyden spark through quartz. — From
the voltaic arc. — It is occasioned by the more refrangible rays. — Im-
perviousness of glass. — Professor Henry's experiments. — Comparison
of the chemical and phosphorogenic rays 316

MEMOIR XXIII.

ON A REMARKABLE DIFFERENCE BETWEEN THE RAYS OF INCANDES-
CENT LIME AND THOSE EMITTED BY AN ELECTRIC SPARK.

Non-permeability of glass to sparJc radiations. — Permeability to calcium-
light radiations. — Different refrangibility of the spark and the cal-
cium-light radiations. — Shadows imbedded in Canton's phosphorus. —
Evolution of old shadows in an order of succession 320

MEMOIR XXIY.

ON THE ELECTRO-MOTIVE POWER OF HEAT.

Experimental arrangement to determine the electro-motive power. — Tem-
peratures calculated from quantities of electricity. — Increase of tension

CONTENTS. xxi

with increase of temperature. — Depends on increased resistance to con-
(I net ion. — (Quantity of elect rid t >/ independent of heated surface. — In
ttnrmo- electric piles the quantity of electricity is proportional to
tin' number of pairs. — Best forms of construction of thermo-electric
pairs Page 324

MEMOIK XXV.

ON MICROSCOPIC PHOTOGRAPHY.

Method of making microscopic photographs by condensed sunlight. —
Specimens of the art 338

MEMOIR XXVI.

ON CAPILLARY ATTRACTION AND INTERSTITIAL MOTIONS. — THE

CAUSE OF THE FLOW OF SAP IN PLANTS AND THE

CIRCULATION OF THE BLOOD IN ANIMALS.

Interstitial motions of solids. — Motions of metal in coins. — Movement of
liquids in crevices. — Capillary attraction. — Conditions for a continu-
ous flow. — Capillary attraction an electrical phenomenon. — Motions
of liquid conductors. — Dutrochefs experiment of endosmosis. — Pas-
sage of gases through liquids. — Soap bubbles. — Passage against heavy
pressure. — Distribution of sap in plants. — Circulation in plants due to
sunlight producing gum. — Circulation of blood in animals explained.
— The systemic, the pulmonary, and the portal circulation 342

MEMOIR XXVII.

ON THE EXISTENCE AND EFFECTS OF ALLOTROPISM ON THE CON-
STITUENTS OF LIVING BEINGS.

t

Allotropism of elementary substances. — Frankenheim1 s nomenclature. —
Brought about by light, heat, electricity, and by the nervous principle.
— Explanation of inflammation and congestion 375

MEMOIR XXVIII.

ON THE DISTRIBUTION OF HEAT IN THE SPECTRUM.

Early experiments seeming to prove that the maximum of heat is in the
less refrangible spaces. — Comparison of the dispersion and diffraction
spectra. — Effect of compression in the less refrangible regions and of
il I In fnt ion in the more refrangible. — Measure of heat in the two halves
of the visible dispersion spectrum. — Description of the apparatus em-
ployed.— The different colored spaces are equally warm 383

xxii CONTENTS.

MEMOIR XXIX.

ON THE DISTRIBUTION OF CHEMICAL FORCE IN THE SPECTRUM.

The curves of the calorific, luminous, and chemical spectra. — Their er-
rors.— Inappropriateness of the term actinic rays. — There is no local-
ization of chemical effects. — Every radiation can produce some specific
effect. — Case of the silver compounds. — Bitumens and resins. — Car-
bonic acid. — Colors of flowers. — Law of Grotthus — Chlorine and hy-
drogen.— Bending of stems of plants. — Absorption essential to chemical
action. — Decomposition of silver iodide. — Union of chlorine and
hydrogen. — Angstrom 's law. — General conclusions. — Chemical force
exists in every portion of the spectrum. — Each radiation exercises
chemical influences proper to itself Page 404

MEMOIR XXX.

ON BURNING GLASSES AND MIRRORS — THEIR HEATING AND CHEM-
ICAL EFFECTS.

Can concentrated rays produce new chemical decompositions ? — Effects of
amplitude, frequency, and direction of vibration in the ether-waves. •=—
Clock lenses for long exposures. — Decomposition of water by chlorine.
— Attempt to decompose it by bromine and iodine. — Use of absorbing
troughs. — Dry silver iodide not decomposed by light. — Decompositions
under water. — Decompositions in a spherical concave. — Effect of ex-
traneous mixtures. — They do not make collodion more sensitive. —
Antagonization of radiations. — Case of electric spark. — Effects of
polarized light. — Attempts to polarize light by an electro-magnet. —
Mechanical cause of decompositions by light 436

SCIENTIFIC MEMOIRS.

MEMOIK I.

EXAMINATION OF THE RADIATIONS OF RED-HOT BODIES.
THE PRODUCTION OF LIGHT BY HEAT.

From the American Journal of Science and Arts, Second Series, Vol. IV., 1847;
London, Edinburgh, and Dublin Philosophical Magazine, May, 1847 ; Harper's
New Monthly Magazine, No. 322.

CONTENTS : — Ascertainment of the temperature at which bodies become
self-luminous; it is 977° Fahr. — Proof that all solids begin to shine
at the same degree. — The spectrum of incandescent solids has no fixed
lines. — The reference spectrum. — Colors of light emitted as the heat
increases are in the order of the spectrum. — Frequency of vibration
increases with the temperature. — Intensity of the light emitted. — In-
tensity of the heat radiated.

ALTHOUGH the phenomenon of the production of light
by all solid bodies, when their temperature is raised to
a certain degree, is one of the most familiar, no person
so far as I know has hitherto attempted a critical in-
vestigation of it. The difficulties environing the inquiry
are so great that even among the most eminent philoso-
phers a diversity of opinion has prevailed respecting
some of the leading facts. Thus Sir Isaac Newton fixed

O

the temperature at which bodies become self-luminous
as 635°; Sir Humphrey Davy at 812°; Mr. Wedgwood
at 947°; and Mr. Daniel at 980°. As respects the nature
of the licrht emitted, there are similar contradictions. In

O »

some philosophical works of considerable repute it is

24 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

stated that when a solid begins to shine it first emits

O

red and then white rays; in others it is asserted that a
mixture of blue and red light is the first that appears.

I have succeeded in escaping or overcoming many of
fche difficulties of this problem, and have arrived at
satisfactory solutions of the main points; and as the
experiments now to be described lead to some striking
and perhaps unexpected analogies between light and
heat, they commend themselves to our attention, as hav-
ing a bearing on the question of the identity of those
principles. It is known that heretofore I have been led
to believe in the existence of cardinal distinctions not
only between these, but also other imponderable agents,
and I may therefore state that when this investigation
was first undertaken it was in the expectation that it
would lead to results very different from those which
have actually arisen.

The following are the points on which I propose to
treat :

1. To determine the point of incandescence of plati-
num, and to prove that different bodies become incan-
descent at the same temperature. '

2. To determine the color of the rays emitted by self-
luminous bodies at different temperatures. This is clone
by the only reliable method — analysis by the prism.

From these experiments it will appear that as the
temperature rises the light increases in refrangibility;
and making due allowance for the physiological imper-
fection of the eye, the true order of the colors is red,
orange, yellow, green, blue, indigo, violet.

3. To determine the relation between the brilliancy
of the light emitted by a shining body and its tempera-
ture.

Here we shall find that the intensity of the light in-
creases far more rapidly than the temperature. For

MKMOIK I.] THE RADIATIONS OF IGNITED BODIES.

25

example, platinum at 2600° emits almost forty times as
much light as it does at 1900°.

The source of light I have employed is in all in-
stances a very thin strip of platinum, 1.35 inch long,
and .05 of an inch wide, brought to the temperature
under investigation by a voltaic current. Platinum was
selected from its indisposition to oxidize, and its power
of resisting a high temperature without fusion.

The strip of platinum thus to be brought to different
temperatures by an electric current of the proper force
was fastened at one end to an inflexible support, and
at the other was connected with a delicate lever- index,
which enabled me to determine its expansion, and there-
by its temperature. For this purpose I have used the
coefficient of dilatation of Dulong and Petit. The tem-
pi -ratures here given are upon the hypothesis of the in-
variability of that coefficient at all thermometric degrees;
they are therefore to some extent in error.

In Fig. 1, a b represents the strip of platinum, the
upper end of which is soldered to
a stout and short copper pin, a,
firmly sunk in a block of wood, <?,
which is immovably fastened to the
basis, d d, of the instrument. A
cavity, e, half
an inch in di-
ameter is sunk
in the block <?,
and into this
cavity the pin a projects, so that when the cavity is filled
with mercury a voltaic current may be passed through
the pin and down the platinum. The other extremity
of the platinum, #, is fastened to a delicate lever, b f,
which plays on an axis at </, the axis working in brass
holes supported on a block, li. Immediately beneath

Fig. 1.

26 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

the platinum strip, and in metallic communication with
it, a straight copper wire dips down into the mercury
cup m; on this wire there is a metal ball, n, weighing
about 100 grains. The further end of the index plays
over a graduated ivory scale, p p, supported on a block,
q / the scale can be moved a little up and down, so as to
bring its zero to coincide with the index at common
temperatures.

The action of the instrument is readily understood.
In the mercury cup e let there dip one of the wires, N,
of a Grove's battery of three or four pairs, the other
wire, P, being dipped into the cup m. The current
passes through the platinum, which immediately ex-
pands, the weight n lightly stretching it. The index/
moves promptly over the scale, indicating the amount of
expansion, and therefore the degree of heat. If the wire
N be removed out of its mercury cup e, the platinum
instantly becomes cold, and pulls the lever to the zero
point.

When the platinum is thin, so as to be quite flexible
at the point #, where it is fastened to the index, the
movements take place with such promptitude and pre-
cision as to leave nothing to be desired. When the
heat has been very high and long continued, the limit of
elasticity of the platinum is somewhat overpassed, and
it suffers a slight permanent extension. But as the
ivory scale p p can slide up and down a little, the index
is readily readjusted to the zero point.

The temperature of the platinum depends entirely on
the force of the current passed through it. By inter-
vening coils of brass wire of lengths adjusted before-
hand, so as to resist the current to a given extent, any
desired temperature may be reached. I found it con-
venient to intervene in the course of the current a
rheostat, so as to be able to bring the index with pre-

MIM..IH!.] TI1K KADIAT1UNS OF IGN1TKI)

<-i>ioii to any degree, notwithstanding slight changes in
tin- f'onv <>f the voltaic battery.

The following are the dimensions and measures of the
instrument I have used: Length of the platinum strip,
1.35 inch; length of the part actually ignited, 1.14 inch;
width of ditto, -fa of an inch; length of the index from
its centre of motion to the scale, 7.19 inches; distance
of the centre of motion of index from the insertion of
the platinum at the point b, .22 inch ; multiplying effect
of the index, 32.68 times; length of each division on the
ivory scale, .021 inch. From this it would appear by a
simple calculation, using the coefficient of dilatation of
platinum given by Dulong and Petit, that each of the
divisions here used is equal to 114.5 Fahrenheit degrees.
For the sake of perspicuity I have generally taken them
at 115°.

The Grove's battery I have employed has platinum
plates thr.ee inches long and three quarters wide; the
zinc cylinders are two inches and a half in diameter,
three high, and one third thick. As used in these ex-
periments it could maintain a current nearly uniform for
an hour. I commonly employed four pairs.

By the aid of resisting wires of different lengths, or
the rheostat, the force of the current in the platinum
could be varied, and therefore its temperature. The first
attempt was, of course, to discover the degree at which
the metal began to emit light.

The platinum and the voltaic battery were placed in
a dark room, the temperature of which was 60° Fahr.;
an«l after I had remained therein a sufficient length of
time to enable my eyes to become sensible to feeble
impressions of light, I caused the current to pass, gradu-
ally increasing its force until the platinum was visible.
In several repetitions of this experiment it was uniform-

28 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

ly found that the index to which the platinum was at-
tached stood at the eighth division when this took place.
The metal had therefore dilated ^ °f ^s length ; the
elevation of its temperature was about 917°, which, add-
ed to the existing height of the thermometer, 60°, gave
for the temperature of incandescence 977° Fahr.

To the correctness of this number it might be objected
that, owing to the narrowness of the metallic strip, it was
not well calculated to make an impression on the eye
when the light emitted was feeble, and that we ought
not to take the dilatations given by the index as repre-
senting the uniform temperature of the whole platinum,
which must necessarily be colder near its points of sup-
port, on account of the conducting power of the metals
to which it was attached.

Physiological considerations might also lead to a suspi-
cion that the self-luminous temperature must vary as es-
timated by different eyes. The experiments of Bouguer,
hereafter to be referred to, indisputably show that some
persons are much more sensitive to the impressions of
light than others. So far as my limited investigation of
this matter has gone, I have not, however, found appreci-
able differences in the estimation of the temperature of
incandescence. Different individuals observing the plat-
inum have uniformly perceived it at the same time.

Against the number 977°, it may also be objected that
antimony melts at a much lower temperature, and yet
emits light before it fuses. If this statement were true,
it would lead us to believe that all bodies have not the
same point of incandescence. But I think that the ex-
periments of Mr. Wedgwood on gold and earthenware
are decisive in this particular; and, moreover, I have
reason to believe that the melting-point of antimony is
much higher than commonly supposed.

With a view of determining directly whether different

MEMOIH I.] THE RADIATK ».Nx OF IGNITED BODIES. 29

bodies vary in their point of incandescence, I took a clean
gun-bunvl, and having closed the touch-hole, exposed the
following substances in it to the action of a fire: plati-
num, chalk, marble, fluor-spar, brass, antimony, gas-carbon,
lead ; each specimen was small : the platinum was in the
form of a coil of stout wire.

When one of these bodies was placed in the gun-bar-
rel and the temperature raised, it is clear that any differ-
ence in their point of incandescence could be detected by
the eye. Thus if the ignition of platinum required a
higher degree than that of iron, on looking down the
ban-el the coil of wire should be dark when the barrel
itself had begun to shine; or, if the platinum was incan-
descent first, the wire should be seen before the barrel
had become visibly hot, and these results might be cor-
roborated by observing the inverse phenomena, when the
barrel was taken from the fire and suffered to cool.

In Fig. 2, a I is the gun-barrel passing through a hole,
<?, of suitable size in the side
of a stove. At the bottom of
the barrel, £», the substances to
be examined are placed. Their
ignition is observed by looking
in at the projecting end, a.

With respect to platinum,
brass, antimony, gas - carbon,
and lead, they all became in-
candescent at the same time as
the iron barrel itself. I could
not discover the slightest dif-

O

ference among them, either in heating or cooling; and it
is worthy of remark that the lead was, of course, in the
liquid condition. But the chalk and marble were vis-
ible before the barrel was red-hot, emitting: a faint white
light; and the fluor-spar still more strikingly so, its light

30 THE RADIATIONS OF IGNITED BODIES. [MEMOIB I.

being of a beautiful blue ; and even when the barrel had
become bright red I could still see the spar, which had
decrepitated to a coarse powder, by its faint blue rays.
In these cases, however, it was not incandescence, but
phosphorescence that was taking place. I infer, then,
that all solids, and probably melted metals, begin to
shine at the same thermometric point.

(When phosphorescent substances are to be examined,
they must be first exposed to a high temperature and
carefully guarded from access of light until they are
placed in the gun-barrel. A diamond which, among oth-
er bodies, had been thus tried, would recover its quality
of phosphorescing by a very short access of light after it
had been cooled, but if that had been carefully avoided,
it began to shine at the same time as other specimens
with which it was placed in the barrel.)

The temperature of incandescence seems to be a natu-
ral fixed point for the thermometer; and it is very inter-
esting to remark how nearly this point coincides with
1000° of the Fahrenheit scale when Laplace's coefficient
for the dilatation of platinum is used. Upon that coef-
ficient the point of incandescence is 1006° Fahr.

In view of these considerations, and recollecting that
the number given by Daniel is 980°, and that of Wedg-
wood 947°, I believe that 977° is not verv far from the

/ «/

true temperature at which solids begin to shine. It is
to be understood, of course, that this is in a very dark
room.

I pass now to the second proposition. The rays emit-
ted by the incandescent platinum strip were received on
a flint-glass prism, placed so as to give the minimum de-
viation, and, after dispersion, viewed in a small telescope.
A movement could be given to the telescope, which was
read off on a graduated circle. However, instead of
bringing the parts of the spectrum under measurement

I.I T11K KAI>IATK»N> 01 IGNITED BODIES.

31

to coincide with the cross wires in the field of the instru-
ment, it was found more satisfactory to determine tin-in
by bringing them to one or other of the edges of the
field — a process by which the extreme rays could be bet-
ter ascertained, their faint light being thus more easily
perceived in the darkness by which it was surrounded.
It would scarcely be possible to see them accurately
while the rest of a bright spectrum was in view.

In Fig. 3,0 b is the ignited platinum strip, c the prism,
d e the telescope,
moving upon the
centre of a gradu-
ated circular ta-

We,//

As it was ab-
solutely necessa-
ry to have fixed
points of refer-
ence, that all the
observations
might be brought to a common standard of comparison,
and as there are no fixed lines in the liorht of incandes-

O

cence such as are in the sunshine and daylight, I there-
fore previously determined the position of the fixed lines
in a spectrum formed by a ray of reflected daylight
which passed through a fissure -^ of an inch wide, and
one inch long, occupying exactly the position subsequent-
ly to be occupied by the incandescent platinum. In Fig.
4, 1 represents the result.*

(I expected to use the Fraunhofer lines for this pur-
pose, and was not a little surprised to find that they are
not to be seen in the spectrum of ignited solid bodies.

* The letters used to indicate the fixed lines are those employed at that
time, 1847.

32 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

Thus was discovered one of the fundamental facts in
spectrum analysis, a fact that has become of the highest
importance in astronomy, as furnishing a means for de-
termining the physical condition of the heavenly bodies,
and a test for the nebular hypothesis. An ignited solid
will give a continuous spectrum, or one devoid of fixed
lines; an ignited gas will give a discontinuous spectrum,
one broken up by lines or bands or spaces.

About twenty years subsequently to this discovery,
Mr. Huggins (1864) made an examination of a nebula
in the constellation of Draco. It proved to be gaseous.
Subsequently, of sixty nebulae examined, nineteen gave
discontinuous or gaseous spectra, the remainder contin-
uous ones.

It may therefore be admitted that physical evidence
has through this means been obtained demonstrating the

o o

existence of vast masses of matter in a gaseous condition,
and at a temperature of incandescence. The nebular hy-
pothesis of Laplace and Herschel has thus a firm basis.)
The strip of platinum was now placed in the position
.„„ n **„.,„ Hi? °f the slit which had

J]a**tt given the spectrum rep-
resented at 1, and its

temperature was raised

2.

— 7 o by the passage of a vol-

— j taic current. Though the

=j metal could be distinctly

- 12130° seen by the naked eye

Fig. 4. when the temperature

Spectra of dnylight and of incandescent platinum had 1'Cached about 1000°
at different temperature,

in passing the prism and telescope was so great that it
was necessary to carry the temperature to 1210° before
a satisfactory observation could be made. At this de-
gree the spectrum extended from the position of the fixed

MI.M.IIK I.] TIIK KAIMATION.s OF IGNITED BOD1IX 33

lint- I') in tlie reil almost as far as the line F in the green,
the colors present being red, orange, and a tint which
may be designated as gray. There was nothing answer-
ing to yellow. The rays first visible through this ap-
paratus may therefore be designated as red and greenish-
<rr:iy ; the former commencing at the line B, and the lat-
ter continuing to F, as at 3.

The voltaic current was now increased, and the tem-
perature rose to 1325°. The red end of the spectrum
remained nearly as before, but the more refrangible ex-
tremity reached the position of the little fixed line d.
Traces of yellow were now visible, and, with a certain
degree of distinctness, the red, orange, yellow, green, and
a fringe of blue could be seen ; 4 shows the result.

The temperature was now carried to 1440°. The red
extremity appeared to be advancing towards the line A ;
the blue had undergone a well-marked increase. It
reached considerably beyond the line G, as shown at 5.

On bringing the platinum to 2130°, all the colors were
present, and exhibited considerable brilliancy. Their ex-
tent was somewhat shorter than that of the daylight
spectrum, as seen at 6.

Having thus by repeated experiments ascertained the
continued extension of the more refrangible end as the
temperature rose, it became necessary to obtain obser-
vations for degrees below 1210, the limit of visibility
through the telescope. I therefore carried the prism
nearer to the platinum, and looking with the unassisted
eye directly through it at the refracted image, found that
it could be distinctly seen at a temperature as low as
1095°. Under these circumstances, the total length
could not be compared by direct measurement with the
other observations, and the result given at 2 is as cor-
rect as could be obtained. The colors were red and
greenish-gray.

C

34 THE EADIATIONS OF IGNITED BODIES. [MEMOIR I.

The gray rays emitted by platinum just beginning to
shine appear to be more intense than the red ; at all
events, the wires in the field of the telescope are more
distinctly seen upon them than upon the other color.
The designation of gray may be given them, for they ap-
pear to approach that tint more closely than any other,
and yet it is to be remarked that they are occupying the
position of the yellow and green regions.

Already we have encountered a fact of considerable
importance. The conclusion that as the temperature of
a body rises it emits rays of increasing refrangibility has
obviously to be taken with a certain restriction. Instead
of first the red, then the orange, then the yellow rays,
etc., in succession making their appearance, in which case
the spectrum should regularly increase in length as the
temperature rises, we here find that at the very first mo-
ment it is visible to the eye it reaches from the fixed
line B nearly to F, that is to say, it is equal to about two
thirds of the whole length of the diffraction spectrum,
and almost one half of the prismatic.

It is to be remarked that while the more refrangible
end undergoes a great expansion, the other extremity ex-
hibits a corresponding though a less change. As very
important theoretical conclusions depend on the proper
interpretation of this fact, it must not be forgotten that
to a certain extent it may be an optical deception, arising
from the increased brilliancy of the light. While the
rays are yet feeble, the extreme terminations may be so
faint that the eye cannot detect them, but as the inten-
sity rises they become better marked, and an apparent
elongation of the spectrum is the consequence.

It is agreed among optical writers that to the human
eye the yellow is the brightest of the rays. In the pris-
matic spectrum the true relationship of the colors is not
perceived, because the less refrangible are crowded to-

MI.M..IK 1.] T11K 1{.U>IATH>N> 01 H.MT1-.I) 1J«)|)I1-.S. 35

, and the more refrangible unduly spread out. But
in the diffraction spectrum, where the colors aiv arranged
side by side in the order of their wave-lengths, the centre
• vupied by the most luminous portion of the yellow,
and from this point the light declines away on one side
in the red, and on the other in the violet, the termina-
tions being equidistant from the centre of the yellow
space.

Now if the rays coming from shining platinum were
passed through a piece of glass on which parallel lines
had been ruled with a diamond point, so as to give a
diffraction spectrum, even admitting the general results
of the foregoing experiments to be true, viz., that as the
temperature rises rays of a higher refrangibility are
emitted, it is obvious that it by DO means follows that
the ray first visible should be the extreme red. Our
power of seeing that depends on its having a certain in-
tensity. Even when it has assumed the utmost brillian-
cy which it has in a solar beam, it is barely visible. We
ought, therefore, to expect that rays of a higher refrangi-
bility should be first seen, because they act more ener-
getically on our organ of vision ; and as the temperature
rises, the spectrum should undergo a partial elongation
in the direction of its red extremity.

I may here remark that the general result of these
experiments coincides exactly with that of M. Melloni
respecting heat at lower thermometric points. In his
second Memoir (Taylor's " Scientific Memoirs," vol. i.,
p. 50) he shows that when rays from copper at 390° and
from incandescent platinum are compared by transmis-
sion through a rock-salt prism, as the temperature rises
the refrangibility of the calorific emanations correspond-
ingly increases. Those who regard light and heat as the
same agent will therefore see in this coincidence another
argument in favor of their opinion.

36 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

In view of the fore^oin^ facts, I conclude that as the

O O '

temperature of an incandescent body rises, it emits rays of
light of an increasing refrangibility, and that the appar-
ent departure from this law, discovered by an accurate
prismatic analysis, is due to the special action of the eye
in performing the function of vision. And as the lumi-
nous effects are undoubtedly owing to a vibratory move-
ment executed by the molecules of the platinum, it seems
from the foregoing facts to follow that the frequency of
those vibrations increases with the temperature.

In this observation I am led by the principle that " to
a particular color there ever belongs a particular wave-
length, and to a particular wave-length there ever be-
longs a particular color;" but in the analysis of the spec-
trum made by Sir D. Brewster by the aid of absorption
media, this principle is indirectly controverted, that emi-
nent philosopher showing that red, yellow, blue, and con-
sequently white light, exist in every part of the spectrum.
This must necessarily take place, when a prism which
has a refracting face of considerable magnitude is used ;
for it is obvious that a ray falling near the edge and one
falling near the back, after dispersion, will depict their
several spectra on the screen ; the colors of the one not
coinciding with, but overlapping the colors of the other.
In such a spectrum there must undoubtedly be a general
commixture of the rays ; but may we not fairly inquire,
whether, if an elementary prism were used, the same facts
would hold good ; or if the anterior face of the prism
were covered by a screen, so as to expose a narrow fis-
sure parallel to the axis of the instrument, would there
be found in the spectrum it gave every color in every
part, as in Sir David Brewster's original experiment?
M. Melloni has shown how this very consideration com-
plicates the phenomena of radiant heat; and it would
seem a very plausible suggestion thai; the effect here

MIM..IU l.J THK RADIATIONS UF IGNITED 1JODIKS. 37

pointed out must occur in an analogous manner for the
phenomena of light.

I next pass to the third branch of this investigation —
to examine the relation between the temperatures of self-
luminous bodies and the intensity of the light they emit,
premising it with the following considerations :

The close analogy which lias been traced between the
phenomena of light and radiant heat lends countenance
to the supposition that the law which regulates the es-
cape of heat from a body will also determine its rate of
emission of light. Sir Isaac Newton supposed that while
the temperature of a body rose in an arithmetical pro-
gression, the amount of heat escaping from it increased
in a geometrical progression. The error of this was sub-
sequently shown by Martin, Erxleben, and Delaroche,
and finally Dulong and Petit gave the true law : " When
a hot body cools in vacuo, surrounded by a medium the
temperature of which is constant, the velocity of cooling
for excess of temperature in arithmetical progression in-
creases as the terms of a geometrical progression, dimin-
ished by a constant quantity." The introduction of this
constant depends on the operation of the theory of the
exchanges of heat ; for a body when cooling under the
circumstances here supposed is simultaneously receiving
back a constant amount of heat from the medium of con-
stant temperature.

While Newton's law represents the rate of cooling of
bodies, and therefore the quantities of heat they emit
when the range of temperature is limited, and the law of
Dulong and Petit holds to a wider extent, there are in

O *

the present inquiry certain circumstances to be taken
into account not contemplated by those philosophers.
Dulong and Petit, throughout their memoir, regard radi-
ant heat as a homogeneous agent, and look upon the
theory of exchanges, which is indeed their starting-point

38 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

and guide, as a very simple affair. But the progress of
this department of knowledge since their time has shown
that precisely the same modifications found in the colors
of light occur also for heat; a fact conveniently desig-
nated by the phrase " ideal coloration of heat," and, fur-
ther, that the wave-length of the heat emitted depends
upon the temperature of the radiating source. It is one
thing to investigate the phenomena of the exchanges of
heat-rays of the same color, and another when the colors
are different. A complete theory of the exchanges of
heat must include this principle, and, of course, so too
must a law of cooling applicable to any temperature.

There is another fact to some extent considered by
Dulong and Petit, but not of such weight in their in-
vestigations, where the range of temperature was small,
as in these where it rises as high as nearly 3000° Fahr.
This is the difference of specific heat of the same body
at different temperatures. At the high temperatures
herein employed, there cannot be a doubt that the
capacity of platinum for heat is far greater than that at
a low point. This, therefore, must affect its rate of calo-
rific emission, and probably that for light also.

From these and similar considerations we should be
led to expect that as the temperature of an incandescent
solid rises, the intensity of the light emitted increases
very rapidly.

I pass now to the experimental proofs which substan-
tiate the foregoing reasoning.

The apparatus employed as the source of the light
and measure of the temperature was the same as in the
preceding experiments — a strip of platinum brought to
a known temperature by the passage of a voltaic current
of the proper force, and connected with an index which
measured its expansion.

The principle upon which the intensities of the light

MI.M..IB I.] Till-: KAI)IATlu.\> u| HiXITEI) HODll.S. ;;.,

were determined was that originally described by H«ni-
gu«T, ami subsequently u><-d liy Masson. After many
experiments, I found that it is the most accurate method
known.

Any one who will endeavor to determine the intensi-
ties of light by Rumford's method of contrasting shad-
ows, or by that of equally illuminated surfaces, will find,
\\lieii every precaution has been used, that the results
of repeated experiments do not accord. There is, more-
over, the great defect that when the lights differ in color,
it is impossible to obtain reliable results except by re-
sorting to such contrivances as that described in the

o

Philosophical Magazine, August, 1844.

Bouguer's principle is far more exact; and where the
lights differ in color, that difference actually tends to
make the result more correct. As it is not generally
known, I will indicate the nature of it briefly :

Let there be placed at a certain distance from a sheet
of white paper a candle, so arranged as to throw the
shadow of an opaque body, such as a rod of metal, on
the sheet. If a second candle be placed also in front of
the paper and nearer than the former, there is a certain
distance at which its light completely obliterates all
traces of the shadow. This distance is readily found,
for the disappearance of the shadow can be determined
with considerable exactness. When the lights are equal,
Bouguer ascertained that the relative distances were as
1 : 8, and therefore inferred correctly that in the case of
his eye the effect of a given light was imperceptible
when it was in presence of another sixty-four times as
intense. The precise number differs according to the
sensibility of different eyes, but for the same organ it is
constant.

Upon a paper screen I threw the shadow of a rod of
copper, which intercepted the rays of the incandescent

40

THE RADIATIONS OF IGNITED BODIES. [MEMOIR 1.

platinum ; then taking an Argand lamp, surrounded by
a cylindrical metal shade, through an aperture in which
the light passed, and the flame of which I had found by
previous trial would continue for an hour of almost the
same intensity, I approached it to the paper sheet, until
the shadow cast by the copper disappeared. The dis-
tance at which this took place was then measured, and
the temperature of the platinum determined.

The temperature of the platinum was now raised, the
shadow became more intense, and it was necessary to
bring the Argand lamp nearer before it was effaced.
When this took place, the distance of the lamp was
again measured, and the temperature of the platinum
again determined.

In Fig. 5, a b is the strip of ignited platinum. It casts
a shadow, h, of the metal rod e on the white screen fg;
c is a metallic cylinder containing an Argand lamp, the
light of which issues through an aperture, d, and extin-
guishes the shadow on the screen.

Fig. 5.

In this manner I obtained several series of results, one
of which is given in the following table. They exhib-
ited a more perfect accordance among each other than I
had anticipated.

The intensity of the light of the platinum is of course
inversely proportional to the square of the distance of

Mi.M.nii I.) TIIK UADIATIONS OF K.Mll 1) BODIES.

41

<>/ ///>• Intensity of Liykt emitted by Platinum at Different
'/'< /literatures.

Ti-MipiTiituic
nf tin- pUiliiiiim.

Distance of Around lump.

Mean.

lutetiHily of
light.

Experiment I.

Experiment II.

880°

0.00

1900

f)4.0()

64.00

64,00

o.:;l

2016

89.00

41. (K)

40.00

0.81

2180

24. (X)

24.00

24.00

1.78

2245

18.00

1 !).()()

18.50

2.112

2:5<;o

14.60

16.60

15.00

4.40

L>47.->

11.60

L2.00

11.75

7.24

•_'.-•! to

J>.00

<».oo

B.OO

12.34

the Argand lamp at the moment of the extinction of the
shadow.

In this table the first column gives the temperatures
under examination in Fahrenheit degrees; the second
and third the distances of the Argand lamp from the
screen in English inches, in two different sets of experi-
ments; the fourth the mean of the two; and the fifth
the corresponding intensity of the light.

The results thus obtained proved that the increase in
the intensity of the light of the ignited platinum, though
slow at first, became very rapid as the temperature rose.
At 2590° the brilliancy of the light was more than
thirty-six times as great as it was at 1900°.

Thus, therefore, the theoretical anticipation founded
on the analogy of light and heat was completely veri-
fied, the emission of light by a self-luminous solid as its
temperature rises being in greater proportion than would
correspond to mere difference of temperature.

To place this in a more striking point of view, I made
some corresponding experiments in relation to the heat
emitted. No one thus fur had published results for high
temperatures, or had endeavored to establish through an
extensive scale the principle of Delaroche, that " the
quantity of heat which a hot body gives off in a given
time, by way of radiation to a cold body situated at a

42 THE KADIATIONS OF IGNITED BODIES. [MEMOIR I.

distance, increases, other things being equal, in a progres-
sion more rapid than the excess of the temperature of
the first above that of the second."

As the object thus proposed was mainly to illustrate
the remarkable analogy between light and heat, the ex-
periments now to be related were arranged so as to re-
semble the foregoing; that is to say, as in determining
the intensities of light emitted by a shining body at
different temperatures, I had received the rays upon a
screen placed at an invariable distance, and then deter-
mined their value by photometric methods, so in this
case I received the rays of heat upon a screen placed at
an invariable distance, and measured their intensity by
thermometric methods. In this instance the screen em-
ployed was, in fact, the blackened surface of a thermo-
electric pile. It was arranged at a distance of about one
inch from the strip of ignited platinum, a distance suffi-
cient t*> keep it from any disturbance from the stream
of hot air arising from the metal ; care was also taken
that the multiplier itself was placed so far from the rest
of the apparatus that its astatic needles could not be af-
fected by the voltaic current igniting the platinum, or
the electro-magnetic action of the wires or rheostat used

O

to modify the degrees of heat.

In Fig. 6, a 1) is the ignited platinum strip, c the ther-
mo-electric pile, d d the multiplier.

The experiments were conducted as follows: The
needles of the thermo- multiplier standing at the zero
of their scale, the voltaic current was passed through
the platinum, which immediately rose to the correspond-
ing temperature, and radiated its heat to the face of the
pile. The instant this current passed, the needles of the
multiplier moved, and kept steadily advancing on the
scale. At the close of one minute the deviation of the
needle and the temperature of the platinum were si-

MKMOIU 1.1 TI1K RADIATIONS OF IGNITED BOI>IF>.

multaneously noted,
and then the voltaic
current was stopped.

Sufficient time was
now given for the
needles of the multiplier to come back to zero. This
time varied in the different cases, according to the in-
tensity of the heat to which the pile had been exposed ;
in no instance, however, did it exceed six minutes, and
in most cases was much less. A little consideration will
show that the usual artifice employed to drive the needles
back to zero by warming the opposite face of the pile was
not admissible in these experiments.

The needles having regained their zero, the platinum
was brought again to a given temperature, and the ex-
periment conducted as before. The following table ex-
hibits a series of these results.

In this table the first column gives the temperatures
of the platinum in Fahrenheit degrees; the second and
third, two series of experiments expressing the arc passed
over by the needle at the close of a radiation lasting one
minute, each number being the mean of several suc-
cessive trials, and the fourth the mean of the two. It
therefore gives the radiant effect of the incandescent
platinum on the thermo-multiplier for the different tem-
peratures.

Of course it is understood that I here take the angu-

44

THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

Table of the Intensity of Radiant Heat emitted by Platinum at Dif-
ferent Temperatures.

Temperature
of the platinum.

Intensity of heat emitted.

Mean.

Experiment I.

Experiment II.

980°

.75

1.00

.87

1095

1.00

1.20

1.10

1210

1.40

1.60

1.50

1325

1.60

2.00

1.80

1440

2.20

2.20

2.20

1555

2.75

2.85

2.80

1670

3.65

3.75

3.70

1785

5.00

5.00

5.00

1900

6.70

6.90

6.80

2015

8.60

8.60

8.60

2130

10.00

10.00

10.00

2245

12.50

12.50

12.50

2360

15.50

15.50

15.50

lar deviations of the needle as expressing the force of
the thermo-electric current, or, in other words, as being
proportional to the temperatures. This hypothesis, it is
known, is admissible.

It therefore appears that if the quantity of heat radi-
ated by platinum at 980° be taken as unity, it will have
increased at 1440° to 2.5, at 1900° to 7.8, and at 2360°
to 17.8, nearly. The rate of increase is, therefore, very
rapid. Further, it may be remarked, as illustrative of
the same fact, that the quantity of heat radiated by a
mass of platinum in passing from 1000° to 1300° is near-
ly equal to the amount it gives out in passing from com-
mon temperatures up to 1000°.

I cannot here express myself with too much emphasis
on the remarkable analogy between light and heat which
these experiments reveal. The march of the phenomena
in all their leading points is the same in both cases. The
rapid increase of effect as the temperature rises is com-
mon to both.

It is not to be forgotten, however, that in the case of
light we necessarily measure its effects by an apparatus
which possesses special peculiarities. The eye is insen-

MI.M..IU I.] TI1K KADIAT1UNS OF IGNITKI) I'.oDIES. 45

sil)le to rays not comprehended within certain limits of
refrangibility. In these experiments it is requisite to
raise the temperature of the platinum almost to 1000°
before we can discover the first traces of light. Meas-
ures obtained under such circumstances are dependent
on the physiological action of the visual organ itself, and
hence their analogy with those obtained by the thermom-
eter becomes more striking, because we should scarcely
have anticipated that it could be so complete.

Among writers on Optics it has been a desideratum to
obtain an artificial light of standard brilliancy. The
preceding experiments furnish an easy means of supply-
ing that want, and give us what might be termed a " unit
lamp." A surface of platinum of standard dimensions,
raised to a standard temperature by a voltaic current,
will always emit a constant light. A strip of that metal,
one inch long and -fa of an inch wide, connected with a
lever by which its expansion might be measured, would
yield at 2000° a light suitable for most purposes. More-
over, it would be very easy to form from it a photome-
ter by screening portions of the shining surface. An in-
genious artist would have very little difficulty, by taking
advantage of the movements of the lever, in making a
self-acting apparatus in which the platinum should be
maintained at a uniform temperature, notwithstanding
any change taking place in the voltaic current.

UNIVERSITY OF NEW YORK, Feb. 27, 1 847.

NOTE. — The experiments related in the foregoing pages
were made by me between 1844 and 1847. They were
published in May in the latter year.

In the following July, M. Melloni, who was at that
time recognized as the chief authority on the subject of

46 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

radiant heat, read before the Royal Academy of Sciences
at Naples a memoir entitled "Researches on the Radi-
ations of Incandescent Bodies and on the Elementary
Colors of the Solar Spectrum." This was translated into
French from the Italian, and published in the BilMo-
tlieque Universelle of Geneva. It was also translated into
English, and published both in England and America.

M. Melloni commences his memoir as follows : " Among
the more recent scientific publications will be found a
memoir by the American professor J. W. Draper ' On the
Production of Light by Heat,' which appears to me to
merit the attentive consideration of those who interest
themselves in the progress of the natural sciences. The
author treats in a very ingenious manner some questions
allied to my own researches on light and radiant heat.
In reading this interesting work, several ideas have pre-
sented themselves to me, which I have submitted to the
test of experiment. I believe that an analysis of the
memoir of M. Draper, accompanied with a brief account
of what I have done, will not be without interest.

"Every one knows that heat, when it accumulates in
bodies, at last renders them incandescent, that is to say,
more or less luminous and visible in the dark. Is the
temperature necessary to produce this state of incandes-
cence always the same, or does it vary with the nature
of the body ? In either case, what is its degree, and what
is the succession of colored lights emitted by a given sub-
stance when brought to temperatures more and more ele-
vated? Finally, what is the relation that subsists at
different periods of incandescence between the tempera-
ture and the quantity of light and of heat emitted by a
body?"

Melloni then describes the apparatus and the processes
I had used to determine the thermometric degree of in-
candescence and its uniformity for different substances.

MI.M..IU I.J Till. KADIATIONS OF IGNITED BO1MI S. 47

Hi- dwells on the fact that melted metals, such as lead,
have the same point of ignition. He agrees in excepting
tin- phenomena of phosphorescence, and those in which
light is developed in chemical combinations. He re-
marks that "some philosophers of the highest eminence,
among them M. Biot, suppose that the first light disen-
gaged by incandescent bodies is blue, and they have ac-
counted for this on the principle of a theory now univer-
sally abandoned. But these cases," he adds, "ought to
be carefully distinguished from incandescence properly
speaking, which arises directly and solely from an eleva-
tion of temperature in the body, and which always com-
mences with a red light.

" As to the exact degree of this temperature, the ob-
jections which might be raised against the mode em-
ployed by M. Draper are of very little importance. If
we compare the results at which he arrives with those
that have been obtained by Wedgwood and Daniel, the
difference is only 30° in excess in the first case, and 3° too
little in the second. The differences are much greater
when compared with the deductions of Davy and Sir I.
Newton, which gave 812° and 635°, respectively. But
those numbers, and especially the latter, were obtained
by methods too imperfect to be trustworthy. Conse-
quently the number 977° Fahr., given by M. Draper,
must approach very closely the degree of heat which
produces the first incandescence of bodies."

Melloni then describes the method I had resorted to
for investigating the nature of the colors which are de-
veloped by an ignited body as its temperature is in-
creased.

He dwells on the employment of a reference spectrum,
which was resorted to in consequence of the spectrum of
a solid having no fixed lines — a discovery which has
become of the utmost value in astronomical spectrum

48 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

analysis. He states the results given in the foregoing
pages, and adds : " In other words, the spectrum of the
strip of platinum which corresponds to the red extremity
of the prismatic spectrum is at first very short, and con-
tains only the less refrangible colors ; but as the tempera-
ture rises, the spectrum of incandescence extends towards
the violet extremity, obtaining the more refrangible tints,
and at last acquiring all the colors and all the extent of
the solar spectrum, except the terminal rays at the two
extremities, which escape the observer evidently on ac-
count of their extreme feebleness. The same cause (in-
sensibility due to a want of luminous energy) makes the
first spectrum appear at the red end a little shorter than
the last, because the less refrangible rays of that color
are, as is well known, so feeble even in the solar spec-
trum that we are unable to perceive them, unless they
are isolated in a place that is totally dark. Much more,
therefere, ought they to remain invisible to the observer
when the spectrum arises from luminous agencies so little
energetic as are those of the first periods of incandescence.

" To a perfectly sensitive eye the variations of length
would evidently have taken place in the direction of the
more refrangible rays only, and all the spectra would
have commenced at the extreme limit of the red rays.

" It results from all these observations that when the
incandescence of a body becomes more and more vivid
and brilliant by the elevation of its temperature, there is
not only an augmentation in the intensity of the result-
ing light, but also in the variety of elementary colors
which compose it ; there is, too, an addition of rays so
much the more refrangible as the temperature of the in-
candescent body is higher. In this there is, therefore,
established an intimate analogy between the progressive
development of light and that of heat. Indeed,'1 M. Mel-
loni adds, "as soon as I had convinced myself of the

MIM..UI!.] T1IK KAIMATIOXS OF KJXITED IIODIKS 49

immediate transmission of every variety of radiant lieat
through rock-salt, I availed myself of that valuaMc prop-
erty to study the refraction of heat from various sources,
and I discovered that radiations coming from those of
a high temperature contain elements more refrangible
than those which are derived from sources that are not

SO ll«)t."

M. Melloni then passes to a criticism of the methods I
had used in investigating the law of the increase of the
luminous and calorific radiations, according as the tem-
perature of the source of heat is elevated.

He adds: "The method invented by Bouguer to deter-
mine the relative intensities of different luminous sources,
and employed by Draper to measure the quantities of
light emitted by a strip of platinum brought to different
degrees of incandescence, is the only one by which we
could hope for a successful result. The method of the
equality of shadows, well known under the name of
Rumford's method, would have furnished in the research-
es of the learned American uncertain data, on account of
the difficulty of establishing an exact comparison between
the accidental green tint introduced into the shadow en-
lightened by the yellow rays of the lamp and the red
light emitted by the ignited metal. As to the measures
of the radiant heat, they were determined by the aid of
the thermo-multiplier, that admirable instrument which
has revealed to science so many new properties of calo-
rific radiations, and which still is rendering eminent
services in the hands of able chemists far beyond the
Alps.

" The numbers obtained by M. Draper show evidently
that the augmentations of both light and heat, though
treble at first, become very rapid at last, from which it
results that the radiations both of light and heat follow
in the progression of qiHtntity the same analogy that we

D

50 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I.

have just observed in the progression of quality. These
researches then conduct, as do others heretofore known
on light and radiant heat, to a perfect analogy between
the general laws which govern these two great agents of
nature."

The law of the radiation of heat, as illustrated by the
foregoing experiments with an ignited strip of platinum,
has been applied in recent discussions respecting the age
of the earth. Geological evidence has satisfactorily es-
tablished that the temperature of the earth was formerly
much higher than now, and the decline that has hap-
pened could only have taken place by radiation into
space. Considering how slow the cooling now is — a
scarcely perceptible fraction of a degree in the course of
many centuries — it would seem that to accomplish the
whole descent, if even wre go no further back than the
paleozoic era, an amazing lapse of time would be re-
quired. And if we accept the nebular hypothesis, since
the original temperature must have been at least that of
the surface of the sun, the time must be correspondingly
extended. Even if numbers could be given, the ima^ina-

o f O

tion would altogether fail to appreciate them.

But we have here experimental proof that the higher
the temperature of a body, the more rapidly it cools. A
descent through a given number of degrees is more quick-
ly made when a body is at a high than when it is at a
low temperature. Anciently the cooling of the earth
was more rapid than it is now. Not that there was any
change or breach in the general law under which the op-
eration was taking place, for the same mathematical ex-
pression applies to all temperatures, no matter how high
or how low they may be. Mr. Croll, in his recent re-
searches on the distribution of heat over the globe, points
out the bearing of these experiments.

I.J T1IK ICAIUATlnN.x OF UiMTKD IJODIKS. QJ

Our estimate of the age of the earth, as deduced from
the cooling she has undergone, must therefore, in vit-u
of these considerations, be diminished — a result insi.-tcd
upon by many recent authors. Too much weight must,
however, not be given to this conclusion, since it ought
to be borne in mind that the cooling was not taking
place by radiation into space from the earth alone as a
solitary body. She was in presence of a high extrane-
ous temperature, which diminished her speed of cooling,
and correspondingly increased the time.

Though the problem of the age of the earth, as inves-
tigated through the changes of her temperature, may not
at present be capable of exact solution, it must be ad-
mitted that the time required to bring her heat to its
present degree must have been inconceivably long.

52 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

MEMOIK II.

SPECTRUM ANALYSIS OF FLAMES. PRODUCTION OF LIGHT
BY CHEMICAL ACTION.

From the American Journal of Science, Second Series, Vol. V., 1848; Philosophical
Magazine, London and Edinburgh, Feb., 1848; Harper's New Monthly Maga-
zine, No. 323.

CONTENTS : — Spectrum analysis of a candle flame. — Examination of
various other flames. — Spectrum analysis of the light of a burning
solid. — Flames consist of a succession of shells. — Spectrum of cyano-
gen.— Combustion of flames in oxygen. — Effect of air in the interior
of a flame. — The blowpipe cone. — General result, the more energetic
the chemical action the higher the refrangibility of the resulting light;
the vibrations increase in frequency as the chemical action is more
violent. — Fraunhofer 's fixed lines.

THE production of light and heat by the combustion
of various bodies is, of all chemical processes, that which
ministers most to the comfort and well-beirisj of man.

^u

By it the rigor of winter is abated, and night made
almost as available for our purposes as day.

One would suppose that, of a phenomenon on which
so much of our personal and social happiness depends,
and which must have been witnessed by every one, all
the particulars ought to have been long ago known.
Among scientific men its importance has been universal-
ly recognized. The earlier theories of chemistry, such as
those of Stahl and Lavoisier, are essentially theories of
combustion.

It is nevertheless remarkable how little positive
knowledge, until quite recently, was possessed on this
subject. Some chemists thought that the light emitted
by flames is due to electric discharges ; others, regarding

MI.MOIII 11.] Sl'KCTHUM ANALYSIS oF FLAMKS. 53

li^lit and lieat as material bodies which can be incor-
porated or united with ponderable substances, supposed
that they arc discii^-ip-d as rhnniral changes ^<> on. In
this confusion of opinions a multitude of interesting ami
hitherto unanswered questions present themselves. It
is known that different substances, when burning, emit
rays of different colors. Thus sulphur and carbonic
oxide burn blue, wax yellow, and cyanogen lilac. What
are the chemical conditions that determine these singular
differences? How is it that, by changing the conditions
of combustion, we can vary the nature of the light ?
We turn aside the flame of a candle by means of a
blow-pipe, and a neat blue cone appears. Why does it
shine with a blue lio-ht?

O

Such inquiries might be multiplied without end ; but
a little consideration shows that their various answers
depend on the determination of a much more general
problem, viz., Can any connection be traced between the
chemical nature of a substance, or the conditions under
which it burns, and the nature of the lio'ht it emits? It

O

is to the discussion of that problem that this memoir is
devoted.

Sir H. Davy has already furnished us with two im-
portant facts in relation to the nature of flame: 1st, All
common flames are incandescent shells, the interior of
which is dark ; 2d, the relative quantity of light emitted
depends upon the temporary disengagement of solid par-
ticles of carbon.

It is only by a veiy general examination of the light
arising from various solids, vapors, and gases, when
burning, that we can expect to obtain data for a true
theory of combustion. This is what I shall endeavor
to furnish on the present occasion.

As was foreseen by all the older chemists, the true
theory of combustion, whatever it may prove to be,

54 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

must necessarily be one of the fundamental theories of
chemistry. It must include the nature of all chemical
changes whatsoever. The subject is therefore not alone
interesting in a popular sense, but of great importance
in its scientific connections.

I. Prismatic analyses of the flames of various vapors
and gases ; proving that they yield all the colors of the
spectrum.

I commenced this investigation of the nature of flame
and of combustion generally by an optical examination
of various bodies in the act of burning. Some authors
have asserted that certain flames yield monochromatic
light. It is necessary to verify this assertion, if true, or
set it aside if false.

The instrumental arrangement I resorted to for the
determination of the structure of a flame may be thus
described : The rays of the flame of which the examina-
tion was to be made passed through a horizontal slit,
one thirtieth of an inch wide and one inch long, in a
metallic screen placed near to the flame, and were re-
ceived at a distance of six or eight feet on a flint-glass
prism, the axis of which was parallel to the slit. After
passing the prism, they entered a telescope, which had a
divided micrometer and parallel wires in its eye-piece.
Through this telescope the resulting spectrum was
viewed. In this form of spectroscope no collimating
lens was used.

In Fig. 7, a is the candle, or flame which is to be ex-
amined, b the screen with a horizontal slit, c the prism,
d d the telescope.

If it be the flame of a lamp of any kind that is to be
examined, by using a movable stand we are able to raise
or lower it, and thus analyze different horizontal elements
in its lower, its middle, or its upper parts at pleasure.

M..MOIK II.] SPECTRUM ANALYSIS OK FLAMES.

If, instead of a horizontal, we wish to examine a vertical
< I, nitnt of the flame, the slit and the prism must, of
course, be set vertically. The former mode possesses
great advantages, as will be presently pointed out. It
is to be understood in all cases that the eye-piece of the
telescope is adjusted to give a sharp image of the slit,
and the prism is at its angle of minimum deviation.

By this arrangement I have examined a great number
of different flames, as those of oil, alcohol, solutions of
boracic acid and nitrate of strontian in alcohol, phos-
phorus, sulphur, carbonic oxide, hydrogen, cyanogen,
arseniuretted hydrogen, etc. Among these, it will be
noticed that different colors occur. Oil gives a yellow
flame, alcohol a pale blue, boracic acid green, strontian
red, phosphorus yellowish -white, sulphur and carbonic
oxide blue, hydrogen pale yellow, cyanogen lilac, arseni-
uretted hydrogen white, etc.

Notwithstanding this diversity of color, all these
flames, as well as many others I have tried, yield the
same result: every prismatic color is found in them.
Even in those cases where the flame is very faint, as in
alcohol and hydrogen gas, not only may red, yellow,
green, blue, and violet light be traced, but even bright
Fraunhoferian Hues of different colors.

56 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

This observation liolds good for those flames reputed
to be monochromatic ; for example, alcohol burned from
a wick imbued with common salt. It is not only a
yellow light which is evolved ; the other colors plainly,
though more faintly, appear.

All flames, no matter what their special colors may
be, evolve all the prismatic rays. Their special tints
arise from the preponderance of one class of rays over
another; thus in cyanogen the red predominates, and
in sulphur the blue.

(Later experiments have proved that the spectrum
thus containing all the prismatic colors, and acting as a
background to the bright fixed lines, is not due, except
indirectly, to the flame under examination, but to other
causes, not here taken into account, especially the acci-
dental presence of daylight, combustion of dust in the
air, etc. The statements here given are, however, in
accordance with observations actually made. These are
the eifects that will be seen in a partially illuminated
room such as the laboratory in which these experiments
were made. In a dark room the spectrum background
disappears.)

The production of light in the case of flames is thus
proved to be a very complex phenomenon. The chem-
ical conditions under which the burning takes place are
likewise very complex. The combustible vapor is sur-
rounded on all sides by atmospheric air; diffusion oc-
curs, and rapid currents are established by the high
temperature. Such circumstances complicate the result;
and it is only by observing the burning of an elementary
solid, in which most of these disturbances are cut off,
that we can hope to effect a proper resolution of the
problem.

II. Prismatic analysis of the light of an elementary

MiM.iiuII.J Sl'KCTIil ,.M ANALYSIS OF 1 LAMKS. 57

hurtling at different temperatures; proving that as
f//> ft nijx rut n re /v'.svx tin- more refrangible rays appear.

I took from the fire a piece of burning anthracite coal
—the fuel ordinarily used in domestic economy in New
York — and which from its compactness, the intense heat
it evolves, and other properties, appears to be well fitted
for these investigations. This coal was placed on a
support so as to present a plane surface to the slit in
the metal screen. The rays coming from it and passing
the slit were received on the flint-glass prism, and viewed
through the telescope.

When the coal was first taken from the fire, and was
burning very intensely, on looking through the telescope
all the colored rays of the spectrum were seen in their
proper order. I had previously passed through the slit
a beam of sunlight reflected from a mirror, so as to have
a reference spectrum with fixed lines. Now when the
coal was burning at its utmost vigor, the spectrum it
gave did not seem to differ, either as respects length or
the distribution of its colors, from the spectrum of sun-

light; but as the combustion declined and the coal

~ *

burned less brightly, its spectrum became less and less,
the shortening taking place first at the more refrangible
extremity, one ray after another disappearing in due
succession. First the violet became extinct, then the
indigo, then the blue, then the green, until at last the
red, with an ash-gray light occupying the place of the
yellow, was alone visible, and presently this also went
out.

From numerous experiments of this kind, I conclude
that there is a connection between the refrangibility of the
light which a burning body yields and the intensity of the
r/K mical action going on, and that the refrangibility al-
ways increases as the chemical action increases. It might,
pi-rhaps, be objected that, in the form of experiment here

58 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

introduced, two totally different things are confounded,
and that the burning coal not only gives forth its rays
as a combustible body, strictly speaking, but also as an
incandescent mass.

To avoid this objection as far as possible, and also to
reach a much higher temperature than could have been
otherwise obtained, I threw a stream of oxygen gas on
that part of the anthracite which was opposite the slit ;
but my expectations were disappointed, for, instead of
the combustion being increased, the coal was actually
extinguished by the jet playing on it. I therefore re-
placed the anthracite with a flat piece of well-burned
charcoal, kindled at the portion opposite the slit, and
throwing a stream of oxygen on this part, the combus-
tion was greatly increased. A spectrum rivalling that
of the sunbeam in brilliancy was produced, all the col-
ors from the extreme red to the extreme violet being
present.

On shutting off the supply of oxygen, the combustion,
of course, declined, and while this was going on the vio-
let, the indigo, the blue, the green, etc., faded away in
succession. By merely turning the gas on or off, the
original colors could be re-established or made to decline.

O

It was very interesting to see with what regularity, as
the chemical action became more intense, the more re-
frangible colors were developed, and how, as it declined,
they disappeared in due succession; the final tint being
red and that ash-gray in the position of the yellow which
has been described in the preceding memoir.

In the form of experiment here made the combustion
is, of course, merely superficial ; and the rays come from
the charcoal not as an incandescent, but as a burning
body.

III. Of the constitution of flames; proving that they

MIM..IU II.] SI'KCTKr.M ANALYSIS OF FLAMES. 59

/x/.s/ of a series of concentric and differently colored

I regard the foregoing experiments as affording the
means of explanation of the much more complicated
phenomena of flames, and proceed to inquire whether
the principle I have just brought forward of the co-or-
dinate increase of refrangibility and of chemical action
will hold good; premising the experiments now to be
detailed with the following considerations.

All common flames, as is well known, consist of a thin
shell of ignited matter, the interior being dark, the com-
bustion taking effect on those points only which are in
contact with the air. From the circumstances under
which the air is usually supplied, this ignited shell can-
not be a mere mathematical superficies, but must have a
sensible thickness. If we imagine it to consist of a series
of cone-like strata, it is obvious that the phenomena of
combustion are different in each. The outer stratum is
in contact with the air, and there the combustion is most
perfect; but by reason of the rapid diffusion of gases
into one another, currents, and other such causes, the at-
mospheric air must necessarily pervade the burning shell
to a certain depth, and in the successive strata, as we ad-
vance inward, the activity of the burning must decline.
On the exterior stratum oxygen is in excess, at the in-
terior the combustible vapor, and between these limits
there must be an admixture of the two, which differs
at different depths. Admitting the results of the fore-
going experiments with anthracite coal and charcoal to
be true, viz., that as the combustion is more active, rays
of a higher degree of refrangibility are evolved, it fol-
lows that each point of the superficies of such a flame
must yield all the colors of the spectrum, the violet coming
from the outer strata, the yellow from the intermediate,
the red from those within. If we could isolate an ele-

60 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

mentary horizontal section of such a flame, it would ex-
hibit the appearance of a rainbow ring, and when those
compound rays are received on the face of a prism, the
constituent colors are parted out by reason of their dif-
ferent refrangibility, and the eye is thus made sensible
of their actual existence.

When thus by the aid of a prism we analyze the light
coming from any portion of the superficies of a flame, we
in effect dissect out in a convenient manner and arrange
together side by side rays that have come from different
strata of the burning shell. These, without the prism,
would have pursued the same normal path, and pro-
duced a commixed effect as white light on the eye, but
with it are separated transversely, and each becomes
perceptible.

It is immaterial whether we impute the light emitted
by an ordinary flame to the liberation of solid particles
of ca»'bon in an ignited condition, and becoming hotter
and hotter as they pass outwardly towards the surface,
or consider these particles to be in a state of combus-
tion. The experiments of an ignited wire in one case,
and of charcoal in presence of oxygen in the other, lead
to the same explanation. We are not to suppose that
it is simply a gas which is burning; we are examining
the light emitted by an incandescent solid — the carbon
particles that for the moment are set free.

(This explanation, that the luminosity of a flame is
due to the temporary extrication of solid carbon, was
given by Sir H. Davy. It has been called in question
by Frankland. Experiments and criticisms have since
been offered by Deville, Knapp, Stein, Blockmann, and
others, but Davy's theory still remains substantially un-
affected. This is the conclusion to which Heumann has
come in his recently published researches on luminous
flames (1876).)

MIMOIR II.] sl'KCTRUM ANALYSIS OF FLAMES. (Jl

It might be supposed that in the familiar instance of
an oil lamp, if we put any check on the supply of the
air, and thereby check the intensity of the combustion,
we ought to produce a flame emitting rays of light the
refrangibility of which becomes less and less, and which,
from their being originally white, should pass through
various shades of orange, and end in a dull red. But
tin- compound nature of the burning vapor interferes
with that result; for when a certain point is gained the
hydrogen, for the most part, alone burns, the carbon be-
ing set free as smoke, and such a flame cannot support
itself in strict accordance with the principle given.

We must, then, search for other conditions under which
carbon is found which are free from this difficulty. Two
at once present themselves: they are carbonic oxide
and cyanogen gas. In the former the carbon is already
united with half the oxygen required for maximum ox-
idation : its complete combustion can therefore be carried
on with a limited supply of atmospheric air; in the lat-
ter the carbon is united with nitrogen, which during the
combustion is set free, and interferes with the process by
cutting off the more complete access of the atmosphere.

In place of the burning coal of the former experiments
I substituted a jet-pipe through which the various gases
might be made to pass, and the rays emitted by their
flames enter the telescope after passing through the slit
and prism. In this arrangement the slit should be hori-
zontal and not vertical. So far from it being immaterial
which of the two positions is selected, very great advan-
tages arise from the former. If the slit be vertical, the
prism, it is true, will separate the constituent colors from
one another, but it fails to show their relative positions.
If it be horizontal, the relative positions of the different
colors can be demonstrated, and it can be proved that a
horizontal section of a flame is in reality, as has been al-

52 SPECTRUM ANALYSIS OF FLAMES. [MEMOIK II.

ready remarked, a colored ring, the red being the inner-
most color, and the violet outside; for if this be the
order in which the colors occur, the red ring must neces-
sarily have a less diameter than the green, and the green
less than the violet ; and when the prism, set in a hori-
zontal position, separates those colors from each other,
the sides of the resulting spectrum ought not to be par-
allel, but inclined to one another, the breadth being least
in the red and increasing towards the violet end. This
increasing breadth proves that the constituent colored
shells of the flame envelop each other, the violet being
outermost, and therefore broadest. This valuable indi-
cation would be wholly lost if the slit were vertical.

This being understood, I may illustrate the facts now
to be brought forward by an example of the prismatic
analysis of a horizontal element of the flame of a spirit-
lamp ; it being understood that the prism is at its angle
of minimum deviation, and the spectrum seen through
the telescope. All the prismatic colors, in their proper
order, are visible ; the sides of the spectrum not being par-
allel, the inclination being quite rapid towards the red
extremity, the rays of which come from the interior of
the flame, where the diameter is less. Mere inspection is
sufficient to show the rapid approach of the red sides to
each other, and I satisfied myself that even in the more
refrangible regions there is the same want of parallelism,
by rotating the telescope on its vertical axis so that the
vertical wires in its eye-piece might coincide with first
one and then the other side of the spectrum. It will be
understood that I took the proper precaution not to be
deceived by a partial want of achromaticity in the tele-
scope, which might have led to a mistake.

But further, the yellow space of such a spirit -flame
spectrum is crossed by a bright fixed line — Brewster's
monochromatic ray. It is a beautiful example of the

MI.M..IK II.] Sl'ECTHUM ANALYSIS OF FLAMES. <;;',

principles just pointed out in this method of horizontal
analysis, being of much greater width than the rest of
the spectrum, ami recalling to the imagination the ap-
pearance of Saturn's ring when nearly closed and seen
through a telescope of moderate power. This ray, from
its superior breadth, must necessarily come from that
pale, tawny light which invests the bright part of the
flame. This, which is readily seen when the flame is
large, envelops the middle and upper parts, but cannot
so easily be detected low down. It is to be attributed
to the carbonic acid and steam that have risen at a high
temperature in the burning shell, and are escaping at a
degree above that of incandescence into the air, and are
mingled with oxygen diffusing from the air into them.
A similar tawny cloak surrounds the upper part of the
flame of a candle ; it answers to the oxidizing flame of
the blow-pipe, and yields Brewster's monochromatic yel-
low light.

(A few years subsequently this yellow ray was dis-
covered by Swan to be due to sodium. It is now known
as the lines D. At the date of this memoir it was not
suspected that sodium is so universally diffused.)

IV. Explanation of the nature of colored flames, show-
ing, for example, why carbonic oxide burns blue, and cy-
anogen red.

To return now to carbonic oxide and cyanogen. Fig.
8, No. 1, represents the solar spectrum with its fixed
lines ; No. 3 represents the spectrum of carbonic oxide
burning in the air. It begins in the red region short of
the fixed line C, and terminates between the lines G and
H. It yields, therefore, rays of every color, and this in
accordance with the principles we have laid down ; but
when the relative quantity and force of the rays are es-
timated in comparison with the sunlight spectrum, the

SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

Spectra of Various Flames.

J_

1 1

! Solar
| spectrum.

1

I Spirit-lamp.

1 Carbonic oxide.

j

UUUUUL

nnnnnr

Illllll Illl

Cyanogen In air.

Illllll Illl

11 11

j Cyanogen
j tn oxygen.

j Oil-lamp In air.

I

iOll-Iamp
ilu oxygen.

I Hydrogen In oxygen.

DOOOOt
[

m g mb i Nitrate of etrontlan.

] D D

Brewster's yellow ray.

Fig

1 Blowpipe cone.

.8.

red and orange are deficient, and the more refrangible
colors predominate, and, indeed, it is the excess of these
that, gives the flame its characteristic blue tint. This
agrees with what has been observed as to anthracite and

O

charcoal ; for with carbonic oxide a limited supply of
oxygen can bring about the maximum chemical action,
and therefore liberate in abundance rays of maximum
refrangibility.

This condition of things is inverted in the case of cy-
anogen. It is the nature of its flame to be enveloped, as
it were, in a sheet of nitrogen arising from its own burn-
ing, and this necessarily impedes the access of air and
checks the intensity of the chemical change: a check
which is at once betokened by the emission of a predom-
inant number of rays of low refrangibility or of a red
color.

But there is a striking difference in the chemical con-
ditions under which carbonic oxide and cyanogen burn.
In the case of the former the whole gas is combustible,
in the latter the carbon alone, and we have, in reality,

MKMOIU II.] SPECTRUM ANALYSIS OF FLAMUS. £5

introduced an incombustible element into the flame; for
as the carbon burns the incombustible nitrogen is set
free. It occurred to me, in selecting the gas for experi-
mriit, that this condition should impress a physical char-
acteristic on the flame. I thought it was not impossible
that dark lines in its spectrum might be the result, be-
cause there must be a peculiar arrangement of the burn-
ing strata which together make up the shell of the flame,
every two atoms of carbon setting free one of nitrogen.
I did not know, until subsequently, that this flame had
already been examined by Faraday. Having therefore
confined some cyanogen, made from cyanide of mercury,
in a glass gas-holder filled with a saturated solution of
common salt, I burned it from the jet-pipe, and found
that what I had surmised was actually the fact. There
was a spectrum so beautiful that it is impossible to de-
scribe it by words, or depict it in colors. It was crossed
throughout its extent by black lines, separating it into
well-marked divisions. I could plainly count four great
red rays of definite refrangibility, followed by one or-
ange, one yellow, and seven green rays ; while in the more
refrangible spaces were two extensive groups of black
lines, recalling somewhat from their position, but greatly
exceeding in extent, Fraunhofer's lines G and H in the
sun-rays. I shall return to the consideration of this
spectrum and to the relation of fixed lines presently,
here only making the remark that the burning of cyan-
ogen, both as respects the color of the light and the oc-
currence of fixed lines, is a direct consequence of the
principle I am establishing.

The unassisted eye detects two well-marked regions
in the cyanogen flame : a greenish-gray stratum on the
outside, and a lilac-colored nucleus writhin. Decomposed
by the prism, a horizontal element of this flame shows
that the exterior shell contains all the prismatic colors,

E

(]g SPECTRUM ANALYSIS OF FLAMES. [MEMOIK II.

except, perhaps, the yellow; but the green, the blue, and
the violet greatly predominate. The interior lilac flame
is the source of the bright spectrum with fixed lines just
described.

V. Continuation of the same principle in the case in
which combustion is carried on in oxygen gas instead of
atmospheric air.

If the principle that high refrangibility is connected
with intense chemical action be true, it must hold good
when the nature of the atmosphere in which the burning
is carried forward is chano-ed. If, instead of beino: the

O o

common air, it is oxygen gas, we ought to be able to
foresee the result. Carbonic oxide, when made to burn
in that gas, should not change its tint ; because if the air
can carry on the process to its maximum effect, oxygen
can do no more. But the result should be just the re-
verse with cyanogen, which, if made to burn in oxygen,
should be capable of emitting rays of higher refrangi-
bility.

Foreseeing this result, I submitted the two gases to
experiment, and first arranged the carbonic oxide so
that its spectrum might be examined in the telescope
as already described; then causing a clean bell -jar full
of oxygen to be inverted over it, the flame diminished
somewhat in size, emitted a slight crackling sound, but
retained its color unchanged. Its spectrum appeared pre-
cisely the same both as respects extent and the distribu-
tion of color, whether the burning took place in oxygen
gas or in atmospheric air.

If cyanogen be made to burn in oxygen, we should
expect that it would lose to a great extent its character-
istic lilac tint, and emit a whiter lio;ht. It was therefore

O

very interesting to find that the moment the flame was
immersed in oxygen it lost much of its pinkish color,

MKMOIK II.] M'l'.i Till M ANALYSIS OF FLAMKs. (57

ami became of a dazzling brilliancy; and on examina-
tion through tlie telescope, though all the colors had
increased in brightness, the most remarkable effect took
place among the extreme refrangible rays. Far out of
the limits of the ordinary spectrum a ray of great purity
and force was developed, as represented in the Fig. at
No. 5. Its color is violet.

I have made similar experiments on many other flames
Besides those here mentioned. It is not necessary to re-
late them in detail, for they give the same results. In
every instance of combustion in the air, when the flame
is bright enough, all the colors are visible; and when
the combustion takes place in oxygen they are increased
in intensity. With hydrogen gas and alcohol the light
is so feeble that the eye cannot catch the terminal rays ;
but as soon as the combustion is made in oxygen the
red and the violet both appear, the latter, however, pre-
dominating. Several of these spectra both in air and
oxygen are represented in Fig. 8. In No. 9, the letters
in // and m b indicate a maximum of green and blue
lirHit in the form of bright lines.

O O

It does not require the use of a prism to satisfy one's
self of the chancre of tint that flames exhibit when the

O

chemical action increases. In reality it is only necessary
to compare by eye-sight the color of the light emitted
in air and in oxygen gas. In the latter case rays of a
higher refrangibility uniformly arise.

On the evidence furnished by the foregoing experi-
ments I regard a common flame as consisting of a shell
of ignited matter in which combustion is going on with
different degrees of rapidity at different depths, being
most rapid at the exterior, where there is a more perfect
contact with the atmosphere, and diminishing inward.
In a horizontal section, the interior space consisting of
unlmrned vapor is black; this is surrounded by a ring

68 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

where the combustion is incipient, and from which red
light issues; then follow orange, yellow, green, blue,
indigo, and violet circles in succession, the production
of each of these tints being dependent on the rapidity
with which chemical action is going forward — that is,
on the amount of oxygen present — the tints gradually
shading off into one another and forming, as I have said,
a circular rainbow. An eye placed on the exterior of
such a flame sees all the colors conjointly, and from their
general admixture arises the predominant tint.

An examination of the flame of a candle vertically
confirms this conclusion, for the red projects on the top
of the flame, and the blue towards the bottom.

From this, which may be regarded as the normal
flame, the flame of cyanogen differs. It must consist of
as many concentric shells as the prism separates it into
regions of definite refrangibility. Its interior part is
tlu lefore divided into four red layers, followed by one
of orange, one of yellow, seven of green, etc. There are
two great inactive spaces towards the outside of the
flame, corresponding to the two great groups of fixed
lines. Perhaps through all these inactive parts the in-
combustible nitrogen chiefly escapes.

VI. Effects of tlie introduction of air into the interior
of a flame, producing the destruction of the red and
orange strata, and converting them into violet.

It now becomes a curious subject to determine what
takes place when an ordinary flame is disturbed by the
introduction of air into its interior. When a blow-pipe
jet is thrown through the flame of an oil-lamp, the sharp
blue cone which forms indicates, on the principles here
set forth, that the combustion is much more active. But
if the colors of the common flame come from different
depths, the red being the innermost, it is clear that the

MIM..II: II.] SPECTRUM ANALYSIS OF FLA MIX gfl

introduction of a jet of air by a blow-pipe should make
the combustion rapid where before it was slowest, and
tin- less refrangible colors ought to be destroyed. A
prismatic analysis should exhibit the spectrum of a
blow-pipe flame without any red or orange.

In this examination no slit was required, as in the
former experiments, for the cone itself, when at a dis-
tance of six or eight feet, was narrow enough for the
purpose. It yielded a very extraordinary spectrum. As
I anticipated, all the red rays were gone; not a vestige
of either them or of the orange could be found. But
the spectrum was divided into five well-marked regions,
separated from one another by dark spaces. There were
five distinct images of the blue cone : one yellow, two
green, one blue, and one violet. In Fig. 9 this result is
represented.

This experiment may be verified without a telescope.
On looking through a prism, set horizontally at its angle
of minimum deviation, at a blow-pipe cone some six or
eight feet distant, there will be seen a spectrum of that
part of the flame which does not join in the production
of the blue cone. It contains, of course, all the prismatic
colors. But projecting from this are five colored images
of the cone — one yellow, two green, one blue, and one
violet. They are entirely distinct from one another, and
are parted by dark spaces.

Such is the effect of introducing air into the interior
of a flame, and destroying those
strata that yield the red and
orange colors. The effect of a
blow -pipe is to produce two
>trata of blue light, one being
external, the other internal; also
two strata of green, one again
external, the other internal, and the escaping products

70 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

of combustion, steam and carbonic acid, mingled with
atmospheric air, constitute the oxidizing flame, which
envelops the blue cone, and emits Brewster's mono-
chromatic yellow light. That the yellow light comes
from this flame is proved by the greater length of its
image.

VII. Physical cause of the production of light by chem-
ical action .

Do not the various facts here brought forward prove
that chemical combinations are attended by a rapid
vibratory motion of the particles of the combining
bodies, which vibrations become more frequent as the
chemical action is more intense?

The burning particles constituting the inner shell of
a flame are executing about four hundred billions of
vibrations in one second ; those in the middle about
six hundred billions, and those on the exterior, in con-
tact with the air, about eight hundred billions in the
same time. The quality of the emitted light, as re-
spects its color, depending on the frequency with which
these vibrations are accomplished, increases in refran-
gibility as the energy of the chemical action becomes
greater.

The parts of all material bodies are in a state of
incessant vibration ; that which we call temperature de-
pends on the frequency and amplitude of these vibra-
tions conjointly. If by any process, as by chemical
agencies, we increase that frequency to between four
and eisfht hundred billions of vibrations in one second,

O 7

ignition or combustion results. In the case of the for-

O

mer of these numbers, the temperature is 9Y7° Fahr.
At this temperature the waves propagated in the ether
impress the organ of vision with a red light. This also
is the temperature of the innermost shell of a flame. If

Mi sum: II.] Sl'ECTRJJM ANALVMS ()F FLAMKS. ^\

tin- frequency of vibration still increases, the temperature
correspondingly rises, and the light successively becomes
oraiiiM-, yelloNv, green, blue, etc., and this condition ob-
tains in the successive strata of a flame, as we pass from
its interior to its exterior surface.

The general principle at which I thus arrive, as the
result of this experimental investigation, viz., that there
is a connection between the energy with which chemical
affinity is satisfied and the refrangibility of the resulting
light, assumes the position of a simple consequence of
the undulatory theory. Is it not very natural, if all
chemical changes are attended by vibratory motions in
the particles of the bodies engaged, that those vibrations
should increase in frequency as the action becomes more
violent ? But an increased frequency of vibration is the
same thing as an increased refraugibility.

VIII. On the physical cause of Fraunliofer'1 s dark lines.

Although I have extended this memoir to so great a
length, I have omitted many facts which have been
made the subject of experiment. I cannot conclude,
however, without offering some remarks on the artificial
production and cause of Fraunhofer's fixed lines.

It has been stated that I was led to expect the pro-
duction of these lines in the flame of cyanogen, from
considering the circumstances under which its combus-
tion takes place. Returning to this phenomenon, I
shall here point out a very remarkable numerical re-
lation existing among the fixed lines of the solar spec-
trum.

The following table contains Fraunhofer's determina-
tion of the wave-lengths of the seven great fixed lines
of the spectrum, which are designated by the capital
letters of the alphabet from B to H. I have added the
wave-length of A from my own experiments.

72

SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II.

Table of wave-lengths corresponding to the eight great fixed lines of
the solar spectrum, the Paris inch being supposed to be divided into
one hundred millions of equal parts.

A = 26GO
B = 2541
C = 2422
D = 2175

E = 1945
F = 1794
G = 1587
H = 1464

An examination of this table proves that —

The wave-length of B is 119 parts less than A ;
C 238

238
485
715
866
1073
1196

and these differences of length are obviously very nearly
as the whole numbers 1, 2, 4, 6, 7, 9, 10. This coincidence
is far too striking to be merely accidental. Moreover,
it must not be forgotten that the observed numbers as
determined by Fraunhofer are wholly independent of
any hypothesis.

If the relation of whole numbers were rigorously true,
the numbers in the foregoing table would stand as fol-
lows: 119, 238, 476, 714, 833, 1071, 1190.

The wave-length of the most luminous portion of the
spectrum, the centre of the yellow space, is 2060 parts.
If we take this as an optical centre, it will be found that
the great lines are situated symmetrically in relation to
it. E and D are equidistant above and below it; the
same observation applies to G and B, and also to H and
A. The only departure from this symmetry is in the
case of F, which is not symmetrical with C. It will be
understood that I am here speaking of one of those
spectra which are formed when a grating or ruled sur-
face is used. In this the colors are arranged side by
side, according to their wave-length, the centre of the
spectrum, which is its most luminous portion, is occupied

MIN , II.] sri.CIIM M ANALYSIS OF II.AMKS. 73

by the centre of the yellow sj.. •!<•<•, ;m<l the light termi-
nates at equal distances in the violet and red.

Do not these observations lead us to conclude that
the cause, whatever it may be, which produces these
fixed lines is periodic in its action?

What that cause in reality is we have not now facts
sufficient to determine. I would not affirm that the
disengagement of incombustible matter by a flame will
always give rise to dark lines. But this is very clear,
that in all those cases, as cyanogen, alcoholic solutions
of nitrate of strontia, of boracic acid, etc., in which these
lines are developed, incombustible matter is uniformly
disengaged.

UNIVERSITY OF NEW YORK, Dec. 25, 1847.

74; INVISIBLE LINES IN THE SUN'S SPECTRUM. [MJSMOIK HI.

MEMOIR III.

ON INVISIBLE FIXED LINES IN THE SUN's SPECTRUM DE-
TECTED BY PHOTOGRAPHY.

From the Philosophical Magazine, May, 1843.

CONTENTS: — The photography of Fraunhofer* s fixed lines. — The lines in
the red due to absorption by the earth's atmosphere. — Nomenclature of
the Fraunhofer lines. — Discovery of the invisible fixed lines. — Origi-
nal map of them. — The new ultra spectral red lines a, /3, y. — Redis-
covered by Foucault and Fizeau. — M. E. BecquereVs discovery. — Ex-
periments in 1834.

WHEN a beam of the sun's light, directed horizontally
by a heliostat, is admitted into a dark room, and, passing-
through a slit with parallel edges, is received on the sur-
face of a flint-glass prism, which retracts it at the angle
of minimum deviation, and, after its passage through the
prism, is converged to a focal image on a white screen
by the action of an achromatic lens, the resulting spec-
trum is given in great purity, and Fraunhofer's lines are
very distinct. If a photographic surface be set in the
place of the white screen, it will exhibit the representa-
tion of multitudes of dark lines, varying greatly in di-
mensions.

After several attempts last summer, I succeeded in
discovering these lines, and have obtained impressions
of them sufficiently perfect.

Before proceeding to the description of the mode to be
followed, and of the characters of the lines themselves, I
cannot avoid calling attention to the remarkable circum-
stance which has frequently presented itself to me of a

Mi-Mum III.J INVISIBLE LINES IN THE SUN'S SPECTRUM. 75

change in the relative visibility of Fraunhofer's
lines when seen on different occasions. There are times
at which the strong lines seen in the red ray are so fee-
ble that the eye can barely catch them, and then again
tht'V come out as dark as though marked with India-ink
on the paper. During these changes the other lines may
or may not undergo corresponding variations. The same
remark applies equally to the blue and yellow rays. It
has seemed to me that the lines in the red are more visi-
ble as the sun approaches the horizon, and those at the
more refrangible end of the spectrum are plainer in the
middle of the day.

(I subsequently substantiated this remark, and satis-
fied myself that many of the lines in the red are due to
absorption by the earth's atmosphere, and therefore more
distinct with a rising or setting sun. Those in the more
refrangible regions, the blue, the indigo, and the violet,
are due to absorption by the atmosphere of the sun.)

A sunbeam, passing horizontally from a heliostat mir-
ror into a dark room, was received on a metal plate with
a slit in its centre, the slit being formed by a pair of
parallel knife edges, one of which was movable by a
micrometer screw, the instrument being, in fact, the com-
mon one used for showing diffracted fringes. The screw
was adjusted so as to give an aperture ^ inch wide, and
the light passing through fell upon an equiangular flint-
glass prism placed at a distance of eleven feet. Imme-
diately on the posterior face of the prism the ray was
received on an achromatic lens, the object-glass of a tele-
scope, and brought to a focus at the distance of six feet
six inches, at which an arrangement was adjusted for ex-
posing white paper screens, on which the greater fixed
lines might be seen, or sensitive plates substituted for
the screens, occupying precisely the same position. The
lines on the screens could, therefore, be compared with

76 INVISIBLE LINES IN THE SUN'S SPECTRUM. [MEMOIR III.

those on the sensitive surfaces as to position and magni-
tude with considerable accuracy.

In order to identify these lines I have made use of
the map of the spectrum published by Professor Powell
in the Keport of the British Association for 1839. With
the apparatus, as above described, they are exceedingly
distinct ; no difficulty arises in the identification of the
more prominent ones. The spectrum with which I have
worked occupies upon the screen a space of nearly four
inches and a quarter in length from the red to the violet,
or, more correctly speaking, from the ray marked in that
map A to the one marked &. In stating, however, that
no difficulty arises in identifying these lines, I ought to
add that I am referring to that particular map. In
the figure annexed to Sir John Herschers "Treatise on
Light," in the " Encyclopaedia Metropolitana," the rays
marked Gr seem to differ from that in the report. But
Professor Powell's map being drawn from his personal
observations, with reference to these very difficulties, and
as it agrees with my own observations and measures, I
have employed it, and therefore take the letters he gives.
(I may add that in all the earlier of these memoirs his
nomenclature of the fixed lines is used. It differs a lit-
tle from that now employed by spectroscopists.)

It will be understood that the whole spectrum and all
its lines cannot be obtained at one impression. The dif-
ficulty is that the different regions of the spectrum act
with different power in producing the proper effect.
Thus, if on common yellow iodide of silver the attempt
were made to obtain all the lines at one trial, it would
be found that the blue region would have passed to a
state of high solarization, and that all its fine lines were
extinguished by being overdone long before any well-
marked action could be traced in the less refrangible ex-

o

tremity. It is necessary, therefore, to examine the differ-

Mi M. .tit HI.] INVlSlMLi; LINKS IN TIIK SUN'S SPECTRUM.

ent regions in succession, exposing the
>niMti\r surface to each for a suitable
li'iiirth of time.

O

In Fig. 10 I have given on the left
siilt- a representation of the larger Hues
of Fraunhofer; the right side gives them
as obtained on a daguerreotype plate
which has been iodized to a yellow,
brought by the vapor of bromine to a
red, and then slightly exposed to the
vapor of chloride of iodine. The pho-
tograph is so adjusted as to have its
lines by the side of those of Fraunhofer
which have the same name. It will be
seen that there are beyond the red ray
three extra spectral lines, which I have
marked a, j3, 7. These, however, I have
only occasionally found, for from the
general diminution of effect in that re-
gion they do not always come out in a
plain and striking manner. None of
Fraurihofer's lines in the yellow and
green are given, but G and its compan-
ions are very strongly impressed, as also
the group about i. But by far the most
striking in the whole photograph are
those marked H and k. Then passing
beyond the violet and out of the visible
limits of the spectrum, four very strik-
ing groups make their appearance.

Fig. 10.

To the first line of
each of these, in continuation of Fraunhofer's nomencla-
ture, I gave the designations M, N, O, P. In I there are
three lines, in M eight, in N three, in O four, and in P
five.

Besides these larger groups, the photographs were

78 INVISIBLE LINES IN THE SUN'S SPECTRUM. [MEMOIR III.

crossed by hundreds of minuter lines, so that it was im-
possible to count them. If nearly six hundred have been
counted between A and H, I should think there must be
quite as many between H and P. In speaking of these
lines as though they were strong individual ones, the
statement is to be taken with some limitation. It is
quite likely that each of these bolder lines is made up
of a great number that are excessively narrow and close
together.

If the absorptive action of the sun's atmosphere be the
cause of this phenomenon, that action must take place
much more powerfully on the more refrangible and ex-
tra-spectral region. The lines exhibited there are bold
and strongly developed ; they are crowded in groups to-
gether.

(Scarcely was the paper from which the foregoing ex-
tracts are made published in the Philosophical Magazine,
when -I learned that in France M. E. Becquerel had al-
ready photographed the more refrangible lines, and pub-
lished statements to that effect. But he had not ob-
served those in the less refrangible regions, designated
by me a, )3, 7.

In fact, the process I was using was one I had recently
discovered: it consisted in permitting the daylight to
fall along with the sun rays on the photographic surface.
The daylight and the sunlight antagonized each other,
and these hitherto undiscovered lines made their appear-
ance as positive photographs. The peculiarities of this
singular and interesting process I will describe hereafter
in one of these memoirs.

In 1846, MM. Foucault and Fizeau, having repeated the
experiment thus originally made by me, presented a com-
munication to the French Academy of Sciences. They
had observed the antagonizing action above described,
and had seen the ultra spectrum heat lines a, j3, 7. They

MIM..IU III.] IN VISIT, I. K LINKS IN THK SUN'S SPECTRUM. 79

had taken the precaution to deposit with the Academy
a scaled envelope containing an account of their discov-
ITV, not knowing that it had been made and published
long previously in America.

Hereupon M. E. Becquerel communicated to the same
Academy a criticism on their paper. In this he remarks:
"M. Draper, in examining the image produced by the
action of the spectrum on plates of iodized silver, an-
nounced before those gentlemen the existence of protect-
ing rays antagonizing the action of the solar rays, and
even acting negatively on iodide of silver." He strength-
ened his views by adding some observations that had
been made by Sir J. Herschel, who did not assent to the
existence of this protecting action, but thought that the
daguerreotype impressions could be explained on New-
ton's theory of the colors of thin plates.

Herschel had made some investigations on the distri-
bution of heat in the spectrum, using paper blackened on
one side and moistened with alcohol on the other. He
obtained a series of spots or patches, commencing above
the yellow and extending far below the red. Some writ-
ers on this subject have considered that these observa-
tions imply a discovery of the lines a, )3, 7 ; they forget,
however, that Herschel did not use a slit, but the direct
image of the sun — an image which was more than a

o o

quarter of an inch in diameter, as I know from the speci-
mens he sent me, and which are still in my possession.
Under such circumstances it was physically impossible
that these or any other of the fixed lines should be seen.
As I had thus been unsuccessful in obtaining impres-
sions of the fixed lines D and E — once only I thought I
perceived a line corresponding to Frauuhofer's F, but it
was exceedingly faint and on the whole doubtful — I sup-
posed that this furnished an argument for the physical
independence of the luminous and actinic rays, as they

gO INVISIBLE LINES IN THE SUN'S SPECTRUM. [MEMOIR III.

were subsequently called. The lines D, E, F were, how-
ever, afterwards photographed by my son, Henry Draper,
and so the argument fell to the ground.)

In 1834, when my attention was first drawn to these
subjects, and I began to make prismatic analyses by the
aid of sensitive paper, some of my earliest attempts were
directed to the detection of these fixed lines. At that
time I was employing sensitive paper, made with bro-
mide of silver, precisely as has been subsequently done
in Europe — a number of the results were published in
the American journals during the year 1837. In the de-
tection of the fixed lines I failed at that time entirely ;
but the bromuretted paper enabled me at that early pe-
riod, when the attention of no other chemist was as yet
turned to these matters, to trace the blackening action
from far beyond the confines of the violet, down almost
to the other end of the spectrum. I distinctly made out
that the dark rays underwent interference, after the man-
ner of their luminous companions, a result originally due
to Arago, and printed some long papers in proof of the
physical independence of the chemical rays and light and
heat throughout the spectrum.

MEMOIR IV.] CONDITION OF THE SUN'S SURFACE. 31

MEMOIR IV.

ON THE NATURE OF FLAME AND ON THE CONDITION OF
THE SUN'S SURFACE.

From the American Journal of Science and Arts, Second Series, Vol. XXVI., 1858 ;
Philosophical Magazine, Feb., 1858.

CONTENTS: — Dove's experiments on electric light. — Dark lines replaced
by bright ones. — Electric spark between metallic surfaces. — The lines
depend on the chemical nature of the substance from which the light is-
sues.— They may be used for determining the physical condition of the
sun and stars. — Three hypotheses of the condition of the surfs surface
examined.

AMONG the more recent publications on Photo-chem-
istry, there is one by Professor Dove on the electric light
(Philosophical Magazine, Nov., 1857) which will doubt-
less attract the attention of those interested in that
branch of science. Examination by the prism, and by
absorbing and reflecting colored bodies, leads him to the
conclusion that it is necessary to consider the luminous
appearance as having two distinct sources: 1st, the igni-
tion or incandescence of the material substances bodily
passing in the course of the discharge ; 2d, the proper
electrical light itself. As respects the former, he illus-
trates its method of increase from low to high tempera-
tures by supposing a screen to be withdrawn from the
red end of the spectrum through the colored spaces suc-
cessively towards the violet ; and that of the latter from
the bluish brush to the bright Leydeii spark, by a like
screen drawn from the violet towards the red.

The true electric light exhibits properties resembling
those observed in actual combustions, as though there

F

82 CONDITION OF THE SUN'S SURFACE. [MEMOIR IV.

were an oxidation of a portion of the translated matter
when the spark is taken in air. The order of evolution
of rays in this instance happens to be the same as in the
second illustration of Professor Dove, that is, from the
violet to the red. There are certain facts connected
with these appearances of color which are not generally
known, and deserve to be pointed out.

(I then give an abstract of the preceding memoir, and,
after speaking of the production of dark lines in the cy-
anogen flame, continue as follows :)

In other cases dark lines are replaced by bright ones,
as in the well-known instance of the electric spark be-
tween metallic surfaces. The occurrence of lines, whether
bright or dark, is hence connected with the chemical nature
of the substance producing the flame. For this reason
these lines merit a much more critical examination than
has yet been given to them, for by their aid we may be
able-to ascertain points of great interest in other depart-
ments of science. Thus if we are ever able to acquire
certain knowledge respecting the physical state of the
sun and other stars, it will be by an examination of the
light they emit. Even at present, by the aid of the few
facts before us, we can see our way pretty clearly to cer-
tain conclusions respecting the sun. For since substances
which are incandescent, or in an ignited state, through
the accumulation of heat in them, show no fixed lines,
their prismatic spectrum being uninterrupted from end
to end, it would appear to follow that the luminous con-
dition of our sun, whose light contains fixed lines, cannot
be referred to such incandescence or ignition. At vari-
ous times those who have studied this subject have of-
fered different hypotheses: one regarding the sun as a
solid or perhaps liquid mass in a condition of ignition ;
another considering the light to be electrical; a third
supposing him to be the seat of a fierce combustion.

MIM..IU IV.] CONDITION OF THE SUN'S SUKFACE. ,s;{

Of sucli hypotheses we have given reasons for declining
thr first. Prismatic analysis, which demonstrates no re-
semblance between the light of the sun and that of any
form of electric discharges with which we are familiar,
enables us in like manner to reject the second ; and, upon
the whole, facts seem most strongly to prepossess us in
favor of the third ; in artificial combustions similar fixed
lines being observed. If such is to be regarded as the
physical condition of the sun, we can no longer contem-
plate him as an immense mass slowly and tranquilly
cooling in the lapse of countless centuries by radiation
into space, as so many considerations drawn from other
I tranches of science have hitherto led us to suppose, but
he must be regarded as the seat of chemical changes
going on upon a prodigious scale, and with inconceiv-
able energy.

If the law designated above, that the more energetic
the chemical action in combustion the more refrangible
the emitted light, be translated into the conceptions of the
undulatory theory, it not only puts us in possession of
a distinct idea of the manner in which the combustive
union of bodies is accomplished, the quickness of vibra-
tion increasing with the chemical energy, but it also en-
ables us to transfer for the use of chemistry some of the
most interesting numerical determinations of optics.

UNIVERSITY OF NEW YORK, Dec. 10, 1857.

NOTE. — I have thus presented the four preceding
memoirs as early contributions to the history of spec-
trum analysis, and applications of spectroscopic researches
to solar physics or astronomical problems. I think that
perhaps they will not be less interesting to the scien-
tific reader from the circumstance of their imperfections.

84 CONDITION OF THE SUN'S SURFACE. [MEMOIR IV.

He will not look upon them from the present elevated
point of view, but regard them as results gathered with
much labor by a pioneer — results that have had some-
thing to do with the development of the subject.

The history of science shows that there have some-
times been occasions on which one investigator, writing
at an opportune moment, has carried off from his pred-
ecessors all the credit they were entitled to, and, per-
haps without any definite intention on his part, the
world has unjustly awarded it to him. In America, as
the foregoing memoirs show, some attention had been
paid to the use of the spectroscope long previously to
the time when M. Kirchoff occupied himself with the
subject.

Thirteen years after the publication of the first of
these memoirs, M. Kirchoff (1860), in a memoir regarded
at that time as the origin of spectrum analysis, and en-
titled, " On the Relation between the Radiating and Ab-
sorbing Powers of Different Bodies for Light and Heat,"
published, under the guise of mathematical deductions,
many of these facts as discoveries of his own. This
memoir appeared in German in PoggendorfFs Annalen,
Vol. CIX., p. 275, and was translated into English in the
Philosophical Magazine, July, 1860.

Among these deductions are the following. I quote
M. Kirchoff's own language :

" If a body (a platinum wire, for example) be gradu-
ally heated up to a certain temperature, it only emits
rays consisting of waves longer than those of the visible
rays. Beyond that point waves of the length of the ex-
treme red begin to appear, and as the temperature rises,
shorter and shorter waves are added, so that, for every
temperature, rays of a corresponding length of wave are
originated, while the intensity of the rays of greater
wave-length is increased.

V.] CONDITION «)!•' 111H SUN'S SURFACE. .s;,

" Whence, applying the same proposition to other
1 indies, it follows that all bodies, when their tempera-
ture is gradually raised, begin to emit waves of the same
length at the same temperature," etc. (Draper, Phil.
Mag., Vol. XXX., p. 345. Berl., 1847.)

"For the same temperature the magnitude (I) is a
continuous function of the wave-length, except for such
values of the latter as render (I) evanescent. The truth
of this assertion may be concluded from the continuity
of the spectrum of a red-hot platinum wire, provided it
be admitted that the power of absorption of such a body
is a continuous function of the length of the waves of
the incident rays."

These, together with other facts, were presented by M.
Kirchoff, not as experimental, but as mathematical results.
No allusion was made to the fact that the whole subject
had been extensively investigated, as shown in the pre-
ceding pages, many years before, the only reference to
such investigation being; that contained, as shown above,

O O '

in a parenthesis, which in the original is in a foot-note,
and even this was omitted in an historical memoir on the
subject shortly afterwards published by M. Kirchoff.

As an example of the effect of this, I may quote from
the Cours de Physique de FJZcole Poly technique, of Paris,
by Professor Jamin :

"M. Kirchoff has deduced the following important
consequences :

" Black bodies begin to emit at 977° Fahr. red radia-
tions, to which are added successively and continuously
other rays of increasing refrangibility as the temperature
rises.

" All substances begin to be red-hot at the same tem-
perature in the same enclosure.

" The spectrum of solids and liquids contains no fixed
lines."

gg CONDITION OF THE SUN'S SURFACE. [MEMOIR IV.

Subsequently, in the Philosophical Magazine (April,
1863), a memoir appeared; under the title of "Contribu-
tions toward the History of Spectrum Analysis and of
the Analysis of the Solar Atmosphere," by M. Kirchoff.
In this all allusion to the foregoing memoirs is avoided.

MIM..IHV.] INTEUFEKENCE OF RADIATIONS.

MEMOIR V.

ON THE NEGATIVE OB PROTECTING RAYS OF THE SUN.

From the Philosophical Magazine, Feb., 1847.

CONTENTS: — Original discovery of protecting radiations. — Case of a
daguerreotype plate. — Spectrum-photographs made in Virginia. — Pro-
tecting action of the less refrangible rays. — Protecting action of the
extreme violet. — Variations of the protecting action. — Spectrum of
darkness and spectrum of daylight. — Interference of rays of different
colors. — Action of waves of red, yellow, and violet light with wave-
lengths 2, l£, 1. — Use of the diffraction spectrum.

IN a letter published in the Philosophical Magazine
Nov., 1842, I had occasion to make some incidental re-
marks respecting a class of rays existing in the sunlight
which have the quality of exerting a negative or antag-
onizing action upon those engaged in producing daguerre-
otype results.

In October last, MM. Foucault and Fizeau having
made a communication to the French Academy of
Sciences to a similar effect, and M. Edniond Becquerel,
in criticising their results, having referred to me as the
original author of the fact, I may on this occasion be
excused for offering a few observations on this, which
perhaps is destined to become one of the most important
phenomena in relation to the chemical action of the sun-
light.

That the opposite ends of the solar spectrum possess
opposite qualities is an idea which has been floating
among chemists for many years. The first distinct
statement in relation to it with which I am acquainted
occurs in a work published by Mr. B.Wilson, the second

gg INTERFERENCE OF RADIATIONS. [MEMOIR V.

edition of which dates as early as 1776. It is entitled
"A Series of Experiments on Phosphori." He shows
that it is the more refrangible rays which excite the
phosphorescence of sulphide of lime, but the less refran-
gible ones extinguish it when shining.

In 1801 Ritter found that chloride of silver which
had been blackened in the violet rays had its color par-
tially restored when placed in the red. He states also
that phosphorus, which is oxidized with the production
of fumes in the invisible red, is instantly extinguished
in the violet.

The well-known experiments of Wollaston with guaia-
cum- served to show the opposite relations of the red
and violet rays. It is remarkable that he subsequently
abandoned this interpretation of the phenomenon, on
discovering that green guaiacum changed its color by
the application of a hot silver spoon.

In 1839 Sir J. Herschel encountered the same action
in the case of some of the preparations of silver. His
first idea was that of a positive and negative polarity
of the spectrum ; but this was subsequently modified
for the reasons set forth in his memoir (Phil. Trans.,
1840, § 60, etc.).

In 1842 I had obtained some very fine daguerreotype
impressions of the solar spectrum in Virginia, and since
that time have never doubted the actual existence of
these negative or protecting rays; and on this occasion,
when that existence is reasserted by Lerebours, Fizeau,
and Foucault, I will make known certain new facts,
premising that I do not think the views taken by M.
Becquerel are correct. They are founded on what seems
to me to be a misapprehension of the phenomenon of
the daguerreotype.

A daguerreotype plate can exhibit three different
varieties of surface: 1st, a black aspect on those re-

V.J INTKHFKUKXCE OF HADIATIONS. fc<j

where it lias been unaffected by light; 2d, various
shades of white; 3d, a colored blackness, the tint of
which may be of a deep watch-spring lustre, or some-
times of an olive shade. Persons familiar with the proc-
ess will understand completely what I mean. The first
of these conditions is represented in the deep shadows
of such a photograph, the places where the light never
acted ; the second is exhibited in the various intensities
of whiteness, which constitute the figures of the picture,
the whiteness varying in intensity according to the in-
tensity of the light; the third is the solarized or overdone
condition, which arises from too long an exposure to the
rays. Like the first, this may be spoken of as a black-
ness, but in reality it is a dark green or blue or tawny
tint. It is this solarized condition of surface which M.
Becquerel confounds with the first, the blackness arising
from the unchanged state; and it is precisely on this
point that the whole argument turns. For the sake of
bavins distinctive words to mark out these three con-

O

ditious, I will call the first the unaffected state, the sec-
ond the white state, and the third the solarized state.

The observations I made in Virginia were as follows :
That if a solar spectrum be received on a daguerreotype
plate on which a weak daylight was simultaneously
acting, the red, orange, yellow, green, and part of the
blue rays arrested the action of the daylight on that
portion of the plate on which they fell, and maintained
it in the unaffected state; while the residue of the blue,
the indigo and violet, carried their part of the plate to
a completely solarized condition. This therefore seemed
to justify the assertion that the less refrangible rays
protect Daguerre's preparation from the action of a dif-
fused daylight.

It was also found that if the plate were exposed to
the daylight for a few seconds, so that had it been then

90 INTERFERENCE OF RADIATIONS. [MEMOIR V.

mercurialized it would have whitened uniformly all
over, on being made to receive the spectrum the less
refrangible rays actually carried it back to the unaffect-
ed condition, reversing what had been already done.
While the more refrangible rays were forcing it on to
the solarized state, these were returning it into the con-
dition of shadow : they therefore not only protect, but
seem even to exert a negative or antagonistic action.

Sir J. Herschel has critically examined one of these
specimens, and has suggested an explanation of their
appearance on the Newtonian principle of the tints ex-
hibited by thin films (Phil Mag., Feb., 1843). But I
found that it was immaterial whether the exposure to
the spectrum was for thirty seconds or one hour — the
result was the same. The final action had been pro-
duced, the less refrangible rays had carried their region
to the unaffected state, while the more refrangible had
solarized theirs. Now if the phenomenon were due, as
M. Becquerel supposes, to an unequal action of the same
kind in the different rays, it is obvious that the final
result ought to depend on the time of exposure; the
red ray, aided by the daylight, should carry its portion
through the various shades of white, and solarize it at
last. But this in the longest exposure never takes place;
that part of the plate remains as though a ray of light
had never fallen upon it.

Such are the facts I observed, and they seem to have
been reproduced by MM. Foucault and Fizeau; but
there are also others of a much more singular nature.
In these Virginia specimens the same protecting action
reappears beyond the violet.

The only impressions in which I have ever seen this
protecting action beyond the violet are those made in
Virginia in 1842; they were made in the month of July.
Struck with this peculiarity, on my return to New York

MEMOIR V.] INTERFERENCE OF RADIATIONS. 91

the following August I made many attempts to obtain
Minilar specimens, but in no instance could the extra-
violet protecting action be traced, though the analogous
action of the red, orange, yellow, green, and blue was
perfectly given. Supposing, therefore, that the differ-
ence must be due either to impurities in the iodine or
to differences in the method of conducting the experi-
ment, I tried it again and again in every possible way.
To my surprise I soon found that the negative effect was
gradually disappearing ; and on Sept. 29 it could no
longer be traced, except at the highest part correspond-
ing to the yellow and green rays. In December it had
become still more imperfect, but on the 19th of the fol-
lowing March the red and orange rays had recovered
their original protective power. It seemed, therefore,
that in the early part of the year a protective action
had made its appearance in the red ray, and about July
extended over all the less refrangible regions, and as the
year went on it had retreated upwards.

Are there, then, periodic changes in the nature of the
sun's light? The absorptive action of the earth's atmos-
phere is out of the question : if that were the cause, the
character of these spectrum impressions should vary
with the hour of the day. Or is it not more probable
that these singular phenomena rather depend on inci-
dental changes in the experiment, such as external tem-
perature, variations of moisture, the color of the sky, etc.?

Under proper circumstances there is no difficulty in
exhibiting the power which the less refrangible rays
exert in arresting the action of the daylight : under such
circumstances a daguerreotype impression of the sun's
spectrum yields all three of the varieties of suiface be-
fore alluded to. The plate in the less refrangible and
extreme violet region is imaffected ; a narrow space of
white separates these unaffected portions from the in-

92 INTERFERENCE OF RADIATIONS. [MEMOIR V.

digo and violet spaces, which are in a highly solarized
condition.

But a totally different result is obtained when the
daylight is not allowed to fall on the plate, either before
or during its exposure to the spectrum. Under these
circumstances the rays which would otherwise protect
now act on the plate and slowly whiten it. A daguerre-
otype spectrum formed in darkness and without pre-
vious exposure to the light exhibits a white stain over
all the less refrangible regions, and bears a marked con-
trast to one formed under the simultaneous action of a
weak daylight. For brevity I will call the former the
spectrum of darkness, and the latter the spectrum of day-
light. The following are some additional observations :

In the spectrum of darkness there is in the white stain
a point of maximum action. This corresponds with the
maximum of protection in the spectrum of daylight.

The white stain of the spectrum of darkness is appar-
ently narrower than the protected space in the spectrum
of daylight.

Rays of luminous or of non-luminous heat projected
on the darkness or daylight spectra during their forma-
tion appear to exert no kind of special influence on the
result.

The white fringe which borders the solarized portion
is not due to anything analogous to conduction. These
chemical changes, unlike thermal changes, cannot be con-
ducted.

By interposing between the prism and the daguerreo-
type plate a small convex lens of short focus, so as to
intercept in succession each of the colored rays, I threw
all over the plate, while the spectrum was in the act of
being impressed upon it, red, orange, yellow, and other
lights in succession ; the object being to ascertain how
far the impressed spectrum would change when these

MI.MOIB V.] ISTI.KI I 1,'I.NCE OF RADIATIONS. c,;>

monochromatic rays were used along with daylight,
Herschel having previously shown in similar experi-
ments that new phenomena arise during the conjoint ac-
tion of rays (Phil. Trans., 1840, § 64). The following
are some of the observations I made ; their date is Sept.
24, 1842.

The red ray when projected increases the length of
the solarized portion, and also of its white extremities.

The yellow ray shortens the solarized portion.

The green ray exerts a greater action of the same kind.

The indigo ray gives a most remarkable result. It to-
tally inverts the action of the less refrangible rays, and
they solarize the plate, acting in the same way that the
more refrangible rays commonly do, causing it to exhibit
a watch-spring lustre.

I further found that when different rays are brought
to act upon each other, the result does not alone depend
upon their intrinsic differences, but also on their relative
intensities. Thus the green and lower half of the blue
rays, when of a certain intensity, protect the plate from
the action of the daylight ; but if of a less intensity, they
aid the daylight.

The red and orange rays, when of a certain intensity,
increase the action of daylight on the plate ; but if of a
less intensity, they restrain it.

These facts seem to be connected with the circum-
stance that there is often to be traced on daguerreotype
plates a remarkable difference between the central and
lateral parts of a spectrum. Thus if a line be drawn
through the centre of such a spectrum and a parallel to
it on one of the edges, the action at any point on the cen-
tral line is the reverse of that at the corresponding point
on the edge. A similar remark, as respects impressions
on paper, has been previously made by Herschel.

Such are the chief facts I have observed in relation to

94 INTERFERENCE OF RADIATIONS. [MEMOIR V.

the daguerreotype spectrum. It would seem at first
sight that their diversity is so great that we can have
but little hope of reducing them to a common system of
results originating in the same cause. I have, however,
been long led to believe that the explanation is to be
met with in the great and fertile principle of interference.
From this point of view I regard the action of rays of
every kind as being essentially positive, and that action
mainly consists in impressing a vibratory movement on
the atoms of the decomposing substance. It is to my
mind a fact of no common significance, that in those Vir-
ginia specimens the places of maximum protection in the
less and more refrangible regions fall where the lengths
of the luminous waves have the extraordinary relation
of 2 : 1. Then, when we also see that, before a perfect
neutralization of action between two rays ensues, those
rays must be adjusted in intensity to each other, does it
not show that interference of some kind is going on?
Again, the yellow ray is in numerous instances the ray
which most completely antagonizes those at the red and
violet extremes of the spectrum : to use the language of
Herschel, "This ray may be considered as marking a
sort of chemical centre, a point of equilibrium, or rather
a change of action in the spectrum." I cannot avoid see-
ing that these phenomena are connected with the remark-
able fact that the waves of red, yellow, and violet light
are of lengths which correspond to 2, 1£, 1.

If, then, a powerful yellow ray can hold in check a fee-
ble violet one, and prevent it from decomposing iodide
of silver merely because their relation of length is in
the ratio of 1| : 1, it should follow on the same princi-
ples that a red ray acting conjointly with a violet should
give rise to an increased effect, because the lengths have
now become 2 : 1. And that this is in reality the case I
found by direct experiment; for on projecting the red

MEMOIR V.] INTERFERENCE OF RADIATIONS. 95

upon the violet, so that the colors should half overlap
i-ui'li other, I found that at the point of concourse the
plate instantly solarized, and assumed a splendid green
metallic color.

I have now explained the acceptation in which I re-
ceive the negative ray as a synonym (in this instance of
iodide of silver) for the yellow ray, and alluded to the
mechanism which seems to be the cause of protecting ac-
tion generally. Perhaps on a review of his own experi-
ments M. Becquerel may find reason to believe that there
are in reality antagonizing actions in different parts of
the spectrum ; actions not limited to the daguerreotype,
but occurring in all kinds of cases. They have been met
with by every one who has examined the spectrum with
sensitive papers, and, in a different series of phenomena,
M. Becquerel has himself furnished a conclusive illustra-
tion. He shows that when sulphide of lime and other
phosphorescent bodies in a shining state are exposed to
the spectrum, the more refrangible rays increase the glow,
but the less extinguish it.

It is proper to observe that some of the phenomena
recorded in this communication which seem to be in op-
position to the principle set forth are not so in reality.
All reasonings founded on the decomposition of light by
the prism, and the action of the prismatic spectrum oil
changeable surfaces, are liable to error. The only meth-
od free from these difficulties is to employ the diffraction
spectrum formed by a ruled suiface or a grating, a meth-
od which was proposed eight years ago by Herschel
with a view of getting rid of the disturbing agencies
arising from the ideal coloration of glass, and which I
first carried into effect in 1844 with so much success
that the resulting daguerreotype impressions contained
Fraunhofer's lines, even with microscopic minuteness.
With this spectrum we avoid a far more serious diffi-

96 INTERFERENCE OF RADIATIONS. [MEMOIR V.

culty than that of the ideal coloration of glass, a difficulty
arising from the magnitude of the refracting faces of the
prism. It is this which makes a prismatic spectrum
blacken paper, made sensitive with the bromide of silver,
from the red to the violet end; whereas the diffraction
spectrum shows that the true action is confined to the
more refrangible side, and stops short of the centre of
the yellow space.

UNIVERSITY OF NEW YORK, Dec.. 24, 1846.

MI.M..IK VI.] Till: DIF1 KA< Tl< >N Sl'KCTUUM. 97

MEMOIR VI.

ON THE DIFFRACTION SPECTRUM.
From the Philosophical Magazine, March, 1857.

CONTENTS: — Mode of obtaining the diffraction spectrum. — The yellow in
in its middle ; it is a centre of chemical action. — It is also the hottest
ray. — Diffraction spectra by reflection. — Cold lines. — Dilatation of the
more refrangible rays in the prismatic spectrum, compression of the less
refrangible. — Action of the diffraction spectrum on salts of silver. — Use
of wave-lengths for spectrum division.

M. EISENLOHR having published in Poggendorff s An-
nalen for June and August, 1856, some researches on the
diftraction spectrum, from which it appeared that he was
not aware of the results I had obtained and published in
1844, 1 thought it well to call attention to them, and this
I did in a letter to the editors of the Philosophical Mag-
azine, which they published in their journal for March,
1857, and from which the following extracts are made:

At first I obtained the diffraction spectrum very much
in the same manner that M. Eisenlohr has done, by pass-
ing a beam of light directed through a vertical slit by a
heliostat, and through a piece of ruled glass at twelve
feet distance. It was then received on an achromatic
lens of about four-feet focus, which gave a sharply de-
fined spectrum on a ground-glass or photographic sur-
face. Subsequently I found that it was much better to
silver the ruled surface with tin amalgam, in the manner
of a looking-glass, and thus employ a reflected spectrum.
This is far more brilliant than the transmitted one, and
the silvering acts so perfectly that the most minute fixed
lines may be seen.

G

98 THE DIFFRACTION SPECTRUM. [MEMOIK VI.

In a work published in 1844 "On the Forces which
Produce the Organization of Plants," the results I ob-
tained were illustrated by steel engravings, one of them
colored, with a view of comparing the prismatic and dif-
fraction spectra, and of showing the photographic ac-
tion on iodide, chloride, and bromide of silver. These
results are substantially the same as those of M. Eisen-
lohr. They give the wave-lengths at which action begins
and ceases on each of those substances. Some of the de-
tails may be found in a former number of the Philosoph-
ical Magazine (June, 1845), and in that article it is par-
ticularly recommended to use wave-lengths for designat-
ing rays, instead of titles of color, as red, yellow, etc.

The chemical action of the diffraction spectrum is,
however, only given in part in M. Eisenlohr's publica-
tion. He speaks of what occurs in the more refrangible
regions, and takes no notice of the action in the centre of

O /

the spectrum, and in its less refrangible space.

I will state in detail what I here mean. It is well
known that in such a spectrum the yellow occupies the
middle space, and that the light grades off in one direc-
tion to the red, and in the other to the violet. These
terminate at equal distances from the yellow, so that
the wave-lengths for the extreme violet, the centre of
the yellow, and the extreme red, are as 1, 1^, 2.

Now from the extreme violet to the yellow the spec-
trum blackens silver preparations, and this is what is
commonly understood by its chemical action by those
who have written on photographic subjects. But from
the yellow to the extreme red the spectrum is also active,
though in a different way. This half is in antagonism
with the other half. It can suspend or arrest the black-
ening which would be caused by contemporaneously act-
ing diffused light; nay, even more, it can undo what such
light may have done some time before. Some remarks

.MiM-.iuVI.] TIIK DIl'TUACTION Sl'ECTltUM. •,«»

on this topic may be seen in the Philosophical Magazine,
1-Vbruary, is i 7 (and in Memoir V. of the present work).

The centre of the yellow space is therefore the point
of a change in photographic action. From the commence-
ment of the red to that point the action is negative; from
that point to the extreme violet it is positive. The phase
of action changes in the centre of the yellow.

M. Eisenlohr has made an allusion to the heating pow- .
er of the spectrum, and to that point my attention has
also been directed. This is the result at which I event-
ually arrived — that the centre of the yellow is the hottest
space, and that the heat declines equally to the two ends
of the spectrum. I attempted to form a diffraction spec-
trum without the use of any dioptric media, endeavoring
to get rid of all the disturbances which arise through
the absorptive action of glasses by using as the grating
a polished surface of steel, on which parallel lines had
been drawn with a diamond, and employing a concave mir-
ror instead of an achromatic lens; and though my results
were imperfect and incomplete, I saw enough to convince
me that this is the right course to be taken in investi-
gating the problem. It is absolutely necessary to employ
a spectrum which has been formed by reflection alone.

I will also add that, in the experiments here referred
to, a method was employed for determining the tempera-
tures of narrow spaces, which may be recommended to
those who are disposed to resume such inquiries. It is
to use a blackened platinum wire, f of an inch long, and
•fa of an inch in diameter, one end of which is fastened
to the end of a bar of bismuth, and the other end to the
end of a bar of antimony, each of these bars being £ of
an inch square on the end and 4 inches long, their dis-
tant extremities communicating with a galvanometer.
By carrying the platinum wire transversely along the
spectrum, I expected not only to determine the distribu-

100 THE DIFFRACTION SPECTRUM. [MEMOIR VI.

tion of heat, but also to ascertain whether the heat spec-
trum has fixed lines, like the luminous and chemical ones.

This was at a time when I first began to suspect that
the essential difference between light and heat is this,
viz., that while the vibrations of light are transverse,
those of heat are normal, and its waves in that respect,
analogous to the waves of sound ; but so feeble is the
.intensity of these spectra that I could do no more than
satisfy myself that in the diffraction spectrum the centre
of the yellow is really the hottest space, as well as the
most luminous. I believe that there is a cold line in the
spectrum answering in position to the dark line H, but
I could not absolutely demonstrate it.

Nevertheless, so certain does it appear that the distri-
bution of heat corresponds to the distribution of light,
that I have not hesitated since that time to look upon
the centre of the yellow space as the point of maximum
heat, .from which there is a decline to each end of the
spectrum. And I accordingly made this the basis of a
theory of vision, published in my Treatise on " Human
Physiology." Such a view has the advantage of being
sustained by many facts of comparative anatomy. It
obliges us, it is true, to return to the opinions entertained
of the functions of the black pigment more than a cen-
tury ago; but then it gives a very elegant explanation
of the uses of the different parts of the retina, examined
in its radial section after the manner of M. H. Miiller,
and of those of the choroid coat — structures which are
without meaning on the ordinary theory of vision.

UNIVERSITY OF NEW YORK, Jan. 26, 1857.

NOTE. — To the foregoing paragraphs I may add
the following as contributions to the theory of vision.

MKMOIK VI.] HYPOTHESES OF VISION. 101

I may also refer to my Treatise on "Human Physiology"
(1856).

There are many rays emitted by the sun and other
shining bodies to which our eyes are entirely blind.

Two different reasons may be alleged for our inability
to perceive such rays : first, they may not be able to reach
the retina, the media of the eye not transmitting them ;
second, the retina may be so constituted as to be unable
to receive their impressions.

It has long been known that rays which come from
sheet-iron heated by a lamp cannot pass either through
the cornea or through the crystalline lens. Even of
those that are furnished by an Argand flame, used as a
luminous source of heat, less than one fifth pass through
the cornea alone, and scarcely one fiftieth when the crys-
talline lens is interposed. Cima showed that of the heat-
rays emitted by a flame, less than one tenth pass through
the cornea, lens, and vitreous humor conjointly. Janssen,
using a flame, compared the heat transparency of the
separate media of the eye with that of water included
between glass plates, showing that there is a perfect
accordance between them if taken of equal thickness.
From this it is to be concluded that invisible rays to a
certain extent reach the retina. Franz, by carefully con-
ducted experiments with a thermo-electric pile, came to
the conclusion that a quantity of obscure rays detectable
by the thermometer can reach the retina, which there-
fore must be so constituted as not to be able to per-
ceive them.

This settles the question so far as the less refrangible
or ultra-red rays are concerned. We have then to de-
termine how it is with those at the opposite or more re-
frangible end of the spectrum. Do these pass through
the media of the eye, or are they arrested and never
reach the retina?

102 HYPOTHESES OF VISION. [MEMOIR VI.

I made a series of experiments on these rays, and
found that they passed through the different media of
the eye, examined separately, and what is more to the
point, through them all collectively, with but little loss.
There was no difficulty in obtaining a dark stain on pa-
per made sensitive with chloride of silver, and placed at
the back of the eye of an ox, from which the sclerotic
and pigment had been suitably removed. In a gen-
eral manner the media of the eye act like water on the
trausmissibility of these rays.

Admitting from these experiments that invisible as
well as visible rays reach the retina, we may next con-
sider the nature of the impression made upon'it, and are
thus brought directly to an investigation of the act of
vision.

There are three hypotheses to be considered:

1. That rays falling on the retina or black pigment
impact to those structures a rise of temperature. This
may be termed the calorific hypothesis.

2. That rays falling on the retina occasion a chemical
change or metamorphosis in its structure, implying the
occurrence of waste in it, and therefore the necessity of
repair. This may be termed the chemical hypothesis.

3. That rays falling on the retina throw its parts into
a vibratory movement, not necessarily attended by any
metamorphosis of tissue, as waves of sound occasion con-
sentaneous pulsations in the auditory apparatus of the
ear. This may be termed the mechanical hypothesis.

FIRST, of the calorific hypothesis of vision. Compara-
tive anatomy offers certain facts which lend plausibility
to this hypothesis. Some of the most remarkable of
these relate to the construction of the eye in lower ani-
mals. The ocelli, which consist of dark-colored or black
spots, or black cup-shaped membranes containing within
them the rudiment of an optic nerve, are the beginning

MI.M..IU VI.] HYPOTHESES OF VISION. 103

of an organ of vision. There being no optical apparatus
for the production of images, the luminous impression
must be felt as heat. For this the dark pigment is well
designed. It is an old physical experiment to lay upon
the snow on a sunshiny winter day pieces of differently
colored cloth. They will melt their way to a greater
depth in proportion as their tint is deeper; the black,
becoming the warmest, sinks deepest; the white, reflect-
ing most of the heat, scarcely melts the snow at all.
Now an animal destitute of any visual organ can only
be affected by the impressions of light in a very doubt-
ful manner; but if there be upon its exterior a black
spot, not only is there a much higher sensitiveness be-
cause of the increased absorptive power for heat, but
the sphere of its consciousness is greatly extended, from
the possibility of acquiring a knowledge of directions in
space — a knowledge that becomes more and more exact
with the increasing number and symmetrical arrange-
ment of these ocelli.

If we apply these principles to a more perfect form of
eye, as that of man, we are led to a new interpretation
of the function of some of its parts. The black pigment
becomes the receiving surface for images of external
things, and rays falling upon it, in their diversity of col-
or, brightness, and shade, in the act of becoming extin-
guished, engender heat. As with the tip of the finger
passing over an object we can discover, even in the dark,
spaces that are warm and those that are cool, so the rods
and cones of Jacob's membrane, acting as tactile organs,
convey to the brain a knowledge of the momentary dis-
tribution of heat on the dark concave of the eye. The
pigment has therefore a far more important office to dis-
charge than that of merely extinguishing stray light and
darkening the inside of the globe.

But this calorific hypothesis is not without great dif-

104 HYPOTHESES OF VISION. [MEMOIR VI.

ficulties. Heat suffers conduction. If this black pig-
ment officiated as a transformer of light rays into heat
by producing extinction, there must unavoidably be a
lateral spread from the boundaries of warm to cooler
spaces, the edges of images must be nebulous and with-
out sharpness of contour. Moreover, there is reason to
believe that the visual apparatus cannot take cognizance
of heat merely as such. Calorific rays reach the black
pigment and raise its temperature without the retina
being affected.

Such considerations seem, therefore, to exclude the cal-
orific hypothesis, and prepare us for an examination of
the chemical.

SECOND, of the chemical hypothesis of vision. Numer-
ous discoveries made of late years in relation to the
chemical action of light put us in possession of many
facts having a bearing on this hypothesis. A majority
of compound substances, both inorganic and organic, suf-
fer chemical modifications when exposed to the access of
light, and, what is very significant, these changes are oc-
casioned by definite classes of rays. One substance finds
its maximum of action in the violet region, another in
the yellow, another in the red. The effect in every in-
stance grades off towards the less and more refrangible
spaces respectively.

In these actions of decomposition there is nothing like
lateral spreading, nothing answering to conduction. No
better proof of this is necessary than the exquisite sharp-
ness of photographic pictures — a sharpness only limited
by the optical imperfections of the lens with which they
are made. The molecules on which the light falls are
the only ones that experience change ; there is no propa-
gation of effect from part to part — an important particu-
lar, because it is what we observe in the case of sight.

The retina, the nervous expansion of the eye, is so con-

I.] II YPOTIIKSES OF VISION.

stitutetl that a maximum effect upon it is occasioned by
tlic yellow ray, the action declining on one side to the
red, and on the other towards the violet, and ceasing
at the extremes of those rays. For this reason, when a
solar spectrum is examined by the eye, the yellow is the
most brilliant space, there being a decline in intensity
t'r<>m it to the two extremes.

In my experiments on the decomposition of carbonic
acid by plants in the sunlight, to be described hereafter
in these memoirs, the maximum of action was found to
be in the yellow, with a gradation of effect towards the
red and violet ends of the spectrum respectively. From
this it would appear that a relation exists between light
and compounds having a carbon nucleus, answering to
that observed in the case of the retina of the eye. Such
a relation is very well illustrated in the case of other
chemical elements, as silver, a metal which is the basis
of all ordinary photographic preparations. The ray of
maximum action is in the indigo space. Objects viewed
by a retina having a silver sensitive nucleus would pre-
sent an appearance altogether unlike that they would
offer to a carbon nucleus. The order of brilliancy in
the lights would be no longer the same. The red and
yellow parts of objects would be black, that is to say,
invisible, and other rays beyond the violet would come
into view.

Among experiments I have made on this subject,
there is one of much physiological interest. The ele-
ment phosphorus finds its maximum impression in the
more refrangible portion of the spectrum, in that respect
resembling silver. Upon a portion of translucent phos-
phorus, enclosed out of contact of air in a flattened glass
tube, into which it had been drawn while melted, and
then suffered to solidify, a solar spectrum was cast. The
effect of light upon this kind of phosphorus is to turn it

106 HYPOTHESES OF VISION. [MEMOIR VI.

eventually to a deep mahogany red, and chemically to
throw it from an active into an inactive state. As amor-
phous phosphorus, otherwise prepared, it ceases to shine
in the dark. In the experiments now alluded to, it ap-
peared that this reddening takes place in the indigo and
violet spaces, so that the fixed lines known by spectro-
scopists as those about H were beautifully depicted.
Now some physiologists have supposed that nerve ves-
icle tissue owes its property to the presence of uuox-
idized phosphorus, but if the principles we are contem-
plating be correct, and this were the case, the most brill-
iant ray in the spectrum should be the indigo, and not
the yellow. Therefore, if vision be performed by chem-
ical change in the substance of the retina, it is carbon
and not phosphorus that is concerned.

If we admit that during the act of vision the retina,
as a structure with a carbon nucleus, undergoes meta-
morphosis, the principles of photo-chemistry would lead
us to expect that the yellow must be the brightest ray,
and a harmony is thus established between this and
other functional changes in the body. We also perceive
the significance of certain structures of the eye which
otherwise would appear to be without meaning. The
rapid retrograde metamorphosis which must be taking
place in the retina involves the provision of some means
for moving away the wasted products and of supplying
nutrition with the utmost quickness. And this is the
office discharged by the choroid.

But such removals and supplies require time. Time,
therefore, enters as an element in the visual operation.
Sis;ht commences instantaneouslv, but the ima^e of an

O »/ 7 O

object may be seen long after the reality has disap-
peared. This instantaneous commencement of a retinal
impression may be very strikingly illustrated. The
spark of a Leyden-jar, though it does not last, as is af-

.MIM..IK VI.] HYPOTHESES OF VISION. JQ7

firmed, the millionth of a second, can without any diffi-
culty be photographed even on so sluggish a compound
as silver iodide. On the far more sensitive retina the
chemical impression must be practically contemporane-
ous with the impinging of the light.

If after the eyelids have been closed for some time,
we suddenly and steadfastly gaze at a bright object,
and then quickly close the lids again, a phantom image
is perceived existing in the indefinite darkness before
us. By degrees the image becomes less and less distinct;
in a minute or two it has disappeared.

The chemical hypothesis renders a very clear explana-
tion of this effect — an explanation that commends itself
to our attention as casting light in many cases on the
curious phenomena of apparitions: phenomena that have
been not without influence on the history of mankind.

The duration and gradual extinction of the retinal
phantoms correspond to the destruction and renovation
taking place in the retina itself. The blood supply is
very ample, as are likewise the channels for the removal
of waste, but the operations require time to be accom-
plished. As in machines contrived by man, so in natural
organs, the practical working does not always come up
to the theoretical standard. Theoretically, as the retina
suffers change under the incident light, the removal of
waste and nutrition should go on in an equal manner
both as to time and quantity. A marvellous approach
to the ideal perfection is attained, for though the action
of light must necessarily be cumulative, that is, increas-
ing with the continuance of exposure, objects do not
become brighter and brighter as we look at them, but
they attain their predestined distinctness at once. The
action of the li^ht, the removal of the waste it is occa-

O '

sioning, and the supply for renovation are all contem-
poraneously going on with an equal step, or so nearly

108 HYPOTHESES OF VISION. [MEMOIR VI.

so that such may be considered to be the practical
effect.

THIKD, of the mechanical hypothesis of vision. There
is a growing belief among those who are cultivating
photo-chemistry that the mode of operation of a ray of
light in accomplishing chemical changes is by establish-
ing vibratory movements among the molecules of the
substance affected. As has been affirmed, perhaps fanci-
fully, of certain singers, that they could cause a glass
goblet to fly to pieces by a proper intonation of their
voice, through the attempt of the glass by resonance to
execute incompatible vibrations, so it is thought that an
incident ray may break asunder a group of molecules by
establishing among them discordant agitations. Chem-
ical decompositions by radiations become thus connected
theoretically with vibratory movements.

But these are vibrations not necessarily attended by
any destruction of tissue. Waves of sound occasion such
pulsations in the apparatus of the ear without producing
any chemical change in the auditory nerve.

If we consider the retina as an elastic shell, of which
the parts are put into a purely mechanical movement
by the pulsations of light, we abandon without explana-
tion some of the most interesting portions of the struct-
ure of the eye. Of what use is that wonderful net- work
of vessels constituting the choroid ? It is a principle in
physiology that the supply of blood to a part is propor-
tional to its functional activity. The elaborate vascular
mechanism in juxtaposition with the retina will bear no
other interpretation than that that tissue is the seat of
incessant chemical changes.

Moreover, physical science in its present state is not
sufficiently advanced to furnish the means of clearly
comprehending such purely mechanical motions exe-
cuted by the ultimate particles of things. We may

MKMOIB VI.] HYPOTHESES OF VISION. 109

conceive of the comparatively slow swaying of groups
of molecules under the influence of normal pulsations in
the air, but not of the dance of atoms disturbed by trans-
verse vibrations in the ether. If, therefore, there were
no arguments of an anatomical kind to be presented
against the admission of this hypothesis, we should be
compelled to turn aside from it because of the inade-
quacy of our knowledge in tracing its conditions to their
applications.

This, therefore, is the conclusion at which we finally
arrive — that vision depends on chemical changes, especial-
ly of oxidation, in the retina, and that they approach in
their nature those that we speak of as photographic.
There is no difficulty in understanding how such changes
may give rise to an influence transmitted along the optic
nerve to the brain, when we reflect that the oxidation
of a few particles of zinc may accomplish specific me-
chanical results through many miles of intervening tele-
graphic wire, producing mechanical motions as in the
telegraph of Morse, or chemical changes as in that of
Bain.

We have remarked that a critical study of the func-
tion of vision cannot fail to lead to interesting results
respecting the nervous system generally. Guided by
that remark, we may perhaps profitably consider further
the vestiges of visual impressions, and the physical con-
ditions under which they disturb us or spontaneously
obtrude themselves on our attention.

The perception of external objects depends on the
rays of light entering the eye, and converging so as to
produce images, which make an impression on the retina,
and through the optic nerve are delivered to the brain.
The direction of these influences, so far as the observer
is concerned, is from without to within, from the object
to the brain.

INVERSE VISION. [MEMOIR VI.

But the inverse of this is possible. Impressions ex-
isting in the brain may take, as it were, an outward
direction, and be projected or localized among external
forms ; or if the eyes be closed, as in sleep, or the ob-
server be in darkness, they will fill up the empty space
before him with scenery of their own.

Inverse vision depends primarily on the condition
that former impressions, enclosed in the optic thalami, or
registering ganglia at the base of the brain, assume such
a degree of relative intensity that they can arrest the
attention of the mind. The moment that an equality
is established between the intensity of these vestiges
and sensations contemporaneously received from the
outer world, or that the latter are wholly extinguished,
as in sleep, inverse sight occurs, presenting, as the occa-
sions may vary, apparitions, visions, dreams.

From the moral effect that arises, we are very liable
to connect these with the supernatural. In truth, how-
ever, they are the natural results of the action of the
nervous mechanism, which of necessity produces them
whenever it is placed, either by normal or morbid or
artificial causes, in the proper condition. It confounds
the subjective and the objective together. It can act
either directly, as in ordinary vision, or inversely, as in
cerebral sight, and in this respect resembles those instru-
ments wrhich equally yield a musical note whether the
air is blown through them or drawn in.

The hours of sleep continually present us, in a state
of perfect health, illusions that address themselves to
the eye rather than to any other organ of sense, and
these commonly combine into moving and acting scenes,
a dream being truly a drama of the night. In certain
states of health appearances of a like nature intrude
themselves before us even in the open day, but these,
being corrected by the realities by which they are sur-

M.MOIB VI.] INVKRSK VISION. HI

mundrd, impress us very differently. The want of uni-
son between such images and the things among which
tlit-y have intruded themselves, the anachronism of their
advent, or other obvious incongruities, restrain the mind
from delivering itself up to that absolute belief in their
reality which so completely possesses us in our dreams.
Yet, nevertheless, such is the constitution of man, the
bravest and the wisest encounter these fictions of their
«>wn organization with awe.

The visions of an Arab merchant have ended in tinct-
uring the daily life of half the people of Asia and Africa
for a thousand years. A spectre that came into the
camp at Sardis the night before the battle of Philippi
unnerved the heart of Brutus, and thereby put an end
to the political system that had made the Roman repub-
lic the arbiter of the wrorld. A phantom that appeared
to Constantine strengthened his hand to that most diffi-

O

cult of all the tasks of a statesman, the destruction of an
ancient faith.

Hallucinations are of two kinds — those seen when the
eyes are open and those perceived when they are closed.
To the former the designation of apparitions, to the lat-
ter that of visions, may be given.

In a physiological sense, simple apparitions may be
considered as arising from disturbances or diseases of
the retina; visions, from the traces of impressions en-
closed at a former time in the corpora quadrigemina and
optic thai ami.

From flying specks floating before us, the first rudi-
ments of apparitions, it is but a step to the intercalation
of simple or even grotesque images among the real ob-
jects at which we are looking; and indeed this is the
manner in which they always offer themselves, as resting
or moving among the actually existing things.

THE DIFFRACTION SPECTRUM. [MEMOIR VI.

The method by which the first photograph of the dif-
fraction spectrum was obtained is described as follows
in the Philosophical Magazine, June, 1845.

The prismatic spectrum, even when every precaution
has been used to obtain it in a state of purity, its fixed
lines being visible, is liable to lead us into many errors.
As respects its luminous or photic properties, we cannot
determine the distribution or intensity of the light be-
cause the violet extremity is unduly dilated. As respects
the chemical effects, the same difficulty occurs, for these
are necessarily controlled and disturbed by the law of
distribution. All chemical actions occurring; in the more

O

refrangible regions, by being spread over a great space
appear to be- more feeble than what they actually are.

In a perfect spectrum the most luminous portion of the
yellow should be in the centre; and from this the inten-
sity of the light should gradually decline, fading away
on one side in the red, and on the other in the violet.
At equal distances from the middle yellow point on
either hand the intensity of the light should be equal.
These beautiful results are due to Mosotti, who also
states that the length of the extreme red wave is to that

O

of the extreme violet in the simple ratio of 2 : 1.

The prismatic spectrum does not exhibit these facts.
The yellow is not in the centre ; the blue, indigo, violet
are abnormally spread out, the spectrum having its own
law of distribution. But the diffraction spectrum en-
ables us to observe them.

By the aid of a heliostat, I arranged horizontally in a
dark room a narrow ribbon of light, coming through a slit
3*0 of an inch wide, set vertically. At the distance of
twelve feet it fell perpendicularly on a piece of flat glass,
the surface of which was ruled with equidistant parallel
lines by a diamond ; and having been silvered with tin
foil, after the manner of a mirror, it served the purpose

Mi .M. -IK VI.]

TliK DIFFRACTION SPECTRUM.

113

of a grating. The reflected beam went out through the
slit at which it entered, and on either side of it to the
riu'ht and left the well-known double series of diffraction
spectra made their appearance. I selected, for the obvi-
ous reason that it was not overlapped by its successor,
the first of the series, and intercepting it by an achro-
matic object-glass, placed in the focus a frame capable
of holding a ground -glass or sensitive suiface. This
frame was adjusted until the fixed lines were distinctly
depicted upon it.

For a further description of the reflected diffraction
spectrum I may refer to any of the elementary works on
optics. It is sufficient for my purpose here to recall that
the angular deviations of any two colors from the primi-
tive incident ray are to one another as the lengths of
their respective undulations.

On the ground-glass we see the fixed lines, and the
length of waves corresponding to those lines has been
carefully determined by Fraunhofer. The following
table is extracted from Herschel's treatise on Light, the
Paris inch being divided into one hundred millions of
equal parts:

Length of wave corresponding to the fixed line B, 2541 parts.

C, '2422

D, 2175

E, 1945

F, 1794

G, 1587
H, 1464

When, therefore, we have any chemical, luminous, ther-
mic, phosphorogenic, or any other effect under discussion,
in relation to the spectrum, we have only to determine
its place among the fixed lines, remembering in the dif-
fraction spectrum the simple law that connects the devi-
ations and wave-lengths. In the mode of operation here
described absolute exactitude is not reached because our

H

THE DIFFRACTION SPECTRUM. [MEMOIR VI.

measures are obtained from a flat surface, and not upon
a circular arc.

To the diffraction spectrum thus formed I exposed for
half an hour a daguerreotype plate, rendered sensitive
by iodine and then by bromine. It resulted that the
bromide of silver is decomposed at a maximum by a
wave which is 0.00001538 of a Paris inch in length. The

o

action does not extend equally, as we might have sup-
posed, towards the more and less refrangible regions.

I exposed a silver plate which had been prepared by
iodine, bromine, and chloride of iodine successively. The
point of maximum fell as before at 0.00001538. The
time of exposure one hour. The decomposition com-
menced by a wave in the green space, the length of
which was 0.00002007, and was terminated by one in
the violet, the length of which was 0.00001257. The
point of maximum action, therefore, inclined to the vio-
let,-and was not midway between the extremities of the
photograph. The absolute length of the stain depends,
however, on the time of exposure.

I need not multiply these results. It is sufficient to
add that in several trials I obtained in these delicate ex-
periments photographs of the diffraction spectrum on
different surfaces in great perfection. The fixed lines
which are crowded close together were beautifully dis-
tinct.

I would suggest, therefore, that when we wish to in-
dicate spectrum regions with precision, we should use
wave-lengths. By doing this we shall connect the vari-
ous actinic phenomena with a great many of the numer-
ical results of optics, and have fixed points of comparison.

UNIVERSITY OF NEW YORK, June, 1845.

MEMOIR VII.] STUDIES IN THE DIFFRACTION bUECTliUM.

MEMOIR VII.

STUDIES IN THE DIFFRACTION SPECTRUM.
A popular exposition condensed from Harper's New Monthly Magazine, Vol. LV.

CONTENTS : — Elementary description of the diffraction spectrum. —
Young's discovery of interference. — FresneCs discovery of transverse
vibrations. — Gratings and the spectra they yield. — Gratings on reflect-
ing surfaces. — Optical action of a grating. — Its spectra of different
orders. — Interpretation of wave-lengths by the mind. — Mental appreci-
ation of multiple wave-lengths. — Refutation of the principle that to
every color there belongs an invariable wave-length. — Increase in the
range of perception in the eye. — Extension to photographic impressions.
— Encroachment on the first dark space. — Photographs of the diffrac-
tion spectrum. — Proposal to use wave-lengths for spectrum divisions. —
Replacement of imaginary imponderable principles by wave-lengths.

WHAT is a diffraction spectrum ? Every person who
has read a book on light is familiar with the prismatic
spectrum, in the study of which Newton displayed his
transcendent philosophical powers. The diffraction I
have had occasion to refer to several times, and since it
is less known, will now describe it. Some very curious
phenomena connected with it I have personally exam-
ined. It carries us to a true interpretation of the rela-
tions of heat, light, and actinism; it offers some impor-
tant suggestions respecting the mode of action of that
most wonderful of all organs, the brain, and therefore
commends itself to our most earnest attention.

If we look at a candle flame placed ten or a dozen feet
distant, the eyelids being so nearly closed that the eye-
lashes intercept the incoming rays, we see on either side
of the true image of the flame a succession of colored
ones — rainbow streaks or fringes, as it were. Examining

STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

these particularly, we find that each of them is blue on
the side nearest to the true image, and red on the more
distant. Our investigation will be simplified if we con-
sider the action of a single eyelash. We can then reason
from that to the conjoint action of all.

It is necessary, however, in the first place, to recall
some facts connected writh the wave theory of light.
The foundations of this theory were laid by Huyghens,
the great Dutch philosopher, contemporary with Newton,
but its construction advanced very slowly, being opposed
by the great authority of Newton, who favored the cor-
puscular or emission theory, and regarded light as con-
sisting of particles emitted with excessive velocity from
shining bodies. Although there were facts, such as those
connected with double refraction, easily accounted for
by the system of undulations, but inexplicable on the
emission theory, these were put aside, in the expectation
thaf they would in the course of time be successfully
dealt with. It was not until the publication of a
course of lectures on natural and experimental philos-
ophy by Dr. Young in 1802, in which he announced the
great discovery of the interference of light, that the un-
dulatory theory could no longer be overlooked. This
discovery wras, however, still ridiculed by the Edinburgh
Review, and Young's explanations so bitterly attacked
that he was constrained to publish a pamphlet in reply.
Of this it is said that only a single copy was sold.

In 1819, a memoir by Fresnel was crowned by the
French Academy of Sciences. He discovered that the
vibratory movements in the ether constituting light are
transverse to the course of the ray. His views are em-
bodied in what is now known as the theory of trans-
verse vibrations.

The conflict between the rival theories was eventually
settled by the experiments of Fizeau and Foucault. On

MEMOIB VII.] STUDIES IN THE DIFFRACTION SPECTRUM. Jjf

Newton's principles the particles of light should move
taster through water than through air; on the theory of
Huyghenfe, waves of light must move slower in water
than in ajr. The experiments of the French physicists
proved that the latter is the case. This may, then,
be considered as the successful establishment of the un-
dulatory theory. It hast, moreover, given that strik-
ing proof of its truth which may be considered as the
criterion of any theory — the ability to foretell results.
This it did in the case of the discovery of conical re-
fraction.

Light, therefore, consists in the transference of energy
or force, not in the transference of matter.

The grating I employed in the experiments hereinafter
related was made for me by Mr. Saxton, at the United
States Mint in Philadelphia, more than thirty years ago.
Though from the work it did for me I cannot but speak
of it with admiration — it enabled me to make the first
photograph that was ever executed of the diffraction
spectrum — yet it was far from being equal to the mag-
nificent ones of Mr. Rutherfurd. This grating was five
eighths of an inch long and one third of an inch in
breadth. Mr. Rutherford's gratings have in some speci-
mens 17,240 lines to the inch. I had found previously
to 1843 that it is more advantageous practically to use a
reflecting than a transparent grating, arid accordingly I
silvered mine with mercury-tin amalgam, such as is used
in ordinary looking-glasses. Mr. Rutherfurd's reflecting
gratings are coated with pure silver, by an operation
more recently discovered.

I will now relate the use of these gratings, and de-
scribe some of the important discoveries made by them.

Let a beam of light, S A', Fig. 11, pass through a nar-
row slit, S, and fall perpendicularly on the ruled grating,

118 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

S

G, the lines of which are parallel to the sides of the slit.
Concentric with the middle line of the grating let there
be placed a circular zone or screen, Q', Q", Q'", Q"", of
white paper, through which there is an opening at A, to
admit the intromitted beam.

A beam of parallel rays passing along S G will give a
bright image of the slit S when it impinges on the screen
at A'. This is the image by transmission. It would
also give another similar image at A, were it not for the
opening arranged there. This is the image by reflection.
Also from G, as from a central axis, there falls upon the
cylindrical paper zone, covering its surface all over, an
infinite number of radiations.

These effects are seen with much more precision if

Mi M..IR VII.] STUDIES IN THE DIFFRACTION SPECTRUM.

tin re be placed behind the grating a convex lens, or, still
better, if the lens be the objective of a telescope.

Now the eye can only be impressed by special radia-
tions consisting of waves of a determinate length. Its
vision is limited to those that impart to it a sensation of
red on one hand, and of violet on the other. To all oth-
ers it is blind. Then, though the whole paper zone is
receiving radiations of every kind, the eye selects out
only those that it can perceive, and, as a result, sees in
the four quadrants, Q', Q", Q'", Q"", those only for which
it is fitted.

It follows, therefore, that at A' there is a white image
of the slit S, and to the right and left of this there are
equal spaces, p, p', completely dark. Beyond, and sym-
metrically on each side, there is a series of spectra, v r,
v' /, v" /', etc., of which the violet ends are nearest A',
and the red ends most distant. These spectra are des-
ignated respectively as being of the 1st, 2d, 3d, etc., order.
On each side the 1st spectrum is separated from the 2d
by an obscure space, r v', which is shorter than the first
dark spaces, p,p', and the red end of the 2d spectrum is
overlapped by the violet of the 3d. In like manner the
3d is overlapped by the 4th, etc. If the intromitted ray
be of sunlight, and a convex lens or small telescope be
used, the dark Fraunhofer lines are seen in these spectra.

Such are the results seen in the quadrants Q'", Q"",
from the light transmitted through the grating. In the
quadrants Q', Q", exactly the same train of phenomena
will be disco vered — dark spaces and spectra, the latter
having their violet ends nearest to A, and the overlap-
ping of successive ones taking place in the manner above
described.

Since the results are thus symmetrical in all the four
quadrants, it is sufficient to select one of them for de-
tailed examination. Let it be the quadrant Q"".

120 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

Selecting one of the fixed lines, that in the yellow
space, the sodium line D, for example, in the successive
spectra, it will be found that the distance which inter-
venes between it and the middle of the white image A'
is in the second double, in the third triple, etc., the dis-
tance it is in the first. These angular distances are des-
ignated as the deviations of the ray under examination.
Fraimhofer proved that

(1) The deviation of the same ray, e. g., D, depends on
the sum of the width of a groove in the grating and of a
transparent interval, being in the inverse ratio of that
sum.

(2) The deviation of any one of the colors of the spec-
trum of the first order, multiplied by the sum of a trans-
parent interval and a groove, gives the length of a wave
of light of that color.

(3) The deviations of the same color in the successive
spectra increase as the whole numbers 1, 2, 3, 4, etc.

(4) The deviations of two colors in the same spectrum
are to each other as the length of their undulations.
Hence in all the violet is nearest to A', and the red the
most distant.

The undulatory theory gives a rigorous explanation
of all these facts. The lengths of the waves of light
have hence been most critically and accurately deter-
mined.

We may now examine more closely the spectrum that
is nearest to A' — the spectrum of the first order. Be-
ing completely separated from the others, it presents the
special facts most distinctly. At the point where the
light first becomes visible, the violet or inner end of this

o .

spectrum, the wave-length of the incident radiation is, as
Angstrom has proved, 3933, and the wave-length of the
last visible radiation at the outer or red end is 7604,
ten millionths of a millimeter. If we accept the velocity

MEMOIR VII.] STUDIES IN THE DIFFRACTION Sl'KCTliUM.

of light as determined by the experiments of Foucault,
the number of vibrations made by the ether in the for-
mer of these radiations is 754 millions of millions in one
second, and the number in the latter case is 392 millions
of millions in one second.

Or, to quote measures which are perhaps more familiar,
and numbers as given by Herschel, though not so exact
as those of Angstrom, the number of undulations con-
tained in one English inch at the extreme violet end is
59,750, and the number of vibrations executed in one
second is 727,000,000,000,000. The number of undu-
lations in one English inch at the extreme red end is
37,640, the number of vibrations executed there in one
second being 458,000,000,000,000. The velocity of light
used in these computations is 192,000 miles per second,
that used in the preceding paragraph, 186,000.

Knowing the rate at which light moves in a second,
and the wave-length of any particular color, it is easy to
compute the number of vibrations made by the ether
in one second for the production of that color. This
is obtained by dividing the distance that light passes
over in one second by the wave-length of the color in
question.

The numbers we thus obtain give us an idea of the
scale of space and time upon which Nature carries for-
ward her works among the particles of matter. They
also indicate to us the amazing activity of those portions
of the brain which execute motions in accordance with
those scales.

The distribution of the colored spaces in the diffrac-
tion spectrum is not the same as in the prismatic. In
the former the yellow space, which is the most luminous
radiation, is in the middle of the spectrum, and is not
crowded down or compressed towards the red end, as in

122 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

the latter. So the maximum intensity or illuminating
power is, as Mosotti first observed, in the centre, the in-
tensity of the light declining symmetrically on each side
to the end.

The Italians have a clear perception, a quick apprecia-
tion of the symmetrical and beautiful. When Mosotti
first stated this peculiarity of the diffraction spectrum, at
a meeting of one of the Italian scientific societies, the an-
nouncement was received by the audience with loud ac-
clamations of joy.

I may now describe some of my own studies of these
beautiful spectra.

Kecalling, then, the principle that the wave-length of
an incident radiation is proportional to its deviation, let
us select upon the paper zone previously described the
point where a ray is falling having a wave-length 7866.
It is, of course, twice as far from A' as was the violet
end <&f the first spectrum, for the selected deviation is
double. If we inquire what interpretation the mind
will give of a radiation having such a wave-length, an
inspection of the zone shows that not only is it visible,
but that it is regarded as being of a violet color.

This is an important fact. We find that a radiation
consisting of waves of a given length which is visible
will also be visible when the constituent waves are twice
that length. And in like manner it might be shown
that the same will hold good when they are three, four,
five, etc., times that length. Moreover, in all these cases
the color impression imparted to the mind will be the
same.

Again, let us select upon the paper zone another point
where the wave-length is 15,208. It will have double
the deviation of the red end of the first spectrum. Now,
agreeably to the foregoing remarks, this point should be
visible to the eye, and, for anything that has thus far

MEMOIK VII.] STUDIES IN THE DIFFRACTION SPECTRUM. J23

been said, it should be interpreted by the mind as red
liirlit. its wave-length being twice that of the red of the

O ' O O

first spectrum. But it is obvious that here a new con-
sideration must enter into account. If this radiation
has double the wave-length of the first red, it has triple
the wave-length of the first yellow-green. On the prin-
ciple just laid down, the mind may interpret it as red
light or as yellow-green. Which will it do?

Examination of the paper zone, or, better still, through
a telescope, shows that the mind adopts both these in-
terpretations, and the same principle applying to other
wave-lengths, this constitutes what we have spoken of
as the overlapping of the second spectrum by the third,
etc. At the point here specially considered, both red
and yellow-green light are seen.

From what has here been presented, it follows that
the principle considered as established in optics, that
to every color there belongs a determinate wave-length,
must be modified, since the same color impression will
be given to the mind by waves that have twice, thrice,
etc., that determinate wave-length. But should the
wave-lengths under consideration answer to multiples
of that of some other color, the mind will interpret them
as being of that color too.

Moreover, these observations lead us to extend the
range of perception of the eye. The prism would lead
us to infer that it can only be affected by waves the
length of which is between 3933 and 7604. Compari-
sons have hence been drawn between the organ of vision
and the organ of hearing, to the disparagement of the
former. The ear, it is said, can embrace a range of sev-
eral octaves, but the eye is influenced by less than one.
The grating, however, leads us to reject the restriction,
and to place the eye more nearly on an equality with
the ear.

124

STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

It is also to be borne in mind that by using very con-
densed sunlight, or by resorting to fluorescent or other
optical contrivances, as several experimenters have done,
the range of vision may be carried beyond the proper
violet limit.

The principles here indicated must not be restricted
to the luminous radiations; they apply to all others too.
Thus if a photographic sensitive surface be made to re-
ceive the first spectrum, it will be impressed by certain
of its radiations, chiefly by those above the line Gr. If
it be exposed in the second, third, etc., spectra, it will
again be impressed by the corresponding undulations,
having two, three, etc., times the former length. From
this it may, therefore, be inferred that a chemical decom-
position of a given substance, brought about by undula-
tions of a certain length, will also be accomplished by
radiations that are octaves of the first.

It has been stated that a dark space, j9, intervenes be-
tween the violet end of the first spectrum and the bright
streak A'. This dark space is at present an attractive
and wonderful field of optical investigation.

A'

Fig. 12.

In Fig. 12, let A' represent the wrhite streak in the
position of A' in Fig. 11 ; then from A' to v is the first
dark space, j9 ; from v to r, the spectrum of the first order,
its violet end, v, nearest to A', its red end, r, more dis-
tant; from r to v' the second dark space; and from v'
to vf the spectrum of the second order. The third spec-
trum overlaps this second, and the fourth the third, etc. ;

MKXOIK VII.] sriDll-S IN TIIK DII FRACTION SPECTRUM. 125

but these it is not uecessary to consider. The two dark
spaces, and especially the first, are the objects mainly to
be examined.

Previously to 1844 I had attempted to obtain diffrac-
tion photographs with the grating that Mr. Saxton gave
me, and had met with great success. In that year I
published engravings of them, the originals having been
made on silver daguerreotype tablets, in use at that time.
By these I carried spectrum impressions as far as the
wave-length 3800, and therefore encroached considerably
on the dark space p, towards A'. But collodion, since
introduced, is a much more sensitive preparation. It
has enabled Henry Draper, who has produced superb
photographs of the more refrangible regions, to carry
the impressions as far as 3032.

According to M. Mascart, waves are emitted by incan-
descent cadmium having a length not exceeding 2200.
These stand still further in the dark space p.

In these excursions into the dark space the experi-
ments of Professor Stokes on the long spectrum of elec-
tric light become not only interesting, but very impor-
tant ; for as we gradually approach A7, the wave-length
of the incident radiation is continually diminishing, and
at A' it becomes zero. That point is the supreme limit,
beyond which no radiant manifestation of any kind is
possible.

The goal towards which experimental investigation is
tending is therefore obvious. We are gradually groping
the way across the dark space, and expect one day to
reach the bright streak that lies at its terminus. At

O

every step of advance the ether waves are becoming
shorter and shorter, and the vibrations more and more
rapid. When the journey is accomplished, a region will
have been gained in which the waves are infinitely short,
and the vibrations infinitely rapid.

126 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIK VII.

Several years before the announcement of the discov-
ery of photography by Daguerre and Talbot (1839) I
had made use of that process for the purpose of ascer-
taining whether the so-called chemical rays exhibited
interference, and in 1837 published the results in the
Journal of the Franklin Institute, Philadelphia (July,
1837, p. 45). In this, as will be seen by consulting that
publication, I was successful.

Encouraged by this result, I some years subsequently
attempted to photograph the diffraction spectrum itself.

The following is an extract from the publication I
made of this experiment in 1844: "Through a narrow
fissure or slit, «, Fig. 13, I direct a beam of light hori-
zontally, and at a distance of twelve feet receive it on a
grating, b c, the lines of which are parallel to the slit.
Having found that there are advantages in using a re-
flecting grating, I silvered this with tin amalgam, which
copies the ruling perfectly. There is no difficulty in
placing b c so that the ray coming from a falls perpen-
dicularly on it, for all that is required is to move the
grating into such a position that the light, after reflection
from it, goes back through the fissure a. At a distance
from b c of
six inches I
place an ach-
romatic ob-
ject-glass, d,

in such a po- s^" Fi£-13-

sition that it

shall receive perpendicularly the reflected rays of the
spectrum of the first order. The lens is brought as near
to the grating as possible without its edge intercepting
the ray coming from a. In the focus of this lens, at ef,
a ground glass is placed. This portion of the apparatus
is, however, nothing more than the sliding part of a com-

MEMOIII VII.] STUDIES IX THE DIFFRACTION SPECTRUM.

mon pliotograpbic camera, which contains the ground
Lrlass and shields for sensitive preparations."

In the publication above referred to I gave engravings
of the results thus obtained; the fixed lines were marked
by their wave-lengths. The photographs were very clear
and beautiful ; they bore magnifying six or eight times
without injury to their sharpness.

I may here be permitted to add that it was on the
publication of these researches in 1844 that I first made
the suggestion to describe spectrum effects by wave-
lengths, or what, perhaps, is still better, by ether vibra-
tions— a method now generally adopted. I may give
the following extracts:

" In the earlier discussions of the chemical effects of
light, the different regions of the spectrum were marked
out by the designations of the different colored rays, and
effects were described as taking place in the red or yel-
low or violet regions respectively. An improved plan
was proposed by Herschel, and followed by him in his
various writings. It consists essentially in dividing the
space which exists between the red and yellow ray as
insulated by cobalt blue glass into 13.30 parts, taking
the centre of the yellow ray as the zero point, and
continuing the divisions equally into the more and less
refrangible regions.

u Over these methods the use of the fixed lines pos-
sesses very great advantages, inasmuch as we make refer-
ence to actually visible points existing in the spectrum.

"It has been stated that the deviations of the different
fixed lines in the diffraction spectrum are proportional
to the lengths of the undulations which they respective-
ly represent. By designating the different points of the
spectrum by their wave-lengths, the subdivision may be
carried to any degree of minuteness, the measures of one
author will compare with those of another, and the dif-

128 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

ferent phenomena of chemical changes occurring through
the agency of light become at once allied to a multitude
of other optical results. If it were necessary, by a very
simple arithmetical process we could determine the num-
ber of vibrations executed by a ray bringing about a
given decomposition in billionths of a second. The
fixed lines used in this way enable us at once to divide
the diffraction spectrum into any number of parts, and,
by comparing wave-lengths and the velocity of light, to
indicate effects either in space or in time."

The diffraction spectrum, as wre have seen, differs
strikingly from the prismatic in the arrangement of its
colored spaces. In the latter, the less refrangible parts
are compressed more and more in proportion as their
refrangibility is less. Now there is reason to believe
that in the former the colored spaces are equally warm,
though so feeble is the calorific effect that all attempts
at the direct measurement of the heat have proved
unsatisfactory. I first made such attempts with very
delicate thermo-electric apparatus, but could not obtain
sufficiently striking results. Admitting, however, that
every ray, irrespective of its color, in the act of extinc-
tion will generate the same amount of heat, it necessarily
follows that in the prismatic spectrum the heat should
appear to increase steadily from the more to the less
refrangible end, because in it the compression of the
colored spaces is becoming greater and greater, and this
is what is actually observed.

These considerations respecting the distribution of
heat in the spectrum lead naturally to the examination
of a much more comprehensive problem — indeed, one of
the most important problems that science presents — viz.,
the constitution of the sunbeam.

Until the time of Newton it was universally admitted

MEMOIR VII.J STUDIES IN THE DIFFRACTION SPECTRUM. 129

that light is a pure un decomposable elementary principle.
He showed that this conclusion must be modified. No
one, except Leibnitz, in those days, and no one for a long
time subsequently, discerned the ominous import of the
discoveries of this Prince of geometers. Of his detec-
tion of the origin of Kepler's laws, and its necessary
consequence of the mode in which the government of
the universe is conducted, I have nothing here to say.
Let us see how it was with his discoveries concerning
light.

His interpretation of the experiment he made in the
"dark chamber" was this, that light is not an undecom-
posable element, as was at that time supposed, but that
in reality it consists of not fewer than seven different
constituents, recognizable by their color. These, if mixed
in any manner together, whether by grinding tinted pig-
ments or revolving parti • colored sectors, or converging
the spectrum through a convex lens, would, by their
union, produce white light. His felicitous experiment
with the two reversed prisms silenced the carping critics
of that day, who had declared that the colored tints
with which he was working had no such origin as he
affirmed — in difference of refrangibility — but were anal-
ogous to the iridescent play of light on a pigeon's breast,
or the more gorgeous lustre of a peacock's tail. It cost
only a short struggle, and the theory of the composite
nature of light made good its ground.

When, therefore, Herschel, in his examination of the
sun's surface through colored glasses, came to the conclu-
sion that the heat emitted by the sun is essentially and
intrinsically distinct from the light, and that these ele-
ments may be parted from each other by refraction, he
did no more than develop the principle that had been
announced by Newton ; and when, at a later period,
Melloni extended these researches, and it was universal-

I

130 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

ly admitted that there are heat rays which, like light
rays, have various refrangibilities, this conclusion was
quite accordant with Newton's results. Heat was con-
sidered as existing in the solar beam independent and
irrespective of light. In fact, the one might be easily
separated from the other.

When, again, the Swedish chemist Scheele, investigat-
ing the chemical action of light, showed that there are
rays invisible to the eye, and of greater refraugibility
than the violet, which can produce the decomposition
of certain compounds of silver, these were considered to
be an additional element, and passed under the designa-
tion of chemical rays, deoxidizing rays, etc. Treated of
in the works of physics of those times as imponderable
bodies, there seemed to be no necessary limit to their
number. More than half a hundred ponderable sub-
stances were known. Why, then, should there not be
as many of these imponderable ones? This was the
view universally entertained at the time I began the
experimental study of radiations. For such as are con-
cerned in producing chemical changes I suggested a
special designation, which, however, did not find accept-
ance : the inappropriate and unmeaning appellation, ac-
tinic rays, was preferred.

Meantime, however, the undulatory theory of light
had been steadily making its way. It was exhibiting
all the aspect of a great physical truth, in not only
rendering an explanation of known facts, but also in
predicting the occurrence of other facts previously un-
known. Persons who were in the front of the scientific
movement in this direction had thus their attention
forcibly drawn to a contemplation of the whole subject
from this new point of view. They very soon perceived
that from it bonds of interconnection between facts
hitherto supposed to be isolated might be discerned;

MEMOIK VII.] STUDIES IN THE DIFFRACTION SPECTRUM.

tilings that were fragmentary and confused spontaneous-
ly fell into an orderly arrangement.

While the theory of optics was making this great
advance, another important science, physiology, was pre-
senting a similar development. It was casting off the
Vital Force of the older medical authors, and acknowl-
edging the dominion of chemical and physical forces.
It had become plain that the interpretation of many
phenomena, as hitherto received, must be changed.

We may apparently have heat without light, and
licrht without heat. In the darkest room we cannot

o

perceive vessels filled with boiling water, yet the warmth
we experience on approaching them assures us that they
are emitting radiations. Is not this heat without light?
If we stand in the rays of the full moon, we cannot de-
tect any increase of temperature. Is this not light with-
out heat? It is true that in this latter instance we are
mistaken as to the fact; but overlooking that — for the
heat to be detected in the moonbeams requires the most
sensitive apparatus — do not such observations assure us
that heat and light are independent of each other, phys-
ical principles having an existence separate from each
other ?

Such were some of the arguments on which was sus-
tained the hypothesis of the intrinsic difference of light
and heat. In this, no account was taken of the optical
functions of the eye. Qualities were incorrectly attrib-
uted to radiations which, in truth, were due to peculiari-
ties in the organ of vision.

The great service which the diffraction spectrum has
rendered to science is the abolishment of all these im-
aginary independent existences — heat, light, actinism,
etc. — and the substitution for them of the simpler con-
ception of vibratory motions in the ether. The only
difference existing among the radiations that issue from

132 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII.

a grating, in the manner we have been describing, is in
their wave-lengths, or what comes to the same thing, in
their times of vibration. The diversity of effects pro-
duced depends on the quality of the surface on which
they fall. If on a dark surface, and the more so in pro-
portion to its blackness, they engender heat ; if on the
retina, they are interpreted by the mind as light ; if on
photographic preparations, they produce decompositions,
designated actinic effects.

Heat, light, actinism, are, then, not natural principles
existing independently of each other, but effects arising
in bodies from the reception of motions in the ether,
motions which differ from each other in their rapidity.
Of those that the eye can take cognizance of, the most
rapid impart to the mind the sensation of violet light,
the slowest the sensation of red, and intermediate ones
the intermediate optical tints. Colors, like light itself,
are nothing existing exteriorly. They are merely mental
interpretations of modes of motion in the ether, and in
this they represent musical sounds, which exist only as
interpretations by the mind of waves in the air.

MKMOIH VIII.] THE PHOSPHORESCENCE OF BODIES. 133

MEMOIR VIII.

ON THE PHOSPHORESCENCE OF BODIES.

From the Philosophical Magazine, Feb., 1851 ; Harper's New Monthly Magazine,

No. 325.

CONTENTS: — Early observations on phosphorescence. — The diamond. —
Duration of shadows. — Lemery's theory. — Du Fay's theory. — Quali-
ties of diamond and fluor-spar. — The volume of a phosphorescent body
does not change during its glow. — A structural change accompanies the
phosphorescence of bodies ; there is a minute disengagement of heat. —
Phosphorescence is not communicable. — Absolute quantity of light
emitted.

THERE are some surfaces on which if a shadow falls, it
can be brought into view a long time subsequently.

A belief in the existence of the carbuncle, a stone
supposed to have the property of shining in the dark,
appears to have been current from the very infancy
of chemistry. It gave rise to many legends among the
alchemists, and early travellers relate marvellous stories
respecting self-shining mountains and gems. Thus it
was said that the King of Pegu wore a carbuncle so
brilliant that if any of his subjects looked upon him in
the dark, his countenance seemed as though it was irra-
diated by the sun, and that in a certain part of North
America there was a mountain which illuminated the
country for many miles, and served by its rays to guide
the Indians at night. The story seems to indicate that
the locality of this wonder was somewhere in the west-
ern part of Pennsylvania. Mr. Boyle relates that a gov-
ernor of one of the American colonies imparted this fact
to him at a time when he was charged with the superin-

134 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

teridence of those important settlements, and that an ex-
pedition had been despatched to ascertain the facts cor-
rectly. It saw the shining wonder from afar, but the
light diminished as the place was approached, and be-
coming at length invisible, the locality could not be
determined with certainty.

These legends had for some time been passing into
discredit, when Viucenzio Cascariola, a cobbler of Bolog-
na in Italy, who had abandoned the mending of shoes
for the purpose of finding the philosopher's stone, discov-
ered his celebrated phosphorus, the Bolognian stone, or,
as it was then designated, sun-stone (lapis Solaris). He
had seduced himself into the expectation that a heavy
mineral he had met with — barium sulphate — contained
silver, and in an attempt to melt out that precious metal
was astonished to see that the burned substance shone
like an ignited coal in the dark. This was in the year
1605.

Some time afterwards a Saxon of the name of Baldwin
conceived the idea of obtaining the soul of the world by
distilling in a retort chalk which had been dissolved in
aqua fortis. In this extraordinary pursuit accident led
him to observe that the substance he was working with
possessed the quality of shining in the dark after it had
been exposed to the light of the sun. The alchemist
Kunckel, who relates the incident, tells us with gravity
how he stole a piece of this substance on the occasion of
a visit he made to Baldwin one night when that adept
was trying to make his phosphorus shine by the light
absorbed from a candle, and also from its image reflect-
ed by a concave mirror. In consequence of this theft,
Kunckel succeeded in discovering what the substance
was, and made known the method of its preparation.

The special condition under which these preparations
shine in the dark was very quickly detected. Isidore,

MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 135

of Seville, speaking of the "lightning-stone," says, "Si
sub clivo positus fuerit fulgorem rapit sidereum." That
condition is previous exposure to light.

The discovery of the elementary substance now known
as phosphorus drew the attention of the cultivators of
natural science to this singular property, and under the
names of sun-stones, light magnets, noctilucas, etc., vari-
ous shining bodies were introduced. But the first truly
scientific examination of the subject was made by Boyle,
on the occasion of observing that a certain diamond be-
longing to Mr. Clayton, and subsequently purchased by
Charles II., emitted li^ht in the dark. Though he does

O O

not seem to have been aware of it, the fact itself was not
new, for the alchemist Albertus Magnus says in the thir-
teenth century that he had seen a diamond which glowed
when it was put into warm water. A diamond rubbed
upon gold becomes beautifully luminous; as Bernouilli
remarks, it shines like a burning coal excited by the bel-
lows.

A diamond rubbed upon gold emitting light! the
imaginary or intrinsic value of the substances employed
adds to the glory of the phenomenon. A light, too, that
cannot be extinguished by water, and yet so ethereal and
pure that it can set nothing on fire. Here certainly were
facts of interest enough to excite the philosophers of the
last century.

The chief points ascertained by Boyle respecting the
diamond were that it shone by friction with various
bodies, and at the same time displayed electrical devel-
opment ; that it also glowed when warmed by a candle,
the fire, a hot iron, or even when placed on the skin.
Under the latter circumstances it exhibited no electricity,
being unable to attract a hair held near to it. He also
found that it would shine under water, various acid or
alkaline liquids, or when covered with saliva, and that

136 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

the glow was increased when the gem was put into hot
water.

These results led to the conclusion that though under
certain circumstances the light was accompanied by elec-
trical development, as when friction had been used, there
was no necessary connection between the two properties.
The gem would shine without the least trace of attract-
ive power.

(Among substances endowed with this property, one
of the best was discovered about a century ago by Can-
ton. Still known as Canton's phosphorus, it is easily
made by burning oyster-shells in an open fire until they
have become white ; then, having pulverized them with
about a quarter of their weight of flowers of sulphur,
they are once more brought to a dull red heat in a cru-
cible. This completes the preparation. A convenient
mode of using the substance is to provide a piece of tin
plate-two or three inches square, brush over one side of
it with gum or glue water, then dust upon it from a fine
sieve some of the powdered phosphorus. In this manner
a uniform white surface is procured, well adapted for
experiments.

If on such a surface a key or other opaque object be
laid, and it then be exposed for a moment to daylight,
on carrying it into a dark room and removing the key,
a spectral shadow will be seen, depicted in black, and its
contour marked out by the brilliantly glowing phos-
phorus surrounding it. After continuing to shine for
some minutes, the light gradually fades, and finally be-
comes extinct. If, this having been accomplished, the
phosphorized plate be put away in a box or drawer
where not a ray of light can reach it, and kept therein
for days or even weeks, on exposing it in a dark room,
on a plate of warm metal, the phantom shadow will
emerge, perhaps even more strongly than at first.

MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES.

A wonderful experiment, truly. Shadows, then, are
not such fleeting, such fugitive things as poets say.
They may bury themselves in stony substances, and be
made to come forth at our pleasure.

The persistence of such surface phantoms may be strik-
ingly illustrated by a simple experiment in which light
is not concerned. If on a cold polished metal, as a new
razor, an object such as a small coin be laid, and the
metal be then breathed upon, and when the moisture
has had time to disappear, the coin be thrown off, though
now the most critical inspection of the polished surface
can discover no trace of any form, if we breathe once
more upon it a spectral image of the coin comes plainly
into view. And this may be done again and again.
Nay, more, if the razor be put carefully aside where noth-
ing can deteriorate its surface, and be so kept for many
mouths, on breathing again upon it the shadowy form
emerges.)

Early in the last century two hypotheses were intro-
duced for the explanation of the various cases of phos-
phorescence :

1. That phosphorescent bodies act like sponges to
light, absorbing it, and retaining it by so feeble a power
that very trivial causes suffice for its extrication. This
was the view of Lemery, and was published in 1709.

2. That phosphorescence arises from an actual combus-
tion taking place in the sulphureous parts of the glow-
ing body. It is to be remembered that sulphur figured
largely in the chemistry of those days. This was the
hypothesis of Du Fay.

To this celebrated electrician we owe a very able in-
vestigation of the phosphorescence of various bodies, and
especially of the diamond. He recognized the fact, over-
looked by Boyle, that the gem must first be exposed to

138 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

the light ; and then, when taken into a dark place, it
shines for a time, the light gradually fading away. But
the glow can be re-established by raising the tempera-
ture, and an exposure of a single second to the sun is
quite enough to commence the process.

To recognize feeble degrees of luminosity, it is neces-
sary for the observer to remain in the dark until the
pupil of the eye is quite dilated, and the impression of
light to which the retina has been exposed is worn off.
Du Fay gives a singular but very serviceable practical
process. He recommends the experimenter to keep one
eye bound up or closed for the purpose of observing in
the dark, and to use the other in conducting his process-
es in the light. He remarks the curious fact that the
eye which has been shut will not have the delicacy of
its indications affected by that which has been exposed
to the light.

In~this manner Du Fay found that of four hundred
yellow diamonds all were phosphorescent; but some
that were white or rose-colored or blue or green were
not. Nor was there any external indication by which
it could be told whether any given one of these kinds
would shine. He discovered, too, that the glow took
place under various - colored media, as stained - glass,
water, milk, but not under ink. He also made attempts
to compel the gem to preserve its light by enveloping it
in opaque media, such as ink, black wax, etc., under the
idea that the light could not get out, and concluded that
he had partially succeeded, because in some instances the
diamonds would shine after being so shut up for six or
twelve hours. He verified Boyle's fact on the effects of
hot water and heating generally, and carried his tem-
peratures to far higher degrees, even above a white heat,
finding that the stone had lost none of its qualities, for
it would take light again when it was cold on a momen-

MI.M..III VIII.] THE PHOSPHORESCENCE OF BODIES.

tnry exposure to the sun. He also investigated how far
tin glow was connected with electrical relations, and
showed its perfect independence. He also greatly in-
creased the list of phosphori, asserting that, so far from
the quality being a peculiarity of the Bolognian stone,
Baldwin's compound, the diamond, all solid substances
except the metals, are phosphorescent when rightly treat-
ed, and even these he believed would eventually be
found to have the same property.

There is one point upon which Du Fay dwells that
deserves more than a passing remark — the connection
between phosphorescence and temperature. He proved
that phosphori cannot absorb light so well when they
are warm as when they are cold, and that a rise of tem-
perature always makes them disengage their light.

It is obvious that these early investigators labored
under great difficulties arising from the imperfect chem-
ical science of those times. They confounded together
things that were essentially different, such as the shining
of urine-phosphorus with the glow of the diamond, and
this again with the electrical light arising when friction
has been employed. Then, again, their erroneous views
of the composition of bodies were constantly leading
them astray. Thus Du Fay, finding that the Bolognian
stone (barium sulphate) emitted a sulphury smell, and
thinking that it shone because of the burning of the

o o

sulphur, transferred the same explanation to the case of
yellow diamonds, and asserted that they also glowed
through the combustion of the sulphur that discolored
them.

I do not intend now to give a review of the subse-
quent discoveries and hypotheses brought forward by
the numerous experimenters of the last century, or by
Heinrich, the Becquerels, Biot, Poggendorff, Pearsall, and
many others in this. I may, however, recall attention to

140 TIIE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

a very elaborate memoir by Osann (PoggendorfFs Anna-
len, 1834, vol. xxxiii., p. 405), in which he discusses the
various theories of combustion, absorption, excitation,
etc., and gives many new facts.

(Respecting the phosphorescence of diamonds, I have
recently had an opportunity of making a curious obser-
vation. A lady, a relative of mine, has a pair of ear-
rings in which are set two large and beautiful gems,
both of which phosphoresce after exposure to an electric-
al spark; she has also another pair in which both the
diamonds in like manner phosphoresce. Judging from
these four instances, one might regard this property as
very common. Curiously enough, the necklace belong-
ing to this set, containing thirty-eight stones of very fine
water, has only one that will phosphoresce. This neck-
lace would, therefore, lead us to reverse the conclusion
to which the ear-rings had led us, and to infer that phos-
phorescing diamonds are comparatively rare.)

All solid substances, except the metals, possess the
phosphorescent quality. We may, however, by making
a judicious selection of the bodies which are to serve as
our means of experiment, disembarrass the inquiry of
many of its complications. If we employ the Bolognian
stone (barium sulphate) or Canton's phosphorus (cal-
cium sulphide), or, indeed, any other substance liable
to undergo chemical changes in the air, we introduce
unnecessary phenomena, and cannot distinctly prove
whether the shining is due to a direct combustion of
the parts or to other causes.

Among selections that might be made, diamond and
fluor-spar possess qualities rendering them very eligible
for these purposes — unchangeability in the air and
under water. Even between these there is a choice, for
fluor-spar possesses all the good qualities of diamond.
It might be said, considering the chemical relationships

MEMOIR VIII.] TIIK PHOSPHORESCENCE OF BODIES.

of diamond, that when it glows it undergoes a kind of
surface combustion, which is the cause of the light ; but
though direct experiments prove that this is not the
case, it is much better to resort to fluor-spar, which is
free from such an objection. It is absolutely incombus-
tible. Besides, it can be obtained perfectly transparent
or nearly opaque ; it occurs of many tints of color ; can
be easily cut and polished to any figure, and obtained in
pieces of any required size. Its phosphorescent powers
are very high ; indeed, it yields, when properly treated,
to no other substance, not even to Canton's phosphorus,
in that respect, and greatly exceeds the potash sulphate,
a substance which, however, possesses many eligible
qualities. It will therefore be understood that in select-
ing fluor-spar and its varieties as the subjects for experi-
ment, it has been done with a view of bringing the re-
sults to their simplest conditions. In such inquiries
Canton's phosphorus and bodies chemically changeable
are wholly inadmissible.

The specimens of fluor-spar employed by me were de-
rived from many different sources, American and Euro-
pean. The color of the light they emitted was in some
cases blue, in some green, in some yellow. Among them
was an American variety of chlorophane of a pale flesh-
colored aspect, translucent on the edges, and excelling
all the others in the splendor of its light. It equalled
the best Canton's phosphorus in power, yielding a superb
emerald green light when it received the rays of the sun
or of an electric spark, or had its temperature raised.
The warmth of the hand in a dark place made it shine.
Considering the facility with which we can regulate the
intensity of an electric spark, measuring out the quanti-
ties of light used in a given experiment, it is clear that
there are great advantages in resorting to it in prefer-
ence to the variable rays of the sun. Our choice of a

142 THE PHOSPHORESCENCE OF BODIES. [MKMOIR VIII.

substance should be controlled by these conditions, and
fluor-spar completely fulfils all the indications.

It will be seen, however, that I have not restricted
myself to the use of this body, but whenever other sub-
stances could be compared with it, have resorted to them
also. The general principles here set forth as applicable
to fluor-spar may be likewise extended to them.

To what cause are we to attribute phosphorescence?
What are the changes taking place in the glowing
body?

We have already seen that a century ago two different
answers had been given to these questions. Lemery
supposed that all bodies act towards light as they do to
heat, absorbing it and then giving it out : Du Fay that
all phosphorescences are cases of combustion.

Before we can reach a decision there are evidently
many preliminary points to be settled. If -chemical
changes between the glowing body and the air are dis-
posed of, and the action is recognized to be of a purely
physical or molecular kind, it is necessary to determine
(1) whether there is any expansion or contraction of the
shining body during its glow ; (2) whether there is any
structural change; (3) whether there is any evolution
of heat along with the light ; or (4) any development of
electricity. These inquiries will now be taken up in
succession.

I. Is there any change of volume in a phosphorescent
body during its glow f

I attempted to ascertain this by causing various bodies
to shine brightly when enclosed in glass vessels filled
with water, so that if there were any expansion the
water might be pressed out into a slender tube, and the

MKM..IU VIII. J Till-: I'linsi'IlOKESCENCE OF BODIES.

143

of dilatation thereby determined. The arrange-
ment was as follows :

A glass tube about two inches long and three quar-
tt-is of an inch in diameter was closed at its upper end
by means of a plate of polished quartz, cemented air-
tight. Immediately beneath the quartz the phosphores-
cent body was supported. Through a cork which closed
the other end of the tube there passed a piece of ther-
mometer tube bent on one side, and to it was affixed a
scale. The arrangement was supported on a suitable
stand, so that the quartz was uppermost, and at a little
distance above it the spark from a Leyden-jar could be
passed between a pair of stout iron wires maintained at
an invariable distance, and thus produced phosphores-
cence in the body. It may be remarked that these ef-
fects of an electric spark do not take place well through
glass, and hence a plate of quartz, which readily trans-
mits them, must be used.

In Fig. 14, a a is the glass tube, b b the plate of pol-
ished quartz, c the phosphorescent body,
d d the cork closing the lower end of the
tube, e e the bent tube,/ its scale, g g the
iron wires connected with a Leyden-jar,
and giving a spark. The index drop at
li refers not to this, but to a subsequent
experiment.

The large tube containing the phosphor-
escent body must be filled quite
full of water, free from air, as also
must be the thermometer tube to
a given mark on its scale. If an
electric spark be now passed between the wires to make
the phosphorus shine, it is clear that if there be any ex-
pansion or contraction of its volume, there will be a corre-
sponding movement in the water of the thermometer tube.

144 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

On making the trial, and using in succession a crystal
of violet-colored fluor-spar, a piece of flesh-colored chloro-
phane, and a mass of Canton's phosphorus, the result in
all cases was negative; for, though these different sub-
stances glowed very brilliantly as soon as the spark
passed, there was not the smallest movement perceptible
in the index liquid of the thermometer tube.

With a view of estimating the delicacy of the means
thus used for determining any change in the volume of
the spar, the solid content of a piece of chlorophane was
determined by weighing in water; also the value of each
division of the scale was ascertained. The value of each
such division was equal to -j-gW of the volume of the
spar, and a movement equal to one tenth of that value
could have been detected.

It may therefore be concluded that a phosphorescent
body, when at its maximum of glow, has not changed its
volimie perceptibly.

The conclusion thus arrived at is strengthened by an-
other mode of experiment. If change of volume be con-
nected with this evolution of light, it might reasonably
be expected that a sudden, severe, but equable compres-
sion, exerted on a piece of spar, the light of which is just
fading out, would compel it to regain a portion of its
brilliancy. A piece of chlorophane in that condition was
placed in water contained in the apparatus known as
Oersted's instrument for measuring the compressibility
of water, and which is described in most of the treatises
on physics ; but though, by suitably turning the screw,
pressures varying from one to four atmospheres were
suddenly put on the spar and as suddenly removed, no
change whatever was seen in the glowing mass, the light
of which continued steadily to die away.

In Fig. 15, Oersted's instrument for proving the com-
pressibility of water, a a is the glass cylinder filled with

145

MEMOIR ViJL] T1IK 1'IIOSPHORESCENCE OF BODlKs.

water, b the pressure screw, c the phosphores-
cent spar or substance.

These experiments have a bearing on Le*-
inrry's theory. A mass of iron suddenly com-
pressed grows hot; so, too, does atmospheric
air. It would, therefore, not be unreasonable
to expect that if a phosphorus acted like a
sponge to light, and were thus pressed upon,
it would yield up its light. But conceptions
derived from the old theories of specific heat Fig. 16.
are perhaps scarcely applicable here.

When unequal pressure is applied, the result is differ-
ent. A piece of chlorophane pressed by a forceps glows
brightly ; if crushed, the fragments sparkle like little
fire-works as they fly through the air. If the spar be
previously powdered, a shining is still produced, and
when the pulverization is conducted in an agate mortar
in the dark, bright eddies of light follow the track of
the pestle. In these cases, however, the separation of
the laminae of the crystal and the heat produced by
friction probably determine the result. Canton's phos-
phorus did not shine when compressed or submitted to
friction.

2. Does any structural change accompany the phos-
phorescence of bodies f

The foregoing experiments appearing to prove that
if there be any expansion of a phosphorescing body, it
is to a very small amount, I next endeavored to de-
termine whether there is any molecular change, or new
structural arrangement, which can be detected by polar-
ized light.

A flat piece of fluor-spar, polished on both sides, was
placed in a polariscope, and a pair of blunt iron wires
connected with a Leyden-jar were adjusted near the

K

146 TIIE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

front of it, so that when the spark passed, a brilliant
glow arose in the spar, which was at once viewed
through the analyzer of the instrument. But though
the experiments were made both by daylight and lamp-
light, no kind of effect could be detected. Had any
molecular change occurred, it could not have escaped
notice.

In Fig. 16, a b is the polariscope, c the flat piece of

fluor-spar, b the analyzer, d d
the wires giving the electric
spark.

These experiments were first
made by using as the analyzer
a doubly refracting achromatic
prism ; they were, however, re-
peated with a Nicol, in which
the eye is not disturbed by a
bright image as in the other
case. Having fixed the plate of polished fluor in the
polariscope, it was readily perceived that it possessed
naturally a structural arrangement, for there were cloudy
spaces or lines in it which contrasted with the faint
white light passing in the adjacent parts. It was also
seen that this structural arrangement could be deranged
in a transient manner, either by pressure or an unequal
warming, as is well known of other bodies ; but when a
powerful electric discharge was passed near the spar,
and a brilliant phosphorescence took place, no impres-
sion could be detected. Even when the iron wires rest-
ed on the spar, and the explosion passed over its surface,
nothing was perceptible except along the line between
the ends of the wires, where the surface was roughened
or abraded by the force of the discharge.

But though these experiments with polarized light
give a negative result, or, at all events, prove that a

MEMOIR VIII.] THE PHOSPHOKESCENCE OF BODIES. 147

phosphorus when shining has its molecular condition so
little disturbed that the change cannot be detected in
this way, there can be no doubt that if the means of
testing were more delicate, such a change would be dis-
covered, for many years ago Mr. Pearsall found that
specimens of iluor, not possessing phosphorescence natu-
rally, might have that quality communicated to them by
repeated exposure to many powerful electric discharges,
which also gave rise to a change in their natural color.
Now there can be no doubt that such an alteration of
tint implies an alteration of structure.

Besides the test by polarized light, there is another
which may be resorted to for the detection of structural
changes when they are merely superficial ; it is the
mode in which various vapors will condense. I de-
scribed several such cases in the Philosophical Magazine
for September, 1840, some time previously to the pub-
lications on the subject that were made by M. Moser.
They were brought forward at that time as an illustra-
tion of the manner in which mercurial vapors condense
on a daguerreotype plate and develop images which it
has received. Proceeding on this principle, a large plate
of fluor-spar, the surface of which was finely polished,
was made to phosphoresce brightly along a given line
determined by the ends of two iron wires, which served
as a discharger for a Leyden spark, and were placed
near to the polished surface. The spar was forthwith
suspended in the mercurial box of a daguerreotype ap-
paratus and kept there an hour. The mercury condensed
upon it faintly in the manner it would have done on a
daguerreotype pi-ate, especially on and in the vicinity
of those parts that were more immediately exposed to
the spark. This, therefore, seems to prove that there is
in these cases a molecular modification of the shining
surface.

148 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

3. When a phosphorescent body glows, does it likewise
emit heat?

A very thin bulb, half an inch in diameter, was blown
on a piece of thermometer tube, and after being washed
over with gum-water, finely powdered chlorophane was
dusted on until it was neatly coated all over. A drop
of water was then introduced into the tube to serve as
an index. Although the instrument was very sensitive
to heat, when the chlorophane was made to shine and
emit a gorgeous emerald green light by the passage of
a powerful electric spark near it, no movement whatever
of the index ensued. From this it would appear that
the quantity of heat developed by phosphorescence must
be very small.

In Fig. 17, a a is the glass bulb covered with a coat-
ing of powdered chlorophane, b
a drop of water serving as an
index.

A modification of this exper-
iment, which appeared to offer

several advantages, was tried. The instrument repre-
sented in Fig. 14 was emptied of its water, and a single
drop, h, put into the index tube. It was supposed that
when the rays of the electric spark passed through the
quartz and made the phosphorus shine, the air contained
in the tube, warmed thereby, would expand, and a move-
ment in the index liquid of the thermometer tube take
place. But in several trials, in which different bodies —
chlorophane, Canton's phosphorus, etc. — were employed,
the results were uniformly negative; for though these
different substances glowed splendidly as soon as the
spark passed, there was not the slightest rise of tem-
perature perceptible.

A further attempt was made as follows : The disk of
quartz being removed and replaced by a cork, through

MEMOIR VIII.] THE PHOSPHOIIESCENCE OF BODIES.

which a pair of iron wires to serve as a discharger passed
air-tiurht, and descended to within a short distance of the

O '

phosphorus, sufficient time was allowed in various repeti-
tions for the index liquid to come to rest. It was hoped
that this form of experiment would have advantages over
the preceding, because the discharging wires could be
brought nearer to the phosphorus, and the effect take
place without the intervention of the quartz. When
the spark was made to pass, there was a great move-
ment in the index tube, as in the instrument known as
Kinnersley's electrometer, but the liquid immediately re-
turned to within a short distance of its first place ; then
a slow dilatation occurred, as though the air was gradu-
ally warming. Thus in one experiment the liquid stood
at 24°, after the explosion it returned to 26°, and then
there was a gradual dilatation to 32°.

To eliminate the various disturbing causes in this ex-
periment, it was repeated many times, the spar being al-
ternately introduced into the glass tube, and alternate-
ly removed. It was found that whenever the spar was
present the gradual dilatation alluded to took place;
but when the spar was not in the tube, instead of a
dilatation, there was a gradual contraction until the in-
dex liquid recovered its original position.

From this it appears that with the evolution of light
there is a feeble extrication of heat.

The quantities of heat thus liberated are so small, and
the causes of error are so numerous, that I endeavored
by other methods to obtain more trustworthy results.
Thus I attempted to determine the surface temperature
of a flat piece of chlorophane while phosphorescing by
means of the thermo-electric multiplier. The thermo-
pile was placed in a vertical position, and the spar
having been attached to a piece of wood, which served
as a handle, intense phosphorescence was communicated

150

THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

by a Leyden spark, and the flat and shining surface
instantly put on the upper face of the pile. But there
was no movement of the astatic needles.

Then, taking the stone by its handle, it was touched
with the tip of the finger for one second, and quickly
placed on the pile. A prompt movement of the needles,
amounting to four degrees, ensued. These experiments
were repeatedly tried, and the results were uniformly
the same.

In Fig. 18, a a is the thermo-electric pile, b b the

plate of chlorophaue, c the
handle.

On considering these re-
sults, it appears that as the
temperature of the air near
the multiplier in one of
the experiments was 53°,
and the estimated temper-
ature of the skin 94°, the
amount of heat which the
stone received from the
touch of the finger must
have been very small. I
made a comparative trial
by touching the bulb of a
thermometer for the same space of time, in the same
way, and found that there was a rise of about 1^°. But
the conductibility of quicksilver is much greater than
that of chlorophane.

It is to be inferred, therefore, that the quantity of
heat set free during phosphorescence is very small,
and that the surface of the chlorophane does not
change its temperature by one fourth of a degree ;
for had it done so, the multiplier would have instant-
ly detected it.

Fig. 18.

MBMOIR VIII.] THE PHOSPHORESCENCE OF BODIES.

4. TV photpliorescence accompanied with a development
of electricity ?

It has been stated already that the experimenters of
the last century paid a good deal of attention to this
point. Du Fay established the fact that though in many
cases of phosphorescence there is a development of elec-
tricity, there are many others in which the light seems
to be wholly unattended by any disturbance of that
kind.

I have repeated some of these experiments, and with
the same result, proper care being taken to avoid fric-
tion and other obvious causes of electrical excitement.
Thus a flat piece of chlorophane, phosphorescing power-
fully, was put on the cap of a very delicate gold-leaf
electroscope, but no disturbance whatever was percep-
tible.

A large crystal of fluor-spar was made to phosphoresce
brilliantly along a line about half an inch in length by
passing the spark of a Leyden-jar between two blunt
iron wires, the ends of which were that distance apart,
and resting on the face of the crystal. Over this line of
blue light, which was pretty sharply marked, and which
lasted for several minutes, a fine hair was held. This
would have been readily attracted and repelled by the
feeblest excitation of sealing-wax, but in this case it
wholly failed to yield any indication whatsoever.

In connection with the foregoing experiments, I may
mention some miscellaneous facts. Some attempts' were
made to determine whether phosphorescent bodies in
the field of a powerful electro-magnet would exhibit any
change of property. Six Grove's pairs were caused to
magnetize a good electro-magnet ; the power they could
give to it would enable the keeper to support about
half a ton. Between its polar pieces chlorophane, Can-

152 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

ton's phosphorus, etc., which were made to glow by ex-
posure to a Leyden spark, were placed. But it made no
difference in the light whether the magnetism was on
or not.

It was also found that the electric spark from a con-
tact-breaker would communicate phosphorescence to all
the various bodies in use in these experiments, and that
up to a certain point the intensity of the light increased
with the number of sparks received.

Phosphorescence is not communicable from one body
to another. Having provided two polished plates of
fluor-spar, one of them was made to glow by an electric
spark, and the other was immediately put upon it. No
communication of phosphorescence took place ; the sec-
ond piece remained perfectly dark.

Some authors state that fluor-spar does not become
phosphorescent by exposure to the sun ; but this re-
mark does not apply to all varieties of it. Thus some
chlorophane, which had been ignited in a glass tube till
it had ceased to shine, was pulverized and again ignit-
ed in a platinum crucible. It emitted an emerald light.
A slip of wood was now put on it to screen a part of
its surface, and it was exposed to the sun for a few
minutes. On ignition, it shone again finely, with a green
light, the shadow of the wood being beautifully depict-
ed. The same having been repeated a great many
times, it appeared that the phosphorescence at last be-
gan to decrease, perhaps by frequent ignition causing a
change.

A screen of yellow glass intervening between the sun
and some powdered chlorophane prevented phosphores-
cence, but it took place through a plate of polished fluor-
spar. When the light of an electric spark was used in-
stead of the sunshine in this experiment, the fluor-spar
prevented phosphorescence.

MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. J53

GENERAL CONCLUSIONS.

The results to which the foregoing experiments bring
us are therefore—

1st. That the methods employed in these experiments
are not sufficiently delicate to detect any increase in the
dimensions of a phosphorus while it is in a glowing state.

2d. No structural change can be discovered by resort-
ing to polarized light ; but there is reason to believe
from the change of color which certain bodies exhibit

^j

when the quality of shining is communicated to them,
and from the manner in which vapors condense on their
surfaces, that such has actually taken place.

3d. That phosphorescence is attended with a minute
rise of temperature.

4th. That it is not necessarily connected with any elec-
trical disturbance.

On comparing these conclusions, it is obvious that if
the third be correct there must necessarily be a change
of volume, and that the reason the dilatation is not dis-
covered by direct experiment is owing to the insufficiency
of the means employed.

The general definition given of phosphorescence is that
it is the extrication of light without heat (Gmelin).
But these results show that that definition is essentially
incorrect; for if the experiment be made with due care,
a rise of temperature can be detected, though its absolute
amount may be very small.

Determination of the absolute quantity of light emitted
by phosphori.

And now we may inquire how it is with the light it-
self? do we not deceive ourselves respecting it? We
ought to recollect that it is barely perceptible in the
open day, and that these experiments require to be made

154 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

in the dark. We should also recollect the great sensi-
tiveness of the eye, and how feeble a luminous impres-
sion it can detect. Impressed with these facts, I have
endeavored to compare the absolute quantity of light
given by the most brilliant phosphori with some well-
known standards. The result of these experiments puts
a new view on the whole subject.

The first attempts I made for this purpose were con-
ducted on the principle of comparing the stains formed
on a daguerreotype plate by the phosphorus under trial,
and by an oil-lamp, receiving the rays from each on a
concave metallic mirror eight inches in diameter and
fifteen in focus, arranged as a reflecting camera obscura.
There were set, side by side, a small oil-lamp, a piece of
white paper illuminated by the lamp, and a fragment of
chlorophane, arranging things in such a manner that the
chlorophane might be illuminated by rays coming from
a contact-breaker worked by two Grove's pairs. The
contact-breaker was kept in action fifteen minutes, and
then, to prove the sensitiveness of the plate, the lamp
was moved for one minute to a new position, and the
experiment closed.

On developing, it was found that the impressions of
the lamp had solarized, both that of fifteen minutes and
that of one, proving that such a light in one minute is
amply sufficient to change the plate to its maximum.
Also the electric spark of the contact-breaker was solar-
ized, and the image of the piece of white paper beauti-
fully given of a clear white; but the phosphorescing
spar had made no impression, except from one portion
where it had reflected the rays of the spark.

Suspecting that the spark from the contact-breaker
might not have been powerful enough, I repeated the
experiment, using sparks from a Leyden-jar. The oil-
lamp was exposed in front of the mirror one minute, and

MEMOIR VIII.] THE I'lIOSl'IIOKESCENCE OF BODIES.

• 155

thru removed; then ten strong sparks were passed over
the spar, each of which made it emit an emerald light;
but during the moment of the passage of each spark a
screen was interposed, that no direct or reflected light,
and, indeed, none but that of the phosphorus, could reach
the mirror and sensitive plate.

On mercurializing, it was found as before that the
lamp was beautifully depicted but the spar was invisible.

In Fig. 19, a is the oil-lamp, b the white paper, c the
chlorophane, cut and polished, d the contact-breaker
with its wires,// the concave mirror. Its concavity
faces the above-named objects, and reflects their images
inverted and reversed on a sensitive plate, e. The mir-
ror and sensitive plate are enclosed in a darkened box
not shown in the figure.

Fig. 19.

Estimated, therefore, by the chemical effects they can
produce, the light from chlorophane is incomparably less
intense than that from a common lamp. For there can
be no doubt that each of the ten Leyden sparks gave a
light which made the spar phosphoresce brilliantly for six
seconds, and the whole phosphorescence was equal in du-
ration to that produced by the light of the lamp; yet the
latter had changed the plate to a maximum, while the
former had not made the smallest perceptible impression.

As the foregoing attempt to obtain photographic ef-

156 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

fects had failed, I varied the experiment as follows : In a
Bohemian glass tube a quantity of chlorophane in coarse
fragments, sufficient to occupy about three inches in
length of the tube, was placed. The reflecting camera
with its sensitive silver plate wras set in a proper po-
sition. When everything was arranged, a spirit-lamp
was applied to the chlorophane, which soon emitted a
superb emerald light, and continued to do so for about
two minutes. As the light began to decline, the spar
splintered by decrepitation. The process went on in a
very satisfactory way. An oil-lamp was then placed in
front of the camera for five seconds. On developing, the
image of the lamp-flame came out, but no trace whatever
of the chlorophaue could be detected. Thus it appears
that the splendid green light emitted when the spar is
heated is at least twenty-four times less intense than the
light emitted by a small oil flame. It should be remem-
bered that this is a .measure of absolute intensity, and
not of illuminating power.

But as it is known that green light is not very ef-
ficient in changing a sensitive surface, I tried to deter-
mine the intensity of the light emitted by chlorophane
by the optical method of Bouguer, described in the first
of these Memoirs, p. 39.

The spar being heated by a current of hot air arising
from the flame of a spirit-lamp, the light of which was
carefully screened by a chimney and other contrivances
of sheet-iron, a comparison was made with a very small
oil-lamp, the flame of which was about six tenths of an

inch hisrh and the wick one sixth of an inch thick. It

~

was covered with a glass shade.

The spar, when it began to glow, cast a reddish shad-
ow on the paper, which shadow was extinguished when
at its maximum by the lamp at about 25 inches, the
spar being at 5 inches.

MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. J57

The distances of the chlorophane and lamp from the
paper were, therefore, as 1:5. The illuminating effect is
as the squares of those numbers, and therefore 1 : 25.
But for extinction it requires that one light should be
sixty times as intense as the other; it follows, therefore,
that at those distances the illuminating power of the
lamp is fifteen hundred times as intense as the illumi-
nating effect of the spar.

But the quantity of spar used in this experiment ex-
posed a surface much greater than that of the flame ; it
was estimated to be at least twice as great. This, there-
fore, would bring us to the conclusion that the intrinsic
brilliancy of the chlorophaue is not y^ part that of
the lamp.

This experiment was several times repeated. Thus it
was found that the lamp extinguished the shadow from
the spar when the relative distances were 1 : 4. The
lamp at 4 was therefore sixty times as luminous as the
spar at 1 ; that is, their illuminating power was as 1 : 960.
But it was estimated that the surface of the spar was 3£
times that of the flame of the lamp, so this would make
the intrinsic brilliancy -g^Vs* a result of the same order
as the preceding.

From this we conclude that the intrinsic brilliancy of
pJiospJiori is very small ; a fine specimen of chlorophane,
at its maximum of brightness, yielding a light three thou-
sand times less intense than the flame of a very small oil-
lamp.

It was stated above that these photometric experi-
ments put a new view on the whole subject; in fact,
they explain all the difficulties of the foregoing inquiries.
How could we expect to be able to measure the heat of
phosphorescence? The radiant heat of the little oil-
lamp here employed would require at such distances a
very delicate thermometer to measure it. Is it likely,

158 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII.

then, that we could detect that of a source three thou-
sand times less intense ?

I conclude, therefore, that all phosphoric bodies emit
radiant heat as well as light ; but that its quantity is so
small that we have no means delicate enough to measure

O

it, though the eye is so sensitive that it can detect the
light, the absolute intensity of which has, however, hith-
erto been greatly overrated. I believe that the quanti-
ties of both are of the same order, and if this be true we
should scarcely expect to discover any dilatation of the
glowing body unless means much more refined than
those here resorted to are employed.

MEMOIR IX.] EFFECTS OF HEAT ON PHOSl'HOKKSCENCE. 159

MEMOIR IX.

ON THE EFFECTS OF HEAT ON PHOSPHORESCENCE.

From the Philosophical Magazine, Feb., 1851.

CONTENTS: — Experiment of Albertus Magnus. — Degree of phosphores-
cence at different temperatures. — The quantity of light a substance can
retain is inversely as its temperature ; the quantity it can receive is di-
rectly as the intensity and quantity of light to which it has been ex-
posed.— Phosphorescent images of the moon. — Action of ether waves. —
Effects of cohesion. — Reason that gases, liquids, and metals are non-
phosphorescent.

IT has been already observed that the effect of heat
in promoting the disengagement of light is an old dis-
covery. Albertus Magnus remarked it in the case of
a diamond plunged into hot water.

It is customary in later works which treat systemat-
ically on phosphorescence to group the different facts
under two heads — 1st, phosphorescence produced by in-
solation ; 2d, by heat. An example of this is offered in
the standard work on chemistry by Gmelin.

A division of this kind brings the whole subject into
confusion. It assigns different causes for things that are
essentially allied. It leads to the inference that as un-
der certain circumstances the sunlight or an electric

O

spark can make bodies glow, so under other circumstan-
ces heat will produce the same effect, and this wholly
independent of incandescence.

But what are the facts ? If a yellow diamond placed
upon ice be submitted to the sun, arid then brought into
a dark room the temperature of which is 60°, for a time
there is a glow, but presently the light dies out. If the

160 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX.

diamond be now put into water at 100°, it shines again,
and again its light dies away. If next it be removed
from that water and suffered to cool, and then be re-
immersed, it will not shine again ; but if the water be
heated to 200°, and the diamond be dropped into it,
again it glows, and again its light dies away.

There is, therefore, a correspondence between the light
disengaged and the temperature. We are not to con-
clude from the foregoing illustration that when the dia-
mond has its temperature raised from 100° to 200° the
light is due to the heat. On the contrary, the light is
unquestionably due to the primitive exposure to the
sun ; just as in Lemery's illustration of the sponge, if we
exert a little pressure a portion of the water flows out ;
if a stronger pressure, still more ; and for each degree of
pressure there will be a corresponding quantity of water
expelled.

The connection between phosphorescence and temper-
ature may be instructively illustrated as follows:

Suppose that three yellow diamonds, a, b, c, have been
simultaneously exposed to the sun, a being kept at 32°,
b at 60°, c at 100°, and that they are then simultaneously
removed to a bath of water at 100° in a dark room ; it
will be found that a emits a bright lisrht, b shines more

O O '

feebly, and c scarcely at all. This is what ought to be
expected from the principle laid down above ; for if at a
particular temperature a certain quantity of light is set
free, it is clear that a has the advantage of b, so that it
will disengage all the light to be set free between 32°
and 60°.

From such experiments and considerations it is to be
inferred that there is an intimate connection between
temperature and phosphorescence which may be conven-
iently expressed in the following terms. The quantity of
light a substance can retain is inversely as its temperature.

MI.M..IK IX ] 1.1 1 KCTS OF HEAT ON PHOSPHORESCENCE.

This principle furnishes the explanation of a multitude
of facts. Thus Du Fay discovered that the Bolognian
stone shines brighter when exposed to the sky than to
the sun. In the latter case the temperature rises, and
the quantity of light retained is less. Under violet
and other glasses, stained with such colors as impede
the warming effect, phosphorescence is even more vivid
than when no glass has intervened. On the same prin-
ciple we have an explanation of Du Fay's apparently
successful attempt to prevent the escape of light from
diamonds by putting them in ink or covering them with
black wax. When removed from the ink and brought
into the air, they became somewhat warmer — perhaps
the touch of the finger aided the effect — and a corre-

O

•ponding quantity of light was set free.

But though temperature is a controlling, it is not the
only condition involved. If it were, phosphorescence
after insolation should occur only after a rise of tempera-
ture. The fundamental fact of the whole inquiry proves
that a glowing body can retain more light in presence
of a lucid surface than it can in the dark.

Is not this fact analogous to what we meet with in
the exchanges of heat? A substance can retain more
heat in presence of a hot body than a cold one. The
brilliancy and quantity of light to which a phosphorus
is exposed goes very far to determine the intensity of
the subsequent glow. Thus I found that a piece of
chlorophane exposed to one spark of a contact-breaker
shone feebly, but if it had received one hundred sparks,
its light was very vivid ; and it has long been known
that in delicate phosphori a certain degree of luminosity
can be communicated by the moonbeams, a more intense
one by lamp-light, and one still more brilliant by the
sunshine or a Leyden spark. This, therefore, leads to
the conclusion that the quantity of light a phosphorus

L

162 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX.

can receive is directly as the intensity and quantity of
light to wJiicli it has been exposed.

(With respect to the light of the moon, I have suc-
ceeded in obtaining an image of that satellite on Canton's
phosphorus, by the aid of a concave metallic mirror.)

The various facts herein cited indicate that when a
ray of light foils on a surface, it throws the particles
thereof into vibration. An examination of the action of
the differently colored rays dispersed by a prism shows
that in general the greater the frequency of vibration
of the impinging ray, the more brilliant is the phosphor-
escence. But in such a prismatic examination we have
constantly to bear in mind the disturbing agencies which
are present, and especially the antagonizing effects of
heat ; that this determines the amount of light that a
phosphorus can receive, and also the rate of its subse-
quent extrication. In Memoir V., p. 87, I have shown
how* the photographic action of light betrays the general
principle of an interference of vibratory movements, and
the production of antagonizing results in different parts
of the solar spectrum. An argument is there brought
forward to the effect that as the violet end produces
phosphorescence and the red extinguishes it, this is a
proof of opposition of action. In explaining this fact,
M. E. Becquerel supposes the darkening power of the
red rays to be due to the more rapid disengagement of
the phosphorescence by reason of the heat produced by
those rays, arid the apparent antagonization is not at-
tributable to the supposition of vibratory movements of
light rays of different frequency, but to the relations of
caloric and light. The force of this explanation, how-
ever, disappears when it is understood that light and
heat, the chemical and phosphorogenic rays, are, accord-
ing to the principles of the able experimenter, all mani-
festations of the same agent. It avails us nothing to say

MEMOIK IX.] EFFECT'S OF HEAT ON PHOSPHORESCENCE.

that a want of phosphorescence at the less refrangible end
of the spectrum is due to the heat-giving powers of those
rays, when that very heat-giving power is under the hy-
pothesis dependent on their comparative rapidity of vi-
bration.

We may, therefore, in the explanation of phosphor-
escence, abandon expressions derived from the material
theory of light, and assume that whenever a radiation
falls upon any surface, it throws the particles thereof
into a state of vibration, just as in the experiment of
Fracaster, in which a stretched string is made to vibrate
in sympathy with a distant sound, and yield harmonics,
and form nodes. Such a view includes at once the facts
of the radiation of heat and the theory of calorific ex-
changes ; it also offers an explanation of the connection
of the atomic weights of bodies and their specific heats.
It suggests that all cases of decomposition of compound
molecules under the influence of a radiation are owing
to a want of consentaneousness in the vibrations of the
impinging ray and those of the molecular group, which,
unable to maintain itself, is broken down under the pe-
riodic impulses it is receiving into other groups, which
can vibrate along with the ray.

If a hot body, a, be placed in presence of a cold body,
£>, the theory of the exchanges of heat teaches that the
temperature of the latter will steadily rise until equi-
librium between the two takes place. The molecules of
a communicate their vibratory movement to the ether,
and this in its turn imparts an analogous movement to
the molecules of b. For, as the ethereal medium is of
vastly less density than the vibrating molecules, each of
these oscillations will produce in it a determinate wave,
which is propagated through it according to the ordi-
nary laws of undulations, in such a way that the ether
would be in repose after the wave had passed, were it

1(54 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX.

not for the recurrence of the continuing vibration of the
molecules. At each vibration the molecules of a lose a
part of their vis viva, by the quantity they have com-
municated to the ethereal wave, the intensity or ampli-
tude of the wave becoming less and less as this abstrac-
tion of force is going on. But the ether being of uniform
density and elasticity throughout, each of its particles
communicates the whole vis viva it has received to the
next adjacent, and would instantly come to rest were it
not again disturbed by the vibrations of the material
molecules. These elementary considerations show how
it is that a wave of sound passes through the air, or of
light through the ether, and the particles of those media
come instantly to rest; but a hot body or a vibrating
string persists in its motions, which only undergo a grad-
ual decline. If the vibrating molecule were in a medium
of the same density, it would impart to it all its motion
at once, and in the same way that a heavy molecule grad-
ually communicates its motion to the ether, so in its turn
does the ether to other systems of molecules.

Upon these principles we may explain the phenomena
of phosphorescence. From a shining body undulations
are propagated in the ether, and these, impinging on a
phosphorescent surface, throw its molecules into a vi-
bratory movement. These in their turn impress on the
ether undulations ; but by reason of the difference of its
density compared with that of the molecules, they do
not lose their motion at once; it continues for a time,
gradually declining away and ceasing when the vis viva
of the molecules is exhausted.

When a phosphorescent surface is exposed to the lu-
minous source, it necessarily undergoes a rise of temper-
ature, and the cohesion of its parts is diminished ; but
after its removal from that source, as the temperature
declines and radiation goes on, the cohesion increases,
and a restraint is put on those motions.

MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE.

Now let the phosphorus have its temperature raised,
and the cohesion of its molecules be thereby weakened,
and the restraint on their motions abated. At once they
resume their oscillations, and continue them to an extent
that belongs to the temperature used. When this has
passed away, a still higher temperature will release them
once more, and the glowing will again be resumed.

What would be the result if we could cause the sur-
face of a mass of water on which circular waves are rising
and falling to be instantaneously congealed? It might
be kept in that condition for a thousand years, and then,
if instantaneously thawed, the waves would resume their
ancient motion from the point at which it was arrested,
and it would now go on to its completion.

So with these phosphor! Exposed to light of a suit-
able intensity, their parts begin to vibrate ; but the free-
dom of those motions is interfered with by their cohe-
sion. Amplitude of vibration must always be affected
by cohesion, and if the ray be removed and the tem-
perature be permitted to decline, the restraint becomes
greater and greater, and they pass into a condition some-
what like that which has just been illustrated. It mat-
ters not how long a time may intervene, rise of tempera-
ture will enable them to resume their motions.

These principles give an explanation of all the facts
we observe. We see how it is that as we advance from
one temperature to another the phosphorus will resume
its glow, and that there is, as it were, for every degree
a certain amount of vibratory movement that can be ac-
complished, or, to use a different phrase, a certain amount
of light that can be set free. It also necessarily follows
that different solids will display these motions with dif-
ferent degrees of facility, and hence shine for a longer or
shorter time, and with lights of different intensities.

But in liquids and gases, which want that particular

EFFECTS OF HEAT ON PHOSPHOKESCENCE. [MEMOIR IX.

condition of cohesion characteristic of the solid state, and
the parts of which move freely among each other, phos-
phorescence cannot take place, for it depends on the in-
fluence that cohesion has had in restraining the vibra-
tory movements.

Further, the condition of opacity does not permit phos-
phorescence to be established. The exciting ray cannot
find access to disturb the interior layers of the mass, and
even if it did, and phosphorescence ensued, how could
we expect to be able to discover it through the impervi-
ous veil of the superficial layers? The light of the most
brilliant phosphorus cannot be seen through the thin-
nest gold-leaf. Its intensity is vastly too small. These
are the reasons that no one has ever yet succeeded in
detecting phosphorescence in metals and black bodies.

It will be gathered from this explanation that I am
led to believe that all the facts of phosphorescence can
be fally explained on the principles of the communica-
tion of vibratory motion through the ether ; that as upon
that theory an incandescent body maintained at incan-
descence would eventually compel a cold body in its
presence to come up to its own temperature by making
its particles execute movements like those of its own, so
the sunshine or the flash of an electric spark compels a
vibratory movement in the bodies on which its rays fall ;
that these vibrations are interfered with by cohesion in
the case of solids, but that they are instantly established
and almost as instantly cease in the case of liquids and
gases ; that reducing the cohesion of a solid by raising
its temperature permits a resumption of the movement ;
and that the condition of opacity, whether metallic or
otherwise, is a bar to the whole phenomenon.

MEJJOIK X.] THE DECOMPOSITION OF CAUBONIC-ACIU GAS.

MEMOIR X.

ON THE DECOMPOSITION OF CARBONIC • ACID GAS BY
PLANTS IN THE PRISMATIC SPECTRUM.

From the Proceedings of the American Philosophical Society, May, 1843; Phil-
osophical Magazine, Sept., 1843; American Journal of Science and Arts, Vol.
XXVI., 1843; Philosophical Magazine, Sept., 1844.

CONTENTS: — The decomposition of carbonic acid by light formerly at-
tributed to the violet ray. — It can be successfully accomplished in the
prismatic spectrum. — It takes place not in the violet but in the yellow
ray. — Decomposition by yellow absorbent media. — Analysis of the gas
evolved ; it always contains nitrogen. — Decomposition of alkaline car-
bonates and bicarbonates. — Analysis of the gas evolved in different rays.

FOR many years it has been known that the green
parts of plants, under the influence of sunlight, possess
the power of decomposing carbonic acid, and setting
free its oxygen. It is remarkable that this, which is a
fundamental fact in vegetable physiology, should not
have been investigated in an accurate manner. The
statements met with in the books are often far from
being correct. It is sometimes said that pure oxygen
gas is evolved, that the decomposition is brought about
by the so-called " chemical rays ;" these and a multitude
of such errors pass current. So far as my reading goes,
no one has yet attempted an examination of the phe-
nomena by the aid of the prism, the only way in which
it can be correctly discussed.

In a paper by Dr. Daubeny, inserted in the Philosoph-
ical Transactions for 1836, two facts, which I shall verify
in this communication, are fully established. These are,
1st, the occurrence of nitrogen gas in mixture with the

163 THE DECOMPOSITION OF CARBONIC- ACID GAS. [MEMOIR X.

oxygen, an observation originally due to Saussure, or
some earlier writer; and, 2d, that the act of decompo-
sition is due to the LIGHT of the sun. This latter ef-
fect, obtained by employing colored glasses or absorb-
ent media, has not been generally received. Doubt will
always hang about results obtained in that way, and
nothing but an examination by the prism can be satis-
factory.

In its connection with organic chemistry and physi-
ology the experiment of the decomposition of carbonic
acid by leaves assumes extraordinary interest. When
we remember that this decomposition is the starting-
point for organization out of dead matter, that com-
mencing with this action of the leaf the series of or-
ganized atoms goes forward in increasing complexity,
and blood and flesh and cerebral matter are at its ter-
minus, it is clear that unusual importance belongs to
precke views of this the commencing change. The rays
of the sun are the authors of all organization.

There is but one way by which the question can be
finally settled — it is by conducting the experiment in
the prismatic spectrum itself. When we consider the
feebleness of effect which takes place by reason of the
dispersion of the incident beam through the action of
the prism, and the great loss of light through reflection
from its surface, it might appear to be a difficult opera-
tion to effect a determination in that way. Encouraged,
however, by the purity of the skies in America, I made
the trial, and met with complete success.

When the leaves of plants are placed in water from
which all air has been expelled by boiling, and exposed
to the sun's rays, no gas whatever is evolved from them.
When they are placed in common spring or pump wa-
ter, bubbles quickly form, which, when collected and
analyzed, prove to be a mixture of oxygen and nitro-

X.J llli: DECOMPOSITION OF CARBONIC- ACID GAS.

gen gases; from a given quantity of water a definite
quantity of air is produced. When they are exposed
in water which has been boiled, and then impregnated
with carbonic acid, the decomposition goes on with ra-
pidity, and large quantities of gas are evolved.

The obvious inference seeming to arise from these
facts is that all the oxygen collected is derived from
the direct decomposition of carbonic acid.

Having by long boiling and subsequent cooling ob-
tained water free from dissolved air, I saturated it with
carbonic-acid gas. Some grass leaves, the surfaces of
which were carefully freed from adhering bubbles or
films of air by having been kept in carbonated water
for three or four days, were provided. Seven glass
tubes, each half an inch in diameter and six inches
long, were filled with carbonated water, and into the
upper part of each the same number of blades of grass
were placed, care being taken to have all as near as could
be alike. The tubes were placed side by side in a small
pneumatic trough. It is to be particularly remarked that
the leaves were of a pure green aspect as seen in the wa-
ter; no glistening air-film such as is always on freshly
gathered leaves nor any air -bubbles were attached to
them. Great care was taken to secure this perfect free-
dom from air at the outset of the experiments.

The little trough was now placed in such a position
that a solar spectrum, kept motionless by a heliostat,
and dispersed in a horizontal direction b}^ a flint-glass
prism, fell upon the tubes. By bringing the trough
nearer to the prism or moving it farther off, the different
colored spaces could be made to fall at pleasure on the
inverted tubes. The beam of light was about three
fourths of an inch in width.

In a few minutes after the beginning of the experi-
ment, the tubes on which the orange, yellow, and green

170 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X.

rays fell commenced giving off minute gas-bubbles, and
in about an hour and a half a quantity was collected
sufficient for accurate measurement.

In Fig. 20, a a represents the trough, and K, O, Y, etc.,
the tubes containing the leaves and carbonated water
placed so as to receive the spectrum, b b.

R o

Fig. 20.

The gas thus collected in each tube having been trans-
ferred to another vessel and its quantity determined, the
little trough with all its tubes was freely exposed to the
sunshine. All the tubes now commenced actively evolv-
ing gas, which, when collected and measured, served to
show the capacity of each tube for carrying on the proc-
ess. If the leaves in one were more sluggish or exposed
a smaller surface than the others, the quantity of gas
evolved in that tube was correspondingly less. And
though I could never get the tubes to act precisely
alike, after a little practice I brought them sufficiently
near for my purpose. In no instance was this testing
process of the power of each tube for evolving gas omit-
ted after the experiment in the spectrum was over.

From the following table it appears that the rays which
cause the decomposition of carbonic-acid gas are the or-
ange, the yellow, the green ; the extreme red, the blue, the
indigo, and the violet exerting no perceptible effect. We

MEMOIR X.] THE DECOMPOSITION OF CARBONIC-ACID GAS.

Table of the Decomposition of Carbonic Acid by Light of Di/erent

Colors.

Experiment I.

Experiment II.

Name of ray.

Volume of gas.

Name of ray.

Volume of K»B.

Kxtreme red

.83

20.00
86.00
.10
.00
.00
.00

Kxtreme red and red.
Red and orange

.00

24.75
43.7',
4.10
1.00
.00
.00

Red and orange. . .
Yellow and green.
Green and blue. . .
Blue

Yellow and green

Green and blue

Blue

Indigo

Violet

Violet

should therefore expect that in a beam, passing through
absorbent media of such a nature that the extreme red,
the blue, the indigo, and violet are absorbed, this de-
composition should nevertheless go on. A solution of
bichromate of potash nearly fulfils these conditions. It
transmits the luminous rays in question, except a trace
of those which correspond to the more refrangible yel-
low and less refrangible green.

A remarkable proof of the correctness of the foregoing
prismatic analysis comes out when leaves are made to
act on carbonated water in light which has passed
through a solution of bichromate of potash. I took a
wooden box of about a cubic foot in dimensions, and
having removed its bottom, adjusted to it a trough made
of pieces of plate-glass. The box being set on one side,
its lid served as a door, and the trough being filled with
a solution of bichromate of potash, the sun's beams came
through it, and in the interior of the box leaves and car-
bonated water could be exposed to the rays that had
escaped absorption. The thickness of the liquid stra-
tum was about half an inch. I had several such boxes
made, so that I might compare the simultaneous effect
of light that had undergone absorption by different me-
dia. They formed, as it were, little closets, in which
bodies could be exposed to parti-colored light — blue,
yellow, red, etc.

Fig. 21.

172 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X.

Fig. 21 represents one of these closets; a a is its side

of stained -glass, or the
trough containing the ab-
sorbing solution, bichro-
mate of potash, etc. ; b b,
the door.

Whenever an experi-
ment was commenced in
these closets, a similar one
was simultaneously com-
menced in the unobstruct-
ed sunshine. It is needless to state that in all these
care was taken to have the different arrangements for
decomposition as nearly alike as possible.

On comparing the amount of gas evolved in unab-
sorbed light and in light that had undergone absorption
by bichromate of potash, in three out of five trials the
gas collected under the latter circumstances exceeded in
volume that collected under the former; this was prob-
ably due to a higher temperature existing in the box.

On comparing the volume of gas collected under bi-
chromate of potash and under litmus -water, the latter
was not equal to half the former.

I compared the gas evolved in unobstructed light,
under bichromate of potash, and under ammonia sul-
phate of copper; the results were as follows:

Unobstructed light 4.75

Bichromate of potash 4.25

Ammonia sulphate of copper 75

It therefore appears that light which has passed
through bichromate of potash, by which the chemical
rays have been absorbed, can accomplish this decompo-
sition ; but light which has passed through ammonia
sulphate of copper, which transmits the chemical rays,
fails to produce that effect.

Mi .MM IK X.] THK DECOMPOSITION OF CARBONIC-ACID GAS. 173

For these reasons I conclude that the decomposition
of carbonic acid by the leaves of plants is brought about
by the rays of light, and that the calorific and so-called
chemical rays do not participate in the phenomenon.
The rays of light are therefore as much entitled to the
appellation of chemical rays as those which have here-
tofore passed under that name.

Next I examined the constitution of the gaseous mixt-
ure given off during these decompositions. It proved to
be not pure oxygen, but a variable mixture of oxygen,
nitrogen, and carbonic acid. Omitting the carbonic acid,
which diffused from the solution in variable quantities,
the following table represents the composition of the
gaseous mixture :

Analyses of Gas evolved from Carbonated Water.

Exp.

Name of plant.

Oxygen.

Nitrogen.

1

Pinus tacda.

16.16

8.:$4

2

(4

27.16

13.84

3

II

22.33

21.67

4

Poa annua.

90.00

10.00

5

* 4

77.90

22.10

This table contains a few out of a great number of
experiments, all of which might have been quoted as ex-
amples of the conclusions which I wish to deduce. 1st.
They all coincide in this, that the oxygen is never
evolved without the simultaneous appearance of nitro-
gen ; 2d. That when certain leaves are employed, as those
of the Pinus taeda, there seems to be a very simple rela-
tion between the volumes of oxygen and nitrogen. In
the first and second of these experiments, the volume of
oxygen is to that of nitrogen as two to one; in the third,
as one to one. In certain cases this apparent simplicity
of proportion is departed from ; but from its frequent oc-
currence in many analyses I have made it seems to de-

174 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X.

mand attentive consideration. Moreover, in other plants,
as in experiments 4 and 5, the amount of oxygen is rela-
tively greater, and between it and the nitrogen there
does not appear any exact proportion.

In order to ascertain whether decompositions taking
place under absorbent media, as bichromate of potash,
yield the same results as indicated in the foregoing
table, I made several analyses of gas collected under
those circumstances. The presence of the absorbent me-
dium did not seem to exert any influence whatever, the
general results coming out as though it had not been
employed.

It was also found that the alkaline carbonates and
bicarbonates could be decomposed by leaves in yellow
light. The alkaline bicarbouates, as is well known, un-
dergo decomposition by a slight elevation of temperature.
When boiled in water they gradually give off their sec-
ond atom of carbonic acid, and slowly pass into the con-
dition of neutral carbonate. In the experiments I made
with them the boiling was not continued long enough to
affect to any extent the constitution of the salt, and in
each case any portion of carbonic acid extricated during
cooling was removed by the air-pump. A few leaves
placed in this solution showed no effect if kept in the
dark, but if brought into the sunshine there was a copi-
ous evolution of gas-bubbles, which, on detonation with
hydrogen, proved to be rich in oxygen gas. One experi-
ment gave 88 of oxygen, 12 of nitrogen.

In a subsequent communication to the Philosophical
Magazine (Sept., 1844), I gave additional analyses of
the gas emitted in yellow light as follows :

Five tubes, each three eighths of an inch in diameter
and six inches long, were inverted in a small trough of
water containing carbonic acid, with which the tubes
were also filled. Some blades of grass nearly of the

MEMOIR X.] THE DECOMPOSITION OF CARBONIC-ACID GAS. J 75

same size and volume were placed in each tube. This
grass had been kept for two days in the dark in a bottle
filled with carbonated water. During this time the film
of air which envelops all new leaves was removed, the
grass became perfectly free from all adhering gaseous
matter, and when in the carbonated water exhibited a
dark-green aspect.

I have previously found that leaves thus soaked emit
under the influence of light a larger amount of nitrogen
than usual ; this comes from the incipient decay of some
of their nitrogenized constituents. When under these
circumstances they are placed in the sunshine, this nitro-
gen comes off along with the gas liberated from the car-
bonic acid.

In the experiment I am now relating, a tube arranged
like one of the foregoing five evolved in the open sun-
shine a certain volume of gas which was composed of —

Oxygen 41 }

Nitrogen 59 '- 100

Carbonic acid 00 )

The five tubes were placed in the spectrum in the fol-
lowing colors, and emitted the quantities of gas repre-
sented in the following table :

1

Extreme red and red.

0.0

2

Orange and yellow

19.8

3

Yellow and green

27.4

4

Blue " "

0.5

5

Indigo and violet

0.0

The gas in tube 2, which had been in the orange and
yellow ray, was then washed with a solution of caustic
potassa. After this it still measured 19.8. It contained,
therefore, no perceptible quantity of carbonic acid. It
was next examined for oxygen, and with the following
result :

176 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X.

Constitution of Gas emitted in Orange and Yellow Light.

Oxygen 8.0 } f Oxygen 40.4

Nitrogen 11.8^- or •< Nitrogen 59.6

Carbonic acid.. 0.0 ) ( Carbonic acid. . 00.0

19.8 100.0

The gas evolved by the yellow and green rays was
next analyzed. Like the former, it underwent no dimi-
nution by washing with caustic potash. After this
treatment it therefore measured 27.4, and on being ex-
amined for oxygen yielded as follows :

Constitution of Gas emitted in Yellow and Green Light.

Oxygen 12.5 ~\ /'Oxygen 45.6

Nitrogen 14.9 >• or -(Nitrogen 54.4

Carbonic acid.. 00. o) ( Carbonic acid .. 00.0

27.4 100.0

In explanation of the large and variable amount of
nitrogen occurring in these analyses, it will scarcely be
necessary to remind the vegetable physiologist that it
arises from the mode of conducting the experiment. In
order to be absolutely certain that no atmospheric air
infilmed the leaves, they were soaked in water, and then
when brought into the sunlight the nitrogen which had
accumulated in their tissues from incipient decay dif-
fused out with the first portions of oxygen. As, there-
fore, more and more gas was evolved, the relative amount
of the nitrogen diminished. Thus the reason that the
third tube appeared to be richer in oxygen than the sec-
ond was owing to its containing more gas. Any person,
however, who is familiar with the physiological action
of leaves will understand these things without any fur-
ther explanation.

UNIVERSITY OF NEW YORK, June 15, 1844.

MKMOIU XL] THE FORCE INCLUDED IN 1'LANTS. 177

MEMOIR XI.

OF THE FORCE INCLUDED IN PLANTS.

Collected and condensed from Memoirs in the Journal of the Franklin Institute and
Harper's Monthly Magazine.

CONTENTS: — Growth of a seed in darkness and in light. — Action of
plants and animals respectively on the atmosphere. — Examination of
Rumford's experiments. — Dark heat rays cannot decompose carbonic
acid. — Germination in colored rays. — Greening of leaves takes place in
the yellow and adjacent rays. — The essential condition of all chemical
changes by radiation is absorption. — Plants absorb force from the sun ;
it is associated with their combustible parts, and is disengaged by oxi-
dation.

I HAVE given in Memoir II. a description of the struct-
ure of an ordinary flame. Its light is derived from par-
ticles of solid carbon issuing from combustible matters
with which the wick or the gas jet is fed ; these solid
particles, passing from a low temperature to a white
heat, and undergoing eventually complete oxidation, es-
cape into the atmosphere as carbonic-acid gas.

We now encounter a question of imposing interest:
Whence has the force which thus manifests itself as heat
and light been derived ? Force cannot be created ; it
cannot spring forth spontaneously out of nothing.

It may be said, without much error, of such flame-giv-
ing compounds as we are here considering, that they are
for the most part compounds of carbon with hydrogen.
To regard them as such will very much simplify the
facts we have now to present.

Under the form of oils and fats these combustible
substances are derived directly or indirectly from the
vegetable world ; directly, as, for instance, in the case of

M

178 THE FORCE INCLUDED IN PLANTS. [MEMOIR XI.

olive-oil ; indirectly, as in the case of animal oils and fats.
These have been collected by the animals from which
we obtain them out of their vegetable food. Even fats
derived from the carnivora have been procured from the
herbivora, and came originally from plants.

This brings us therefore to a consideration of the
chemical facts connected with the life of plants.

If a seed be planted in moist earth, the air having ac-
cess and the temperature that of a pleasant spring day,
germination in the course of a few hours will take place.
Should the process be conducted in total darkness, as in
a closet, the young plant shoots upward, pale, or at most
of a faint tawny tint. We can easily verify this state-
ment by placing a few turnip seeds in a flower-pot con-
taining earth, put into a closet or drawer from which
light has been carefully excluded.

A sickly-looking plant thus springs from a seed in the
dark. It is etiolated, as botanists say. If we examine
it carefully, making allowance for the water it contains,
we shall find that no matter how tall it may be, its
weight has not increased beyond the original weight of
the seed from which it came. It has been developing at
the expense of the seed, the substance of which has been
suffering exhaustion for its supply of nourishment. We
cannot continue this development in the dark indefinite-
ly, for the seed-supply is soon exhausted, and then the
shoot dies.

But if instead of exposing the seed which is the sub-
ject of our experiment to darkness, we cause the germi-
nation to take place in the open day, a very different
train of consequences ensues. There is no longer that
immoderate extension of a sickly etiolated stem upward,
but the parts emerging into the light turn green. Very
soon, to use a significant expression, they are wreaned
from the seed ; they no longer use the material collected

MKMOIH XL] THE FORCE INCLUDED IN PLANTS. j ;•»

for them in the preceding year by the parent plant, and
stored up for their use, but their leaves, expanding, turn
ijiven, and expose themselves to receive the rays of the
sun. If they be now examined as in the previous in-
stance, making allowance for the water they contain, it
will be found that from day to day their weight is in-
creasing; they are living independently of the seed.
They are obtaining carbon and hydrogen, the former
from carbonic acid, and the latter from water and am-
monia— compounds existing in the air or furnished from
the ground.

If a seedling, germinated in darkness and permitted to
grow to a certain extent, be then exposed to light, pro-
vided its dark -life has not continued too long, its etio-
lated aspect will soon disappear; it turns green, and as-
sumes all the characters of a healthy plant. This is in
effect the natural process. For we bury seeds a little
under the surface, covering them lightly with earth, the
opacity of which secures the necessary darkness ; but the
mould being moist, the air having a ready access, and
the temperature of the season suitable, all the conditions
needful for germination — water, air, warmth, darkness —
are present. The plumule, or shoot, makes its way out
of the obscurity into the light; its reliance for nutrition
on the seed ends; its independent life begins. It obtains
carbon, hydrogen, oxygen, nitrogen from the air, and sa-
line substances and water from the soil.

The facts which we thus bring into relief, as necessary
for the further exposition of the subject, are these : In
the first stage of the life of a plant, its dark -life, there
is, excluding water, a diminution in the weight ; in the
second, or light-life, there is an increase, due very largely
to the appropriation of carbon from the air. The atmos-
pheric carbonic acid has been decomposed, its oxygen set
free and, for the most part, permitted to escape, its other

180 THE FORCE INCLUDED IN PLANTS. [MEMOIK XI.

constituent, carbon, now ministering to the growth of the
plant.

A stone trough standing in a garden received the
waste water from a pump. There had accumulated on
its sides a green slimy growth (conferva). From this
growth, on the west side of the trough, which was receiv-
ing the morning rays of the sun, bubbles of gas were con-
tinually forming ; and these, as they attained a sufficient
size, rose through the water and escaped into the air.
This effect on the west side diminished as the sun passed
towards the meridian, but at mid-day the north side of the
trough was in full activity. As evening came on, that in
its turn gave forth fewer bubbles, and was succeeded in
activity by the east side. At first it was thought that
these bubbles were nothing more than the gas which is
dissolved in all water, and analogous in composition to
atmospheric air, but closer examination showed that it
was "oxygen, nearly pure. During the night no gas what-
ever was disengaged.

Priestley, Ingenhousz, Rumford, and other experiment-
ers of the last century investigated these facts carefully.
The conclusions to which they came may be thus sum-
marized : All ordinary natural waters contain carbonic
acid in solution ; leaves or other green parts of plants
placed in such water and kept in darkness exert no action
upon it, but in the sunshine they decompose the carbonic
acid, appropriating its carbon and setting its oxygen free,
as gas. Soon, however, the supply in the sample of water
is exhausted, and the action even in the sunlight ceases.
It is again resumed if more carbonic acid be artificially
dissolved in the water; and since the air expired from
the lungs in the act of breathing contains much of that
gas, it is sufficient, by the aid of a tube, or in any other
suitable manner, to conduct such expired air into the
water for the disengagement of oxygen to go on.

MKM..IU XL] THE FORCE INCLUDED IN PLANTS.

The experiments of these earlier chemists had thus
established the important fact that from carbonic acid,
which is extensively diffused through the atmosphere
and in water, and even in the soil, through the influence
of sunlight, oxygen is obtained. The sunlight, then, is
the force which carries into effect the decomposition.

There is thus a perpetual drain on the supply of car-
bonic acid, a perpetual tendency to its diminution, and
hence, for the order of nature to continue, there must be
an incessant supply. The source of that supply was very
strikingly indicated by some of Priestley's experiments.
" Having rendered a quantity of air thoroughly noxious
by mice breathing and dying in it, he divided it into
two receivers inverted in water, introducing a few green
leaves into one, and keeping the other receiver unaltered.
The former was placed in light, the latter in darkness.
After a certain time he found that the air in the former
had become respirable, for a mouse lived very well in it;
but that in the latter was still noxious, for a mouse died
the moment it was put into it."

To Priestley chiefly, though he was aided by other in-
vestigators, we must refer the honor of one of the great-
est discoveries of the last century. It was this, that the
two great kingdoms of nature, the animal and the vege-
table, stand at once in antagonism and alliance. What
is done by the one is undone by the other. Each is ab-
solutely essential to the existence of the other. There is
a never-ending cycle through which material atoms run.
Now they are in the atmosphere, then they are parts
of plants, then they are transferred to animals, and by
them they are conducted back to the atmosphere, to run
through the same cycle of changes a^ain. The sunlight

•J O O O

supplies the force that carries them through these revo-
lutions.

Previously to 1834 I had turned my attention to this

182 THE FORCE INCLUDED IN PLANTS. [MEMOIR XL

interesting subject. It had been asserted by Rumford
that many other substances besides the leaves of plants
would evolve oxygen gas. He specified raw silk and
cotton fibres. My investigation commenced by an exam-
ination of this assertion. I soon found that two totally
distinct things had been confounded. Ordinary water
contains, as has been said, carbonic acid in solution, but
it also necessarily contains the ingredients of atmospheric
air. To this, for the sake of distinctness, the perhaps in-
correct designation of water-gas may be given. Since
oxygen is very much more soluble than nitrogen, this
dissolved gas differs in composition from atmospheric
air. It is relatively richer in oxygen.

I very soon found, on exposing raw silk, spun glass,
and other such fibres, immersed in water, to the sun, that
Rumford's assertion was correct — gas -bubbles were set
free; but his inference was incorrect — the gas did not
comfrfrom decomposed carbonic acid; it was merely the
water-gas of the water. I was thus able to separate the
true from the false portion of the experiment. And
though I thus dispose of the subject in a few words, it is
perhaps due to the labor that was expended to say that
it cost several weeks of uninterrupted work and many
scores of analyses before I felt absolutely certain that
this was the indisputable interpretation of Rumford's
experiments.

Now as a guide to a correct exposition of the experi-
ments that I have to relate, it must be borne in mind that
the assumption of a green color by a germinating plant
and the decomposition of carbonic acid by it are identical
events. Or, perhaps, to speak more correctly, the latter
is the cause, the former the effect.

At that time very incorrect views of the nature of the
sun -rays were entertained. It was believed that they
contained three distinct principles: (1) heat, (2) light, (3)

MKMIHU XI.]

THE FORCE INCLUDED IN PLANTS.

a

chemical or deoxidizing radiations. In common with all
other chemists I accepted this view, and proposed to my-
self to determine to which of these principles the decom-
position of carbonic acid and the greening of plant leaves
are due.

And first, to ascertain if it were the heat radiation, I
converged, by the aid of a large metallic mirror, the dark
or invisible radiations emitted by an iron stove on some
leaves placed in water. The gas which was set free was
nothing more than the water-gas; there was no decom-
position of carbonic acid.

In Fig. 22, a is the concave mirror, I an inverted flask
containing the leaves and
water; it dips into a glass,
c, also containing water,
and receives the reflected
radiations of the stove, d d.

Then I tried a similar
experiment, using the radi-
ations emitted by a bright-
ly burning wood fire. The
result was the same as the
preceding, and I even push-
ed the experiment so far
that the water became very
hot, a portion of its carbonic acid effervesced from it, and
the leaves lost their bright green color. Still no decom-
position of the carbonic acid could be detected.

At this time it was generally received that the essen-
tial characteristic of the more refrangible — the violet —
rays is that they produce deoxidation. In accordance
with this opinion a name — deoxidizing — had been given
them. Now since the decomposition of carbonic acid is
an effect of deoxidation, I was not surprised at the issue
of the foregoing experiments, and expected to find that

Fig. 2*.

184 THE FORCE INCLUDED IN PLANTS. [MEMOIR XI.

though the less refrangible radiations, those of heat,
were inoperative, the more refrangible, the chemical or
deoxidizing, would decompose carbonic acid readily.

But some collateral experiments had thrown a diffi-
culty in the way of this conclusion. I had caused seeds
to germinate in three little closets (Fig. 21, page 1^2), into
which, by means of panes of colored glass, or through
troughs filled with colored liquids — red, yellow, and vio-
let— light respectively could be admitted I remarked
with very great surprise that the seeds in the red-light
closet and those in the violet one were just as much etio-
lated as they would have been had they grown in dark-
ness; those in the yellow closet promptly assumed a
green color, and developed themselves as well as if grow-
ing under natural circumstances.

But the light that comes through stained glass and
colored solutions is far from being homogeneous ; it con-
tains" rays of many refrangibilities. I therefore deter-
mined to attempt the greening of plants and the decom-
position of carbonic acid by their leaves — phenomena
which, as has been said, are equivalent — in the solar
spectrum itself.

I arranged things so as to have a horizontal solar spec-
trum of several inches in length kept motionless by a he-
liostat. I had previously caused to germinate in a wooden
box filled with earth, and of corresponding length, a crop
of seeds. They were etiolated, or blanched, for the ger-
mination had taken place in the dark. These young
plants I placed so as to receive the spectrum. Very soon
those that were in the yellow space turned green, but
those in the extreme red and extreme violet underwent
no change, though the exposure might be kept up the
whole day.

In Fig. 23, a a is the box containing the germinating
seeds, and placed so as to receive the colored spaces R,

Mi MOIR XL] THE FOHCE INCLUDED IN PLANTS.
R 0 Y G B I

185

Fig- &».

O, Y, G, B, I, V — red, orange, yellow, green, blue, indigo,
violet — of the spectrum.

Many repetitions of this experiment satisfied me that
it is the yellow and adjacent regions of the spectrum
which occasion the greening of plants; the heat rays
and the chemical rays have nothing whatever to do
with it.

Next I attempted the decomposition of carbonic acid
in the spectrum, and succeeded. I read before the Amer-
ican Philosophical Society in Philadelphia, at its centen-
nial celebration in 1843, an account of this experiment
as published in Memoir X.

In addition to the special interest of these experiments
on plant life, they had a very important bearing on the
general principles of actino - chemistry. They proved
that it is altogether incorrect to suppose that chemical
changes are brought about by the more refrangible rays
only. They showed that every ray has its proper chem-
ical function ; for instance, the violet in the decomposi-
tion of compounds of silver, the yellow in the case of car-
bonic acid. And hence I proposed to abandon the con-
ception of a tripartite division of the spectrum into heat,
light, and chemical radiations, and to designate radia-
tions by their wave-lengths, or, better still, by their mini-

186 THE FORCE INCLUDED IN PLANTS. [MEMOIR XI.

her of vibrations — a method now universally adopted in
spectrum analysis.

By other experiments — a narrative of which would be
too long for the present occasion — I established this
result: that for any ray to produce a chemical effect, it
must be absorbed. For instance, when a ray has passed
through a mixture of chlorine and hydrogen gases, and
by causing them to unite has produced hydrochloric acid,
it can no longer produce the same effect if made to pass
through a second portion of the same mixture; its act-
ing part has been detained or absorbed by the first. So,
too, the radiations which have fallen on a daguerreotype
plate, and impressed their image upon it, have lost the
quality of producing a similar effect on a second plate
that may be placed to receive them. Their active por-
tion has been taken up or absorbed by the first. The es-
sential preliminary of all chemical changes by radiations
is absorption.

But it must not be supposed that the rays thus ab-
sorbed are annihilated or lost. They are simply held in
reserve, ready to be surrendered again, undiminished and
unimpaired, if the conditions under which they were ab-
sorbed are reversed. They may appear under some other
form — as heat, electricity, motion — but their absolute
energy remains unchanged. This is a necessary conse-
quence of the theory of the Conservation and Correla-
tion of Force.

From this point of view how interesting is that great
discovery made by Angstrom, that an ignited gas emits
the same rays it absorbs — a discovery that explained
the Fraunhofer lines of the solar spectrum, and consti-
tuted an epoch in the history of spectrum analysis.

I have now presented the facts that are requisite for
answering the question proposed on one of the foregoing

MKMOIU XI. J THE FOIiCE INCLUDED IN PLANTS.

pages: "Whence has the force which manifests itself as
heat and light in a flame been derived? Force cannot
be created ; it cannot spring forth spontaneously out of
nothing."

The answer is, it came from THE SUN.

Under the influence of his rays the growing plant de-
composed carbonic acid obtained from the atmosphere,
appropriating its carbon and setting its oxygen free. To
accomplish this decomposition, this appropriation, it was
necessary that a portion of the energy contained in those
rays should be absorbed. Associated with this, the car-
bon could now form part of the plant, and, indeed, con-
stituted the solid basis of which it was composed.

But the force thus associated with the carbon atoms
was not annihilated ; it was only concealed : through
countless ages it might remain in this latent state, ready
at any moment to come forth. All that is requisite is
to oxidize the carbon, to turn it into carbonic acid, and
the associated energy, under the form of heat and light,
is set free.

When we read by gas or by the rays of a petroleum
lamp, the light we use was derived from the sun perhaps
millions of years ago. The plants of those ancient days,
acting, as plants do now, under the influence of sunshine,
separated carbon from the carbonic acid of the atmos-
phere by associating it with the radiant energy they had
absorbed, and this remained for an indefinite time en-
closed, as it were, in the now combustible material, ready
to be disengaged as soon as the reverse action, oxidation,
takes place, returning then to commingle as heat with
the active forces of the world.

Much of what has here been said applies to hydrogen
as well as to carbon. Hydrogen is derived, under similar
conditions, from the decomposition of water or ammonia.
When its oxidation recurs, it delivers up, under the form

THE FORCE INCLUDED IN PLANTS. [MEMOIK XI.

of heat, the energy it had absorbed. As, however, I am
here speaking of the source of light in flames, in which
carbon takes the leading arid hydrogen only an indirect
or subordinate part, it is not necessary to trace in further
detail the action of the latter element.

A very interesting illustration of the principles here
under consideration occurs in the case of the decomposi-
tion of water by an electric current. The constituents
of the water, hydrogen and oxygen, are set free in the
gaseous form. But for them to assume that form they
must be furnished with caloric of elasticity. The current
supplies them with this, and, indeed, the decomposition
can only go on at the rate which is regulated by that
supply. The heat they have thus assumed remains in-
sensible in them, imparting to them their elastic or gas-
eous condition, until they are caused to reunite and re-
form water, when it is at once given up. The part that
is planted by that portion of the electric current which is
thus transformed into heat — and furnishing their caloric
of elasticity to the evolving gases is absolutely essential
to the decomposition — has been hitherto too much over-
looked by chemists.

Nature thus offers us in the instance we have been
considering in this Memoir a striking illustration of the
transmigration of matter and of force. Plants obtain

^

carbon from the atmosphere ; it constitutes the basis of
their combustible portions. Sooner or later it suffers ox-
idation, turns back into the condition of carbonic acid,
and is diffused again into the atmosphere. There is a
never-ending series of cycles through which it runs: now
it is in the air, now a part of a plant, now back again
in the air. And the same is true as regards the energy
with which it was associated. Derived from the sun-
beam, it lay hidden in the plant, awaiting re-oxidation ;
then it was delivered, escaping under the form of heat or

MKMOIK XI. J THE FORCE INCLUDED IN PLANTS.

liglit, and remingling with the universal cosmic force
from which it had been of old derived by the sun, or
from which, perhaps more correctly speaking, the sun
himself was derived, for he is the issue of nebular con-
densation.

I cannot close this Memoir without making reference
to a point of surpassing interest. What goes on in the
case of a flame, goes on in the case of an animal. Either
from other animals or from plants, combustible material
is obtained and used as food. Directly or indirectly it
undergoes oxidation in the system, brought about by
the air introduced through the process of respiration.
Speaking in a general manner, though there are many
intermediate products, the issue of this chemical action
is the evolution of carbonic acid, ammonia, water, which
pass into a common receptacle, the atmosphere. Thence
their ingredients are taken by plants, and, under the
agency of the sunlight, combustible material — food — is
re-formed. The same particle is, therefore, now in the
air, now in the plant, now in the animal, now back again
in the air. It suffers a perpetual transmigration.

But in the case of an animal the oxidation may not be
so sudden, so complete, as it is in the case of a flame. It
may, and indeed generally does, go on stage by stage,
step by step, partial oxidations occurring. It is thus
that, from one original hydrocarbon, a long catalogue of
fatty and oily substances may arise ; the inevitable issue,
however, is total oxidation. As the partial degradations
go on, in corresponding degrees the latent energy or force
is set free. It may assume any correlated form, as mus-
cular motion ; or, as heat, it may give warmth to the
body; in certain fishes, as the gyinnotus, it may turn
into an electrical or nerve current; in certain insects,
such as the fire-fly, into light.

As a cataract is only a form which any river may as-

190 THE FORCE, INCLUDED IN PLANTS. [MEMOIR XL

sume if it comes to a precipitous descent — a form which,
though it may be outwardly unchanging, is interiorly
never for two successive moments the same, for it is per-
petually fed from above and is wasting away below — so
the flame of a lamp is only a form, the aspect of which is
determined by its environment. The changes it is under-
going issue in the liberation, the escape, of force, chiefly
under the aspect of light and heat. Its life is very tran-
sitory. It dies out as soon as the oil that fed it is ex-
hausted. We blow upon it, and it passes into nonentity.
And so, too, with an animal, the appearance of identity
it presents is altogether deceptive. At no two succes-
sive moments are its parts the same. In a very short
time all the old have been removed, and new ones have
taken their places. The force that it derived from its
food has been manifested in various ways, such as mus-
cular motion or heat. But the material particles have
not been destroyed ; they have merely gone back into
the atmosphere, and will be used by nature for the fab-
rication of other plant and animal forms over and over
again. And so, too, the energy they have displayed — it
has not ceased to exist ; the heat, for instance, that once
vivified them has merely mingled with that of the outer
world, and is ready to discharge its special functions
again and again. In the world there is thus an unceas-
ing transmigration of matter, an unceasing transmigra-
tion of force.

MKMOIH Xll.J THE MAGNETIC EFFECTS OF LIGHT.

MEMOIR XII.

EXPERIMENTS TO DETERMINE WHETHER LIGHT PRODUCES
ANY MAGNETIC EFFECTS.

From the Journal of the Franklin Institute, Feb., 1835.

CONTENTS : — Mrs. Somerville's experiments. — Christie"1 s experiment. —
Their results cannot be substantiated. — The violet ray has no effect. —
Blue glasses and blue ribbons also ineffective.

" THE more refrangible rays of light are said to pos-
sess the property of rendering iron and steel magnetic.
The existence of this property was first asserted by Dr.
Morichini of Rome. Other observers subsequently failed
in obtaining the same results, but in the year 1825 the
fact appeared to be decisively established by the learned
and accomplished Mrs. Somerville in an Essay published
in the Transactions of tlie Royal Society. In her experi-
ments sewing-needles were rendered magnetic by expos-
ure for two hours to the violet ray, and the magnetic
virtue was communicated in still shorter time when the
violet rays were concentrated by a lens. The indigo
rays were found to possess a magnetizing power almost
to the same extent as the violet; and it was observed,
though in a less degree, in the blue and green rays. It
is wanting in the yellow, orange, and red. Needles were
likewise rendered magnetic by the sun's rays transmitted
through green and blue glass. These results have been
verified by M. Zantedeschi, of Pavia (Bibl. Univ., May,
1829), but their accuracy has been doubted by Riess
and Moser, who consider that the means employed by
Mrs. Somerville for ascertaining the magnetic state of

192 THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII.

the needles were not sufficiently exact. They found the
oscillation of the needles to be wholly unaffected by ex-
posure to the prismatic colors (firewater'1 & Journal, Vol.
II., p. 225, N. S.). This must still be regarded, there-
fore, as one of the disputed points in science " (Turner's
Chemistry).

It has been supposed that these contradictory results
arose entirely from local circumstances. A hazy atmos-
phere, such as is met with in the northern and middle
countries of Europe, might perhaps influence in some
manner this peculiar property of light when the clearer
sky of Italy permitted the experiment to succeed. Some,
indeed, have thought that the observers who were said
to have verified the alleged results were deceived in not
having previously ascertained the magnetic state of the
needles they used.

During the past summer (1834) I have attempted to
satisfy myself whether the more refrangible rays really
exert any magnetic influence; and happening to reside
in the south of Virginia, on the same parallel of latitude
as Tunis and the more northerly African kingdoms, I
thought the situation too favorable to suffer such an op-
portunity to pass without endeavoring to gain some in-
formation on this contested point.

In 1824 Mr. Christie found that a needle six inches
long, contained in a brass compass -box with a glass
cover, suspended by a hair and made to vibrate alter-
nately shaded and exposed to the sun, came to rest much
sooner in the latter than in the former case. That this
was not occasioned by an increase of temperature was
proved by the needle vibrating more rapidly when its
temperature was raised by other means.

On repeating this experiment, I very quickly found
that it depended in a great measure on the mode of
suspension of the needle, and its position with respect to

MSKOIBXII.] THE MAGNETIC EFFECTS 0? HOOT. 193

the incident light, what results would be obtained. If
the needle was suspended on a point, or by a thread
without torsion, the time and the number of vibrations
were the same whether the needle was exposed to the
sunbeam or not. But if the needle was suspended by a
hair or other organic substance having torsion, the sun-
beam \vould occasion a twist iu the hair on its first
exposure to light ; and if the direction of that twist hap-
pened to coincide with the direction of the needle's
motion, the momentum of the needle was increased and
the vibrations continued longer. A needle which vi-
brated forty-four times in one minute would occasional-
ly, owing to this cause, vibrate forty-six when suspended
by a hair; but if by a silk fibre, its vibrations were al-
ways forty-four, the first arc of vibration being in every
instance 40°.

Thinking to obtain more decisive effects, I concen-
trated the sunbeam with a lens on the south pole of the
suspended needle, and found that the needle was thrown
into a rapid, tremulous motion. But here the hot air
ascending from the needle acts upon it as upon the sail
of a windmill ; and the same effect ought to take place
to a certain extent on simple exposure of the half of a
vibrating needle to direct light. But I found that a
needle suspended in the vacuum of an air-pump by a
thread without torsion is in no way affected by exposure
to solar light.

It is said in the account of Christie's experiment that
the needle was contained in a brass compass -box. It
might have been that electrical currents were excited in
that box which were the cause of the derangement in

o

question. I therefore vibrated a needle under similar
circumstances, with the result above stated. I should
mention that this was done in a solid cylinder or ring
of brass without any seam or soldered junction ; but as

N

194

THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII.

Fig. 24.

compass-boxes are generally made of sheet-brass with a
soldered seam in the side, it was possible that the fine
line of solder acted with the brass as a thermo-electric
couple, capable of excitation by the warmth of the sun-
beam. I therefore made a compound cylinder, half of

copper, the other half of zinc, c z,
Fig. 24, the edges of which, at a
and I respectively, are soldered to-
gether, one junction, b, being pol-
ished, the other, #, blackened. The
needle was suspended in an ex-
hausted receiver by a silk fibre, </,
^ and a ray of light, d, coming from
an aperture half an inch wide in
the shutter fell upon the junction.
The needle used in this experiment
was of watch-spring. Its first vi-
bration was performed in an arc of 40°, and when the
compound cylinder was taken away it made thirty-two
vibrations in sixty seconds in vacuo. On placing it con-
centrically with the compound cylinder, and suffering
the ray to impinge on the polished junction, the moment
that the arc of vibration had become 40° the number of
oscillations in one minute was observed ; six experiments
gave severally thirty -two. On turning the blackened
junction to the light the result was still thirty-two, and
on substituting the solid brass cylinder three consecutive
trials gave thirty -two. The thermometer stood in the
sunshine at 103°.

By some this magnetic action of light has been at-
tributed to the violet or more refrangible rays only. A
needle of watch-spring about four inches long, which in
an exhausted receiver suspended by a filament of silk
exhibited no polarity, had one half exposed to the violet
ray dispersed by a flint-glass prism. This ray was sepa-

MEMOIB XII.] THE MAGNETIC EFFECTS OF LIGHT.

rated from others of the spectrum by passing through a
slit in a metallic screen, and half the needle was screened
from its action by a piece of paper. After two hours'
exposure to the sun it was suspended again in the ex-
hausted receiver, but still showed no polarity; it was
then exposed to the other rays successively, with the
same result. The needle was now slightly touched, and,
slowly vibrating, arranged itself in the magnetic merid-
ian. The first vibration was performed in a semicircular
arc, the number of vibrations in one hundred seconds
was twenty -seven. But after four hours' exposure to
the violet ray, as before, no evidence of any change
either increasing or diminishing the number of oscilla-
tions could be obtained. A beam of violet light passing
through a disk of stained glass was concentrated on one
end of a sewing-needle by means of a lens without pro-
ducing any change in the number of vibrations made in
one minute. This needle on some occasions would, how-
ever, give different results: when its first vibration was
performed in a semicircle, the number varied from forty-
one to forty-three in sixty seconds. On vibrating it in
vacuo, its results uniformly gave the latter number very
nearly.

The position of the needle to the incident ray is not
of any consequence, whether it receives it obliquely, in
the direction of the light, or across it. If soft iron be
substituted for steel, the results are still negative, even
if the needle be arranged in the magnetic meridian, the
line of dip, or any other position. I therefore came to
the conclusion that the violet ray exerts no influence on
the magnetic needle, and that all the other rays are
equally inert.

But as Mrs. Somerville found that a needle placed
under a piece of glass or blue ribbon, having half its
length protected by paper, became in a short time mag-

196 THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII.

netic, I tried the same experiment, but in every instance
failed to make the needle magnetic. When suspended
by a silk fibre in vacuo needles showed no disposition
to arrange themselves in any particular direction, and
when they came to rest were found cutting the magnetic
meridian at every angle, though the temperature of the
sunbeam to which they had been exposed on one occa-
sion was 124°. Great care was taken to ascertain the
previous non- magnetic state of the needles, and they
were suspended by a fibre without torsion.

MKMOIU XI11.] EXPERIMENTS MADE IN VIRGINIA, 197

MEMOIR XIII.

AN ACCOUNT OF SOME EXPERIMENTS ON THE LIGHT OF
THE SUN, MADE IN THE SOUTH OF VIRGINIA.

Abstract from the Journal of the Franklin Institute of Philadelphia for June, July,
August, and September, 1837 ; Philosophical Magazine, Feb., 1840.

CONTENTS: — Absorption of luminous radiations. — The reference spec-
trum.— Absorption of heat radiations ; the apparatus employed. — Ab-
sorption of chemical radiations. — Screen of bromide of silver. — Color-
-ation of chloride and bromide of silver by radiations that have passed
through correspondingly colored solutions. — Early application of pho-
tography to the investigation of physical problems. — Crystallization of
camphor towards the light. — The side of vessels towards the sky is the
colder.

IF a beam of the sun's light be passed through a so-
lution of chromate of potassa, it can no longer blacken a
piece of sensitive paper — paper covered over with chlo-
ride or bromide of silver. If the light which has thus
passed through a stratum of this liquid be converged by
means of a lens, the chloride of silver will remain for a
long time without change in the focus.

I made many such experiments with a view of deter-
mining the effect of absorbent media on the luminous,
calorific, and chemical rays. Such, at that time, was the
accepted subdivision of the solar radiations. A ray of
the sun was caused to pass through a trough with paral-
lel sides containing the absorbent solutions, and for the
sake of exactness a reference spectrum was employed,
such as is now used in spectroscopic experiments. In
this manner the particular luminous rays absorbed by
any given solution could be determined, the absorbed

198

EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII.

Fig. 25.

heat -rays could be ascertained by an air-thermometer,
and the acting chemical rays by papers made sensitive
by chloride of silver.

Into a darkened chamber, the shutter of which is rep-
resented in section at A A, Fig. 25, a beam of the sun's

light is made to pass hori-
zontally by means of a mir-
ror of silvered glass, B. The
mirror I use is one belong-
ing to a solar microscope,
and by turning the milled
screws, e 6, it can be brought
into any position required
to throw a beam horizontally into the room. A brass
tube,/, two inches in diameter, can be screwed into the
position figured. There is also a lens, a, Fig. 31, which
may be introduced ; its focus is nine inches, its diameter
abofct two inches, and the diameter of the sun's image it
gives nearly -^ of an inch.

A piece of sheet-lead about a quarter of an inch thick
is to be cut into the form of a horse-shoe of such size
that a circle one inch in diameter may be inscribed in it.
Upon this lead two pieces of glass are cemented, so as to
form a trough for containing liquids. In Fig. 26, a a is
the wooden basis or foot of the trough ;

re O '

c c c, the leaden horse -shoe; b b, the
glass plates.

A thin metallic plate three or four
inches square, having a longitudinal slit
in it about an inch long
and -j^ °f an incn wide, is
to be provided. In Fig. 27, a a is the slit.
The lens, a, Fig. 31, having been removed,
a beam of light is thrown horizontally into
the room, as at Fig. 25 ; the slit, Fig. 27,

Fig. 26.

Fig. 2T.

MEMOIR XIII.] EXPERIMENTS MADK IN VIRGINIA.

199

being placed as at a, Fig. 25, a narrow streak of light
passes through. The trough, Fig. 26, is then placed
behind in such a position that half the light coming
through the slit may pass through the liquid in the
trough, and the other half pass by its side unintercepted.
Behind the trough is placed a flint-glass prism, as in Fig.
•_}r>, and further still a white pasteboard screen,^ a is
the slit plate, b the trough, d the prism.

The action of this arrangement is as follows: The
beam of light cast by the mirror into the room is inter-
cepted, except the small portion that passes the slit. Of
this a part passes through the trough and a part on one
side of it. Two beams of lio-ht, therefore, fall on the

O '

prism, one of which has passed through the trough
and suffered absorption, the other has not. There are,
therefore, on the pasteboard screen two spectra side
by side.

Suppose, for example, that the trough is filled with
distilled water; the two spectra on the screen are alike,
as in Fig. 28. If it be filled with a solution of chromate
of potassa, in that coming from the light that has passed
through the trough the blue, indigo, and violet rays are
missing, as in Fig. 29. If the trough be filled with am-
monia-sulphate of copper, the red, the orange, the yellow
have disappeared or been absorbed, as in Fig. 30. The

Fig. 28.

Fig. 20.

Fig.sa

reference or undisturbed spectrum enables us to deter-
mine what rays have been absorbed with precision.
For the investigation of the absorption of heat the

200

EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII.

Fig. 31.

following apparatus was used. The mirror being placed

on the shutter, as in Fig. 31, a
plano-convex lens, #, is screwed
into the tube so as to bring the
rays to a focus on one of the
bulbs of a delicate differential
thermometer ; this gives the heat

r O

of the sunbeam as concentrated
by the lens. To find the effect
of any liquid medium in absorbing heat-rays, the trough
filled with the substance under trial is placed as at <?,
Fig. 31. The cone of rays, converging from the lens, «,
on the blackened bulb, 5, forms an image upon it, and
the differential thermometer yields a corresponding in-
dication. In trying different solutions the same trough

*J O O

is always to be used, so that the solutions may always
be of the same thickness. It is also requisite to cover
the thermometer with a very thin shade of clear glass,
e e, to prevent disturbance from air-currents.

I may select as examples the following : A solution of
sulphate of copper and ammonia, absorbing the red and
yellow light, transmitted twenty rays out of every hun-
dred that fell on it.

A thin stratum of pitch enclosed between two plates
of crown-glass, and transmitting a homogeneous red
light, but absorbing all the other colors of the spec-
trum, allowed only 19 per cent, of the heat to pass
through it.

In examinations of the transmissive powers of vapors
and gases, a cubical bottle was used instead of the
trough. The vapor of iodine was thus found to absorb
two thirds of the heat falling on it ; but the same bottle
filled with nitrous acid, and which, therefore, was in a
stratum of the same thickness, permitted much more
heat to pass.

EXPERIMENTS MADE IN VIRGINIA.

For the investigation of chemical radiations, there was
placed upon the screen,/, Fig. 25, a paper covered with
l>rmnide of silver. It darkened in those parts on which
the more refrangible rays fell.

Or, having removed the differential thermometer and
its shade, Fig. 31, the cone of light converging from the
lens passed through a solution of sulphate of copper and
ammonia in the trough, and a piece of paper painted
with chloride of silver was placed so as to receive the
focus. Though but little heat was transmitted through
the solution, a dark spot was at once produced charac-
teristic of the blackening of the silver salts by the sun-
rays. Though, therefore, this double salt transmits the
rays of heat with difficulty, the rays of chemical action
pass with facility. If in the trough there be placed a
strong solution of bichromate of potassa, a far greater
quantity of light will pass, and much more heat ; but a
paper painted with chloride of silver being placed in the
focus, no chemical change whatever goes onj the chloride
retaining its usual whiteness.

NOTE. — The foregoing experiments were made be-
tween 1834 and 1837. The reader of this volume must
not regard them from the present elevated view of Radi-
ations. At that time very little respecting these radi-
ations was known. They present to me personally this
point of interest — that they were the beginning of a
series of researches to which I devoted many subsequent
years.

The following is a list of solutions possessing an ab-
sorptive action on the more refrangible or chemical radi-
ations :

202 EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII.

Bichromate of potassa,

Chromate of potassa,

Yellow hydrosulphide of ammonia,

Hydrosulphide of lime,

Chloride of iron,

Chloride of gold,

Chloride of platinum.

It is to be remarked that all these solutions are yel-
low. I also found that a great many vegetable colored
infusions, especially those which had a yellow tint, would
in like manner absorb the chemical -rays.

When pieces of paper covered with chloride or bro-
mide of silver were exposed to a beam which had passed
through a solution of red sulpho- cyanide of iron, the
paper became of a brick-red color ; if to a beam which
had passed through a solution of ammonia- sulphate of
copper, it became of a blue-brown ; and on exposing a
piece for five days to light acted on by bichromate of
potassa, it became perceptibly of a faint yellowish green.

A -beam which has passed through bichromate of po-
tassa does not cause the union of a mixture of chlorine
and hydrogen. I kept such a mixture for several hours
in it, and could not perceive any change.

Hitter asserted that the opposite extremities of the
spectrum possess opposite powers of chemical action : he
states that phosphorus will emit fumes in the red ray,
but if the violet be thrown on it, it ceases to smoke.
This experiment I repeated often, and under favorable
circumstances, but could not make it succeed.

I could succeed, however, in showing very beautifully
the interference of that class of chemical rays which
'blacken chloride and bromide of silver, but failed for
want of proper apparatus in trying to produce their
polarization. An electric current circulating in a wire
does not seem to have any influence on these chemical
rays. I found that the same neat magnified image of
the wire was obtained on chloride paper when it was

MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA.

placed in a beam diverging from a lens, whether the
current was made to pass or was stopped.

NOTE. — These were instances of the application of
photography to the solution of physical problems long
before the announcement of the discoveries of Daguerre
or Talbot.

The following are some mechanical results of solar
light:

(a.) Having made a large air-pump jar clean and diy,
a few pieces of camphor were placed on the plate of the
pump, and the jar exhausted. The pump and its re-
ceiver were then set in the sunshine, and very soon all
that side towards the sun was covered with crystals ; but
few or none were on the farther side.

(#.) A torricellian vacuum having been made in a
tube half an inch or more in diameter, and upwards of
thirty inches long, a fragment of camphor was passed up
through the mercury into it. The tube might be kept
for any length of time in the dark without anything
happening, but on bringing it into the beams of the sun
crystallization in a few minutes took place on that side
of the tube next the luminary.

(c.) On the inside of an air-pump
jar a circle of tinfoil an inch in diam-
eter was pasted ; and having oper-
ated as in experiment («), that side
was exposed to the sun. Crystals
soon formed, but the tinfoil protected
the glass in its vicinity, and none
were found within a certain space
round the metallic circle. Ficr. 32. rig. 39.

204 EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII.

(d.) Crystallization is not necessarily connected with
these results; the vapor of mercury in a torricellian
void is condensed towards the light ; so also the dew
which settles on the inside of a jar containing water
is always on the side nearest the window. The rays of
the sun have also the power of decomposing chloride of
gold : the metalline spangles are deposited on the side
of the glass nearest to the light.

Artificial light gives none of these results.
(61.) Having removed the piece of tinfoil iised in ex-
periment (<?), it was placed on a stand in front of the re-
ceiver; it hindered crystallization taking place on the
parts on which its shadow was cast, and also for a cer-
tain space in the vicinity.

(/.) A piece of tinfoil was placed before a jar that had
been already coated with crystals. It removed all those
crystals that were within its shadow.

(</. ^Instead of using a piece of tinfoil as in experiment
(<?), the receiver was made hot, and a piece of resin
rubbed upon it, so as to leave a transparent circle of
that substance on it. On exposure to the light it was
found that the resin could not protect the glass.

(h.) Along the inside of a vessel about to be exposed
to the sun a glass rod was rubbed. Rows of crystals
w7ere deposited on the lines described by the end of the
rod. But the vessel must be very dry for this experi-
ment to succeed. This curious fact was first
observed in the case of an exhausted vessel
having a small siphon gauge shut up in it,
the extremity of which rested against the
glass. By accident the gauge was moved
half round the glass, and shortly afterwards a
line of crystals was observed coinciding with
the line of motion. The appearance was such as is rep-
resented in Fig. 33.

MI.M..IH XIII. J EXPERIMENTS MADE IN VIRGINIA. OQ5

Now can we explain these singular results, excepting
the last, on any other known principle than this : that
the side of the jar nearest the sun radiates freely the
heat it receives back again, while radiation is interfered
with at the other side ? that, in point of fact, the anterior
side is the colder, and the other side the hotter ?

206 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV.

MEMOIR XIV.

EXAMINATION OF THE PROCESS OF DAGUERREOTYPE.

NOTE ON LUNAR PHOTOGRAPHY.

From the Philosophical Magazine, Sept., 1840; Smithsonian Contributions to
Knowledge, No. 180.

CONTENTS : — Description of the process. — Cause of the deposition of mer-
cury and production of the light parts of the picture. — Polishing of
the plate. — The operation of iodizing. — Effect of keeping the iodide. —
The achromatic lens. — Reduction of focal length in the non- achro-
matic.— The development. — Fixing by hyposulphite and galvanism. —
Necessity of heating the plates. — Lunar impressions. — Artificial light.
— Note on lunar photography — The first photographs of the moon.

M$RE than one hundred instances are recorded in Ber-
zelius's Chemistry in which the agency of light brings
about changes in bodies; these are of all kinds: for-

o

mations of new compounds, re-arrangernents of elements
already in union, changes of crystallographic character,
decompositions, and mechanical modifications.

The process of the daguerreotype is to expose a sur-
face of pure silver to the action of the vapor of iodine, so
as to give rise to a peculiar iodide of silver, which under
certain circumstances is exceedingly sensitive to light.
The different operations of polishing, washing with nitric
acid, exposure to heat, etc., are only to secure a pure silver
surface ; the operation of hyposulphite of soda, and the
process, which I shall presently describe, of galvaniza-
tion, are to free the plate from its sensitive coating, and
in nowise affect the depth of the shadows, as some of
the French chemists at first supposed.

There is but one part of the daguerreotype which

MI.M..IK XIV.] THE PROCESS OF DAGUERUEOTYl'K. 207

does not yield to theory: on one point alone there is ol>-
M-urity. Why does the vapor of mercury condense in a
white form on those portions of the film of iodide which
have been exposed to the influence of light ? — condense
to an amount almost proportional to the quantity of
incident light?

Even on this point there are facts which appear to
have a bearing.

(a.) It has long been known that if a piece of soap-
stone or agalmatolite be made use of as a pencil to write
with on glass, though the letters that may have been
formed are invisible, and though the surface of the glass
may subsequently have been well cleaned, yet they will
come into view as soon as the glass is breathed on.

(£».) I have often noticed that if a piece of very clear
and cool glass, or, what is better, a cold polished metallic
reflector, has a little object such as a piece of metal laid
upon it, and the surface be breathed over once, the object
being then carefully removed, as often as you breathe
again on the surface a spectral image of it may be seen,
and this singular phenomenon may be exhibited for
many days after the first trial was made.

(o.) Again, in the common experiment of engraving on
glass by hydrofluoric acid, if the vapor has been very
weak, no traces will be perceived on the glass after the
wax has been removed ; but on breathing over it, the
moisture condenses in such a way as to bring the object
into view.

(d.) In a former Memoir (XIII.) I described a phe-
nomenon relating to the crystallization of camphor on
surfaces of dry glass, on which invisible traces have
been made by the pressure of a glass rod ; this also ap-
pears to belong to the same class of effects.

Berzelius (Timte,Vo\. II., p. 186) has attempted to ex-
plain (a) and (<?) on this principle, that the changed and

208 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV.

unchanged surfaces radiate heat unequally. There may
be strong doubts as to the correctness of this, but is not
the daguerreotype due to the same cause, whatever it
may be ?

We must separate carefully the chemical changes
which iodide of silver undergoes in the sunbeam from
the mechanical changes which happen to the sensitive
film : iodide of silver turns black in the solar ray — the
whole success of the daguerreotype artist depends on
his checking the process before that change shall have
supervened.

The coating of iodine is not immediately necessary to
the production of images by the mercurial vapor. The
condition seems to be traceable to the metallic surface.
If you take a daguerreotype, clean off the mercury, polish
the plate thoroughly with rottenstone, wash it with nitric
acid, and bring it to a brilliant surface, yet if it has not
been^exposed to heat, the original picture will reappear
on exposure to the mercurial vapor. Is not this a result
of the same kind as those just referred to ?

As a polishing material for the daguerreotype plate,
common rottenstoue and oil answer very well. The
plate having been planished by the workman, is to be
rubbed down to a good surface, and as high a polish
given to it as possible; it is to be heated and washed
with nitric acid, as indicated in the French account, and
finished by being rubbed with whiting (creta prcepa-
rata) in the state of a very fine powder, going over it for
the last time with a piece of clean dry cotton ; this
gives an intensely black lustre, that cannot be obtained
by rottenstone alone, and thoroughly removes any film
which nitric acid may have left.

To coat writh iodine, I make use of a box about two
inches deep, in the bottom of which that substance in
coarse flakes is deposited ; no cloth intervenes, but the

M. M..IK XIV.J THE riiOCESS OF DAGUERREOTYPE. 209

silvered plate, with a temporary handle attached to it, is
brought within half an inch of the crystals, and it be-
comes perfectly coated in the course of from one to three
minutes ; no metallic strips are necessary to insure this
effect ; if the edges and corners are thoroughly clean, the
golden hue will appear uniformly.

M. Daguerre recommends that the plate, after being
iodized, shall be placed in the camera without loss of
time. The longest interval, he says, ought not to exceed
an hour. " Beyond this space the action of the iodine
and silver no longer possesses the requisite photogenic
properties."

There may be something peculiar in the preparation
of the plate as I have described it, but it is certain that
this observation must be received with some limitation.
A plate which has been iodized does not appear so
quickly to lose its sensitiveness. On the other hand, by
keeping it in the dark for twelve or twenty-four hours,
its sensitiveness is often remarkably increased. Other
advantages also accrue. Those who have made many of
these photogenic experiments will have had frequent oc-
casion to remark that the film of iodine is not equally
sensitive all over, that there are spots or cloudy places
which do not evolve any impression, and often the whole
is in that condition that the bright parts alone come out,
while the parts that are in shadow do not evolve corre-
spondingly, nor can they be well developed, except at the
risk of solarizing the picture. Now a plate that has
been kept for several hours is by no means so liable to
these effects: I do not pretend to give any reason for
this, but merely mention it as a fact, of considerable im-
portance to the travelling daguerreotyper ; he will find
that the iodine does not lose its sensitiveness in many
days.

In a paper read before the Royal Society, Herschel

O

210 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV.

states that there is an absolute necessity of a perfect
achromaticity in the object-glass of a photographic cam-
era. M. Daguerre appears to have been under the same
impression, and recommends in his published account
such an object-glass.

All the rays of light, with perhaps the exception of the
yellow, leave an impression on the iodide of silver. The
less refrangible rays, however, act much more slowly
than those at the opposite end of the spectrum. In the
common kinds of glass, the most energetic action takes
place in the indigo, or on the boundary of the blue.
Now the retina receives an impression with equal facility
from each of the different rays, the yellow light acting as
quickly upon it as the red or the blue. Vision is there-
fore performed independently of time, the eye catching
all the colors of the spectrum with equal facility and
with equal speed. But it is not so with these photo-
genic preparations. In the action of light upon them,
time enters as an element ; the blue ray may have effect-
ed its full change, while the red is yet only beginning
slowly to act ; and the red may have completed its
change before the yellow has made any sensible impres-
sion. On these principles, it is plain that an achromatic
object-glass is by no means essential for the production
of fine photographs ; for if the plate be withdrawn at a
certain period, when the rays that have a maximum
energy have just completed their action, those that are
more dispersed but of slower effect will not have had
time to leave any stain. We work, in fact, with a tem-
porary monochromatic light.

Upon these principles I constructed the camera I am
in the habit of using, with a double convex non-achro-
matic lens. Some of the finest proofs were procured
with a common spectacle lens, of fourteen inches focus,
arranged at the end of a ci^ar-box as a camera; a lens

o O '

M..M...K XIV.] THIC DAGUERREOTYPE. f>ll

of this diameter answers very well for plates four inches
by three, reproducing the objects with the most admi-
rable finish, copper-plate engravings being represented in
tin- minutest particulars, and the marks of the tool be-
coming quite distinct under a magnifier.

In this instance, it is true, owing to the magnitude of
the focal length compared with the aperture, but little
difficulty ensues from chromatic aberration ; but when
with the same focal length the aperture is increased to
three or four inches, the dispersion becomes very sen-
sible, and yet good proofs can be procured by working
in the method here indicated, the chief difficulty then
arising from spherical aberration.

It has already been stated that the ray of maximum
action for the daguerreotype, when colorless French
plate-glass is used, lies probably within the indigo space:
it therefore follows that the length of the camera should
be diminished, after arranging it to the luminous focus.
The importance of this has been pointed out in a paper
by Mr. Towson ; I was, however, in the habit of using
that adjustment before reading the suggestions contain-
ed in his communication. The amount of shortening to

O

be given to the camera, where the lens is fifteen inches
focus, does not commonly exceed three tenths of an inch.
If the luminous focus be used, the proof comes out indis-
tinctly.

In the subsequent process of mercurializing, it is of lit-
tle importance what the angular position may be. Sev-
eral experimenters were for a time under the idea that
an angle of 45° or 48° is a necessary inclination, in order
that the plate should take the vapor; this arose from a
misinterpretation of the printed account. Plates mercu-
rialize equally well in a horizontal as in any other posi-
tion ; perhaps a slight inclination may be of advantage,
in allowing the vapor to flow with uniformity over the

212 THE DAGUERREOTYPE. [MEMOIR XIV.

iodized surface, but the chief use of an angle of 45° is to
allow the operator to inspect the process through the
glass of the mercury-box.

Sometimes it is advantageous to heat the mercury a
second time, when the proof is not distinctly evolved at
first. Indeed, it occasionally happens that a proof which
did not evolve at all at first will come out quite fairly
on raising the temperature of the mercury again.

M. Daguerre recommends two methods of removing
the sensitive coating from the plate — by washes of hypo-
sulphite of soda, and by a solution of common salt. The
former answers perfectly, the second only indifferently
well. There is, however, another process, which is very
simple, and has an advantage over the former of these in
cheapness. It adds not a little to the magic of the whole
operation, in the eyes of those who are unaccustomed to
chemical results. The plate, having been dipped into
cold*"water, is placed into a solution of common salt, of
moderate strength ; it remains without being acted upon
at all; but if it be now touched on one corner with a
piece of zinc, which has been scraped bright, the yellow
coat of iodide moves off like a wave and disappears. It
is a very pretty process. The zinc and silver forming
together a voltaic couple, with the salt water intervening,
oxidation of the zinc takes place, and the silver surface
commences to evolve hydrogen gas; while this is in a nas-
cent condition it decomposes the film of iodide of silver,
giving rise to the production of hydriodic acid, which is
very soluble in water, and hence instantly removed.

This process, therefore, differs from that with hyposul-
phite. The latter acts by dissolving the iodide of silver,
the former by decomposing it. It is necessary not to
leave the zinc in contact too long, or it deposits stains,
and in large plates the contact should be made at the
four corners successively, to avoid this accident.

MEMOIK XIV.] PICTURES OF THE MOON. 213

After the proof is washed, all the defects in the prep-
aration of the plate become apparent. If a film of mer-
cury has existed on it, due to its not having been burned
sutlieiently long, there will be found a want of distinct-
ness in the shadows ; or if the plate has not been burn-
ed at all, perhaps the former impressions will reappear.
This accident frequently happened in my earlier trials,
when care had not been taken to give a due exposure
each time to the spirit-flame. Spectral appearances of
former objects, on different parts of it, emerged — an in-
terior with Paul Pry coming out, when the camera had
been pointed at a church.

There is no difficulty in procuring impressions of
the moon by the daguerreotype beyond that arising
from her motion. By the aid of a lens and a heliostat,
I caused the moonbeams to converge on a plate, the lens
being three inches in diameter. In half an hour a very
strong impression was obtained. With another arrange-
ment of lenses I obtained a stain nearly an inch in diam-
eter, and of the general figure of the moon, in which the
places of the dark spots might be indistinctly traced.

An iodized plate, being exposed for fifteen seconds
only close to the flame of a gas-light, was very distinctly
stained ; in one minute there was a very strong impres-
sion.

On receiving the image of a gas-light, eight feet dis-
tant, in the camera, for half an hour, a good representa-
tion was obtained.

The flame of a gas-lamp was arranged within a magic-
lantern, and a portion of the image of a grotesque on
one of the slides received on a plate ; a very good repre-
sentation was procured.

With Drummond's light, and the rays from a lime-pea
in the oxy-hydrogen blowpipe, the same results were ob-
tained.

214 LUNAR PHOTOGRAPHY. [MEMOIR XIV.

NOTE ON LUNAR PHOTOGRAPHY. — To the foregoing
paragraph respecting photographs of the moon, I may
here add an extract from a publication by my son,
Dr. Henry Draper, in the Smithsonian Contributions to
Knowledge, No. 180, p. 33, entitled, " On the Construction
of a Silvered Glass Telescope, and its Use in Celestial
Photography :"

''The first photographic representations of the moon
ever made were taken by my father, Professor John W.
Draper, and a notice of them published in his quarto
work ' On the Forces that Organize Plants,' and also in
the September number (1840) of the London, Edin-
burgh, and Dublin Philosophical Magazine. He pre-
sented the specimens to the New York Lyceum of Nat-
ural History. The Secretary of that association has
sent me the following extract from their minutes :

"'March 23d, 1840. Dr. Draper announced that he
had succeeded in getting a representation of the moon's
surface by the daguerreotype. . . . The time occupied was
twenty minutes, and the size of the figure about one inch
in diameter. Daguerre had attempted the same thing,
but did not succeed. This is the first time that any-
thing like a distinct representation of the moon's surface
has been obtained. — ROBERT H. BEOWNNE, Secretary? "

MKMOIK XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 215

MEMOIR XV.

ON THE TAKING OF PORTRAITS FROM LIFE BY PHO-
TOGRAPHY.

From the Philosophical Mngnzine, September, 1840.

CONTENTS: — History of the invention. — First attempts by whitening the
face. — Use of reflecting mirrors. — Use of a blue-colored trough. — Kind
of camera necessary. — The seat or support. — The background. — Ap-
propriate dresses. — Ladies' dresses. — Arrangement of the shadow. —
Reflecting camera.

HISTORICAL NOTE. — This Memoir contains the first
published description of the process for taking daguer-
reotype portraits. Of late, since the introduction of col-
lodion, this art has been much cultivated and improved.
It now forms an important branch of industrial occupa-
tion. That it was possible by photogenic processes, such
as the daguerreotype, to obtain likenesses from the life,
was first announced by the author of this volume in a
note to the editors of the Philosophical Magazine, dated
March 31, 1840, as may be seen in that journal for June,
1840, p. 535. The first portraits to which allusion is made
in the following Memoir were produced in 1839, almost
immediately after Daguerre's discovery was known in
America.

In the Edinburgh Review for January, 1843, there is
an important article on Photography. In that the in-
vention of the art of taking photographic portraits is
attributed to its true source — the author of this book.
It says : " He was the first, we believe, who, under the
brilliant summer sun of New York, took portraits by
the daguerreotype. This branch of photography seems

216 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV.

not to have been regarded as a possible application of
Daguerre's invention, and no notice is taken of it in the
reports made to the legislative bodies of France. We
have been told that Daguerre had not at that period
taken any portraits; and when we consider the period
of time — twenty or twenty-five minutes — which was
then deemed necessary to get a daguerreotype landscape,
we do not wonder at the observation of a French author,
who describes the taking of portraits as ' Toujours un
terrain un pen fabuleux pour le Daguerreotype? "

Very soon after M. Daguerre's remarkable process for
photogenic drawing was known in America, I made at-
tempts to accomplish its application to the taking of
portraits from the life. M. Arago had already stated in
his address to the Chamber of Deputies that M. Da-
guerre expected by a slight advance to meet with suc-
cess, but as yet no account had reached us of that object
being attained.

In the first experiments I made for obtaining portraits
from the life, the face of the sitter was dusted with a
white powder, under an idea that otherwise no impres-
sion could be obtained. A very few trials showed the
error of this ; for even when the sun was only dimly
shining there was no difficulty in delineating the feat-
ures.

When the sun, the sitter, and the camera are situated
in the same vertical plane, if a double convex non-achro-
matic lens, four inches in diameter and fourteen in focus,
be employed, perfect miniatures can be obtained in the
open air, in a period varying with the character of the
light, from twenty to ninety seconds. The dress is admi-
rably given, even if it should be black ; the slight differ-
ences of illumination are sufficient to characterize it, as
well as to show each button, button-hole, and every fold.

XV.J TAKIXCJ OF PORTRAITS BY PHOTOGRAPHY. 917

Partly owing to the intensity of such light, which can-
not be endured without a distortion of the features, but
chiefly owing to the circumstance that the rays descend
at too great an angle, such pictures have the disadvan-
t.i-v of not exhibiting the eyes with distinctness, the
shadow from the eyebrows and forehead encroaching on
them.

To procure fine proofs, the best position is to have the
line joining the head of the sitter and the camera so ar-
ranged as to make an angle with the incident rays of less
than ten degrees, so that all the space beneath the eye-
brows shall be illuminated, and a slight shadow cast from
the nose. This involves obviously the use of reflecting
mirrors to direct the ray. A single mirror would answer,
and would economize time, but in practice it is often con-
venient to employ two; one placed, with a suitable mech-
anism, to direct the rays in vertical lines ; and the second
above it, to direct them in an invariable course towards
the sitter.

On a bright day, and with a sensitive plate, portraits
can be obtained in the course of five or seven minutes, in
the diffused daylight. The advantages, however, which
might be supposed to accrue from the features being
more composed, and of a more natural aspect, are more
than counterbalanced by the difficulty of retaining them
so long in one constant mode of expression.

But in the reflected sunshine the eye cannot .support
the effulgence of the rays. It is therefore absolutely
necessary to pass them through some blue medium, which
shall abstract from them their heat, and take away their
offensive brilliancy. I have used for this purpose blue
glass, and also ammonia-sulphate of copper, contained in
a large trough of plate glass, the interstice being about
an inch thick, and the fluid diluted to such a point as to
permit the eye to bear the light, and yet to intercept

218 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV.

no more than was necessary. It is not requisite, when
colored glass is employed, to make use of a large sur-
face ; for if the camera operation be carried on until the
proof almost solarizes, no traces can be seen in the por-
trait of its edges and boundaries ; but if the process is
stopped at an earlier interval, there will commonly be
found a stain, corresponding to the figure of the glass.

The camera I have used, though much better ones
might be constructed, has for its objective two double
convex lenses, the united focus of which for parallel rays
is only eight inches ; they are four inches in diameter in
the clear, and are mounted in a tube, in front of which
the aperture is narrowed down to 3^ inches, after the
manner of Daguerre's.

The chair in which the sitter is placed has a staff at
its back, terminating in an iron ring, which supports the
head, so arranged as to have motion in directions to suit
any Stature and any attitude. By simply resting the
back or side of the head against this ring, it may be kept
sufficiently still to allow the minutest marks on the face
to be copied. The hands should never rest upon the
chest, for the motion of respiration disturbs them so
much as to make them of a thick and clumsy appear-
ance, destroying also the representation of the veins on
the back, which, if they are held motionless, are copied
with surprising beauty.

It has already been stated that certain pictorial advan-
tages attend an arrangement in which the light is thrown
upon the face at a small angle. This also allows us to
get rid entirely of the shadow on the background, or to
compose it more gracefully in the picture; for this it is
well that the chair should be brought forward from the
background from three to six feet.

Those who undertake daguerreotype portraitures will
of course arrange the backgrounds of their pictures ac-

MKXOIK XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY.

cording to their own tastes. When one that is quite
uniform is desired, a blanket, or a cloth of a drab color,
properly suspended, will be found to answer very well.
Attention must be paid to the tint : white, reflecting too
much light, would solarize upon the proof before the face
had had time to come out, and owing to its reflecting all
the different rays, a blur or irradiation would appear on
all edges, due to chromatic aberration. It will be readily
understood, that if it be desired to introduce a vase, an
urn, or other ornament, it must not be arranged against
the background, but brought forward until it appears
perfectly distinct on the ground-glass of the camera.

Different parts of the dress, for the same reason, re-
quire intervals differing considerably, to be fairly copied
—the white parts of a costume passing on to solarization
before the yellow or black parts have made any decisive
representation. We have therefore to make use of tem-
porary expedients. A person dressed in a black coat
and open waistcoat of the same color must put on a tem-
porary front of a drab or flesh color, or by the time that
his face and the fine shadows of his woollen clothing are
evolved, his shirt will be solarized, and be blue or even
black, with a white halo around it. Where, however,
the white parts of the dress do not expose much surface,
or expose it obliquely, these precautions are not essential;
the white shirt collar will scarcely solarize until the face
is passing into the same condition.

Precautions of the same kind are necessary in ladies'
dresses, which should not be selected of tints contrasting
strongly.

It will now be readily understood that the whole art
of taking daguerreotype miniatures, consists in directing
an almost horizontal beam of light, through a blue-col-
ored medium, upon the face of the sitter, who is re-
tained in an unconstrained posture, by an appropriate

220 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV.

but simple mechanism, at such a distance from the back-
ground, or so arranged with respect to the camera, that
his shadow shall not be copied as a part of his body ;
the aperture of the camera should be three and a half or
four inches at least, indeed the larger the better, if the
objective be aplanatic.

If two mirrors be made use of, the time actually occu-
pied by the camera operation varies from forty seconds to
two minutes, according to the intensity of the light. If
only one mirror is employed, the time is about one fourth
shorter. In the direct sunshine, and out in the open air,
the time varies upwards from half a minute.

Looking-glasses which are used to direct the solar
rays after a short time undergo a serious deterioration,
the silvering assuming a dull granular aspect, and losing
its black brilliancy. Hence the time, in copying, becomes
gradually prolonged.

The arrangement of the camera above indicated gives
reversed pictures, the right and left sides changing places.
Mr. Woolcott, an ingenious mechanician of this city, has
taken out a patent for the use of an elliptical mirror for
portraiture; it is about seven inches in aperture, and
allows him to work conveniently with plates two inches
square. The concave mirror possesses this capital advan-
tage over the convex lens, that the proof is given in its
right position, that is to say, not reversed; but it has the
serious inconveniences of limiting the size of the plate,
and representing parts that are at all distant from the
centre in a very confused manner. With the lens, plates
might be worked a foot square, or even larger.

Miniatures procured in the manner here laid down are
in most cases striking likenesses, though not in all. They
give, of course, all the individual peculiarities — a mole, a
freckle, a wart. Owing to the circumstance that yellow
and yellowish browns are long before they impress the

MKMOIII XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 221

substance of the daguerreotype, persons whose faces are
freckled all over give rise to the most ludicrous results, a
white portrait mottled with just as many black dots as
the sitter had yellow ones. The eye appears beautifully :
the iris with sharpness, and the white dot of light upon
it with such strength and so much of reality and life as
to surprise those who have never before seen it. Many
are persuaded that the pencil of the painter has been
secretly employed to give this finishing touch.

UNIVERSITY OF NEW YORK, September, 1840.

222 CONDITION or A DAGUERREOTYPE SURFACE. [MEMO™ xvi.

MEMOIR XVI.

ON THE CHEMICAL CONDITION OF A DAGUERREOTYPE
SURFACE.

From the Philosophical Magazine, September, 1841.

CONTENTS: — Mercury exists all over a daguerreotype surface. — There is
no superposition of the parts. — The shadows have metallic mercury ;
the lights silver amalgam. — No iodine is ever evolved from the plate.
— Action of a solution of gum and one of gelatine in tearing off the
films. — The starch experiment. — The etching of daguerreotypes.

As many of the results to be given in the next Mem-
oir depend on the use of daguerreotype tablets, I shall
in this examine the chemical and physical condition of
those surfaces and offer proof of the following facts :

1st. That metallic mercury exists all over the surface
of an ordinary daguerreotype — in the shadows as well
as in the lights — in the shadows as metallic mercury,
in the lights as silver amalgam.

2d. That in an iodized daguerreotype as taken from
the mercury-bath there is no order of superposition of
the parts, that is to say, the iodide is neither upon nor
beneath the mercury, but both are, as it were, in the same
plane.

3d. That when a ray of light falls upon the surface of
this preparation, through all the intervening steps, and
up to the point of maximum action, no iodine is evolved
from the plate, but that in the common daguerreotype
the light communicates a tendency to the atoms of the
iodide to yield up to the mercurial vapor their silver,
while the iodine retires and combines with the unaf-

Mi M..IU XVI.] CONDITION OF A DAGUERUEOTYPK KUUFACE. 993

fected silver beneath. It follows that when such a plate
is withdrawn from the mercurial vapor, there is all over
it a uniform film of iodide of silver of the very same
thickness as at first, and that this has happened through
a direct corrosion of the silver by the iodine, while it
was undergoing the mercurial operation.

I pass at once to the proofs of these several propo-
sitions, and, 1st, That metallic mercury exists all over the
tturface of an ordinary daguerreotype, in the shadows as
well as in the lights — in the shadows it is as metallic mer-
cu?*y, in the lights as silver amalgam.

I took a plated copper three inches by four in surface,
and having prepared it with care, I exposed half of it to
the diffused light of the day, screening the other half; it
was then mercurialized at 170° Fahr., the iodide removed
by hyposulphite of soda, and washed. And now, a plate
on which a gold-leaf was spread was placed over it, but
separated, as shown in Fig. 34, in
the points «, #, <?, by three slips of '•' ' »•• i 'fT

glass. By means of a spirit- lamp
the photographic plate d e was heated, and the gilded
plate g k kept cool by occasionally wetting it. On part-
ing the plates, it was perceived that faint but distinct
traces of whitening were visible all over the gold, as well
on that part which was over the whitened half of the
photograph as over that which was unchanged.

But as it might happen that the mercury diffused
itself laterally past the imperfect obstacle b, I made the
following decisive trials :

I iodized three silver plates, A, B, C, each three inches
by four in surface, conducting the processes for each in the
same way ; and having exposed each for two minutes to a
faint daylight, I laid them aside in the dark, to be pres-
ently used as test-plates in lieu of the gilded plate (g &).

224 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIR XVI.

Then I took three other plates, D, E, F, of the same
size, and conducting the preparatory processes for each
as before, I iodized D in the dark, and mercurialized it
forthwith at 170° Fahr., taking the utmost care that not
a ray of light should be suffered to impinge upon it.

E was iodized and exposed for two minutes to dif-
fused daylight, and then mercurialized at 170° Fahr.

F was iodized and exposed to the sun until it began
to turn brown, an effect occurring almost at once. It
was then mercurialized at 170° Fahr.

All these plates then had their sensitive coating re-
moved by hyposulphite, and were thoroughly washed in
distilled water and dried.

I had, therefore, three plates, representing accurately
the conditions proposed to be investigated. D was in
the condition of the most perfect shadows; E in that of
the highest lights, and F solarized. In appearance; D
was black, E was white, and F bluish-gray.

Upon D, E, F, I placed A, B, C, respectively, separat-
ing each pair of plates one sixteenth of an inch, or there-
abouts, by slips of glass. Then I laid them on the level
surface of the sand-bath, the test-plates being kept cool
by sponging occasionally with water. Temperature of
the sand, 200° Fahr. ; duration of the experiment, fifteen
minutes.

On examination, A, B, C, were all found powerfully
mercurialized, nor did there seem to be any difference
between them.

I consider, therefore, that the shadows, the demi-tints,
the lights, and the solarized portions of a daguerreotype,
are covered with mercury ; for at a temperature of 200°
Fahr. they all evolve it alike, a sufficiency of vapor rising
from the parts that have not been exposed to the light
to bring a plate that has been so exposed to its max-
imum of whiteness.

MI.M..IH XVI. 1 CONDITION OF A DAGUERREOTYPE SURFACE. 225

In Memoir XIV. I described a remarkable effect which
1 had noticed in these investigations, that if an object
such as a wafer be laid upon a piece of cold glass or
metal, and you breathe once on it, and as soon as the
moisture has disappeared remove the object and breathe
again on the glass, a spectral image of the wafer will
make its appearance. The impression thus communi-
cated to the surface, under certain conditions, remains
there a long time. During the cold weather last winter
I produced such an image on the mirror of my heliostat ;
it could be revived by breathing on the metal many
weeks afterwards, nor did it finally disappear until the
end of several months.

I do not at present know what is the explanation of
this result, but the analogy between it and the arrange-
ment of mercurial globules covering the surface of a da-
guerreotype is too striking to be overlooked. It proves
that surfaces may assume such a condition as to affect
the deposition of vapors upon them, so as to produce
the reproduction of appearances of external forms. I
gave, therefore, particular attention to this point, but
eventually found that silver exists in an ordinary da-
guerreotype, in connection with the mercury all over the
plate, in a less proportion in the shadows, and in a
greater proportion in the lights. This result was, how-
ever, only obtained after the following fact was discov-
ered — that the mucilage of gum-arabic, when slowly
dried in a thin layer on the surface of a daguerreotype,
splits up in shivers, bringing along with it the white
portions of the picture, and leaving the plate clean.

Having therefore prepared three plates, D, E, F, ex-
actly as before, I poured on them a solution of gum,
drained them so as to leave only a small quantity, and
let them dry slowly over the sand-bath. The gum sepa-
rated readily, and lay in chips on the surface of each

P

226 CONDITION OF A DAGUERREOTYPE SURFACE. [MKMOIK XVI.

plate; it was easily removed to three sheets of paper, by
tapping with the finger on the back of the plate. Each
was then treated alike as follows :

The gummy matter wras incinerated on a platinum
leaf, and the remaining ashes transferred to a test-tube

' O

half an inch in diameter. One drop of nitric acid and
one drop of water were added ; it was boiled over a
small flame, and diluted with a little water. Dilute hy-
drochloric acid was now added, and the chloride of silver
immediately fell. In repeating this, it is necessary to
attend to the state of dilution of the acid, for if too
strong it wholly dissolves the minute quantity of chlo-
ride of silver generated.

As, from the minuteness of that quantity, it was
impossible to obtain a direct quantitative analysis, I
adopted the foregoing method, and added the dilute
acid to all three tubes at the same time. In D there
was^a faint opalescence, in E and F a cloud; but I
could not always determine whether the deposit of E
or F was most copious, sometimes the one and some-
times the other appearing to have a slight advantage.

I conclude, therefore, that while the whole surface of
the plate is coated with mercury, it exists as silver amal-
gam chiefly in the lights, and as uncombined mercury
chiefly in the shadows, and in a mixed proportion in the
demi-tiuts; and that when a plate is solarized, both free
mercury and amalgam are present.

Such is the state of surface in a daguerreotype, recently
formed. In the course of time, however, a great portion
of the mercury that is in the shadows, and also free in
the lights, evaporates away. When the picture has thus
changed, the shadows are metallic silver, and the lights
silver amalgam.

2d. That in an iodized daguerreotype, as taken from
the mercury-bath, there is no order of superposition of the

MI.M..IK XVI.J CONDITION OF A DAtifKKKKOTYrE SURFACE. 227'

<, flat w to say, the iodide is neither upon nor beneath
tin mercury, but both are, an it were, in tlw same plane.

Soon after 1 had ascertained the action of gum-arabic,
some of it was applied to the surface of a plate on which
an impression had just been formed in the mercury-bath.
This was without removing the coat of iodine. On dry-
ing it, the gum chipped up, as was expected, bringing
away with it all the lights of the picture, and leaving
a uniform coat of yellow iodide of silver beneath. It
seems, therefore, that the film of iodide coheres more
strongly to the metal plate than the amalgam ; and, fur-
ther, from this result we should judge that the amalgam
is on the surface of the iodide.

But this is not true; for on three different occasions
I found that when Russian isinglass was employed in-
stead of gum for the purposes presently to be related,
the isinglass, from its stronger cohesive power, chipped
off in the act of drying, tearing up the yellow film from
end to end of the plate, and leaving the amalgam con-
stituting the lights undisturbed. It is here to be under-
stood that this action takes place without the smallest
disturbance of the lights and demi-tints, the plate remain-
ing in all the beauty and brilliancy and perfection that
it would have had if it had been carefully washed in
hyposulphite of soda.

This is a result, however, which cannot be produced
with uniformity. Most commonly the lights are torn
up with the iodide. Had it occurred but once, I should
still have cited it with decision, for from the very char-
acter of it, it is impossible to be mistaken or to commit
an error of judgment. It proves that the film of iodide
may be mechanically torn off from the metallic surface
as perfectly as it can be dissolved off by chemical agents
—a singular fact.

This result, therefore, proving that we can tear off

228 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIR XVI.

the film of iodide and leave the amalgam, can only be co-
ordinated with that by gum-water, in which the amalgam
is removed and the iodide left, by supposing that there
is not anything like a direct superposition in the case,
and that the particles of amalgam and iodide lie, as it
were, side by side.

3d. That when a ray of light falls upon the surface
of this, etc.

There is no difficulty in proving this directly, and the
indirect evidence is copious. If we lay a piece of paper
imbued with starch on an iodized plate, and expose it
to the sun, although the plate presently assumes a dark
olive-green color, the starch remains uncolored.

This dark substance is probably a subiodide of silver;
the iodine therefore which has been disengaged from it,
not having been set free, must have necessarily united
with the adjacent metallic silver — this, for very obvious
reasons, there is no difficulty in admitting.

Now, therefore, when a photogenic impression existing
on the surface of a plate in an invisible state is brought
out by the action of mercury vapor, we easily understand
how this is effected. No iodine is ever evolved. But
each atom of iodide of silver that has been acted on by
the light yields to the attraction of the mercury its atom
of silver, and the iodine thus set free unites with the me-
tallic silver particles around it, reproducing the same
yellow iodide by a direct corrosion of the plate: the
proofs that we have of this are two in number:

1st. Dry some mucilage of gum-arabic on a daguerre-
otype just brought from the mercury-bath ; when it has
split up, we perceive that the white amalgam of silver
is removed, and a uniform coat of yellow iodide of sil-
ver, of the very same thickness as at first, as is proved
by its color, is left.

2d. Dry upon the same plate a solution of Russian

MI.M..IU XVI.] CONDITION OF A DAOUBBBEOTTPK SURFACE. 229

isinglass, and when it has split up, it will be seen that
it uniformly rends off with it the yellow iodide, leaving
the metallic plate with an exquisite polish ; and wher-
ever the li«>ht has touched, there it in corroded.

O

These two facts, taken together, prove that in mercu-
rializing a plate no iodine is evolved, but that a new film
of iodide of the same thickness is formed, at the expense
of the metallic surface.

From these facts we readily gather that on the pres-
ence of the metallic silver the sensitiveness of this prep-
aration mainly depends, for to the tendency which the
light has impressed on the elements of the iodide to sep-
arate is added the strong attraction of metallic silver for
uascent iodine.

This corrosion or biting in of the silver plates, by the
conjoint action of the mercury and iodine, gives rise to
etchings that have an inexpressible charm. Could any
plan be hit upon of forcing the iodine to continue its ac-
tion, the problem of producing engraved daguerreotypes
would be solved. By another process, which will be
described hereafter, I have succeeded in producing deep
etchings from daguerreotypes.

UNIVERSITY OF NEW YORK, September, 1841.

230 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII.

MEMOIR XVII.

ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE
CHEMICAL RAYS AND THOSE OF RADIANT HEAT.

From the Philosophical Magazine, September, 1841.

CONTENTS: — The chemical rays are absorbed. — Photographic effects are
transient. — The chemical rays are not conducted ; they become latent.
— Optical qualities control chemical action. — The active rays are ab-
sorbed and the complementary reflected. — Relation of optical conditions
and chemical affinities.

IT is the object of this Memoir to establish some strik-
ing analogies existing between the phenomena of the
chemical rays and those of radiant heat.

Without saying anything respecting the laws of re-
flection, refraction, polarization, and interference, to which
the chemical rays are subject, the study of which I com-
menced more than five years ago on paper rendered sen-
sitive by bromide of silver, further than that a general
similitude holds in all these cases between the rays of
heat and the chemical rays, I shall at present confine my
observations to establishing the following propositions:

1st. That the chemical action produced by the rays of
light depends upon the absorption of those rays by sen-
sitive bodies; just as an increase of temperature is pro-
duced by the absorption of those of heat.

2d. That as a body warmed by the rays of the sun
gradually loses its heat by radiation, or conduction, or
contact with other bodies, so likewise, by some unknown
process, photographic effects produced on sensitive sur-
faces are only transient, and gradually disappear.

MKMOIII XVII. ] C11KMICAL HAYS AND KADI ANT IIKAT.

3d. That, as when rays of heat fall on a mass of cold
ice its temperature rises degree by degree until it reaches
32° Fahr. and there stops until a certain molecular
diMiige (liquefaction) is accomplished, and after that
rises again, so also the chemical rays impress certain
changes proportional to their quantity, up to a certain
point, and there a pause ensues — a very large amount of
lisjht being now rendered latent or absorbed, without

o o

any indication thereof being given by the sensitive prep-
aration (as the heat of fluidity is latent to the thermom-
eter) ; a molecular change then setting in, the incre-
ments of the quantity of light are again indicated by
changes in the sensitive preparation.

4th. That it depends on the CHEMICAL nature of the
ponderable material what rays shall be absorbed.

5th. That while the specific rays thus absorbed de-
pend upon the chemical nature of the body, the absolute
amount is regulated by its OPTICAL qualities, such as de-
pend on the condition of its surfaces and interior ar-
rangement.

6th. It will be proved from this that the SENSITIVE-
NESS of any given substance depends on its chemical
nature and optical qualities conjointly, and that it is
possible to exalt or diminish the sensitiveness of any
chemical compound by changing the character of its op-
tical relations.

7th. That, as when radiant heat falls on the surface of
an opaque body, the number of rays reflected is the com-
plement of those that are absorbed, so in the case of
a sensitive preparation, the number of rays reflected
from the surface is the complement of those that are
absorbed.

I now commence with the proofs of the propositions
of tli is Memoir.

1st. That Hie chemical action produced by the ray*

232 CHEMICAL RAYS AND RADIANT HEAT. [MKMOIK XVII.

of light depends upon the absorption of those rays by
sensitive bodies, etc.

I iodized a plate to a golden yellow color, and exposed
it to the diffused light of day, setting it in such a po-
sition that it reflected specularly the light falling upon
it from the window to the objective of the eamera-obscu-
ra, which formed an image upon a second sensitive plate.
The rays falling upon the sensitive plate of course ex-
erted their usual influence upon the iodide, which, after
the lapse of a short time, began to turn brown. As
soon as this effect was observed, I closed the aperture of
the camera, and, taking out its plate, mercurialized it;
but it was found that the rays reflected from the sensi-
tive plate, although they had been converged by a lens
four inches in diameter, and formed a very bright image,
had lost the quality of changing the iodide of silver.

We see, therefore, that a ray of light which has im-
pinged on the surface of yellow iodide of silver has lost
the power of causing any further change on a second
similar plate on which it may fall.

In the practice of photography this observation is of
much importance, especially when lenses having large
apertures are used ; the rays converging upon the sensi-
tive plate are reflected by it in all directions, and the
camera is full of liojht; its sides reflect back asrain in all

O ' O

directions on the surface of the plate these rays, which,
if they were effective, must stain the plate in the shad-
ows. But if the plate has been iodized to the proper
tint, this light is wholly without action, and hence the
proof comes out neat and clean.

Upon an iodized plate I received a solar spectrum
formed by a flint-glass prism, the ray being kept motion-
less by reflection from a heliostat, and the plate so ar-
ranged as to receive the refracted rays perpendicularly.
After five minutes it was mercurialized, and the resulting

MIM..III XVII.] U1KMICAL HAYS AND RADIANT HEAT. 233

proof exhibited the place of the more refrangible colors
iu the most brilliant hues. The less refrangible colors
had also left their impress of a whitish aspect, but the
region of the yellow was unaltered. All the different
rays, therefore, except the yellow, have the power of
changing this particular preparation. Now when sev-
eral pieces of cloth of different colors are placed in the
sunbeam, they absorb heat in proportion as their color
is deeper. A black cloth, which does not reflect any of
those calorific rays, becomes presently hot ; and in the
same way Daguerre's sensitive preparation absorbs all
the rays having any chemical action on it, and reflects
the yellow only, which does not affect it. In this par-
ticular lies the secret of its sensitiveness, compared with
the common preparations of the chloride arid bromide
of silver.

2d. That as a body warmed by the rays of the sun, etc.

After a beam of light has made its impression on the
iodide, if the plate be laid aside in the dark before mer-
curializing, that impression decays away with more or
less rapidity; first the faint lights disappear, then those
that are stronger.

Having brought

~ o

three plates to the
same condition of
iodization, and re-
ceived the image
of a gas -flame in
the camera on each
for three minutes, I
mercurialized one, A, forthwith ; the second, B, I kept
an hour, the third, C, forty-eight hours. The relative ap-
pearance of these three images is represented in Fig. 35.

Those who are in the habit of taking daguerreotypes
know how much they suffer when the process of mercu-

234 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII.

rialization is deferred. To show this effect in the ex-
treme, I took four plates, and having prepared all alike,
I exposed half of the surface of each to a bright sky for
eight seconds.

No. 1 mercurialized immediately, came out black solarized.

" 2 " in five hours, " white.

" 3 " twenty-two hours, " same effect.

" 4 " one hundred and forty-four hours, no effect.

This last plate, on being submitted twice more to the
vapor of mercury, gave an indistinct mark. On exposing
a corner of it to the sun it blackened instantly, these re-
sults showing that the peculiar condition brought on by
the action of the light gradually disappears, the com-
pound all the time retaining its sensitiveness.

Similar results are mentioned by Daguerre in the case
of the changes produced on surfaces of resinous bodies,
and I have noticed them in a variety of other cases.
Now if) whatever cause these phenomena are due, whether
to anything analogous to radiation, conduction, etc., it is
most active during the first moment after the light has
exerted its agency, but it must also take effect even at
the very time of exposure; and it is for these reasons
that it comes to pass that when light of a double inten-
sity is thrown upon a metallic plate the time required
to produce a given effect is less than one half.

I could conceive the intensity of a ray so adjusted that
in falling upon a given sensitive preparation the loss
from this cause, this casting off of the active agent, should
exactly balance the primitive effect, and hence no observ-
able change result. Hereafter we shall find that one

O

cause of the non-sensitiveness of a number of bodies is
to be traced directly to the circumstance that they yield
up these rays as fast as they receive them.

It needs no other observation than a critical examina-
tion of the sharp lines of a daguerreotype proof with a

M.MMIK XVII. J ( IIKMK'AI- KAYS AM) KADIANT IIKAT. 235

magnifying glass to show that the influence of the chem-
ical rays is not propagated laterally on the yellow iodide
of silver. Of the manifestation* which these rays may
exhibit, after they have lost their radiant form and be-
come absorbed, we know but little. If they conform to
the analogous laws for heat, and if the absorbing action
of bodies for this agent is inversely as their conducting
power, we perceive at once why a photographic effect
produced on yellow iodide of silver retains the utmost
sharpness without any lateral spreading; the absorbing
power is almost perfect, the conducting should therefore
be zero.

3d. That, as lolien rays of heat fall on a mass of cold
ice, etc.

Although in the sun the iodide of silver blackens at
once, this is only the result of a series of preliminary
operations.

When we look at a daguerreotype, we are struck with
the remarkable gradation of tint, and we naturally infer
that the amount of whitening induced by mercurializa-
tion is in direct proportion to the amount of incident
light; otherwise it would hardly seem that the grada-
tion of tones could be so perfect.

But in truth it is not so. When the rays begin to
act on it, the iodide commences changing, and is capable
of being whitened by mercury. Step by step this proc-
ess goes on, an increased whiteness resulting from the
prolonged action or increased brilliancy of the light, until
a certain point is gained, and now the iodide of silver
apparently undergoes no further visible change ; but an-
other point being gained, it begins to assume, when mer-
curialized, a pale blue tint, becoming deeper and deeper,
until it at last assumes the brilliant blue of a watch-
spring. This incipient blueness goes under the technical
name of solarization.

236

CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII.

The successful practice of the art of daguerreotyping,
therefore, depends oil limiting the action of the sun-ray
to the first moments of change in the iodide; for if the
exposure be continued too long, the high lights become
stationary, while the shadows increase unduly in white-
ness, and all this happens long before solarization sets in.
Let us examine this important phenomenon more mi-
nutely. Having carefully cleaned and iodized a silver
plate, three inches by four in size, it is to be kept in the
dark an hour or two.

By a suitable set of tin-foil screens, rectangular por-
tions of its surface, half an inch by one eighth, are to be
exposed at a constant distance to the rays of an Argand
gas-burner (the one I have used is a common twelve-
holed burner), the first portion being exposed fifteen sec-
onds, the second thirty seconds, the third forty-five sec-
onds, the fourth sixty seconds, etc.

We have thus a series of
spaces upon the plate, «, 5,
c, d, Fig. 36, each of which
has been affected by known
quantities of light ; b being
affected twice as much as «,
having received a double
quantity of light; c thrice
as much as «, having re-
ceived a triple quantity,
etc.

The plate is now exposed
to the vapor of mercury at
170° Fahr. for ten minutes;
the spaces all come out
in their proper order, and
nothing remains but to remove the iodide.

An examination of one of these plates thus prepared

Fig. 36.

MKMOIK XVII.] CHEMICAL RAYS AND RADIANT HEAT.

237

shows* that, commencing with the first space, «, we dis-
cover a gradual increase of whitening effect until we
reach the seventh ; that a perfect whiteness is there at-
tained ; that, passing on to the sixteenth, no increase of
whitening is to he perceived, although the quantities of
lififht that have been incident and absorbed have been con-

O

tin mil ly increasing; but as soon as the light thus latent
has reached a certain quantity, visible decomposition sets
in, indicated by a blueness, and the sensitive surface once
more renders evident the increments of incident li^ht.

O

Or, by presenting a plate covered with a screen to a
sky that is clear or uniformly obscured, and with a regu-
lar motion withdrawing the screen deliberately from one
end to the other, and then
suddenly screening the
whole, it is plain that those
parts first uncovered will
have received the greatest
quantity of light, and the
others less and less. On
mercurializing, it will be
seen that a stain will be
evolved on the plate, as is
represented in Fig. 37;
from a to b the changes

O

have been successive ; from

b to c no variation in the

amount of whitening is per-

ceptible; at d solarization

is commencing, which becomes deeper and deeper to the

end, e, of the stain.

* It is impossible to represent these chanpes in a drawing which is simply bl:ick
ami wtiite ; it will be understood that the characteristic distinction of the srwce*
from the sixteenth to the twentieth, for example, depends on their assuming a blue
tint, which continually deepens in intensity.

238 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII.

The plate from which the drawing of Fig. 37 is taken
gives from a to b ten parts, from b to c seventeen parts,
from d to e twelve parts; we perceive, therefore, how
large an amount of light is absorbed, and its effects ren-
dered latent, between the maximum of whiteness beins;

' O

gained and solarization setting in.

4th. That it depends on the CHEMICAL nature of the
ponderable material what rays shall be absorbed.

I had prepared a number of observations in proof of
this, very much of the same kind as those which have
some time ago been published in the Phil. Trans, by
Herschel. These refer chiefly to the variable lengths of
the stains impressed by the prismatic solar spectrum on
different chemical bodies, and the points of maximum ac-
tion noticed in them. For the present I content myself
with referring to that Memoir for proofs substantiating
this proposition.

5th* That while the specific rays thus absorbed de-
pend upon the chemical nature of the body, the absolute
amount is regulated by its OPTICAL QUALITIES, such as
depend on the condition of its surfaces and interior ar-
rangement.

I took a polished silver plate, and having exposed it to
the vapor of iodine, found that it passed through the fol-
lowing changes of color : 1st, lemon yellow ; 2d, golden
yellow; 3d, reddish yellow; 4th, blue; 5th, lavender; 6th,
metallic ; 7th, yellow ; 8th, reddish ; 9th, green, etc., the
differences of color being produced by the differences of
thickness in the film of iodide, and not by any difference
of chemical composition.

It is a common remark, originally made by Daguerre,
that of these different tints that marked 2 is the most
sensitive, and photogenic draughtsmen generally suppose
that the others are less efficient from the circumstance
of the film of iodide being too thick. Some suppose,

MKMOIK XVII.] CHEMICAL KAYS AND RADIANT HEAT. 239

indeed, that the first yellow alone is sensitive to light.
We shall see in a few moments that this is very far
from being the case.

Having brought nine different plates to the different
colors just indicated, I received in the camera on each
the image of a uniform gas-flame, treating all as nearly
alike as the case permitted. I readily found that in
No. 1 there was a well-marked action, No. 2 still stronger,
but that the rays had less and less influence down to
No. 6, in which they appeared to be almost without
action ; but in No. 7 they had recovered their original
power,' being as energetic as in No. 2, and from that
declining again. This is shown in Fig. 38.

Yellow. Blue. Lavender. 2d Yellow.

No. 2. No. 4. No. 6. No. 7.

Fig. 38.

Hence we see that the sensitiveness of the iodide of
silver is by no means constant; that it observes period-
ical changes depending on the optical qualities of the
film and not on its chemical composition; and that by
bringing the iodide into those circumstances that it re-
flects the blue rays we greatly reduce its sensitiveness,
and still more so when we adjust its thickness so as to
give it a gray metallic aspect. But the moment we go
beyond this, and restore by an increased thickness its
original color, we restore also its sensitiveness. Here,
then, in this remarkable result we again perceive a cor-
roboration of our first proposition.

040 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII.

I may, however, observe in passing, that although I am
describing these actions as if there were an actual ab-
sorption of the rays, and that films on metallic plates ex-
hibit colors,not through any mechanism like interference,
but simply because they have the power of absorbing
this or that ray, there is no difficulty in translating these
observations into the language of that hypothesis. When
the diffracted fringes given by a hair or wire in a cone
of diverging light are received on these plates, corre-
sponding marks are obtained, a dark stripe occupying
the place of a yellow fringe, and a white that of a blue.
I found, more than four years ago, that this held in the
case of bromide of silver paper, and have since verified it
in a more exact way with this French preparation. Sim-
ilar phenomena of interference may be exhibited with
the chloride of silver.

We have it, therefore, in our power to exalt or de-
press4he sensitiveness of any compound by changing its
optical character. Until now, it has been supposed that
the amount of change taking place in different bodies,
by the action of the rays of light, depends wholly on
their chemical constitution, and hence comparisons have
been instituted as to the relative sensitiveness of the
chlorides, bromides,oxides, and iodides of silver, etc. But
it seems the liability to change depends also on other
principles which, being liable to variation, the sensitive-
ness of a given body varies with them. Thus this very
iodide of silver, when in a thin yellow film, is decomposed
by the feeblest rays of a taper, and even moonlight acts
with energy ; yet simply by altering the thickness of its
film it becomes sluggish, blackening even in the sunlight
tardily, and recovering its sensitiveness again on recov-
ering its yellow hue.

We have now no difficulty in understanding how, in
the preparation of ordinary sensitive paper, great varia-

MEMOIR XV1I.J C1IKM1CAL KAYS AND HAD1ANT HEAT. 241

tions ensue, by modifying the process slightly, and how
even on a sheet which is apparently washed uniformly
over, large blotches appear, which are either inordinately
sensitive or not sensitive at all. If, without altering
the chemical composition of a film on metallic silver, or
even its mode of aggregation, such striking changes re-
sult by difference of thickness, how much more may we
expect that the great changes in molecular condition,
which apparently trivial causes must bring about on sen-
sitive paper, should elevate or depress its capability of
being acted on by light. If I mistake not, it is upon these
principles that an explanation is to be given of the suc-
cessful modes of preparation which Talbot and Hunt have
described, and the action of the mordants of Herschel.

I therefore infer—

6th. That the SENSITIVENESS of any given preparation
depends on its chemical nature and its optical qualities
conjointly, and that it is possible to exalt or diminish the
sensitiveness of a given compound by clianging its optical
relations.

7th. That, as when radiant heat falls on the surface of
an opaque body, the number of rays reflected is the comple-
ment of those tliat are absorbed, so in the case of a sensitive
preparation, the number of chemical rays reflected from
tlie surface is the complement of those that are absorbed.

This important proposition I prove A

in the following way: I take a plate,
A G, Fig. 39, three inches by four, and
by partially screening its surface while
in the act of iodizing with a piece of
flat glass, I produce upon it five trans-
verse bands, b, c, d, e, f ; the fifth,/,
which has been longest exposed, is of
a pale lavender color, the fourth a
bright blue, the third a red, the sec-

Q

Metallic.

Yellow.

Red.

Bloc.

Lavender.

242 CHEMICAL KAYS AND EADIANT HEAT. [MEMOIR XVII.

ond a golden yellow, and the first uniodized metal ; the
object of this arrangement being to expose at the same
time and on the same plate a series of films of different
colors and of different thicknesses, and to examine the
action of the rays impinging on them and the rays re-
flected by them.

Having prepared a second plate, B, and iodized it uni-
formly to a yellow, I deposit it in the camera, and now,
placing the first plate, A G, so that the rays coming on it
from the sky through the window shall be specularly
reflected to the object-glass of the camera, and the image
of A G form upon B, I allow the exposure to continue
until the yellow of A G is beginning to turn brown;
then I shut the camera and mercurialize both plates.

In accordance with what has been said, it will be
readily understood that of the bands on A G, the first
one, which is the bare metal, does not whiten in the mer-
cury *Vapor; the second, which is yellow, mercurializes
powerfully ; the third, which is red, is less affected ; the
fourth, which is blue, still less ; and the fifth, which is
lavender, hardly perceptibly.

But the changes on B, which have been brought about
by the rays reflected from A G, are precisely the con-
verse ; the band which is the image of b is mercurialized
powerfully; that of c is untouched and absolutely black,
d faintly stained, e whitened, and / mercurialized but lit-
tle less than b.

It follows from this that a white stripe on B corre-
sponds to a black one on A G, and the converse ; and for
the depth of tint of the intermediate stripes those of the
one are perfectly complementary to the corresponding
ones of the other.

By the aid of these results we are now able to give
an account of the^variability of sensitiveness in photo-
genic preparations ; the yellow iodide of silver is exces-

MEMOIB XVII.] CHEMICAL RAYS AND RADIANT HEAT. 243

sively sensitive, because it absorbs all the chemical rays
that can disturb it, while the lavender is insensitive, be-
cause it reflects them. Under this point of view, sensi-
tiveness therefore is directly as absorption and inversely
as reflection.

The superiority of Daguerre's preparation over com-
mon sensitive paper may now be readily understood. It
absorbs all the rays that can affect it, but the chloride
of silver, spread upon paper, reflects many of the active
rays. The former, when placed in the camera, gives rise
to no reflections that can be injurious; the latter fills it
with active light, and stains the proof all over. Hence
the daguerreotype has a sharpness and mathematical ac-
curacy about its lines, and a depth in its shadows, which
is unapproachable by the other. Moreover, the translu-
cency of the white chloride of silver, as well as its high
reflecting power, permits of particles lying out of the
lines of light being affected, the light becoming diffused
in the paper.

The fact, therefore, that a given compound remains un-
changed even in the direct rays of the sun is DO proof
that light cannot decompose it; it may reflect or trans-
mit the active rays as fast as it receives them. It results
from this that optical conditions can control and even
check the play of chemical affinities. While thus it ap-
pears that there are points of analogy between this chem-
ical agent and radiant heat, we must not too hastily infer
that the laws which regulate the one obtain exclusively
also with the other. As is well known, there are strik-
ing analogies between radiant heat and light, but there
are also points of difference, the convertibility of heat of
one degree of refrangibility to another does not occur
with light ; there are also dissimilitudes in the phenom-
ena of radiation and its consequences.

From the phenomena of the interference of these rays,

244 CHEMICAL KAYS AND EADIANT HEAT. [MEMOIR XVII.

of the sensitiveness or non - sensitiveness of the same
chemical compound being determined merely by the fact
of its thickness or thinness, these, and many other similar
results obviously depending upon mechanical principles,
it seems to me that very powerful evidence may be
drawn against the materiality of light, and its entering
into chemical union with ponderable atoms. Those phi-
losophers who have adopted the undulatory theory will
probably find in studying these subjects evidence in
favor of their doctrines.

MEMOIK XVIII.J THE CilLuK-IlYDUOGEN PIIOTOMETLK. 045

MEMOIR XVItt

DESCRIPTION OF THE CIILOR-HYDROGEN PHOTOMETER.

From the American Journal of Science and Art, Vol. XLVI. ; Philosophical Mag-
azine, December, 1843.

CONTENTS : — Properties of a mixture of chlorine and hydrogen. — It is
acted upon by lamplight, an electric spark at a distance, etc. — The
gases unite in proportion to the amount of light. — Mode of measuring
out known quantities of radiations. — The maximum action is in the
indigo space. — Construction of the instrument. — The gases are evolved
by electricity and combined by light. — Theoretical conditions of equi-
librium.— Preliminary adjustment. — Method of continuous observa-
tion.— Method of interrupted observation. — Remarkable contraction
and expansion.

I HAVE invented an instrument for measuring the
force of the chemical rays found at a maximum in the
indigo space, and which from that point gradually fade
away to each end of the spectrum. The sensitiveness,
speed of action, and exactitude of this instrument will
bring it to rank as a means of physical research with the
thermo-multiplier of Melloni.

The methods hitherto available in optics for measuring
intensities of light by a relative illumination of spaces
or contrast of shadows are admitted to be inexact. The
great desideratum in that science is a photometer which
can mark down effects by movements over a graduated
scale. With those optical contrivances may be classed
the methods hitherto adopted for determining the force
of the chemical rays by stains on daguerreotype plates
or the darkening of sensitive papers. As deductions
drawn in this way depend on the opinion of the observ-

246 THE CHLOK-HYDKOGEN PHOTOMETER. [MEMOIR XVIII.

er, they can never be perfectly satisfactory, nor bear any
comparison with thermometric results.

Impressed with the importance of possessing for the
study of the properties of the chemical rays some means
of accurate measurement, I have resorted in vain to many
contrivances; and, after much labor, have obtained at
last the instrument which it is the object of this Memoir
to describe.

This photometer consists essentially of a mixture of
equal measures of chlorine and hydrogen gases, evolved
from and confined by a fluid which absorbs neither.
This mixture is kept in a graduated tube, so arranged
that the gaseous surface exposed to the rays never varies
in extent, notwithstanding the contraction that may be
going on in its volume, and the hydrochloric acid result-
ing from its union is removed by rapid absorption.

The theoretical conditions of the instrument are, there-
fore, sufficiently simple; but, when we come to put them
into practice, obstacles appearing at first sight insur-
mountable are met with. The means of obtaining chlo-
rine are all troublesome ; no liquid is known which will
perfectly confine it; it is a matter of great difficulty to
mix it in the true proportion with hydrogen, and have
no excess of either. Nor is it at all an easy affair to
obtain pure hydrogen speedily, and both these gases
diffuse with rapidity through water into air.

Without dwelling further on the long catalogue of
difficulties thus to be encountered, I shall first give an
account of the capabilities of the instrument in the form
now described, which will show to what an extent all
those difficulties are already overcome. In a course of
experiments on the union of chlorine and hydrogen,
some of which were read at the last meeting of the Brit-
ish Association, I found that the sensitiveness of their
mixture had been greatly underrated. The statement

MI.N : XVIII. J T11K ( IlI.oK HYDKoi.l.N PHOTOMETER. 247

made in the books of chemistry, that artificial light will
not affect it, is wholly erroneous. The feeblest gleams
of a taper produce a change. No further proof of this
is required than the tables given in this communication,
in which the radiant source was an oil-lamp. For speed
of action no compound can approach it : a light which
perhaps does not endure the millionth part of a second
affects it energetically, as will be hereafter shown.

Proofs of the sensitiveness of tlie instrument. — The fol-
lowing illustrations will show that this instrument is
promptly affected by rays of the feeblest intensity and
of the briefest duration.

When, on the sentient tube, the image of a flame
formed by a convex lens is caused to fall, the liquid in-
stantly begins to move over the scale, and continues its
motions as long as the exposure is continued. It does
not answer to expose the tube to the direct emanations
of the lamp without first absorbing the radiant heat,
or the calorific effect will mask the true result. By the
interposition of a lens this heat is absorbed, and the
chemical rays alone act.

If the photometer be exposed to daylight coming
through a window, and the hand or a shade of any kind
be passed in front of it, its movement is in an instant ar-
rested ; nor can the shade be passed so rapidly that the
instrument will fail to give the proper indication.

The experimenter may further assure himself of the
extreme sensitiveness of this mixture by placing the in-
strument before a window and endeavoring to remove

^j

and replace its screen so quickly that it shall fail to give
any indication : he will find that it cannot be done.

Charge a Leyden-jar, and place the photometer at a
little distance from it, keeping the eye steadily fixed on
the scale ; discharge the jar, and the rays from the spark

248 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII.

will be seen to exert a very powerful effect, the move-
ment taking place and ceasing in an instant.

This remarkable experiment not only serves to prove
sensitiveness, but also brings before us new views of the
powers of that extraordinary agent electricity. That
energetic chemical effects can thus be produced at a
distance by an electric spark in its momentary passage,
effects which are of a totally different kind from the
common manifestations of electricity, is thus proved ;
these phenomena being distinct from those of induction
or molecular movements taking place in the line of dis-
charge, they are of a radiant character; and we are led at
once to infer that the well-known changes brought about
by passing an electric spark through gaseous mixtures,
as when oxygen and hydrogen are combined into wrater,
or chlorine and hydrogen into hydrochloric acid, arise
from a very different cause than those condensations and
percussions by which they are often explained — a cause
far more purely chemical in its kind. If chlorine and
hydrogen can be made to unite silently by an electric
spark passing outside the vessel which contains them, at
a distance of several inches, there is no difficulty in un-
derstanding why a similar effect should take place with
a violent explosion when the discharge is made through
their midst, nor how a great many mixtures may be
made to unite under the same treatment. A flash of
lightning cannot take place, nor an electric spark be dis-
charged, without chemical changes being brought about
by the radiations emitted.

Proofs of the exactness of the indications of this pho-
tometer.— The foregoing examples may serve to illustrate
the extreme sensitiveness of this instrument. I shall
next furnish proofs that its indications are exactly pro-
portional to the quantities of light incident on it.

xvill.] THE CHLOR-HYDBOGBN PHOTOMETER.

249

As it is necessary, owing to the variable force of day-
light, to resort to artificial means of illumination, it will
be found advantageous to employ the following method
of obtaining a flame of suitable intensity.

Let A B, Fig. 40, be an Argand oil-lamp of which the

Fig. 40.

wick is C. Over the wick, at a distance of half an inch
or thereabouts, place a plate of thin sheet-copper, three
inches in diameter, perforated in its centre with a circu-
lar hole of the same diameter as the wick, and concentric
therewith. This piece of copper is represented at d d\
it should have some contrivance for raising or depressing
it through a small space, the proper height being deter-
mined by trial. On this plate the glass cylinder, e, an
inch and three quarters in diameter and eight or ten
inches long, rests.

When the lamp is lighted, provided the distance be-
tween the plate d d and the top of the wick be properly
adjusted, on putting on the glass cylinder the flame in-
stantly assumes an intense whiteness; by raising the
wick it may be elongated to six inches or more, and
becomes exceedingly brilliant. Lamps constructed on
these principles may be purchased in the shops. I have,
however, contented myself with using a common Argand
study -lamp, supporting the perforated plate d d at a
proper height by a retort stand. It will be easily un-
derstood that the great increase of light arises from the
circumstance that the flame is drawn violently through

250 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII.

the aperture in the plate by the current established in
the cylinder.

As much radiant heat is emitted by this flame, in
order to diminish its action and also to increase the
chemical effect I adopt the following arrangement: Let
A B (Fig. 40) be the lamp ; the rays emitted by it are
received on a convex lens, D, four inches and three quar-
ters in diameter, that which I use being the large lens of
a lucernal microscope. This, placed at a distance of
twenty-one inches from the lamp, gives an image of the
flame at a distance of thirteen inches, which is received
on the sentient tube F ; between it and the lens there is
a screen, E.

Things being thus arranged, and the lamp lighted so
as to give a flame about three inches and a half long,
the experiments may be proceeded with. It is conven-
ient always to work with the flame at a constant height,
whicli. may be determined by a mark on the glass cylin-
der. At a given instant, by a seconds watch, the screen
E is removed, and immediately the liquid begins to de-
scend. When the first minute is elapsed the position on
the scale is read off and registered ; at the close of the
second minute the same is done, and so on with the
third, etc. And now, if these numbers be compared,
casting aside the first, they will be found equal to one
another, as the following table of experiments, made at
different times and with different instruments, shows.

From this it will be perceived that, taking the first
experiment as an example, if at the end of 30s the pho-
tometer has moved 7.00, at the end of 60s it has moved
8.00 more ; at the end of 90s, 7.50 more ; at the end of
120s, 7.75 more; the numbers set down in the vertical
column representing the amount of motion for each 30s.
And, when it is recollected that the readings are all
made with the instrument in motion, the differences

MEMOIR XVIII.] Till. < HI.oK IIVDKoGEN PHOTOMETER.

251

between the numbers do not greatly exceed the possible
errors of observation. It may be remarked that the
third aud fourth experiments were made with a different
lamp.

TABLE I.

Showing that when the radiant source is constant, the amount of move-
ment in the photometer is directly proportional to the times of exposure.

Experiments.

1.

2.

3.

4.

5.

8.

80

7.00

7.00

10.25

5.25

60

8.00

7.75

11.50

11.75

6.50

90

7.50

8.00

11.50

C.'.T)

120

7.75

7.75

11.50

13.00

6.00

150

7.75

7.25

• • •

6.00

180

...

• •

12.00

6.00

210

6 00

Mean. . .

7.60

7.55

11.19

12.25

6.00

Though a certain amount of radiant heat from a source

O

so highly incandescent as that here used will pass the
lens, its effects can never be mistaken for those of the
chemical rays. This is easily understood when we re-
member that the effect of such transmitted heat would
be to expand the gaseous mixture, but the chemical ef-
fect is to contract it.

Next, the indications of the photometer are strictly
proportional to the quantity of rays that have impinged
upon it ; a double quantity producing a double effect, a
triple quantity a threefold effect, etc.

A slight modification in the arrangement (Fig. 40)
enables us to prove this in a satisfactory way. The lens,
D, being mounted in a square wooden frame, can easily
be converted into an instrument for delivering at its
focal point, where the sentient tube is placed, measured
quantities of the chemical rays, and thus becomes an
invaluable auxiliary in those researches which require
known and predetermined quantities of radiations to be

252

THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII.

measured out. The method of doing this will be de-
scribed in a subsequent part of this Memoir.

In order, therefore, to prove that the indications of
the photometer are proportional to the quantity of im-
pinging rays, place this measuring lens in the position
D, setting its screens at an angle of 90°. Remove the
screen E, and determine the effect on the photometer for
one minute. At the close of the minute, and without
loss of time, turn one of the screens so as to give an
anirle of 180°, and now the effect will be found double

O 7

what it was before, as in the following table :

TABLE II.

Showing that the indications of the photometer are proportional to the
quantity of incident rays.

Quantities.

Experiment 1.

Experiment 2.

Observed.

Calculated.

Observed.

Calculated.

o
90
180
270
360

2.18
4.27
6.70

8.90

2.22
4'. 45
6.67
8.90

2.69
5.75

8.25
11.00

2.75

5.50
8.25
11.00

I have stated in the commencement of this paper that
the action upon the photometer is limited to a ray which
corresponds in refrangibility to the indigo, or, rather, that
in the indigo space its maximum action is found. The
table on the following page serves at once to prove
this fact, and also to illustrate the chemical force of the
different regions of the spectrum.

In this table the spaces are equal ; the centre of the
red, as insulated by cobalt blue glass, is marked as
unity ; the centre of the yellow, insulated by the same,
being marked 3 : the intervening region being divided
into two equal spaces, and divisions of the same value
carried on to each end of the spectrum.

As instruments will no doubt be hereafter invented
for measuring the phenomena of different classes of rays,

MI.M..II; XVIII.] THE CHLOR-IIYDROGEN PHOTOMETER.

258

TABLE III.

Showing that the maximum for the photometer is in the indigo space of

the spectrum.

Space.

Ray.

Force.

Space.

Kay.

Force.

0

KM iv! iir red.. . .

.83

8

Uliie-intligo

•>(\\ 00

1

Red

.50

9

L'l'i 00

2

.75

10

Violet

I'M 00

3

Yellow

2.75

11

Violet

79 00

4

Green

10.00

12

Violet

48 00

5

(Jreen-blue

54.00

13

Violet

24 00

6

Blue

108.00

II

Extra-spectral. .

r> oo

7

Blue

144.00 i

it may prove convenient to designate the precise ray to
which they apply. Perhaps the most simple mode is to
affix the name of the ray itself. Under that nomencla-
ture the instrument described in this paper would take
the name of indigo-photometer.

There is no difficulty in adapting this instrument to
the determination of questions relating to absorption,
reflection, and transmission. Thus I found that a piece
of colorless French plate-glass transmitted 866 rays out
of 1000.

Description of the Instru-
ment. First, of tlie Glass
Part. — The chlor- hydrogen
photometer consists of a glass
tube bent into the form of a
siphon, in which chlorine and
hydrogen can be evolved
from hydrochloric acid con-
taining chlorine in solution
by the agency of a voltaic
current. It is represented by
Fig. 41, where a I c is a clear
and thin tube four tenths of
an inch in external diameter,

Fig. 41.

254 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVII L

closed at the end a. At d, a circular piece of metal an
inch in diameter, which may be called the stage, is
fastened on the tube, the distance from d to a being
2.9 inches. At the point x, which is two inches and a
quarter from d, two platinum wires, x and y, are fused
into the glass, and entering into the interior of the tube,
are destined to furnish the supply of chlorine and hy-
drogen ; from the stage d to the point Z», the inner bend
of the tube, is 2.6 inches, and from that point to the top
of the siphon c the distance is three inches and a half.
Through the glass at £, three quarters of an inch from c,
a third platinum wire is passed ; this wire terminates in
the little mercury-cup r, and x and y in the cups p and
q respectively.

A stout tube, six inches long and one tenth of an inch
interior diameter, ef, is fused on at c. Its lower end
opens into the main siphon tube; its upper end is turned
overMit/, and is narrowed to a fine termination so as
barely to admit a pin, but is not closed. This serves
to keep out dust, and in case of a little acid passing out,
it does not flow over the scale and deface the divisions.
At the back of this tube a scale is placed, divided into
tenths of an inch, beins; numbered from above down-

/ C

wards. Fifty of these divisions are as many as will be
required. Fig. 2 shows the termination of the narrow
tube bent over the scale.

From a point one fourth of an inch above the stage <7,
downwards beyond the bend, and to within half an inch
of the wire z, the whole tube is carefully painted with
India-ink, so as to allow no light to pass; but all the
space from a fourth of an inch above the stage d to the
top of the tube a is kept as clear and transparent as
possible. This portion constitutes the sentient part of
the instrument. A light metallic or pasteboard cap, A D,
closed at the top and open at the bottom, three inches

Mi M..IK XVIII.] THE CHLOIMIYDROGEN PHOTOMETER 053

long and six tenths of an inch in diameter, blackened on
its interior, may be dropped over this sentient tube; it
being the office of the stage d to receive the lower end
of the cap when it is dropped on the tube so as to shut
out the light.

The foot of the instrument, Ic I, is of brass ; it screws
into the block w, which may be made of hard wood or
ivory; in this three holes, p, q, r, are made to serve as
mercury-cups ; they should be deep and of small diam-
eter, that the metal may not flow out when it inclines
for the purpose of transferring. A brass cylindrical cov-
er, L M, L M, may be put over the whole when it is de-
sirable to preserve it in total darkness.

Things being thus arranged, the instrument is filled
with its fluid, prepared as will presently be described;
and as the tubes a b, b c are not parallel to each other,
but include an angle of a few degrees, in the same way
that lire's eudiometer is arranged, there is no difficulty
in transferring the liquid to the sealed side. Enough is
admitted to fill the sealed tube and the open one par-
tially, leaving an empty space to the top of the tube at
c of two and three quarter inches.

Secondly, of the Fluid Part. — The fluid from which
the mixture of chlorine and hydrogen is evolved, and by
which it is confined, is yellow commercial hydrochloric
acid, holding such a quantity of chlorine in solution that
it exerts no action on the mixed gases as they are pro-
duced. From the mode of its preparation it always con-
tains a certain quantity of chloride of platinum, which
gives it a deep golden color, a condition of considerable
incidental importance.

When hydrochloric acid is decomposed by voltaic
electricity its chlorine is not evolved, but is taken up in
very large quantity and held in solution ; perhaps a bi-
chloride of hydrogen results. If through such a solution

256 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII.

hydrogen gas is passed in minute bubbles, it removes
with it a certain proportion of the chlorine. From this,
therefore, it is plain that hydrochloric acid thus decom-
posed will not yield equal measures of chlorine and hy-
drogen unless it has been previously impregnated with
a certain volume of the former gas. Nor is it possible
to obtain that degree of saturation by voltaic action, no
matter how long the electrolysis is continued, if the hy-
drogen be allowed to pass through the liquid.

Practically, therefore, to obtain the photometric liquid
we are obliged to decompose commercial hydrochloric
acid in a glass vessel, the positive electrode being at the
bottom of the vessel and the negative at the surface of

o

the liquid. Under these circumstances, the chlorine as
it is disengaged is rapidly taken up, and the hydrogen
being set free without its bubbles passing through the
mass, the impregnation is carried to the point required.

Although this chlorinated hydrochloric acid cannot of
course be kept in contact with the platinum wires with-
out acting on them, the action is much slower than
might have been anticipated. I have examined the
wires of photometers that had been in active use for
four months, and could not perceive the platinum sen-
sibly destroyed. It is well, however, to put a piece of
platinum foil in the bottle in which the supply of chlo-
rinated hydrochloric acid is kept ; it communicates to it
slowly the proper golden tint.

The liquid being impregnated with chlorine in this
manner until it exhales the odor of that gas is to be
transferred to the siphon a b c of the photometer, and its
constitution finally adjusted as hereafter shown.

Thirdly, of the Voltaic Battery. — The battery which
will be found most applicable for these purposes con-
sists of two Grove's cells, the zinc surrounding the plati-
num.

Mi M..IK XVIII. J THE CIlLOIMIYDllOUKN PHOTOMETER.

The following are the dimensions of the pairs which I
The platinum plate is half an inch wide and two
long; it dips into a cylinder of porous biscuit-
ware of the same dimensions, which contains nitric acid.
Outside this porous vessel is the zinc, which is a cylin-
der one inch in diameter, two inches long, and two tenths
thick; it is amalgamated. The whole is contained in a
cup, two inches in diameter and two deep, which also re-
ceives the dilute sulphuric acid.

The force of this battery is abundantly sufficient both
for preparing the fluid originally and for carrying on
the photometric operations. It can decompose hydro-
chloric acid with rapidity, and will last with ordinary
care a long time.

Before passing to the mode of using this photometer,
it is absolutely necessary to understand certain theoret-
ical conditions of its equilibrium. These in the next
place I shall describe.

Theoretical Conditions of Equilibrium. — This photom-
eter depends for its sensitiveness on the exact proportion
of the mixed gases. If either one or the other is in ex-
cess a great diminution of delicacy is the result. The
comparison of its indications at different times depends
on the certainty of evolving the gases in exact, or, at all
events, known proportions.

Whatever, therefore, affects the constitution of the sen-
tient gases alters at the same time their indications. Be-
tween those gases and the fluid that confines them cer-
tain relations subsist the nature of which can be easily
traced. Thus, if we had equal measures of chlorine and
hydrogen, and the liquid not saturated with the former,
it would be impossible to keep them without change, for
by degrees a portion of chlorine would be dissolved and
an excess of hydrogen remain ; or if the liquid was over-

R

258 THE CHLOR-HYDBOGEN PHOTOMETER. [MEMOIR XVIII.

charged with chlorine, an excess of that gas would ac-
cumulate in the sentient tube.

It is absolutely necessary, therefore, that there should
be an equilibrium between the gaseous mixture and the
confining fluid.

As has been said, when hydrochloric acid is decom-
posed by a voltaic current, all the chlorine is absorbed
by the liquid and accumulates therein ; the hydrogen
bubbles, however, as they rise withdraw a certain pro-
portion, and hence pure hydrogen passed up through the
photometric fluid becomes exceedingly sensitive to the
light.

There are certain circumstances connected with the
constitution and use of the photometer which continu-
ally tend to change the nature of its liquid. The plat-
inum wires immersed in it by slow degrees give rise to
a chloride of platinum. It is true that this takes place
verygradually, and by far the most formidable difficulty
arises from a direct exhalation of chlorine from the nar-
row tube ef, for each time that the liquid descends, a
volume of air is introduced, which receives a certain
amount of chlorine which with it is expelled the next
time the battery raises the column to zero; and this
going on time after time finally impresses a marked
change on the liquid. I have tried to correct it in
various ways, as by terminating the end f with a bulb ;
but this entails great inconvenience, as may be discov-
ered by any one wrho will reflect on its operation.

When by the battery we have raised the index to its
zero point, if the gas and liquid are not in equilibrium
that zero is liable to a slight change. If there be hydro-
gen in excess, the zero will rise; if chlorine, the zero
will fall.

In making what will be termed " interrupted experi-
ments," we must not too hastily determine the position

Mi Mum XV11I.] THE CHLUR-HYDKOGEN I'HOTO.MKTKK.

of the index on the scale at the end of a trial. It is to
be remembered that the cause of movement over the
scale arises from a condensation of hydrochloric acid, but
that condensation, though very rapid, is not instanta-
neous. Where time is valuable and the instrument in
pel feet equilibrium, this condensation may be instanta-
neously effected by simply inclining the instrument so
that its liquid may pass down to the closed end #, but
not so much as to allow gas to escape into the other
side — the inclination of the two sides to each other
makes this a very easy manipulation — and the gas thus
brought into contact with an extensive liquid suiface
yields up its hydrochloric acid in an instant.

Directions for using the Photometer. Preliminary ad*
justment. — Having transferred the liquid to the sealed
end of the siphon, and placed the cap on the sentient ex-
tremity, the voltaic battery being prepared, the operator
dips its polar wires into the cups/>, <7, which are in con-
nection with the wires a?, y. Decomposition immediately
takes place, chlorine and hydrogen rising through the
liquid and gradually depressing it, while, of course, a
corresponding elevation takes place in the other limb.
This operation is continued until the liquid has risen to
the zero. It takes but a few seconds for this to be ac-
complished.

The polar wires having been disengaged, the photom-
eter is removed opposite a window, care being taken
that the light is not too strong. The cap is now lifted
off the sentient extremity a d, and immediately the liquid
ascends. This exposure is allowed to continue, and the
liquid suffered to rise as much as it will to the end a.
And now, if the gases have been properly adjusted, an
entire condensation will take place, the sentient tube a d
filling completely. In practice this precision is not how-

0(30 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIU XVIII.

ever obtained, and if a bubble as large as a pepper-corn
be left, the operator will be abundantly satisfied with
the sensitiveness of his instrument. Commonly, at first,
a large residue of hydrogen gas, occupying perhaps an
inch or more, will be left. It is to be understood that
even this large surplus will disappear in a few hours by
absorbing chlorine. But this is not to be waited for ;
as soon as no further rise takes place, in a minute or two,
the siphon is to be inclined to one side, and the residue
turned into the open side.

Now, recurring to what has been said on the equilib-
rium, it is plain that this excess of hydrogen arises from
a want of chlorine in the photometric liquid. A proper
quantity must therefore be furnished by proceeding as
follows :

The sentient tube being filled with the liquid by in-
clination, connect the polar wires with JP, ^, as before.
Thes^may be called generating wires. Allow the liquid
to rise in b c until the third platinum wire 0, which may
be called the adjusting ivire, is covered an eighth of
an inch deep. Then remove the negative wire from the
cup p into the cup r, and now the conditions for saturat-
ing the liquid are complete; hydrogen escaping from
the surface of the liquid at z, and chlorine continually
accumulating and dissolving between x and d. This
having been carried on for a short time, the gas in a d
is to be turned out by inclination and the instrument
recharged. That a proper quantity is evolved is easily
ascertained by allowing total condensation to take place,
and observing that only a small bubble is left at a.

It will occasionally happen in this preliminary adjust
ment that an excess of chlorine may arise from continu-
ing the process too long. This is easily discovered by
its greenish-yellow tint, and is to be removed by inclin-
ing the instrument and turning it out.

MEMOIK XVIII.] THK CHLOIMIYDKOGEN PHOTOMKTKK.

Thus adjusted, everything is ready to obtain measures
of any effect, there being two different methods by
which this can be done: 1st, by continuous observa-
tion ; 2d, by interrupted observation.

Of the metJiod of continuous observation. — This is best
described by resorting to an example. Suppose, there-
fore, it is required to verify Table I., or, in other words,
to prove that the effect on the photometer is propor-
tional to its time of exposure.

Put on the cap of the sentient tube a d, connect the
polar wires with^>, ^, and raise the liquid to zero.

Place the photometer so that its sentient tube will re-
ceive the rays properly.

At a given instant, marked by a seconds watch, re-
move the cap A D, and the index-liquid at once begins
to descend. At the end of the first minute read off the
division over which it is passing. Suppose it is 7. At
the end of the second do the same: it should be 14 ; at
the end of the third, 21, etc. This may be done until
the fiftieth division is reached, which is the terminus of
the scale.

Recharge the tube by a momentary application of the
polar wires; but it is convenient first to remove any
excess of hydrochloric -acid gas in the sentient tube by
allowing it time for condensation ; or, if that be inadmis-
sible, by inclining a little to one side, so as to give an
extensive liquid contact.

Of the method of interrupted observation. — It frequent-
ly happens that observations cannot be had during a
continuous descent, as when changes have to be made in
parts of apparatus or arrangements. We have then to
resort to interrupted observations.

This method requires that the gas and liquid should
be well adjusted, so that no change can arise in volume
when extensive contact is made by inclination.

262 THE CHLOR-HYBROGEN PHOTOMETER. [MEMOIR XVIII.

The photometer being charged, place it in a proper
position. At a given instant remove its cap, and the
liquid descends. When the time marked by a seconds
watch has elapsed, drop the cap on the sentient tube.
The liquid simultaneously pauses in its descent, but does
not entirely stop, for a little uncondensed hydrochloric
acid still exists, which is slowly disappearing in the sen-
tient tube. Now, incline the instrument for a moment
to one side, so that the liquid may run up to the end #,
but not so much as to let any gas escape. Restore it to
its position and read off on the scale. It is then ready
for a second trial.

The difference between continuous and interrupted ob-
servation .is this, that in the latter we pause to wash out
the hydrochloric acid, and though this is effected by the
simplest of all possible methods, continuous observations

are always to be preferred when they can be obtained.

^

I have extended this Memoir to so great a length that
many points on which remarks might have been made
must be passed over. It is scarcely necessary to say
that the sentient tube must be uniformly and perfectly
clean. As a general rule also, the first observation may
be cast aside, for reasons which I will presently give.
Further, it is to be remarked, as it is an essential prin-
ciple, that during different changes of volume of the gas
its exposed surface must never vary in extent, the liquid
is not to be suffered to rise above the blackened portion
at d. If the measures of the different parts be such as
have been here given, this cannot take place, for the
liquid will fall below the fiftieth division before its
other surface rises above d.

The same original volume of gas in a d will last for
a long time, as we keep replenishing it as often as the
fiftieth division is reached.

MEMOIII XVIII.] THE CHLOU-HYDKOGEN PHOTOMETER 263

The experimenter cannot help remarking that, on sud-
denly exposing the sentient tube to a bright light, the
liquid for an instant rises on the scale, and on dropping
the cap in an instant falls. This important phenomenon,
which is strikingly seen under the action of an electric
spark, I shall consider hereafter.

In conclusion, as to comparing the photometric indi-
cation at different times, if the gases have the same con-
stitution the observations will compare; and if they
have not, the value can from time to time be ascertained
by exposure to a lamp of constant intensity. To this
method 1 commonly resort.

From the space occupied in this description the reader
might be disposed to infer that this photometer is a
very complicated instrument and difficult to use. He
would form, however, an erroneous opinion. The pre-
liminary adjustment can be made in five minutes, and
with it an extensive series of measures obtained. These
long details have been entered into that the theory of
the instrument may be known, and optical artists con-
struct it without difficulty. Though surprisingly sensi-
tive to the action of the indigo ray, it is as manageable
by a careful experimenter as a common differential ther-
mometer.

UNIVERSITY OF NEW YORK, Sept. 26, 1843.

NOTE. (From Haider's Magazine, No. 328.) — Pro-
fessors Bunsen and Roscoe, in their photo-chemical re-
searches, made at the University of Heidelberg, and
communicated to the Royal Society of London, 1856,
say : " The first and only attempt which has been
made to refer the chemical action of light to a stand-
ard measure is to be found in the researches of Draper.

264 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII.

The description of his instrument and mode of observa-
tion employed by him was published in 1843. Even
with this instrument, which, as we shall show, is in many
respects defective, Draper has succeeded in establishing
experimentally some of the most important relations of
the chemical action of light. In these experiments Dra-
per collected hydrogen, evolved by electrolysis over hy-
drochloric acid saturated with chlorine, and to this hy-
drogen he admitted so much chlorine, either by diffusion
from the saturated acid or by electrolysis, that the mixt-
ure consisted of nearly equal volumes of the two gases,
and entirely or almost entirely disappeared on exposure
to light. The alteration in the volume of the gaseous
mixture arising from the absorption of the hydrochloric
acid formed by the action of the light was read off on a
scale, and being within certain limits proportional to the
time of exposure, served as a measure of the chemical
rays.'V

Professors Bunsen and Roscoe, having modified this
instrument to suit the objects they had in view, accord-
ingly used it in their very exhaustive and important
series of researches.

The measuring lens referred to in previous paragraphs
is constructed upon this principle: If half the surface of
a convex lens be screened by an opaque body, as a piece
of blackened card-board, of course only half the quantity
of rays will pass which would have passed had the
screen not been interposed ; if one fourth of the lens be
left uncovered, only one fourth of the quantity will pass.
But in all these instances the focal image remains of the
same size as at first. Therefore by adjusting upon the
frame of the lens two screens, the edges of which pass
through its centre and are capable of rotation thereupon,
we shall cut off all light when the screens are applied

MKMOIR XVIII.] THE FEUUIC-OXALATE PHOTOMETER.

2r,r>

edge to edge opposite each other. We shall have 90°
when they are rotated so as to be at right angles, and
180° when they are superposed with their edges coin-
ciding, or one of them be taken away. Thus, by setting
them in different angular positions, we can have all
quantities, from 0° up to 180°, and by removing them
entirely, reach 360°. The lens will thus give an image
of a visible object always of the same size, its brilliancy
or intensity varying at pleasure in a known proportion.
In Fig. 42, A, B, B, D is a double convex lens set in a
wooden frame, F. Its face can be cov-
ered by two semicircles of blackened
card -board, one of which revolves on the
centre at c. In the figure they are rep-
resented as set at right angles, and the
quarter of the lens at A is uncovered.

Fig. 42.

To the foregoing description of the
chlor-hydrogen photometer I may add a
reference to another which I have very
advantageously used when extreme sen-
sitiveness was not required. It depends
on the employment of an aqueous solu-
tion of ferric oxalate. This substance,
which is of a golden-yellow color, may be kept for many
years without undergoing any change, if in total dark-
ness; but on exposure to a lamp or the daylight it
decomposes, carbonic acid escaping, and lemon -yellow
ferrous oxalate precipitating. If set in the sunshine, it
actually hisses through the escape of the gas. The ray
which chiefly affects it is the indigo, the same which
affects the chlor-hydrogen photometer and the silver
compounds used in photography. This ray, to produce
its effect, undergoes absorption, as might be anticipated
from what has been previously said in this Memoir, and

266 THE FERRIC-OX AL ATE PHOTOMETER. [MEMOIII XVIII.

as is easily proved by causing a sunbeam to pass through
two parallel strata of the oxalate, when it will be found
that the light which has gone through the first portion
is inoperative on the second.

Other properties which this solution of ferric oxalate
possesses strongly recommend it as a photometric agent.
Unlike solution of chlorine, it may be very conveniently
confined in glass tubes by mercury. In its use there are
two points which must be attended to: (1) the lemon-
yellow ferrous oxalate must not be permitted to incrust
the side of the glass exposed to the light, and thereby
injure its transparency; (2) the ferric solution must be
kept nearly at a constant temperature, for its color
changes with the heat. At the freezing of water it is
of an emerald-green tint ; at the boiling, of a brownish-
yellow. With these variations of tint its absorptive ac-
tion varies, and therefore its liability to be changed. In
an extensive series of experiments made with it, but
which I have not yet published, I found that it is great-
ly improved by the addition of an aqueous solution of
ferric chloride.

It may be remarked that the oxalate is an excellent
photographic substance. A piece of tissue-paper, made
yellow by being dipped into a neutral solution of it,
when dried in the dark is very sensitive. Its invisible
impression may be developed by a weak solution of
nitrate of silver, two grains dissolved in an ounce of
water answering very well. A weak solution of chlo-
ride of gold is a still more sensitive developer. I have
in my possession photographs made by both these meth-
ods more than thirty years ago, which have undergone
apparently no deterioration.

In the application of ferric oxalate to photometry sev-
eral methods may be followed. The course I have most
commonly taken has been to determine the quantity of

MEMOIW XVIII.] THE FERRIC-OXALATE PHOTOMETER L>,;7

c;u •!)<>! lie acid produced, sometimes by volume, sometimes
by weight. It is to be understood that before any car-
bonic acid can be disengaged the solution must become
saturated therewith, and that before we can correctly
measure the quantity of light by the quantity of acid
produced this dissolved portion must be ascertained.
In one of my photometers the expulsion of the dissolved
gas is accomplished by exposure to a bath of boiling
water, in another by a stream of hydrogen. Both yield
satisfactory results.

But this method by the determination of the produced
carbonic acid is only one of numerous plans; for in-
stance, we might use the weight of certain rnetals which
the solution after exposure will precipitate. Thus a por-
tion which has been made and kept in the dark may be
mixed with chloride of gold without any action ensuing,
but if it has been illuminated, the weight of metallic
gold precipitated is in proportion to the incident light.
On this principle I commenced an attempt to determine
the hourly and diurnal illumination of a certain locality.
At the bottom of a metal tube, arranged as a polar axis,
was placed a bulb containing a standard solution of the
iron salt, and at the close of the proposed periods the
weight of gold it could reduce was ascertained. There
is something fascinating in determining the quantity of
light which the sun yields by the quantity of gold it
can produce. Upon the whole, however, I would rec-
ommend those who are disposed to renew these at-
tempts to select a method depending on the volume of
carbonic acid, for it is always easier to make an obser-
vation than an experiment.

Among the important results which may be expected
from these new modes of photometry are the hourly,
diurnal, and annual quantities of sunlight. These are
important not only in a meteorological point of view,

268 THE FERRIC-OXALATE PHOTOMETER. [MEMOIR XVIII.

but also as respects physical geography and the great
interests of agriculture. The sum of vegetable organi-
zation is in all climates and localities a function of the
light distributed thereto. And so far as heat is con-
cerned, it is not the intensity only, but the absolute
quantity, which is to be measured. To each plant, from,
the moment of its germination to the moment of its
maximum development and the completion of its life, a
definite quantity of heat and of light must be given. As
respects the heat in such inquiries, it is not only the
thermometer but the calorimeter which must be consid-
ered; and as to the light, the photometers herein de-
scribed determine its quantity but not its brilliancy,
and therefore answer the indications required. And
since it is not merely the temperature of a locality, but
also the light, of the sun, which is the effective condition
of vegetable growth, we see how important even in agri-
culture itself these proposed determinations really are.

To those who would devote themselves to such inqui-
ries I recommend as a photometric means a mixture of
chlorine and hydrogen where great sensitiveness is re-
quired, and in other cases ferric oxalate.

The chlor-hydrogen and the ferric oxalate photometers
act by selective absorption, on the principle of the calo-
rimeter; that is to say, they measure the quantity of the
radiations they select.

I may, perhaps not inappropriately, close this Memoir
with a brief allusion to an instrument I formerly used
very much. It measures the intensity of the radiations
it is made to select, and these radiations may be varied
at pleasure. I will describe it first as adjusted for the ra-
diations of which chlor-hydrogeu and ferric oxalate take
charge. The description is extracted from a paper I pub-
lished in the Philosophical Magazine, August, 1844.

MKMOIK XVIIF.] THE SELECTIVE PHOTOMETER.

Fi<;.43.

Let a wooden box, A B (Fig. 43), six inches long, two
wide, and two deep, with perforations at A and B in its
ends, be provided ; in the centre of its top an aperture
three quarters of an inch in diameter is to be made.
The box must be blackened interiorly, and a rectangular
prism of wood, C, be placed in it, with its right angle in
such a position that its edge bisects as a diameter the
circular aperture; over this wooden prism a piece of
white paper is pasted, care being taken that where it
bends over the right angle of the prism it is folded
sharp. So far the reader will recognize in this Ritchie's
photometer. Upon the aperture in the top of the box
a glass trough, g A, is placed ; it is made by cutting a
circular hole an inch in diameter in a piece of plate-glass
one third of an inch thick, and laying on each side of it
a thin piece of plate-glass. This forms a circular trough,
in which a strong solution of sulphate of copper and
ammonia may be enclosed. Over the trough a tube, d,
eight or ten inches long, is placed so that the eye may
see distinctly through the aperture in the top of the box
the disk of paper, and more especially its dividing di-
ameter.

270 THE SELECTIVE PHOTOMETER. [MEMOIR XVIII.

Lights, E, F, set at the opposite ends of the box, A B,
may therefore be compared as regards their photographic
intensity, the calculations being made by the common
photometric law. And by changing the liquid in the
absorbing trough, g h, any radiations may be selected
for examination. The instrument may therefore be des-
ignated as " the selective absorption photometer."

From the facts presented in this and the preceding
paper, as conveying the modern conception of the rela-
tion of luminous and calorific radiations, it may be con-
cluded that the thermometer with a blackened bulb is
an absolute photometer, and that, in accordance with the
principles set forth, many other selective photometers and
thermometers may be devised.

MI.M..IU X1X.J MODIFIED C11LOU1NE. 271

MEMOIR XIX.

ON MODIFIED CHLORINE.

From the Philosophical Magazine, July, 1844.

[This Memoir was read at the meeting of the British Association held at Cork,
1843. The concluding paragraphs were subsequently added.]

CONTENTS : — Description of the experiment. — The change in the chlorine
w not transient. — There are two stages in the phenomenon. — Rays are
absorbed in producing this change. — The indigo ray is absorbed. — The
action is positive from end to end of the spectrum. — The indigo ray
forms hydrochloric acid and also produces the preliminary modifica-
tion.— Change in other elementary bodies. — Verification of these results
with the chlor-hydrogen photometer.

CHLORINE gas which has been exposed to the daylight
or to sunshine possesses qualities not possessed by chlo-
rine made in the dark.

This is shown by the circumstance that chlorine which
has been exposed to the sunshine has obtained from
that exposure the property of speedily uniting with hy-
drogen gas, a property not possessed by chlorine made
and kept in the dark.

This quality gained by the chlorine arises from its
having absorbed chemical rays corresponding in refran-
gibility to the indigo. It is not a transient, but appa-
rently a permanent property, the rays so absorbed be-
coming latent, and the effect lasting for an unknown
period of time. The facts here presented will be inter-
esting to chemists, because they plainly lead us to sus-
pect that the descriptions we have of the properties of
all elementary and compound bodies are either inaccu-
rate or confused. These properties are such as bodies

272 MODIFIED CHLORINE. [MEMOIR XIX.

exhibit after they have been exposed to light ; we still
require to know what are the properties they possess
before exposure to such influences.

Natural philosophers will also find an interest in
these phenomena, for they finally establish for the chem-
ical rays two important facts: 1st, that those rays are
absorbed by ponderable bodies; arid, 2d, that they be-
come latent after the manner of heat. Some years ago
I endeavored to prove that these things held for a com-
pound substance, the iodide of silver (Phil. Mag., Sep-
tember, 1841).

For reasons which will be obvious as the description
proceeds, I shall speak of chlorine which has been ex-
posed to the beams of the sun as modified chlorine.

I. Description of the Experiment.

In two similar glass tubes place equal volumes of
chloidne, made from peroxide of manganese and hydro-
chloric acid by lamplight, and carefully screened from
access of daylight. Expose one of the tubes to the full
sunbeams for some minutes, or, if the light be feeble, for
a quarter of an hour : the chlorine in it becomes modi-
fied. Keep the other tube during this time carefully in
a dark place ; and now, by lamplight, add to both equal
volumes of hydrogen gas. These processes are best car-
ried on in a small porcelain or earthenware trough, filled
with a saturated solution of common salt, which dis-
solves chlorine slowly; and to avoid explosions operate
on limited quantities of the gases. Tubes that are eight
inches long and half an inch in diameter will answer
very well. The two tubes now contain the same gaseous
mixture, and only differ in the circumstance that one is
modified and the other not. Place them, therefore, side
by side before a window, through which the entrance of
daylight can be regulated by opening the shutter; and

MI.M..IU . \IX.J MODIFIED CHLORINE. 273

n«)\v, if this part of the process be conducted properly,
it will be seen that the modified chlorine commences to
unite with the hydrogen and the salt water rises in that
tube. But the unmodified chlorine shows no disposition
to unite with its hydrogen, and the liquid in its tube
remains motionless for a long time. Finally, as it be-
comes slowly modified by the action of the daylight im-
pinging on it, union takes place. From this, therefore,
we perceive that chlorine which has been exposed to
the sun will unite promptly and energetically with hy-
drogen ; but chlorine that has been made and kept in
the dark shows no such property.

As I doubt not this remarkable experiment will be
repeated by chemists, I will add that the only point to
which attention in particular is to be given is in the
final exposure to the light. This must not be too feeble,
or the action will be tedious; but the direct sunbeam
must be sedulously excluded or an explosion will result.
A room illuminated by one small window looking to the
north answers very well. It need scarcely be added that
care must be taken that both tubes are illuminated alike.

II. The Change in (he Chlorine is not Transient.

Now it might be supposed that this apparent exalta-
tion of the electro-negative properties of the chlorine is
only a transient thing, which wrould speedily pass away,
the gas reverting to its original condition.

To show that this is not so, modify some chlorine in a
tube as before. Place it for an hour or two in the dark
alono: with the tube of unmodified chlorine with which

O

it is to be compared, then to both add hydrogen. Ex-
pose them as in the former experiment to the daylight,
and the result will turn out as before; the modified
chlorine forming hydrochloric acid at once, and the un-

modified refusing to do so.

S

274 MODIFIED CHLORINE. [MEMOIR XIX

This, therefore, shows that the change which the sun-
beams impress upon chlorine is to a certain extent a
permanent change, and, unlike a calorific effect, it does
not spontaneously and rapidly pass away.

III. There are Two Stages in the Phenomenon.

Let us now make inquiry into the nature of the
change thus impressed on the chlorine. This, I shall
show, rests in the circumstance of the absorption of rays
which correspond in refrangibility to the indigo, and ap-
pear to become latent.

In a tube, over salt water, mix together equal volumes
of unmodified chlorine and hydrogen gas. Expose it to
the daylight, marking the time at which the exposure
commences. Watch the level of the liquid in the tube
narrowly, and, though stationary for a considerable time,
after a certain period has elapsed it will be seen on a
sudfteri to start and commence rising. Observe now
how far it will rise during a period equal to the time
that elapsed between the first exposure and the begin-
ning of the rise, and it will be seen that one fourth or

O

half the gases will disappear.

It is obvious that from the first moment of exposure
the rays must have been exerting their influences on the
mixture. As will presently be proved, absorption has
been all along taking place. There are, therefore, two
distinct phenomena exhibited by this experiment. There
is a period during which, though large quantities of the
dark rays are disappearing, no visible change is pro-
duced ; there is a second period, during which absorp-
tion is accompanied by a remarkable chemical effect—
the production of hydrochloric acid. From these things
we gather that a definite amount of the chemical rays
must disappear and become latent before hydrochloric
acid can form. The phenomenon is not unlike that of the

MLM..M: XIX.J MODIFIED CIILOUINE. o —

disappearance of a definite quantity of heat in the pas-
sage of ice into the condition of water.

A mixture of chlorine and hydrogen does not, therefore,
instantly give rise to the production of hydrochloric acid
on exposure to the light, but as a preliminary condition
a certain definite amount of absorption must take place.

Now if this were a mere molecular disturbance, such
as might be brought about by the action of heat, we
should expect to find it transient and speedily passing
away. Such, however, is far from being the case. As
with simple chlorine, so with this mixture : after it has
been modified it loses its quality very slowly. I have
observed that after a week or more has elapsed since it
was first exposed to the light, it commences to contract
when placed in a feeble gleam.

IV. Rays are Absorbed in Producing this Clianye.

I have thus far assumed that the rays which bring
about these changes are absorbed; the following is the
proof which I have to offer:

Over a tube half an inch in diameter and six inches
long, closed at its upper extremity and open at its lower,
invert a jar of the same length and one inch and a half
in diameter. Fill the tube and the jar at the salt-water
trough, about two thirds full, with the same mixture of
chlorine and hydrogen. Expose them to diffuse day-
light. Now it is clear that no rays can gain access to
the tube except after having passed through the gaseous
mixture in the jar. After a certain space of time the
level of the liquid in the jar commences to rise, but that
in the tube will remain much longer wholly stationary.

It therefore appears that a beam which has passed
through a mixture of chlorine and hydrogen has lost, to
a great extent, the quality of bringing about the union
of a second portion of the mixed gases through which it

276 MODIFIED CHLORINE. [MEMOIR XIX.

may be caused to traverse. The active rays have been
absorbed ; they disappear from the beam and are lost in
producing their first effect.

A beam of light loses its energy in producing a chem-
ical effect ; the beam, as well as the medium on which it
acts, becomes changed. I have a series of results which
proves that this takes place for a great variety of com-
pound bodies.

V. It is the Indigo Ray which is Absorbed.

As has been said, it is a ray corresponding in refrangi-
bility to the indigo which produces these results.

In a small porcelain trough I inverted, side by side,
ten tubes, each of which was three inches long and one
third of an inch in diameter, the trough being filled with
salt water. I passed into each tube a certain quantity
of unmodified chlorine and hydrogen. A beam of the
sun^being directed by a heliostat into a dark room, was
dispersed horizontally by a flint-glass prism, and the
trough with its tubes so placed as to offer an exposure
to the different colored rays. The aperture admitting
the beam was about half an inch in diameter. For a
while no movement was observed in any of the tubes ;
but as soon as the preliminary absorption previously de-
scribed was over, the level of the liquid began to rise.
In the red and in the orange no movement could be

o

perceived, in the violet only after a time ; but first of all
the tube that was immersed in the indigo light was in
action, and exhibited finally a very rapid rise; this was
soon followed by the tube that was in the space where
the indigo and violet joined, then by that in the violet
and that in the blue ; the tube in the green was next in
order. The following table gives the numerical results
obtained by observing the time which elapsed before
movement took place in each tube :

.Mi.M.uu X1X.J

MODIFIED C1ILOK1M-:.

TABLE I.

277

Name •>( rny.

Time.

Name of ray.

Time.

Kxtreino red

*

liuli^o

1.50

lied and orange. ...
Yellow and green

+ 100.00

r>2 oo

Indigo nnd violet..
Violet

2.00

') '>r,

Green nnd blue

4 00

Viuk-t

r> oo

Ulne

2 33

Extreme violet. . . .

5 r>o '

Many years ago M. Berard made experiments on the
explosion of chlorine and hydrogen, and concluded from
his results that it was brought about by the violet ray.
This was at a time when the methods of making these
experiments were less exactly known. It is a very easy
matter to prove that in reality the indigo is the active
ray, and that from a maximum point which is in the in-
digo, but towards the blue, the effect gradually dimin-
ishes to each end of the spectrum.

The following table gives the calculated approximate
intensity of the chemical force for each ray, deduced
from the foregoing experiment :

TABLE II.

Name of ray.

Force.

Name of ray.

Force.

Indigo . . .

66.60

Red nnd orange. ...

1.00
1.90

Indigo and violet. .
Violet

fiO.OO
44 40

95 00

Violet

20 00

Blue

4L> IX)

Extreme violet. . . .

18 10

There is a great advantage which experiments con-
ducted in this way possess over those depending for
their indication on the stains impressed on daguerre-
otype plates or sensitive papers. In those cases we ob-
tain merely a comparative contrast for different regions
of the spectrum ; in this we have absolute measures de-
termined by a definite chemical effect and the rise of a

* Even after the longest exposure I hnd the means of giving it, no movement took
place in the tube in the extreme red, nnd I nm doubtful about that in the red nnd
orange.

278 MODIFIED CHLORINE. [MEMOIR XIX.

liquid in a graduated tube; and from this we gain just-
er views of the true constitution of the spectrum. On
studying the numbers in the foregoing table, or, better
still, if we project them, it will appear what an enormous
difference there is in the chemical force of the different
rays. In the experiment from which I have deduced
this table it appears that the force of the indigo ray
exceeds that of the orange in a greater ratio than 66 to
1 ; and from the circumstances under which the experi-
ment is made this difference must be greatly underrated.
There is always diffused light in the room coming from
the intromitted beam, and this accelerates the rise in the
less refrangible tubes ; then, again, it is impossible that
the tube which gives the greatest elevation shall coin-
cide mathematically with the maximum point and ex-
press the maximum effect.

From some estimates I have made I am led to believe
that In point of chemical force, for this mixture of chlo-
rine and hydrogen, the indigo ray exceeds the red in a
higher ratio than 500 to 1.

o

VI. The Action is Positive from End to End of the

Spectrum.

M. Becquerel found that for an iodized silver plate the
red, the orange, and the yellow rays possess the quality
of continuing the action be^un by the more refrangible

o o «/ o

colors ; he therefore names these " rayons continuateursT
For the same compound I found that those rays, acting
conjointly with the diffused daylight, exerted a negative
agency. It is therefore desirable to understand whether,
with respect to the gases now under consideration, the
less refrangible rays exert anything in the way of an ac-
tion of depression or hindrance to union. By direct ex-
periment I found that this was not the case, the action
being positive from end to end of the spectrum. This

MKM..III XIX.] MODIFIED CHI.oKlNi: 279

can be shown by removing the tubes, after they have
been in the spectrum for an hour or two, into the gleams
of daylight. One by one they exhibit after a time a
rise, the order being the green first, then the yellow and
the orange, and at last the red. And if at the same time
a tube which has been kept in the dark be exposed
along with them, they will all rise before it, showing
that modification had set in and been going on in them
all ; that it had been more active in the green than in
the yellow, in the yellow than in the orange, in the or-
ange than in the red; and, had the exposure to the spec-
trum been long enough, the liquid in every one of the
tubes would have risen.

VII. The Indigo Ray forms the Hydrochloric Acid as
well as produces the Preliminary Modification.

It only remains now to inquire whether the rays caus-
ing the production of the hydrochloric acid are those
which effect the modification of the chlorine ; in other
words, whether the first stage of the process is brought
about by the same agent which carries on the second.
The experiment I have just described shows that modifi-
cation is most actively produced by the indigo ray, and
it is easy to show that it is the same ray which carries
on the second part of the process ; for, if before placing
the tubes in the prismatic spectrum we expose them to
the daylight, so that the liquid has just commenced to
rise in each, and then to the spectrum, it will be found
that the liquid of the tube in the indigo rises most rap-
idly, and the others in the order stated before. There-
fore we perceive that the same ray commences, carries
on, and completes the process.

Few substances can exceed in sensitiveness to licrht a

^j

mixture of chlorine and hydrogen previously modified.
Brought into the obscure daylight of a gloomy chamber,

280 MODIFIED CHLOKINE. [MEMOIR XIX.

it is remarkable how promptly the level of the liquid in
the tube rises; how, when the shutters are successively
thrown open, the action becomes more and more ener-
getic ; and how, in an instant, it stops when the instru-
ment is shaded by a screen.

I have not recorded in this communication a multi-
tude of experiments of detail supporting the conclusions
here drawn. It has been my object on this occasion to
call attention to the fact that chlorine, an elementary
body, undergoes a change after exposure to the light ; a
change which appears to produce an exaltation of its
electro-negative properties, as is shown by its power of
uniting more energetically with hydrogen. This change
must not be confounded with those transient elevations
of activity due to increased temperature, inasmuch as
this is more permanent in its character. It arises from
the absorption of rays existing most abundantly in the
indigo space of the spectrum. That the phenomenon is
due to a true absorption is fully shown by the circum-
stance that a beam which has produced this effect has
lost the quality of ever after producing a similar result.
This is borne out by what we observe to take place
when a feeble light falls on a mixture of chlorine and
hydrogen prepared in the dark. A certain space of
time elapses before any formation of hydrochloric acid
occurs, during which the absorption in question is going
on ; and when that is completed, and the mixture is
modified, union of the gases begins and hydrochloric
acid forms. From end to end of the spectrum the action
is positive, and differs only in intensity ; but this differ-
ence in intensity opens before us new views of the con-
stitution and character of the solar beam,

UNIVERSITY OF NEW YORK, June 20, 1813.

M..MUIU XIX.] MODIFIED CHLORINE. 281

Paragraphs subsequently added.

Chlorine is not the only elementary substance in
which the radiations produce a change. In his chapter
on phosphorus, Berzelius remarks: "Light produces in it
(phosphorus) a peculiar change, of which the intimate
nature is unknown; and which, so far as we can judge
at present, does not alter its weight. It makes it take a
red tint. This phenomenon occurs not only in a vac-
uum, even in that of a barometer, but also in nitrogen
gas, in carburetted hydrogen, under water, alcohol, oil,
and other liquids. When we expose to the sunlight
phosphorus dissolved in ether, oil, or hydrogen gas, it in-
stantly separates under the form of red phosphorus; it
undergoes very rapidly this modification in violet light,
or in glass vessels of a violet color. The light of the
sun makes it easily enter into fusion in nitrogen gas, but
it does not melt in hydrogen, and in the torricellian
vacuum it sublimes in the form of brilliant red scales "
(Berzelius, Traite, torn, i., p. 258).

Again, when speaking of phosphuretted hydrogen, lie
says: "Exposed to the influences of the direct solar light
this gas is decomposed, a part of the phosphorus sep-
arates under the form of red phosphorus, and is depos-
ited on the interior surface of the glass. If we cover
the vessel which contains the gas imperfectly, no phos-
phorus is deposited on the covered spaces " (/£., torn, i.,
p. 265).

As Berzelius does not give these experiments as his
own, and I do not know to whom we are indebted for
them, I repeated some of them. Among other corrobo-
rative results it appeared that a piece of phosphorus of
a pale or whitish color, in a vessel filled with pure and
dry carbonic- acid gas, placed in the sunshine, rapidly
exhibited the phenomenon in question. Eventually the

282

MODIFIED CHLORINE.

[MEMOIR XIX.

phosphorus became of a deep blood -red color, and on
the sides of the glass towards the light feathery crystals
formed, the tint of which bore a close resemblance to
that of the red prussiate of potash.'

Since the invention of the chlor-hydrogen photometer
I have been able to observe more closely the habitudes
of chlorine. In the description given of that instrument
it is recommended to cast aside the first observation, be-
cause it never gives an accurate estimate of the true
effect. When a mixture of chlorine and hydrogen is
exposed, hydrochloric acid does not immediately form ;
but a preliminary absorption is necessary, and then at the
end of a certain period contraction begins to take place.

Such a photometer exposed to the daylight is much
too powerfully affected to allow the successive stages
of change to be distinctly made out; the preliminary
modification is accomplished so rapidly that the indica-
tions of it are merged and lost in the contraction which

O

instantly follows. It is necessary therefore that we
should operate with a small lamp-flame.

To such a flame I exposed a mixture of chlorine and
hydrogen, and marked the number of seconds which
elapsed before contraction, arising from the production
of hydrochloric acid, took place. The first indications
of movement occurred at the close of 600 seconds.

The index then moved through the first degree in 480 seconds ;

second

third

fourth

fifth

sixth

165

130

95

93

93

and continued to move with regularity at the same rate.

These observations, therefore, prove that a very large
amount of radiant matter is absorbed before chemical
combination takes place, and that in the case of chlorine
and hydrogen the total action is divisible into two

MI.M..IK XIX.] MODIFIED CHLORINE. 283

periods: the first during which a simple absorption is
taking place without a chemical effect, the second durum;
which absorption is attended with the production of
hydrochloric acid.

The facts which I am endeavoring to set forth promi-
nently in this Memoir are — 1st, the preliminary modifi-
cations just discussed; and, 2d, the persistent character
of the change impressed upon chlorine wrhen it has been
exposed to the sun, an effect wholly unlike a calorific
effect, which would soon disappear.

By resorting to the chlor-hydrogen photometer we ob-
tain information equally distinct upon the second point,
that the preliminary modification is not a transient effect
which at once passes away, but is, on the contrary, a per-
sistent change.

I modified the chlorine and hydrogen contained in the
instrument, and kept it in the dark for ten hours. On
exposure to the lamp-rays it moved after a few seconds,
showing, therefore, that the change impressed on the
chlorine was not lost. In the former case 600 seconds
had elapsed before any movement was visible.

When, however, we remember that the invisible im-
ages on daguerreotype plates, and even photographic im-
pressions on surfaces of resin, and probably all other
similar changes, are slowly effaced, it would be prema-
ture to conclude that modified chlorine does not revert
to its original condition. I have sometimes thought
that there were in several of my experiments indications
that this was taking place, but would not be understood
as asserting it positively. Whether it be so or not, one
thing is certain, that the taking on of this condition and
the loss of it is a very different affair from any transient
exaltation of action due to a temporary elevation of tem-
perature, or the contrary effect produced by cooling.

April 26, 1844.

284 THE ALLOTKOPISM OF CHLORINE. [MEMOIR XX.

MEMOIR XX.

ON THE ALLOTKOPISM OF CHLOEINE AS CONNECTED WITH
THE THEOEY OF SUBSTITUTIONS.

From the American Journal of Science and Art, Vol. XLIX. ; Philosophical Mag-
azine, Nov., 1845.

CONTENTS : — Chlorine exists in two states, active and passive. — Decompo-
sition of water by it in the sunlight. — Facts connected with this de-
composition.— The relations of chlorine and hydrogen. — The allotro-
pism of chlorine. — Connection of these facts with the theory of substi-
tutions.

THE researches of M. Dumas on chemical types have
shown that between chlorine and hydrogen remarkable
relations exist, indicating that the electrical characters
of elementary atoms are not essential, but rather inci-
dental properties. The extension of these researches
has given much weight to the opinion that the electro-
chemical theory may be regarded as failing to account
for the replacement of such bodies as hydrogen by chlo-
rine, bromine, oxygen, etc. I do not know that as yet
any direct evidence has been offered that the electrical
character of an atom is not an essential quality, but one
that changes with circumstances. It appears to be
rather a matter of inference than of absolute demonstra-
tion.

It is the object of this Memoir to furnish such direct
evidence, and to show that chlorine, the substance which
has given rise to the discussions connected with the
theory of substitutions, under the very circumstances
contemplated, has its electro-chemical relations changed.

More than two years ago I brought before the British

MEJIOIK XX.] THE ALLOTROPISM OF CIILOHINK. 035

Association some of the facts. The connection of these
•zperiments with the discussion between the theory of
substitutions and the electro-chemical theory is obvious.

Very recently M. Berzelius has published an impor-
tant paper on the allotropism of simple bodies, the ob-
ject of which is to point out that many of those bodies
can assume different qualities by being subjected to cer-
tain modes of treatment. Thus carbon furnishes three
forms — charcoal, plumbago, and diamond.

To a certain extent these views coincide with those
which have offered themselves to me from the study of
the properties of chlorine. They are not, however, al-
together the same. M. Berzelius infers that elementary
bodies can, as has been said, assume under varying cir-
cumstances different qualities. The idea which it is at-
tempted to communicate in this Memoir is simply this,
that a given substance, such as chlorine, can pass from a
state of high activity, in which it possesses all its well-
known properties, to a state of complete inactivity, in
which even its most energetic affinities disappear. And
that between these extremes there are innumerable in-
termediate points. Between the two views there is,
therefore, this essential difference: from the former it
does not appear what the nature of the newly assumed
properties may be ; from the latter they must obviously
be of the same character, and differ only in intensity or
degree — diminishing from stage to stage until complete
inactivity results.

In the case of chlorine the same activity which is com-
municated by the indigo rays can also be communicated
by a high temperature, or by the action of platinum.

The points which this Memoir is intended to establish
are —

I. That chlorine gas can exist under two forms. In
the same way that metallic iron can exist as active or

286 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

passive iron, chlorine can assume the active or passive
state.

II. Having established the fact of the allotropism of
chlorine, I shall then show its connection with the the-
ory of substitutions of M. Dumas, and how the most re-
markable points in that theory may be easily accounted
for.

The time, perhaps, has not yet arrived for offering a
complete mechanical explanation of the assumption of
an active or passive state. It may be remarked that a
very trivial modification of our admitted views of the re-
lation between atoms and their properties is all that is
required to give a consistent explanation of every one of
these facts. Instead of regarding the specific qualities
of an atom as appertaining equally to the whole of it in
the aggregate, we have merely to assume that there is a
relation between its properties and its sides, and that
any focce which can make it change its position upon its
own axis will throw it into the active or passive state.
But this is nothing more than the well-known idea of
the polarity of atoms.

Phenomena of the Decomposition of Water by Chlorine
in the Hays of the Sun.

From the various facts which might be employed as
offering the means of establishing the allotropism of
chlorine, I shall select those which arise from an exam-
ination of the phenomena of the decomposition of an
aqueous solution of chlorine by the rays of the sun.

For many years it has been known that an aqueous
solution of chlorine undergoes decomposition by the ac-
tion of the solar rays. Several of the most remarkable
phenomena connected with this decomposition appear to
have been overlooked. Among such may be mentioned
the singular fact that chlorine which has been thus in-

MI.MOIR XX.] llli: ALLOTUOPISM OF CHLOKINK.

fluenced by the sun has obtained the quality of effecting
this decomposition subsequently, to a measured extent,
even in the dark. Not to anticipate what I have to
offer on this point, I shall now proceed in the first place
to establish the various facts connected with the decom-
position in question.

Having provided a number of small glass vessels, con-
sisting of a bulb and neck of the capacity of from 1.5 to
2 cubic inches, I filled them with a solution of chlorine
in recently boiled water, and inverted them in small
glass bottles containing the same solution, as shown ^^
in Fig. 44. With these bulbs the following exper- f~ "\
iments were made : N r

I. An aqueous solution of chlorine does not de- /UN
compose in the dark.

One of the bulbs was shut up in a dark closet,

and kept there for a week, being examined from F's-44-
time to time. No decomposition was perceptible, for no
gas collected in the upper part of the bulb.

II. An aqueous solution of chlorine decomposes in the
light.

One of the bulbs was placed in a beam of the sun re-
flected into the room by a heliostat. For sixteen min-
utes no change was perceptible, then small bubbles of
gas made their appearance; they increased in quantity
for a time, but finally the speed of decomposition became
uniform. On analysis by explosion with hydrogen, after
washing out any chlorine contained in it, this gas was
found to contain 97 per cent, of oxygen.

III. The rapidity of this decomposition depends on
the quantity of the rays and on the temperature.

In various repetitions of these experiments on differ-
ent days I soon convinced myself that the rate of evo-
lution of the oxygen depended on the quantity of the
rays. Among other proofs I may mention this: After

288 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

ascertaining the rate of decomposition in the reflected
beam, if the bulb be set in the direct sunshine the bub-
bles increase in number; the total quantity of oxygen
evolved becoming greater in the same space of time, an
effect obviously clue to the difference of intensity of the
reflected and incident beams. When a certain point is
gained, apparently no further increase of effect takes
place on increasing the brilliancy of the light, as I found
by employing a convex lens.

With respect to the influence of temperature. If,
while one of the bulbs is actively evolving gas in the
sun-rays, it be warmed by the application of a spirit-
lamp, the amount of gas thrown off becomes very much
greater. A difference of a few degrees produces a strik-
ing effect. As an illustration of this I placed in the sun-
shine two bulbs which were nearly alike, except that
one of them was painted black with India-ink on that
portion which was farthest from the sun. The rays
coming through the transparent part had access to the
solution, and then, impinging on the dark side, raised its
temperature. On measuring the quantity of gas col-
lected, it was found —

In the transparent bulb 3.46

In the half-blackened bulb C.19

IV. The decomposition of water, once begun in the
sunbeams, goes on afterwards in the dark.

1st. This very important fact may be established in a
variety of ways. Thus, if a bulb be removed from the
sunshine while it is actively evolving gas, and be placed
in the dark after all the gas has been turned out of it, a
slow evolution continuously goes on, the gas collecting
in the upper part of the bulb.

2d. A bulb, A, Fig. 45, having a neck, b, the end of
which was bent at c upwards at an angle of about 45
degrees, was employed. After exposure to the sun, by

MI.M..II: XX.] TIM-: AU.UTKul'IvM OF CIILOIUM..

289

Fig. 46.

inverting the bulb, and with one finger closing the <-\-
tivmity c, the gas disengaged could be trans- ^—^
fi-rred to a graduated vessel and measured. A( )
I satisfied myself by several variations of this
arrangement that the small quantity of water
introduced from time to time when the gas
bubble escaped from the end of the tube c
exerted no essential influence on the phenom-
enon. The following table shows the amount of gas
evolved in the dark during the periods indicated.

The bulb having been exposed to the sunshine, in ten
minutes the evolution of gas commenced, and in an hour,
0.107 cubic inch having collected, this was thrown away
and the arrangement placed in the dark. To prevent
the undue escape of the chlorine, the flat piece of glass
d was laid on the open end of the tube c. In each suc-
cessive hour the quantity of gas given in the following
table was then evolved:

First hour... .. 0.01 62

Second

o.oi no

Third

0.008(5

Fourth

O.OOGO

Fifth

0.0038

Sixth

. 0.0031

And for four days afterwards gas was collecting in the
bulb in diminished quantities.

V. This evolution of gas in the dark is not merely a
gradual escape of oxygen, originally formed while the
solution was exposed to the sun, but is traceable to an
influence continuously exerted by the chlorine arising in
properties it has acquired during its exposure to the
rays.

If a bulb which has been exposed to the sun be raised
by a spirit-lamp to such a temperature that its gaseous
constituents are rapidly evolved, its extremity dipping
beneath some of the solution in the bottle, after allowing

T

290 THE ALLOTROPISM OF CHLORINE. [MKrtoiR XX.

a sufficient space of time for the disengaged chlorine to
he redissolved, and the oxygen be turned out of the
bulb, it will be found on keeping the arrangement in
the dark that oxygen will slowly disengage as before.

Now there is every reason to believe that any small
amount of oxygen dissolved in the liquid would be ex-
pelled with the chlorine at a high temperature. We
therefore have to infer that the chlorine after this treat-
ment still retains the quality of causing the decompo-
sition to go steadily forward.

The oxygen which thus accumulates in the course of
time in the dark, after an exposure to the sun, does not
arise from any portion of that gas held in a state of
temporary solution, nor from peroxide of hydrogen, nor
from chlorous acid in the liquid undergoing partial de-
composition. From any of these states a high temper-
ature would disenc;ao;e it.

o ~

Vit, The evolution of gas is not of the nature of a
fermentation ; for when it once sets in, the molecular
motion is not propagated from particle to particle, but
affects only those originally exposed to the rays.

Let a bulb be filled with chlorine- water which has
been exposed to the sun, and in a second bulb place a
quantity of the same liquid equal to about one third of
its capacity. Fill up the remaining two thirds with
chlorine -water which has been made and kept in the
dark; and after keeping both bulbs in obscurity for
some days, measure the volumes of gas they contain. If
the qualities of chlorine which has been changed by
exposure were communicable by contact or close prox-
imity from atom to atom, we might expect that both the
bulbs would yield the same quantity of gas; but this is
far from being the case, and in such an experiment I
found that the bulb containing the mixture gave only
one fourteenth of the gas found in the other.

Mi M..IH XX.] THK ALLOTKOPISM OF CHLORINE. 291

VII. The quantity of gas thus collecting in the dark
depends on the intensity of the original disturbance,
which in its turn depends on the time of exposure to
(In- rays, to their intensity, And other such conditions.
In other words, the rays are perfectly definite in their
action — a long exposure giving a larger amount of sub-
sequent decomposition, and short exposure a less amount.

On exposing a bulb filled with chlorine-water to the
rays until bubbles of gas began to appear, and a second
one until the decomposition had been actively going on
for a quarter of an hour, and then transferring both to
the dark and measuring the oxygen collected at the end
of a day, I found in the former one twelfth of what was
collected in the latter.

VIII. In a given quantity of chlorine -water, the de-
composition in the dark corresponding to a given expos-
ure to the light having been performed, and the proper
quantity of oxygen evolved and the phenomenon ended,
it can be re-established from time to time, as long as
any chlorine is found in the liquid, by a renewed expos-
ure to the sun.

In a glass vessel, like Fig. 46, which, indeed, was noth-
ing more than one of Liebi^'s dry-

^j C3 */

ing apparatus, I placed a sufficient
quantity of chlorine -water to fill
the larger vessel, and the vertical
tubes half full. After exposing
this to the light for a certain time, Fi«-46-

until decomposition had fairly set in, I placed it in the
dark and found that for several days it gave off gas —
the quantity continually diminishing. Finally, no more
gas was evolved. But the liquid still contained free
chlorine, as was shown by its color. I therefore again
exposed it to the sun, and, repeating the former observa-
tion, found that it evolved gas for several days in the

292 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

dark. A third exposure was followed by the same re-
sult.

The form of this vessel renders it very convenient for
these experiments; because when sufficient gas has col-
lected for the purpose of observation, it is easily removed
by inclining the instrument, without the necessity of
introducing fresh quantities of liquid.

Having found, as has been said, that the rapidity of
the decomposition depended to a certain extent on the
temperature, it seemed desirable to determine whether
heat alone could bring about the change.

IX. The decomposition of water by chlorine is not
brought about by mere elevation of temperature when
the liquid is set in the sunbeam : although heat accel-
erates, it does riot give rise to the phenomenon.

1st. I raised by a spirit-lamp the temperature of one
of the bulbs nearly to its boiling-point, until so much
gas was given off that all the liquid was expelled from
the tube to the bottle beneath. If at this temperature,
which probably was higher than 200° Fahr., chlorine
had been able to decompose water, an equivalent quan-
tity of oxygen would have been produced ; but on al-
lowing the apparatus to cool, all the gas was reabsorbed
with the exception of a small bubble, amounting in vol-
ume to jo^gy of the water. This bubble, which was left
after the chlorine was recondensed, I found in three dif-
ferent experiments contained 32, 33, and 36 per cent, of
oxygen, the remainder being nitrogen ; but this being
nearly the constitution of the gas dissolved in ordinary
water, the source from which the small bubble came
was inferred to be the water used in these experiments.

2d. One of the bulbs was painted black all over with
India-ink. Its temperature now rose much higher than
in former experiments when it was set in the sun, but
not a bubble of oxygen appeared.

MEMOIR XX.] THE ALLOTROPISM OF CHLOUINK.

808

X. When chlorine- water has been exposed to the sun,
the <»\ygen accumulated in it is readily expelled by rais-
ing the temperature.

Having exposed one of the bulbs used in the last ex-
periment until it was actively evolving gas, I raised its
temperature with the spirit-lamp until the bulb was full
of gas. But on cooling this gas did not all condense as
in the last instance, a large quantity remained behind.
This was oxygen.

These ninth and tenth facts are of further interest, as
bearing upon a question much discussed by chemists—
the nature of the bleaching compounds of chlorine. The
chloride of lime, and other such substances, probably
have the same theoretical constitution as chlorine-water.
Berzelius and Balard suppose that in this solution chlo-
rous or hypochlorous acid exists. It might be inquired,
if this be the condition of things, why does not an ex-
posure to heat alone evolve oxygen, for chlorous acid is
exceedingly liable to decomposition by slight elevation
of temperature, and we should be justified in inferring
that if any of this acid is to be found in chlorine-water
it would be decomposed at the boiling-point. M. Mil-
Ion adopts the view that the bleaching compounds are
metallic chlorides analogous to the corresponding per-
oxides. But the ninth fact seems incompatible with this
view. If chlorine-water be analogous to peroxides of
hydrogen, and this last be what its name imports, and
not merely oxygenated water, it is difficult to understand
why, when chlorine-water is thus boiled, oxygen is not
given off. If the atom of chlorine and the atom of oxy-
gen in this body are placed under the same relations to
the atom of hydrogen, it seems necessary that the chlo-
rine atom at 212° Fahr. should expel the oxygen atom
and hydrochloric acid form. It is probable, indeed, that
the two oxygen atoms in peroxide of hydrogen are re-

294 THE ALLOTKOPISM OF CHLORINE. [MEMOIK XX.

lated to their hydrogen atom with different degrees of
affinity, and that one of them is retained far more loosely
than the other. But this would correspond to our ideas
of oxygenized water and not of peroxide of hydrogen,
and lead us to the conclusion that the solution employed
in this Memoir is strictly a solution of chlorine in water.

XL The decomposition of chlorine-water, when placed
in the sunbeam, does not begin at once, but a certain
space of time intervenes, during which the chlorine is
undergoing its specific change.

I need quote no further instance of the truth of this
than the experiment given in support of the second fact.
This is the same phenomenon which takes place when
chlorine and hydrogen are exposed together; they do
not begin to unite at once, but a certain space of time
elapses, during which the preliminary absorption is tak-
ing place, and when that is over union begins.
«*•

On the Relations of Chlorine and Hydrogen.

We have thus traced the cause of the decomposition
of water, in the case before us, to a change impressed on
the chlorine by exposure to the rays of the sun. In this
decomposition three elementary bodies are involved-
chlorine, oxygen, and hydrogen.

We can therefore reduce the problem under discussion
to simple conditions, and study the relations of each of
these substances to each other and to the solar rays suc-
cessively.

When a mixture of oxygen and hydrogen gases, in the
proportion to form water, is exposed to the most brilliant
radiation converged upon it by convex lenses, union does
not ensue ; the reason being, as I have shown, that those
gases are perfectly transparent to the rays, and do not
possess either real or ideal coloration. For the same
cause, water exposed alone for any length of time to the

M.MoiaXX.] THE ALLOTKOPISM ( )l < II l.i >UIXE. .,;,;,

sun, or to the influence of a large convex lens, does not
decompose. It is transparent, and cannot absorb any of
the rays.

But, as is well known, a mixture of chlorine and hy-
drogen unites, under the same circumstances, with an
explosion. I have formerly proved that this depends on
the absorption of the indigo rays. For in the indigo
space of the spectrum the action goes on with the great-
est activity.

If, therefore, this phenomenon be due to absorption
taking place by the mixture, it is easy to determine
the function discharged by each of its ingredients.

I transmitted a ray of light through hydrogen gas
contained in a tube seven inches long, the ends of which
were terminated by pieces of flat glass; and then, dis-
persing the ray by a flint-glass prism, received the result-
ing spectrum on a daguerreotype plate. Simultaneously,
by the side of it, I received the spectrum of a ray which
had not gone through hydrogen, but through a similar
tube filled with atmospheric air. On comparing the im-
pressions together, I could find no difference between
them.

I therefore infer that hydrogen gas does not exert
any absorptive action on the solar rays.

In one of the foregoing tubes I placed dry chlorine
gas, the other containing atmospheric air as before, and
receiving the two spectra side by side on the same da-
guerreotype plate, I found that a powerful absorption
had been exercised by the chlorine. All the chemical
rays between the fixed line H and the violet termination
of the spectrum were removed, and no impression cor-
responding to their place was left upon the plate. On
repeating this experiment so as to determine with pre-
cision the rays which had been absorbed, I found that
chlorine absorbs all the rays of the spectrum included

296 TIIE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

between the fixed line i and the violet termination, and
is probably affected by all those waves the lengths of
which are between 0.00001587 and 0.00001287 of a
Paris inch ; and inasmuch as it absorbs luminous rays
included between the same limits, it is to this absorption
that its yellow color is due.

In these Memoirs the same result is established by me
in another way. I found that a ray which had passed
through a given thickness of a mixture of equal volumes
of chlorine and hydrogen lost by absorption just half as
much of its original intensity as when it passed through
the same thickness of pure chlorine gas ; a result which
obviously leads to the conclusion that when chlorine
and hydrogen unite under the influence of the sun, they
discharge different functions — the chlorine an active, and
the hydrogen a passive function. The primary action
or disturbance takes place upon the chlorine, and a dis-
positioji is communicated to it enabling it to unite read-
ily with the hydrogen.

By arranging in the spectrum a series of tubes con-
taining a mixture of these gases, it was found that
the gases placed in the indigo space went into union
first.

These various experiments enabling me thus to trace
to the chlorine the source of disturbance, I have next to
remark that chlorine which has been exposed to the rays
of the sun has gained thereby a tendency to unite with
hydrogen not possessed by chlorine made and kept in
the dark. In proof of this fact I may cite an experiment
from Memoir XIX. :

"In two similar glass tubes place equal volumes of
chlorine made from peroxide of manganese and hydro-
chloric acid by lamplight, and carefully screened from
access of daylight Expose one of the tubes to the full
sunbeams for some minutes, or, if the light be feeble, for a

Mi MOIK XX.] TIIK ALLOTROPISM OF C11LOK1NE. 297

quarter of an hour: the chlorine in it becomes modified.
Keep the other tube during this time carefully in a dark
place ; and now, by lamplight, add to both equal volumes
of hydrogen gas. These processes are best carried on
in a small porcelain or earthenware trough, filled with a
saturated solution of common salt, which dissolves chlo-
rine slowly, and to avoid explosions operate on limited
quantities of the gases. Tubes that are eight inches
long and half an inch in diameter will answer very well.
The two tubes now contain the same gaseous mixture,
and only differ in the circumstance that one is modified
and the other not. Place them, therefore, side by side
before a window, through which the entrance of daylight
can be regulated by opening the shutter; and now, if
this part of the process is conducted properly, it will be
seen that the modified chlorine commences to unite with
the hydrogen, and the salt water rises in that tube. But
the unmodified chlorine shows no disposition to unite
with its hydrogen, and the liquid in its tube remains
motionless for a long time. Finally, as it becomes slow-
ly modified by the action of daylight impinging on it,
union takes place. From this, therefore, we perceive
that chlorine which has been exposed to the sun will
unite promptly and energetically with hydrogen ; but
chlorine which has been made and kept in the dark
shows no such property."

This form of experiment may be supposed imperfect,
since the chlorine is in a moist condition and confined
by water. I have therefore made the following vari-
ation :

I took a tube, A, Fig. 47, six inches long and half an
inch in diameter, closed at one end and open at the
other, and cemented its open end on a piece of flat plate-
glass, M N, one inch wide and two long, ground on both
sides, and having a hole, p, one sixth of an inch in diara-

298

THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

Fig. 41.

eter perforated through it. This hole was
not in the centre of the glass, but towards
one side, as shown in the figure. The interior

/ O

of the tube was perfectly clean and dry.

A second tube, B, consisting, as shown in
Fig. 47, of two portions; a wide portion, B,
and a narrower tube, <?, was cemented on an-
other piece of ground plate -glass, similar to
the foregoing in all respects. The tube c
was open at its lower extremity, and the entire capacity
of B and c conjointly was adjusted so as to be equal to
the capacity of A.

Next I filled A with dry chlorine, and B c with dry
hydrogen, and kept them from mixing until the proper
time by operating in the following way: I
placed the ground glasses face to face, as
shown in Fig. 48, with a small quantity of
soft tallow between them, arranging them

O O

in such a way that the aperture which led
to the interior of A was open.

Through this aperture dry chlorine was
conveyed. It was generated by a mixture
of peroxide of manganese and hydrochloric
acid in the flask D, Fig. 49, and passed along a tube, E,
filled with chloride of calcium. A slender glass tube,/,

conveyed it to the bottom of
A, which was then filled by
displacing the atmospheric
air. When A was supposed
to be full of chlorine, it was
slowly lowered so as to bring
the tube out of the aperture,
and as soon as it was disen-
gaged the glass plates were
Fig. 49. moved in such a manner by

MKMOIU XX.] THE ALLOTROl'ISM OF CHLORINE.

sliding them on one another that the aperture leading
into A was shut, but that leading into B was open.
The vessel A was thus filled with dry chlorine and se-
curely closed.

In the next place I filled B with dry hydrogen, which
was done as follows: To a bottle, G, Fig. 50, containing
dilute sulphuric acid and zinc, a dry ing- tube,
K, of chloride of calcium was adjusted, and at
its upper end a cork, h, arranged so as to re-
ceive tightly the tube c. In a short time,
therefore, B became full of dry hydrogen, the
surplus escaping through the open aperture p.
The two ground-glass plates were now moved
on one another in such a manner that they
mutually closed one another. The vessel A
was therefore filled with dry chlorine, and the
vessel B c with an equal volume of dry hydro-
gen, without communicating for the present
with one another.

I had provided two sets of these tubes as
nearly alike as they could be made, and operated with
them in the following manner:

O

In a dark room I filled the tube A of each of them
with dry chlorine in the manner just described, and con-
fined it by sliding the plates. One of the tubes was
retained in the dark room and kept carefully screened
from the light, but the other was set for half an hour in

O '

the sunbeams. The chlorine which was in it underwent
the specific change — the object of this Memoir to de-
scribe.

After restoring this tube to the dark room and wait-
ing a few minutes for it to gain the same temperature
as the other, the tubes B c of each set were filled with
dry hydrogen in the manner described. In each in-
stance, as soon as the plates were moved on each other

Fig. 50.

300

THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

Fis.51.

so as to confine the hydrogen, and they were released
from the cork h of the drying-tube K, Fig. 50, the lower
extremity of each was dipped beneath the surface of
some water contained in a saucer, P, Fig. 51 ; the two

sets of tubes being held
steadily in a proper posi-
tion by the aid of a wooden
frame, Q R. The tubes now
differed from one another
in nothing but the circum-
stance that the chlorine of
one had been exposed to
the sun, and that of the
other had not.
The gases were now brought in contact. This was
easily done by sliding each pair of ground glasses until
their apertures coincided, as shown at p in Fig. 51. The
hydrogen now rose through the hole into the upper ves-
sel, the chlorine descending through it, mutual and per-
fect diffusion of the two gases rapidly taking place.
This \vas done by lamplight in the dark room. And
now it could be ascertained that the gases were at the
same temperature in the different tubes, and that the
experiment had thus far been carried on successfully, by
the water retaining its level at the same point in the
tubes c of both sets. If that which had been in the
sunshine was warmer than the other, as soon as the
apertures coincided a bubble of gas would have escaped
through the water, or at all events the level would have
changed.

It remained now to open the shutter of the dark room,
the tubes having been previously set in such a position
that the light would fall equally on both. As soon as
this was done, the chlorine which had been exposed to
the sun united at once with its hydrogen, and the water

MEMOIR XX.] THE ALLUTliUl'ISM OF CIILOUIXE. ;>n|

rose in the tube c. But in the other, which had not
Uvn exposed to the sun, no movement took place until
the gases had had time to be modified by the light com-
ing through the open shutter.

When care has been taken to have the gases made
quite dry, and, owing to the narrowness of the tube c, no
aqueous vapor has had time to contaminate the gas in B,
so that no water is present to condense the hydrochloric
:u-id as it forms, a little delay may be occasioned in the
liquid rising in the tube, the chlorine of which was ex-
posed to the sun. But after a time a mist arises in the
neighborhood of the water in the narrow tube, due to
the hydrochloric acid condensing, and then the process
goes forward with regularity.

It appears, therefore, that chlorine by exposure to the
sun contracts a tendency to unite with hydrogen which
is not possessed by chlorine which has been kept in the
dark.

On the Allotropism of Chlorine, or its Passive and Act-
ive States.

In what, then, does this remarkable change impressed
by indigo rays upon chlorine consist? This is the ques-
tion immediately arising from the phenomena we have
had under consideration.

To this I answer that when chlorine has been thus in-
fluenced its electro-negative properties are exalted, and
it has passed from an inactive to an active state.

It is now fully established that a great number of the
elementary bodies undergo similar modifications. Many
of them can exist in no less than three different states,
and these peculiarities are impressed on the compounds
to which they give rise. To these peculiarities Berze-
lius directed the attention of chemical philosophers
in his Memoir " On the Allotropism of Simple Bodies,

3Q2 THE ALLOTBOPISM OF CHLORINE. [MKMOIR XX.

and its Relation with Certain Cases of Isomerism in
their Combinations." He shows that of the elementary
bodies now known, many undoubtedly exist in several
allotropic states, and infers that all are liable to anal-
ogous modifications. He indicates that the isomerism
~

of compound bodies is due sometimes to the different
modes in which the atoms of which their constituent
molecules consist are grouped, and sometimes to the dif-
ferent allotropic states in which one or the other of those
elements is found. Thus, as M. Millon has remarked,
the intrinsic difference between carburetted hydrogen
gas (CH) and ottar of roses (CH), which are isomeric
bodies, may perhaps consist in this, that in the former
the carbon is under the form of common charcoal, and in
the latter under the form of diamond.

The following instances from Berzelius may serve as
examples of these allotropic states :

Caj'bon is known under three forms — charcoal, plum-
bago, and diamond. They differ in specific gravity, in
specific heat, and in their conducting power as respects
caloric and electricity. In their relations to light, the
first perfectly absorbs it, the second reflects it like a
metal, the third transmits it like glass. In their rela-
tions with oxygen they also differ surprisingly ; there
are varieties of charcoal that spontaneously take fire in
the air, but the diamond can only be burned with diffi-
culty at a high temperature in pure oxygen gas. The
second and third varieties do not belong to the same
crystalline form.

Silicon exists also under two forms. In its first it
burns with facility in the air under a slight elevation of
temperature. But if it be previously exposed to a strong
red heat it changes into the second variety and becomes
incombustible, so that it will not oxidize when placed
with nitrate of potash in the hottest part of a blowpipe

Mi M..H: XX. J THE ALLOTROl'ISM OF CHLOHINK.

808

flame. As is well known, there are two forms of silicic
MrM: one soluble in water and hydrochloric acid, but
passing into the insoluble state by being previously
made red-hot. The silicon therefore carries in its com-
bination the same properties that it exhibits in the free
state.

In the same manner it might be shown that sulphur,
selenium, phosphorus, titanium, chromium, uranium, tin,
iridium, osmium, copper, nickel, cobalt, and a variety of
other bodies, exist under several different forms, with
distinctive properties that are often well marked. In
several of them the influence of this allotropic condition
is plainly carried into the compounds, as is well shown
in the two varieties of arsenic which give rise to the two
arsenious acids.

The passage from one allotropic state to another takes
place commonly through the agency of apparently very
trivial causes, such as a slight elevation of temperature
and the contact of certain bodies. Thus iron, which is
so easily oxidized under ordinary circumstances, appears
to lose its affinity for oxygen after it has been touched
under the surface of nitric acid by a piece of platinum.
It then puts on the attributes of a noble metal, and sim-
ulates the properties of platinum and gold.

This remarkable instance of the passage from an act-
ive to a passive state, as Berzelius remarks, may lead to
a conjecture respecting the true condition of certain
gases. No one can reflect on the inactivity of nitrogen
gas under ordinary circumstances, contrasted with its
equally extraordinary activity as a constituent of organ-
ic bodies, without being struck with the apparent con-
nection of that phenomenon with these of allotropisui.
And though Berzelius with his customary caution mere-
ly insinuates that nitrogen can exist under two forms,
the facts here developed in relation to chlorine appear to

304 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

show that that opinion rests on something more solid
than conjecture. The habitudes of many of the gaseous
bodies strengthen this conclusion. Oxygen refuses to
unite when mixed with hydrogen precisely in the man-
ner of chlorine, and it requires a certain modification to
be made in the electro-negativ7e element before water or
hydrochloric acid can result.

Just, therefore, in the same manner that so many ele-
mentary bodies can put on under the influence of exter-
nal causes an active or passive condition, I infer, as the
result of the experiments brought forward in this Mem-
oir, that chlorine is one of these allotropic bodies, having
a double form of existence. That, as commonly prepared,
it is in its passive state; but that on exposure to the
indigo rays or other causes it changes and assumes an
active form. That, in this latter state, its affinity for
hydrogen becomes so great that it decomposes water
without difficulty, as in the experiment which this Mem-
oir is designed to illustrate.

On the Relation of the Preceding Conclusions with the
Theory of Substitutions.

Having thus explained the facts which appear to indi-
cate the allotropism of chlorine, I shall now offer some
considerations on its connection with the theory of sub-
stitutions of M. Dumas.

Admitting the fact that the electro-negative qualities
of chlorine are exalted upon its exposure to the indigo
rays, and that the resulting effect is not a temporary
change, but one lasting for a considerable period of time,
we can give a very plain and simple account of the de-
composition of water by this gaseous substance under
the influence of sunshine.

On the same principle that a mixture of chlorine and
hydrogen may be kept in the dark without union for a

MIM..IK XX.] T11K ALLOTHOPISM OF CHLORINE.

long time, so may n solution of chlorine in water be pre-
served. The chlorine is in an inactive state.

But if anything be done to make the chlorine take on
its other form and pass to the active condition — if it be,
for example, set in the sunshine — its affinity for hydro-
gen is exhibited and decomposition is the result.

The qualities thus communicated to the chlorine not
being of a transient kind, but remaining for a length of
time, we see how it is that after an exposure to the sun
decomposition is subsequently carried forward in the
dark.

The indisposition of chlorine to unite with carbon,
which has been regarded as a singular quality, is not
more remarkable than its indisposition to unite with hy-
drogen in the dark.

D

If the power assumed by chlorine of uniting with hy-
drogen and carbon depends on a change in its electrical
relations — a passage from the passive to the active state
—we might expect that those various causes which in
the case of other elementary bodies bring about anal-
ogous changes, and throw them from one allotropic con-
dition to another, would here also exercise a perceptible
action. Among such causes we may enumerate the ac-
tion of a high temperature and the contact or presence
of other bodies.

It may be remarked, in the instances to which Berze-
lius has referred, that exposure to a high temperature is
one of the most frequent causes of allotropic change. In
the case of chlorine the remark holds good, for, as is
well known, when a mixture of chlorine and hydrogen is
passed through a red-hot tube, hydrochloric acid forms
with rapidity. The high temperature, therefore, impress-
es on chlorine the same tendency to unite with hydrogen
which is communicated by the solar rays.

But the contact of other bodies frequently determines

U

306 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

in a given substance an allotropic change. Thus, when
a piece of iron is placed in nitric acid in contact with
platinum, the iron becomes less electro-positive, or, what
is the same thing, more electro-negative, than it was be-
fore, and the acid can no longer oxidize it. The contact
of the very same substance, platinum, determines an an-
alogous change in chlorine — giving it at once the capac-
ity of uniting with hydrogen. The porous condition of
spongy platinum is not essential to the result, for clean
platinum foil exhibits the same phenomenon.

In the case of iron, the action of a high temperature
or the contact of platinum throws the metal from the
active to the passive state ; in the case of chlorine the
same causes apparently produce the opposite result,
throwing the gas from the passive to the active state.
But the difference is rather in appearance than in real-
ity. In both cases it amounts to the same thing, and is
an exaltation of the electro-negative qualities of either
substance respectively.

The same causes, therefore, which produce allotropic
changes in other bodies produce analogous changes in
chlorine.

Now, among the physical facts connected with the
theory of types and substitutions, two are prominent :
1st. The union of chlorine with hydrogen, giving rise to
the removal of that hydrogen as hydrochloric acid. 2d.
The subsequent function discharged by the chlorine,
which has entered as an integrant portion of the mole-
cules, and occupies the place of the hydrogen removed.
This function is in many instances that of the hydrogen
itself, and it is this fact which is the remarkable point in
the phenomena of substitution — that an intensely elec-
tro-negative body can act the part of a positive body.
It is this fact which is leading chemists to the conclusion
that the properties of compound bodies arise as much

MEMOIU XX.] THE ALLOTROPISM OF CHLORINE. 307

from tlu- mode of grouping of their constituent atoms as
from the qualities of those atoms themselves.

But, if it be admitted that the experiments related in
this Memoir establish the allotropism of chlorine, then it
is plain that a very different and perhaps satisfactory
account of the phenomena of substitution may be given.

As has been already said, no difficulty can arise in
accounting for the removal of hydrogen from organic
bodies, or for the first fact just alluded to. This remov-
al will ensue whenever processes are resorted to which
bring the chlorine into an active state. When we ex-
pose acetic acid and chlorine to the sun, the latter be-
comes active, gains the quality of uniting with hydrogen,
and chlor-acetic acid forms. Probably the same change
could be brought about by the aid of spongy platinum
and heat.

Upon the second fact — the similarity of function dis-
charged by the chlorine which has replaced the hydro-
gen atoms with the function of those atoms themselves —
a flood of light is thrown by other phenomena of allot-
ropism. If a piece of iron be dipped in hydrated nitric
acid, though it may be acted on for a few moments, it
rapidly becomes passive. And so with the chlorine
atoms which have substituted the hydrogen. In the
circumstances in which they are placed they rapidly re-
vert from the active to the passive state. They are no
longer endued with an intense electro-negative quality
—they have assumed the condition of inactivity. The
fact that chlorine in chlor-acetic acid simulates the func-
tions of hydrogen in acetic acid is not more remarkable
than that iron touched by platinum under nitric acid
simulates the properties of that noble metal.

Do not, therefore, these circumstances seem to point
out that if we admit the fact that simple substances can
exist in different states, in a passive and an active form,

308 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX.

the phenomena of substitution are deprived of much of
their singularity.

Thus, to recall once more the example to which I have
before referred, and which has been so well illustrated
by the researches of M. Dumas, the transmutation of
acetic into chlor-acetic acid exhibits a double phenom-
enon. 1st. The existence of active chlorine, expressed
by the removal of hydrogen, activity having been com-
municated by the rays of the sun, or by some other ap-
propriate method. 2d. The existence of passive chlorine
in the particles of chlor-acetic acid.

I consider that, were no other instances known, the
two cases cited by Berzelius of the double forms of silicic
acid and arsenious acid establish the fact that a given
allotropic condition may be continued by an elementary
atom when it goes into union with other bodies. And
I regard the various cases in which hydrogen is replaced
by iedine, bromine, etc., in which, in the resulting com-
pound, those energetic electro-negative elements fail to
give any expression of their presence and activity, as
analogous to other common and too much overlooked
facts. Chlorine which is in the dark may be kept in
contact with hydrogen without exhibiting any of its
latent energies. Touched by an indigo ray, it instantly
assumes the active state, and a violent explosion is the
result.

To use, therefore, the same nomenclature to which
Berzelius has resorted in the case of other allotropisins,
we may designate the ordinary form of chlorine made
by the action of hydrochloric acid on peroxide of man-
ganese as Cl/3, and admit that this passes into the con-
dition Cla by the action of the solar rays, contact of plat-
inum, or a high temperature; and that in any case of
substitution the hydrogen is removed under the con-
dition Cla, and the resulting compound contains Cl/3,

MKM..IU XX.J THE A I.I. uTKoPISM OF CHLORINE. ;>n'.l

the assumption of the passive state disguising the pres-
ence of the electro-negative atom.

The explanation here given of the phenomena of
substitutions involves the position that chlorine when
brought in relation with carbon under certain circum-

O

stances is thrown into the passive state, the state Cl/3.
We naturally look for direct evidence that this is the
case. It seems to me that there are many well-known
chemical facts tending to establish the passive condition.
In the first case to which we turn, the chlorides of car-
bon, the inactive state is established in a striking man-
ner. The affinity existing between chlorine and carbon
is apparently feeble ; yet when these bodies have once
united the chlorine is brought into such a condition
that it has lost the quality of being detected by the or-
dinary tests which determine its presence. How strong-
ly does this contrast with the case of hydrochloric acid !
A feeble affinity unites carbon and chlorine, an intense
affinity unites hydrogen and chlorine ; yet in the former
case the chlorine is undiscoverable by the commonest
tests, in the latter it yields to them all. And the causes
are obvious ; in the one case it is in the passive, in the
other in the active condition.

I have hitherto spoken of the active and passive
states as though they were fixed points in elementary
bodies, and as though the transition from one to the
other were abrupt and sudden. I have done this that
the views here offered might be unembarrassed and dis-
tinct. But there are many facts which serve to show
that the passage from a state of complete activity to a
state of complete inactivity takes place through gradu-
al steps. Thus, in carbon itself, there are undoubtedly
many intermediate stages between the almost spontane-
ously inflammable varieties and diamond, which, under
common circumstances, is incombustible. Berzelius ad-

310 THE ALLOTROPISM OF CHLORINE. [MKMOIR XX.

mits three allotropic conditions of this body, Ca, C/3, C-y.
Between the first and last terras of this series it is prob-
able that several intermediate bodies besides plumbago
might be found, their existence establishing the gradual
passage from one to the other state.

For similar reasons, in this Memoir the illustrations
and arguments given have for the most part been re-
stricted to one subject, chlorine. It need scarcely be
pointed out, in conclusion, that if the views here offered
be true, very much of this reasoning may be transferred
to other bodies, as oxygen, nitrogen, hydrogen, sulphur,
etc. When oxygen and hydrogen are mixed, there is no
disposition exhibited by them to unite; and this does
not arise from their happening to have the gaseous form.
As in the instance we have been considering, if they are
exposed to a high temperature, or to the influence of
platinum, the active condition is assumed with prompti-
tude, *and union takes place.

The power possessed by carbon of throwing bodies
into a completely passive state is fur from being limited
to chlorine. It reappears in the case of sulphur. The
sulphide of carbon yields to none of the tests to which
we commonly resort for determining the presence of sul-
phur, for the simple reason that its sulphur is in an inac-
tive state. This substance, moreover, serves to illustrate
what has been said of the gradual passage of bodies
from a state of complete activity to one of complete in-
activity. Berzelius recognizes for it three different allo-
tropic states — an alpha, beta, and gamma condition. In
none of these is it in that condition of absolute inactiv-
ity which it assumes in the sulphide of carbon.*

* For these examples, the chloride and sulphide of carbon, I am indebted to M.
Millon's paper, " Remarks on the Elements which Compose Organic Substances, and
on their Mode of Combination, "in the Comptes Rendus, t. xix., p. 799. That chem-
ist, however, gives a very different explanation of the phenomena involved.

MEMOIR XX.] TIIK ALLOTROPISM OF CHLOHIM!

In offering these experiments and arguments to the
consideration of chemists, I am fully aware of the mag-
nitude of the change which would be impressed on the
science generally, and especially on several modern theo-
ries, by their reception. The long -established idea of
the immutability of the properties of elementary bodies
would, to a certain extent, be sacrificed ; and it is prob-
able that before these results are conceded more cogent
evidence of the main principle will be required. In the
meantime, however, it is plain that the admission of
these doctrines throws much light on theories now ex-
tensively attracting attention, and for that reason they
commend themselves to our consideration. I have of-
fered no opinion here on the atomic mechanism involved
in these changes from an active to a passive state,
though it is impossible to deal with these things with-
out the reflection arising in our minds that here we are
on the brink of an extensive system of evidence con-
nected with the polarity of atoms — an idea which, under
a variety of forms, is now occurring in every department
of natural philosophy.

UNIVERSITY OF NEW YORK, July 29, 1845.

312 INFLUENCE OF LIGHT UPON CHLORINE. [MEMOIR XXI.

MEMOIR XXL

ON THE INFLUENCE OF LIGHT UPON CHLORINE, AND SOME
REMARKS ON ALCHEMY.

From the Philosophical Magazine, November, 1857.

CONTENTS : — Modification of chlorine by the sun-rays. — The modification
is not transient. — Alchemical attempts to modify metals. — Exposure
of silver chloride to a burning-lens. — The resulting silver is not acted
upon by nitric acid.

SEVERAL years ago I observed that when a mixture
of chlorine and hydrogen is exposed to light, union does
not occur at once, but that a certain time must elapse
during which absorption takes place, the combination
then proceeding in a uniform manner.

It is by the chlorine that this absorptive agency is ex-
ercised, the indigo ray being mainly concerned. And
not only is it that ray which is thus absorbed : to it
must be attributed also the subsequent combination.

Among several other facts connected with this subject,
which may be found in the Philosophical Magazine
(July, 1844), The American Jownal of /Science, Vol.
XLIX., and other publications of that time, there is one
to which I would particularly direct attention. Chlo-
rine which has been exposed to the sun has obtained
properties not possessed by chlorine which has been
made and kept in the dark, and the change is by no
means transient. It lasts for many hours, and even days.

In their recent examination of this fact, Professor Bun-
sen and Dr. Roscoe do not appear to regard the modifi-
cation in question as being of so permanent a nature.

MEMOIK XXI.] INFLUKNCE OF LIGHT UPON CHLORINE. ;; 1 ;>

Perhaps it may have been that the insolation to which
they submitted the chlorine was not continued sufficient-
ly long, or perhaps the light was not sufficiently intense.
My opinion was founded on three different conditions of
the experiment — 1st. On the behavior of chlorine itself
confined over salt water ; 2d. On the effect of a mixture
of chlorine and hydrogen in equal volumes as disengaged
from hydrochloric acid by a voltaic current; 3d. On the
action of a solution of pure chlorine in distilled water.
In each of these instances, the active properties imparted
to the chlorine by exposure to light were plainly per-
ceptible for a long time after. Indeed, I infer from the
experiments of those chemists that they found the effects
to continue for a certain brief period. If they do so
continue, though only in a momentary manner, after the
light has been shut off, I do not see in what other man-
ner we are to explain the result than on the principle of
a change in the relations of the chlorine. In this inter-

o

pretation it is very well known that Berzelius coincided
in his account of my experiments in the Annual Report
for 1847.

At first I thought that there was a general analogy
between the case of chlorine thus thrown into an active
state and that of iron in its passive condition. An iron
wire which has been made passive will quickly revert
to the condition of activity if submitted to any jarring,
vibration, or other trivial disturbance; its passive state
being in one sense permanent, though very easily lost.
But subsequently I found many reasons for supposing
that the impression is of a much more lasting nature,
and resembles that on phosphorus after a similar expos-
ure to the indigo rays. As an illustration of what is
here meant, I may relate that having obtained a thin
stratum of perfectly white phosphorus between two
pieces of glass, I exposed it to a motionless solar spec-

314 INFLUENCE OF LIGHT UPON CHLORINE. [MEMOIR XXI.

trum, and found that it turned to a dark-brown color in
those spaces on which the more refrangible rays fell, the
effect reaching a maximum under the indigo ray. The
fixed lines of Fraunhofer were very prettily depicted, as
white streaks, particularly the large ones at H. I kept
the sample of phosphorus for several years without its
showing any disposition to resume the active state.

Professor Buuseu and Dr. Roscoe dwell very appro-
priately on the disturbing effects of minute quantities of
extraneous gases mingled with chlorine on photo-chem-
ical induction. No one who has used a chlor-hydrogen
photometer can have failed to make a similar remark.
My attention has been directed to that subject in its
more general aspect, and I will ingenuously confess that
I have made several attempts at the transmutation of
metals, on the principle of compelling them, by the aid
of solar light, to be disengaged from states of combina-
tion hi the midst of resisting or disturbing media.

The following is a description of one of these alchem-
ical attempts: In the focus of a burning -lens, twelve
inches in diameter, was placed a glass flask, two inches
in diameter, containing nitric acid diluted with its own

/ O

volume of water. Into the nitric acid were poured alter-
nately small quantities of a solution of nitrate of silver
and of hydrochloric acid, the object being to cause the
chloride of silver to form in a minutely divided state, so
as to produce a milky liquid, into the interior of which
the brilliant converging cone of light might pass, and the
currents generated in the flask by the heat might drift
all the chloride successively through the light. The
chloride, if otherwise exposed to the sun, blackens mere-
ly upon the surface, the interior parts undergoing no
change ; this difficulty I therefore hoped to avoid. The
burning-glass promptly brings on a decomposition of the
salt, evolving on the one hand chlorine, and disengaging

MEMOIR XXL] INFLUENCE OF LIGHT UPON CHLORINE. 315

a metal on the other. In one experiment the exposure
lasted from 1 1 A.M. to 1 P.M. ; this was, therefore, equal
to a continuous mid-day sun of seventy-two hours. The
metal was disengaged readily. But what is it ? It
cannot be silver, since nitric acid has no action upon it.
It burnished in an agate mortar, but its reflection is not
like the reflection of silver: it is yellow. The light
must, therefore, have so transmuted the original silver
as to enable it to exist in the presence of nitric acid. In
1837 I published some experiments on the nature of
this decomposition in the Journal of tlie Franklin In-
stitute.

Though this experiment, and several modifications of
it which I might relate, fail to establish any permanent
chancre in the metal under trial, in the sense of an actual

D

transmutation, it does not follow that we should despair
of final success. It is not likely that Nature has made
fifty elementary substances of a metallic form, many of
them so closely resembling each other as to be with dif-
ficulty distinguished. Moreover, chlorine and other ele-
mentary substances can be changed by the sunlight in
some respects permanently ; and if silver has not thus far
been transmuted into a more noble metal, as platinum
or gold, it has at all events been made transiently into
something which is not silver. Those who will reflect

O

a little on the matter cannot fail to observe that the
sun-rays really possess many of the powers once fabu-
lously imputed to the powder of projection and the phi-
losopher's stone.

316 THE ACTION OF GLASS AND QUARTZ. [MEMOIR XXII.

MEMOIR XXII.

ON THE ACTION OF GLASS AND QUAETZ ON THE KADI-
ATIONS THAT PRODUCE PHOSPHORESCENCE.

From the Philosophical Magazine, Aug., 1844.

CONTENTS : — Phosphorescence by rays from a Leyden spark through
quartz. — From the voltaic arc. — It is occasioned by the more refran-
gible rays. — Imperviousness of glass. — Professor Henry's experiments.
— Comparison of the chemical and phosphorogenic rays.

AT the distance of six inches from the terminations of
two blunt wires, between which the spark from a Ley-
den-jar was caused to pass, I placed a lens of quartz, the
focus»pf which for parallel rays was six inches, and then
intercepted the resulting beam by a diaphragm, with a
circular aperture in it one third of an inch in diameter.
I had caused an equiangular prism of quartz to be cut
and polished from a large and faultless crystal; it was
cut transverse to the axis. This prism I placed in such
a way that, in dispersing the beam coming through the
circular aperture, I got rid of double refraction and ob-
tained only one spectrum ; this was received on a metal
plate, which, having been washed over with gum-water
and sulphide of lime dusted on it, offered a uniform
phosphorescent surface which might be set in a vertical
plane. When the spark passed, I saw that the plate
was phosphorescing on those portions where the more
refrangible rays had fallen.

But the transient light of a Leyden spark did not last
long enough, nor was the phosphorescence it produced
powerful enough to enable me to conduct the experiment

MEMOIR XXII.] THE ACTION OF GLASS AND QUARTZ.

in a way entirely satisfactory. I resorted, therefore, to
the brilliant light obtained when a piece of metal, or,
what is better, the hard variety of carbon obtained from
gas-retorts, is lowered upon mercury entirely filling a
very small open porcelain cup, and the continuous dis-
charge of a voltaic battery passed. The battery used
contained fifty pairs of Grove's cells, but a smaller num-
ber would have been amply sufficient.

As soon as the light was emitted, I marked on the
lime sulphide the beginning of the red, the centre of the
yellow, and the termination of the visible violet ray.
Then, stopping the current, I examined on what parts
the plate was phosphorescing. The commencement of
the glow was between the indigo and the blue ; towards
the blue it extended far beyond the visible boundaries
of the spectrum. I could not see any divisions or points
of maximum in it. The surface of the plate shone all
over except in the region of the less refrangible rays,
and there were traces of the negative action which M.
Becquerel has illustrated in the cases of the solar ema-
nations— rays which, however, were first observed in the
last century.

It is necessary to remark that the rays from the voltaic
discharge resemble those from an electric spark in their
inability to pass through glass. On this observation all
the value of the foregoing experiment depends.

But it can nevertheless be easily proved that although
glass is impervious to the phosphorogenic emanation
coming from the voltaic deflagration of any metallic bod-
ies, the observation applies to transient discharges only.
A voltaic light which lasts but a moment fails to cause
phosphorescence through glass in the same way that an
electric spark does; but if the discharge be continued
the surface presently begins to glow, and if maintained
for several minutes it shines as brightly as though a
piece of quartz had been used.

318 THE ACTION OF GLASS AND QUARTZ. [MEMOIR XXII.

The inability of an electric spark to cause phosphor-
escence is connected with its transient duration. The
voltaic light enables us at pleasure to imitate the effects
of an electric spark or those of the sun. The phosphor-
ogenic rays, whether they originate in an electric spark
or from the sun, occupying thus the same place in the
spectrum, and even exhibiting the same peculiarities as
the chemical rays on iodide of silver, we have next to de-
termine whether this is an apparent or a positive identity.

Professor Henry, of Princeton, read a paper before the
American Philosophical Society, in May, 1843, in which
he discussed all the leading mechanical properties of the
phosphorogenic rays, and, among other important exper-
iments, made some with a view of determining this par-
ticular question. A daguerreotype plate and some lime
sulphide were simultaneously exposed to the sky: the
plate was stained, but no effect was produced on the
lime. *,.A daguerreotype plate and some lime sulphide
were exposed to the light of an electric spark : the lime
was observed to glow, but no impression was produced
on the plate. When the plate was exposed to a succes-
sion of sparks for ten minutes with a sheet of mica inter-
posed, an impression was made. Lime exposed to the
moon did not phosphoresce, but a sensitive plate under
the circumstances is said to be stained. In view of
these different facts, Professor Henry observes : " These
experiments, although not sufficiently extensive, appear
to indicate that the phosphorogenic emanation is distinct
from the chemical, and that it exists in a much greater
quantity in the electric spark than either the luminous
or chemical radiation."

From Wilson's experiments it appears that he was
aware that when the phosphorescent surface is warmed
so as to hasten the disengagement of light, the moon-
beams may be found to have left traces of action upon

MEMOIK XXII.] THE ACTION OF GLASS AND QUARTZ.

it, feeble, it is true, but nevertheless very apparent. We
have seen, also, that the peculiarity of an electric spark
is due to its transient duration. Before, therefore, a
final decision can be obtained on this point, we are re-
quired to examine the effects of the chemical rays and
phosphorogenic emanations under circumstances which
are precisely similar as to intensity and time.

For the transient rays of an electric spark, quartz is
transparent and glass is nearly opaque. Having pre-
pared a bromo-iodized silver plate so as to be exceeding-
ly sensitive, I set in front of it at the distance of about one
third of an inch a disk of quartz and one of crown-glass
of equal thickness, and between a pair of copper wires,
the interval of which was three eighths of an inch, I
passed the spark of a Leyden-jar fifteen times ; the dis-
tance between this spark and the sensitive plate was
about two inches. On mercurializing this plate, it was
deeply whitened all over, equally so through the glass,
through the quartz, and on the uncovered spaces ; but a
spot of sealing-wax which I had put on the glass left its
shadow on the plate beautifully depicted, so also were
the edges of the glass and the quartz. The two disks
overlapped one another to a certain extent, but the cor-
responding portion of the silver plate was as deeply
stained there as anywhere else.

Next I put a surface of lime sulphide in the place of
the daguerreotype plate, everything else remaining as be-
fore. On passing fifteen sparks the lime phosphoresced
powerfully under the quartz, but not under the glass, so
that the difference between its shadow and that of the
spot of wax could not be distinctly seen.

For these reasons, therefore, I adopt the view ex-
pressed by Professor Henry, that the phosphorogeuic
emanation and the chemical rays are distinct. Under
the same circumstances glass is transparent to the one,
to the other it is opaque.

320 RAYS OF INCANDESCENT LIME, ETC. [MEMOIR XXIII.

MEMOIR XXIII.

ON A REMARKABLE DIFFERENCE BETWEEN THE EAYS OF

INCANDESCENT LIME AND THOSE EMITTED BY AN

ELECTRIC SPARK.

From the Philosophical Magazine, December, 1845.

CONTENTS : — Non-permeability of glass to spark radiations. — Permeabil-
ity to calcium-light radiations. — Different refrangibility of the spark
and the calcium -light radiations. — Shadows imbedded in Canton's
phosphorus. — Evolution of old shadows in an order of succession.

SOME years ago M. Becquerel discovered that the rays
of an electric spark, if transmitted through a screen of
glass^could not excite -the phosphorescence of lime sul-
phide.

To make this experiment, wash a metallic plate over
with gum-water. Dust upon it from a fine sieve a quan-
tity of Canton's phosphorus (oyster-shells calcined with
sulphur), and allow the plate to dry. A uniform surface
is thus obtained suitable for these purposes. Place be-
fore that surface a piece of glass and a piece of polished
quartz, and discharge a Leyden-jar a few inches off, so
that the rays of its spark may fall on the plate. It will
be found that under the quartz the phosphorus will
shine as much as on the spaces that have not been cov-
ered, but under the glass it will remain almost entirely
dark.

Last winter I observed the curious fact that when this
experiment is made with a piece of lime incandescing in
a stream of oxygen directed through the flame of a spirit-
lamp, the glass, so far from being unable to transmit the

Mi M..IK XXIII.] KAYS OF INCANDESCENT LIME, ETC. 321

rays, appmrs to be as transparent to them as quartz or
atmospheric air.

A screen of glass is opaque to the phosphorogenic
rays of an electric spark, but it is quite transparent to
those of incandescent lime.

It might be supposed that the very brief duration of
an electric spark has something to do with this phenom-
enon, but the voltaic arc passing between charcoal points
gives the same results. I caused its rays to impinge on
a plate for thirty seconds, and observed the obstructing
effect of glass in a very satisfactory manner. That a
certain portion of the rays passes through may be
shown by continuing the light for a few minutes, when
the phosphorus will begin to shine under the glass.

I came to the conclusion, also, that the transient dura-
tion of the light had nothing to do with the phenomenon,
because the lime light occasions phosphorescence through
glass in the space of a single second, but in that time
the rays from a voltaic arc could not traverse a piece of
glass so as to produce a sensible effect ; the phosphorus
beneath it appearing quite dark, and yet this light is in-
comparably brighter than the lime light.

The blue light emitted when a platinum wire in con-
nection with one pole of the voltaic battery is brought
down upon some mercury in connection with the other,
and the green light obtained when the copper wires are
the medium of discharge, appear to produce the same
effect as charcoal points.

It is therefore neither the color nor the duration of
the light that determines this result. It seems to depend
on a peculiarity of the electric discharge.

Some time ago I determined the refrangibility of the
rays of an electric spark which excite phosphorescence
in lime sulphide ; they are found at the violet extremity
of the spectrum. I have made attempts to ascertain the

X

322 KAYS OF INCANDESCENT LIME, ETC. [MEMOIR XXIII.

position of the active rays of incandescent lime. They
cannot pass through the blue solution of ammouio-sul-
phate of copper, but through the red solution of sulpho-
cyanide of iron, and also through a strong solution of
bichromate of potash, they pass ; in the latter case al-
most as copiously as through atmospheric air.

The phosphorogenic rays of an electric spark are in
the violet space, but those of incandescent lime are at
the other extremity of the spectrum.

An Argand lamp, when made to burn very brightly,
emits phosphorogenic rays which traverse glass. As
has been proved long ago, the sun-rays possess the same
property.

Thus, therefore, the rays of incandescent lime, of an oil-
lamp, and of the sun can excite phosphorescence through
glass, and differ from those of an electric spark or voltaic
discharge, in which that peculiarity is deficient.

I iave also remarked some curious cases of spectral
appearances. They are analogous to those instances to
which I first drew public attention in 1840, and which
at a later date M. Moaer brought before the British As-
sociation. They are interesting as affording an ocular
proof of secondary radiation. The following experiments
may serve as illustrations :

Place a key or any other opaque object before a sensi-
tive phosphorescent surface, and having made that sur-
face glow intensely by a vo1 ^aic discharge between char-
coal points continued for two or three minutes, on re-
moving the key an image of it will of course be seen.
This image in a short time will disappear. Then shut
the plate in a dark place, where no light can have access
to it in the daytime. If in a day or two the surface be
carefully inspected in the dark, no trace of anything will
be visible upon it ; but if it be laid on a piece of warm
iron, a spectral image of the key is suddenly evolved.

MKMOIK XXIII.] RAYS OF INCANDESCENT LI.MK, IK

It is still more curious that a number of these latent
images may co-exist on the same surface. Provide u
phosphorescent surface on which the latent image of a
key impressed a day or two before by a voltaic discharge
is known to exist. Take some other object, as a metal
ring, and setting it before the surface, discharge at a
short distance a Leyden-jar. The phosphorus shines all
over, save on those portions shaded by the ring; it ex-
hibits, therefore, an image of that body. This image
soon fades away and totally disappears. Set the plate
now upon a piece of warm iron ; it soon begins to glow,
and the image of the ring is first reproduced, and as it
declines away the spectral form of the key gradually un-
folds itself, and after a time it totally vanishes.

A series of spectral images may thus exist together
on a phosphorescent surface, and after remaining there
latent for a length of time, they will come forth in their
proper order on raising the temperature of the surface.

The idea that phosphorescence is merely the light of
an electric discharge from particle to particle seems to
me wholly incompatible with such results.

324 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV.

MEMOIR XXIV.

ON THE ELECTRO-MOTIVE POWER OF HEAT.

From the Philosophical Magazine, June, 1840 ; Harper's Magazine, No. 328.

CONTENTS: — Experimental arrangement to determine the electro-motive
power. — Temperatures calculated from quantities of electricity. — In-
crease of tension with increase of temperature. — Depends on increased
resistance to conduction. — Quantity of electricity independent of heated
surface. — In thermo-electric piles the quantity of electricity is propor-
tional to the number of pairs. — Best forms of construction of thermo-
electric pairs.

FROM the Memoir of M. Melloni "On the Polarization
of Heat," inserted in the second part of the first volume
of Tailor's " Scientific Memoirs," we learn that M. Bec-
querel, as well as himself, had made experiments to de-
termine the quantities of electricity set in motion by
known increments of heat. From these experiments
they conclude that through the whole range of the ther-
mometric scale those quantities are directly proportional
to each other.

But as thermo-electric currents are now employed in
a variety of delicate physical investigations, and as there
appears to be much misconception as to their character,
I propose in this Memoir to show —

1st. That equal increments of heat do not set in mo-
tion equal quantities of electricity.

2d. That the tension undergoes a slight increase with
increase of temperature, a phenomenon due to the in-
creased resistance to conduction of metals when their
temperature rises.

3d. That the quantity of electricity evolved at any

MKMOIH XXIV. J T1IK KLECTRO-MOTIVE POWER OF HEAT. 305

given temperature is independent of the amount of In-at
ed surface-, a mere point being just as efficacious as an
indefinitely extended surface.

4th. That the quantity of electricity evolved in a pile
of pairs is directly proportional to the number of the
elements.

First, then, as to the comparative march of electric de-
velopment with the rise of temperature in the case of
pairs of different metals.

The experimental arrangement which I have em-
ployed is represented
in Fig. 52. A A is a
glass vessel, about
three inches in diame-
ter, with a wide neck,
through which can be

O

inserted a mercurial
thermometer, />, and
one extremity of a pair
of electro-motoric wires. The wires I have employed
have generally been a foot long and one sixteenth of an
inch in diameter. The extremities, S, of the wires thus
introduced into the vessel ought to be soldered with hard
solder; their free extremities dip into the glass cups r/,
(I, filled with mercury, and immersed in a trough, et con-
taining water and pounded ice. By means of the cop-
per wires//, one sixth of an inch thick, communication
is established with the mercury-cups of the galvanom-
eter. The coil of this galvanometer is of copper wire
one eighth of an inch thick, and making twelve turns
only round the needles, which are astatic. The devi-
ations were determined by the torsion of a glass thread.
It is surprising to those who have never before seen
the experiment with what promptitude and accuracy a

Pig. 62.

326 THE ELECTRO-MOTIVE TOWER OF HEAT. [MEMOIR XXIV.

copper and iron wire soldered thus together will indi-
cate temperatures.

In the arrangement now described, when an experi-
ment has to be made, the vessel A A is to be filled
two thirds full of water, the bulb of the thermometer
being so adjusted as to be in its middle, the soldered
extremity S of the two wires being placed in contact
with it, and a small cover with suitable apertures ad-
justed on the top of the vessel, so that the steam as it
is generated may rush up alongside of the tube of the
thermometer and bring the mercurial column in it to
a uniform temperature. If the extremity of the thermo-
electric pair be allowed to rest on the bottom of the
glass vessel, no accurate results can be obtained ; the
pair does not then indicate the temperature of the
water. The communicating wires// are then placed in
che cups, the trough e filled with water and pounded ice,
and carefully surrounded with a flannel cloth. The wa-
ter in the vessel A A is then gradually raised to the
boiling-point by means of a spirit-lamp, and kept at that
temperature until the galvanometer needles and the ther-
mometer are quite steady. The same, plan must be fol-
lowed when any other temperature tnan 212° is under
trial, for the thermo-electric wires changing their temper-
ature more rapidly than the mercury in the thermom-
eter, it is absolutely necessary to continue the experi-
ment for some minutes to bring both to the same state
of equilibrium.

When a temperature higher than 212° Fahr., but un-
der a red heat, is required, I substitute in place of the
vessel A A a tabulated retort, the tubulure of wrhich is
large enough to allow the passage of the bulb of the
thermometer and the wires. A quantity of mercury,
sufficient to fill the retort half- full, is then introduced,
and the tubulure being closed by appropriate pieces of

MKJUOIR XX1V.J THE KLECTRO- MOTIVE TOWER OF III \ I 307

soapstone, the neck of the retort is inclined upwards, so
that the vapor as it rises may condense and drop hack
again without incommoding the operator. As in the
former case, it is here also necessary to continue each
experiment for a few minutes, to bring the thermometer
and thermal pair to the same condition. There is not
much difficulty in obtaining any required temperature,
by raising or lowering the wick of the lamp.

The metals I have tried were in the form of wires.
They were in the state found in commerce, and therefore
not pure; they were obtained in the shops of Philadel-
phia.

TABLE I.

Names of the Pairs of Metals.

Temperatures (Kahr.).

32° F.

122° F.

212° F.

662° F. <C

Copper and iron

0

0
0
0
0
0

93

f>r>

112

11

89

28

176
147
223
26
137
aC

233 }
6I3/ s?E
631 I £2
122/^2,
244 E
248 J ?

Silver nud palladium

Iron and palladium

Platinum and copper

Iron nnd silver

Iron ami platinum

In this table I have estimated the temperature of
boiling mercury at 662° Fahr. The quantities of elec-
tricity evolved, as estimated by the torsion of a glass
thread, are rangea in columns under their corresponding
temperatures. Each series of numbers is the mean of
four trials, the differences of which were often impercep-
tible, and hardly ever
amounted to more than
one degree.

Now if this table be
constructed, the tem-
peratures being ranged
alone: the axis of abscis-

o

sas, and the quantities
of electricity being rep-
resented by correspond-

328 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV.

ing ordinates, we shall have results similar to those given
in Fig. 53, in which it is to be observed that the curves
given by the systems of silver and iron, copper and iron,
and palladium and iron are concave to the axis of ab-
scissas; but those given by platinum and copper, silver
and palladium, and platinum and iron are convex.

Let us now apply the numbers obtained by these sev-
eral pairs for the calculation of temperatures, which will
set their action in a more striking point of view. The
following table contains such a calculation, on the sup-
position that for the 90 degrees from 32° Fahr. to 122°
Fahr. the increments of electricity are proportional to the

temperatures.

TABLE II.

Temperatures by the Mercurial Thermometer.

32° F.

12-2° F.

Water boils.

Mercury boils.

& 1 Copper and iron

32
32
32
32
32
32

122
122
122
122
122
122

202
235
211
244
170
212

257
880
539
1030
279
829

^tr \Silverandpalladium..
§4- J Iron and palladium...
«& ) Platinum and copper. .
c " / Iron and silver

g \ Iron and platinum

We are therefore led to the general conclusion that in
these six different systems of metals the developments of
electricity do not increase proportionally with the temper-
atures, but in some with greater rapidity and in others
with l-ess.

The results here given I have corroborated in a vari-
ety of ways and with a variety of wires. A pair con-
sisting of copper and platinum gave for the temperature
of tin when in the act of congealing 452° Fahr. instead
of 442° Fahr., the point usually taken. For the melting-
point of lead it gave 942^° Fahr. instead of 612° Fahr.
The melting-points of tin, lead, zinc, and occasionally of
antimony and bismuth, were in this manner employed,
for they allow time for the working of the torsion bal-
ance, and with the exception of bismuth, their tempera-

Fig. 54.

MI.M..IIC XXIV.J Till-: Ml. l.i Miii MOTIVE 1'oWKU <>1 lll.Al.

ture appears to be steady all the while they are in a
granular condition before they finally solidity. The ac-
tion of these metals on the thermo-electric pair is easily
prevented by dipping it into a cream of pipe-clay.

A pair of copper and platinum gave for a dull red
heat 1416° Fahr., and for a bright red 2103° Fahr.

A pair of palladium and platinum gave for a dull red
1850° Fahr., and for a bright red 2923° Falir.

Some of the combinations into which iron enters as
an element give rise to remarkable results. Thus, if \v«
project the curve given by a system of copper and iron
we shall find it resem-
bling Fig. 54, where
the maximum ordinate
b occurs at a temper-
ature of about 650°
Fahr. ; the point c ap-
pears to be given between 700 and 800 degrees; d by a
dull red heat; e is very nearly the point at 'which an
alloy of equal parts of brass and silver melts, for if the
pair be soldered with this substance, it fuses when the
needles have returned almost exactly to the zero point.
With harder solders or with wires simply twisted, the
curve may be traced on the opposite side of the axis
towards ft its ordinates increasing with regularity. At
60° Fahr., taking the length of the ordinate correspond-
ing to a temperature of 212° Fahr. as unity, the length
of the maximum ordinate at b is 1.85, very nearly.

A system of silver and iron gives also a similar curve,
the point b occurring at a temperature rather higher
than the analogous one for the preceding system, but
still below the boiling-point of mercury.

Now, all these things serve to show that we cannot
determine with accuracy unknown temperatures by the
aid of thermo-electric currents, on the supposition that

330 THE ELECTRO-MOTIVE POWER OF HEAT. [MKMOIR XXIV.

the increments of the quantities of electricity are propor-
tional to the increments of temperature throughout the
range of the mercurial thermometer.

Let us now pass to the second proposition : "That the
tension undergoes a slight increase with increase of tem-
perature— a phenomenon due to the increased resistance
to conduction of metals when their temperature rises."

It will be seen, on consulting the following table, that
pairs of different metals at the same temperature have
tensions which are apparently very different.

The currents the tensions ot which are here indicated
were generated by keeping one end of the thermal pair
in boiling water, the other ends being maintained at a
temperature of 32° Fahr.

TABLE III.

A pair of

Tension.

A pair of

Tension.

Antimony and bismuth. .
Copper and iron

.137
.183
.307
.313
.380

Platinum and iron

.470
.473
.500
.518
.567

Copper and platinum
Platinum and palladium..
Tin and iron

Silver anfl lend

Lead and palladium
Silver and platinum

Platinum and tin

We perceive, therefore, that there apparently exist
specific differences in the qualities of electric currents
derived from different sources. If. for example, we take
a pair of platinum and palladium and expose it to a tem-
perature which shall generate a current capable of de-
flecting the torsion balance through 1000°, and then
obstruct it by a wire of such dimensions as to stop one
half, or only allow 500° to pass, and repeat the experi-
ment with a current generated by bismuth and antimo-
ny, the temperature being still so adjusted as to give a
deflection of 1000°, on making this pass through the
same intercepting wire, perhaps not much more than one
eighth of it will go through the galvanometer.

This peculiarity of thermo-electric currents depends

XXIV. J TIIK KLECTKO-MOTIVK I'OWKlt OF HEAT.

on the conducting resistance of the system that generates
them. It is possible to give a current a higher or a
lower tension by simply making use of thin or thick
wires to generate or to carry it. In the foregoing table
the current from platinum and palladium had a high
tension, because slender wires of those metals happened
to be used to generate it ; and the current from antimo-
ny and bismuth had a low tension because thick bars
of those substances were employed. In the former case,
the conducting resistance was greater than in the latter,
and hence the tension of the current was higher.

That this is strictly true will appear on examining the
current evolved by any number of systems under the
same condition of resistance to conduction.

In a great number of trials I failed in getting any
trustworthy results as respects tension of currents at
high temperatures, on account of the difficulty of main-
taining the thermo-electric pair at the same degree with-
out variation. By employing, however, a small black-
lead furnace, to which was adapted a covered sand-bath
into which the wires could be plunged, I succeeded at
last ; for with this arrangement a regulated temperature
could be kept up for a length of time.

The experiment was made with care in the case of
two systems of metals: 1st, copper and platinum; 2d,
copper and iron.

1st. At the boiling-point of water, a pair of copper and
platinum, the unexcited extremity of which was carefully
maintained at 67° Fahr., evolved as a mean of four trials,
three of which were absolutely identical, 123° of electric-
ity, of which 23° could pass a secondary wire.

Then, by the aid of the furnace and sand-bath, the
temperature was raised until the pair evolved 783° as a
mean of four trials; of these 163° could pass the second-
ary wire. Now,

332 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV.
As 783 : 163 :: 123 : 25^ instead of 23,

showing, therefore, a slight rise of tension.

2d. The pair of copper and iron gave at the boiling-
point of water 300°, of which 57° passed the secondary
wire. The temperature was now raised, with the follow-
ing results :

490 degrees passing the primary, 95 the secondary wire.
553 " " " ' 113 " "

545 " " " 112 " "

493 " " " 110 " "

It will be understood that although the quantities of
electricity indicated in the first column do not regularly
increase, the temperatures were, notwithstanding, going
regularly upwards: to this peculiarity of the systems
into which iron enters I have already alluded. Let us
now compare these measures with those obtained for the
boiling-point of water :

As 490 : 95
553 : 113
545 : 1 1 2
493 : 1 10

300 : 58 instead of 57.
300 : 61 "
300 : 61 "
300 : 67 "

We find, therefore, that in the case of both these systems
of metals the tension slowly rises with increase of tem-
perature, being much better marked in the latter than in
the former instance.

The increase of tension here detected depends unques-
tionably on increased resistance to conduction, which the
wires exhibit as their temperature rises, as the following
experiments show.

A pair of copper and iron evolved a current at the
boiling-point of water, which, passing through a wire of
copper eight feet long, was determined at the galvanom-
eter to be 176°. Having twisted a part of this wire into
a spiral, so as to go over the flame of a spirit-lamp, eight
inches of it were thereby brought to a red heat; the de-

MKMOIH XXIV.] THE ELECTRO-MOTIVE 1'OWKK OK HKAT.

viation of the needle fell now to 165°, being a deficit of
11°. In this experiment care was taken that no heat
should be transmitted along the wire to the connecting

flips.

The same was repeated with an iron wire of the same
length and under the same circumstances. The current
at first being 90°, as soon as the spiral was made red-hot
it fell to 61°, being a deficit, therefore, of nearly one third
the whole amount.

To the increased resistance to conduction, occasioned
by an increased temperature, we are to impute the slight
rise of tension observed in thermo-electric currents. The
quantities are of the same order.

We have next to show " that the quantity of electric-
ity evolved at any given temperature is independent of
the amount of heated surface, a mere point being just
as efficacious as an indefinitely extended surface."

The quantities of electricity evolved by hydro-electric
pairs increase with the surfaces, but it is not so in ther-
mo-electric arrangements. A pair of disks of copper and
iron, two inches in diameter, were soldered together;
they had continuous straps projecting from them, which
served to connect them with the galvanometer cups. At
the boiling-point of water they gave 62° ; on being cut
down to half an inch in diameter, they still gave 62°.
On the disk being entirely removed, and the copper
made to touch the iron by a mere point, its extremity
being roughly sharpened, the deflection was still 62°.

By means of a common deflecting multiplier, I ob-
tained the following results: 1st. A copper wire being
placed in a bath of mercury, the temperature of which
was 240° Fahr., I dipped into it a second copper wire,
the temperature of which was about 60° Fahr. ; the gal-
vanometer needles moved through 15°.

334 THE ELECTRO-MOTIVE POWER OF HEAT. [MKMOIK XXIV.

2d. The cold wire being sharpened to a point and
plunged deliberately into the mercury to the bottom of
the bath, the deflection was 19°.

3d. But when I touched the surface of the mercury
with the very point of the cold wire, there was a deflec-
tion of 60°.

Having laid a plate of tinned iron upon the surface
of some hot mercury, it was touched with the point of
the cold wire. There was a strong deflection of the
needles in the opposite direction to what would have
been the case had the mercury been touched and not the
iron. The under surface of the iron was, therefore, act-
ing as a hot face, and the parts round the point as a cold
face, being temporarily chilled by the touch of the wire.

These results explain the anomalies observed by some
of those who investigated the course of thermo-electric
currents by means of small metallic fragments.

It Ay^ould therefore seem that when wires of the same
metal are used as electromotors, the quantity of electric-
ity evolved depends on the quantity of caloric that can
be communicated in a given time. Time, therefore, under
these circumstances, must enter as an element of thermo-
electric action. In the case of a single metal, the maxi-
mum effect would be produced when the hot element is
a mass and the cold one a point.

And, lastly, " that the quantities of electricity evolved
in a pile of pairs are directly proportional to the number
of the elements."

In the first trials I made to determine the effect of in-
creasing the number of pairs in a pile, the results ob-
tained were contradictory ; by operating, however, in the
following way, the proposition was at last satisfactorily
determined.

1st. The resistance to conduction was made nearly

MI.M..H: XXIV.] THE ELECTHO-MOTIVE 1'OWKU OF HEAT.

constant by uniting all the pairs intended to be worked
with at once. The current, therefore, whether general. -.1
by one, two, three, four, or more pairs, had always to run
through the same length of wire, and experienced in all
cases a uniform resistance.

iM. By making each individual element of consider-
able length, the liability of transmission from the hot to
the cold extremity was diminished.

Having, therefore, taken six pairs of copper and iron
wires, -fa of an inch thick and each element 38 inches
long, I formed them, by soldering their alternate ends,
into a continuous battery. Then I successively immersed
in boiling water one, two, three, etc., of the extremities,
their length allowing freedom of motion, and the other

*^ O »

extremities not differing perceptibly from the temper-
ature of the room.

The following table exhibits one of this series of ex-
periments :

TABLE IV.

No. of Pairs.

Calculated Deviations.

Observed Deviations.

1

~>~>

r>5

2

110

111

3

M5

165

4

220

220

5

27r>

272

6

no

:w_>

Hence there cannot be any doubt that the quantities
of electricity evolved by compound batteries at the samt<
temperature are directly proportional to the number of
the pairs.

With some general remarks, arising from the forego-
ing subjects, I shall conclude this Memoir.

1. It is of importance to remember that thermo-electric
currents traverse metallic masses only on account of dif-
ferences of temperature existing at different points.

336 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV.

2. When a current of electricity, flowing from the poles
of a battery, is made to traverse a metallic sheet, the
whole of it does not pass in a straight line from one pole
to the other, but diffuses itself through the metal, di-
verging from one point and converging to the other.
The greater part of the current is found, however, to
take the shortest route.

3. Combining, therefore, the foregoing observations
(1, 2), we perceive that there are certain forms of con-
struction which will give to thermo-electric arrangements
peculiar advantages. For example, the surfaces united

by soldering must not be too mass-
ive. Let A, Fig. 55, be a semi- ring
of antimony, and B a semi-ring of
bismuth ; let them be soldered to-
gether along the line C H, and at
the point H let the temperature be
raised : a current is immediately ex-
cited ; but this does not pass around
the ring A B, inasmuch as it finds a
shorter and readier channel through the metals, between
H and d, circulating therefore as indicated by the arrows.
Nor will the whole current pass round the ring until the
temperature of the soldered surface has become uniform.
An obvious improvement in such a combination is
shown at H in Fig. 56, which con-
sists of the former arrangement cut A
out alone: the dotted lines : here the

o

whole current so soon as it exists is
forced to pass along the ring. And
because the mass of metal has been diminished at the
line of junction, such a pair will change
its temperature very quickly.

One of the best forms for a ther mo-
Fig. 57. electric couple is given in Fig. 57, where

XXIV.] THE ELECTRO-MOTIVK l'<>\\ Kit OF HEAT. 337

A is the section of a semieylindrical bar of antimony, B
of one of bismuth, united together by the opposite cor-
iHT.s of a lozenge -shaped piece of copper, C. From its
exposing so much surface, the copper becomes hot and
cold with the greatest promptitude, and from its good
conducting power it may be made very thin without
injury to the current. With a pair of bars three fourths
of an inch thick, and a circular cop-
per plate, as at D, Fig. 58, having both jfSHjS^I&tL
surfaces blackened, I have repeated A
the greater part of those experiments - '\_^

which M. Melloni made with his mul-
tiplier. A platinum wire, as at E, Fig. 59, may sometimes

be very advantageously used.

4. The currents circulating in a steel

magnet are to all appearance perpetual.
pln, og I thought for some time it might be

possible to procure similar perpetual
currents by compound thermo- electric arrangements.
Let a, b, c be wires of three different metals soldered to-
gether so as to form a triangle. Now if these metals
were selected, so that a and b could form a more power-
ful thermo-electric pair than a and c, or b and 6', it might
be expected that at all temperatures an incessant current
would run round the system. Such, however, will not
be found to be the case. In effect, any one of these three
serves simply as a connecting solder to the other two,
and hence no current is excited ; for the ends that have
the third metal between them, although that metal inter-
venes, are under exactly the same condition as the other
ends which are in contact.

Y

338 MICROSCOPIC PHOTOGRAPHY. [MEMOIR XXV.

MEMOIR XXV.

ON MICROSCOPIC PHOTOGRAPHY.

CONTENTS : — Method of making microscopic photographs by condensed
sunlight. — Specimens of the art.

WHEN giving courses of lectures on Physiology in
the University of New York, I found it very desirable
to have photographic representations of various micro-
scopic objects. There are many such objects, costing
much time and trouble in their preparation, which it is
very difficult, if not impossible, to preserve. A photo-
graph is their best substitute.

I used as the sensitive surface daguerreotype plates,
for this was previous to the invention of the collodion
process. The daguerreotype plate is, however, much less
sensitive than the collodion film. My first attempts
were to copy the objects by lamplight, but this was
found to be not sufficiently intense. Even after very
long exposures with low powers the impressions ob-
tained are unsatisfactory.

On employing a beam of sunlight impressions could
be quickly obtained, and by concentrating the beam
with a condensing lens the object could be sufficiently
illuminated to suit any magnifying power. A fine mi-
croscope was used to give the image.

Two difficulties were, however, to be provided for:
1st. If the condensing lens be large, the heat at its focal
point may be so high as to injure or even destroy the
object. 2d. It is not easy to find the chemical focus, for
it does not coincide with the visual one.

MEMOIU XXV.] MICROSCOPIC I'HOTOGKAI'HV. ;;;;;,

But these difficulties may be completely overcome by
causing the illuminating beam to pass through a solu-
tion of sulphate of copper and ammonia. This transmits
all the radiations that affect a photographic surface, but
absorbs all others. With the absorbed rays the heat so
nearly disappears that the most delicate preparation may
be left at the focal point for any length of time without
risk.

The solution of sulphate of copper and ammonia also
enables us to ascertain the focal point with precision.
On receiving the image on the ground glass of a camera,
it is, of course, of a blue color, and when brought to a
sharp focus its photograph will be equally sharp.

The best results are obtained when the blue image
of the sun, formed by the condenser, coincides with the
object.

In Fig. 60, a a is a heliostat mirror reflecting the sun's

Fig. CO.

rays horizontally ; c a condensing lens three inches in
aperture; there are adapted to it different diaphragms,
I, to regulate the quantity of light used, since it is not
well to have too bright an image on the photographic
plate ; d is a glass cell, formed by cutting a circular hole
three inches in diameter in a glass plate a quarter of an
inch thick ; on each face of this plate a flat piece of glass
is laid, thus forming a receptacle in which the sulphate
of copper and ammonia solution may be placed. It
holds the liquid without risk of leakage ; e is the object-
stage of the microscope,/; </, the camera with its ground
glass or sensitive plate.

340 MICROSCOPIC PHOTOGRAPHY. [MEMOIR XXV.

In this manner I made many microscopic daguerreo-
types. Some of them, such as were needed, were subse-
quently cut in wood to serve as illustrations for my
Treatise on " Human Physiology." From that work I
extract the specimens on the opposite page.

When the collodion process was invented, I made
many of these photographs in that way. Some of my
old original daguerreotypes were exhibited in the Cen-
tennial Exposition at Philadelphia, as illustrations of the
early history of the art. Collodion photographs have, of
course, the great advantage over daguerreotypes in this,
that they may be magnified by the lantern and used for
demonstration before a large class.

Hln'nx.KAIMIY.

341

Pig. 61.-SpIracle of insect, magnified T5 diame- Fig. «2.-Elliptic blood-cells of frog, magnified
tere- 250 diameters.

Pig. 63.— Ultimate muscular fibre, magnified 200 Fi
diameters.

ve.8els of bauaDfli milguifled 50
diameters.

Pig. 68.— Woody fibre <>t pine, magnified 60 di-
ameters.

*-— Mucous nieinbrnne ofthe fioniach of a
carnivorous beetle, magnified 00 diameters.

342 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

MEMOIR XXVI.

ON CAPILLARY ATTRACTION AND INTERSTITIAL MOTIONS.

THE CAUSE OF THE FLOW OF SAP IN PLANTS AND

THE CIRCULATION OF THE BLOOD IN ANIMALS.

Extracted and condensed from various Memoirs as follows : On Capillary Attrac-
tion, Franklin Institute Journal, Philadelphia, September, 1834. On the Tidal
Motions of Movable Electric Conductors, Franklin Institute Journal, January,
1836. Experiments on Endosmosis, Franklin Institute Journal, March, 183(5.
On Endosmosis through Water and Soap Bubbles, Franklin Institute Journal, July,
1836. On Interstitial Movements, American Journal of Medical Sciences, May,
1836. On the Physical Theory of Capillary Attraction, American Journal of
Medical Sciences, February, 1 838. On the great Mechanical Force Generated by
the Condensing Action of Tissues, American Journal of Medical Sciences, May,
1838. On the Physical Theory of Endosmosis, American Journal of Medical
Sciences, August, 1838. Analysis of Some Coins, Silliman's American Journal,
VoL^CXIX., 1836. On the Circulation of the Blood, Silliman's American Journal,
Second Series, Vol. II., 1846. On the Constitution of the Atmosphere, London
and Edinburgh Philosophical Magazine, October, 1838. On Capillary Attraction,
London and Edinburgh Philosophical Magazine, March, 1845. On the Circula-
tion of the Blood, London and Edinburgh Philosophical Magazine, March, 1846.
A Treatise on the Forces that Produce the Organization of Plants, New York,
1844. A Treatise on Human Physiology, New York, 1856.

NOTE. — These Memoirs are collectively so voluminous that I could not publish them
in this volume in full. 1 made the following abridgment of them, which was
printed in Harper's Magazine, January, 1878.

CONTENTS: — Interstitial motions of solids. — Motions of metal in coins. —
Movement of liquids in crevices. — Capillary attraction. — Conditions
for a continuous flow. — Capillary attraction an electrical phenomenon.
— Motions of liquid conductors. — Dutrochefs experiment of endosmosis.
— Passage of gases through liquids. — Soap bubbles. — Passage against
heavy pressure. — Distribution of sap in plants. — Circulation in plants
due to sunlight producing gum. — Circulation of blood in animals ex-
plained.— The systemic, the pulmonary, and the portal circulation.

IT is necessary for the life of every organized being
that a liquid should circulate through all its parts. In

MKM..II: XX VI. | CAPILLARY ATTRACTION, KTC. 343

plants that liquid is called the sap; in animals, the
blood.

When the distance through which such a liquid has
to pass is small, there seems to be little difficulty in as-
signing the cause of its movement; but how shall we
explain the rise of the sap in the great sequoia-trees of
California, some of which attain a height of more than
four hundred and thirty feet? To force a column of
water to such an elevation would require a pressure of
more than two hundred pounds on the square inch. Yet
we cannot doubt that the power which overcomes these
enormous resistances is the same as that engaged in the
most insignificant trausudations.

Even in the case of solid mineral substances the parti-
cles are not at rest. Boyle, in his tract on "The Lan-
guid Motions of Bodies," has collected several interesting
instances.

He says : " But what is more extraordinary, a gentle-
man of my acquaintance had a turquois stone wherein
were several spots of different colors, which seemed to
him for many months to move slowly from one part of
the stone to the other. And having the ring wherein it
was set put into my custody, I drew pictures of the
spots at different times ; and by comparing several of the
draughts together, it evidently appeared that they shifted
their places, as if the matter whereof they consisted made
its way through the substance of the stone. And as far
as we observed, the motion of these spots was exceeding-
ly slow and irregular. An experienced jeweller, likewise,
assured me that in a few turquois stones he had ob-
served two different blues in different parts of the same
stone, and that one of these colors would, by slow and
imperceptible degrees, invade and at length overspread
that part of the stone which the other before possessed.
And the same gentleman who lent me the spotted tur-

044 CAPILLARY ATTRACTION, ETC. [.MEMOIR XXVI.

quois also showed me an agate haft of a knife, wherein
was a certain cloud which an ingenious person had for
some years observed to change its place in the stone."

Some years ago, having occasion to make an analysis
of certain Roman silver coins which had been long bur-
ied in the earth, I found that much of the alloying cop-
per had made its way to the surface, constituting the
green patina of antiquarians, and that the silver had be-
come comparatively pure. An interstitial movement in
these denarii must therefore have taken place — a move-
ment so slow that it had required many centuries to
yield the observed result.

If one end of a porous substance, such as a sponge, be
dipped into water, the liquid very soon percolates in all
directions through the mass, which becomes charged with
as much as it can hold. If a piece of glass having a
crack in it be put into water so that the end of the
crackjs immersed, the liquid instantly runs spontaneous-
ly to the other end. And if from the crack other smaller
ones branch forth, along these also the water rapidly
finds its way.

These effects have long been studied by using slender
glass tubes, which operate in the same manner as cracks,
but permit the phenomena to be observed in a more con-
venient and exact manner.

Water will pass through a crevice the width of which
is less than one half of the millionth of an inch. The

proof of this is readily
obtained experimentally.
If we take a convex lens,
Fig'67' a a (Fig. 67), of long fo-

cus, and place it upon a glass plane, b b, there will be seen
at the point of apparent contact, <?, on looking down, a
black spot surrounded by a series of variously colored
concentric circles, the appearance being well known

MKMOIU XXVI.] ( 'A HLLARY ATTRACTION, ETC. 345

among optical writers under the name of Newton's col-
ored rings. At the point c the lens and the plane are,
as Newton has shown, a distance apart of about one half
of the millionth of an inch ; and from this centre, pro-
ceeding outwardly, the distance between the glasses of
course increases. If anywhere at the outer portion a
drop of water be introduced, it extends itself instantly
across all the colored rings, reaching even across the cen-
tral black spot.

If a tube of such diameter that it could lift water ten
inches be broken off so as to be only six inches long, we
might inquire whether the water would overflow from
its top or simply remain suspended there.

Mathematical considerations, as well as direct experi-
ments, prove that in such a case there would be no over-
flow. A capillary tube under these circumstances lifts
the water, but does not produce a continuous current.

But if a removal of the liquid at the top of the tube
take place in any manner, as by evaporation or by being
dissolved in another liquid, a continuous current is pro-
duced.

As illustrating the production of such a continuous
flow we may cite the case of a spirit-lamp, the wick of
which may be regarded as a fagot of capillary tubes. Be-
tween the fibres of the wick there are interspaces that
answer as tubes. If the cover of the lamp be taken off,
all the spirit will eventually pass up the wick and escape
from the reservoir by evaporation. Or, in an oil-lamp,
the wick of which becomes readily saturated with the
oil, but never exhibits an overflow, on the lamp being
lighted, the oil is burned off, a current is established, and
after a time the reservoir is emptied.

The phenomena of capillary tubes are connected with
the adhesion of surfaces. Clairaut showed that if the

346 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

mutual attraction of a solid and liquid amount to less
than half the cohesion of the latter, the liquid will be
depressed in a capillary tube made of the solid ; if it be
equal to half, the liquid will stand level in the tube ; if
it exceed half, the liquid will rise.

I published a paper in the Journal of the Franklin
Institute of Philadelphia for September, 1834, its object
being to show that the adhesion of surfaces, whether
solid or liquid, to each other, and the rise or depression
of liquids in capillary tubes, are strictly electrical phe-
nomena.

When a glass plate is laid on the surface of quicksil-
ver, a considerable force is required to separate them.
On the separation being made, if the substances be ex-
amined by the electroscope, the glass will be found to be
electrified positively, the mercury negatively. Their at-
traction or adhesion is, therefore, a necessary electrical
result**- So intense is this electrical development that if
during the act of separation the mercury be in connec-
tion with a gold-leaf electroscope, the
gold leaves are commonly torn asun-
der. In Fig. 68, a a is a glass dish
containing mercury, c the glass plate,
- 68- d the gold-leaf electroscope.

In like manner, if some melted sulphur be poured into
a conical glass and permitted to solidify, on making the
separation the interior of the glass and the solid sulphur
cone will be found to be in opposite electrical states.
And the same occurs when surfaces of various kinds are
parted from each other. There ought, therefore,
to be adhesion. In Fig. 69, a a is the conical
glass, b the solidified sulphur, with handle for
its removal attached.
pig. 69. ]gut ^ a g}ass plate be laid on a surface of

water, there is no apparent development of electricity on

XXVI.] CAPILLARY ATTRACTION, ETC. ;;j-

separating them. And the reason is obvious, for the
glass has brought away with it a layer of water, and
there has been no true separation of the solid from the
liquid, but only of water from water. The force of ad-
hesion of the glass to the water has exceeded the cohe-
sion of the water for itself.

If a plate of polished zinc be laid on mercury, there
will, again, be no electrical development apparent on
separating them. For, owing to the conductibility of
the zinc, there is nothing to prevent the opposite elec-
tricities from uniting, and all electrical manifestations
must cease.

Whatever can disturb the electrical relations of a
solid and a liquid will disturb their capillarity. On
wetting the interior of a glass tube, so as to form a tem-
porary tube of water, and placing some mercury in it,
the mercury will be depressed below the hydrostatic
level. But on connecting the mercury with the negative
pole of a voltaic battery, and the water with the posi-
tive, the mercury at once rises, their mutual attraction
being increased.

I derived these conclusions from the following experi-
ments :

1. In a watch-glass (Fig. 70) place a quantity of pure
mercury, a b, and upon it a drop of water, c. Bring the
water in contact with the positive
platinum electrode of a voltaic bat-
tery, and touch the mercury with
the negative. The moment the con-
tact is made the drop of water

loses its spherical form and spreads out into a thin cir-
cular disk, wetting the surface of the mercury. The di-
ameter of the disk seems to be greater in proportion as
the battery is more powerful.

Under ordinary circumstances water does not wet

348 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

quicksilver. A drop of water remains on the surface of
quicksilver in the same manner that a drop of oil re-
mains on water. As soon, however, as their electro-
chemical relations are disturbed by the aid of a voltaic
battery, the phenomenon of wetting at once occurs.

2. Take a tube, Fig. 71, in the form of an inverted
siphon, one branch of which, a, is about half an inch
wide, and the other, &, not more than
"* the tenth of an inch. Fill the siphon to
a given height, a b, with mercury; the
metal, of course, does not rise in the nar-
row branch, &, to its hydrostatic level,
for mercury is depressed in a capillary
tube, inasmuch as it cannot wet glass.

r ig. il. o

Introduce a small column of water, b c.
The mercury may now be regarded as being in contact
with a tube of water, because that liquid wets the sides
of the*glass intervening between it and the mercury.

Pass a slender platinum wire, -a?, down the tube, so as
to touch the water ; let it be in communication with the
positive electrode of the voltaic battery ; with the nega-
tive electrode, y, touch the mercury in the wide branch
of the siphon. #, and in an instant the metal will rise in
the narrow tube, and fall again to its former position as
soon as the current is stopped. "

If into a watch-glass fifty or sixty grains of mercury
be poured, and over that as much water acidulated with
sulphuric acid as is sufficient to cover the surface of the
mercury, on the mercury being placed in contact with
the negative pole of a battery, and the water with the
positive, currents are produced both in the water and
the mercury, as was first observed by Erman and Ser-
r nl as.

If the wires be on opposite sides of the mercury, as
shown in Fig. 72, the metal instantaneously elongates,, as

Fig. 73.

MKMOIK XXYI.J CAHI.I-AUY ATI'I; KTi

indicated by the dotted line, and currents also are seeii
playing in the water. If the neg-
ative wire be introduced into the
centre of the metallic globule, and
the positive be brought on one
side, as in Fig. 73, the mercury will bulge out elliptically

at both sides, nearest and farthest
from the positive pole. If, now,
the negative wire be cautiously
raised from its position, so as to
be just out of contact with the surface of the metal, the
mercury is immediately convulsed, its whole suiface be-
ing covered with circular waves. On lowering the neg-
ative wire to its former position and advancing the posi-
tive, the moment it comes to the edge of the mercurial
ellipsoid intense convulsions are produced, which increase
until contact of the mercury and wire takes place.

At the same time that these movements are going on
in the mercury, the surface of the water is ploughed by
gentle currents, exactly resembling those that might be
produced by directing a stream of air
from a blow-pipe slantingly across the
surface (Fig. 74).

The following experiment illustrates
the nature of these effects :

A platinum needle, a c, Fig. 75, is suspended by a
thread of unspun silk from a stand, b bt
in a cup filled with acidulated water as
high as d d. The needle hangs hori-
zontally, its ends being about one fourth
of an inch distant from the platinum
polar wires, j9, n, of a battery. Now the
wire p being positive and n negative,
the extremity a of the suspended nee-
Fig. 75. die would be negative and c positive

Pig. 74.

350 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

by induction. The conjoined effect of the forces thus
brought to bear on the needle causes it to move on its
centre, and take up a position of rest between the polar
wires. To this, if it were turned aside, it would return
after a few slow oscillations.

From this it appears that though the polar wires are
plunged into a conducting medium and the current is
passing, they still act as centres of attraction.

When a globule of mercury under the surface of some
water is brought in presence of a point of attraction, p,
situated at a short distance from its surface, two tides
will be formed on the globule ; one, a,
Fig. 76, directly in front of the point
of attraction, the other, 5, 180° from it.
In the quadrantal regions, c, d, there
will be an ebb. If the point p be
moved round the globule, both tides will follow it, keep-
ing the same relative position that they had at first.
These motions imitate on a small scale the effect which
takes place by the action of the sun and moon in pro-
ducing the tides of the ocean, and the explanation is
the same as in the case of those tides.

If a spring-tide were formed on a spherical ocean, and
the sun and moon then annihilated, the elevation must
sink, pressing the under waters aside, and causing them
to rise where they were depressed. But the motion
would not cease when the level was reached, for the wa-
ter would arrive at that position with an accelerated ve-
locity. It would, therefore, pass that position and form
a high water wrhere it had been low, and low water
where it had been high. And this would be repeated
again and again.

Now this theoretical case may be imitated with the
globule of mercury, for on approaching the positive wire
to it, a position will be reached at which contact will

CAPILLARY ATTRACTION, ETC. ;;;,j

take place between the protuberant tide on the mercury
and the wire. At that moment the cause of attraction
is annihilated, the whole current of electricity now pass-
ing along perfect conductors, and fulfilling the supposed
case of an annihilation of the sun and moon at the time
of high tide. And the same reasoning that held in one
case applies equally in the other — the mercurial tide falls
with an accelerated motion, and the line which before
was the transverse axis of the ellipse becomes the conju-
gate, tides being produced at right angles to the former
ones. But here the strict comparison ends, for as the
mercury ebbs from its protuberant position the metallic
connection breaks, and the wire is again put in action as
a point of attraction. The motion of the ebbing tide is
checked ; it flows once more. Once more the metallic
contact is complete ; and when the tide falls, it is only to
flow again as long as the battery current passes. Tides
take place at right angles to each other, in a series too
rapid to be counted, and the whole surface of the mer-
cury is worked into those various and beautiful undula-
tions which have been before referred to.

With respect to the currents that are observed, as if a
gentle wind were playing over the surface, the explana-
tion is obvious. We have seen that when a voltaic cur-
rent is passed through mercury and water, the pressure
on the surface of contact is changed. Newton has shown
("Principia," Vol. II., Bk. ii., Pr. 41) that if the particles
of a fluid do not lie in a right line, a pressure propagated
through that fluid will not be in a rectilinear direction,
but the particles that are obliquely
posited have a tendency to be urged
out of their position. So the particles
#, 0, Fig. 77, pressing on the particles
£, d, which stand obliquely to them by
reason of the shape of the mass of mercury, M, have a

352 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

tendency to be urged from their places towards e and c
respectively, and the motion thus produced in a fluid
diverges from a rectilinear progress into the unmoved
spaces ; and such a pressure taking place in a liquid free
to move continually returns the moving particles to
their position, after making them describe an elliptical
orbit.

Such is the nature of the evidence that may be brought
forward in proof of the hypothesis that the adhesion of
surfaces and capillary attraction are electrical results.
We may now pass to an exposition and explanation of
the more interesting instances of motion in the cases of
liquids and gases. If, in the lapse of centuries, metallic
copper can part itself from silver with which it has been
alloyed, coming forth from the interior of a coin to form
a new compound on the surface, such movements, it
might be expected, would more readily occur in liquids,
of which the cohesion is but small, and in gases, in which
cohesion perhaps does not exist at all.

And, first, as respects liquids :

The phenomena of endosmosis, first brought to general
notice in the case of liquid substances by M. Dutrochet,
may be explained as follows : If some alcohol be placed
in a bladder, the neck of which is tightly tied, and the
bladder be sunk into a vessel of water, percolation en-
sues, so that the bladder is distended to its utmost
capacity, and may even burst. Or if, instead of tying
the mouth of the bladder, a glass tube, open at both
ends and a foot or two long, be fastened into it without
leakage, as the water introduces itself through the pores
of the bladder to mingle with the alcohol the liquid rises
in the glass tube, and, when it has reached the top of it,
overflows. To express this inward passage of the water
the term endosmosis was introduced ; and since a little of

MI.M..IK XXVI. J CAPILLAKY ATTRACTION, ETC.

78.

the alcohol simultaneously passes outward to mix with
the water, it is said to exhibit exosmosis.

In Fig. 78 is represented the endosmometer of Diitr..
chet. It consists of a small blad-
der, #, tightly tied to a tul »»•,//,
open at both ends, and bent as
seen in the figure at c; the blad-
der being completely filled with
alcohol, and the tube to some
such point as d, the arrangement
is placed in a vessel of water, ee;
almost immediately the level of
the liquid will be seen to be ris-
ing, the bend of the tube is
reached, and one drop after an-
other falls from the open end into the receiver, />. And
this continues until the liquids inside and outside of the
bladder are uniformly commingled.

In these results there is nothing more than should
take place on the ordinary principles of capillary action.
The pores of a bladder are only short capillary tubes,
into which water readily finds its way, because it can
wet the substance surrounding the pores. If the blad-
der be distended with air and sunk under water, al-
though the water will fill the pores, it will not exude
from them and accumulate in the interior of the blad-
der; for, as we have seen, a capillary tube cannot estab-
lish a continued current or flow. But the case becomes
totally different when the bladder is filled with alcohol;
for then as fast as the water presents itself on the inner
end of each pore it is dissolved away by the alcohol, and
the necessary condition for a continuous flow is complied
with. Meantime through the pore itself a little alcohol
passes in the opposite way by infiltrating through tin*
incoming water, provided that the current be not too

354 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

strong, and so the endosmosis of the water and exosmo-
sis of the alcohol take place ; the current of the former
greatly preponderates over that of the latter, and an ac-
cumulation of liquid in the interior of the bladder ensues.

That in all this there is nothing specially dependent
on the organic texture employed is obvious from the fact
that the same results arise when any inorganic porous
body is used. Vessels of unglazed earthenware, pieces
of baked slate or stucco, answer the purpose very well,
as will also a glass vessel with a minute fissure or crack
in it.

An incorrect representation of the conditions under
which endosmosis takes place is often made. It is said
to depend on the relative specific gravities of the liquid,
and that the lighter liquid always moves towards the
denser more abundantly than the denser towards the
lighter. But water endosmoses equally well to alcohol,
which is lighter than it, and to gum-water or salt-water,
which are heavier.

The force with which a liquid will thus pass through
a pore to mingle with another liquid beyond is, as we
shall presently see, very great.

It has sometimes been affirmed that living organic
textures possess a so-called selecting power, permitting
some substances to pass through them and refusing a
passage to others. But this so-called selecting power is
purely physical, as are the separations and apparent de-
compositions to which it gives rise.

If we take a glass tube, a a, Fig. 79, over the lower
end of which a piece of peritoneum or other delicate
membrane, b b, is tightly tied, and half fill it with litmus-
water, and then place it in a glass of alcohol, c c, the lev-
els of the liquids inside and outside being adjusted ac-
cording to their specific gravities, so that there may be

MI.M..IU XXVI.] CAPILLARY ATTRACTION, ETC.

855

Fig. 79.

no hydrostatic pressure either one way or
the other through the pores of the perito-
neum, as soon as the arrangement is com-
pleted, if the observer be so placed as to
view it by transmitted light, he will see
the water descending through the pores of
the peritoneum in striae and streams in a
perfectly colorless state. The membrane
has, therefore, absorbed and transmitted
the water, but has refused to the coloring
matter a passage. Such illustrations may,
therefore, satisfy us that the selecting pow-
er of organic textures, like that of inorganic
ones, is dependent on simple physical cir-
cumstances, and for these reasons we may
exclude from the mechanism of animal ab-
sorption the influence of any vital or other mysterious
principles, and adopt the opinion of Abbe" Hauy, that
"those specious causes and imaginary powers to which
in the Middle Ages all natural phenomena, even those of
an astronomical kind, were referred, but which, through
the genius of Newton and Laplace, have been banished
from the celestial spaces, have taken their last refuge in
the recesses of organic beings, and from these retreats
science is preparing to expel them."

We may next turn to an examination of analogous phe-
nomena in the case of gases. I found that on blowing a
little bubble of melted shellac on the end of a glass tube,
and putting some reddened litmus-water in its interior,
ammoniacal gas, to which it was exposed, could almost
instantly permeate the thin texture or barrier, and, gain-
ing access to the inside of the bubble, turn the litmus
blue.

When the pores of such barriers are of sensible size,

356

CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

Pig. SO.

it is plain that the passing material is influenced by the
substance of which the pore consists only on those por-
tions that come in contact therewith. Through the cen-
tral parts of the pore the material will pass by mere
leakage. If, then, we desire to determine the physical
conditions under which these movements take place, we
must make use of barriers the pores of which are not of
sensible size.

I therefore closed the top of a glass tube, A, Fig. 80,
otherwise open at both ends, with a disk of
paper placed at such a distance down the tube
a as to permit a stratum of water, a a, one eighth
of an inch thick, to be laid upon it. Conveying
the tube to the pneumatic trough, I filled it in
succession writh various gases, and watched their
passage through the water roof, for so it might
be called, into the air. In such an experiment
a column of hydrogen gas half an inch in length
had escaped in twenty-four hours.

But experiments of this kind may be much shortened
by using very thin films instead of a thick stratum. A
glass bell-jar, a a, Fig. 81, was filled with
hydrogen gas, and by the side of it was
placed a small bottle containing atmos-
pheric air. A finger dipped in soap-
water was passed over the mouth of the
bottle, so as to close it with a thin film,
and the glass bell -jar of hydrogen was
then placed over it, as at b. In the course of two min-
utes the film, which was at first horizontal, had become
convex, and eventually swelled into a large spherical
bubble, c. In sixteen minutes it had become so thin
that it was of a dark metallic lustre.

But the action is much more speedy if, instead of these
horizontal films, soap-bubbles are used. Such films are

Fig. 81.

IfttOU \.\VI.] CAPILLARY ATTKAOTION, II'

Fig. 82.

at first too thick, they expose too small a surface to the
atmosphere to which they are subjected, and it is not
until the close of the experiment that the action becoun->
rapid. But a bubble at once exposes a lar^e surthrr.
and by using proper precautions there is no difficulty in
preserving it for an hour, or even much longer.

The quickness of these motions is very well illustrated
by the following experiment. If
a glass flask, a a, be rinsed out
with ammonia, and then by means
of a bent glass tube, b b, narrowed
at its projecting extremity, a soap-
bubble, c, be blown therein, the air
from the bubble being immediate-
ly drawn into the mouth without
a moment's delay, the strong taste
of the ammonia will be perceived.
Or if a rod dipped in hydrochloric
acid be presented to the projecting end of the glass tube,
as the bubble slowly collapsing presses out its contents,
copious white fumes arise. Care must be taken that no
drop of water obstructs the egress of the gas from the
bubble, and that the cork fits the flask with a slight
leakage. This, therefore, shows that va-
pors will pass through barriers having no
proper pores, the transit taking place in-
stantaneously.

I constructed an apparatus (Fig. 83) for
exposing gases to each other with the in-
tervention of a soap-bubble, and subse-
quently measuring and analyzing tlum.
a a is a tin saucer about three inches in
diameter and half an inch deep; into it
water can be poured ; it also serves as a
Fig. SB. platform to support a small bell-glass, b.

358 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

Through its centre at c passes a glass tube,/, one eighth
of an inch in diameter, the upper extremity of which is
cemented into a hole of the same size in a round, thin
piece of copper, d, about half an inch in diameter ; the
other end of the pipe opens into another small bell-glass,
&, through a perforation in its top, the communication be-
ing capable of being cut off by means of a stop-cock, g.
The apparatus is used as follows : The upper bell, being
taken off the platform, is filled with any gas to be tried
— oxygen, for instance — and is placed aside on the shelf
of a pneumatic trough. The lower bell-glass is then filled
with water by depressing it in the trough ; and the stop-
cock being closed, five hundred measures of hydrogen, for
instance, are thrown into it. After seeing that the cop-
per plate d is free from moisture, a drop of water ren-
dered viscid by soap is placed upon it, exactly where the
orifice of the tube f opens. The upper glass containing
the oxygen is now placed upon the tin saucer platform
as in the figure. The lower glass is next depressed in
the pneumatic trough, and as soon as the cock is opened
a bubble of hydrogen containing five hundred measures
expands, the spare oxygen escaping from the edge of the
upper glass through the water in the tin saucer. The
cock is next closed, and the apparatus placed on the
trough shelf as long as the operator desires the experi-
ment to continue. Keeping that position, when the cock
is once more opened the gas passes into the lower bell
until the bubble has entirely collapsed, when the cock is
again closed, the contents of the bubble being now ready
for measurement and analysis. As the gas was passing
from the bubble into the lower bell, the water rose from
the tin saucer into the upper bell, confining the gas that
was outside of the bubble. This, by the common mode
of manipulation, is to be transferred from the tin plat-
form to the shelf of the trough for inspection.

MEXOIU XXVI. J CAPILLAUY A'lTUACTION, ETC. ;V,

By this apparatus it \vas found that one thousand
measures of atmospheric air exposed to atmospheric air
underwent no change either in volume or composition.
The exposure in some cases lasted an hour.

One thousand measures of hydrogen, in the bubble,
were exposed to atmospheric air in the bell. In five
minutes there remained only four hundred and seventy-
two. (It will be understood that the numbers quoted
in this and other succeeding experiments are for the pur-
pose of illustrating the general principle. They change
with the relative proportion of gases inside and outside
of the bubble.)

A reverse action ensues when nitrogen is substituted
for hydrogen. The bubble swells instead of diminish-
ing. Thus one hundred measures of nitrogen in half an
hour became one hundred and seven and a half.

Oxygen decreases in bulk. Two hundred and fifty
measures in ten minutes became one hundred and fifty-
three. This gas passes more rapidly through the bub-
ble than nitrogen.

Carbonic acid passes through the bubble very rapidly.
When five hundred measures were used, the bubble col-
lapsed almost as fast as it had expanded. Under a
water roof half an inch thick and two inches in diame-
ter, five thousand measures escaped into the air in forty-
eight hours. In its place there were found two hundred
measures of atmospheric air, which had passed in the op-
posite direction through the water roof.

By this apparatus it was proved that these motions
through soap-bubbles continue until the gases on both
sides of the bubble have the same chemical composition.

It has long been known that liquids and gases pass
through porous structures though resisted by consider-
able force. Thus, if over the mouth of a cylindrical jar
&, Fisr. 84, a thin sheet of India-rubber be tied, and the

7 O '

360

CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

Fig. 84.

jar exposed to an atmosphere of
carbonic - acid gas, that gas will
force its way through the rubber
and mingle with the atmospheric
air in the jar, the rubber will be
pressed outward, as at b, and even-
tually may be burst. I found
that carbonic acid would thus
force its way against a pressure of
ten pounds on the square inch. If
the conditions be reversed, the jar being filled with car-
bonic acid and then exposed to the atmosphere, the In-
dia-rubber will be depressed as at a, and stretch so as
almost to sink to the bottom. Dr. Mitchell, of Philadel-
phia, has shown that this percolation will take place
though resisted by many inches of mercury.

The same holds good as respects liquids. Thus water
will readily pass through animal membrane to mix with
alcohol against a pressure of fifteen pounds on the square
inch.

In making experiments for determining the effect of
such pressures it is to be borne in mind that there are
certain disturbing circumstances which may vitiate the
results. Among these is that general leakage which
happens through the open pores of all tissues. Thus in
the experiment first referred to (Fig. 78) it might be
supposed that the force with which water passes through
animal membrane into alcohol is not greater than one
atmosphere, whereas in truth it is much more; but as
soon as the pressure within the vessel had amounted to
about one atmosphere, the alcohol escaped from the ves-
sel by general leakage from the whole surface of the
membrane as rapidly as the water entered. It is obvi-
ous that in a pore of sensible size those parts alone of a
passing liquid in contact with its substance are subjected

M.M.K XXVI.] CAPILLARY ATTRACTION, ETC. ;;,,]

to its influence, and those situated in its central regions
are ready to be influenced by any extraneous pressure.

In an experiment made on the passage of ammonia
into atmospheric air through India-rubber, it was found
that, though the passage of the gas was resisted by a
pressure of seventy-five inches of mercury, or upwards of
two atmospheres and a half, it took place apparently as
readily as if no such resistance had been opposed. The
question at once arises, Whence is this powerful impul-
sive force derived ? Clearly not from the action of one
gas on the other. To the porous tissue or barrier alone
we must refer the seat of this power.

It is well known that porous solids of all kinds and
fluids absorb gaseous matter very readily, in volumes
varying according to circumstances. Water, for exam-
ple, absorbs its own volume of carbonic acid, and four
hundred and eighty times its volume of hydrochloric-
acid gas. In the latter case, therefore, an extremely
great condensation takes place. So, too, a fragment of
porous charcoal absorbs nearly ten times its volume of
oxygen, and ninety times its volume of ammonia. These
gases, therefore, exist in the absorbing substance in a
state of very high compression. And the reasoning
which here applies, applies also in the case of two gases
separated by a tissue. If, for example, we separate by a
medium of this kind a certain volume of ammonia from
a like volume of nitrogen gas, though at the outset of
the experiment both the gases might be existing under
the same pressure, this equality would very rapidly be
lost. Ammonia, being absorbed more rapidly than ni-
trogen, would be presented to this latter gas not uiuK-r
an equal pressure, but in a state of great condensation.
Under such circumstances, the transit of a gas is not
analogous to the case in which it flows under common

O 9

pressure into a vacuum, or into another gas ; but the tis-

362 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

sue, continually acting as a perpetual condensing engine,
brings the two media in contact with each other under
extremely different conditions — the one in a compressed
state, but ready to exert the whole of its elastic force ;
the other in a state perhaps little varying from its nor-
mal condition.

Moist membranes and films of water, by reason of
their affinity for gaseous substances and their
consequent condensing action, become the origin
of great mechanical power. I have seen car-
bonic acid pass into atmospheric air through
India-rubber against a pressure of ten atmos-
pheres, and sulphuretted hydrogen against a
pressure of twenty-five atmospheres.

The apparatus with which these results were
obtained may be thus described. A strong
glass tube, a &, Fig. 85, seven inches or more
in length and half an inch in diameter, is her-
metically closed at one end, through which a
pair of platinum wires, I, c, pass to the interior,
parallel but not touching. The other end, a #,
has a lip or rim turned on it. Between the
platinum wires a gauge tube, d, is dropped.
On the top of the gauge tube a small test glass, f, is
placed, to contain a reagent suited to the gas under trial,
as lime-water for carbonic acid, acetate of lead for sul-
phuretted hydrogen, litmus water for sulphurous acid.
Sometimes, instead of this test tube, a piece of paper
soaked in the proper reagent was employed. The large
tube was then filled with water to the height e e, and
over its lip a thin sheet of India-rubber was tightly tied,
and over this again, to give strength, a very stout piece
of silk. Everything being thus arranged, the projecting
wires #, <?, were connected with a voltaic battery ; decom-
position of the water ensued, oxygen and hydrogen be-

MEMOIR XXVI.] CAPILLARY ATTKACTION, ETC. 353

ing disengaged, and a condensed mixture of atmospheric
air and those gases accumulated in the space a a, e e, the
gauge tube showing the extent to which Ihe condensa-
tion had gone. Now if the little cup/ had been filled
previously with lime-water, and the whole arrangement
introduced into a jar of carbonic-acid gas, the upper part
of the lime-water presently became milky, and after a
time a copious precipitate of carbonate of lime subsided.
This would readily take place when the gauge was indi-
cating a pressure of ten atmospheres. In like manner,
when a piece of paper covered with carbonate of lead
had been introduced, and a pressure of twenty-four and
a half atmospheres accumulated, on introducing the in-
strument into a vessel of sulphuretted hydrogen the pa-
per quickly became brown. So sulphuretted hydrogen
can pass through a sheet of India-rubber, and diffuse
into an atmosphere of oxygen, hydrogen, and atmos-
pheric air beyond, though it be resisted by a pressure
equal to that of eight hundred feet of water.

The method of condensation here employed, because
of its freedom from mechanical concussions, enabled me
to continue these researches up to pressures of fifty at-
mospheres, without leakage, in comparatively slender
tubes, and even under these circumstances gaseous diffu-
sion seemed to take place without any restraint.

In passing, it may be remarked that when water is
enclosed hermetically in a vessel, and a voltaic current
passed through it, decomposition ensues, a portion of the
gases making their appearance in the gaseous form, fill-
ing the small space occupied by the decomposed water,
and the remainder being absorbed by that liquid as fast
as it is given off. When the pressure is high, the di-
mensions of the vessel become sensibly greater, and the
little bubble of air accumulated exceeds in bulk the vol-
ume of the decomposed water.

364 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVL

If, as it thus appears, no pressure we can command be
sufficient to restrain one gas from passing into another,
we next inquire what obstacle the condensed gas pre-
sents. There is abundant evidence to show that this
medium bears the same relation to the percolating gas
that a vacuum would do, inasmuch as the rate of dis-
charge into it is the same as it is into a vacuum. If the
particles of different gases possess no repulsive tendency
as respects each other, if the presence of one makes no
difference in, nor produces any retardation in, the par-
ticles of the other, then it is immaterial how many of
such particles are condensed together in a given space.
The vacuum is not less a vacuum because it is contained
under smaller dimensions, any more than a torricellian
vacuum is less perfect when the mercury is made to rise
nearly to the top of a barometer tube than it was when
there was a vacant space several inches in length. This
would therefore indicate that these diffusions will take
place under all pressures, provided the gaseous condition
subsists; and this conclusion is abundantly borne out
by the experiments herein detailed.

The explanation we thus give of the action of con-
densing barriers rests upon a fundamental principle of
dynamics, that when the moving force and the matter
to be moved vary in the same proportion, the resulting
velocity will always be the same. Thus, if a cylinder
filled with air and fitted with a piston communicate with
a vacuum through an aperture, it is immaterial whether
the air be allowed to flow into the void without any
pressure, or whether it be urged by a direct action on
the piston — its velocity, as it goes into the void, will be
the same in both cases; for if it be compressed, the ac-
tion of the piston is to reduce the air to such a density
that its elasticity is equal to the compressing force ; and
because the elasticity varies as the density, the density

Mi M.-.K XXVI.] CAPILLARY ATTRACTION, ETC. 355

of the air increases with the impelling force. The mat-
ter to be moved is increased, therefore, in the same pro-
portion as the pressure, and therefore the final velocity
is the same, and the same takes place in the case of a
tissue which is compressing a gas.

Such is the case while the gases are engaged with
each other in the tissue; but as soon as they pass from
it, the condensed gas, being no longer under its compres-
sion, expands freely, and, when measured, gives a result
differing from that which it would have been had not
the tissue compressed it.

Accordingly, when carbonic acid and air are separated
by a screen of stucco, which absorbs each to a very small
extent, they diffuse, according to the law of the square
roots of their density, one volume of air replacing 0.8091
of carbonic acid, the volume on that side of the screen
where the carbonic acid was increasing in quantity.
But if a thin sheet of India-rubber be used as a screen,
since it can condense one atmosphere of carbonic acid
while it does not act upon the air, though the same rate
of exchange ensues, there is a diminution of the gaseous
matter on the side containing the acid, and one volume
of air replaces 1.6182 of the acid.

In every plant two prominent operations are carried
forward: (1) the production of organic matter; (2) its
distribution through the various parts of the vegetable
system.

In 1844 I published a work under the title of "A
Treatise on the Forces that Produce the Organization of
Plants." It was a monograph chiefly devoted to the il-
lustration or investigation of those operations, and com-
bated the existence of the Vital Force of physiologists
as a homogeneous and distinct power.

The progress of science shows plainly that living

366 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

structures, far from being the products of one such homo-
geneous power, are rather the resultants of the action
of a multitude of material forces. Gravity, cohesion,
elasticity, the agency of the imponderables, and all other
powers which operate both on masses and atoms, are
called into action, and hence it is that the very evolution
of a living form depends on the condition that all these
various agents conspire. Organized beings and organ-
ized bodies spring forth in those positions only to which
the rays of the sun have access. They are, therefore,
limited, to the atmosphere, the sea, and the surface of the
earth.

If we expose some spring -water to the sunshine,
though it may have been clear and transparent at first,
it presently begins to assume a greenish tint, and after a
while flocks of green matter collect on the sides of the
vessel in which it is contained. On these flocks, when-
ever the sun is shining, bubbles of gas may be seen,
which, if collected, prove to be a mixture of oxygen and
nitrogen, their proportion being variable. Meantime
the green matter rapidly grows, its new parts, as they
are developed, being all day long covered with air- bells,
wrhich disappear as soon as the sun is set. If these ob-
servations be made on a stream of water the current of
which runs slowly, it will be found that the green mat-
ter serves as food for thousands of aquatic insects which
make their habitations in it. These insects are indued
with powers of rapid locomotion, and possess a highly
organized structure. In their turn they fall a prey to
the fishes which frequent such streams.

Thus by the influence of the sunlight organic matter
is added to vegetable systems, the action being accom-
panied by a variety of chemical decompositions and in-
terstitial diffusions. The substances arising are such as
are necessary for the uses of the plant ; and in order to

MKM..IK XXVI.] CAPILLARY ATTRACTION, ETC.

distribute them, mechanical motion has to take place.
This, in the more highly organized plants, goes under
the designation of the flow of the sap.

The flow of the sap in plants and the circulation of
the blood in animals are probably due to the same
physical cause. And bringing into view the experi-
ments already related respecting capillary attraction, I
considered, in the work above referred to, the conditions
necessary for producing a continual flow, such as evap-
oration, decomposition, solution, developing the general
law of those movements, and illustrating the great force
with which they are accomplished. I showed that these
motions depend on this physical principle : " That if two
liquids communicate with one another in a capillary
tube, or in a porous or parenchymatous structure, and
have for that tube or structure different chemical affin-
ities, movement will ensue; that liquid which has the
most energetic affinity will move with the greatest veloc-
ity, and may even diive the other liquid entirely before
it ;" and that this is due to common capillary attraction,
which, in its turn, is due to electric excitement.

Applying this principle to the case of plants, the
liquid of which the ascending sap is constituted is de-
lived from the ground by the action of the spongioles,
and consists of water holding in solution the different
saline bodies necessary to the plant, along with carbonic
acid, etc. This passes upwards by the woody fibre and
ducts of the alburnum, making its way to the leaf, on
the upper surface of which, in common cases, a change in
its chemical constitution occurs through the influence of
the sunlight. It obtains a quantity of carbon. This
elaborated sap, or latex, now returns to the bark, and
descends through its cellular tissue and intercellular

O

spaces, finding its way by the route of the medullary
rays to all parts of the plant. During its descent the

3(58 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

different vegetable principles necessary for the economy
of the plant are removed from it, and a certain quantity
goes down to the roots, partly to aid in their growth,
and partly to throw new quantities of ascending sap
into the tree. In this descent the elaborated sap moves
through a system of vessels which anastomose with one
another in the same manner as the capillary vessels of
animals.

There are, therefore, two points in this circulation
which require attentive consideration — the spongiole
and the leaf. The spongioles are nothing but the young
succulent extremities of the roots, which have been re-
cently formed from portions of the descending sap, and
that sap is itself a species of mucilaginous solution. Pre-
cisely, therefore, as water will pass through the tissue of
a bladder the interior of which is filled with gum-water,
so will moisture from the ground flow through the spon-
giole. There is no difficulty in thus accounting for the
rise of the ascending sap on the principles of capillary
attraction, arid indeed this is the explanation generally
received by vegetable physiologists.

Guided by the principle above laid down, I offered
the following as an explanation of the action of the leaf.
The ascending sap, which we may assume to be a weak,
watery solution, rises to the upper face of the leaf. It
there obtains carbonic acid from the air ; of this the sun-
light effects the decomposition, with the production of
gum, the result being a change from water to a muci-
laginous solution. In the tissue of the leaf we have,
therefore, two liquids engaged — water and a mucilagi-
nous solution. On the principle above indicated, the
water will drive the mucilaginous solution before it, and

O

force it back along its proper vessels into the stem.

What, then, is the reason that the light of the sun
controls the rapidity with which the ascending current

MI.M..IK XXVI.] CAl'lLLAHY ATTKACT1ON, ETC.

comes ? Because it controls the amount of carbonic acid
reduced, and, therefore, the amount of elaborated sap
formed. Why is it that the upward flow diminishes
when changes are befalling the leaves, and why does it
stop in the winter ? Because the mucilaginous solution
made by light diminishes in quantity, or ceases to be
formed altogether.

There are, therefore, two sources of force in a flower-
ing plant — the spongiole and the leaf; and they derive
their power from ordinary physical principles. What-
ever has been said respecting the movements of sap in
exogenous plants applies also to the case of endogenous,
and indeed to flowerless plants too.

It has been clearly established by the researches of
comparative anatomists that the presence of a circula-
tory mechanism is determined by the centralization of
the nutritive and respiratory apparatus. In exogenous
and endogenous plants, from the circumstance that liquid
and solid materials are introduced at distant points, chan-
nels of communication from one to the other, and indeed
to every part, are required, and hence the introduction
of a circulatory apparatus. In the lower tribes of vege-
table life, where the separation of function does not ex-
ist, the circulatory mechanism is correspondingly absent.
Sea-weeds absorb on their whole surface, and nutrition
is directly carried forward at the points of reception.
In lichens there is the first appearance of a transfusory
mechanism, arising from the circumstance that on those
parts which are shaded from the light, absorption most
rapidly takes place : here, probably, however, the chan-
nels of movement are the interspaces between the cells,
and the cause simple capillary attraction. In mush-
rooms there is a closer approximation to the mechanism
more fully developed in the higher plants, for in them
the rootlets absorb nutrient matter from the soil, from

AA

370 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

which it passes by capillary action to every part of the
system.

The cause of the movement of the sap in flowering
plants, both of the rise of the crude sap and the descent
of the elaborated sap, is the light of the sun, which ef-
fects the decomposition of carbonic acid.

From this explanation of the causes of the movement
of sap in plants, I turn to the circulation of the blood in
animals.

In man there are three chief circulations — the sys-
temic, the pulmonary, the portal. Bearing the above-
mentioned general principle in mind, I presented the
following explanation of these circulations:

1. The Systemic. — The arterial blood, which moves
along the various aortic branches, and is distributed to
every part of the system, contains oxygen which it has
received during its passage through the lungs. Its color
is crimson. As soon as it has reached its destination in
the minute capillary vessels, it begins to carry on its
proper process of oxidation, attacking in a measured
way the tissues through which it is flowing. The di-
rect result of this operation is an evolution of heat.
But while this chemical change in the tissues is going;

o o o

forward, the arterial blood itself is also suffering a
change in giving up its oxygen and gaining in exchange
the results of combustion. From being crimson, it turns
dark ; from being arterial, it changes into venous blood.
Now, under these circumstances, what must take place
in every capillary or each small portion of a porous struct-
ure ? On the arterial side, we have the crimson arterial
blood ; on the venous side, dark venous blood — two dif-
ferent liquids. What, then, is the relation that obtains
between each of these liquids and the walls of the tube,
or the substance of the parenchyma in which they are

MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 371

placed ? Must it not be that the arterial blood bearing
its oxygen has an intense affinity for those structures,
but, those affinities being satisfied, that which was arte-
rial passes into the condition of venous blood ? The af-
finities it had for the structures with which it was in
contact are satisfied and have come to an end. The
arterial blood presented a highly energetic force, which
in the venous has diminished to zero. Under these cir-
cumstances, in accordance with the general principle, the
arterial blood must press the venous blood before it, and
the flow must be from the artery to the vein.

2. The Pulmonary. — In this the venous blood pre-
sents itself in the air-cells to receive oxygen. The sys-
temic circulation deoxidized arterial blood, the pulmo-
nary oxidizes venous. The latter, therefore, is the con-
verse of the former. The venous blood has an affinity
for the oxygen dissolved in the tissues with which it is
in contact, and the arterial blood has none. Movement,
therefore, must ensue; but as the conditions of the affin-
ity are reversed, so also is the direction of the motion,
for now the venous blood drives the arterial before it,
and drives it to the heart.

3. The Portal Circulation. — In this the same physical
principles apply. The blood which flows towards the
liver along the portal vein has been obtained by that
vein from the chylopoietic viscera; it has, therefore, the
same relation to the blood furnished from the different
and corresponding aortic branches as has the general
systemic venous blood. The arterial blood, therefore,
drives it before it in the same way that the general sys-
temic circulation takes place ; and, passing along the por-
tal vein, it is now distributed to the liver. In this or-
gan it also receives the blood which has been brought
by the hepatic artery.

The process of biliary secretion now takes place, and

CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

compounds of carbon and hydrogen with soda are sepa-
rated as bile, and pass along the biliary tubes. In its
final effect, therefore, the chemical action of the liver close-
ly resembles the chemical action of the lungs. Compared
with the blood which passes along the branches of the
hepatic veins and finds its way into the ascending vena
cava, the portal blood differs by containing the elements
of bile.

Two systems of forces now conspire to drive the por-
tal blood out of the liver into the ascending cava.

First, the blood which is coming along the capillary
portal veins and that which is receding by the hepatic
veins, compared together as to their affinities for the
substance of the liver, obviously have this relation : the
portal blood is acted upon by the liver, and there are
separated from it the constituents of the bile ; the affin-
ities that have been at work in producing the result
have all been satisfied ; and the residual blood, over
which the liver can exert no action, constitutes that
which passes into the hepatic veins. Between the por-
tal blood and the substance of the liver there is an ener-
getic affinity, indicated by the circumstance that a chem-
ical decomposition takes place and bile is separated ; but,
that change once completed, the residue, which is no
longer acted upon, forms the venous blood of the he-
patic veins, and hence the portal blood drives before it
the inert blood which is in those veins.

But, in addition to this, the blood of the hepatic ar-
tery, after serving for the economic purposes of the liver,
is thrown into the portal plexus. Hence arises a second
force, which, conspiring in its resultant with the former,
produces movement in the same direction. The pres-
ence of the arterial blood in the hepatic capillaries is not
only sufficient to give a force towards that in the capil-
laries of the portal veins, but also to give it a pressure

MI.M..M: XXVI.J < 'A PI LI.A HY ATTRACTION, I

towards that in the hepatic veins. No regurgitation can
take place backwards through the portal vein upon the
blood arising from the chylopoietic viscera, because along
that channel there is a pressure propagated in the oppo-
site direction arising from the arterial blood of the aortic

O

branches. This pressure conspires with that of the por-
tal blood, and both together join in giving rise to mo-
tion towards the ascending cava.

On the same principle we may explain the circulation
of the blood in other types of life ; for example, in the
case of the model adopted in fishes, the aorta of which
has long been recognized as bearing a strong resem-
blance to the portal vein of the mammalia. To any one,
however, who reflects on the principles here laid down,
there will arise no difficulty in explaining the circula-
tion in any particular case, if this plain precept be con-
stantly kept in mind : that, in consequence of the phys-
ical principle which has been assigned, a pressure will
always be exerted by the liquid which is ready to un-
dergo a change upon that which has already undergone
it — a pressure which, as there is no force to resist it, will
always give rise to motion in a direction from the chang-
ing to the changed liquid.

I then, in the work referred to and in my " Physiol-
ogy," continued the investigation to a determination of
the uses and action of the heart (heretofore considered
as the sole cause of the circulation), the action in as-
phyxia, the case of obstructed trachea, local inflamma-
tion, etc.

By regarding the affinity between the blood and the
tissues with wl>ich it is in contact as the primary cause
of the circulation, we assign a reason for those various
phenomena which cannot be accounted for by Harvey's
doctrine: the motions in the embryo; the periodic and
local variations ; the portal circulation ; the changes in

374 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI.

the current as seen under the microscope ; the movement
in the capillaries after the heart is cut out; the empty
condition of the arteries after death ; the phenomena of
acardiac monsters ; local inflammations and congestions ;
the gangrene of parts while their capillaries are pervi-
ous; the retardation of the current on the application
of cold or of carbonic -acid gas; the results of asphyxia
and death by drowning or hanging; the changes of press-
ure in the arteries and v?eins respectively during a check
on the respiration; the vis a tergo of the veins; the ef-
fects of a ligature on those vessels; the action of irres-
pirable gases when breathed, and the opposite condi-
tions when oxygen gas or protoxide of nitrogen is used.
A doctrine which accounts with simplicity for such a
long list of miscellaneous facts commends itself to our
attention at once. There are, however, considerations of
a still weightier character which must compel us to
adopt it. The affinity between the blood and the parts
with which it is in contact is a chemical fact beyond
contradiction. The pressures and motions I have been
speaking of follow as the inevitable consequences of that
affinity. We therefore cannot gainsay their existence in
the living mechanism, and the only doubt we can enter-
tain is as to whether they are of competent power to
produce all the effects before us. But after what has
already been said respecting the energy of endosmotic
movements against pressures of many atmospheres, we
may abandon those doubts; and since we have here a
force of universality enough and intensity enough, and
in every instance acting in the right direction, it would
be unphilosophical to look farther, since such a force
must, under these conditions, exist in the physical neces-
sity of the case.

MKMOIII XXVII.] THE EFFECTS OF ALLOTROPISM. 375

MEMOIR XXVII.

ON THE EXISTENCE AND EFFECTS OF ALLOTROPISM ON
THE CONSTITUENTS OF LIVING BEINGS.

From the Philosophical Magazine, April, 1849.

CONTENTS : — Allotropism of elementary substances. — Frankenheim '* no-
menclature.— Brought about by light, heat, electricity, and by the ner-
vous principle. — Explanation of inflammation and congestion.

IT has been completely established for the majority
of elementary substances that there are several forms
under which each may occur — forms differing entirely
both in their physical and chemical relations.

Thus, in the case of carbon, many such forms are
known. To three of them Berzelius has directed at-
tention: 1st, ordinary charcoal; 2d, plumbago; 3d, dia-
mond. They are three distinct modifications of the same
element. They differ in specific gravity, in specific heat,
and in conducting power, both for electricity and caloric.
In their relations to light, the first perfectly absorbs it,
and is black ; the second reflects it like a metal ; the
third is transparent like glass. When crystallized, plum-
bago and diamond do not belong to the same system:
their chemical relations are also strikingly different.
Charcoal takes fire with facility, and some varieties of it
are even spontaneously combustible in the air; but cru-
cibles and furnaces are made of plumbago because of its
incombustibility ; and the diamond with difficulty is set
on fire in pure oxygen gas.

It seems immaterial to what class elementary bodies
belong, whether electro -negative or positive: they pre-

376 THE EFFECTS OF ALLOTROPISM. [MEMOIR XXVII.

sent analogous results. Silicon, sulphur, selenium, phos-
phorus, titanium, chromium, uranium, tin, iridium, osmi-
um, copper, nickel, cobalt, iron, oxygen, chlorine, are cases
in point; and the instances which appear as exceptions
are rapidly diminishing in number.

As is well known, to these singular modifications
Berzelius gave the designation of allotropic forms, and
the whole phenomenon passes conveniently under the
designation of allotropism. He shows that the peculiar-
ity assumed is often of such a persistent nature that it is
not lost, even though the substance affected should go
into combination with others. Thus, there are two forms
of silicon — one combustible, and the other remarkably
incombustible. Each, by uniting with oxygen, gives rise
to silicic acid ; the acid in one case beino- soluble in

' O

water and in hydrochloric acid, and in the other the re-
verse. And, in like manner, metallic arsenic, which ex-
hibits the same duality of condition, gives rise to two
different arsenious acids. Of phosphorus there are at
least two modifications ; and, accordingly, we have two
compounds of that body with hydrogen, one of which is
spontaneously inflammable, and the other not ; and at
least two oxygen acids, the monobasic and tribasic, in
which the essential difference rests in the state of the
phosphorus they contain.

It is to be remarked that, so far as observation ex-
tends, the most common cause of producing these singu-
lar differences is the action of that class of agents which
we term imponderable substances. In very many cases,
change of temperature brings about allotropic change;
in others it is the agency of light, as in chlorine and
phosphorus ; and again, in others, association with for-
eign bodies, which apparently establish new voltaic rela-
tions. Heat, light, and electricity seem to be the general
modifying agents.

.MIM..IU xxvn. i TIII: EFFECTS OF ALLOTKOHSM. 377

Berzelius, following the suggestion of Frankenheim,
proposes a nomenclature for pointing out the peculiar
form referred to in any special case. It depends on the
use of Greek letters. Thus, we have the three forms
of carbon just alluded to designated on these principles
by Co, Cj3, Cy. But in a Memoir republished in this
work, p. 285, it is remarked that we may often with
greater convenience use the simple expressions "active"
and "passive." Thus, active chlorine is that which will
decompose water in the dark, passive chlorine failing to
do so. In this Memoir the same expressions will be em-
ployed.

Hitherto, allotropism has only been considered as af-
fecting inorganic states of matter, but its influence can
be plainly traced in the far more interesting case of or-
ganic beings ; and this, when placed in a proper point
of view, yields a remarkable explanation of some of the
most obscure but important facts in physiology and
pathology. These explanations I propose now to point
out.

In the Philosophical Magazine (March, 1846, p. 178)
there is a paper by me explanatory of the causes of the
circulation of the blood in the capillary vessels. It is
merely an abridgment of a lecture which for eight years
past has been delivered in this university. The doctrine
there set forth has been generally received in America,
and introduced into some of the standard works on
physiology published in England. The principle on
which it essentially depends, and which has been abun-
dantly confirmed by direct experiment, is briefly this —
that if there be twro fluids occupying a capillary tube, or
a porous structure of any kind, under the condition that
one of them has a stronger chemical affinity for the sub-
stance of that tube or structure than the other, a move-
ment of the liquids will at once ensue, that which has

378 THE EFFECTS OF ALLOTROPISM. [Moicm XXVII.

the stronger affinity driving the other before it. On this
principle a clear account of the systemic circulation of
animals may be given ; for the arterial blood, an oxidiz-
ing liquid, having a stronger affinity for the soft tissues
with which it is in contact than the venous blood, the
affinities of which have been satisfied, and therefore no
longer exist, necessarily exerts such a pressure that mo-
tion must ensue, the arterial blood forcing the venous
before it.

An application of the same principles shows that in
the pulmonary circulation the motions must necessarily
be in the opposite direction, or from the venous to the
arterial side, as is actually the case. It also explains
clearly the conditions of the portal circulation, in which
the direct action of the heart could hardly be expected
to be felt. With the generality which ought to belong
to a true theory, it meets all the cases which occur in the
lower orders of animal life, such as the greater circula-
tion in fishes, in which there is no systemic heart ; the
movements which take place in the vascular system of
insects ; and even the extreme case of the rise and de-
scent of sap in plants.

In this doctrine everything depends on the relation-
ship between the nutritive fluid, or blood, and the solid
parts with which it is brought in contact ; and whatever
changes that relationship must impress a corresponding
change on the circulation itself.

From experiments which I made some time ago, I have
been led to suppose that the arterialization of the blood,
as it takes place on the cell-walls of the lungs, bears a
strong analogy to the oxidation of white indigo. The
loose hold which the coloring matter of the blood retains
on the oxygen, coupled but not combined with it, is not
unlike what is witnessed in other nitrogenized coloring
matters, such as indigo, which oxidizes and deoxidizes

M; MO.U XXV11.J TilK EFFECTS OF ALLOTliOl'lSM. 379

with the utmost facility. Charged with the oxygen it
has thus obtained, the arterial blood passes to all parts
of the system; and now arises that striking but all-im-
portant physiological fact, that it does not attack indis-
criminately all those parts of the soft solids which it first
encounters, but, proceeding in a measured way, exerts its
action on such particles alone as have become effete, and,
accomplishing the great process of interstitial death, re-
solves those particles into other forms, so that they can
be eliminated from the system by the lungs, the kidneys,
and the skin.

Now why is it that things proceed in this way ? What
is it that regulates this interstitial death ? Why is one
atom preserved and another surrendered ?

It is upon these obscure points that the recent discov-
eries in allotropism shed a flood of light.

The three leading neutral uitrogeuized bodies, fibrine,
albumen, and caseiue, are characterized by exhibiting al-
lotropism in a most remarkable degree, and that in a
double sense. 1st. Though so different from one another
in their physical and chemical relations, it is admitted
on all hands that they are mutually convertible : the al-
bumen of the egg, during incubation, gives rise to fibrine
and other allied bodies; from caseine in the milk with
which the young mammalia are nourished, the albumi-
nous and fibrinous constituents of their systems arise; the
nurse fed on fibrine and albumen secretes caseine from
the mammary gland. Indeed, there is no more reason
to regard these three bodies as essentially distinct sub-
stances than there is to apply the same conclusion to
charcoal, plumbago, and diamond. Between the two
cases there is the most complete analogy ; and if char-
coal, plumbago, and diamond are merely allotropic forms
of one substance, the same holds good for fibrine, albu-
men, and caseine. But, 2d, each of these three com-

380 THE EFFECTS OF ALLOTROPISM. [MEMOIR XXVII.

pounds betrays a disposition under trivial causes to as-
sume new forms; as with silicic acid so with fibrine,
there are two varieties — one soluble in water, the other
not. A difference of a few degrees of heat turns trans-
parent albumen into the porcellanous variety, and analo-
gous observations might be made respecting caseine.

It may therefore be asserted that these proteine bod-
ies exhibit a propensity to allotropism in a far more re-
markable manner than any other substances known ; not
only passing indiscriminately into one another, but also
exhibiting special variations under the influence of the
most trivial causes.

And now we may recall the fact that, of the agents
which in the inorganic kingdom bring about these
changes, the so-called imponderable principles are pre-
eminent. I transfer this observation to the case of or-
ganized beings, and infer that the nervous system has
the power of throwing organized atoms into the active
or passive state; that this is the fundamental fact on
which all the laws of interstitial death depend ; and that
upon this principle — its existing allotropic condition—
an organized molecule either submits to the oxidizing
influence of arterial blood, or successfully resists that
action.

But it has been stated that there are certain patho-
logical conditions which upon these views meet with
a clear explanation — conditions which, though long and
laboriously studied by physicians, remain involved in
contradictions and obscurity. The conditions to which
I refer are those known as inflammation and congestion.

It is agreed among chemists that during the preva-
lence of these conditions the urine assumes a peculiar
constitution. In inflammatory actions the relative quan-
tity of urea and sulphuric acid is much above the nor-
mal standard, while in congestive cases the reverse holds

Mi lion XXVII.] TI1K EFFECTS OF ALLOTROl'ISM.

good, and the urea and sulphuric acid are below the
standard. What is the interpretation of these remark-
able facts? We shall find they are very significant.

The quantity of urea and sulphuric acid in the urine
undoubtedly expresses the quantity of proteine matter
that has undergone oxidation in the system. In all
cases where that quantity is above the normal standard,
the destruction of proteine matter has been correspond-
ingly accelerated ; and where it is deficient, the destruc-
tion has been reduced. The result of inflammations cor-
responds to the first of these cases, and of congestions to
the second.

Recalling now what has been said respecting the cause
of the capillary circulation, we see how all these appar-
ently disconnected facts group themselves together in
the attitude of dependent effects. In inflammation there
has been that allotropic change in the soft solids in-
volved that they have assumed a disposition for rapid
oxidation — they are active. Their relations with arterial
blood have become highly exalted ; and the blood flows,
on the principles I have set forth, to the affected part
with energy. Redness of that part and a higher temper-
ature are the result. Oxidation goes on with prompti-
tude, and urea and sulphuric acid begin to accumulate
in the urine.

But in congestive cases it is the reverse; the parts af-
fected are thrown into a more passive state. Oxidation
goes on in a reluctant way, the amount of tissue meta-
morphosed diminishes, the urea and sulphuric acid di-
minish in the urine ; and, on the principles which I have
endeavored to explain respecting the capillary circula-
tion, we perceive that an immediate action must be ex-
erted on the flow of the blood : the passive condition of
the tissues and diminished capacity for oxidation re-
strain the flow from the arteries, and, there being now

382

THE EFFECTS OF ALLOTROPISM. [MEMOIR XXVII.

less pressure on the contents of the veins, engorgement
of those vessels is the result ; and this condition of
things is what a physician designates as congestion.

In this manner, if we admit the existence of allotro-
pism in organic atoms, we can give a very clear explana-
tion of the condition of the circulation in the patholog-
ical states of inflammation and congestion, and also of

O

the peculiarities which in those states belong to the con-
stitution of the urine.

UNIVERSITY OF NEW YORK, Feb. 17, 1849.

MI.M..IK XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 333

MEMOIR XXVIII.

ON THE DISTRIBUTION OF HEAT IN THE SPECTRUM.

From the American Journal of Science and Arts for 1872, Third Series, Vol. CIV.,
No. 1G1 ; Philosophical Magazine, August, 1872.

CONTENTS : — Early experiments seeming to prove that the maximum of
heat is in the less refrangible spaces. — Comparison of the dispersion
and diffraction spectra. — Effect of compression in the less refrangible
regions and of dilatation in the more refrangible. — Measure of heat in
the two halves of the visible dispersion spectrum. — Description of the
apparatus employed. — The different colored spaces are equally warm.

MANY experimenters, at various times, have occupied
themselves with the problem of the Distribution of Heat
in the Spectrum. At first it was supposed that there is
a coincidence between the luminous and calorific radia-
tions, and that the maximum of intensity in both occurs
at the same point — that is, in the yellow space. This
view was abandoned on the publication of the well-
known experiments of Sir W. Herschel, who showed that
in certain cases the maximum is below the red. Subse-
quently Melloni, having discovered the singular heat-
transparency of rock-salt, proved that when a prism of
that substance is used, the maximum in question is as far
below the red as the red is below the yellow ; but that if
the light has passed through flint-glass, the maximum ap-
proaches the red; if through crown-glass, it passes into
the red ; if through water or alcohol, it enters the yellow.

In the case of the sun's spectrum the distribution of
heat was more closely examined by Professor Miiller,
whose results in a general manner confirmed the views
then held, that the invisible radiation below the red

384 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

greatly exceeds that in the visible spectrum; and still
more recently Dr. Tyndall, examining the spectrum of
the electric light through rock - salt, showed that the
curve indicating the distribution " in the region of the
dark rays beneath the red shoots suddenly upward in a
steep arid massive peak, a kind of Matterhorn of heat,
which quite dwarfs by its magnitude the portion of the
diagram representing the visible radiation." These in-
vestigations were made under unexceptionable circum-
stances ; the beam of electric light had practically under-
gone no atmospheric absorption, and the optical refract-
ing train was of rock-salt.

Sir J. Herschel had shown in 1840 that when the
sun's rays are dispersed by a flint-glass prism, the dis-
tribution of the heat towards the less refrangible region
is not continuous, but there are three maximum points.
These points, as shown by Dr. Tyndall, do not exist in
the spectrum of electric light, the decline of which is
continuous; they are therefore to be attributed to the
absorptive disturbance which the sun's rays have under-
gone. Quite recently (1871), M. Lamansky has succeed-
ed in identifying these interruptions by the aid of the
thermo- multiplier. In his Memoir he states that, with
the exception of Foucault and Fizeau, in their well-
known experiments on the interference of heat, no one
has made reference to these lines, and that all experi-
menters describe the heat -curve as a continuous one
(Philosophical Magazine, April, 1872).

I may therefore be excused for remarking at this point
that the three lines in question were not only observed
by me nearly thirty years ago, but that an engraving
of them was published in that journal, in a memoir an-
nouncing the discovery of fixed lines in the invisible
portions of the spectrum (May, 1843). It will be seen,
from an inspection of that engraving, that these lines

M..M ...it XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 335

are marked a, /5, 7, They were impressed on daguerre-
otype plates by resorting to the well-known processes
for obtaining photographs of the less refrangible regions
of the spectrum. The paper is republished in the pres-
ent volume, Memoir III.

In view of the preceding statement and others that
might be given, it may, I think, be affirmed that the
general opinion held at the present day as to the consti-
tution of the spectrum is this, that there exists a heat
spectrum in the less refrangible regions, a light spectrum
in the intermediate, and a spectrum producing chemical
action in the more refrangible regions. An experimental
attempt to correct this view, and to introduce a more ac-
curate interpretation, will not be without interest, espe-
cially as it is necessarily and directly connected with the
important subject of photometry. In this Memoir I shall
offer some experiments and suggestions respecting the
heat of the spectrum, and in another, shortly to be pub-
lished, shall consider the distribution of the so-called
chemical rays. Among the numerous problems of actiuo-
chemistry, there are none more important than these.

All the experiments hitherto made on the heat of the
spectrum have been conducted on the principle of ex-
posing a thermometer in the differently colored spaces.
Such was Sir W. Herschel's method. Leslie used a dif-
ferential with small bulbs. Melloni, Muller, Tyndall, a
thermo-electric pile, the form preferred being the linear.
This was advanced successively through all the radia-
tions, and the deflections of the multiplier noted.

Is not this method essentially defective ? Does it not
necessarily lead to incorrect results?

" There is an inherent defect in the prismatic spectrum
—a defect originating in the very cause which gives rise
to that spectrum itself — unequal refrangibility. Of two
groups of rays compared together, one taken in the red,

BB

386 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

the other in the violet region, it is clear that in the same
spectrum, from the very circumstance of their greater re-
frangibility, those in the violet will be relatively more
separated from each other than those in the red. The
result of this increased separation in the more refrangi-
ble regions is to give an apparent dilatation to them,
while the less refrangible are concentrated. The relative
position of the colors must also vary : the fixed lines
must be placed at distances greater than their true dis-
tances as the violet end is approached." I am quoting
from the fifth chapter of a work " On the Forces which
Produce the Organization of Plants," published by me in
1844. In this chapter, one of the chief points insisted on
is the necessity of using wave-lengths in the measure-
ment and discussion of spectrum results — a suggestion
which, I believe, I was the first to make, and which I re-
newed in a memoir in the Philosophical Magazine (June,
1845).

The importance of these remarks respecting the pecul-
iarities of the prismatic or dispersion spectrum may per-
haps be most satisfactorily recognized on examining such
a spectrum by the side of a diffraction or interference
one. By the aid of Fig. 86 this may be done.

Regarding the space between the fixed lines D and E
as representing the central region, in each the fixed lines

D and E are made

A BC D E F G H

coincident. The

GH

other lines are
laid off in the
prismatic as they

Fig. 86. , -i

appear through

the flint-glass prism of the spectroscope; those of the
diffraction are arranged according to their wave-lengths.
It thus appears that in the prismatic, from the fixed line
D to A, the yellow, orange, and red regions occupy but

MI.M..IU XXV11I.J DJ8TBIBUTION OF HEAT1N THE Sl'KcntUlt

little more than half the space they do in the diffraction ;
while the green, blue, indigo, and violet, from the fixed
line E to H, occupy nearly double the space in the pris-
matic that they do in the diffraction spectrum. The gen-
eral result is, that in the prismatic the less refrangible
regions are much compressed, and the more refrangible
much dilated. And it is plain that the same will hold
good in a still greater degree for any invisible rays that
are below the red and above the violet respectively.

Now, if a thermometer of any kind were carried in
succession from the greatly dilated, more refrangible re-
gions to the greatly condensed, less refrangible, could the
measures obtained be accepted as expressing the true
distribution ? The thermometric surface being invariable,
would it not receive in the less refrangible spaces more
than its proper amount of heat, and in the more refrangi-
ble less than its proper amount ?

If we should admit that the distribution of heat in a
correctly formed spectrum is uniform, it is plain that
measures made by the use of a prism would not substan-
tiate that admission. The concentration to which I have
alluded as taking place in the less refrangible region
would give an exaggerated, an increased heat for that
region ; and, on the contrary, the dilatation of the more
refrangible would give an exaggerated diminution of
heat for that space. But if it were possible to make sat-
isfactory heat measures on the diffraction spectrum, in
which the colored spaces and fixed lines are arranged ac-
cording to their wave-lengths, the admission would be
substantiated.

In view of these facts, I did attempt, many years ago,
to make heat measures on the diffraction spectrum. But
so small is the heat that, as may be seen in the Philo-
sophical Magazine (March, 1857), the results were un-
satisfactory. More recently I have tried another meth-

388 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

od of investigation, on principles which I will now ex-
plain.

For the sake of clearness, restricting our thoughts for
the moment to the more familiar case of the visible spec-
trum, if we desired to ascertain the true distribution of
heat, would not the proper method be to collect all the
more refrangible rays into one focal group, and all the
less refrangible into another -focal group, and then meas-
ure the heat that each gave ? If the view currently re-
ceived be correct, would not nearly all the heat observed
be found in the latter of these foci, and little, if indeed
any, be found in the former? But if all the various re-
gions of the spectrum possess equal heat-giving powers,
would not the heat in each of these foci be the same?

Let ns give greater precision to this idea. Using Ang-
strom's wave-lengths — the length at the line A is 7604,
and that at H2 3933, and these lines are not very far
from the less and more refrangible ends of the visible
spectrum respectively. The middle point of this spec-
trum is at 5768, which may therefore be called its optical
centre. This is a little beyond the sodium line D, which
is 5892. Now, if by suitable means we reunite all the
rays between 7604 and 5768 into one focus, and all the
rays between 5768 and 3933 into another focus, are we
not in a position to determine the true distribution of
the heat? Should the heat at these two foci be sensibly
the same, must not the conclusion at present held be
abandoned ?

If in these investigations the rays of the sun be used,
it is necessary to restrict the examination to the visible
spectrum, excluding the invisible red and invisible violet
radiations. On these the earth's atmosphere exerts not
only a very powerful but a very variable action, and,
what is still more, an action the result of which we can-
not see, so that we are literally working in the dark.

MEMOIR XXVIII.] DISTRIBUTION OF HEAT IN THK Sl'KCTRUM. 339

There are days on which, owing to the excessive absorp-
tion taking place among the ultra-red rays, a rock-salt
train has no advantage over one of glass. But if it is
the visible spectrum alone that we are using, and the
prisms are of a material colorless to the eye, we may be
certain that they are exerting no selective absorption on
any of the radiations of that spectrum, and that the in-
dications they are giving are reliable.

This variable absorptive action of the atmosphere de-
pends partly on changes in the amount of water vapor,
and partly on the altitude of the sun. At midday and
at midsummer it is at a minimum. The disturbance
is not merely a thermochrose, for both ends of the spec-
trum are attacked. It is a matter of common observa-
tion that the horizontal sun has but little photographic
power, owing to atmospheric absorption of the ultra-vio-
let rays, and under the same circumstances his heating
power is diminished, owing to the absorption of the ultra-
red rays. But if the day be clear, and the sun's altitude
sufficient, the visible spectrum may be considered as un-
affected.

It should be borne in mind that the envelopes of the
sun himself exert an absorptive action, which is power-
fully felt in the ultra-violet region, as is indicated by
the numerous fixed lines crowded together in that region.
The force of this remark will be appreciated on examin-
ing the plate above referred to in the Philosophical Mag-
azine for May, 1843.

It seems, then, that all the conditions necessary for the
solution of this problem will be closely approached if we
make use of prisms constituted of any substance which is
completely colorless to tJie eye, and confine our measures to
the visible spectrum, collecting all the radiations between
the fixed line A and the centre of that spectrum just be-
vond D into one focus, and all the radiations between

390 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

that centre and H2 into another focus, and by the thermo-
pile, or any other suitable means, measuring the heat of
these foci.

Such is the method I have followed in obtaining the
measures now to be presented : but before giving them,
there are certain preparatory facts which I wish to sub-
mit to the consideration of the reader.

(1.) In the mode of experiment hitherto adopted, no
special care has been taken to ascertain with accuracy the
position of the " extreme red," yet that is held to be the
point from which, on one side, we are to estimate the in-
visible, and, on the other, the visible spectrum. Differ-
ent persons, perhaps because of a different sensitiveness
of their eyes, will estimate that position differently. The
red light shades off gradually — it is almost impossible
to tell when it really comes to an end. A linear thermo-
pile, such as is commonly used, is liable, under these cir-
cumstances, to give deceptive results, and any error in its
indications counts in a double manner. It not only di-
minishes the value of one spectrum, but it adds that dim-
inution to the value of the other. The force of this re-
mark will be understood by considering the best experi-
ments hitherto made on this subject, those of Dr. Tyndall,
as related in his " Heat a Mode of Motion" (London edi-
tion, 1870, p. 420, etc.). In the case of the electric light,
the result yielded by those experiments was that the
heat in the invisible is eio;ht times that of the visible

O

region. But had there been an error in estimating the
position of the extreme red by only two millimeters, so
much would have been taken from the invisible and add-
ed to the visible that they would have been brought to
equality, and then the slightest turn of the screw that
carried the pile towards the dark space would have
given a preponderance to the visible. It is obvious,
therefore, that there cannot be certainty in such meas-

MKMOIK XXVIII. J DISTRIBUTION OF HEAT IN TIIK SI'KC'l Hl.M.

ures, unless the fixed lines be resorted to as standard
points.

(2.) A ray which has passed through a solution of
sulphate of copper and ammonia possesses no insignifi-
cant heating power. I took a stratum of a solution of
that salt, of such strength that it only permitted waves
to pass which are of less length than 4860. Seen in the
spectroscope, the colors transmitted through it com-
menced with a thin green fringe, followed by blue, in-
digo, violet. It therefore gave rays in which, according
to the accepted views, little or no heat should be detect-
ed. Yet I found that such rays produced one ninth of
the heat of the solar beam. Does not this indisputably
show that the more refrangible rays have a higher ca-
lorific power than is commonly imputed to them ?

(3.) Again, by the use of the apparatus presently to
be described, I found no difficulty in recognizing heat in
the violet region. But in the mode of conducting the
experiment heretofore resorted to, it could not be detect-
ed in rays more refrangible than the blue. It was this
result which gave so much weight to the conclusion, that

O O *

in the more refrangible regions the calorific power is re-
placed by chemical force, and strengthened the idea com-
monly entertained that the solar radiations consist of
three distinct principles — heat, light, and actinism. In
the Memoir above referred to, as soon to be published, I
shall present some facts which apparently make this view
indefensible.

(4.) If waves of light falling upon an absolutely black
surface, and becoming extinct thereby, be transmuted
into heat, if the warming of surfaces by incident light
be nothing more than the conversion of motion into
heat — an illustration of the modern doctrine of the corre-
lation of forces, heat itself being only " a mode of mo-
tion"— it would seem extraordinary that the conversion

392 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

should cease in the green or blue or in any middle ray.
On the contrary, calorific effects ought to be traceable
throughout the entire length of the spectrum. These
views on the transference of motion from the ether to the
particles of ponderable bodies, and conversely, I endeavor-
ed to explain in detail in a memoir on Phosphorescence
inserted in the Philosophical Magazine, February, 1851,
p. 98, etc. (Memoir VIII.). I had previously indicated
them in the same journal, February, 1847 (Memoir V.).
A given series of waves of red light impinging upon an
extinguishing surface will produce a definite amount of
heat, and similar series of violet waves should produce
the same amount; for though an undulation of the lat-
ter may have only half the length of one of the former,
and therefore only half its vis viva, yet, in consequence
of the equal velocity of waves of every color, the impacts,
or impulses, of the violet series will be twice as frequent
as those of the red. The same principle applies to any
intermediate color, and hence it follows that every color
ought to have the same heating power.

Description of the Apparatus employed.

The optical arrangement I have employed for carrying
the foregoing suggestions into practice is represented by
Fig. 87, and in a horizontal section by Fig. 88.

A ray of sunlight reflected by a Silberman's heliostat
comes into a dark room through a slit, a, one millimeter

O I'

wide. It then passes through a prism, b. On the front
face of this prism is a black paper screen, c c, having a
rectangular opening, just sufficient to permit the light of
the slit to pass. After refraction the dispersed rays fall
as a spectrum on a concave metallic mirror, dd, nine inch-
es in aperture and eleven in focus for parallel rays. I
have sometimes used one of speculum metal, but more
frequently one silvered on its front face. In front of this

MEMOIH XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 393

mirror there are therefore three foci. At a distance of
eleven inches there is one, e, Fig. 88, giving a spectrum
image of the sun. Still farther there is a second,/, which
is a spectrum image of the slit, tf, in which, if the prism
be at its angle of minimum deviation, and the other ad-
justments be correctly made, will be seen the Fraunhofer
lines. Again, still farther off, at g, is a focal image of
the rectangular opening of the black paper, c c, on the
front face of the
prism. This im-

age

, arising from

Fig. 87.

the recombination
of all the dispersed
rays, is consequent-
ly white. The sec-
ond and third foci
are at distances
from the mirror de-
pending on the dis-
tance of the slit, «,
and the black pa-
per, c c, respect i vely.

With the intention of being certain that the light com-
ing through the slit, #, is falling properly on the rectangu-
lar opening in the prism screen, c c, a small looking-glass
is placed at p. The experimenter, sitting near the mul-
tiplier, m, can then see distinctly the reflected image of
that opening.

At the place where the second focal image, with its
Fraunhofer lines, forms, two screens of white pasteboard,
h, i, are arranged. By suitably placing the former of
these, 7i, the more refrangible rays may be intercepted,
and in like manner by the other, i, the less refrangible.
In using these screens, and particularly 7i, care must be
taken that no rays passing from the prism to the mirror

394 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

be obstructed, a remark that applies especially to the in-
visible rays of less refrangibility than the red. For this
reason the mirror, d d, must be placed at such an obliquity
to its incident rays as to throw the focal images sufficient-
ly on one side. Yet this obliquity must not be greater
than is actually necessary for that purpose, or the purity
of the second spectrum, with its Fraunhofer lines, will be
interfered with. At the place of the third focus, arising

from the reunion of the
dispersed rays, is the ther-
mopile, <7, connected by its
wires, k k, with the multi-
plier, m.

Whenever any of the
visible rays of the Fraun-
hofer spectrum are inter-

cepted by advancing either
of the screens h, i, the im-
age on the face of the pile
ceases to be white. It be-
comes of a superb tint, an-
swering to the combination of the non-intercepted rays.
A slip of white paper placed for a moment in front of the
pile will satisfy the experimenter howr magnificent these
colors are. It is evident, therefore, that by this arrange-
ment the pile will enable us to measure the heat of any
particular ray, or of any selected combination of rays.
The screens can be arranged so as to reach any desig-
nated Fraunhofer line.

The pile I have used is of the common square form ;
a linear pile would not answer. The focal image on the
pile is of very much greater width than the slit «, on ac-
count of the obliquity of the front face of the prism.

By removing the screen 7^, and placing the screen i so
that its edge coincides with the line A of the Fraunhofer

MI.M..IK XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTHUM. 395

spectrum, all the invisible heat radiations of less refran-
gihility than the red are cut off, except the contaminat-
ing ones arising from the general diffusion of light by
the substance of the prism. Under these circumstances
the image on the pile will be white, and the multiplier
will give a deflection representing the heat of the visible
and the extra-violet regions. If then the screen be ad-
vanced still farther, until it has intercepted all the less
refrangible regions up to the sodium line D or a little
beyond — that is, to the optical centre of the spectrum —
the tint on the face of the pile will be greenish - blue,
and the multiplier will give a measure of the heat of
the more refrangible half of the visible spectrum, to-
gether with that of the ultra-violet rays : the latter por-
tion may, however, be eliminated by properly using the
other screen, k.

Besides the error arising from stray heat diffused
through the spectrum in consequence of the optical im-
perfection of the prism, there is another which may be
recognized on recollecting the relative positions of the
prism, the concave mirror, and the face of the pile. It is
evident that the prism, considered as a warm or a cool
mass, is a source of disturbance, for the mirror reflects its
image — that is, the image of the prism itself — to the pile.
After the intromitted sunbeam has passed through the
prism for a short time, the temperature of that mass has
risen, and the heat from this source has become inter-
mingled with the proper spectrum heat. But this error
is very easily eliminated. It is only necessary to put a
screen, n, in the path of the incoming ray, between the
slit and the prism, and note the deflection of the multi-
plier. Used as we are here supposing, the multiplier
has two zeros. The first, which may be termed the
magnetic, is the position in which the needles will stand
when no current is passing through the coil. The scale

396 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIIF.

of the instrument should be set to this. The other,
which may be termed the working zero, is formed by
coupling the pile and the multiplier together, and intro-
ducing the screen n between the intromitting slit and
the prism. On doing this it will probably be found
that the index will deviate a few divisions. Its po-
sition should be accurately marked at the beginning
and close of each set of measures, and the proper correc-
tion for them made. The disturbing influences of the
mass of the prism, of the mirror, and of the pile itself,
are thus eliminated. As respects the last, it should not
be forgotten that it may be affected by changes in the
position of the person of the experimenter himself.
With the intention of diminishing these errors, I have

O '

usually covered the upper and lower portions of the con-
cave mirror, dd, with pieces of black paper, so arranged as
to leave a band of sufficient width to receive and reflect
the entire spectrum, a a, Fig. 89, is the upper paper, b b
the lower, c c the uncovered reflecting
band, receiving the spectrum, r v. Had
the spaces thus covered been permitted to
reflect, they would have rendered more in-
tense the image of the prism with its ex-
Fl'g-89- traneous heat.

As regards the multiplier, care must be taken to avoid
disturbance from aerial currents. I have one of these
instruments of French construction, which could not be
used in these delicate researches until proper arrange-
ments were applied. It was covered with a glass shade.
The slightest cause occasioned currents in its included

O

air, which perpetually drifted and disturbed the needles.
For this reason, and also for more accurate reading, it is
best to view the position of the index through a small
telescope.

The combination of needles being nearly astatic, at-

MKMoiuXXYlII.] DISTRIBUTION <>1 1 1 KAT IN THE SPECTRUM. 397

tention must be paid to tbeir magnetic perturbations,
win-tiler arising from local or other causes; and, since
the vibrations are very slow, ample time must be given
before the reading is ascertained.

The condition of the face of the pile is of importance.
It must be such as to extinguish as completely as possible
all the incident rays. To paint it with lamp-black, mix-
ed with gum, will not answer — the surface so produced
is too glossy and reflecting. The plan I have found best
is to take a crlass tube half an inch in diameter and six

o

inches long, open at both ends, and use it as a chimney.
A. piece of camphor being set on fire at the lower end,
and the face of the pile to be blackened being held for a
moment at the upper, it is covered with a dense black
film, without any risk of injury to the pile. Even at the
best, when this has been done, there is an unavoidable
source of error in the want of perfect blackness of the
lamp-black. It is sufficient to inspect the face of the pile
when receiving rays from the concave mirror to be satis-
fied how large a portion of light is reflected. The ex-
periments of Dr. Tyridall show that this substance also
transmits a considerable percentage of the heat falling
on it. Its quality of transmitting light is well known
to every one who has looked at the sun through a
smoked glass.

The galvanometer I have used is calibrated according
to the usual method. The numbers given in this Memoir
do not represent the angles of deflection, but their corre-
sponding forces.

The proper position of the intercepting screens #, i, can
often be verified with precision by looking through blue
cobalt glass. This glass insulates a definite red, an
orange, and a yellow ray in the less refrangible regions,
and then, commencing with the green, gives a continuous
band to the end of the violet. Its red ray begins at the

398 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

less refrangible end of the spectrum, and ends near C ;
it includes the fixed lines A, B, C. Its orange ray lies
wholly between C and D, including neither of those lines.
Its yellow ray begins near 5894, and ends about 5581 ;
the line D is therefore near its point of beginning. The
remaining continuous band begins about 5425 ; it there-
fore includes the lines E, b, F, G, H. I have found this
glass of much use in determining how far the screen i
has been pushed. It is convenient to select a light kind
of it, and by looking through one, two, or three pieces,
the depth of color can be regulated at pleasure.

The optical train which has acted on the sunbeam un-
der examination is, therefore, (1) the sun's atmosphere ;
(2) the earth's atmosphere ; (3) the heliostat mirror of
speculum metal ; (4) the prism ; (5) the concave mirror
of silvered glass ; (6) the blackened face of the thermo-
pile.

Results obtained by the Apparatus.

We are now ready to examine the results which this
optical apparatus yields, it having been of course pre-
viously ascertained that the reflecting band of the con-
cave mirror, d d, is sufficient to receive all the radiations
coming from the prism, and that none are escaping past
its edges.

The operations required are as follows :

The heliostat is to be set, and its reflected ray brought
into the proper position. The optical train is adjusted,
the prism being at its minimum deviation, and the con-
cave mirror giving a white image on the face of the
pile.

The screen h is then to be placed so that, without in-
tercepting any rays coming from the prism to the mirror,
it cuts off all the Fraunhofer spectrum above H2.

The screen i is so placed as to cut off all rays less re-
frangible than the sodium line D. More correctly, the

MEMOIR XXVIII.] DISTRIBUTION OF HEAT IX THE SPECTRUM. 399

screen should be a little beyond D. The light on the
face of the pile will now be greenish-blue.

The screen n is then placed so as to intercept the in-
troraitted beam. When the needles of the multiplier
come to rest, they give the working zero, which must be
noted.

The intromitting screen n being now removed, the mul-
tiplier will indicate all the heat of the more refrangible
rays ; that is, from a little beyond D up to H2. The force
corrected for the working zero is to be noted.

o

The screen * is then removed to the line A, so as to give
all the radiations between the lines A and H2. The light
on the face of the pile is white, and the multiplier gives
the whole heat of the visible spectrum. By subtracting
the foregoing measure from this, we have the heat of the
less refrangible region ; that is, from A to the centre of
the spectrum.

As a matter of curiosity the experimenter may now, if
he please, remove the screens ^,z; the light on the face
of the pile will still be white, and the multiplier will
give the force of the entire radiations, except so far as
they are disturbed by the thermochrose of the media.
These measures, as they do not bear upon the prob-
lem under consideration, I do not give in the following
tables.

Instead of advancing the screen i from the less tow-
ards the more refrangible regions, I have very frequently
moved li from the more refrangible to the less. When it

o

is brought down from H8 to the centre of the spectrum,
the light on the face of the pile is of an intense orange-
red — it might perhaps be called a bromine-red. I need
not give further details of this mode of experimentation,
as I did not find that its results differed in any impor-
tant degree from those obtained as just described.

The variation in different experiments may generally

400 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

be traced to errors in placing the screen i with exactness
on the centre of the spectrum and on the line A.

For the sake of more convenient comparison, I have re-
duced all the different sets of experiments to the stand-
ard of 100 for the whole visible spectrum.

I have made use of four prisms: (1) rock-salt; (2)
flint-glass ; (3) bisulphide of carbon ; (4) quartz, cut out
of the crystal so as to give a single image.

All the observations here recorded were made on days
when there was a cloudless sky.

TABLE I. — Distribution of Heat by Hock-salt.

Series I. Series II.

(1) Heat of the whole visible spectrum 100 100

(2) " more refrangible region 53 51

(3) " less " " 47 49

In this table the column marked Series I. gives the
mean of four sets of measures, and that marked II. of
three. At the beginning of each set the rock-salt was
repolished.

TABLE II. — Distribution of Heat by Flint-glass.

Series I. Series II.

(1 ) Heat of the whole visible spectrum 100 1 00

(2) " more refrangible region 49 52

(3) " less " " 51 48

Series I. gives the mean of ten sets of measures, Series
II. of eight.

TABLE III. — Distribution of Heat by Bisulphide of Carbon.

Series I. Series II.

(1) Heat of the whole visible spectrum, 100 100

(2) more refrangible region 52 49

(3) " less " " 48 51

The sulphide employed was devoid of any yellowish
tinge ; it was quite clear. Series I. is the mean of eight
experiments, Series II. of ten.

MI.M..II: \\vin.j DISTRIBUTION OF HE AT D? THB 8PECTBDM.

TABLE IV. — Distribution of Heat by Quartz.

Series I. Series IL

(1) Heat of the whole visible spectrum 100 100

(2) " more refrangible region 4!» 68

(5) " less " " 51 47

Series I. represents twenty -seveu experiments, Series
II. twelve. In the former two quartz prisms were used
to increase the dispersion; in the latter only one was
employed.

Perhaps it may not be unnecessary for me to say that
I have repeated these experiments many hundred times
during a period of several months, including the winter
and the summer, varying the conditions as to the hour of
the day, arrangement of the apparatus, etc., as much as I
could, and present the foregoing tables as fair examples
of the results. Apprehending that the heliostat mirror,
which was of speculum metal, might exert some disturb-
ing influence on account of its faint reddish tinge, I re-
placed it with one of glass silvered on the front face, but
could not detect any substantial difference in the results.

The important fact clearly brought into view by these
experiments is, that if the visible spectrum be divided
into two equal portions, the ray having a wave-length of
5768 being considered as the optical centre of such a
spectrum, these portions will present heating powers so
nearly equal that we may impute the differences to errors
of experimentation. Assuming this as true, it necessarily
follows that in the spectrum any two series of undula-
tions will have the same heating power, no matter what
their wave-lengths may be.

But this conclusion leads unavoidably to a most im-
portant modification of the views now universally held
as regards the constitution of the spectrum. When a
ray falls on an extinguishing surface, heat is produced ;
but that heat did not pre-exist in the ray. It arose from
the stoppage of ether waves, and is a pure instance of

Cc

402 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII.

the conversion of motion into heat — an illustration of
the modern doctrines of the conservation and transmu-
tation of force.

From this point of view, the conception that there ex-
ist in an incident ray various principles disappears alto-
gether. We have to consider an incident ray as consist-
ing solely of ethereal vibrations, which, when they are
checked by an extinguishing substance, lose their vis viva.
The effect that ensues depends on the quality of the sub-
stance. The vibrations imparted to it may be manifest-
ed by the production of heat, as in the case of lamp-
black, or by chemical changes, as in the case of many of
the salts of silver. In the parallel instance of acoustics
clear views have long ago been attained, and are firmly
held. No one supposes that sound is one of the ingre-
dients of the atmosphere, and it would not be more in-
correct to assert that it is something emitted by the
sounding body than it is to affirm that light or heat, or
actinism, is emitted by the sun.

The progress of actino-chemistry would be greatly ac-
celerated if there could be steadfastly maintained a clear
conception of the distinction between the mechanism of
a ray and the effects to which that ray may give rise.
The evolution of heat, the sensation of light, the produc-
tion of chemical changes, are merely effects — manifesta-
tions of the motions imparted to ponderable atoms; and
these in their turn can give rise to converse results, as
when we gradually raise the temperature of a substance
the oscillating movements of its molecules are imparted
to the ether, and waves of less and less length are suc-
cessively engendered.

In the title of this Memoir I have employed the phrase
"distribution of heat" in accordance with general usage;
but if the conclusion arrived at be true, it is plain that
this should be exchanged for " production of heat." The

MI.M..IU XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 4Q3

heat observed did not pre-exist in the incident rays: it is
the result of their extinction.

The remark has been made that these results are es-
sentially connected with photometry. In fact, any ther-
mometer is converted into a photometer if its ball or
other receiving surface be coated with a perfectly opaque
non-reflecting substance.

404 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

MEMOIR XXIX.

ON THE DISTRIBUTION OF CHEMICAL FORCE IN THE
SPECTRUM.

From the American Journal of Science and Arts for December, 1872, and January,
1873; Philosophical Magazine, December, 1872.

CONTENTS: — The curves of the calorific, luminous, and chemical spec-
tra.— Their errors. — Inappropriateness of the term actinic rays. — •
There is no localization of chemical effects. — Every radiation can pro-
duce some specific effect. — Case of the silver compounds. — Bitumens
and resins. — Carbonic acid. — Colors of flowers. — Law of Grotthus —
Chlorine and hydrogen. — Bending of stems of plants. — Absorption es-
sential to chemical action. — Decomposition of silver iodide. — Union of
chlorine and hydrogen. — Angstrom's law. — General conclusions. —
Chemical force exists in every portion of the spectrum. — Each radia*
tion exercises chemical influences proper to itself.

WITH scarcely an exception, the most recent works on
the chemical action of radiations and spectrum analysis
describe a tripartite arrangement of the spectrum, illus-
trated by an engraving of three curves, exhibiting the
supposed relations of the calorific, the luminous, and the
chemical spectra. This view, which by a mass of evi-
dence may be shown to be erroneous, is exerting a very
prejudicial effect on the progress of actino-cheniistry.

I propose now to present certain facts which may aid
in correcting this error. For this purpose it is necessary
to show that chemical effects — decompositions and com-
binations— may take place in any part of the spectrum.
The points to be established may be thus distinctly
stated :

1st. That so far from chemical influences being re-

MEMOIK XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 405

stricted to the more refrangible rays, every part of the
spectrum, visible and invisible, can give rise to chemical
changes, of modify the molecular arrangement of bodies.

2d. That the ray effective in producing chemical or
molecular changes in any special substance is determined
by the absorptive property of that substance.

I may here remark that both these propositions were
maintained by me many years ago : an example of the
first will be found in the Philosophical Magazine (Dec.,
1842), and of the second in a paper in the same journal,
" On some Analogies between the Phenomena of the
Chemical Rays and those of Radiant Heat " (Sept., 1841),
Memoir XVII.

The opinion commonly held respecting the distribu-
tion of chemical force in the spectrum is mainly founded
on the behavior of some of the compounds of silver.
These darken when exposed to the more refrangible
rays, and, unless correct methods of examination be re-
sorted to, seem to be unaffected by the less refrangible.
Hence it has been supposed that in the higher parts of
the spectrum a special principle prevails, to which the
designation of " actinic rays " is often applied — an inap-
propriate iteration. In these pages I use the derivatives
of aKTiq not in this restricted sense, but as expressive of
radiations of every kind. This is their proper significa-
tion.

Every part of the spectrum, no matter what its re-
frangibility may be, can produce chemical changes, and
therefore there is no special localization of force in any
limited region. Out of a large body of evidence that
might be adduced, I select a few prominent instances.

~[st. — Case of the Compounds of Silver.

Silver is the basis of the most important photographic
sensitive substances. Its iodide, bromide, and chloride,

406 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

darkening with rapidity under the influence of the more
refrangible rays, have mainly been the cause of the mis-
conception above alluded to respecting the tripartite con-
stitution of the spectrum. It is necessary, therefore, to
determine what are really the habitudes of these sub-
stances.

(1.) If a spectrum be received on iodide of silver,
formed on the metallic tablet of the daguerreotype, and
carefully screened from all access of extraneous light,
both before and during the exposure, on developing
with mercury vapor an impression is evolved in all the
more refrangible regions. This stain corresponds in
character and position to the blackening effect which
under like circumstances would be found on any com-
mon sensitive silver paper. It is this which has given
rise to the opinion that the so-called actinic rays exist
only in the upper part of the spectrum. If, however, the
action of the light be long continued, a white stain
makes its appearance over all the less refrangible re-
gions. It has a point of maximum to which I shall
again presently refer.

(2.) But if the metallic tablet during its exposure to
the spectrum be also receiving diffused light of little in-
tensity, as the light of day or of a lamp, it will be found
on developing that the impression obtained differs strik-
ingly from the preceding. Every ray that the prism can
transmit, from below the extreme red to beyond the ex-
treme violet, has been active. The ultra-red lines a, /3, y
are present. It must be borne in mind that the impres-
sion of these lines is a proof of proper spectrum action,
and distinguishes it from that of diffused light, arising
either from the atmosphere or from the imperfect trans-
parency of the prism — a valuable indication. The result-
ing photograph shows two well-marked regions or phase's
of action. On its general surface, which, having con-

.MI.M..IU XXIX.J ( 11KMICAL FOK« K IX Till. BPECTB1 M.

clensed the mercury vapor, has the aspect of the
1 id its of the daguerreotype, and forms, as it were, the
basis for the spectrum picture, there is in the region of
the more refrangible rays a bluish or olive-colored im-
pression, the counterpart of the result described in the
foregoing paragraph. But in the region of the less re-
frangible rays no mercurial deposit has occurred, the
place of those rays being depicted in metallic silver,
dark, and answering to the shadows of the daguerreo-
type. This protected portion, which stands out in bold
relief from the white background, reaches from a little
below G to beyond the extreme red, and encloses the
heat lines above named. They are in the form of white
streaks. Though I speak of them as single lines, they
are in reality groups, or perhaps bands.

The general appearance of the photograph at once
suggests that the less refrangible rays can arrest the ac-
tion of the daylight and protect the silver iodide from
change. A close examination shows that there are three
points — the extreme red, the centre of the yellow, and the
extreme violet — which apparently can hold the daylight
in check. There are also two intervening ones in which
the actions conspire. The point of maximum protection
corresponds to the point of maximum action referred to
above in paragraph (1).

(3.) If the metallic tablet, previously to its exposure
to the spectrum, be submitted for a few moments to a
weak light, so that, were it developed, it would at this
stage whiten all over, the action of the spectrum upon
it will be the same as in the last case (2). But this
change in the mode of the experiment leads to a very
important conclusion. The less refrangible rays can re-
verse or undo the change, in whatever it may consist,
that light has already impressed on the iodide of silver.

Now, bearing in mind the fact that the photographic

408 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

action of diffused light on this iodide is mainly due to
the more refrangible rays it contains, we are brought by
these experiments to the following conclusions :

1st. Every ray in the spectrum acts on silver iodide.

2d. The more refrangible rays apparently promote the
action of daylight on that substance; the less refrangible
apparently arrest it.

3d. For the display of this arresting or antagonizing
effect, it is not necessary that the less and more refran-
gible rays should be acting simultaneously. An interval
may elapse, and they may act successively. Hence the
effect is not due to the contemporaneous interference of
waves of different periods of vibration with one another
— the material particles of the changing substance of
the silver iodide are involved.

I abstain for the moment from giving further details
of these spectrum impressions. That has been very
completely done by Herschel in the case of one I sent
him many years ago. His examination of it, illustrated
by a lithograph, may be found in the Philosophical Mag-
azine (Feb., 1843). I shall have to return to the subject
of the behavior of silver iodide in presence of radiations
on a subsequent page of this Memoir.

The main point at present established is this, that the
iodide under proper treatment is affected by every ray
that a flint-glass prism can transmit, and therefore it is
altogether erroneous to suppose that chemical force is
restricted to the more refrangible portions of the spec-
trum.

2d. — Case of Bitumens and Resins.

These substances are of special interest in the history
of photography, since in the hands of Niepce they prob-
ably were the first on which impressions in the camera
were obtained and fixed. Their use has been abandoned
in consequence, as it seems to me, of .an incorrect opinion

MKMOIK XXIX.] CHEMICAL FORCE IN THE SPECTRUM. j,,;i

of their want of sensitiveness. Properly used, they are
scarcely inferior to chloride of silver.

The theory of their use is very simple. Alcohol, ether,
and various volatile oils, respectively dissolve certain por-
tions of these substances. If such a solution be spread
in a thin film upon glass, as in the collodion operation,
and parts of the surface be then exposed to light, the
portions so exposed become insoluble in the same men-
struum. They may therefore be developed by its use.
Practically, care has to be taken to moderate the solvent
action and to check it at the proper time. The former
is accomplished by dilution with some other appropriate
liquid, the latter by the affusion of a stream of water.

The substance I have used is West India bitumen dis-
solved in benzine, and developed by a mixture of ben-
zine and alcohol.

The bitumen solution being poured on a glass plate
in a dark room, and drained off as in the operation of
collodion, leaves a film sufficiently thin to be iridescent.
This is exposed to the spectrum for five minutes, and
then developed.

The beginning of the impression is below the line A,
its termination beyond H. Every ray in the spectrum
acts. The proof is continuous except where the Fraun-
hofer lines fall. A better illustration that the chemical
action of the spectrum is not restricted to the higher
rays, but is possessed by all, could hardly be adduced.

3d. — Case of Carbonic Acid.

The decomposition of carbonic acid by plants under
the influence of sunshine is undoubtedly the most im-
portant of all actino-chemical facts. The existence of the
vegetable world, and indeed it may be said the existence
of all living beings, depends upon it.

I first effected this decomposition iu the solar spec-

410 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

tram, as may be seen in Memoir X. The results obtain-
ed by me at that time from the direct spectrum experi-
ment, that the decomposition of carbonic acid is effected
by the less, not by the more refrangible rays, have been
confirmed by all recent experimenters, who differ only
as regards the exact position of the maximum. In the
discussions that have arisen this decomposition has often
been incorrectly referred to the green parts of plants.
Plants which have been caused to germinate and grow
to a certain stage in darkness are etiolated. Yet these,
when brought into the sunlight, decompose carbonic
acid, and tJien turn green. The chlorophyl thus pro-
duced is the effect of the decomposition, not its cause.
Facts derived from the visible absorptive action of chlo-
rophyl do not necessarily apply to the decomposition of
carbonic acid. The curve of the production of chlorophyl,
the curve of the destruction of chlorophyl, the curve of
the visible absorption of chlorophyl, and the curve of the
decomposition of carbonic acid, are not all necessarily co-
incident. To confound them together, as is too frequent-
ly done, is to be led to incorrect conclusions.

Two different methods may be resorted to for deter-
mining the rays which accomplish the decomposition of
carbonic acid : 1st, the place of maximum evolution of
oxygen gas in the spectrum may be determined ; 2d,
the place in which young etiolated plants turn green.

I resorted to both these methods, and obtained from
them the same results. The rays which decompose car-
bonic acid are the same which turn etiolated plants
green. They may be designated as the yellow with the
orange on one side and a portion of green on the other.
Though the form of experimentation does not admit of a
close reference to the fixed lines, I think we are almost
justified in supposing that the point of maximum action
is in the yellow. It must be borne in mind that the

MEMOIU XXIX.] CHEMICAL FORCE IN Till; M KCTRUM.

rapidly increasing concentration of the rays occasioned
by the peculiarity of prismatic dispersion towards the
red end will give a deceptive preponderance in that di-
rection. Without entering further into this discussion,
it is sufficient for my present purpose to understand that
the decomposition in question is accomplished by rays
between the fixed lines B and F.

The two absorptive media, potassium bichromate and
cupro-ammonium sulphate, so often and so usefully em-
ployed in actino - chemical researches, corroborate this
conclusion. Plants cannot decompose carbonic acid, nor
can they turn green in rays that have passed through a
solution of the latter salt. They accomplish both those
results in rays that have passed through the former.

The decomposition of carbonic acid, and the produc-
tion of chlorophyl by the less refrangible rays of the
spectrum, afford thus a striking illustration that chem-
ic-al changes may be brought about by other than the
so-called chemical rays.

. — Case of tlie Colors of Flowers.

The production and destruction of vegetable colors by
the agency of light have, of course, long been a matter of
common observation. Little has, however, been done in
the special examination of the facts, and that little for
the most part by Herschel.

We have only to examine his Memoir in the Philo-
sophical Transactions (Part II., 1842) to be satisfied that
nearly every radiation can produce effects. Thus the
yellow stain imparted by the Corchorus Japonica to pa-
per is whitened by the green, blue, indigo, and violet
rays. The rose-red of the Ten-weeks-stock is in like man-
ner changed by the yellow, orange, and red. The rich
blue tint of the Viola odorata, turned green by sodium
carbonate, is bleached by the same group of rays; that is,

412 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

by those less refrangible than the yellow. The green
(chlorophyl) of the Elder leaf is changed by the extreme
red.

It is needless to extend this list of examples. The
foregoing establish the principle that every part of the
spectrum displays activity, some vegetable colors being
affected by one, others by other rays. It is, however,
desirable that the general principle at which Herschel ar-
rived— viz., that the luminous rays are chiefly effective-
should be more closely examined. Some important phys-
iological explanations turn on that principle. These so-
called luminous rays are such as can impress the retina,
which, like organic colors, is a carbon compound. There
are strong reasons for inferring that carbon is affected
mainly by rays the wave-lengths of which are between
those of the extreme red and extreme violet, the maxi-
mum being in the yellow.

It is, however, to a former experimenter, Grotthus, that
we owe the discovery of the law under which these de-
compositions of the colors of flowers take place. This
law in repeated instances was verified by Herschel, and
more recently by myself. It may be thus expressed :
" The rays which are effective in the destruction of any
given vegetable color are those which by their union
produce a tint complementary to the color destroyed."
Even the partial establishment of this law, already ac-
complished, is sufficient to prove that chemical effects
are not limited to the more refrangible portions of the
spectrum, but can be occasioned by any ray.

5th. — Case of the Union of Chlorine and Hydrogen.

In Memoir XVIII. may be found the description of
an actinometer invented by me, depending for its indi-
cations on the combination of chlorine and hydrogen,
these gases having been evolved in equal volumes from

Mi M..IK XXIX.J CHEMICAL FORCE IN THE SPECTRUM.

hydrochloric acid by a small voltaic battery. This in-
strument, modified to suit their purposes, was used by
Professors Buusen and Roscoe in their photometrical re-
searches. Many of my experiments were repeated by
them (Transactions of the Royal Society, 1856, 1857).

In Table III. of my Memoir above referred to, it is
shown that this mixture is affected by every ray of the
spectrum ; but by different ones with very different en-
ergy. The maximum is in the indigo, the action there
being more than TOO times as powerful as in the ex-
treme red.

6th. — Case of tlie Bending of the Stems of Plants in the

Spectrum.

It is a matter of common observation that plants tend
to grow towards the light. Dr. Gardner was, however,
the first to examine the details of this phenomenon in
the spectrum. His Memoir is in the Philosophical Mag-
azine (Jan., 1844). When seeds are made to germinate
and grow for a few days in darkness, they develop verti-
cal stems, very slender and some inches in length. These,
on being placed so as to receive the spectiaim, soon ex-
hibit a bending motion. The stems in other parts of
the spectrum turn towards the indigo; those in the in-
digo bend to the approaching ray. Removed into dark-
ness, they recover their upright position. These move-
ments are the most striking of all actinic phenomena. I
have often witnessed them with admiration.

Dr. Gardner's experiments were repeated and con-
firmed by M. Dutrochet, who, in a report to the French
Academy of Sciences (Comptes • Rendus, No. 26, June,
1844), added a number of facts respecting the bending
of roots from the light, which he found to be occasioned
by all the colored rays of the spectrum.

In Dr. Gardner's paper there are also some interesting

414 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

facts, respecting the bleaching or decolorization of chlo-
rophyl by light. He used an ethereal solution of that
substance :

" The first action of light is perceived in the mean red
rays, and it attains a maximum incomparably greater at
that point than elsewhere. The next part affected is in
the indigo, and accompanying it there is an action from
+ 10.5 to +36.0 of the same scale (Herschel's) beginning
abruptly in Fraunhofer's blue. So striking is this whole
result that some of my earlier spectra contained a per-
fectly neutral space, from —5.0 to +20.5, in which the
chlorophyl was in no way changed, while the solar
picture in the red was sharp and of a dazzling white.
The maximum in the indigo was also bleached, pro-
ducing a linear spectrum, as follows :

in which the orange, yellow, and green rays are neutral.
These, it will be remembered, are active in forming
chlorophyl. Upon longer exposure, the subordinate
action along the yellow, etc., occurs, but not until the
other portions are perfectly bleached.

" In Sir J. Herschel's experiments there remained a
salmon color after the discharge of the green. This is
not seen when chlorophyl is used, and is due to a color-
ing matter in the leaf, soluble in water, but insoluble in
ether."

I have quoted these results in detail because they il-
lustrate in a striking manner the law that vegetable col-
ors are destroyed by rays complementary to those that
have produced them, and furnish proof that rays of every
refrangibility may be chemically active.

At this point I abstain from adding other instances
showing that chemical changes are brought about in
every part of the spectrum. The list of cases here pre-
sented might be indefinitely extended, if these did not

MI.M..II: XXIX.] CIIK.MICAL FORCE IN Till. Sl'IXTliUM. | j ;,

suffice. But how is it possible to restrict the chemical
I'Tce of the spectrum to the region of the more refrangi-
ble rays, in face of the fact that compounds of silver
such as the iodide, which have heretofore been mainly
relied upon to support that view, and, in fact, originated
it, are now proved to be affected by every ray from the
invisible ultra-red to the invisible ultra-violet; how,
when it is proved that the decomposition of carbonic
acid, by far the most general and most important of the
chemical actions of light, is brought about not by the
more refrangible, but by the yellow rays ? The delicate
colors of flowers, which vary indefinitely in their tints,
originate under the influence of rays of many different
refrangibilities, and are bleached or destroyed by spec-
trum colors complementary to their own, and, therefore,
varying indefinitely in their refrangibility. Towards
the indigo ray the stems of plants incline ; from the red
their roots turn away. There is not a wave of light
that does not leave its impress on bitumens and resins,
some undulations promoting their oxidation, some their
deoxidation. These actions are not limited to decompo-
sition ; they extend to combinations. Every ray in the
spectrum brings on the union of chlorine and hydrogen.

The conclusion to which these facts point is, then,
that it is erroneous to restrict the chemical force of the
spectrum to the more refrangible, or, indeed, to any
special region. There is not a ray, visible or invisible,
that cannot produce a special chemical effect. The dia-
gram so generally used to illustrate the calorific, lumi-
nous, and chemical parts of the spectrum serves only to
mislead.

While thus we find that chemical action may take
place throughout the entire length of the spectrum, the
remarks that have been made in a previous Memoir
(XXVI.) respecting the differences of calorific distribu-

416 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

tion in dispersion and diffraction, apply likewise to the
chemical force. To be satisfied of this it is only neces-
sary to compare photographic impressions given by a
prism and a grating !

I published engravings of such photographs in 1844.
They are referred to in Memoir VI. As they were ob-
tained on silver plates made sensitive by iodine, bro-
mine, and chlorine, they do not extend to the line F.

I had found that certain practical advantages arise
from the use of a reflected instead of a transmitted
spectrum. The ruled glass was therefore silvered upon
its ruled face with tin amalgam, copying the surface per-
fectly. Of the series of spectra I used the first.

The fixed lines were beautifully represented in the
photographs. They were, however, so numerous and so
delicate that I did not attempt to do more than to mark
the prominent ones. These were, I believe, the first dif-
fraction photographs that had ever been obtained. The
wave -lengths assigned were according- to Fraunhofer's

o o o

scale, which represents parts of a Paris inch.

The length of the photographic impression given by the
prism I was then using, from the line H to the ultra-violet
end of the spectrum, was about three times that from H
to G ; but in the spectrum by the grating, though the
exposure was in one instance continued for a whole hour,
the impression beyond H was not more than 1£ times
the length of that to G. In more moderate exposures
the last fixed line in the photograph was about as far
from H on one side as G was on the other. This, there-
fore, showed very clearly the difference of distribution in
the diffraction and prismatic spectra.

Union XXIX. J ( HK.MICAL FORCE IN THE SPECTRUM. 417

OF THE CHEMICAL ACTION OF RADIATIONS ON SUBSTANCES.

Having offered the foregoing evidence in support of
the first proposition considered in this Memoir, which
was to the effect —

" That so far from chemical influences being restricted
to the more refrangible rays, every part of the spectrum,
visible and invisible, can give rise to chemical changes,
or modify the molecular arrangement of bodies," I now
pass to the second, which is —

" That the ray effective in producing chemical or mo-
lecular changes in any special substance is determined
by the absorptive property of that substance."

This involves the conception of selective absorption,
as I have formerly shown (Philosophical Magazine, Sept.,
1841), Memoir XVII. A ray which produces a maxi-
mum effect on one substance may have no effect on an-
other. Thus the rays which change chlorophyl are not
those which change silver iodide.

O

In the examination of this subject I shall select two
well-known instances, presenting the fewest elements and
the simplest conditions. They are, 1st, the decomposition
of silver iodide, the basis of so many photographic prepa-
rations ; 2d, the production of hydrochloric acid by the
union of its two constituents, chlorine and hydrogen — a
mixture of these gases being exceedingly sensitive to
light.

1st. — Of the Decomposition of Silver Iodide.
There are two forms in which the silver iodide has
been used for photographic purposes : 1st, when prepared
by the action of the vapor of iodine on metallic silver, as
in the daguerreotype tablet ; 2d, when nitrate of silver
is decomposed by iodide of potassium, or other metallic
iodides. These preparations differ strikingly in their ac-

DD

418 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

tinic behavior, the former furnishing by far the most in-
teresting series of facts.

When a polished surface of silver is exposed at com-
mon temperatures to the vapor of iodine, it speedily
tarnishes, a film of silver iodide forming. This passes
through several well-marked tints as the exposure con-
tinues and the thickness increases. They may be thus
enumerated in the order of their occurrence : 1, lemon-
yellow; 2, golden -yellow; 3, red; 4, blue; 5, lavender;
6, metallic ; 7, deep yellow ; 8, red ; 9, green.

All these films are sensitive. Under the influence of
radiations they exhibit two phases of modification : 1st,
an invisible modification, which, however, can be made
apparent or developed, as Daguerre discovered, by ex-
posure to the vapor of mercury, the iodide turning white
by the condensation of mercury upon it wherever it has
been exposed to light, but remaining unacted upon in
parts that have been in shadow ; 2d, a visible modifica-
tion, which arises under a longer exposure, the iodide
passing through various shades of olive and blue, and
eventually becoming dark gray.

But though all the variously tinted films of silver
iodide are impressionable, they differ greatly in relative
sensitiveness when compared with each other. This may
be very satisfactorily shown by producing on one silver
tablet bands of all the above-named colors — an effect
readily accomplished by suitably unscreening successive
portions of the tablet during the process of iodizing, and
then exposing all at the same time to a common radiation.
It will be found, on developing with mercury vapor, that
the bands of a yellow color have been the most sensitive,
those of a metallic aspect have been scarcely acted on,
and those of other tints intermediately. It is to be par-
ticularly remarked that the second yellow, numbered 7
in the above series, is equally sensitive with the first
yellow, numbered 2.

MI.M.IIU XX1X.J CHEMICAL FOKCK IX Till-. M'KCTRUM. .j j ,

From this it appears that the sensitiveness of this form
of iodide depends not merely on its chemical constitution,
but also on its optical properties. The explanation of
this different sensitiveness in different films of iodide be-
comes obvious when we cause a tablet prepared, as just
described, with tinted bands to reflect the radiations fall-
ing on it to another tablet iodized to a yellow color, mid
placed in a camera. After due exposure and develop-
ment of both with mercury, it will be found that the im-
age of the first tablet formed on the second consists of
bands of different shades of whiteness. The yellow parts
of the first tablet have scarcely affected the second, but
its metallic and blue parts have acted very powerfully.
On comparing the first plate and its image on the second
together, it will be perceived that the parts that have
been affected on the one are less affected on the other.

It may therefore be inferred that the yellow films are
sensitive because they absorb the incident radiation, and
the metallic and blue are insensitive because they re-
flect it.

The effect, in whatever it may consist, which occurs
during the invisible modification is not durable: it grad-
ually passes away. If tablets that have received im-
pressions be kept for a time before developing, the im-
ages upon them gradually disappear. On these tablets
there is no lateral propagation of effect, nothing answer-
ing to conduction.

On examining the operation of a radiation continuous-
ly applied to one of these sensitive films, it will be dis-
covered that a certain time elapses — that is, a certain
amount of the radiation is consumed — before there is any
perceptible effect. When that is accomplished, the radi-
ation affects the film to a degree proportional to its quan-
tity, until a second stage is reached ; there is then an-
other pause, followed by the second stage, in which vis-

420 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

ible modification or chemical decomposition sets in. The
film begins to darken ; it passes through successive tints
— brown, red, olive, blue — and eventually becomes dark
gray.

I have described in some of the foregoing paragraphs
the action of the spectrum on silver iodide as presented
on the tablet of the daguerreotype, showing the differ-
ence in the impression obtained : 1st, when extraneous
light has been excluded ; and, 2d, when it has been per-
mitted simultaneously or previously to act.

In the latter case, in all that region of the spectrum
from the more refrangible extremity to somewhat below
the line G, the usual darkening effect manifested by silver
compounds is observed ; but beyond this, and to the ex-
treme less refrangible rays, with certain variations of in-
tensity, the action of the extraneous and simultaneously
acting light is checked, and the effect of previously act-
ing light is destroyed.

It happened that in 1842 I obtained two very fine
specimens of the latter spectra; one of these I sent to
Sir J. Herschel, the other is still in my possession.

In the Philosophical Magazine (Feb., 1843), Herschel
gave a detailed description of these spectrum impressions.
He was disposed to refer the appearance they present to
the phenomena of thin films, but at the same time point-
ed out the difficulties in the way of that explanation.
He also sent me three proofs he had obtained on ordi-
nary sensitive paper, darkened by exposure to light, then
washed with a solution of iodide of potassium, and placed
in the spectrum. He described them as follows :

(1.) " Blackened paper from which excess of nitrate of
silver has not been abstracted, under the influence of an
iodic salt. Produced by a November sun. N.B. — View
it also transparently against the light."

(2.) "Blackened paper under the influence of an iodic

Mi M..HC XXIX.J ( III.MICAL FORCE IN THE SPECTRUM. 40 ^

salt when no excess of nitrate of silver exists in the
paper.'1

(3.) "Action of spectrum under iodic influence when
very little nitrate of silver remains in excess in the pa-
per. To be viewed also transparently."

These paper photographs I still preserve. They are as
perfect as when first made. The different colored spaces
of the spectrum are marked upon them with pencil. The
appearances they respectively present are as follows: (1)
is bleached by the more refrangible rays, and blackened
deeply from the yellow to the ultra-red ; (2) is bleached
from the ultra invisible red to the ultra-violet (a maxi-
mum occurs abruptly about the blue) ; (3) has the same
upper spectrum as the others, a bleached dot in the cen-
tre of the yellow, and a darkened space on the extreme
red. The action has reached from the ultra-red to the
ultra-violet.

In Herschel's opinion, these effects in the less refran-
gible region are connected with the drying of the paper.
It is well known that paper in a damp condition is more
sensitive than such as is dry. But obviously this con-
dition does not obtain in the case of the daguerreotype
operation, which is essentially a dry process.

In 1846 MM. Foucault and Fizeau, having repeated
the experiment originally made by me, presented a com-
munication to the French Academy of Sciences to the
effect that when a silver tablet which has been sensitized
by exposure to iodine and bromine, and then impressed
by light, is exposed to the spectrum, the effect is greatly
increased in all the region above the line C, and is neu-
tralized in all that below C. They remarked the dis-
tinctness with which the atmospheric .line A comes out,
and saw the ultra spectrum heat-rays a, /3, 7 described by
me some years previously.

The interpretation given by them is, that the more re-

422 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

frangible rays promote the previous action of light ; the
less neutralize it. The curve representing the chemical
intensity of the different rays would cross the axis of
abscissas about the boundary of the red and orange ; be-
low that point to the ultra-red the ordinates would have
negative values ; above it to the ultra-violet those values
would be positive (Goniptes-Itendw, No. 14, tome xxiii.).
Hereupon M. Becquerel communicated to the same
Academy a criticism on this interpretation, the opinion
maintained by him being that while the more refrangi-
ble rays excite sensitive surfaces, the less refrangible, far
from neutralizing, continue the action so besmu. To the

O * O

former he gave the designation " rayons excitateurs," to
the latter "rayons contiuuateurs " (^Oomptes-Hendus^o.
17, tome xxiii.).

In 1847 M. Claudet communicated a paper to the
Royal Society, subsequently published in the Philosoph-
ical Magazine (Feb., 1848), on this subject. His atten-
tion had been drawn to it by observing that the red im-
age of the sun, during a dense fog, had destroyed the
effect previously produced on a sensitive silver surface,
and that this destruction could be occasioned at pleasure
by the use of red and yellow screens. A surface which
has been impressed by daylight, and the impression then
obliterated by the less refrangible rays, had recovered its
primitive condition. It was ready to be impressed again
by daylight, and again the resulting effect might be de-
stroyed. Claudet found that this excitation and neutral-
ization might be repeated many times, the chemical con-
stitution of the film remaining unchanged to the last.

These facts seem to be inconsistent with Herschel's
opinion, that positive and negative pictures may succeed
each other by the continued action of a radiation, on the
principle of Newton's rings.

On a collodion surface such negative neutralizing or

o o

Mi. Mom XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 403

reversing actions cannot be obtained by the less refran-
gible rays. The spectrum impression developed in the
usual manner by an iron salt presents a sudden maxi-
mum about the line G, and continues thence to the high-
est limit of the spectrum. In the other direction it ex-
tends below F. From E to the ultra-red not a trace of
action can be detected. The lines a, /3, 7 cannot be ob-
tained on collodion. There is, therefore, a difference be-
tween its behavior under exposure to light and that of
a daguerreotype tablet.

The reversals that are obtained on collodion by the
use of certain haloid compounds are altogether different
from the reversals on the thin films of a silver tablet.
They are produced by the more refrangible rays.

On exposing a collodion surface (prepared in the usual
manner) to daylight long enough to stain it completely,
then washing off the free nitrate, and in succession dip-
ping the plate into a weak solution of iodide of potas-
sium, exposing it to the spectrum, washing, again dip-
ping it into the nitrate bath, and finally developing, a re-
versed action is obtained. The daylight is perfectly neu-
tralized, but not after the manner in a daguerreotype.
In the region about G, the place of maximum action in
collodion, the impression of the light is totally removed
by an exposure of five seconds. In twelve seconds the
protected space is much larger; in thirty seconds it has
spread from F to H. It is, however, to be particularly
remarked that the less refrangible rays show no action.

The results are substantially the same when, instead
of iodide of potassium, the chloride of sodium, corrosive
sublimate, bromide of potassium, or fluoride of potassium
is used. In all these the reversing action is from F to
H, and has its maximum somewhere about G. That is,
the reversing action coincides with the direct action :
there is no protection in the lower portion of the spec-

424 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

trum, as in the daguerreotype. The effect is altogether
due to the change of composition of the sensitive film.
Ordinarily it contains free nitrate; now it contains free
iodide, chloride, etc.

The silver compounds of collodion absorb the radia-
tions falling on them which are capable of producing a
photographic effect. Yet, sensitive as it is, collodion is
very far from having its maximum sensitiveness, as is
shown by the following experiment, which is of no small
interest to photographers : I took five dry collodion
plates, prepared by what is known as the tannin process,
and, having made a pile of them, caused the rays of a gas-
flame to pass through them all at the same time. On
developing, it was found that the first plate was strongly
impressed, and the second, which had been behind it, ap-
parently quite as much. Even the fifth was considerably
stained. From this it follows that the collodion film, as
ordinarily used, absorbs only a fractional part of the
rays that can affect it. Could it be made to absorb the
whole, its sensitiveness would be correspondingly in-
creased.

Radiations that have suffered complete absorption can
bring about no further change. Partial absorption, aris-
ing from inadequate thickness, may leave a ray possessed
of a portion of its power. There must be a correspond-
ence between the intensity of the incident ray and the
thickness of the absorbing medium to produce a maxi-
mum effect.

Though the silver iodide is affected by radiations of
every refrangibility, it is decomposed so that a subiodide
results only by those of which the wave-length is less
than 5000. If in presence of metallic silver, as on the
daguerreotype tablet, the iodine disengaged unites with
the free silver beneath. The rays of high refrangibility
occasion in it chemical decomposition ; those of less re-

Mi M..IIS XXIX.] ( IIKMICAL FORCE IN THE SPECTRUM. 425

tangibility, physical modification. In the language of
the older theories of actino-chemistry, this substance may
be said to exert a selective absorption. In this it illus-
trates the general principle, that it depends on the nat-
ure of the ponderable material presented to radiations
which of them shall be absorbed.

2d. — Of the Union of Chlorine and Hydrogen.

An interesting experiment, illustrating the fact that
chlorine gas absorbs the radiations which bring about
its combination with hydrogen, may be made by cover-
ing a test-tube containing an explosive mixture of equal
volumes of those gases with a large jar filled with chlo-
rine. This arrangement may be exposed in the open
daylight without risk of exploding the mixture ; but if
the experiment be made with a covering jar containing
atmospheric air instead of chlorine, the gases immediate-
ly unite, and commonly with an explosion.

I placed a mixture of equal volumes of chlorine and
hydrogen in a vessel made of plate-glass, the edges of
the pieces being cemented together. This vessel was so
arranged on a small porcelain trough, containing a satu-
rated solution of common salt, that it could be used as a
gas-jar. The radiations of a lamp were caused to pass
through it, so as to be submitted to the selective absorp-
tion of the mixture. They were then received on a chlor-
hydrogen actinometer.

Successive experiments were then made: 1st, with the
radiations of a lamp after passing through the absorption
vessel ; 2d, with the same radiations after the vessel had
been removed.

Two facts were now apparent: 1st, the mixture of
chlorine and hydrogen in the absorption vessel began to
unite under the influence of the rays of the lamp; *J«1,
the rays which had passed through that mixture had lost

426 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

very much of their chemical force. It was not totally
extinct, but the actinometer showed that it had under-
gone a very great diminution.

From this it follows that, on its passage through a
mixture of chlorine and hydrogen, the radiation had suf-
fered absorption, and as respects the mixture under trial
had become deactiuized. Simultaneously the mixture
itself had been affected, its constituent gases uniting.
And thus it appears that the radiation had undergone a
change in producing a change in the ponderable matter.

The following modification of this experiment shows
the part played by the chlorine and hydrogen respective-
ly when they are in the act of uniting :

(«) The glass absorption vessel above described was
filled with atmospheric air, and the chemical force of the
radiation passing from the lamp through it was deter-
mined. It was measured by the time required to cause
the index of the actinometer to descend through one di-

o

vision. This was 12 seconds.

(I) The absorption vessel was now half filled with
chlorine, obtained from hydrochloric acid and peroxide
of manganese. The chemical force of the ray after pass-
ing through it was determined as before. It was now
represented by 25£ seconds.

(<?) To the chlorine an equal volume of hydrogen was
added, the absorption vessel being consequently full of
the mixture. The radiation was now passing through a
stratum of chlorine diluted with hydrogen, and the point
to be determined was whether it had undergone the
same, or a greater, or less loss than in the preceding case,
since the chlorine was now uniting with the hydrogen.
On measuring the force, it was found to be represented
by 19 seconds.

(d) Lastly, the first (a) of these measures was re-
peated with a view of ascertaining whether the inten-

MKMOIH XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 427

sity of the lamp had changed. It gave 12 seconds as
before.

From these observations it may be concluded that the
addition of hydrogen to chlorine does not increase its
absorptive power. Moreover, it is obvious that the ac-
tion of the radiation is expended primarily on the chlo-
rine, giving it a disposition to unite with the hydrogen,
and that the functions discharged by the chlorine and
by the hydrogen respectively are altogether different.
The ray itself also undergoes a change ; it suffers absorp-
tion and loss of a part of its via viva.

As to the ray which is thus absorbed. In 1835 I
found that a radiation which had passed through a solu-
tion of potassium bichromate failed to accomplish the
union of chlorine and hydrogen ; but one which had
passed through ammonia sulphate of copper could do it
energetically. This indicates that the effective rays are
among the more refrangible. On exposing these gases
in the spectrum, the maximum action takes place in the
indigo rays (Philosophical Magazine, Dec., 1843), Memoir
XVIII.

Recently (1871) some suggestions have been made by
M. Budde respecting the action of light upon chlorine.
Admitting the correctness of the theorem that the mole-

o

cules of most elementary gases consist of two atoms, he
conceives that the effect of li^ht on chlorine is to tend

O

to divide, or actually to divide, its molecule into isolated
atoms. These atoms, if the gas be kept in the dark,
may reunite into molecules.

The chlorine molecule cannot unite with hydrogen ;
the chlorine atom can ; hence insolation brings on com-
bination. But if the chlorine be unmixed, there will, as
a consequence of insolation, be a certain proportion of
uncombined atoms; and from this, together with Avo-
gadro's theorem, is drawn the conclusion that this gas

428 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

through insolation increases in specific volume. More-
over, as the reunion of the chlorine atoms probably
produces heat, rays of high refrangibility will cause
chlorine to expand ; but it will contract to its original
volume when no longer under the influence of light.

In corroboration of this conclusion, Budde found that
a differential thermometer filled with chlorine showed a
certain expansion when placed in the red or yellow rays,
but it gave an expansion six or seven times greater
when in the violet rays. With carbonic acid and ether
no such effect took place.

It should not be forgotten, however, in considering
the bearing of these experiments, that chlorine merely
because it is yellowish -green will absorb rays of a com-
plementary— that is, of an indigo and violet — color and
become heated thereby.

It has next to be determined whether the points of
maximum action — that is, the points of maximum ab-
sorption— correspond to the rays of emission of either or
both these gases, as they apparently ought to do under
Angstrom's law: "A gas when luminous emits rays of
light of the same refrangibility as those which it has the
power to absorb."

Of the four rays characteristic of hydrogen there is
one the wave-length of which is 4340. It is in the in-
digo space.

Pliicker gives for chlorine a ray nearly answering to
this. Its wave-length is 4338, and also another, 4346, the
latter being one of the best marked of the chlorine lines.

There are, therefore, rays in the indigo which are ab-
sorbed both by hydrogen and by chlorine. The place
of these rays in the spectrum corresponds to that in
which the gases unite — the place of maximum action for
their mixture.

But the absorptive action of chlorine is not limited

MI.M..IK XXIX. j CI1K.MICAL FORCE IN Till: M'KCTRUM. |-j;»

to a few isolated lines. The gas removes a very large
portion of the spectrum. Subsequent experiments must
determine whether each of these lines of absorption is
also a line of maximum chemical action.

The chlor- hydrogen actinometer furnishes the means
of ascertaining many facts respecting the combination of
those substances advantageously, since it gives accurate
quantitative measures.

By referring to my papers in the Philosophical Mag-
<cine (Dec., 1843; July, 1844; Nov., 1845; Nov., 1857) it
will be found that chlorine and hydrogen do not unite
in the dark at any ordinary temperature or in any
length of time ; but if exposed to a feeble radiation, such
as that of a lamp, they are strongly affected. The phe-
nomena present two phases: 1st, for a brief period there
is no recognizable chemical effect, a preliminary actiniza-
tion, or, as Professors Bunsen and Roscoe subsequently
termed it, photo -chemical induction, taking place (it is
manifested by an expansion and contraction of the mixt-
ure) ; 2d, the combination of the gases begins, it steadily
increases, and soon acquires uniformity. In obtaining
measures by the use of these gases we must, therefore,
wait until this preliminary actinization is completed.
That accomplished, the hydrochloric acid arising from
the union of the gases is absorbed so quickly that the
movements of the index-liquid over the graduated scale
give trustworthy indications.

As regards the duration of the effect produced on the
gases by this preliminary actinization, I found that it
continued some time — several hours (Philosophical Mag-
<i::ine, July, 1844), Memoir XIX. Professors Bunsen and
Roscoe, however, in their Memoir read before the Royal
Society, state that it is quite transient (Transact ion*
of the Royal Society, 1856).

This preliminary actinization completed, the quantity

430 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

of hydrochloric acid produced measures the quantity of
the acting radiation. This I proved by using a gas-
flame of standard height, and a measuring lens (page
265) consisting of a double convex, five inches in diame-
ter, sectors of which could be uncovered by the rotation
of pasteboard screens upon its centre, the quantity of hy-
drochloric acid produced in a given time being propor-
tional to the area of the sector uncovered. The same was
also proved by using a standard flame and exposing the
gases during different periods of time. The quantity
of hydrochloric acid produced is proportional to the time.
The following experiment illustrates the phenomena
arising durino; the actinization of a mixture of chlorine

O O

and hydrogen, and substantiates several of the foregoing
statements.

The diverging rays of a lamp were made parallel by a
suitable combination of convex lenses. In the resulting
beam a chlor- hydrogen actinometer was placed, there
being in front of it a metallic screen, so arranged that it
could be easily removed or replaced, and thus permit the
rays of the lamp to fall on the actinometer or intercept
them.

On removing the screen and allowing the rays to fall
on the sensitive mixture in the actinometer, an expansion
amounting to half a degree was observed. In 60 sec-
onds this expansion ceased.

The volume of the mixture now remained stationary,
no apparent change going on in it. At length, after the
close of 270 seconds, it was beginning to contract and
hydrochloric acid to form.

At the end of 45 seconds more a contraction of half a
degree had occurred : the volume of the mixture was
therefore now the same as when the experiment began,
this half degree of contraction compensating for the half
degree of expansion.

MKMOIR XXIX.] CHEMICAL FORCE IN Till. spr.CTRUM.

The rate of contraction of the gaseous mixture — that
K the rate at which its constituents were uniting — was
then ascertained.

From these observations it appeared that when chlo-
rine and hydrogen unite, under the influence of a radia-
tion, there are four distinct periods of action :

1st. For a brief period the mixture expands.

2d. For a much longer period it then remains station-
ary in volume, though still absorbing rays.

3d. Contraction arising from the production of hydro-
chloric acid begins. At first it goes on slowly, then
more and more rapidly.

4th. After that contraction is fully established, it pro-
ceeds with uniformity, equal quantities of hydrochloric
acid being produced in equal times by the action of
equal quantities of the rays.

The prominent phenomena exhibited by a mixture of
chlorine and hydrogen are a preliminary absorption and
a subsequent definite action.

It may be remarked, since a similar preliminary ab-
sorption occurs in the case of other sensitive substances,
that there is in practical photography an advantage, both
as respects time and correctness in light and shadow,
gained by submitting a sensitive surface to a brief ex-
posure in a dim light, so as to pass it through its pre-
liminary stage.

The expansion referred to as taking place during the
first of these periods may be advantageously observed
when the disturbing radiation is very intense. It is well
seen when a Leyden-jar is discharged in the vicinity of
the actinometer. Though this light lasts but a very
small fraction of a second, it produces an instantaneous ex-
pansion, followed by an instantaneous contraction. Not
nnfrequently the gases unite with an explosion. I have
had several of these instruments destroyed in that manner.

432 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

It might be supposed that this instantaneous expansion
is due to a heat disturbance arising from the absorption
of rays that are not engaged in producing the chemical
effect. But this interpretation seems to be incompatible
with the instantaneously following contraction. Though
it is admissible that heat should be instantaneously dis-
engaged by the preliminary actinization, it is difficult to
conceive how it can so instantaneously disappear.

When the radiation is withdrawn and the hydrochloric
acid absorbed, there is no after-combining. The action is
perfectly definite. For a given amount of chemical ac-
tion, an equivalent quantity of the radiation is absorbed.

The instances I have cited in this discussion of the
mode of action of radiations are, one of decomposition, in
the case of the silver iodide ; and one of combination, in
the case of hydrochloric acid. I might have introduced
another — the dissociation of ferric oxalate, which I have
closely studied — but it would have made the Memoir of
undue length. From the facts herein considered the fol-
lowing deductions may be drawn :

When a radiation impinges on a material substance, it
imparts to that substance more or less of its vis viva, and
therefore undergoes a change itself. The substance also
is disturbed. Its physical and chemical properties de-
termine the resulting phenomena.

(1st.) If the substance be black and undecomposable,
the radiation establishes vibrations among the molecules
it encounters. We interpret these vibrations as radiant
heat. The molecules of the medium do not lose the vis
viva they have acquired at once, since they are of greater
density than the ether. Each becomes a centre of agita-
tion, and heat-radiation and conduction in all directions
are the result. The undulations thus set up are com-
monly of longer waves ; and as the movements gradually

MBMUH: XXIX.] ( 11KMK Al. JOB I IN llll. BPK 1 Kl'M.

dec-line, tlie shorter waves of these are the first to b<
Anguished, the longer ones the last. This, therefore, i-
in accordance with what I found to be the case in the
-i ;ulual warming of a solid body, in which the long waves
pertain to a low temperature, the short ones arising as
the temperature ascends (Memoir I.).

In some cases, however, instead of the disturbing un-
dulation giving rise to longer waves, it produces shorter
ones, as is shown when a platinum wire is put into a
hydrogen flame, or by Tyndall's experiments, in which
invisible undulations below the red give rise to the igni-
tion of platinum.

(2d.) If the substance be colored and undecomposable,
it will extinguish rays complementary to its own tint.
The temperature will rise correspondingly.

(3d.) If the substance be decomposable, those portions
of the radiation presented to it which are of a comple-
mentary tint will be extinguished. The force thus dis-
appearing will not be expended in establishing vibrations
in the arresting particles, but in breaking down the union
of those which have arrested them from associated par-
ticles. No vibrations, therefore, are originated ; no heat
is produced ; there is no lateral conduction.

In actinic decompositions the effects may be conven-
iently divided into two phases: 1st, physical ; 2d, chemi-
cal. The physical phase precedes the chemical. It con-
sists in a preliminary disturbance of the group of mol-
ecules about to be decomposed. Up to a certain point
the dislocation taking place may be retraced or reduced,
and things brought back to their original condition.
But that point once gained, decomposition ensues, and
the result is permanent.

I may perhaps illustrate this by a familiar example.
If a sheet of paper be held before a fire, its surface will

EE

434 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX.

gradually warm ; and if the exposure be not too long or
the fire too hot, on removing it the paper will gradually
cool, recovering its former condition without any perma-
nent change. One could conceive that the laws of ab-
sorption and radiation might not only be studied, but
again and again illustrated by the exposure and removal
of such a sheet. But a certain point of temperature or
exposure gained, the paper scorches — that is, undergoes
chemical change — and then there is no restoration, no re-
covery of its original condition. Hence it may be said
of such a sheet of paper that it exhibits two phases, in
the first of which a return to the original condition is
possible ; in the second such a return is impossible, be-
cause of the supervening of the chemical change.

An investigation of the facts produced by a ray pre-
sents, then, these two separate and distinct phases — the
physical and the chemical.

General Conclusions.

The facts presented in the preceding and the present
Memoir suggest the following conclusions:

1st. That the concentration of heat heretofore observed
in the less refrangible portion of the prismatic spectrum
arises from the special action of the prism, and would not
be perceived in a diffraction spectrum.

2d. From the long-observed and unquestionable fact
that there is in the prismatic spectrum a gradual dimi-
nution in the heat-measures from a maximum below the
red to a minimum in the violet, coupled with the fact
now presented by me (that the heat of the upper half of
the spectrum is equal to that of the lower half), it follows
that the true distribution of heat throughout the spaces
of the spectrum is equal. In consequence of the equal ve-
locity of ether-waves, they will, on complete extinction by
a receiving surface, generate equal quantities of heat, no

Mi.M.iut XXIX.J ClIKMICAL FORCE IN THE SPECTRUM. 435

matter what their length may be, provided that their
extinction takes place without producing any chemical
effect.

3d. That it is incorrect to restrict to the upper portion
of the spectrum the property of producing chemical
changes. Such changes may be produced by waves of
any refrangibility.

4th. That every chemical effect observed in the spec-
trum is in consequence of the absorption of a specific
radiation, the absorbed or acting radiation being deter-
mined by the properties of the substance undergoing
change.

5th. That the figure so generally employed in works
on actino-chemistry to indicate the distribution of heat,
light, and actinism in the spectrum serves only to mis-
lead. The heat curve is determined by the action of the
prism, not by the properties of calorific radiations ; the
actinic curve does not represent any special peculiarities
of the spectrum, but the habitudes of certain compounds
of silver.

436 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

MEMOIR XXX.

ON BUENING GLASSES AND MIEROES THEIE HEATING AND

CHEMICAL EFFECTS.

Collected and condensed from the Philosophical Magazine, May, 1851; Harper's
Monthly Magazine, No. 329.

CONTENTS : — Can concentrated rays produce new chemical decompositions?
— Effects of amplitude, frequency, and direction of vibration in the
ether-waves. — Clock lenses for long exposures. — Decomposition of water
by chlorine. — Attempt to decompose it by bromine and iodine. — Use of
absorbing troughs. — Dry silver iodide not decomposed by light. — De-
compositions under water. — Decompositions in a spherical concave. —
Effect of extraneous mixtures. — They do not make collodion more sen-
sitive.— Antagonization of radiations. — Case of electric spark. — Effects
of polarized light. — Attempts to polarize light by an electro-magnet. —
Mechanical cause of decompositions by light.

WHEN, many years ago, I commenced an experimental
examination of the chemical action of light, I entertained
great expectations of what might be accomplished by the
use of burning-glasses. It seemed reasonable to suppose
that if the direct sun -rays could occasion so many de-
compositions, their chemical force would be incompara-
bly greater when their brilliancy was exalted by a mirror
or a lens. Of the two, a concave metallic mirror should
produce a more characteristic effect, since it returns the
rays as it receives them, but a special and very impor-
tant portion of them is absorbed by the selective action
of the lens.

I had not, however, at that time the means of making
these experiments in a satisfactory manner, and, though
very much disappointed with the result, postponed the

MEMOIH XXX.] ON III KNIN<; CLASSICS AND MlHU<>l;>

prosecution of them to a more favorable opportunity.
Obtaining from time to time several isolated facts, I was
led, in meditating upon them, to what seemed to be some
general conclusions respecting the chemical action of ra-
diations. Several of these were published, in a desultory
manner, in various periodicals ; but it was not until May,
1851, that they were collected in the Philosophical Mag-
<i;/ne, under the title of a "Memoir on the Chemical Ac-
tion of Light." Of this, the following is an abstract.

The general discussion of the problem of the chemical
action of a ray involves the following considerations:

1. In what manner does the ray act, and what are the
changes it undergoes ?

2. What is the nature of the impression made on the
material group, the decomposition of which ensues?

Many facts justify the supposition that the parts of all
material substances are in a state of incessant vibration.
To each particular thermometric degree there belongs a
particular frequency of vibration. As soon as these mo-
tions approach four hundred billions in a second, red
light is emitted, and the temperature is near 1000° Fahr.
As the frequency increases, rays of a higher refrangibility
are in succession evolved, and the temperature correspond-
ingly rises. On the other hand, when these oscillatory
movements decline, the temperature of the body falls.

These principles lead to a ready explanation of the
nature of the exchanges of heat and the cause of the
equilibrium of temperature. The vibratory molecular
motions are necessarily propagated to the ether, through
which medium they are again transferred to the particles
of other bodies, on which the ethereal waves impinge, as
a vibrating string excites undulations in the air, and
these, in their turn, can give birth to analogous motions
in other strings at a distance.

There is an analogy between the relations of a hot

438 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

and cold body and those of two strings, one of which is
emitting a musical sound and compelling the other to
execute synchronous movements. The ether in the one
case and the air in the other are the media through which
their motions pass.

Equilibrium of temperature takes place when the mol-
ecules of the substances concerned are in synchronous
and equal vibration. A hot body in presence of a cold
one compels the latter to hasten its rate of motion, its
own rate all the time declining, and this continues until
both have the same frequency ; then equilibrium of tem-
perature results. The theory of the exchanges of heat is,
therefore, only an expression for the exchanges of vibra-
tions through the ether.

But temperature in thermotics is the equivalent term
for brilliancy in optics. Both refer to compound quali-
ties, depending not only on frequency of vibration, but
also on its amplitude. As the degree of heat of a mass
rises, the mass expands, the increase in its volume indi-
cating that not only do its parts vibrate more swiftly,
but also that their individual excursions are increased.
It follows, therefore, that every mass will have a deter-
minate volume for every degree of heat, the volume in-
creasing as the temperature rises. On this view the ex-
planation of the expansion of bodies by heat is that their
parts are not only vibrating more quickly, but also that
the individual excursions are greater.

The atoms of the chemical elements differ in weight.
We therefore should not expect that the ethereal vibra-
tions would throw them into movement with equal facil-
ity, but that some would yield more readily than others.
Is not this what we express in chemistry by the term
specific heat ? — a body, the capacity of which is great, re-
quiring a prolonged application of ethereal pulses before
a consentaneous motion is reached, and in its turn im-

Mi M..IK \XX.J ON BURNING GLASSKS AM) MIKI:-

pressing on the ether during cooling a correspondingly
prolonged series of motions. And is not this tin- i-au>«
of that remarkable relation between the atomic \\ fights
of elementary bodies and their specific heats, discovered
by Dulong and Petit?

These considerations may lead us to inquire whether
the general cause of the decomposition of compound bod-
ies by radiations is due to the circumstance that all the
atoms of which their molecules are composed take on the
vibratory motion with unequal facility. Thus if a cer-
tain compound molecule be submitted to the influence of
an intense radiation, some of its constituent particles may
vibrate consentaneously at once, and others more tardily.
Under these circumstances, the continued existence of
the group may become impossible, and decomposition en-
sue in the necessity of the case.

In entering upon the experimental analysis of the ac-
tion of a ray upon a decomposable body, there are three
different points to be considered, so far as the ray itself
is concerned : 1. To what extent and in what manner is
the result affected by the intensity of the ray, or by the
amplitude of the vibrating excursions ? 2. How is it af-
fected by the frequency of the pulsatory impressions?
and, 3. How by the direction in which the vibrations
arc made, as involved in the idea of polarization? I
shall now examine these in succession.

1. ^77? what extent and in what manner is tlie decompo-
sition of a compound body affected by the INTENSITY of a
ray or the amplitude of the vibrating excursions f

If the different degrees of facility with which atoms
receive the impression of ethereal vibrations be the true
cause of decomposition by light, we should expect that
many such changes would become possible under the in-
fluence of a burning-lens which are not so in the direct
rays of the sun.

440 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

This idea is favored by what we find in the case of heat.
The burning-glass has long had celebrity in that respect,
and in former times was the most powerful means of
reaching a high temperature.

The effect of the glass is due to the rapidity with
which it can supply caloric, contrasted with the loss by
conduction, radiation, etc. Thus an object of any kind
exposed to the sun receives heat at a certain rate; but
it is simultaneously experiencing a loss by conduction,
radiation, and currents in the air. Exposed to the focus
of a lens, the supply becomes, in a given time, greater
than before ; and the temperature rising, great effects are
the necessary result.

But changes brought about by light are in a different
predicament. Here conduction is entirely absent, as is
also loss by currents in the air. The cumulative effects
of a long exposure give the same action as a highly con-
centrated ray furnishes in a brief period of time. In this
case, therefore, everything will depend on the absorptive
power of the substance.

When a piece of polished silver is placed in the focus
of a burning -lens, it remains quite cold, because of its
high reflecting power ; but if blackened, it melts in an
instant. And so with chemical changes. A body which,
like chlorine, can exert an absorptive action on the ray
becomes modified, and induces changes; but if, like oxy-
gen, it has not that property, it will remain indifferent
and unaffected by the most intense radiation.

Considering, however, that the calorific effects of the
converged solar rays are so striking, we may reasonably
inquire whether, in like manner, the chemical action can
be increased. There is a very general impression that
the intense radiation of tropical climates accomplishes
changes which cannot be imitated by the feebler light
of higher latitudes, and perhaps decompositions may be

Mi^ : X\\ ] OX lUKMNC CLASSICS AND MIHUORS.

ill

1 nought about by a large convex lens which the direct
rays of the sun arc wholly inadequate to produce.

A very brilliant beam may possibly break up a given
combination, which a far greater quantity of light, acting
through a long period, might be inadequate to touch.
Sir R. Kane states that he, with M. Dumas, could remove
two atoms of hydrogen from acetone by the action of
chlorine in the sunshine at Paris, but in Dublin only
one.

In Fig. 90, a is a convex burning -lens supported in
ribbed frame, b b ; there is at c a second lens to hasten

the convergence; (Id, a cir-
cular arc for directing the

O

lens towards the sun ; e g,
a stand on which objects
may be exposed to the fo-

Fig. 90. .

cal point,/. It is earned
by a stout bar, m n, attached to the frame.

By this instrument I endeavored to collect a series
of facts which might set this part of the question in
its true light. The lens a was of very fine and thin
French plate -glass, twelve inches in diameter in the
clear. Its goodness was such that on a fine day plat-
inum might be melted in its focus. It was ground

442 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

and polished for me by the late Mr. Fitz, whose skill
was shown in the large and excellent telescopic objec-
tives that he made. He mounted it on a suitable sup-
port ; it required, however, to be guided by the hand as
the sun moved. When the college building of the Med-
ical Department of the University of New York was de-
stroyed by fire in 1865, 1 had to regret the loss of this
instrument, with much other apparatus, and many docu-
ments that were of uu appreciable value to me. Mounted
as the lens was, its use was attended with considerable
risk to the eyes, on account of the excessive brilliancy 6f
the focus. Screens and dark spectacles were found to be
very unsatisfactory, and an illness which I consequently
contracted admonished me either to abandon the subject
or pursue it in some other way.

The following experiments were made with a smaller
glass, consisting of a combination of two similar lenses,
their diameter being five inches and focal distance eight.
It was, in fact, the large lens of an old-fashioned lucerual
microscope, such as was made in London a century ago.
I had it fixed on a polar axis, as shown in Fig. 91, and
by the aid of a clock it could follow the motion of the
sun with such accuracy that, when once set in the morn-
ing, an object might be exposed in its focus, if desirable,
for a whole day. It had a contrivance on the frame car-
rying the lens for supporting small crucibles, glass mat-
rasses (Fig. 92), charcoal supports, etc., at the proper
point, which might be either at the focus or at any other
distance from the lens, as the circumstances of the exper-
iment required. Among these instruments were ther-
mometers, blackened or otherwise so arranged as to ex-
ercise any desired selective absorption. At the outset
of any experiment, the whole face of the lens could be
covered with a blackened pasteboard screen, with a hole
half an inch in diameter. Through this a sufficient

.MI.M..H: xxx.] ON m KMM, <.L.\.NM;> AND MIKI

11:;

amount of light could be transmitted to enable on«- t««
arranuv the various details of tin- proposed «.-.\j)criin<-iit :
and when everything was ready, the screen wa^
moved, and in the concentrated and brilliant focus tin-
art ion went on. I found that this simple contrivance
was an invaluable relief to the eyes.

In Fig. 91, a a is the heliostat clock; b, its polar axis;
d d, a frame carrying the lens, <?, and
having an arrangement at f for support-
ing flasks, crucibles, or other apparatus.
This turns on a double joint at <?, so
that the lens may be directed to the
sun.

In Fig. 92, # is a small flask receiving
the converging rays, £, at their focus, f.

The lens being five inches in di-
ameter, and the space covered by
the solar focal image, owing to
want of achromaticity and spherical aberration, one fifth
of an inch, the multiplying effect
would be 625 times, if the glass were
perfectly transparent, and there were
no loss by reflection from its sur-
faces. On a summer day of average
brightness, with the thermometer at
68° in the shade, and the bulb, not
being' blackened, at 108° in the sun,
this lens could fuse copper instantly, the bead oxidizing
only superficially, and cutting readily after fusion. Black
oxide of copper in a little crucible of platinum foil melt-
ed into a slaty-looking substance at once. Wrought iron
did not melt alone; but if exposed on a charcoal support
in a globule of microcosm ic salt, previously fused by the
lens, it gave a clear, round bead, which readily extended
when beaten upon an anvil. The globule of flux turned

Fig. 93.

444 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

black. The specimen employed was cut from a piece of
good iron wire ; and though it might be thought that ex-
posure on the charcoal would tend to turn it into cast
iron, its subsequent complete malleability seems to dis-
prove this. Spongy platinum did not melt alone, nor
even if enclosed in a globule of fused microcosrnic salt.
We may therefore estimate the working power of this
lens on a substance placed in its focus as somewhat
above the point of fusion of wrought iron, and lower
than the point of fusion of platinum. This refers to tem-
perature only. The power of the lens as to light must
be enormously greater.

We may now examine the chemical eifects produced
by this lens.

Two small glass matrasses, the bulbs of which were
about half an inch in diameter, were filled with chlorine
water, the one being exposed to the direct rays of the
sun, the other to the converging rays of the lens. De-
composition of the water occurred in both, but with far
more activity in that placed in the focal point. The dif-
ference was at once so striking to the eye that I made no
attempt to measure it. It is plain that the greater the
quantity of incident light, the more rapid the decomposi-
tion ; though, after the first moment of action, the solu-
tions being no longer the same in constitution, the quan-
tities of gas disengaged are no longer proportional to the
incident liojht.

•^j

There is thus no difficulty in effecting rapidly the de-
composition of water by chlorine under the influence of
the sun, but under the same circumstances iodine and
bromine are inadequate to produce such an effect.

A solution of bromine in water was prepared, the wa-
ter being first boiled to expel the air contained in it. It
was placed in a half inch matrass (Fig. 93), and exposed
to the focus of the lens. As the temperature rose rap-

MKM..IU XXX.) ON IH'RNINt; (il.ASSKS AND .MIUK<H;-v J J ;,

i«ll\, the water was depressed in the hull) by the steam
and bromine vapor winch occupied the upju-r part, the
bulb being placed uppermost, and the tube dipping into
a phial which served as a reservoir. After the exposure
had continued for two hours and a half, the matrass was
removed from the lens and suffered to cool. There re-
mained uncondensed a little bubble, measuring about yta
of a cubic inch ; but this was probably nothing more
than the atmospheric air which had found access to the
water, for on submitting the same specimen to another
exposure for three hours, after the gas had been decanted
from it, a little bubble, the diameter of which was esti-
mated at one fiftieth of an inch, was all that could be
procured.

In Fig. 93, a is the flask containing the bromine water ;
b, a phial serving as a reservoir. It is half filled
with the same water.

In like manner I endeavored to decompose wa-
ter by iodine, and with the same negative result, /r\
even when the exposure to the focal point lasted
four hours. When proper care had been taken

to remove from the solution all traces of air, no rig. 93.
gas was evolved.

To reduce the heating effect of the lens, and allow the
more refrangible rays alone to act, there was interposed
between the lens and its focus a stratum of a solution of
sulphate of copper and ammonia one third of an inch
thick, included between two flat plates of glass, suitably
arranged and carried along with the other parts by the
movement of the clock. The cone of solar rays now
passed through this absorbent medium (Fig. 94).

In Fig. 94, #, b is the flask and reservoir, but the con-
verging rays pass through an absorbent trough, c c,
shown in front view at d, e being the circular cell con-
taining the blue solution.

446

ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

Fig. 94.

Ill the focus of blue light thus formed there was ex-
posed for two and a half
hours (from 7| to 10£ A.M.,
June 13, 1848) an inverted
half -inch bulb containing
iodine water, with a few
particles of iodine. Tem-
perature in the shade, 64° ;
in the sun, 86°. At the
end of that time there was
found an insignificant bub-

O

ble of air, estimated at one
thirtieth of an inch in diameter. It could, of course, be
nothing but atmospheric air.

The absorbing medium was now removed, and the
full rays of the sun permitted to converge on the mat-
rass. The temperature of the water quickly ran up to
the boiling-point, and the bulb was filled with steam
and the purple vapor of iodine. Everything seemed
favorable for the decomposition of the water to take
place, if the iodine could accomplish it under so intense
a radiation. At first I thought that the experiment had
succeeded, for the color of the bulb became paler — a
result that ought to have ensued if hydriodic acid
was forming and oxygen being eliminated. The action,
therefore, was kept up for four hours ; but as soon as
the sun was screened from the lens and the bulb began
to cool, the water returned and filled it almost entirely.
This, therefore, shows that under a most intense radia-
tion iodine cannot decompose water.

A similar experiment was tried with bromine, and
with the same result. It failed to decompose water.

Some silver chloride, carefully purified, was exposed
in a little crucible of platinum foil (Fig. 95) so inclined
that the cone of rays could come in at its mouth. The

M..M..IK XXX.] ON lU'KMNti QLA88B9 AND MIKKnUv j j-

trough \va> not used. Though tlie sun's r.i\>
wi-re not brilliant, the chloride at once melted, forming a
reddish -looking liquid. It was kept in that condition
all day. When cool it proved to be in the state of
horn-silver, easily cut by a knife. When the rays first
touched it a fume was disengaged, due* probably to the
escape of vapor of water. It seems, therefore, that this
substance when perfectly dry is not decomposable by-
sunlight, though so sensitive at common temperatures
when moist.

In Fig. 95, a is the platinum crucible ; &, the place of the
material experimented upon, re-
ceiving at the focus the converg-
ing rays, c.

I must refer to the original
Memoir for the detail of numer-
ous experiments on many metal-
lic compounds, the general re-
sult of these being that, no mat-

ter how brilliant a ray may be, it cannot carry a decom-
position farther than a feeble one acting for a corre-
sponding longer period of time could do. Compounds
that can resist the force of an ordinary ray cannot be
broken down by the intense illumination of the focal
point of a burning -lens. That instrument cannot do
what the voltaic pile has done — effect decompositions
which had never been effected before.

To reduce the disturbing effect of heat as far as pos-
sible, and give every advantage to the condensed lumi-
nous focus, I received the cone of rays coining from the
twelve -inch burning -lens on a glass globe (Fig. 90) six
inches in diameter, filled with water. This increased
the converging of the rays, and brought them more
quickly to a focus. Then through the neck of the globe
was introduced to the focus, in a matrass, spoon, or other

448 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

suitable support, the substance to be experimented upon.
The mass of water kept the temperature down, and in
some cases the hot water was removed by an aspirator
and cold water introduced below. A spoon could be
used when powders were employed of so great a specific
gravity as not to drift too high from the focus in the
ascending current of hot water.

In Fig. 96, a a is a matrass filled with water, through

which come in the converging
rays, c. Through the neck at d
a spoon, b, may be passed down
to the focal point,/.

The result was, however, the
same as before. The focus of a
burning-lens cannot cause any
Fig-96- chemical change which the un-

converged sun-rays are incompetent to produce. It mere-
ly hastens the effect.

Upon the whole, we may therefore conclude that it is
not the intensity of a beam which determines its decom-
posing power, and that we cannot produce greater chem-
ical effects by the action of converging mirrors and lenses
than we can by the application of the simple sunbeam,
continued for an equivalent period of time.

In estimating the influence of light on different solu-
tions, we should constantly bear in mind that the maxi-
mum effect is never produced unless complete absorption
has taken place. When the color of a solution is pale,
it may require considerable thickness before complete
absorption is accomplished. Thus if two equal tubes,
containing equal quantities of the same solution of chlo-
rine in water, be exposed to the rays, they will evolve
equal quantities of oxygen gas; but if behind one of
them a piece of looking-glass be placed, the effect is im-
mediately increased. The rays that have passed through

MEMOIR XXX.] ON BURNING GLASSES AND MIKK( »KS. .j.jc,

the solution are compelled to cross it again, and, if not
already exhausted, thus to act once more. The follow-
ing illustrations are examples of the kind :

Two small bulbs of equal size containing chlorine
water were exposed to the rays of the sun ; behind one
of them a concave hemispherical mirror was placed, so
that the rays which had crossed the solution were com-
pelled to cross it again. The amount of oxygen set free
in this bulb was about one fourth greater than that in
the other.

The same was repeated, the exposure being to the sky
light instead of the sun-rays. The quantity of oxygen
set free in the two bulbs was as 18 to 55.

It might be supposed that part of this increased effect
is due to the rise of temperature, from the mirror ob-
structing radiation. To exert a cooling action the fol-
lowing modification was therefore tried. In a glass jar
(Fig. 97) full of quicksilver a half-inch bulb containing
chlorine water was placed in such a way that a small
portion of its surface, about one eighth of an inch in di-
ameter, projected above the surface of the liquid metal.
On this part the solar focus from a burning-glass was
thrown. The rays therefore gained access to the interior
of the bulb, and were thrown about in all directions,
crossing and recrossing the liquid in every way by the
numerous reflections they underwent — the mercury, as it
applied itself to the outer surface of the glass, acting like
a spherical concave mirror, and, from its mass and high
conducting power, effectually keeping the temperature
down. The quantity of pxygen emitted in a given time
was measured. The same experiment was then repeated
with the bulb removed from the mercury. After the
close of the same time, on measuring the oxygen set free,
it was found that the reflecting action of the mercury
had nearly tripled the effect.

FF

450 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

In Fig. 97, a a is the vessel filled with mercury ; I d, the
glass flask immersed in it, but having
at its upper part a small portion un-
covered, through which the converging
rays, <?, may come in.

The power of a ray thus depending
on the degree of absorption exerted
upon it, I was led to inquire whether,
by admixture with other suitable sub-
stances in a solution undergoing de-
composition, the effect could be increased. Chlorine wa-
ter decomposes more rapidly as its yellow tint is deeper.
Four equal bulbs were therefore taken — a, containing
chlorine water ; &, the same, deepened with chloride of
gold ; #, chlorine water, with commercial hydrochloric
acid of a yellow tint ; d, chlorine water, with tincture of
iodine. These were all exposed together to the sun. It
was at once obvious that a was giving off most oxygen,
and eventually it was found that b yielded a much
smaller quantity, and c and d none at all. The presence
of these bodies, therefore, exerted a prejudicial effect.

A system of vibrating molecules will solicit an adja-
cent one to execute similar motions through the medium
of the intervening ether. A rise of temperature is due
to an increased rapidity or intensity of the oscillations
of the groups of vibrating molecules, but chemical de-
composition is due to the dislocation of their parts. It,
of course, by no means follows that when a compound
molecule is undergoing entire disruption, those in the
neighborhood should be compelled to pass into a similar
state. For the very reason that chemical decomposition
takes place is because the group that receives the pro-
voking ray cannot vibrate consentaneously with it; and
if that group cannot assume the motion in question, how
can it possibly transmit it to any other ?

MI.M..IU XXX.] ON BURNING Li I. \D Ml KK< >K>. j ;, j

Any artificial coloration by the addition of extraneous
bodies does not increase the rate of decomposition, but
retards it. This is precisely what ought to be exp«-ct«-d.
A compound atom has its grouping destroyed by the
action of light upon its own parts, and is in no manner
concerned in what is taking place in other atoms around.
They therefore cannot increase the effect on it; but, on
the contrary, they may greatly diminish the action on
the mass by exerting a special absorption themselves.
Thus the chloride of gold retards the decomposition of
chlorine water, when mixed therewith, in the same man-
ner as if it were placed in a trough in front of the water,
and intercepted the impinging beam.

Experiments similar to the foregoing were made with
a solution of ferric oxalate mixed with alcohol, ammonia
citrate of iron, tincture of turmeric, sodic chloride, etc.
In every instance it was clear that the action of the light
is strictly molecular; that it is impressed on the group of
atoms, and not on the mass ; and that when various bod-
ies are conjointly exposed to the sun, each one undergoes
its own specific change, independently of and unaffected
by all the rest.

These experiments, with others of a like kind, made
many years ago, have an important bearing on some
recently published by Professor Vogel, Captain Abney,
Captain Waterhouse, and others on imparting increased
sensitiveness to collodion by mixing it with variously
colored substances. I repeated their experiments as caiv-
fully as I could, and should have thought that my want
of success was due to unskilfulness had I not borne in
mind the foregoing considerations.

2. We may next inquire, To what extent ami in
manner is tlie decomposition of a wmpound /W//

bij the FREQUENCY of Vibration of a fit'//

452 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

From the beginning of optical chemistry, investigations
have been made for the purpose of determining the ac-
tion of rays of different refrangibilities. Almost a hun-
dred years ago it had been shown, in special cases, that
there is an antagonism between the opposite ends of the
spectrum. Thus the phosphorescence excited in Can-
ton's preparation by the violet end of the spectrum is ex-
tinguished by the red. As respects colored compounds,
Grotthus showed that the active ray is very commonly
of the tint complementary to that which it destroys.

More recently this branch of the subject has been ex-
amined to a great extent, and the behavior of all kinds
of substances in the solar spectrum made known. The
general result is this, that on wave-length — or, what is
the same thing, frequency of vibration, the number of
impulses it can communicate in a given period of time —
depends the power of a ray to break down the union of
any group of atoms. A compound that may resist a
slowly recurring motion may be unable to maintain it-
self when the impulses increase in frequency.

So numerous and well known are the photographic
and other changes brought on by light that I need not
occupy space with a description of them here. I shall
only refer to some curious instances of antagonism or in-
terference, the details of which will be found in the orig-
inal Memoir. Hitherto they have been very much over-
looked.

Two rays may be so placed in relation to each other
that their motions may conspire or may antagonize ; and
as one or other of these conditions ensues, the chemical
result will correspond.

When iodine vapor is permitted to have access to a
surface of polished silver, the silver tarnishes, the tar-
nished film increasing in thickness as the exposure to
the iodine is prolonged. It assumes in succession colors

M.M..IU XXX.] ON BURNING GLASSES AND MIKKoUS.

which undoubtedly arise from the interference of tin- in-
cident light with the light reflected from the metal at
the back of the film. They are the colors of thin plates,
like those of a soap-bubble.

Now, there is a great difference in the action of light
upon these differently colored films, though chemically
they are the self-same silver iodide. Some have been un-
acted upon ; in them the effect of the incident light has
been destroyed or reversed by the effect of the light re-
flected from the back of the film. Some have been pow-
erfully acted upon ; in them the chemical effect is at a
maximum — the incident and reflected rays have con-
spired.

If any proof were required that these maxima and
minima of chemical effect arise from the superposition of
similar or contrary motions, it is found in the relative
thickness of the films which have been acted or unacted
upon. Those in which there has been maximum action
have thicknesses as 2:1; that showing the minimum
action is 1£.

If the daylight and simple spectrum rays be permitted
to act together on a daguerreotype plate, the rays of
which the times of vibration are as 1,2, etc., aid the day-
light; but those of wiiich the times of vibration are as
£, 1^, 2£, etc., interfere with it and destroy its effect.

In these numbers we may discern the suggestion of
some very important facts.

One of the most striking instances of this positive and
negative action I discovered in the case of the electric
spark. Let there be placed over a daguerreotype plate
(Fig. 98) two metal balls, connected respectively with
the inside and outside of a Leyden-jar in such a way that
the discharge may pass from one of the balls at about
half an inch distance to the sensitive plate, and from the
sensitive plate to the other ball at about the same dis-

454

ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

tance. One spark is sufficient. The experiment should
be made in a dark room.

In Fig. 98, a a, the metal photographic plate ; #, <?, the

brass balls connected with
the Leyden - phial. The
spark passes between them
and the metal plate. At
d, e, the effect is shown, and
again more plainly in Fig.
99.

On developing it will be
found that on the point
which received the spark
there is a blue-white spot about one fortieth of an inch
in diameter (Fig. 99). Immediately around this an an-
nular space which is perfectly black,
the rays of the spark having there had
no action ; then follows a white ring,
and then another black one. Finally
succeeds a whitish stain of an indistinct

circular form, which can be traced bv

*/

inclining the plate as having a diame-
ter of about 1£ inches.

That part of the plate from which the spark escaped
shows a repetition of the same phenomenon.

How shall we account for the production of these al-
ternate white and black spaces — rings of action and in-
action ? Some persons might at first be led to suppose
that this is only an interesting form of Priestley's exper-
iment of "the fairy rings," formed by receiving the shock
of a battery on a polished steel surface, when, by the ox-
idation that ensues, a film is formed of variable thickness,
and giving the colors of thin plates. But a little con-
sideration will show that this is impossible, and the facts
are only to be explained on the principles of interference.

Jfig. 99.

MEMOIU XXX.J ON BURNING GLASSES AM) MIH1;

3. /// wKatnumntr istfodtoonpoqition »f n <•<,/,//„,///,,/

affected by the condition of POLARIZATION of t/i-
tnrl>in(j ray?

A beam of light passing through a circular aperture
one inch in diameter was received on the achromatic
lens of a camera-obscura, and then fell on a doubly re-
fracting prism, so placed as to give on the ground glass
two circular images of the aperture, one third of an inch
in diameter, and overlapping each other to a small ex-
tent. In these images the light was, of course, polarized
at right angles respectively.

When paper rendered sensitive by being washed with
ferric oxalate was placed so as to receive them — the
light permitted to act nine minutes, and its effect devel-
oped by chloride of gold — the images (Fig. 100) were
found of equal blackness, and the lenticular space tbrnu-d
by their overlapping of greater depth. This was repeat-
ed with several different photographic compounds, and
always with the same result. It shows that plane polar-
ized light acts precisely like common light, and with a
rapidity proportional to its intensity.

In Fig. 100, a, b, the disks of plane polarized light, po-
larized at right angles to each other;
at <?, the place of overlapping.

While thus attempting to detect a
difference in the decomposing action
of common and polarized light, I made
some inquiries as to the possibility of polarizing light by

a magnet.

A great many experiments have been made at differ-
ent times for the purpose of producing disturbance on a
ray of light by magnets. There are two methods which
may be resorted to. The one hitherto followed has 1
to intercept the ray in its course, and submit it to mag-
netic action ; but the principle on which my attempts

456 ON BURNING GLASSES AND M1RROUS. [MEMOIR XXX.

have been founded is to attack it at its origin, and at-
tempt to produce an impression on the shining bod}7.
These methods are essentially distinct. To borrow an
illustration from acoustics, it is one thing to try to
modify a sound on its passage through the air, and a
very different thing to exert an influence on the sound-
ing body.

When Bancalari's experiment on the influence of the
poles of a powerful magnet on a flame was first publish-
ed, I repeated it at once, expecting that the oscillations
of the shining particles were constrained to take place in
one plane by the magnetism, and that the light emitted
would be polarized. The result, however, did not seem
to prove this.

A similar experiment was then made with the electric
spark from the prime conductor of a machine. It was
compelled to cross between the poles of a powerful elec-
tro-magnet. But when the magnetism was on it did not

O O

seem that the light was polarized.

De la Rive has shown that the voltaic arc between
charcoal points is greatly disturbed when it passes be-
tween the poles of a powerful electro-magnet. In the
hope that this would produce the expected disturbance,
I examined an arc formed between points of copper, plat-
inum, and gas carbon; but though the sounds emitted
were strong, resembling the sudden tearing of a piece of
cloth, I could not perceive that the light was polarized.

In like manner the induction spark from a contact-
breaker and the phosphorescent light from fluor-spar were
tried without success. I still think, however, that with
better means than those thus employed the experiment
would succeed.

At the commencement of this Memoir it was stated
that we should consider, 1st, the manner in which a ray

MEMOIK XXX.] ON BURNING GLASSES AND MIRRORS. \ ;,;

of light acts in bringing about decomposition, and the
changes it undergoes; 2d, the nature of the impiv.ssiou
made on the material group, the decomposition of wh'u-h
ensues. The observations I proposed ottering in relation
to the former of these points being completed, I may pass
to some remarks respecting the latter.

An examination of many cases of the decomposition
of bodies by light has led me to the conclusion that its
cause is to be attributed to the inability of the group of
molecules affected to withstand th.e periodic impulses
communicated to them. Of those molecules some, per-
haps, take on a vibratory motion more readily than the
others, and the continuance of a given group becoming
impossible, a rearrangement ensues.

But in other cases the mechanism of decomposition is
undoubtedly different : a change is impressed on one of
the elements acted upon, which weakens its affinity for
the others. Thus, under the influence of the sunshine,
plants can decompose many bodies, such as carbonic, sul-
phuric, and phosphoric acids.

The nature of these changes may be best illustrated by
tracing the complete course through which any one of
these substances passes. The chief facts may be seen in
the case of phosphorus. This substance, when freshly
made, commonly exhibits a white waxy appearance, but
when exposed to the sunshine, it turns to a deep mahog-
any-red. If the exposure has been long continued, or the
effect hastened by the action of a burning-lens, the change
of aspect is very striking. It is analogous to that which
sulphur exhibits when heated to 400° or 500°. I havr a
specimen which has been kept for many years in an at-
mosphere of dry carbonic acid ; the sides of the v«
are iucrusted with crystals, which have slowly sublimed,
and which in color resemble the ferrid cyanide of
tassium.

458 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

The chemical properties of these two varieties of phos-
phorus are very different; indeed, there is scarcely a
point in which they may not be said to be unlike. The
common kind shines in the dark ; the red does not. The
common is soluble in a variety of menstrua, which do not
act on the other: thus one of the methods of preparing
red phosphorus is to expose a solution of the common in
sulphuric ether to light — a red powder, the substance in
question, precipitates. Compared together, the one dis-
plays a range of affinity which the other does not ; nor do
these properties seem to leave them when they are united
with other bodies. Thus the active or white phosphorus,
when united with hydrogen, yields a gas which is spon-
taneously combustible in the air; the red or passive va-
riety yields a hydrogen compound of the same constitu-
tion, but devoid of the property of spontaneous combus-
tibility.

It should be understood that though other agents — as

o O

a high temperature — can impress this remarkable change
upon phosphorus, none can do it with more energy or
more completely than the solar rays. I found by expos-
ing a surface of white or active phosphorus to the pris-
matic spectrum that it is the more refrangible rays that
are the most effective. Thus the rays most efficient in set-
ting oxygen free from the bodies with which it is united
have also the quality of impressing such a change on
those bodies that they oxidize subsequently with diffi-
culty. It follows that the true cause of such decompo-
sitions is the impression which the light makes on the
elementary substance. Thus if phosphoric acid be de-
composed by the solar rays, the decomposition is owing
to the phosphorus being thrown into the red or passive
state — a state in which its affinity for oxygen has almost
entirely disappeared.

These considerations enable us to explain what takes

MI.M.MK \XX.J ON IU USING GLASSES AND MIKK

place in the economy of plants. The water of the soil is
always charged with carbonic acid, which communicates
to it the ([iiality of dissolving bone-earth. The solution
]>:i-<ing throiiirh the spongioles goes to the leaves as as-
cending sap. Here it is exposed to light, the effect of
which is, aided by the cell-growth there taking place, to
set the phosphoric acid free, and turn its photpborm into
the passive state. Its continued union with oxygen as
an acid compound thus becomes imporaible, and it is
now associated with the proteine and oily bodies form-
ing in the plant. Nor does it again unite with oxygen
until it has passed into the systems of animals as a con-
stituent of their nervous and muscular tissues. At the
moment of activity of these, and especially of the former,
it is oxidized, the change being apparently an immediate
consequence of that activity, and, reverting to the acid
state, it is finally dismissed from the system under the
form of phosphate of soda and ammonia.

In the same manner might be explained the decompo-
sition of carbonic acid by plants in the sunshine ; for car-
bon, like phosphorus, and, indeed, like all other elemen-
tary bodies, has its active and passive states, as is exem-
plified in the contrast between diamond and lamp-black.
The sunlight enables the leaves of plants to bring the
carbon into the inactive state, and decomposition ensues
as a secondary result. The carbon compounds arising
form the food of various animals; nor does this element
recover its active state until it has given rise to the proc-
esses of life, when it suddenly unites with oxygen brought
by the arterial blood, and the compounds it then form*
are dismissed from the system by the lungs and kidi
conjointly. It might seem that the mechanism of decora-
position by vibratory movement is essentially different
from that by these allotropic changes, but a more detail-
ed examination will show that this is not necessarily so.

460 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX.

In this Memoir I have endeavored to examine how
far the decomposing action of a radiation is dependent
on the amplitude, the frequency, or the direction of its
vibrations. The result arrived at is that decompositions
are not determined by amplitude — that is, brilliancy—
since a faint light continued long enough can produce
precisely the same effect as the more concentrated ray of
a burning-lens applied for a shorter time. Nor does the
direction of motion, as involved in the idea of polariza-
tion, whether plane or circular, exert any effect, but it is
the frequency of the periodic impulses that is the sole
determining cause. And the phenomena of interference
from the superposition of such small motions occur ex-
actly as might have been predicted.

The immediate cause assigned for such decompositions
is that a ray forcing the material particles on which it
falls into a state of rapid vibration, it comes to pass in
many compound molecules that their constituent atoms
can no longer exist together as the same group, because
of the impossibility of their being animated by consenta-
neous or conspiring motions, and dislocation, rearrange-
ment, or decomposition is the result.

In this Memoir I have spoken of heat and light as
though they were distinct agencies, and considered such
facts as conductibility, etc., displayed by the one and not
by the other. But if we recall what has been said in
preceding Memoirs to the effect that these are only modes
of motion, and that the difference of the effects they dis-
play turns on the character of the receiving surface or
substance, there will be no difficulty in translating this
commoner language into terms that are more exact, and
in presenting the phenomena in question under a more
rigidly scientific point of view. Familiar expressions
very frequently convey to the mind clearer ideas than
others which, perhaps, may be more strictly correct.

THE RUMFOKD MEDAL.

APPENDIX.

" In 1796, Count Rumford presented to the Royal Society of London
£1000, the interest of which was to be spent in striking two medals
hoth in the same die, one of gold and one of silver, of the value of the
interest of the donation for two years, and to be given biennially for the
most important discovery or improvement relating to light and heat that
h.-itl l>een made during the preceding two years in any part of Europe."

At the same time Count Rumford made " a corresponding donation
to the American Academy of Arts and Science of $5000, the interest of
\\hirli was to be used in like manner as regards American discoveries.
It was provided that if this term passed without any discovery or im-
provement being made that should be deemed worthy of the award, the
accruing interest was to be added to the principal, and the augmented
income thus arising was to be added to the medals when the next award
was made."

The Royal Society accepted the trust, and made the first award to
Rumford himself in 1802; in 1804, to Leslie; in 1806, to Murdock ;
in 1810, to Malus; in 1814, to Wells; in 1816, to Davy; in 1818, to
Brewster; in 1824, to Fresnel; in 1834, to Melloni ; in 1838, to Forbes;
in 1840, to Biot; in 1842, to Fox-Talbot; in 1 846, to Faraday ; in 1S4S,
to Regnault ; in 1850, to Arago ; in 1852, to Stokes ; in 1854, to Arnott :
in 1856, to Pasteur; in 1858, to Jamin ; in 1860, to Clerk-Maxwell ; in
1862, to Kirchhoff; in 1864, to Tyndall ; in 1866, to Fizeau; in 1868,
t < > I ialfour Stewart, etc.

These awards covered the most important discoveries that had been
made in relation to heat and light — the production of heat by friction,
the correlation of forces, the effects of surfaces on radiation, the polari-
zation of light, the theory of the formation of dew, the tran>\er-e \ i-
lirations of light, the application of the thermopile to the study of radi-
ant heat, the refraction of dark rays, the invention of photography, the
discovery of fluorescence, the fixed lines of the spectrum, the di>tri tuition
of heat in the spectrum, the velocity of light, etc.

The American Academy permitted several years to pass without mak-
ing any award. Meantime the fund had so greatly increased that the

4(54 APPENDIX.

Academy had to apply to the Legislature for authority to depart from
the strict letter of the endowment, and use the funds with more freedom
in the interest of advancing knowledge. In 1839, the Academy gave
from the interest of the fund the sum of $600 to Hare for the invention
of the oxyhydrogen blowpipe and improvements in galvanic apparatus.
In 1862, it awarded the medal to Ericsson for his caloric engine; in
1865, to Treadwell for improvements in the management of heat; in
1867, to Alvan Clark for improvements in the lenses of refracting tele-
scopes ; in 1870, to Corliss for improvements in steam-engines. The fund
had now reached $42,000. It will be seen that the American awards
had been mainly for inventions ; the English for discoveries.

At the six hundred and eighty-ninth meeting of the American Acad-
emy, held March 8, 1876, the chairman of the Rumford committee in-
troduced the special business of the evening, and handed to the presi-
dent, Hon. Charles Francis Adams, the Rumford medals (in gold and
silver), on each of which had been engraved the following inscription :
" Awarded by the American Academy of Arts and Science to John
William Draper for his Researches in Radiant Energy, May 25, 1875."

In presenting the medals, the president gave a brief history of the
fund, and announced that, " after a careful review of the services of Pro-
fessor Draper in this great field of inquiry, the committee having the
subject in their charge have, for reasons given by them, recommended
through their chairman that the medals prescribed in the deed of trust
should be presented to him, as having fully deserved them."

The president then recapitulated some of these reasons :

"In 1840, Dr. Draper independently discovered the peculiar phenom-
ena commonly known as Moser's images, which are formed when a medal
or coin is placed upon a polished surface of glass or metal. These im-
ages remain, as it were, latent, until a vapor is allowed to condense upon
the surface, when the image is developed and becomes visible.

" At a later period he devised the method of measuring the intensity
of the chemical action of light, afterwards perfected and employed by
Bunsen and Roscoe in their elaborate investigations. This method con-
sists in exposing to the source of light a mixture of equal volumes of
chlorine and hydrogen gases. Combination takes place more or less
rapidly, and the intensity of the chemical action of the light is measured
by the diminution in volume. No other known method compares with
this in accuracy, and most valuable results have been obtained by its
use.

"In an elaborate investigation, published in 1847, Dr. Draper estab-
lished experimentally the following important facts :

A1TKN1MX.

" 1. All solid substances, ami probably liquids, become incandescent at
the same temperature.

"2. The therm. .metric point at which substances become iv.l-h.,t i-
:il...ut 977° Fuhp.

" :!. The spectrum of an incandescent solid is continuous; it contains
neither bright nor dark fixed lines.

"4. From common temperatures nearly up to 977° Fahr., tin-
emitted by a solid are invisible. At that temperature they are n d, and
the heat of the incandescing body being made continuously to inci
other rays are added, increasing in refrangibility as the temperature

"5. While the addition of rays, so much the more refrangible as the
temperature is higher, is taking place, there is an increase in the in-
tensity of those already existing. Thirteen years afterwards Kirch hoff
published his celebrated memoir on the relations between the coeffi-
cients of emission and absorption of bodies for light and heat, in which
he established mathematically the same facts, and announced them as
new,

"6. Dr. Draper claims, and we believe with justice, to have been the
first to apply the daguerreotype process to taking portraits.

" 7. Dr. Draper applied ruled glasses and specula to produce spectra
for the study of the chemical action of light. The employment of
ruled metallic specula for this purpose enabled him to avoid the ab-
sorbent action of glass and other transparent media, as well as t
tablish the points of maximum and minimum intensity with reference to
portions of the spectrum defined by their wave-lengths. He obtained
also the advantage of employing a normal spectrum in place of one
which is abnormally condensed at one end and expanded at the other.

" 8. We owe to him valuable and original researches on the nature of
the rays absorbed in the growth of plants in sunlight. These researches
prove that the maximum action is produced by the yellow rays, and they
have been fully confirmed by more recent investigations.

"9. We owe to him, further, an elaborate discussion of the chemical
action of light, supported in a great measure by his own experiment-.
and proving conclusively, and, as we believe, for the first time, that rays
of all wave-lengths arc capable of producing chemical changes, and that
too little account has hitherto been taken of the nature of the substance
in which the decomposition is produced.

" 10. Finally, Dr. Draper has recently published researches on the dis-
tribution of heat in the spectrum, which are of the highest interest, and
\\hieh have largely contributed to the advancement of our knowledge
of the subject of radiant enei

G o

466

APPENDIX.

" And now, in the absence of Dr. Draper, unable at this inclement
season to execute a fatiguing journey, it gives me pleasure to recognize
you, Mr. Quincy, as his worthy and competent representative.

" I pray you, in receiving these two medals on his behalf, in accordance
with the terms of the original trust, to assure him, on the part of the
Academy, of the high satisfaction taken by all its Fellows in doing
honor to those who, like him, take a prominent rank in the advance of
science throughout the world."

Mr. Quincy, on receiving the medals, said :

" MR. PRESIDENT, — In the name and on the behalf of Dr. Draper, I
have the honor to receive the Rumford medals in gold and silver which
the Academy has been pleased to award to him, and I will have them safe-
ly conveyed to him to-morrow, together with the assurances of the sat-
isfaction of the Academy in this action which you wish me to com-
municate to him. In common with yourself, sir, and all the Fellows
present, I regret that that eminent person is unable to attend this meet-
ing and receive the medals himself. And, personally, I regret the ab-
sence of Dr. Wolcott Gibbs, who had promised to perform this grate-
ful service for his friend, and who would have been able to make a more
suitable reply to the able discourse with which you have accompanied the
presentation of the medals, and to have done more justice to the claims
of Dr. Draper to this distinction, than I can pretend to do. Dr. Gibbs
having also been unavoidably prevented from being present this evening,
I have now the honor to read a communication from Dr. Draper to the
Academy, in acknowledgment of this testimony to his services to science."'

Mr. Quincy then read the following letter :

"To THE AMERICAN ACADEMY OF ARTS AND SCIENCES: Your favor-
able appreciation of my researches on radiations, expressed to-day by the
award of the Rumford medals — the highest testimonial of approbation
that American science has to bestow on those who have devoted them-
selves to the enlargement of knowledge — is to me a most acceptable re-
turn for the attention I have given to that subject through a period of
more than forty years, and I deeply regret that through ill-health I am
unable to receive it in person.

" Sir David Brewster, to whom science is under so many obligations
for the discoveries he made, once said to me that the solar spectrum is a
world in itself, and that the study of it will never be completed. His
remark is perfectly just.

" But the spectrum is only a single manifestation of that infinite ether
which makes known to us the presence of the universe, and in which
whatever exists— if I may be permitted to say so — lives and moves and
has its being.

AITI.MHX.

167

" What object, then, can be offered t.. us more worthy of contempla-
tion than the attributes of this intermedium between ourselves and the
outer world?

"Its existence, the modes of motion through it, its transverse vihra-
tions, tlu-ir creation of the ideas of light and colors in the mind, tin- in-
terferences of it> wa\es, polari/ation, the conception of radiations and
their physical and chemical effects— t hex,. have occupied the thought- ,,f
men of the highest order. The observational powers of science have
been greatly extended through the consequent invention of tli«»-<-
in-truinents, the telescope, the microscope, the spectrometer.
tln-e \\e ha\e ohtaiiied more majestic views of the nature of the nni-
pene. Through these we are able to contemplate the >t rue tun- and
genesis of other systems of worlds, and are gathering information as to
the chemical constitution and history of the stars.

"In this noble advancement of science you, through some of your
members, have taken no inconspicuous part. It adds imprecisely to
the honor you have this day conferred on me that your action i^ the
deliberate determination of competent, severe, impartial judges. I can-
not adequately express my feelings of gratitude in such a presence, pub-
licly pronouncing its approval on what I have done.
" I am, gentlemen, very truly yours,

"JOHN W. DRAPER."

The investigations and memoirs referred to by the committee of the
Academy are contained in this volume.

INDEX.

A.

Absorbent media, action of I'.i7

Absorption of chemical rays 2.". I

of radiant heat 200

" of radiations, necessity of. 186

Achromaticity of camera 210

Actinic rays 130-132

Adhesion and capillary attraction. . . 346

Air in interior of Hame G8

Albertns Magnus on phosphores-
cence I.'!."., 1 "»!>

Alchemy 313-315

Alcohol flame spectrum, analysis of. ,V">

Allotropism in living beings 375

its successive stages. ... 310

" of chlorine L'st

Analogies of heat and chemical rays. 230

Angstrom's discovery 186

" law 428

Animal and flame, analogy of. 18i)

Animals, circulation in 370

Antagonism in spectra 1»S

Anthracite, burning, analysis of ~>7

Arseninretted hydrogen 55

Arterialization of blood 378

Atmospheric lines 75

Attraction, capillary 34 '2

B.

Baldwin's phosphorus 134

Bancalnri's experiment l.MI

Becquerel on rayons continuateurs. . . 278

on ultra red lines 7'.t

Bending of stems of plants I 1 :;

Bcr/.elius on allotropism :'.?•'>

on carbon compounds .'{():{

Bitumens and resins 40S

Blood, arieriali/.ution of :',7>

Blow-pipe flame 68-G9

Bolognian stone 1 34, 1(?(J

Boracic acid, spectrum of. .".."•

Bouguer's photometric method.. . .'!'.». l.'.i;
" " criticised

l.y Mel-
loni.... 49
Boyle, experiments of, on diamond. . !:;.">

" on movements in solids 343

Brewster's experiments, criticism on. 36

Bromide of silver as test 202

Bromine cannot decompose water. . 11.'.
Bubbles, soap, endosmosis through.

Budde on chlorine 427

Bunsen and Koscoe, experiments of. 4211

" " " quoted

Burning glasses and mirrors... 436-441

" driven by clock 443

C.

Calcium light, radiations of 320

Calorific hypothecs of vision 102

( 'amplior, experiments with 20.'5

Canton's phosphorus I .'{(!

Capillary attraction .".12

" " an electric phe-
nomenon 346

Carbon, allotropism of

" nucleus in vision KC.

" solid, disengaged in flame .. ii"
Carbonic acid decomposed l>y color-
ed solutions 172

" decon»po>ed l>y light., hi;
". " decomposed in spec-
trum it;:*. 4in

Carbonic oxide, spectrum analysis

of '.

Carbuncle, legend of. 133

Chemical force, distribution of, in

spectrum 404

" hypothesis of vision !«>."•

" radiations, absorption of. . 2»>1
" rays,action of silver iodide

on

("hlor-hydrogen jihotometer 24."»

Chlorine and hvdrogen. relation to

light 24s. •_-_'. 12.-,, 4:U

Chlorine, active and inactive 271

" duration of its moditica-

tion 27:'.. _'-::. :'.i::

indigo rays absorlied by. .

its allotropi>m 2-1

in nlitii-d 271

substitutes hvdrogen

two stages in its modifica-
tion

" water decomposed by ...
Chlorophnne, its phosphorescent prop-
erly 141

470

INDEX.

Chlorophyl 410

Choroid, function of 106

Christie on magnetism of light 192

Circulation in animals 370

in plants 367

Claudet on red rays 422

Clayton's phosphorescing diamond.. 135

Coins, slow movement in 344

Colored flames, nature of 63

Congestion, explanation of. 381

Conical refraction 117

Conservation of force 186

Continuous spectra of ignited solids. 32

Cooling, law of 37

" of the earth 50

Correlation of force 186

Cracks, movement of liquids in 344

Crevice, movement of liquids in 344

Croll on age of earth 50

Currents, electrical, in water 351

" in capillary tubes 345

Cyanogen flame spectrum, analysis
of 55,65,67

D.

Daguerreotype, condition of surface. 222

" engraving of. 229

fading of 233

" plate, its properties. 89

" process of 206-208

Dark heat rays 131

" spaces of spectrum 124

Darkness, spectrum of 92

Daubeny's experiments 167

Davy on flame 53

Daylight in spectrum analysis. . . . 56, 91

Deoxidizing rays 183

Diamond, effects of heat in its phos-
phorescence 160

" phosphorescence of 140

Diffraction and dispersion spec-
tra 386, 416

Diffraction spectrum, ad vantages of. . 95
how obtained. . 97

laws of 120

photograph-
ed.. 112,113,126

studies in 115

what it is 115

Distribution of colors in spectrum... 121

of heat in spectrum 99

Dove, experiments of. 81

Draper, H., on diffraction-spectrum

photographs 125

" " on fixed lines 80

Dufay on Bolognian stone 161

" on diamonds 137

" on theory of phosphorescence. 137

Dulong and Petit, law of cooling 37

Dumas, experiment by 308

Dumas, theory of substitution of. 304

Dutrochet on bending of roots 413

" on endosmosis 352

Earth, age of. 50

Eisenlohr's experiments 97

Electric spark, interference of 454

" its phosphorescent

rays 321

Electromotive power of heat 324

Endosmosis 352

" decompositions bv 355

" force of. ." 362

" force of through India-
rubber 359

Engraving of daguerreotypes 22!)

Etiolated plants turn green 410

Eye and ear, analogies of 123

F.

Fading of daguerreotypes 233

Ferric oxaiate for photographs 266

" its properties 2(55

Fishes, circulation in 373

Fixed lines, atmospheric 75

" " identification of 77

" " nature of. 82

" " ultra red 77

" " ultraviolet 74

Fixing of daguerreotype 212

Flame and animals, analogy of. 189

" Heumann on 60

" passes into nonentity 190

Flames composed of shells 59, 68

" spectrum analyses of 52,64

Flowers, their colors 411

Fluorescence, spectrum examined by. 124
Fluor-spar, its phosphorescent quali-
ties 141

Force derived from the sun 187

" included in plants 177

Foucault and Fizeau, their experi-
ments 421

" on fixed lines 78
" " " on wave the-
ory 117

Frankenheim on allotropism 377

Frankland on flame 60

Fraunhofer lines not in ignited solids. 31
Frequency of undulation, effect

of 439, 452

Fresnel on transverse vibration 116

G.

Gardner on bending of stems 413

Germination of seeds. . . , 184

Grating 35

" Rutherfurd's 117

Grotthus, his law 412

INDEX.

171

drouth of plant- in darkness 177

" of plants in light

(iiiln on daguerreotype L'L'.'i

linn-barrel ignition expei imeut !.".»

II.
Heat distribution in spectrum... 100,383

' elVeet Of, On pllOSpho:

•nice ir.ii, H;L', it;:,

" invisible action of, on carbonic

acid 183

" measurement of, in spectrum. . IK)
" radiation <it', at different tem-
peratures 43

" rays and cliemical rays, their

analogies •_':'.( i

Henry on phosphorescence 318

Herschel on hent radiations 7'.»

" on photographic spec-
trum 418,420

" on spectrum scale 127

Henmann on flame GO

Ilori/uutal clement of flames ana-
lyzed :-l, 1:1

Huggins on nebula in Draco 82

Hydrogen flame, spectrum analysis of ~>~>

" its relation to chlorine L'iH

" substituted by chlorine. . . .'!»>•',

" union of, with chlorine 'Jiis

Hypothesis of vision 101

I.

Ignited solids, their spectra 31

Ignition of platinum, apparatus for. . 26

" " " on Laplace's co-
efficient 30

Ignition, temperature of, by Davy.. . . 23

4 ' " by Newton . 23

Incandescence, temperature of 23

India-rubber, endosmosis through... .'>."/ 7

Inflammation, explanation of 380

Ingenhousz, experiments of 180

Intensity of light, effect of. 439, 448

Interference of radiations. . . !»:.', L'4o. 4.~i4

Inverse vision Ill

Iodide of silver, its decomposition. . . 40C>

Iodine cannot decompose water 4 I "•

" corrodes daguerreotype plate. L"_'>
Isinglass on daguerreotype plate L'l'7

J.

Jamin, quotation from 85

K.

Kirchoff on fixed lines 84

" on spectrum analysis 86

Kunckel, the alchemist 134

L.

Lamansky on fixed lines 384

Lavoixier, chemical theory r.f 62

l.cm.-rv, theory i.f pln,-p!i .,re»-

• •••ii. •>• ;7, J04I

Leyden upark

I. .in.-, incandescence of, compared

with electric spark

Light, action of, on daguerreotype.

dccoliqH»«.es chlorine Uatn

" intensity of, at different tem-
peratures 41

" magnetism of i;i|

" tran-li'ivnce of energy 117

Lunar photography -_'14

M.

Magnetism of light

Mascart on cadmium !!.'."•

Measuring lens •_'(;.-,

Mechanical hypothesis nf vision in*

Melloni on heat radiations l-.'9

Melloni's critique 46

Mercuriali/.ation of daguerreotype. . . 211

Mercury, tides on

Metals not phosphorescent 140

Microscopic photography

Milloii on bleaching com|>ounds L".','{

" on carbon com]M>umls

Mirrors, action of light on '_'•_'<>

Modified chlorine 1'7I

Moon, phosphorescent picture of 1«I2

4 ' photographic picture of. . 2 1 M. •_' 1 4

" rays of heat of. i::i

Mosotti on wave-lengths 1 1 '2

Mllller on heat distribution 883

N.

Nebulae, spectra of 32

Negative rays 87

Newton on decomposition of light. .. 128

" on temperature of ignition
Nitrogenized bodies, allotropi-m <>t
Nonentity, flume passes into 1'JO

O.

Ocelli, their use in vision 1 1 >•-'

Octaves, effect of, in decomposition . 1 •J^

" several seen by eye 1 L'.'J

Oil-flame, spectrum aiiah -is ,,f

( (ptical qualities, control of.

Organic matter, production of. ....
Oxvgcn, combustion in 68

P.

Passive and active states oi chlorine. 801
" condition, etlec:

Phantom images

" subjective 107

Phosphore-ceuce 183

bodies, ignition

their volume. 142

472

INDEX.

Phosphorescent bodies, effect of heat

on.... 159-161
" effect of mag-
netism on.. 151
" effect of press-
ure on 144

" effect of press-
ure on, by
forceps.... 145
" electricity of. 151
" emit heat. 148-150
" examined by
Bouguer's
method.... 156
" " examined by
photogra-
phy 155

" examined by
the polari-

scope 146

" examined by

vapors 147

" quantity of
light of

1 53, 155, 157
" structural

change of.. 146

Phosphorescent images 322

Phosphorogenic experiments by Hen-
ry 318

Phosphorus, action of light on 105

modified by light.. 281, 313

Phosphuretted hydrogen 281

Photo decomposition, cause of. 457

Photographic portraits 215

Photography, microscopic 338

" specimens

of. 341

Photometer, chlor-hydrogen — 245, 253
" its sensi-

tive-
ness . . 247

ferric-oxalate 267

" selective 269

Photometric methods. 39

Pigment, dark, its use in vision 103

Plants and animals, balance of 181

" circulation in 367

' ' decomposition of carbonic acid

by 169

" force included in 177

Platinum, emission of light by 25

Polarization bv magnetism 456

effect of 455

Portal circulation 371

Portraits from life by photogra-
phy 215

Priestley's experiments 181

Prismatic spectrum, its defects 112

Prisms, distribution of heat by 400

Protecting rays 87

Pulmonary circulation 371

Q.

Quartz and glass in phosphorescence. 316

R.

Radiant heat, absorption of 200

Radiations, interference of 89

' ' of red-hot bodies 23

Red heat 28

" rays 406,421

Reference spectrum 31, 57, 199

Reflections, repeated, effects of 449

" in globe under

mercury 450

Refrangibility and chemical energy

connected 70

increases with temper-
ature 36

Resistance in thermo-electricity 330

Hetina, function of. 103

Reversals in the spectrum. 423

Ritter's experiments 202

Roscoe and Bunsen, quotation from. 264
Rumford's experiments on plants. ... 1 80

photometric method 39

Rutherfurd's gratings 117

S.

Salt in flame 56

Sap, circulation of 367

Scheele on chemical radiations 130

Seeds, germination of 184

Selective photometer 269

Sensitiveness, cause of 233

control of 241

Shadows on phospliori 137

Shell construction of flame 59

" inner temperature of, of flame.. 70

Silk, evolution of gas by 1 82

Silver chloride, action of burning-lens

on 446

" salts, their decomposition. 406, 417

Soap-bubble, endosmosis through.... 356

Sodium line 62

Solids have same point of ignition.. . 30
' ' spectrum analysis of radiations

of. 57

Somerville's experiments 191

Spectres evoked by breathing 137

Spectroscope of 1 848 54

Spectrum analysis of flames 52-54

dark spaces of 124

effect of daylight in 56

length increases with tem-
perature 33

length of. in ignited solids. 31

prismatic, its defects 112

structure of 402

INDEX,

Stahl, chemical theory of 52

Stems lu'iitt to indigo rays 413

Sti>k.'> mi e|e,-trie liijlil l'2't

Strontian, Hume of 55

Sul»iitiiiion, theory of 104

Sun, force derived t'nun I-V

" surface of, its condition 81-88

Swan on yellow my 63

Systemic circulation ;57<>

T.

Thermo-electric couples 827

" currents 328

" " pairs, effect of num-
ber of 334

" " pairs, improved

forms of :•.:•.«;

" quantity "M:\

" " thermometer .'J'-'T

Thickness, effect of, in sensitivene>-

Tiili-s in mercury 346

Towson on camera 211

Transmission from retina to brain. . . Hi'.'

Tyndall on distribution of heat 385

" on electric light 3W

" on ignition of platinum !:'>'•'•

" on spectrum 384

U.

Undulations and colors 123

" interpretation of, by

brain 122

" length of. l-'l

Undulntory theory, power of the 117

Unit lamp 45

V.

Vertical clement of flame analyzed.. 65
Vibrations coimci-t.-d with tempera-
ture 36

" "f, on retina 1<>H

" spectrum meaxure of . . .

Virginia, CXJ-IM iin.-nis made in l'.»7

sjicctrum -

Vision and photographic impressions. 210

" hypothesis of ln|

" inverse II!

Vital force, rejection of

Vogel, 1'rofessor, on increased sen-
Mti\eness 4.".1

\V.

Water, decomposition of. 188

" decomposition of, under press-
ure 363

" not decomposed by light — i".U
Wave-lengths as spectrum meas-

ui-es 11::. IL'7

" " interpretation of, by

brain ". 1L"J

" " measures of 121

Wedgwood on ignition of gold and

earthenware 28

Wilson's experiments on phospho-
ri... .. 318

Wollaston on spectnim 88

Y.

Yellow ray most luminous 1 '>"•

Young discovers interference 116

THE END.

VALUABLE AND INTERESTING VTOBK8

FOR

PUBLISHKD HY HAUTE!! & HKOTI I ! l;s. ftm*

For a full Lint «f KiHik* xnitiMf f»r Libntrirx, tee HABPK* & Buormtu'

TI:M>I:-!,IWI nwl » 'ATAl.oiiHK, ir/i/c/i /mil/ Ix Innl 'initial-

tn the l^ublishers pemoiiatly, or bit •< 1,1

f3T~ II A. tin: it & BROTIIKUS wi'/J ««»<f Mcir publication* by mail, postage prepaid [ex-
ffjitiii't n-rtain books trhoxe weight tcould rxclwle them from tfie. tmn/j, <>n
receipt of tin- /!••/,•,•. HAUPKR & BKOTHKBB' School and Collcye Tr*'
are marked in this list irith an asterisk (*).

FIKST CI.N'ITRY OF THE REPUBLIC. A Review of American

Progress. 8vo, Cloth, $5 00 ; Sheep, $5 50 ; Hull' Morocco, .* 7

Contents.

Introduction : I. Colonial Progress. By EUOKNC LAWBKXDE.— II. Mcchnnlcal
Progress. By EKWAEK H. K.MOHT.— III. I'm^n-Mj iu Manufitcture. By the
Hon. DAVIP A. \\'i I.T>. 1\'. Agricultural Progress. I!y I'mfi---..! WII.I.IAM
H. BREWEB.— V. The Devcloptiu-iil of our Mineral BaMMieM. Mv I'i..ri---.ir
'!'. STKRRV HUNT. — VI. C'liinnicivial Dcvflopnifiit. 15y BDWAXD Al
— VII. (Jrowih and Dmtribution of I'opiilntion. By the Hon. Ki: \
WAI.KF.R.— VIII. Monetary Development By Profo-or WII.I.IAM •
NF.R.— IX. The Experiment "f the rnion.with its Preparations. 1!\ 'I 1>.
WHOI.M v. D.I)., LL.I).— X. Kdneational I'ro^re-s. By KciiKsi: I, A -
—XI. Scientific 1'ro-ress: 1. The Exa.-t Sciein-e-. By V. \. P. BAKNARII,
D.D., LL.I). 2. Natural Science. By ProlV««or Tntooon <;m.. XII. \
Century of America n Literature. Bv Ki>« IN 1'. WMUMM.K.— XIII. 1'
of the FIlM Arts. By S. S. i ON AST.— XIV. Medical and Sanilary Progress.
By AUSTIN FLINT, M.I). — XV. American Jurisprudence. By Bmmutn
VAVOHAN AHIIOTT.— XVI. Humanitarian 1'ro^ress. By C'IIARLKH L. BEACK.
—XVII. Religious Development. By the Kev. .Jons K. Hi KS*T, D.I).

MOTLEY'S DITCH KF.1T15LIC. The Rise of the Dutch

A History. Hy JOHN Lomuoi- MOTI.KV. 1.I..D.. D.C.L. \Yitli iv
Portrait of William of Orange. 8 vols., 8vo, Cloth, $10 :>0; Sheej.,
$12 00; Half Calf, $17 26,

MOTLEY'S UNITED NETHEKLANDS. 1 1 i^tory of the United Neth-
erlands : from the Death of William the Silent to the Twelve Years'
Truce — 160l». With a full View of the Ennli-h-Diitch Simple .iK'"i"st
Spain, and of the Origin and Dcstrnetimi ut'tln- S|>anisli Aniiada. By
JOHN LOTHROP MOTI.KY, LL.I).. D.C.I.. I'oi traits. 4 vols., 8vo,
Cloth, $14 00; Sheep, $16 00; Half Calf, $28 00.

MOTLEY'S LIFE AND DEATH OF JOHN OF BAUNFVF.I.D. The
Life and Death of John of Harm-veld, Adv-ratc <>f Holland: with a
View of the Primary Causes and .Movement-. .,(' "The Thirty-
War." Hv JOHN I. <>iiii;<>i- MOTI.KY, LL.I).. D.C.L. Illustrated. In
2 vols., 8vo, Cloth, $7 00; Sheep, $8 00; Half Calf, $11 50.

2 Valuable and Interesting Works for Public and Private Libraries.

"HAYDN'S DICTIONARY OF DATES, relating to all Ages and Na-
tions. For Universal Reference. Edited by BENJAMIN VINCENT, As-
sistant Secretary and Keeper of the Library of tbe Royal Institution of
Great Britain; and Revised for the Use of American Readers. 8vo,
Cloth, $3 50 ; Sheep, $3 94.

HILDRETH'S UNITED STATES. History of the United States.
FIRST SERIES : From the Discovery of the Continent to the Organiza-
tion of the Government under the Federal Constitution. SECOND SE-
RIES : From the Adoption of the Federal Constitution to the End of
the Sixteenth Congress. By RICHARD HILDRETH. 6 vols., 8vo, Cloth,

. $18 00; Sheep, $21 00; Half Calf, $31 50.

HUME'S HISTORY OF ENGLAND. The History of England, from
the Invasion of Julius Coesar to tbe Abdication of James II., 1688. By
DAVID HUME. 6 vols., 12mo, Cloth, $4 80 ; Sheep, $7 20 ; Half Calf,

$15 30.

HUDSON'S HISTORY OF JOURNALISM. Journalism in the United
States, from 1 G90 to 1872. By FREDERIC HUDSON. 8vo, Cloth, $5 00;
Half Calf, $7 25.

JEFFERSON'S DOMESTIC LIFE. The Domestic Life of Thomas
Jefferson : compiled from Family Letters and Reminiscences, by his
Great-granddaughter, SARAH N. RANDOLPH. Illustrated. Crown 8vo,
Cloth, $2 50.

JOHNSON'S COMPLETE WORKS. The Works of Samuel Johnson,
LL.D. With an Essay on his Life and Genius, by ARTHUR MURPHY,
Esq. 2 vols., 8vo, Cloth, $4 00 ; Sheep, $5 00 ; Half Calf, $8 50.

KINGLAKE'S CRIMEAN WAR. The Invasion of the Crimea : its
Origin, and an Account of its Progress down to the Death of Lord Rag-
lan. By ALEXANDER WILLIAM KINGLAKE. With Maps and Plans.
Three Volumes now ready. 12mo, Cloth, $2 00 per vol. ; Half Calf,
$3 75 per vol.

LAMB'S COMPLETE WORKS. The Works of Charles Lamb. Com-
prising his Letters, Poems, Essays of Elia, Essays upon Shakspeare,
Hogarth, &c.,and a Sketch of his Life, with the Final Memorials, by
T. NOON TALFOURD. With Portrait. 2 vols., 12mo, Cloth, $3 00-,
Half Calf, $6 50.

LAWRENCE'S HISTORICAL STUDIES. Historical Studies. Bj
EUGENE LAWRENCE. Containing the following Essays : The Bishops
of Rome. — Leo and Luther. — Loyola and the Jesuits. — Ecumenical
Councils. — The Vaudois. — The Huguenots. — The Church of Jerusalem.
— Dominic and the Inquisition. — The Conquest of Ireland. — The Greek
Church. 8vo, Cloth, uncut edges and gilt tops, $3 00.

MYERS'S REMAINS OF LOST EMPIRES. Remains of Lost Em-
pires : Sketches of the Ruins of Palmyra, Nineveh, Babylon, and Per-
sepolis, with some Notes on India and the Cashmerian Himalayas. B.y
P. V. N. MYERS. Illustrated. 8\o, Cloth, $3 50.

.•table and Interesting Works for Fublit and Private Libraries. 3

LOSSIMiS FIELD- HOOK OF TIIK RK\ '< H.I I l« 'N. IVtoruJ
FieM-Book <>f tin: Revolution : nr, Illustrations l.v I'm and Pencil of
the llistoiy. Biography. Scenery. Relies, and Tradition* of the \\'ur for
Independence. Bv BI.N^IS .1. L.I-MM:. :.' \..|-.. - \<i, ( 'loth, $14 OU'
Sheep or Koiin, $15 00 ; Half Calf, fI8 00.

-!\<!-S FIELD- BOOK <IF TIIK WAR (IF 1ML>. Pictorial
FifM Book of tin- War of 1M:.' : or, Illustrations l.v Ten mid Pencil of
the History, Biography, Scenery. Relics. mid Tradition;, of tin- la-t War
for American Independence. By BI.SMIN .1. I.H--IM;. \\'iih -e\er.d
hundred Engravings on Wood by Los>ing and Hanitt. cliieHy from
Original Skcti-hea by the Autlior. 1088 pages, 8vo, I 'loth, .*7 00;
Sheeji or Koaii, jy« DO; Half Cull, •'rlO 00.

MACACLAVS HiypiKY (»F KM. I. AND. The History of Knglaml
frOIB the Aooevipa of .lanu'S II. l!\ 'rini\i\« I'>MUN..|"\ M \i M i \>.
5 vols.,8vo, Cloth, $1000; Sheep, $!'_' r.O : Half Calf, !?•.'! 25; ll'mo,
Cloth, $4 00; Sheep, $6 00; Half Calf, !*1'.' T."..

MACAVLAVS I. IKK AM) I.KTTF.HS. The Life nnd Letters of Lord
Mncaulay. Uy liis Nt-jilu.-w, (;. Orro TUKVKI.YAN. .M.I'. Wit'.
trait on Stool. Complete in 2 vols., 8vo, Cloth, uncut edges and gilt
tops, $5 (Ml; Slic.-p, fG 00; Half Calf, $9 60. Popular Kdition, '»
vols. in one, 12mo, Cloth, !*1 l'>.

FOKSTKK'S I, IKK OF DEAN SWIFT. The Early Life of Jonathan
Swift (1667-1711). By JOHN FuitHTKit. With 1'ortrait. 8vo, Cloth.
$2 50.

*GKEEN'S SHORT HISTORY OF THE KNULISH 1'KOl'LE. A
Short History of the English People. By J. R. (ii:i i v. .M. A., Exam-
iner in the S'chool of Modern History, Oxford. With Tables and Col-
ored Maps. 8vo, Cloth, $1 :.L'.

IIALLAM'S MIDDLE AUKS. View of the State of Europe during
the Middle Ages. By HI-.NUV HM.LASI. 8vo, Cloth, $i 00 } .Siiee|.,
$2 50; Half Calf, $4 "25.

IIALLAM'S CONSTITUTIONAL HISTORY OF EXCf.AND. The

( 'onstitiitional History of England, from the Acfos>ion ot Homy VII.
to the Death of (icm^c II. Bv Hi. NICY HAI.LAM. 8vo, Cloth, f2 00;
Sheep, $2 r>(); Half C'alf. !?l L'5.

IIALLAM'S LlTERATl'RE. Introduction to the Literature of Europe
during the Fifteenth, Sixteenth. :ind Seventeenth ( Vntini.'s. Bv Hi >-
in II M.I.AM. 2 vols., 8vo, Cloth, $4 00; Sheep, $5 (M); Half Calf.
$8 50.

SCHWEINFURTIKS HEART oK Al RICA. The Heart of Africa.
Tlm-f Year-;' Travels and Adventures in the Unexplored Regions of the
Centre of Africa. From I Mis n, IsTl. By Dr. (iKoit<; S n
i i 1:1 ii. Translated liy EI.LKN E. FRKWKII. With an Introduction t.y
\VINW. .<»!> RKAI>K. I'llustnited by about 130 Woodcuts from Drawing*
made by the Author, and with two Maps. 2 vols., 8vc, Cloth, £f OO.

4 Valuable and Interesting Works for Public and Private Libraries.

M'CLINTOCK & STRONG'S CYCLOPEDIA. Cyclopedia of Bib.
lical, Theological, and Ecclesiastical Literature. Prepared by the Rev.
JOHN M'CLINTOCK, D.D., and JAMES STRONG, S.T.D. 7 vois. now
ready. Roval 8vo. Price per vol., Cloth, $5 00; Sheep, $6 00;
Half Morocco, $8 00.

MOHAMMED AND MOHAMMEDANISM: Lectures Delivered at
the Royal Institution of Great Britain in February and March, 1874.
By R. BOSWOKTH SMITH, M.A., Assistant Master in Harrow School;
late Fellow of Trinity College, Oxford. With an Appendix containing
Emanuel Deutsch's Article on "Islam." 12mo, Cloth, $1 50.

MOSHEIM'S ECCLESIASTICAL HISTORY, Ancient and Modem ;
in which the Rise, Progress, and Variation of Church Power are con-
sidered in their Connection with the State of Learning and Philosophy,
and the Political History of Europe during that Period. Translated,
with Notes, &c., by A. MACLAINE, D.D. Continued to 1826, bv C.
COOTE, LL.D. 2 vols., 8vo, Cloth, $4 00; Sheep, $5 00; Half Calf,
$8 50.

HARPER'S NEW CLASSICAL LIBRARY. Literal Translations.
The following Volumes are now ready. 12mo, Cloth, $1 50 each.

CAESAR. — VIRGIL. — SALLUST. — HORACE. — CICKRO'S ORATIONS. —
CICERO'S OFFICES, &c. — CICERO ON ORATORY AND ORATORS. —
TACITUS (2 vols.). — TERENCE. — SOPHOCLES. — JUVENAL. — XENO-
PHON. — HOMER'S ILIAD. — HOMER'S ODYSSEY. — HERODOTUS. — DE-
MOSTHENES (2 vols.). — THUCYDIDES. — ^ESCHYLUS. — EURIPIDES (2
vols.). — LIVY (2 vols.). — PLATO [Select Dialogues].

LIVINGSTONE'S SOUTH AFRICA. Missionary Travels and Re-
searches in South Africa : including a Sketch of Sixteen Years' Resi-
dence in the Interior of Africa, and a Journey from the Cape of Good
Hope to Loanda on the West Coast ; thence across the Continent, down
the River Zambesi, to the Eastern Ocean. By DAVID LIVINGSTONE,
LL.D., D.C.L. With Portrait, Maps, and Illustrations. 8vo, Cloth,
$4 50 ; Sheep, $5 00 ; Half Calf, $6 75.

LIVINGSTONE'S ZAMBESI. Narrative of an Expedition to the Zam-
besi and its Tributaries, and of the Discovery of the Lakes Shirwa and
Nyassa, 1858-1864. By DAVID and CHARLES LIVINGSTONE. With
Map and Illustrations. 8vo, Cloth, $5 00 ; Sheep, $5 50 ; Half Calf,

$7 25.

LIVINGSTONE'S LAST JOURNALS. The Last Journals of David
Livingstone, in Central Africa, from 1865 to his Death. Continued by
a Narrative of his Last Moments and Sufferings, obtained from his
Faithful Servants Chuma and Susi. By HORACE WALLER, F.R.G.S.,
Rector of Twywell, Northampton. With Portrait, Maps, and Illustra-
tions. 8vo, Cloth, $5 00; Sheep, $5 50; Half Calf, $7 25. Cheap
Popular Edition, 8vo, Cloth, with Map and Illustrations, $2 50.

GROTE'S HISTORY OF GREECE. 12 vols., 12mo, Cloth, $18 00;
Sheep, $22 80 ; Half Calf, $39 00.

Valuable and Interesting Works for Public and Private Ltbrartts, 5

RKCI.I .sS I AKT11. The l.aith: a I », •-, -i iptive Hi Phe-

nomena of tin; Lite ol the lilohe. BjButtfci Ki'ii-. \Y,
and Illustrations, .in.l L':i 1'age Map- punted in ('•.!. us. .-\->, ( loth,
$5 00; Half Culf, $7 :.':..

RKCU'S'S OCKAX. The Ocean, Atmosphere, and Lit'.-. Being the

• n. 1 >rries of a De-i-iiptive Ili-toiv of the Life of ilu- i
l£i.isi i lit . 1.1 -. Profusely lllu.-trated with •_'."»O Maps or 1-iguren, and
1'7 Maps jirintcd in Colors. 8vo, Cloth, $G 00; Half Culf, >

KOKDIIOFF'S rOMMTXlSTH1 SlM'IF.TIKS OF TIIK I'MTKI)
STATKS. The Communistic Societies of the I'nited States, fn.i:
sonal Visit and Observation ; including Detailed Accounts of tlic ,
omists, Xoarites, Shakers, the Ainana. Oneida, Uetliel, Auioni, Icariun,
and other existing Societies. \Vith rarticulars of their ItuliKious Creed*
and Practices, tlieir Social Tlieorie* and Life. Numbers, Industries, and
Present Condition. By CHAKLES NORUIIOFF. Illustrations. 8vo,
Cloth, $4 00.

NOHDHOFF'S CALIFORNIA. California: for Health, Pleasur
Residence. A Book for Travellers and Settlers. Illustrated. 8vo,
Cloth, $2 50.

NOKDIIOFF'S NOIMIIl-.HN CALIFORNIA, ORKGON. AND TIIK
SANDWICH ISLANDS. Northern California, ( )re«on, and the Sand-
wich Islands. By CHARLES NORDHOFF. Illustrated. 8vo, C'loth,
$2 r,n.

PARTON'S CARICATURE. Caricature and Other Comic Art. in All
Times and Many Lands. By J A M i - 1 ' M: i ' >N. With 203 Illustration.-.
8vo, Cloth, Gilt Tops and uncut edges, $5 00.

*KAWLINSOYS MAM A I. o|-' ANCIF.XT HISTORY. A Manual
of Ancient History, from the Earliest Times to the Fall of the \Yestern
Empire. Comprising the History of Chaldiua, Assyria, Media. Hahy-
lonia. Lvdia. I'lia-nieia. Syria, .luda-a. Etrypt, Carthage, Per.-ia, (Jreece,
Macedonia. 1'arthia. and" Koine. l?y (Ji:i>i:i.i. KV\MIN-.N. M.A..
Camden Professor of Ancient History in the L'niversity of Oxford.
12mo, Cloth, $1 46.

NICHOLS'S ART EDUCATION. Art Education applied to Industry.
By GBOBOE WAKI. NICHOLS, Author of "The Story of the Great
Maivh." Illustrated. 8vo, Cloth, $4 00.

BAKER'S ISMAILlA. Ismaili'a: n Narrative of the Expedition t.. Cen-
tral Africa for the Suppression of the Slave-trade, organi/.-d l>y Ismail,
Khedive of Egypt. By Sir SAMTKI. WHITK MAKI:I:. l'v-n\. F.K.S.,
F.R.(i.S. With Maps^ 1'ortraits, and Illustrations. 8vo, Cloth, >
Half Calf, $7 L'.V

HOSWELI/S .IOI1NSON. The Life of Samuel Johnson. 1.I..D.. in-
cluding a Journal of a Tour to the Hehrid.'>. I'.v J\MI- i;..-\vni.
FS.I E.lited l.y J..IIN WlLMMI CMMCBB, I.L.D.. F.K.S. With a l'..r-
trait of Boswell. L' vols., 8vo, C'loth, $4 (M) ; Sheep, f5 00 ; Half Call,
$8 50.

6 Valuable and Interesting Works for Public and Private Libraries.

VAN-LENNEP'S BIBLE LANDS. Bible Lands : their Modern Cus.
toms and Manners Illustrative of Scripture. By the Rev. HENRY J.
VAN-LENNEP, D.D. Illustrated with upward of 350 Wood Engravings
and two Colored Maps. 838 pp., 8vo, Cloth, $5 00; Sheep, $6 00;
Half Morocco, $8 00.

VINCENT'S LAND OF THE WHITE ELEPHANT. The Land of
the White Elephant : Sights and Scenes in Southeastern Asia. A Per-
sonal Narrative of Travel and Adventure in Farther India, embracing
. the Countries of Burma, Siam, Cambodia, and Cochin-China (1871-2).
By FRANK VINCKNT, Jr. Illustrated with Maps, Plans, and Woodculs.
Crown 8vo, Cloth, $3 50.

SHAKSPEARE. The Dramatic Works of William Shakspeare. With
Corrections and Notes. Engravings. 6 vols., 12mo, Cloth, $9 00. 2
vols., 8vo, Cloth, $4 00; Sheep, $5 00. In one vol., 8vo, Sheep,
$4 00.

SMILES'S HISTORY OF THE HUGUENOTS. The Huguenots:
their Settlements, Churches, and Industries in England and Ireland.
By SAMUEL SMILES. With an Appendix relating to the Huguenots in
America. Crown 8vo, Cloth, $2 00.

SMILES'S HUGUENOTS AFTER THE REVOCATION. The Hu-
guenots in France after the Revocation of the Edict of Nantes ; with a
Visit to the Country of the Vaudois. By SAMUEL SMILES. Crown
8vo, Cloth, $2 00.

SMILES'S LIFE OF THE STEPHENSONS. The Life of George
Stephenson, and of his Son, Robert Stephenson ; comprising, also, a
History of the Invention and Introduction of the Railway Locomotive.
Bv SAMUEL SMILES. With Steel Portraits and numerous Illustrations.
8vo, Cloth, $3 00.

SQUIER'S PERU. Pern : Incidents of Travel and Exploration in the
Land of the Incas. By E. GEORGE SQUIER, M. A., F.S. A., late U. S.
Commissioner to Peru, Author of "Nicaragua," "Ancient Monuments
of Mississippi Valley," &c., &c. With Illustrations. 8vo, Cloth, $5 00.

STRICKLAND'S (Miss) QUEENS OF SCOTLAND. Lives of the
Queens of Scotland and English Princesses connected with the Regal
Succession of Great Britain. By AGNES STRICKLAND. 8 vols., 12mo,
Cloth, $12 00; Half Calf, 826 00.

THE -'CHALLENGER" EXPEDITION. The Atlantic: an Account
of the General Results of the Exploring Expedition of H.M.S. "Chal-
lenger." By Sir WYVILLE THOMSON, K.C.B., F.R.S. With numer-
ous Illustrations, Colored Maps, and Charts, from Drawings by J. J.
Wyld, engraved by J. D. Cooper, and Portrait of the Author, engraved
by'C. H. Jeens. 2 vols., 8vo, $12 00.

BOURNE'S LIFE OF JOHN LOCKE. The Life of John Locke. By
H. R. Fox BOURNE. 2 vols., 8vo, Cloth, uncut edges and gilt tops,
$5 00.

CO

o

& -H

3 o

•H I
H 0)
H &

'-O
CD

UNIVERSITY OF TORONTO
LIBRARY

Acme Library Card Pocket

Under 1'at. " Rcf. !
Made ty LIBRARY BUREAU

cn