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THE

COKRELATION AND CONSERVATION

OF

F0ECE8:

Peof. GEO ye, Peof. HELMHOLTZ, De. MAYEE,

De. FAEADAY, Peof. LIEBIG and

De. CAEPENTEE.

INTEODITCTION AND BRIEF BIOGEAPHICAL NOTICES OF THE
CHIEF PEOMOTERS OF THE NEW VIEWS.

EDWARD L. YOUMANS, M. D.

" — The highest law in physical science which our faculties permit us to perceive—tho
Conservation of Force."— Db. Fasadat.

NEW YOEK:
D. APPLETON AND COMPANY,

443 «S5 445 BROADWAY.

1865.

Library of the

I ; A OBSERVATORY.

Kktmuh), according to Act of Congress, In the year 1864, by

D. APPLETON AND COMPANY,

In th« aerk'i Office, of the DiBtrict Court of the United States for the

Southern District of New York.

-13
Y47

TO

JOHN WILLIAM DEAPEE, M.D., LL.D.

propbssor op chemistry and physiology in the
university op new york.

Dear Sir: —

It seems peculiarly appropriate that this volume should be dedicated to
you. Knowing the eminent esteem in which you are held in the circles of
European science, I cannot ' doubt that the distmguished authors of the fol-
lowing essays would cordially approve this connection of your name with
their introduction to the American public.

There is, besides, a further reason for this in that large coincidence of
purpose which is manifest in their labors and your own. For while the per-
vading design of the present collection is to widen the range of thought by
unfolding a broader philosophy of the energies of nature, your own compre-
hensive course of research — ^beginning with an extended series of experi-
mental investigations in chemical physics and physiology, and rising to the
consideration of that splendid problem, the bearing of science upon the His-
tory of the Intellectual Development of Europe — ^has powerfully contributed
to the same noble end ; that of elevatmg the aim and enlarging the scope of
scientific inquiry.

I gladly avail myself of this occasion to say how greatly I am indebted
to your writings, in which accurate and profound instruction is so often and
happily blended with the charms of poetic eloquence. That you may live
long to enjoy your well-won honors, and to contribute still further to the
triumphant advance of scientific truth, is the heartfelt wish of

Yours truly,

E. L. Y.

I

PREFACE.

In his address before the British Association for the Advance-
ment of Science last year, the President remarked that the new
views of the Correlation and Conservation of Forces constitute the
most important discovery of the present century. The remark is
probably just, prolific as has been this period in grand scientific re-
sults. No one can glance through the current scientific publica-
tions without perceiving that these views are attracting the pro-
found attention of the most thoughtful minds. The lively con-
troversy that has been carried on for the last two or three years
respecting the share that different men of different countries have
had in their establishment, still further attests the estimate placed
upon them in the scientific world.

But httle, however, has been published in this country upon the
subject; no complete work, I believe, except the admirable volume
of Prof. Tyndall on " Heat as a Mode of Motion," in which the
new philosophy is adopted, and applied to the explanation of ther-
mal phenomena in a very clear and forcible manner. I have, there-
fore, thought it would be a useful service to the public to reissue
some of the ablest presentations of these views which have ap-
peared in Europe, in a compact and convenient form. The selec-
tion of these discussions has been determined by a desire to com-
bine clearness of exposition with authority of statement. In the
first of these respects the essays will speak for themselves ; in re-
gard to the last I may remark that all the authors quoted stand
high as founders of the new theory of forces. Although I am not

VI PBEFACE.

aware that Prof. Liebig has made any claims in this direction, yet
it can scarcely be doubted that his original researches in Animal
Chemistry tended strongly toward the promotion of the science of
vital dynamics.

The work of Professor Grove, which is here reprinted in full,
has a high European reputation, having passed to the fourth edi-
tion in England, and been translated into several continental lan-
guages. It is hardly to the credit of science in our country, that
this is the first American edition. The eloquent and interesting
paper of Helmholtz, though delivered as a popular lecture, was
translated for the Philosophical Magazine, and has been very highly
appreciated in scientific circles. Th% three articles of Mayer,
which were also translated for the Philosophical Magazine, will
have interest not only because of the great ability with which the
subjects are treated, but as emanating from a man who stands per-
haps preeminent among the explorers in this new tract of inquiry.
The researches of Faraday in this field have been conspicuous and
important, and his argument is marked by the depth and clearness
which characterize, in an eminent degree, the writings of this ex-
traordinary man. The essay of Liebig forms a chapter in the last
edition of his invaluable ' Familiar Letters on Chemistry,' which
has not been republished here ; and, as it touches the relation of the
subject to organic processes, it forms a fit introduction to the final
article of the series by Dr. Carpenter, on the " Correlation of the
Physical and Vital Forces." The eminent English physiologist has
worked out this branch of the subject independently, and the pa-
per quoted gives evidence of being prepared with his usual care
and ability. A certain amount of repetition is of course unavoida-
ble in such a collection, yet the reader will find much less of this
than he might be inclined to look for, as each writer, in elaborating
the subject, has stamped it with his own originality.

In the introduction I have attempted to bring forward certain
facts in the history of these discoveries, in which we as Ameri-
cans have a special interest, and also to indicate several applications
of the new principles which are not treated in the volume. It
seemed best to confine the general discussion to those aspects of the
subject upon which most thought had been expended, and which
may be regarded as settled among advanced scientific men. But
there are other applications of the doctrine, of the highest interest,
which though incomplete are yet certain, and these will bo found

PEEFACE. Vll

briefly noticed in the introductory observations — too briefly, I fear,
to be satisfactory. Those, however, who desire to pursue still
further this branch of the inquiry — the correlation of the vital,
mental, and social forces — are referred to the last edition of Car-
penter's " Principles of Human Physiology ; " Morell's " Outlines
of Mental Philosophy ; " Laycock's " Correlations of Consciousness
and Organization ; " Sir J. K. Shuttleworth's address before the
Social Science Congress of 1860, on the "Correlation of the Moral
and Physical Forces;" Hinton's "Life in Nature," and "First
Principles " of Herbert Spencer's new system of Philosophy. The
first and last of these works are the only ones, it is believed, that
have appeared in an American form, and the last is much the
ablest of all ; I was chiefly indebted to it in preparing the latter
part of the introduction. The biographical notices, brief and im-
perfect as they are, it is hoped may enhance the reader's interest
in the volume.

I have been specially incited to procure the publication of a
work of this kind, by the same motive that has impelled me to
write upon the subject elsewhere ; a conviction of our educational
needs in this direction. The treatment of a vast subject like this
in ordinary school text-books, is at best quite too limited for the
requirements of the active-minded teacher ; to such, a volume like
the present may prove invaluable.

But a more serious difficulty is that, until compelled by the de-
mands of intelligent teachers, the compilers of school-books will
pass new views entirely by, or ^ve them a mere hasty and careless
notice, while continuing to inculcate the old erroneous doctrines.
And thus it is that from inveterate habit, or intellectual sluggish-
ness, or a shrewd calculation of the indifference of teachers, out-
worn and effete ideas continue to drag through school-books for
half a century after they have been exploded in the world of liv-
ing science. He who continues to teach the hypothesis of caloric^
falsifies the present truth of science as absolutely as he would do
in teaching the hypothesis of phlogiston ; in fact, the reasons of-
fered for persisting in the erroneous notions of the materiality of
heat — convenience of teaching, unsettledness of the new vocabu-
lary, &c., are precisely those that were offered for clinging to phlo-
giston, and rejecting the Lavoiserian chemistry of combustion.
Both conceptions have no doubt been of service, but both were
transitional, and having done their work they become hindrances

Vlll PEEFACE.

instead of helps. We can now see that when the true chemistry
of combustion was once reached, the notion of phlogiston was of
no further use, and if retained could only produce confusion and
prevent the reception of correct ideas. So with caloric, and those
false conceptions of the materiality of forces, which it implies : not
only are they errors, but the ideas they involve are radically in-
compatible with the higher truths to which science has advanced ;
80 that while the errors are retained the truths cannot be received.

Nor will it answer merely to mention the new views while
adopting the old, on the plea that the facts are the same in both
cases. The facts are very far from being the same in both cases. It
is precisely because the old ideas are out of harmony with the facts,
and can no longer correctly explain and express them, that new ideas
are sought. Was not phlogiston abandoned because it no longer
agreed with the facts ? So with the conception of the materiality
of the forces ; it contradicts the facts, and therefore, for scientific
purposes, can no longer represent them. In the workshop it may
perhaps be very well to magnify facts, and depreciate their theoreti-
cal explanations, but not in the school-room ; the business is here not
working, but thinking. It is the aim of art to use facts, but of sci-
ence to understand them. And it is simply because science goes
beyond the fact to its explanation, and is ever striving after the
highest truth, that it is fitted to discipline the thinking and reason-
ing faculties, and therefore has imperative educational claims.

In therefore bringing forward these able and authoritative ex-
positions in a form readily accessible to teachers, I trust I am not
only doing them a helpful service, but that they wiU be led to re-
quire of the preparers of school-books a more conscientious per-
formance of their tasks, and that the interests of sound education
will be thereby promoted.

Nxw YoBK, Oct. 1, 1864.

CONTENTS

Preface, ........

Introduction, .......'

THE CORRELATION OF PHYSICAL FORCES, By W. R. Grove.
Preface, .....
I. — ^Introductory Remarks,

V

ad

II.— Motion,

. 25

m.— Heat,

.

39

IV.— Electricity, .

. 83

v.— Light,

.

110

VI.— Magnetism, .

. 142

Vn.— Chemical Affinity,

.

162

VIU.— Other Modes of Force,

. 169

IX. — Concluding Remarks,

.

118

Notes and References,

. 200

ON THE INTERACTION OF NATURAL FORCES, By

Prof.

Helmholtz, ,

.

211

REMARKS ON THE FORCES OF INORGANIC NATURE, By Dr.

J. R. Mayer, . . . . . .251

X CONTENTS.

PAOB

ON CELESTIAL DYNAMICS, By J. R. Mayer, . . 259

I. — Introduction, ..... 259

II.— Sources of Heat, ..... 261

m.— Measure of the Sun's Heat, . . . .264

IV.— Origin of the Sun's Heat, . . .276

V. — Constancy of the Sun's Mass, . . . 282

VI.— The Spots on the Sun's Disc, . . .286

Vn.— The Tidal Wave, . . . . .291

Vin.— The Earth's Interior Heat, ... 300

REMARKS ON THE MECHANICAL EQUIVALENT OF HEAT, By

Dr. J. R. Mayer, . . . . .316

SOME THOUGHTS ON THE CONSERVATION OF FORCE, By Dr.

Faraday, ...... 359

THE CONNECTION AND EQUIVALENCE OF FORCES, By Prof.

LiEBia, . . . . . .387

ON THE CORRELATION OF THE PHYSICAL AND VITAL

FORCES, By Dr. Carpenter, . . .401

I.— Relation of Light and Heat to the Vital Forces of Plants, 401
n.— Relation of Light and Heat to the Vital Forces of Ani-
mals, ...... 420

INTRODUCTION.

There are many wlio deplore what they regard as the material-
izing tendencies of modern science. They maintain that this pro-
found and increasing engrossment of the mind with material ob-
jects is fatal to all refining and spiritualizing influence. The cor-
rectness of this conclusion is open to serious question : indeed, the
history of scientific thought not only falls to justify it, but proves
the reverse to be true. It shows that the tendency of this kind of
inquiry is eyerfrom the material, toward the abstract, the ideal, the
spiritual.

We may appeal to the oldest and most developed of the sciences
for confirmation of this statement. The earliest explanations of
the celestial movements were thoroughly and grossly material, and
all astronomic progress has been toward more refined and ideal
views. The heavenly bodies were at first thought to be supported
and carried round in their courses by solid revolving crystalline
spheres to which they were attached. This notion was afterward
replaced by the more complex and mobile mechanism of epicy-
cles. To this succeeded the hypothesis of Des Cartes', who rejected
the clumsy mechanical explanation of revolving wheelwork, and
proposed the more subtile conception of ethereal currents, which
constantly whirled around in vortices, and bore along the heavenly
bodies. At length the labors of astronomers, terminating with

Xll ENTEODrcnON.

Newton, struck away these crude devices, and substituted the action
of a uniyersal immaterial force. The course of astronomic science
has thus been on a vast scale to withdraw attention from the mate-
rial and sensible, and to fix it upon the invisible and supersensuous.
It has shown that a pure principle forms the immaterial foundation
of the universe. From the baldest materiality we rise at last to a
truth of the spiritual world, of so exalted an order that it has been
said ' to connect the mind of man with the Spirit of God.'

The tendency thus illustrated by astronomy is characteristic in
a marked degree of all modern science. Scientific inquiries are
becoming less and less questions of matter, and more and more
questions of force ; material ideas are giving place to dynamical
ideas. "While the great agencies of change with which it is the
business of science to deal — ^heat, light, electricity, magnetism, and
affinity, have been formerly regarded as kinds of matter ' impon-
derable elements,' in distinction from other material elements,
these notions must now be regarded as outgrown and abandoned,
and in their place we have an order of purely immaterial forces.

Toward the close of the last century the human mind reached
the great principle of the indestructiblity of matter. What the
intellectual activity of ages had failed to establish by all the re-
sources of reasoning and philosophy, was accomplished by the in-
vention of a mechanical implement, the balance of Lavoisier.
When nature was tested in the chemist's scale-pan, it was first
found that never an atom is created or destroyed ; that though
matter changes form with protean facility, traversing a thousand
cycles of change, vanishing and reappearing incessantly, yet it
never wears out or lapses into nothing.

The present age will be memorable in the history of science for
having demonstrated that the same great principle applies also to
forces, and for the establishment of a new philosophy concerning
their nature and relations. Heat, light, electricity, and magnetism
are now no longer regarded as substantive and independent exist-
ences— subtile fluids with peculiar properties, but simply as modes

THE NEW DOCTEINE OF FORCES. Xlll

of motion m ordinary matter ; forms of energy which are capable
of mutual conversion. Heat is a mode of energy manifested by
certain effects. It may be transformed into electricity, which is
another form of force producing different effects. Or the process
may be reversed ; the electricity disappearing and the heat reap-
pearing. Again, mechanical motion, which is a motion of masses,
may be transformed into heat or electricity, which is held to be a
motion of the atoms of matter, while, by a reverse process, the mo-
tion of atoms, that is, heat or electricity, may be turned back again
into mechanical motion. Thus a portion of the heat generated in
a locomotive is converted into the motion of the train, while by
the application of the brakes the motion of the train is changed
back again into the heat of friction.

These mutations are rigidly subject to the laws of quantity. A
given amount of one force produces a definite quantity of another .
so that power or energy, like matter, can neither be created nor
destroyed: though ever changing form, its total quantity in the uni-
verse remains constant and unalterable. Every manifestation of
force must have come from a preexisting equivalent force, and must
^ve rise to a subsequent and equal amount of some other force.
"When, therefore, a force or effect appears, we are not at liberty to
assume that it was self-originated, or came from nothing ; when it
disappears we are forbidden to conclude that it is annihilated : we
must search and find whence it came and whither it has gone ; that
is, what produced it and what effect it has itself produced. These
relations among the modes of energy are currently known by the
phrases Correlation and Conser'cation of Force.

The present condition of the philosophy of forces is perfectly
paralleled by that of the philosophy of matter toward the close of
the last century. So long as it was admitted that matter in its
various changes may be created or destroyed, chemical progress
was impossible. If, in his processes, a portion of the material dis-
appeared, the chemist had a ready explanation — the matter was
destroyed; his analysis was therefore worthless. But when he

XIV INTEODUCTION.

started with the axiom that matter is indestructible, all disappear-
ance of material during his operations was chargeable to their im-
perfection. He was therefore compelled to improve them — to ac-
count in his result for every thousandth of a grain with which he
commenced ; and as a consequence of this inexorable condition,
analytical chemistry advanced to a high perfection, and its conse-
quences to the world are incalculable. Precisely so with the anal-
ysis of forces. So long as they are considered capable of being
created and destroyed, the quest for them will be careless and the
results valueless. But the moment they are determined to be in-
destructible, the investigator becomes bound to account for them ;
all problems of power are at once affected, and the science of dy-
namics enters upon a new era.

The views here briefly stated will be found fully and variously
elucidated in the essays of the present volume ; in these introduc-
tory remarks I propose to offer some observations on their history
and the extended scope of their application.

I have spoken of the principles of Correlation and Conservation
of Forces as established ; it may be well to state the sense in which
this is to be taken. They have been accepted by the leading scien-
tific minds of all nations with remarkable unanimity ; their discus-
sion forms a leading element in scientific literature, while they oc-
cupy the thoughts and guide the investigations of the most philo-
sophical inquirers. But while science holds securely her new pos-
session as a fundamental principle, its various phases are by no
means completely worked out. Not only has there been too little
time for this, even if the views were far less important, but the
questions started lie at the foundation of all branches of science,
and ean only be fully elucidated as these advance in their develop-
ment. The new doctrine of forces is now in much the same con-
dition as was the new astronomy of Copernicus. It is not with-
out its diflficulties, which time alone must be trusted to remove ;
but it simplifies so many problems, clears up so many obscurities.

THE HISTORY OF SCIENTIFIC DISCOVERY. XV

opens so extended a range of new investigations, and contrasts so
strongly with the complexities and incongruities of the older doc-
trines, as to leave little liberty of choice between the opposing theo-
ries. Not only do the reception of these views mark a signal epoch m
the progress of science, but from their comprehensive bearings and
the luminous glimpses which they open into the most elevated re-
gions of speculative inquiry, they have a profound interest for
many thinkers who give little attention to the specialties of exact
science.

In the history of human affairs there is a growing conception
of the action of general causes in the production of events, and a
corresponding conviction that the part played by individuals has
been much exaggerated, and is far less controlling and permanent
than has been hitherto supposed. So also in the history of science
it is now acknowledged that the progress of discovery is much
more independent of the labors of particular persons than has been
formerly admitted. Great discoveries belong not so much to indi- \
viduals as to humanity ; they are less inspirations of genius than j
births of eras. As there has been a definite intellectual progress,
thought has necessarily been limited to the subjects successively
reached. Many minds have been thus occupied at the same time
with similar ideas, and hence the simultaneous discoveries of inde-
pendent inquirers, of which the history of science is so full. Thus
at the close of the sixteenth century, philosophers had entered
upon the investigation of the laws of motion, and accordingly we
find Galileo, Benediti, and Piccolomini proving independently that
all bodies fall to the earth with equal velocity, whatever their size
or weight. A century after, when science had advanced to the
systematic apphcation of the higher mathematics to general phys-
ics, Kewton and Leibnitz discovered independently the differential
calculus. A hundred years later questions of molecular physics
and chemistry were reached, and oxygen was discovered simulta-
neously by Priestley and Scheele, and the composition of water by
Cavendish and Watt. These discoveries were made because the

XVI INTEODUCTION.

periods were ripe for them, and we cannot doubt that if those who
made them had never lived, the labors of others would have speed-
ily attained the same results. The discoverer is, therefore, in a
great degree, but the mouthpiece of his time. Some discern clearly
what is dimly shadowed forth to many ; some work out the results
more completely than others, and some seize the coming thought
so long before it is developed in the general consciousness, that
their announcements are unappreciated and unheeded. This view
by no means robs the discoverer of his honors, but it enables us to
place upon them a juster estimate, and to pass a more enlightened
judgment upon the rival claims which are constantly arising in the
history of science.

Probably the most important event in the general progress of
science was the transition from the speculative to the experimental
period. The ancients were prevented from creating science by a
false intellectual procedure. They believed they could solve all the
problems of the universe by thought alone. The moderns have
found that for this purpose meditation is futile unless accompanied
by observation and experiment. Modern science, therefore, took its
rise in a change of method, and the adoption of the principle that
the discovery of physical truth consists not in its mere logical but
in its experimental estabhshment. It is now an axiom that not he
^ who guesses^ though he guess aright, is to be adjudged the true dis-
' coverer, but he who demonstrates the new truth, and thus compels
its acceptance into the body of valid knowledge.

Now the later doctrines of the constancy and relations of forces,
and that heat is a kind of motion among the minuter parts of mat-
ter, have had their twofold phases of history, corresponding to the
two methods of inquiry. They had an early and vague recognition
among many philosophers, and may be traced in the writings of
Galileo, Bacon, Newton, Locke, Leibnitz, Des Cartes, Bernoulli,
Laplace, and others ; but they were held by these thinkers as un-
veriJSed and fruitless speculations, and the subject awaited the gen-
ius that could deal with it according to the more effective methods
of modern science.

SKETCH OF THE CAEEER OF COUNT RUMFOKD. XVll

It was this country, widely reproached for being over-practical,
which produc'ed just that kind of working ability that was suited
to translate this profound question from the barren to the fruitful
field of inquiry. It is a matter of just national pride that the two
men who first demonstrated the capital propositions of pure sci-
ence, that lightning is but a case of common electricity, and that
heat is but a mode of motion — who first converted these proposi-
tions from conjectures of fancy to facts of science, were not only
Americans by birth and education, but men eminently representa-
tive of the peculiarities of American character — Benjamin Frank-
lin and Benjamin Thompson, afterwards known as Count Eumford.
The latter philosopher is less known than the former, though his
services to science and society were probably quite as great. The
prominence which his name now occupies in connection with the
new views of heat, and the relations of forces, make it desirable to
glance briefly at his career.

Benjamin Thompson was born at Woburn, Mass., in 1753. He
received the rudiments of a common school education ; became a
merchant's apprentice at twelve, and subsequently taught school.
Having a strong taste for mechanical and chemical studies, he cul-
tivated them assiduously during his leisure time. At seventeen he
took charge of an academy in the village of Rumford (now Con-
cord), N. H., and in 1772 married a wealthy widow, by whom he
had one daughter. At the outbreak of revolutionary hostilities he
applied for a commission in the American service, was charged
with toryism, left the country in disgust, and went to England.
His talents were there appreciated, and he took a responsible posi-
tion under the government, which he held for some years.

After receiving the honor of knighthood he left England and
entered the service of the elector of Bavaria. He settled in Mu-
nich in 1784, and was appointed aide-de-camp and chamberlain to
the Prince. The labors which he now undertook were of the most
extensive and laborious character, and could never have been ac-

XVm mTEODUCTION.

compKshed but for the rigorous habits of order which he carried
into all his pursuits. He reorganized the entire military establish-
ment of Bavaria, introduced not only a simpler code of tactics, and
a new system of order, discipline, and economy among the troops
and industrial schools for the soldiers' children, but greatly im-
proved the construction and modes of manufacture of arms and
ordnance. He suppressed the system of beggary which had grown
into a recognized profession in Bavaria, and become an enormous
public evil — one of the most remarkable social reforms on record.
He also devoted himself to various ameliorations, such as improv-
ing the construction and arrangement of the dwellings of the work-
ing classes, providing for them a better education, organizing houses
of industry, introducing superior breeds of horses and cattle, and
promoting landscape-gardening, which he did by converting an old
abandoned hunting-ground near Munich into a park, where, after
his departure, the inhabitants erected a monument to his honor.
For these services Sir Benjamin Thompson received many distinc-
tions, and among others was made Count of the holy Roman Empire.
On receiving this dignity he chose a title in remembrance of the
country of his nativity, and was thenceforth known as Count of
Rumford.

His health failing from excessive labor and what he considered
the unfavorable climate, he came back to England in 1798, and had
serious thoughts of returning to the United States. Having re-
ceived from the American government the compliment of a formal
invitation to revisit his native land, he wrote to an old friend re-
questing him to look out for a "little quiet retreat" for himself
and daughter in the vicinity of Boston. This intention, however,
failed, as he shortly after became involved in the enterprise of
founding the Royal Institution of England.

There was in Rumford's character a happy combination of phi-
lanthropic impulses, executive power in carrying out great projects,
and versatility of talent in physical research. His scientific inves-
tigations were largely guided and determined by his philanthropic

SCIENTIFIC LABORS OF COTTNT EUiMFOED. XIX

plans and public duties. His interest in the more needy classes led
him to the assiduous study of the physical wants of mankind, and
the best methods of relieving them ; the laws and domestic man-
agement of heat accordingly engaged a large share of his attention.
He determined the amount of heat arising from the combustion of
different kinds of fuel, by means of a calorimeter of his own in-
vention. He reconstructed the fireplace, and so improved the
methods of heating apartments and cooking food as to produce a
saving in the precious element, varying from one-half to seven-
eighths of the fuel previously consumed. He improved the con-
struction of stoves, cooking ranges, coal grates, and chimneys;
showed that the non-conducting power of cloth is due to the air
enclosed among its fibres, and first pointed out that mode of action
of heat called convection; indeed he was the first clearly to dis-
criminate between the three modes of propagation of heat — radia-
tion, conduction, and convection. He determined the almost per-
fect non-conducting properties of liquids, investigated the produc-
tion of light, and invented a mode of measuring it. He was the
first to apply steam generally to the warming of fluids and the
culinary art ; he experimented upon the use of gunpowder, the
strength of materials, and the maximum density of water, and
made many valuable and original observations upon an extensive
range of subjects.

Prof. James D. Forbes, in his able Dissertation on the recent
Progress of the Mathematical and Physical Sciences, in the last
edition of the Encyclopedia Britannica, gives a full account of Eum-
ford's contributions to science, and remarks ;

" All Rumford's experiments were made with admirable precis-
ion, and recorded with elaborate fidelity, and in the plainest lan-
guage. Every thing with him was reduced to weight and meas-
ure, and no pains were spared to attain the best results.

" Rumford's name will be ever connected with the progress of

science in England by two circumstances : first, by the foundation

of a perpetual medal and prize in the gift of the council of the
B

XX INTRODUCTION.

Eoyal Society of London, for the reward of discoveries connected
with heat and light; and secondly, by the establishment in 1800 of
the EoyaJ Institution in London, destined, primarily, for the pro-
motion of original discovery, and, secondarily, for the diffusion of a
taste for science among the educated classes. The plan was con-
ceived with the sagacity which characterized Eumford, and its suc-
cess has been greater than could have been anticipated. Davy was
there brought into notice by Eumford himself, and furnished with
the means of prosecuting his admirable experiments. He and Mr.
Faraday have given to that Institution its just celebrity with little
intermission for half a century."

Leaving England, Eumford took up his residence in France, and
the estimation in which he was held may be judged of by the fact
that he was elected one of the eight foreign associates of the Acad-
emy of Sciences.

Count Eumford bequeathed to Harvard University the funds
for endowing its professorship of the Application of Science to the
Art of Living, and instituted a prize to be awarded by the Ameri-
can Academy of Sciences, for the most important discoveries and
improvements relating to heat and light. In 1804 he married the
widow of the celebrated chemist Lavoisier, and with her retired
to the villa of Auteuil, the residence of her former husband, wher©
he died in 1814.

Having thus glanced briefly at his career, I now pass to the dis-
covery upon which Count Eumford's fame in the future will chiefly
rest. It is described in a paper published in the transactions of the
Eoyal Society for 1798.

He was led to it while superintending the operations of the
Munich arsenal, by observing j;he large amount of heat generated
in boring brass cannon. Eeflecting upon this, he proposed to him-
self the following questions : " Whence comes the heat produced
in the mechanical operations above mentioned ? " " Is it furnished
by the metallic chips which are separated from the metal ? "

If lit

XXI

The common hypothesis affirmed that the heat produced had
been latent in the metal, and had been forced out by condensation
of the chips. But if this were the case the capacity for heat of the
parts of metal so reduced to chips ought not only to be changed,
but the change undergone by them should be sufficiently great to
account for all the heat produced. With a fine saw Rumford then
cut away slices of the unheated metal, and found that they had eX'
actly the same capacity for heat as the metallic chips. No change
in this respect had occurred, and it was thus conclusively proved
that the heat generated could not have been held latent in the
chips. Having settled this preliminary point, Rumford proceeds to
his principal experiments.

With the intuition of the tru^ investigator, he remarks that
" very interesting philosophical experiments may often be made,
almost without trouble or expense, by means of machinery con-
trived for mere mechanical purposes of the arts and manufactures."
Accordingly, he mounted a metallic cylinder weighing 113.13
pounds avoirdupois, in a horizontal position. At one end there was
a cavity three and a half inches in diameter, and into this was in-
troduced a borer, a flat piece of hardened steel, four inches long,
0.63 inches thick, and nearly as wide as the cavity, the area of con-
tact of the borer with the cylinder being two and a half inches.
To measure the heat developed, a small round hole was bored in
the cylinder near the bottom of the cavity, for the insertion of a
small mercurial thermometer. The borer was pressed against the
base of the cavity with a force of 10,000 pounds, and the cylinder
made to revolve by horse-power at the rate of thirty-two times per
minute. At the beginning of the experiment the temperature of
the air in the shade and also in the cylinder was 60°F. at the end
of thirty minutes, and after the cylinder had made 960 revolutions
the temperature was found to be ISO'F.

Having taken away the borer, he found that 839 grains of me-
lic dust had been cut away. " Is it possible," he exclaims, " that
the very considerable quantity of heat produced in this experiment

XXii INTEODUCnON.

—a quantity which actually raised the temperature of upward of
113 pounds of gun metal at least 70°, could have been furnished by
so inconsiderable a quantity of metallic dust, and this merely in
consequence of a change in the capacity for heat ? "

To measure more precisely the heat produced, he next sur-
rounded his cylinder by an oblong wooden box in such a manner
that it could turn water-tight in the centre of the box, while the
borer was pressed against the bottom. The box was filled with
water until the entire cylinder was covered, and the apparatus was
set in action. The temperature of the water on commencing was
60°. He remarks, " The result of this beautiful experiment was
very striking, and the pleasure it afforded amply repaid me for all
the trouble I had taken in contriving and arranging the complicated
machinery used in making it. The cyhnder had been in motion
but a short time when I perceived, by putting my hand into the
water and touching the outside of the cylinder, that heat was gen-
erated."

As the work continued the temperature gradually rose ; at two
hours and twenty minutes from the beginning of the operation, the
water was at 200"^, and in ten minutes more it actually boiled !
Upon this result Kumford observes, " It would be diiSicult to de-
scribe the surprise and astonishment expressed in the countenances
of the bystanders, on seeing so large a quantity of water heated
and actually made to boil without any fire. Though there was
nothing that could be considered very surprising in this matter,
yet I acknowledge fairly that it afforded me a degi-ee of childish
pleasure which, were I ambitious of the reputation of a grave phi-
losopher, I ought most certainly rather to hide than to discover."

Rumford estimated the total heat generated as sufficient to raise
26.58 pounds of ice-cold water 180°, or to its boiling point; and he
adds, " from the results of these computations, it appears that the
quantity of heat produced equally or in a continuous stream, if I
may use the expression, by the friction of the blunt steel borer
against the bottom of the hollow metallic cylinder, was greater

XXIU

tlian that produced in the combustion of nine wax candles, each
three-quarters of an inch in diameter, all burning together with
clear bright flames."

" One horse would have been equal to the work performed,
though two were actually employed. Heat may thus be produced
merely by the strength of a horse, and in a case of necessity this
might be used in cooking victuals. But no circumstances could be
imagined in which this method of producing heat could be advan-
iiageous, for more heat might be obtained by using the fodder ne-
cessary for the support of the horse, as fuel.

" By meditating on the results of all these experiments, we are
naturally brought to that great question which has so often been
the subject of speculation among philosophers, namely. What is
heat? Is there such a thing as an igneous fluid? Is there any
thing that with propriety can be called caloric ?

" We have seen that a very considerable quantity of heat may
be excited by the friction of two metallic surfaces, and given off in
a constant stream or flux in all directions, without interruption or
intermission, and without any signs of diminution or exhaustion.
In reasoning on this subject we must not forget that most remar'k'
able circumstance, that the source of the heat generated by friction
in these experiments appeared evidently to be inexhaustible. (The
italics are Kumford's.) It is hardly necessary to add, that any
thing which any insulated body or system of bodies can continue
to furnish without limitation, cannot possibly be a material sub-
stance ; and it appears to me to be extremely difiicult, if not quite
impossible, to form any distinct idea of any thing capable of being
excited and communicated in those experiments, except it be mo-
tion."

No one can read the remarkably able and lucid paper from
which tl^ese extracts are taken, without being struck with the per-
fect distinctness with which the problem to be solved was pre-
sented, and the systematic and conclusive method of its treatment.
Rumford kept strictly within the limits of legitimate inquiry, which

XXIV rNTEODUCTION.

no man can define better than he did. "I am very far from pre-
tending to know how, or by what means or mechanical contri-
vances, that particular kind of motion in bodies, which has been
supposed to constitute heat, is exerted, continued, and propagated,
and I shall not presume to trouble the Society with new conjec-
tures. But although the mechanism of heat should in part be one
one of those mysteries of nature, which are beyond the reach of
human intelligence, this ought by no means to discourage us, or
even lessen our ardor in our attempts to investigate the laws of its
operations. How far can we advance in any of the paths which
science has opened to us, before we find ourselves enveloped in
those thick mists, which on every side bound the horizon of the
human intellect."

Eumford's experiments completely annihilated the material hy-
pothesis of heat, while the modern doctrine was stated in explicit
terms. He moreover advanced the question to its quantitative and
highest stage, proposing to find the numerical relation between
mechanical power and heat, and obtained a result remarkably near
to that finally established. The EngHsh unit of force is the foot-
pound, that is, one pound falling through one foot of space ; the
unit of heat is one pound of water heated 1° F. Just fifty years
subsequently to the experiment of Rumford, Dr. J. P. Joule,* of
Manchester, England, after a most delicate and elaborate series of
experiments, determined that 772 units of force produce one unit
of heat ; that is, 772 pounds falling through one foot produces suf-
ficient heat to raise one pound of water 1° F. This law is known
as the mechanical equivalent of heat. Now, when we throw Rum-
ford's results into these terms, we find that about 940 units of force
produced a unit of heat, and that, therefore, on a large scale, and
at the very first trial, he came within twenty per cent, of the true

* James Pbescott Joule, born December 24th, 1818, at Salford, near Manchester,
England, where ho pursued the occupation of a brewer. Long and deeply devoted
to Bcientiflc inTestigation, he became a member of the Manchester Philosophical So-
ciety in 1842, and of the Eoyal Society of London in 1850.

XXV

statement. No account was taken of the heat lost by radiation,
which, considering the high temperature produced, and the dura-
tion of the experiment, must have been considerable ; so that as
Rumford himself noticed, this value must be too high. The ear-
liest numerical results in science are rarely more than rough ap-
proximations, yet they may guide to the establishment of great
principles. Certainly no one could question Dalton's claim to the
discovery of the law of definite proportions, because of the inac-
curacy (rf the numbers upon which he first rested it.

We are called further to note that Eumford's ideas upon the
general subject of forces were far in advance of his age. He saw
the relation of all friction to heat, and suggested that of fluids, by
churning processes, as a means of producing it — precisely the
method finally employed by Joule in establishing the mechanical
equivalent of heat. He furthermore regarded animals dynami-
cally^ considering their force as the derivative of their food, and
therefore as not created. That Rumford held these views in the
comprehensive and matured sense in which they are now enter-
tained is, of course, not asserted. The advance from his day to
ours has been prodigious. Whole sciences have been created,
which afford the most beautiful exemplifications of the new doc-
trines. Those doctrines have received their subsequent develop-
ment in various directions by many minds, but we may be allowed
to question if the contributions of any of their promoters will sur-
pass, if indeed they will equal, the value and importance which we
must assign to the first great experimental step in the new direc-
tion.

The claims of Rumford may be summarized as follows :

I. He was the man who first took the question of the nature
of heat out of the domain of metaphysics, where it had
been speculated upon since the time of Aristotle, and
placed it upon the true basis of physical experiment.

11. Ho first proved the insuflSciency of the current explanations

XXVI INTKODUCTTON.

of the sources of heat, and demonstrated the falsity of the
prevailing view of its materiality.

HI. He first estimated the quantitative relation between the heat
produced by friction and that by combustion.

rV. He first showed the quantity of heat produced by a definite
amount of mechanical work, and arrived at a result re-
markably near the finally established law.

•
V. He pointed out other methods to be employed in determining
the amount of heat produced by the expenditure of me-
chanical power, instancing particularly the agitation of
water, or other liquids, as in churning.

VI. He regarded the power of animals as due to their food, there-
fore as having a definite source and not created, and thus
apphed his views of force to the organic world.

Vn. Eumford was the first to demonstrate the quantitative con-
vertibility of force in an important case, and the first to
reach, experimentally, the fundamental conclusion that heat
is but a mode of motion.

In his late work upon heat. Prof. Tyndall, after quoting co-
piously from Eumford's paper, remarks : " When the history of the
dynamical theory of heat is written, the man who in opposition to
the scientific belief of his time could experiment, and reason upon
experiment, as did Rumford in the investigation here referred to,
cannot be lightly passed over." Had other English writers been
equally just, there would have been less necessity for the foregoing
exposition of Eumford's labors and claims ; but there has been a
manifest disposition in various quarters to obscure and depreciate
them. Dr. WheweU, in his history of the Inductive Sciences,
treats the subject of thermotics without mentioning him. An em-
inent Edinburgh professor, writing recently in the Philosophical
Magazine, under the confessed influence of * patriotism,' under-

davy'« eelation to the question. xxvii

takes to make the dynamical theory of heat an English monopoly,
due to Sir Isaac Newton, Sir Humphrey Davy, and Dr. J. P.
Joule ; while an able writer in a late number of the IlTorth British
Review, in sketching the historic progress of the new views, puts
Davy forward as their founder, and assigns to Rumford a minor
and subsequent place.

Sir Humphrey Davy, it is well known, early rejected the caloric
hypothesis. In 1799, at the age of twenty-one, he published a
tract at Bristol, describing some ingenious experiments upon the
subject. It was the publication of this pamphlet which brought
him to Rumford's notice, and resulted in his subsequent connection
with the Royal Institution. But Davy's ideas upon the question
were far from clear, and will bear no comparison with those of
Rumford, published the year before. Indeed his eulogist remarks :
"It is certain that even Davy himself was led astray in his argu-
ment by using the hypothesis of change of capacity as the basis
of his reasoning, and that he might have been met successfully by
any able calorist, who, though maintaining the materiality of heat,
might have been willing to throw overboard one or two of the less
essential tenets of his school of philosophy." It was not till 1812
that Davy wrote in his Chemical Philosophy, " The immediate
cause of the phenomena of heat then is motion, and the laws of its
communication are precisely the same as those of the communica-
tion of motion." When, therefore, we remember that Davy's first
publication was subsequent to that of Rumford's, that he confined
himself to the narrowest point of the subject, the simple question
of the existence of caloric ; and that he nowhere gives evidence
of having the slightest notion of the quantitative relation between
mechanical force and heat, the futility of the claim which would
make him the experimental founder of the dynamical theory, is
abundantly apparent.

The inquiries opened by Rumford and Davy were not formally
pursued by the succeeding generation. Even the powerful adhe-
sion of Dr. Thomas Young — perhaps the greatest mind in science

XXviii INTEODUCTION*

since N'ewton — failed to give currency to the new views. But the
salient and impregnable demonstration of Eumford, and the ingen-
ious experiments of Davy, facts which could neither be evaded nor
harmonized with the prevailing errors, were not without influence.
That there was a general, though unconscious tendency toward a
new philosophy of forces, in the early inquiries of the present cen-
tury, is shown by the fact that various scientific men of different
nations, and with no knowledge of each other's labors, gave ex-
pression to the same views at about the same time. Grove and
Joule of England, Mayer of Germany, and Oolding of Denmark^
announced the general doctrine of the mutual relations of the forces,
with more or less explication, about 1842, and Seguin of France,
it is claimed, a little earlier. From this time the subject was closely
pursued, and the names of Helmholtz, Holtzman, Clausius,* Faraday,
Thompson, Rankine,t Tyndall, Carpenter, and others are intimately
associated with its advancement. In this country Professors Henry J
and Leconte § have contributed to illustrate the organic phase of the
doctrine.

I cannot here attempt an estimate of the respective shares
which these men have had in constructing the new theories ; the
reader will gather various intimations upon this point from the
succeeding essays. The foreign periodicals, both scientific and lit-
erary, show that the question is being thoroughly sifted, and mate-
rials accumulating for the future history of the subject. The para-
mount claims are, however, those of Joule, Mayer, and Grove.

♦ CLATTsnrs, Ettdolph JiTLnis Immantel was bom at Coslin, Pommern, January
22, 1822, He became Professor of Philosophy and Physics in the Polytechnic School
at Zurich In 1865, and then Professor of the Zurich University (1857). He was after-
wards teacher of Physics and Artillery in the School of Berlin, and then private
teacher of the University of that place.

t Eankinb, "Wiluam John Macqttoen was born at Edinburgh, July 5, 1820. He
is a civil engineer in Glasgow, a member of the Philosophical Society at that place,
and of the Eoyal Society of London.

t See the article "Meteorology," in the Agricultural Eeport of the Patent OlBce, for
1867.

§ See the American Journal of Science for Nov. 1859.

CLAIMS OF JOULE, GEOVE, AKD MAYER. XXIX

According to the strict rule of science, that in all those cases
where experimental proof is possible, he who first supplies it is the
true discoverer. Dr. Joule must be assigned the foremost place
among the modern investigators of the subject. He dealt with the
whole question upon the basis of experiment. He labored with
great perseverance and skill to determine the mechanical equivalent
of heat— the corner-stone of the edifice ; and in accomplishing this
result in 1850, he may be said to have matured the work of Rumford,
and finally estabhshed upon an experimental basis the great law of
thermo-dynamics, to remain a demonstration of science forever.

Professor Grove has also worked out the subject in his own in-
dependent way. Combining original experimental investigations
of great acuteness, with the philosophic employment of the gen-
eral results of science, he was the first to give complete and system-
atic expression to the new views. His able work, which opens
the present series, is an authoritative exposition, and an acknowl-
edged classic upon the subject.

Again, the claims of Dr. Mayer to an eminent and enviable
place among the pioneers of this great scientific movement, are un-
questionable. There has evidently been, on the part of some Eng-
glish writers, an unworthy inclination to depreciate his merits,
which has given rise to a sharp and searching controversy. The
intellectual rights of the German philosopher have, however, been
decisively vindicated by the chivalric pen of Prof. Tyndall ; and it
is to the pubhc interest thus excited, that we are indebted for the
translation of Mayer's papers, which appear in this volume. Mayer
did not experiment to the extent of Joule and Grove, yet he well
knew its importance, and made such investigations as his apparatus
and the duties of a laborious profession would allow. Yet his
views were not therefore mere ingenious and probable conjectures.
Master of the results of modern science, and of the mathematical
methods of dealing with them, possessing a broad philosophic
grasp, and an extraordinary mental pertinacity. Dr. Mayer entered
early upon the inquiry, and not only has he developed many of its

INTEODUCnON.

prime applications in advance of any other thinker, but he has
done his work under circumstances and in a manner which awa-
kens the highest admiration for his genius.*

An eminent authority has remarked * that these discoveries open
a region which promises possessions richer than any hitherto
granted to the intellect of man.' Involving as they do a revolution
of fiindamental ideas, their consequences must be as comprehen-
sive as the range of human thought. A principle has been devel-
oped of all-pervading application, which brings the diverse and
distant branches of knowledge into more intimate and harmonious
alliance, and affords a profounder insight into the universal order.
Not only is science itself deeply affected by the presentation of its
questions, in new and suggestive lights, but its method is at once
made universal. There is a crude notion in many minds, that it ig
the business of science to occupy itself merely with the study of
matter. When, hitherto, it has pressed its inquiries into the higher

• Prof. Tyndall remarks: "Mayer probably had not the means of making experi-
ments himself^ but he ransacked the records of experimental science for his data, and
thus conferred upon his writings a strength which mere speculation can never possess.
From the extracts which I have given, the reader may infer his strong desire for quan-
t ^ titative accuracy, the clearness of his insight, and the firmness of his grasp. Eegard-
iBg the recognition which will be ultimately accorded to Dr. Mayer, a shade of trouble
doubt has never crossed my mind. Individuals may seek to pull him down, but
•^ ^heir efforts will be unavailing as long as such evidence of his genius exists, and aa
long as the general mind of humanity is influenced by considerations of justice and
truth.

" The paucity of facts in Mayer''s time has been urged as if it were a reproach to
I him; but it ought to be remembered that the quantity of fact necessary to a generaliza-
XmtU **on is different for different minds. ' A word to the wise is sufficient for them,' and
^ \1 8 single t&ct in some minds bears fruit that a hundred cannot produce in others.
X^q Mayer^Tdata were comparatively scanty, but his genius went far to supply the lack of
e^eriment, by enabling him to see clearly the bearing of such facts as he pcJBsessed.
Th«y enabled him to think out the law of conservation, and his conclusions received
the stamp of certainty from the subsequent experimental labors of Mr. Joule. In ref-
erence to their comparative merits, I would say that as Seer and Generalizer, Mayer,
in my opinion, stands first— cw experimentcU philo8op?ter^ Joule."

^

THE TETJE SCOPE OF SCIENCE. XXXI

region of life, mind, society, history, and education, the traditional
custodians of these subjects have bidden it keep within its limits
and stick to matter. But science is not to be hampered by this
narrow conception ; its oflQce is nothing less than to investigate the
laws and universal relations of force, and its domain is therefore
coextensive with the display of power. Indeed, as we know noth-
ing of matter, except through its manifestation of forces, it is ob-
vious that the study of matter itself is at last resolved into the
study of forces. The establishment of a new philosophy of forces,
therefore, by its vast extension of the scope and methods of sci-
ence, constitutes a momentous event of intellectual progress.

The discussions of the present volume will make fully apparent
the importance of the new doctrines in relation to physical science,
but their higher implications are but partially unfolded. In the
concluding article Dr. Carpenter has shown the applicability of the
principle of correlation to vital phenomena. His argument is of
interest, not only because of the facts and principles established,
but as opening an inquiry which must lead to still larger results :
for, if the principle be found operative in fundamental organic
processes, it will undoubtedly be traced in those which are higher ;
if in the lower sphere of life, then throughout that sphere. If the
forces are correlated in organic growth and nutrition, they must bo
in organic action ; and thus human activity, in all its forms, is
brought within the operation of the law. As a creature of or-
ganic nutrition, borrowing matter and force from the outward
world ; as a being of feeling and sensibility, of intellectual power
and multiform activities, man must be regarded as amenable to the
great law that forces are convertible and indestructible; and as
psychology and sociology — the science of mind and the science of
society — ^have to deal constantly with different phases and forms
of human energy, the new principle must be of the profoundest
import in relation to these great subjects.

The forces manifested in the living system are of the most
varied and unlike character, mechanical, thermal, luminous, electric,

XXXU INTRODTJCnON.

chemical, nervous, sensory, emotional, and intellectual. That these
forces are perfectly coordinated — that there is some definite relation
among them which explains the marvellous dynamic unity of the
living organism, does not admit of question. That this relation is
of the same nature as that which is found to exist among the
purely physical forces, and which is expressed by the term ' Correl-
ation,' seems also abundantly evident. From the great complex-
ity of the conditions, the same exactness will not, of course, be
expected here as in the inorganic field, but this is one of the neces-
sary limitations of all physiological and psychological inquiry ; thus
qualified the proofs of the correlation of the nervous and mental
forces with the physical, are as clear and decisive as those for the
physical forces alone.

If a current of electricity is passed through a small wire it
produces heat, while if heat is applied to a certain combination of
metals, it reproduces a current of electricity; these forces are,
therefore, correlated. A current of electricity passed through a
small portion of a motor or sensory nerve will excite the nerve-
force in the remainder, while, on the other hand, as is shown in the
case of the torpedo, the nerve-force may generate electricity.
Nerve-force may produce heat, light, electricity, and, as we con-
stantly experience, mechanical power, and these in their turn may
also excite nerve-force. This form of energy is therefore clearly
entitled to a place in the order of correlated agencies.

Again, if we take the highest form of mental action, viz. : will-
power, we find that while it commands the movements of the sys-
tem, it does not act directly upon the muscles, but upon the cerebral
hemispheres of the brain. There is a dynamic chain of which
voluntary power is but one link. The will is a power which excites
nerve-force in the brain, which again excites mechanical power in
the muscles. Will-power is therefore correlated with nerve-power
in the same manner as the latter is with muscular power. Dr.
Carpenter well observes: "It is difficult to see that the dynamical
agency which we term will is more removed from nerve-force on

COERELATION OF NERVOUS AKD MENTAL FORCES. XXXIU

the one hand than nerve-force is removed from motor force on the
other* Each, in giving origin to the next, is itself expended or
ceases to exist wb such^ and each bears, in its own intensity, a pre-
cise relation to that of its antecedent and its consequent." We have
here only space briefly to trace the principle in its application to
sensations, motions, and intellectual operations.

The physical agencies acting upon inanimate objects in the
external world, change their form and state, and we regard these
changes as transformed manifestations of the forces in action. A
body is heated by hammering ; the heat is but transmuted mechani-
cal force ; or a body is put in motion by heat, a certain quantity
being transformed into mechanical effect, or motion of the mass.
And so it is held that no force can arise except by the expenditure
of a preexisting force. Now, the living system is acted upon by
the same agencies and under the same law. Impressions made
upon the organs of sense give rise to sensations, and we have the
same warrant in this, as in the former case, for regarding the effects
as transformations of the forces in action. If the change of
molecular state in a melted body represents the heat transformed
in fusing it, so the sensation of warmth in a living body must
represent the heat transformed in producing it. The impression on
the retina, as well as that on the photographic tablet, results from
the transmuted impulses of light. And thus impressions made
from moment to moment on all our organs of sense, are directly
correlated with external physical forces. This correlation, further-
more, is quantitative as well as qualitative, l^ot only does the
light-force produce its peculiar sensations, but the intensity of these
sensations corresponds with the intensity of the force ; not only is
atmospheric vibration transmuted into the sense of sound, but the
energy of the vibration determines its loudness. And so in all
other cases ; the quantity of sensation depends upon the quantity
of the force acting to produce it.

Moreover, sensations do not terminate in themselves, or come
to nothing ; they produce certain correlated and equivalent effects.

XXXiv INTBODUCnON.

The feelings of light, heat, sound, odor, taste, pressure, are im-
mediately followed hy physiological effects, as secretion, muscular
action, &c. Sensations increase the contractions of the heart, and
it has heen lately maintained that every sensation contracts the
muscular fibres throughout the whole vascular system. The res-
piratory muscles also respond to sensations ; the rate of breathing
being increased by both pleasurable and painful nerve-impressions.
The quantity of sensation, moreover, controls the quantity of emo-
tion. Loud sounds produce violent starts, disagreeable tastes cause
wry faces, and sharp pains give rise to violent struggles. Even
when groans and cries are suppressed, the clenched hands and set
teeth show that the muscular excitement is only taking another
direction.

Between the emotions and bodily actions the correlation and
equivalence are also equally clear. Moderate actions, like moderate
sensations, excite the heart, the vascular system, and the glandular
organs. As the emotions rise in strength, however, the various
systems of muscles are thrown into action ; and when they reach a
certain pitch of intensity, violent convulsive movements ensue.
Anger frowns and stamps ; grief wrings its hands ; joy dances and
leaps — the amount of sensation determining the quantity of correla-
tive movement.

Dr. Carpenter, in his Physiology, has brought forward numerous
exemplifications of this principle of the conversion of emotion into
movement, as seen in the common workings of human nature.
Most persons have experienced the diflBculty of sitting still under
high excitement of the feelings, and also the rehef afforded by
walking or active exercise ; while, on the other hand, repression of
the movements protracts the emotional excitement. Many irascible
persons get relief from their irritated feelings by a hearty explosion
of oaths, others by a violent slamming of the door, or a prolonged
fit of grumbling. Demonstrative persons habitually expend their
feelings in action, while those who manifest them less retain them
longer: hence the former are more weak and transient in their

CORRELATION OF PHYYSICAL AND MENTAL FOECES. XXXV

attaclimeiits than the latter, whose unexpended emotions become
permanent elements of character. For the same reason, those who
are loud and vehement in their lamentations seldom die of grief;
while the deep-seated emotions of sorrow which others cannot
work off in violent demonstrations, depress the organic functions,
and often wear out the life.

The intellectual operations are also directly correlated with
physical activities. As in the inorganic world we know nothing
of forces except as exhibited by matter, so in the higher intellectual
realm we know nothing of mind-force except through its material
manifestations. Mental operations are dependent upon material
changes in the nervous system ; and it may now be regarded as a
fundamental physiological principle, that " no idea or feeling can
arise, save as the result of some physical force expended in pro-
ducing it." The directness of this dependence is proved by the
fact that any disturbance of the train of cerebral transformations
disturbs mentality, while their arrest destroys it. And here, also,
the correlation is quantitative. Other things being equal there is a
relation between the size of the nerve apparatus and the amount
of mental action of which it is capable. Again, it is dependent
upon the vigor of the circulation ; if this is arrested by the cessation
of the heart's action, total unconsciousness results; if it is enfeebled,
mental action is low ; while if it is quickened, mentality rises, even
to delirium, when the cerebral activity becomes excessive. Again,
the rate of brain activity is dependent upon the special chemical
ingredients of the blood, oxygen and carbon. Increase of oxygen
augments cerebral action, while increase of carbonic acid depresses
it. The degree of mentality is also dependent upon the phosphatic
constituents of the nervous system. The proportion of phosphorus
in the brain is smallest in infancy, idiocy, and old age, and greatest
during the prime of life ; while the quantity of alkaline phosphates
excreted by the kidneys rises and falls with the variations of mental
activity. The equivalence of physical agencies and mental effects
is still further seen in the action of various substances, as alcohol,

XXXvi INTRODUCTTION.

opium, hashish, nitrous oxide, etc., when absorbed into the blood.
Within the limits of their peculiar action upon the nervous centres,
the effect of each is strictly proportionate to the quantity taken.
There is a constant ratio between the antecedents and consequents.
" How this metamorphosis takes place — ^how a force existing
as motion, heat, or light, can become a mode of consciousness —
4 how it is possible for aerial vibrations to generate the sensation
V we call sound, or for the forces liberated by chemical changes in the
brain, to give rise to emotion, these are mysteries which it is im-
possible to fathom. But they are not profounder mysteries than
the transformation of the physical forces into each other. They
are not more completely beyond our comprehension than the
natures of mind and matter. They have simply the same insolu-
bility as all other ultimate questions. We can learn nothing more
than that here is one of the uniformities in the order of phe-

The law of correlation being thus applicable to human energy
as well as to the powers of nature, it must also apply to society,
where we constantly witness the conversion of forces on a compre-
hensive scale. The powers of nature are transformed into the activ-
ities of society; water-power, wind-power, steam-power, and electri-
cal-power are pressed into the social service, reducing human labor,
multiplying resources, and carrying on numberless industrial pro-
cesses : indeed, the conversion of these forces into social activities
is one of the chief triumphs of civilization. The universal forces
of heat and hght are transformed by the vegetable kingdom into
the vital energy of organic compounds, and then, as food, are again
converted into human beings and human power. The very exist-
ence as well as the activity of society are obviously dependent upon
the operations of vegetable growth. When that is abundant, popu-
lation may become dense, and social activities multifarious and
complicated, while a scanty vegetation entails sparse population
and enfeebled social action. Any universal disturbance of the
physical forces, as excessive rains or drouth, by reducing the har-

COEEELATION OF VITAL AND SOCIAL FOKCES. XXXVll

vest, is felt throughout the entire social organism. Where this
effect is marked, and not counteracted by free communication with
more fertile regions, the means of the community become restricted,
business declines, manufactures are reduced, trade slackens, travel
falls off, luxuries are diminished, education is neglected, marriages
are fewer, and a thousand kindred results indicate decline of enter-
prise and depression of the social energies.

In a dynamical point of view there is a strict analogy between
the individual and the social economies — the same law of force
governs the development of both. In the case of the individual,
the amount of energy which he possesses at any time is limited,
and when consumed for one purpose it cannot of course be had for
another. An undue demand in one direction involves a corre-
sponding deficiency elsewhere. For example, excessive action of
the digestive system exhausts the muscular and cerebral systems,
while excessive action of the muscular system is at the expense of
the cerebral and digestive organs ; and again, excessive action of
the brain depresses the digestive and muscular energies. If the
fund of power iij the growing constitutions of children is overdrawn
in any specijil channel, as is often the case by excessive stimulation
of the brain, the undue abstraction of energy from other portions
of the system is sure to entail some form of physiological disaster.
So with the social organism ; its forces being limited, there is but a
definite amount of power to be consumed in the various social
activities. Its appropriation in one way makes impossible its em-
ployment in another, and it can only gain power to perform one
function by the loss of it in other directions. This fact, that social
force cannot be created by enactment, and that when dealing with
the producing, distributing, and commercial activities of the com-
munity, legislation can do little more than interfere with their
natural courses, deserves to be more thoroughly appreciated by the
public.

But the law in question has yet higher bearings. More and
more we are perceiving that the condition of humanity and tho

XXXVlll INTKODUCTION.

progress of civilization are direct resultants of the forces by which
men are controlled. What we term the moral order of society, im-
plies a strict regularity in the action of these forces. Modern sta-
tistics disclose a remarkable constancy in the moral activities man-
ifested in communities of men. Crimes, and even the modes of
crime, have been observed to occur with a uniformity which admits
of their prediction. Each period may therefore be said to have its
definite amount of morality and justice. It has been maintained,
for instance, with good reason, that " the degree of liberty a peo-
ple is capable of in any given age, is a fixed quantity, and that any
artificial extension of it in one direction brings about an equiva-
lent limitation in some other direction. French revolutions show
scarcely any more respect for individual rights than the despotisms
they supplant; and French electors use their freedom to put
themselves again in slavery. So in those communities where State
restraint is feeble, we may expect to find it supplemented by the
sterner restraints of public opinion."

But society like the individual is progressive. Although at
each stage of individual growth the forces of the organism, physi-
ological, intellectual, and passional, have each a certain diefinite
amount of strength, yet these ratios are constantly changing, and
it is in this change that development essentially consists. So with
society ; the measured action of its forces gives rise to a fixed
amount of morality and liberty in each age, but that amount in-
creases with social evolution. The savage is one in whom certain
classes of feelings and emotions predominate, and he becomes civil-
ized just in proportion as these feelings are slowly replaced by oth-
ers of a higher character. Yet the activities which determine
human advancement are various. Not only must we regard the
physiological forces, or those which pertain to man's physical or-
ganization and capacities, and the psychological, or those resulting
from his intellectual and emotional constitution, but the influences
of the external world, and those of the social state, are likewise to
be considered. Man and society, therefore, as viewed by the eye

spencee's contribution to the inquiry, xxxix

of science, present a series of vast and complex dynamical problems,
which are to be studied in the future in the light of the great
law by which, we have reason to believe, all forms and phases of
force are governed.

A further aspect of the subject remains still to be noticed. Mr.
Herbert Spencer has the honor of crowning this sublime inquiry by
showing that the law of the conservation, or as he prefers to term it
the ' Persistence of Force,' as it is the underlying principle of all be-
ing, is also the fundamental truth of all philosophy. With masterly
analytic skill he has shown that this principle of which the human
mind has just become fully conscious, is itself the profoundest law
of the human mind, the deepest foundation of consciousness. He
has demonsti'ated that the law of the Persistence of Force, of which
the most piercing intellects of past times had but partial and un-
satisfying glimpses, and which the latest scientific research has
disclosed as a great principle of nature, has a yet more transcendent
character ; is, in fact, an (I priori truth of the highest order — a
truth which is necessarily involved in our mental organization ;
which is broader than any possible induction, and of higher validity
than any other truth whatever. This principle, which is at once
the highest result of scientific investigation and metaphysical
analysis, Mr. Spencer has made the basis of his new and compre-
hensive System of Philosophy ; and in the first work of the series,
entitled " First Principles, " he has developed the doctrine in its
broadest philosophic aspects. The lucid reasoning by which he
reaches his conclusions cannot be presented here ; a brief extract
or two will, however, serve to indicate the important place assigned
to the law by this acute and profound inquirer :

" We might, indeed, be certain, even in the absence of any such
analysis as the foregoing, that there must exist some principle
which, as being the basis of science, cannot be established by sci-
ence. All reasoned out conclusions whatever must rest on some
postulate. As before shown, we cannot go on merging derivative
truths in these wider and wider truths from which they are de-

•3

INTEODUCnON.

rived, Without reaching at last a widest trath which can be merged

f'" in no other, or derived from no other. And whoever contemplates

5 the relation in which it stands to the truths of science in general,

y-will see that this truth, transcending demonstration, is the Persist-

Vence of force." * * *

X " ■** Such, then, is the foundation of any possible system of posi-
tive knowledge. Deeper than demonstration — deeper even than
definite cognition — deep as the very nature of mind, is the postu-
late at which we have arrived. Its authority transcends all others
^ whatever ; for not only is it given in the constitution of our own
^ consciousness, but it is impossible to imagine a consciousness so
J%3 ^ ^ constituted as not to give it. Thought, involving simply the estab-
3 .^ ** y 'lishment'of relations, may be readily conceived to go on while yet
.. ^ these relations have not been organized into the abstracts we call
C space and time ; and so there is a conceivable kind of consciousness
which does not contain the truths commonly called d priori, in-
"^^ ^ *"' *• volved in the organization of these forms of relations. But thought
^ /T y v( cannot be conceived to go on without some element between which
^ i V ^ its relations may be established ; and so there is no conceivable
J-.^pC kind of consciousness which does not imply continued existence as
^ tv its datum. Consciousness without this or that particular form is
^possible ; but consciousness without contents is impossible.

" The sole truth which transcends experience by underlying it, is
thus the Persistence of force. This being the basis of experience,
must be the basis of any scientific organization of experiences. To
this an ultimate analysis brings us down ; and on this a rational
synthesis must be built up."

To the question, "What then is the value of experimental inves-
tigations upon the subject, if the truth sought cannot be estab-
lished by inductions from them ? Mr. Spencer replies : " They are
of value as disclosing the many particular implications which the
general truth does not specify ; they are of value as teaching us how
much of one mode of force is the equivalent of so much of another
mode ; they are of value as determining unfter what conditions each

STUPENDOUS REACH OF THE LAW. xli

metamorphosis occurs ; and they are of value as leading us to in-
quire in what shape the remnant of force has escaped, when the
apparent results are not equivalent to the cause." And it may be
added, that it is to these investigations that we are indebted for the
clear and comprehensive establishment of the principle as a law of
physical nature ; psychological analysis having only shown that it
extends much further than it is the business of experimental science
to go.

Thus the law characterized by Faraday as the highest in phys-
ical science which our faculties permit us to perceive, has a far
more extended sway; it might well have been proclaimed the
highest 'law of a W science — the most far-reaching principle that
adventuring reason has discovered in the universe. Its stupendous
reach spans all orders of existence. Not only does it govern the
movements of the heavenly bodies, but it presides over the genesis
of the constellations ; not only does it control those radiant floods
of power which fill the eternal spaces, bathing, warming, illumining
and vivifying our planet, but it rules the actions and relations of
men, and regulates the march of terrestrial affairs. Nor is its do-
minion limited to physical phenomena ; it prevails equally in the
world of mind, controlling all the faculties and processes of thought
and feeling. The star-suns of the remoter galaxies dart their ra-
diations across the universe ; and although the distances are so pro-
found that hundreds of centuries may have been required to traverse
them, the impulses of force enter the eye, and impressing an atomic
change upon the nerve, give origin to the sense of sight. Star
and nerve-tissue are parts of the same system — stellar and nervous
forces are correlated. Nay more ; sensation awakens thought and
kindles emotion, so that this wondrous dynamic chain binds into
living unity the realms of matter and mind through measureless
amplitudes of space and time.

And if these high realities are but faint and fitful glimpses
which science has obtained in the dim dawn of discovery, what
must be the glories of the coming day ? If indeed they are but

Xlii INTEODUCnON.

* pebbles' gathered from the shores of the great ocean of truth,
what are the mysteries still hidden in the bosom of the mighty un-
explored? And how far transcending all stretch of thought that
Unknown and Infinite Cause of all to which the human spirit turns
evennore in solemn and mysterious worship !

It remains only to observe, that so immense a step in the pro-
gress of our knowledge of natural agencies as the following pages
disclose, cannot be without effect upon the intellectual culture
of the age. To the adherents of that scholastic and verbal edu-
cation which prefers words to things, and ancient to modern
thought ; which ignores the study of nature, and regards the pro-
gress of science with indifference or hostility, it matters little what
views of the world are entertained, or what changes these views
may undergo. But there is another, and happily an increasing class,
who hold that it is the true destiny of mind to comprehend the
vast order of existence in the midst of which it is placed, and that
the faculties of man are divinely adapted to this sublime task ; who
see that the laws of nature must be understood before they can be
obeyed, and that only through this understanding can man rise to
the mastery of its powers, and bring himself into final harmony
with his conditions. These will recognize that the discovery of
new principles whic^ expand, and elevate, and harmonize our views
of the universe — which involve the workings of the mind Itselt*
open a new chapter in philosophy, and touch the very foundations
of knowledge, cannot be without a determining influence upon the
future course and development of thought, and the spirit and
methods of its acquisition.

V X «^

THE

CORRELATION

OF PHYSICAL FOEOES

By W. R. GROYE, Q.C., M.A., F.R.S.

FIEST AMERIOAIf, FEOM THE FOUETH ENGLISH EDITION.

William Robert Grove, an English lawyer and physicist, was bom at
Swansea, July 14, 1811. He graduated at Oxford in 1834, and during the
next five years was Professor of Natural Philosophy at the London Insti-
tution. Professor Grove is a rare example of the abihty which has achieved
a distinguished eminence in different fields of effort. While pursuing with
marked success the profession of an advocate, he has devoted his leisure to
original scientific researches, and obtained a high distinction both as a dis-
coverer and a philosophic writer upon scientific subjects. In 1852 he was
made Queen's Counsel, and afterwards Vice-President of the Eoyal Society.
He is the inventor of the powerful galvanic battery known by his name, and
his chief researches have been in the field of electricity. Many of his ex-
perimental results are referred to in the following pages, which will also
attest his high position among the founders of the new philosophy of
forces.

^ t^^ Cj^^f^^'-' y /^ aL^ /tv.; /PS-

PREFACE.

THE Phrase ' Correlation of Physical Forces ' in the sense in
which I have used it, having become recognized by a large
number of scientific writers, it would produce confusion were I
now to adopt another title. It would, perhaps, have been better
if I had in the first instance used the term Co-relation, as the
words ' correlate,' ' correlative,' had acquired a peculiar metaphys-
ical sense somewhat differing from that which I attached to the
substantive correlation. The passage in the text (p. 183) explains
the meaning I have given to the term.

Twenty years having elapsed since I promulgated the views
contained in this Essay, which were first advanced in a lecture at
the London Institution in January 1842, and subsequently more
fully developed in a course of lectures in 1843, 1 think it advisable
to add a little to the Preface with reference to other labourers in
the same field.

It has happened with this subject as with many others, that
similar ideas have indejDeudeutly presented themselves to differ-
ent minds about the Same period. In May 1842 a paper was
published by M. Mayer which I had not read when my last edition
was published, and indeed only now know imperfectly by the
vivd-^oce translation of a friend. It deduces very much the same
conclusions to which I had been led, the author starting partly
from d priori reasoning and partly from an experiment by which
water was heated by agitation, and from another, which had, how-
ever, previously been made by Davy, viz. that ice can be melted
by friction, though kept in a medium which is below the freezing
point of water.

In 1843 a paper by Mr. Joule on the mechanical equivalent of

4: PREFACE.

heat appeared, wMch, though not in terms touching on the mutual
and necessary dependence of all the Physical Forces, yet bears
most importantly upon the doctrine.

While my third edition was going through the press I had
the good fortune to make the acquaintance of M. Seguin, who
informed me that his uncle, the eminent Montgolfier, had long
entertained the idea that force was indestructible, though, with
the exception of one sentence, in his paper on the hydraulic ram,
and where he is apparently speaking of mechanical force, he has
left nothing in print on the subject. Not so, however, M. Seguin
himself, who in 1839, in a work on the ' Influence of Railroads,'
has distinctly expressed his uncle's and his own views on the
identity of heat and mechanical force, and has given a calculation
of their equivalent relation, which is not far from the more recent
numerical results of Mayer, Joule, and others.

Several of the great mathematicians of a much earlier period
advocated the idea of what they termed the Conservation of Force,
but although they considered that a body in motion would so
continue for ever, unless arrested by the impact of another body,
and, indeed, in the latter case, would, if elastic, still continue to
move (though deflected jfrom its course) with a force proportion-
ate to its elasticity, yet with inelastic bodies the general, and, as
far as I am aware, the universal belief was, that the motion was
arrested and the force annihilated. Montgolfier went a step far-
ther, and his hydraulic ram was to him a proof of the truth of
his preconceived idea, that the shock or impact of bodies left the
mechanical force imdestroyed.

Previously, however, to the discoveries of the voltaic battery,
electro-magnetism, thermo-electricity, and photography, it was
impossible for any mind to perceive what, in the greater number
of cases, became of the force which was apparently lost. The
phenomena of heat, known from the earliest times, would have
been a mode of accounting for the resulting force in many cases
where motion was arrested, and we find Bacon announcing a
theory that motion was the form, as he quaintly termed it, of
heat. Rumford and Davy adopted this view, the former with a
fair approximate attempt at numerical calculation, but no one of
these philosophers seems to have connected it with the inde-
structibility of force. A passage in the writings of Dr. Roget,

PREFACE. 0

combating the theory that mere contact of dissimilar bodies was
the source of voltaic electricity, philosophically supports his argu-
ment by the idea of non-creation of force.

As I haye introduced into the later editions of my Essay ab-
stracts of the different discoveries which I have found, since my
first lectures, to bear upon the subject, I have been regarded by
many rather as the historian of the progress made in this branch
of thought than as one who has had anything to do with its ini-
tiation. Everyone is but a poor judge where he is himself inter-
ested, and I therefore write with diffidence, but it would be affect-
ing an indifference which I do not feel if I did not state that I
believe myself to have been the first who introduced this subject
as a generalised system of philosophy, and continued to enforce
it in my lectures and writings for many years, during which it
met with the opposition usual and proper to novel ideas.

Avocations necessary to the well-being of others have prevent-
ed my following it up experimentally, to the extent that I once
hoped ; but I trust and believe that this Essay, imperfect though
it be, has helped materially to impress on that portion of the
public which devotes its attention rather to the philosophy of
science than to what is now termed science, the truth of the thesis
advocated.

To show that the work of to-day is not substantially different
from the thoughts I first published on the subject, at a period
when I knew little or nothing of what had been thought before,
I venture to give a few extracts from the printed copy of my
lecture of 1842 :—

Physical Science treats of Matter, and what I shall to-night term its
Affections ; namely, Attraction, Motion, Heat, Light, Electricity, Magnet-
ism, Chemical-Affinity. When these re-act upon matter, they constitute
Forces. The present tendency of theory seems to lead to the opinion that
all these Affections are resolvable into one, namely, Motion; however,
should the theories on these subjects be ultimately so effectually gener-
alised as to become laws, they cannot avoid the necessity for retaining dif-
ferent names for these different Affections ; or, as they would then be called,
different modes of Motion

(Ersted proved that Electricity and Magnetism are two forces which act
upon each other ; not in straight Imes, as all other known forces do, but in
a rectangular direction : that is, that bodies invested with electricity, or the
conduits of an electric-current, tend to place magnets ut right angles to
them ; and, conversely, that magnets tend to place bodies conductiug elec-
tricity at right angles to them

6 PREFACE.

The discovery of (Ersted, by which electricity was made a source of
Magnetism, soon led philosophers to seek the converse effect ; that is, to
educe Electricity from a permanent magnet : — had these experimentalists
succeeded in their expectations of making a stationary magnet a source of
electric-currents, they would have realised the ancient dreams of perpetual
motion, they would have converted statics into dynamics, they would have
produced power without expenditure ; in other- words, they would have be-
come creators. They failed, and Faraday saw their error ; he proved that
to obtain Electricity from Magnetism it was necessary to superadd to this
latter, motion; that magnets while in motion induced electricity in con-
tiguous conductors ; and that the direction of such electric-currents was
tangential to the polar direction of the magnet ; that as Dynamic-electricity
may be made the source of Magnetism and Motion, so Magnetism conjoined
with Motion may be made the source of Electricity. Here originates the
Science of Magneto-electricity, the true converse of Electro-magnetism;
and thus between Electricity and Magnetism is shown to exist a reciprocity
of force such that, considering either as the primary agent, the other be-
comes the re-agent ; viewing one in the relation of cause, the other is the
effect

The Science of Thermo-Electricity connected heat with electricity, and
proved these, like all other natural forces, to be capable of mutual reac-
tion

Voltaic action is Chemical action taking place at a distance or trans-
ferred through a chain of media ; and the Daltonian equivalent numbers
are the exponents of the amount of voltaic action for corresponding chemi-
cal substances. . . .

By regarding the quantity of electrical, as directly proportional to the
efficient chemical action, and by experimentally tracing this principle, I
have been fortunate enough to increase the power of the Voltaic-pile
more than sixteen times, as compared with any combination previously
known

I am strongly disposed to consider that the facts of Catalysis depend
upon voltaic action, to generate which three heterogeneous substances are
always necessary. Induced by this belief I made some experiments on the
subject, and succeeded in forming a voltaic combination by gaseous-oxygen,
gaseous-hydrogen, and platinum ; by which a galvanometer was deflected
and water decomposed

It appears to me that heat and light may be considered as affections ;
or, according to the Undulatory-theory, vibrations of matter itself, and not
of a distinct etherial fluid permeating it : these vibrations would be prop-
agated, just as sound is propagated by vibrations of wood or as waves by
water.^ To my mind, all the consequences of the Undulatory-theory flow
as easily from this, as from the hypothesis of a specific ether ; to suppose
which, namely, to suppose a fluid sui generis^ and of extreme tenuity pene-
trating solid bodies, we must assume, first, the existence of the fluid itself;
secondly, that bodies are without exception porous; thirdly, that these
pores- communicate; fourthly, that matter is limited in expansibiUty.
None of these difficulties apply to the modification of this theory which I
venture to propose ; and no other difficulty applies to it which does not
equally apply to the received hypothesis. With regard to the planetary
spaces, the diminishing periods of comets is a strong argument for the ex-
istence of an universally-diflfused matter : this has the function of resist-

PREFACE. T

ance, and there appears to be no reason to divest it of the functions com-
mon to all matter, or superficially to appropriate it to certain affections.
Again, the phenomena of transparency and opacity are, to my mind, more
easily explicable by the former than by the latter theory ; as resulting from
a difference in the molecular arrangement of the matter affected. In re-
gard to the effects of double-refraction and polarisation, the molecular
gives at once a reason for the effects upon the one theory, while upon the
other we must, in addition to previous assumptions, further assume a dif-
ferent elasticity of the ether in different directions within the doubly-
refracting medium. The same theory is applicable to Electricity and
Magnetism ; my own experiments on the influence of the elastic intermedium
on the voltaic-arc, and those of Faraday on electrical induction, furnish
strong arguments in support of it. My inclination would lead me to de-
tain you on this subject much longer than my judgment deems advisable :
I therefore content myself with offering it to your consideration, and,
should my avocations permit, I may at a future period more fully develope

it

Light, Heat, Electricity, Magnetism, Motion, and Chemical-afl&nity, are
all convertible material affections ; assuming either as the cause, one of
the others will be the effect : thus heat may be said to produce electricity,
electricity to produce heat ; magnetism to produce electricity, electricity
magnetism ; and so of the rest. Cause and effect, therefore, in their - ab-
stract relation to these forces, are words solely of convenience: we are
totally unacquainted with the ultimate generating power of each and all
of them, and probably shall ever remain so ; we can only ascertain the
normae of their action : we must humbly refer their causation to one omni-
present influence, and content ourselves with studying their effects and
developing by experiment their mutual relations.

I have transposed the passages relating to voltaic action and
catalysis, but I have not added a word to the above quotation,
and, as far as I am now aware, the theory that the so-called im-
ponderables are affections of ordinary matter, that they are re-
solvable into motion, that they are to be regarded in their action
on matter as forces, and not as specific entities, and that they are
capable of mutual reaction, thence alternately acting as cause and
effect, had not at that time been publicly advanced.

My original Essays being a record of lectures, and being pub-
lished by the managers of the Institution, I necessarily adhered
to the form and matter which I had orally communicated. In
preparing subsequent editions I found that, without destroying
the identity of the work, I could not alter the style ; although it
would have been less difficult and more satisfactory to me to have
done so, the work would not have been a republication ; and I
was for obvious reasons anxious to preserve as far as I could the
original text, which, though added to, is but little altered.

The form of lectures has necessarily continued the use of the

8 PREFACE.

first person, and I would beg my readers not to attribute to me,
from the modes of expression used, a dogmatism which is far from
my thought. If my opinions are expressed broadly, it is that, if
opinions are always hedged in by qualifications, the style becomes
embarrassed and the meaning frequently unintelligible.

As a course of lectures can only be useful by inducing the
auditor to consult works on the subject he hears treated, so the
object of this Essay is more to induce a particular train of thought
on the known facts of physical science than to enter with minute
criticisms into each separate branch.

In one or two of the reviews of previous editions the general
idea of the work was objected to. I believe, however, that will
not now be the case ; the mathematical labours of Mr. Thompson,
Clausius, and others, though not suitable for insertion in an
Essay such as this, have awakened an interest for many portions
of the subject, which promises much for its future progress.

The short and irregular intervals which my profession permits
me to devote to science so prevent the continuity of attention
necessary for the proper evolution of a train of thought, that I
certainly should not now have courage to publish for the first
time such an Essay ; and it is only the favour it has received
from those whose opinions I highly value, and the, I trust pardon-
able, wish not to let some favourite thoughts, of my youth lose all
connection with my name, that have induced me to reprint it.

My scientific readers will, I hope, excuse the very short notices
of certain branches of science which are introduced, as without
them the work would be unintelligible to many for whom it is
intended. I have endeavoured so to arrange my matter that each
division should form an introduction to those which follow, and
to assume no more preliminary knowledge to be possessed by my
readers than would be expected from persons acquainted with the
elements of physical science.

The notes contain references to the original memoirs in which
the branches of science alluded to are to be found, as well as to
those which bear on the main arguments ; where these memoirs
are numerous, or not easy of access, I have raferred to treatises in
which they are collated. To prevent the reader's attention being
interrupted, I have in the notes referred to the pages of the text,
instead of to interpolated letters.

COEEELATION

OF

PHYSICAL FOEOES.

tf

I.— INTRODUCTORY REMARKS.

"T'TT"-HEN natural phenomena are for the first time ob-
V V ' served, a tendency immediately developes itself to
refer them to something previously known — to bring them
within the range of acknowledged sequences. The mode of
regarding new facts, which is most favourably received by
the public, is that which refers them to recognised views —
stamps them into the mould in which the mind has been al-
ready shaped. The new fact may be far removed from those
to which it is referred, and may belong to a different order
of analogies, but this cannot then be known, as its co-ordi-
nates are wanting. It may be questionable whether the
mind is not so moulded by past events that it is impossible
to advance an entirely new view, but admitting such possi-
bility, the new view, necessarily founded on insufficient data,
is likely to be more incorrect and prejudicial than even a
strained attempt to reconcile the new discovery with known
acts.

The theory consequent upon new facts, whether it be a
co-ordination of them with known ones, or the more difficult
1*

10 COBKELATION' OF PHYSICAL FORCES.

and dangerous attempt at remodelling the public ideas, is
generally enunciated by the discoverers themselves of the
facts, or by those to whose authority the world at the period
of the discovery defers ; others are not bold enough, or if
they be so, are unheeded. The earliest theories thus enuncia-
ted obtain the firmest hold upon the public mind, for at such
a time there is no power of testing, by a sufficient range of
experience, the truth of the theory ; it is accepted solely or
mainly upon authority : there being no means of contradic-
tion, its reception is, in the first instance, attended with some
degree of doubt, but as the time in which it can fairly be in-
vestigated far exceeds that of any lives then in being, and as
neither the individual nor the public mind will long tolerate
a state of abeyance, a theory shortly becomes, for want
of a better, admitted as an established truth : it is handed
from father to son, and gradually takes its place in edu-
cation. Succeeding generations, whose minds are thus
formed to an established view, are much less likely to aban-
don it. They have adopted it in the first instance, upon au-
thority, to them unquestionable, and subsequently to yield up
their faith would involve a laborious remodelling of ideas, a
task which the public as a body will and can rarely under-
take, the frequent occurrence of which is indeed inconsistent
with the very existence of man in a social state, as it would
induce an anarchy of thought — a perpetuity of mental revo-
lutions.

This necessity has its good ; but the prejudicial effect
upon the advance of science is, that by this means, theories the
most immature frequently become the most permanent ; for
no theory can be more immature, none is likely to be so in-
correct, as that which is formed at the first flush of a new
discovery ; and though time exalts the authority of those
from whom it emanated, time can never give to the illustri-
ous dead the means of analysing and correcting erroneous
views which subsequent discoveries confer.

mTRODUOTOBY REMARKS. 11

Take for instance the Ptolemaic System, which we may
almost literally explain by the expression of Shakspeare :
' He that is giddy thinks the world turns round/ We now
see the error of this system, because we have all an immedi-
ate opportunity of refuting it ; but this identical error was
received as a truth for centuries, because, when first promul-
gated, the means of refuting it were not at hand, and when
the means of its refutation became attainable, mankind had
been so educated to the supposed truth, that they rejected the
proof of its fallacy.

I have premised the above for two reasons : first to obtain
a fair hearing, by requesting as far as possible a dismissal
from the mind of my readers of preconceived views by and in
favour of which all are liable to be prejudiced ; and secondly,
to defend myself from the charge of undervaluing authority,
or treating lightly the opinions of those to whom and to
whose memory mankind looks with reverence. Properly to
value authority, we should estimate it together with its means
of information : if ' a dwarf on the shoulders of a giant can
see further than the giant,' he is no less a dwarf in compari-
son with the giant.

The subject on which I am about to treat — ^viz., the rela-
tion of the affections of mattei to each other and to matter —
peculiarly demands an unprejudiced regard. The different
aspects under which these agencies have been contemplated ;
the different views which have been taken of matter itself;
the metaphysical subtleties to which these views unavoidably
lead, if pursued beyond fair inductions from existing expe-
rience, present difficulties almost insurmountable.

The extent of claim which my views on this subject may
have to originality has been stated in the Preface ; they be-
came strongly impressed upon my mind at a period when I
was much engaged in experimental research, and were, as I
then believed, and still believe, regarding them as a system,
new : expressions in the works of different authors, bearing

12 CORRELATION OF PHYSICAL FORCES.

more or less on the subject, have subsequently been pointed
out to me, some of which go back to a distant period. An
attempt to analyse these in detail, and tp trace how far I
have been anticipated by others, would probably but little
interest the reader, and in the course of it I should constantly
have to make distinctions showing wherein I differed, and
wherein I agreed with others. I might cite authorities which
appear to me to oppose, and others which appear to coincide
with certain of the views I have put forth ; but this would
interrupt the consecutive developement of my own ideas, and
might render me liable tathe charge of misconstruing those of
others ; I therefore think it better to avoid such discussion in
the text ; and in addition to the sketch given in the Preface,
to furnish in the notes at the conclusion such references
to different authors as bear upon the subjects treated of,
which I have discovered, or which have been pointed out to
me since the delivery of the lectures of which this essay
is a record.

The more extended our research becomes, the more we
find that knowledge is a thing of slow progression, that the
very notions which appear to ourselves new, have arisen,
though perhaps in a very indirect manner, from successive
modifications of traditional opinions. Each word we utter,
each thought we think, has in it the vestiges, is in itself the
impress, of antecedent words and thoughts. As each ma-
terial form, could we rightly read it, is a book, containing in
itself the past history of the world ; so, different though our
philosophy may now appear to be from that of our progeni-
tors, it is but theirs added to or subtracted from, transmitted
drop by arop through the filter of antecedent, as ours will be
through that of subsequent, ages. — ^The relic is to the past as
is the germ to the future.

Though many valuable facts, and correct deductions from
them, are to be found scattered amongst the voluminous
works of the ancient philosophers ; yet, giving them the

INTKODUCTORY REMAKES. 13

credit which they pre-eminently deserve for having devoted
their lives to purely intellectual pursuits, and for having
thought, seldom frivolously, often profoundly, nothing can be
more difficult than to seize and apprehend the ideas of those
who reasoned from abstraction to abstraction — who, although,
as we now believe, they must have depended upon observa-
tion for their first inductions, afterwards raised upon them
such a complex superstructure of syllogistic deductions, that,
without following the same paths, and tracing the same sinu-
osities which led them to their conclusions, such conclusions
are to us unintelligible. To think as another thought, we
must be placed in the same situation as he was placed : the
errors of commenta,tors generally arise from their reasoning
upon the arguments of their text, either in blind obedience to
its dicta, without considering the circumstances under which
they were uttered, or in viewing the images presented to the
original writer from a different point to that from which he
viewed them. Experimental philosophy keeps in check the
errors both of a priori reasoning and of commentators, and,
at all events, prevents their becoming cumulative ; though
the theories or explanations of a fact be different, the fact
remains the same. It is, moreover, itself the exponent of its
discoverer's thought : the observation of known phenomena
has led him to elicit from nature the new phenomena : and,
though he may be wrong in his deductions from this after its
discovery, the reasonings which conducted him to it are them
selves valuable, and, having led from known to unknown
truths, can seldom be uninstructive.

Very different views existed amongst the ancients as to the
aims to be pursued by physical investigation, and as to the objects
likely to be attained by it. I do not here mean the moral ob-
jects, such as the attainment of the summum honum, &c.
— ^but the acquisitions in knowledge which such investiga-
tions were likely to confer. Utility was one object in view,
and this was to some extent attained by the progress noMde in

14 COKEELATION OF PHYSICAL FORCES.

astronomy and mechanics ; Archimedes, for instance, seems
to have constantly had this end in view ; but, while pursuing
natural knowledge for the sake of knowledge and the power
which it brings with it, the greater number seemed to enter-
tain an expectation of arriving at some ultimate goal, some
point of knowledge, which would give them a mastery over
the mysteries of nature, and would enable them to ascertain
what was the most intimate structure of matter, and the
causes of the changes it exhibits. Where they could not dis-
cover, they speculated. Leucippus, Democritus, and others,
have given us their notions of the ultimate atoms of which
matter was formed, and of the modus agendi of nature in the
various transformations which matter undergoes.

The expectation of arriving at ultimate causes or essences
continued long after the speculations of the ancients had been
abandoned, and continues even to the present day to be a ver^
general notion of the objects to be ultimately attained by
physical science. Francis Bacon, the great remodeller of
science, entertained this notion, and thought that, by experi-
mentally testing natural phenomena, we should be enabled to
trace them to certain primary essences or causes whence the
various phenomena flow. These he speaks of under the
scholastic name of ' forms * — a term derived from the ancient
philosophy, but differently applied. He appears to have un-
derstood by ' form ' the essence of quality — that in which, ab-
stracting everything extraneous, a given quality consists, or
that which, superinduced on any body, would give it its pe-
culiar quality: thus the form of transparency, is that
which constitutes transparency, or that by which, . when dis-
covered, transparency could be produced or superinduced.
To take a specific example of what I may term the syn-
thetic application of his philosophy : — ' In gold there meet
together yellowness, gravity, malleability, fixedness in the fire,
a determinate way of solution, which are the simple natures
in gold ; for he who understands form, and the manner of

INTEODUCTOEY REMARKS. 15

superinducing this yellowness, gravity, ductility, fixedness,
faculty of fusion, solution, &c., with their particular degrees
and proportions, will consider how to join them together in
some body, so that a traiismutation into gold shall follow.'

On the other hand, the analytic method, or, ' the enquiry
from what origin gold or any other metal or stone is generated
from its first fiuid matter or rudiments, up to a perfect min-
eral,' is to be perceived by what Bacon calls the latent pro-
cess, or a search for ' what in every generation or transfor-
mation of bodies, flies off, what remains behind, what is add-
ed, what separated, &c. ; also, in other alterations and mo-
tions, what gives motion, what governs it, and the like.'
Bacon appears to have thought that qualities separate from
the substances themselves were attainable, and if not capable
of physical isolation, were at all events capable of physical
transference and superinduction.

Subsequently to Bacon a belief has generally existed, and
now to a great extent exists, in what are called secondary
causes, or consequential steps, wherein one phenomenon is
supposed necessarily to hang on another, until at last we ar-
rive at an essential cause, subject immediately to the First
Cause. This notion is generally prevalent both on the Con-
tinent and in this country : nothing is more familiar than the
expression ' study the efiTects in order to arrive at the causes.'

Instead of regarding the proper object of physical science
as a search after essential causes, I believe it ought to be, and
must be, a search after facts and relations — ^that although the
word Cause may be used in a secondary and concrete sense,
as meaning antecedent forces, yet in an abstract sense it is to-
tally inapplicable ; we cannot predicate of any physical agency
that it is abstractedly the cause of another ; and if, for the sake
of convenience, the language of secondary causation be per-
missible, it should be only with reference to the special phe-
nomena referred to, as it can never be generalised.

The misuse, or rather varied use, of the term Cause, has

16 CORRELATION OF PHYSICAL FORCES.

been a source of great confusion in physical theories, and
philosophers are even now by no means agreed as to their
conception of causation. The most generally received view
of causation, that of Hume, refers it to invariable antecedence
— i. e., we call that a cause which invariably precedes, that
an effect which invariably succeeds. Many instances of in-
variable sequence might however be selected, which do not
present the relation of cause and eifect : thus as Reed observes,
and Brown does not satisfactorily answer, day invariably
precedes night and yet day is not the cause of night. The seed,
again, precedes the plant, but is not the cause of it ; so that when
we study physical phenomena it becomes difficult to separate the
idea of causation from that of force, and these have been regarded
as identical by some philosophers. To take an example which
will contrast these two views : if a floodgate be raised, the water
flows out ; in ordinary parlance, the water is said to flow he-
cause the floodgate is raised : the sequence is invariable ; no
floodgate, properly so called, can be raised without the water
flowing out, and yet in another, and perhaps more strict, sense,
it- is the gravitation of the water which causes it to flow. But
though we may truly say that, in this instance, gravitation
causes the water to flow, we cannot in truth abstract the pro-
position, and say, generally, that gravitation is the cause of
water flowing, as water may flow from other causes, gaseous
elasticity, for instance, which will cause water to flow from a
receiver full of air into one that is exhausted ; gravitation may
also, under certain circumstances, arrest instead of cause the
flow of water.

Upon neither view, however, can we get at anything like
abstract causation. If we regard causation as invariable se-
quence, we can find no case in which a given antecedent is
the only antecedent to a given sequent : thus if water could
flow from no other cause than the withdrawal of a floodgate,
we might say abstractedly that this was the cause of water
flowing. If, again, adopting the view which looks to causa-

INTEODUCTORY EEMAEKS. 17

tion as a force, we could say that water could be caused to
flow only by gravitation, we might say abstractedly that grav-
itation was the cause of water flowing — ^but this we cannot
say ; and if we seek and examine any other example, we
shall find that causation is only predicable of it in the partic-
ular case, and cannot be supported as an abstract proposition ;
yet this is constantly attempted. Nevertheless, in each jpar-
ticular case where we speak of Cause, we habitually refer to
some antecedent power or force : we never see motion or any
change in matter take effect without regarding it as produced
by some previous change ;^ and when we cannot trace it to its
antecedent, we mentally refer it to one ; but whether this hab-
it be philosophically correct is by no means clear. In other
words, it seems questionable, not only whether cause and ef-
fect are convertible terms with antecedence and sequence, but
whether in fact cause does precede effect, whether force does
precede the change in matter of which it is said to be the
cause.

The actual priority of cause to effect has been doubted,
and their simultaneity argued with much ability. As an in-
stance of this argument it may be said, the attraction which
causes iron to approach the magnet is simultaneous with and
ever accompanies the movement of the iron ; the movement
is evidence of the co-existing cause or force, but there is no
evidence of any interval in time between the one and the oth-
er. On this view time would cease to be a necessary element
in causation ; the idea of cause, except perhaps as referred to
a primeval creation, would cease to exist ; and the same ar-
guments which apply to the simultaneity of cause with effect
would apply to the simultaneity of Force with Motion. We
could not, however, even if we adopted this view, dispense
with the element of time in the sequence of phenomena ; the
effect being thus regarded as ever accompanied simultaneous-
ly by its appropriate cause, we should still refer it to some an-
tecedent effect ; and our reasoning as applied to the succes-
sive production of all natural changes would be the same.

18 COEKELATION OF PHYSICAL FORCES.

Habit and the identification of thoughts with phenomena
so compel the use of recognised terms, that we cannot avoid
using the word cause even in the sense to which objection is
taken ; and if we struck it out of our vocabulary, our lan-
guage, in speaking of successive changes, would be unintelli-
gible to the present generation. The common error, if I am
right in supposing it to be such, consists in the abstraction of
cause, and in supposing in each case a general secondary
cause — a something which is not the first cause, but which, if
we examine it carefully, must have all the attributes of a first
cause, and an existence independent of, and dominant over,
matter.

The relations of electricity and magnetism afford us a
very instructive example of the belief in secondary causa-
tion. Subsequent to the discovery by Oersted of electro-mag-
netism, and prior to that by Faraday of magneto-electricity,
electricity and magnetism were believed by the highest author-
ities to stand in the relation of cause and effect — ^i. e. elec-
tricity was regarded as the cause, and magnetism as the effect ;
and where magnets existed without any apparent electrical
currents to cause their magnetism, hypothetical currents
were supposed, for the purpose of carrying out the caus-
ative view ; but magnetism may now be said with equal
truth to be the cause of electricity, and electrical currents
may be referred to hypothetical magnetic lines : if therefore
electricity cause magnetism, and magnetism cause electricity,
why then electricity causes electricity, which becomes, so to
speak, a reductio ad ahsurdum of the doctrine.

To take another instance, which may render these posi-
tions more intelligible. By heating bars of bismuth and anti-
mony in contact, a current of electricity is produced ; and if
their extremities be united by a fine wire, the wire is heated.
Now here the electricity in the metals is said to be caused
by heat, and the heat in the wire to be caused by electricity,
and in a concrete sense this is true ; but can we thence say

INTRODUCTORY REMARKS. 19

abstractedly that heat is the cause of electricity, or that elec-
tricity is the cause of heat? Certainly not ; for if either be
true, both must be so, and the effect then becomes the cause
of the cause, or, in other words, a thing causes itself. Any
other proposition on this subject will be found to involve sim-
ilar diifrculties, until, at length, the mind will become con-
vinced that abstract secondary causation does not exist, and
that a search after essential causes is vain.

The position which I seek to establish in this Essay is,
that the various affections of matter which constitute the
main objects of experimental physics, viz., heat, light, elec-
tricity, magnetism, chemical affinity, and motion, are all cor-
relative, or have a reciprocal dependence ; that neither, taken
abstractedly, can be said to be the essential cause of the oth-
ers, but that either may produce or be convertible into, any
of the others : thus heat may mediately or inmiediately produce
electricity, electricity may produce heat ; and so of the rest,
each merging itself as the force it produces becomes devel-
oped : and that the same must hold good of other forces, it be-
ing an irresistible inference from observed phenomena that a
force cannot originate otherwise than by devolution from some
pre-existing force or forces.

The term force, although used in very different senses by
different authors, in its limited sense may be defined as that
which produces or resists motion. Although strongly inclined to
believe that the other affections of matter, which I have above
named, are, and will ultimately be resolved into, modes of
motion, many arguments for which will be given in subse-
quent parts of this Essay, it would be going too far, at pre-
sent, to assume their identity with it ; I therefore use the term
force in reference to them, as meaning that active principle
inseparable from matter which is supposed to induce its vari-
ous changes.

The word force and the idea it aims at expressing might
be objected to by the purely physical philosopher on similar

20 CORRELATION OF PHYSICAL FORCES.

grounds to those which apply to the word cause, as it repre-
sents a subtle mental conception, and not a sensuous percep-
tion or phenomenon. The objection would take something
of this form. If the string of a bent bow be cut, the bow
will straighten itself ; we thence say there is an elastic force
in the bow which straightens it ; but if we applied our expres-
sions to this experiment alone, the use of the term force
would be superfluous, and would not add to our knowledge
on the subject. All the information which our minds could
get would be as sufficiently obtained from the expression,
when the string is cut, the bow becomes straight, as from the
expression, the bow becomes straight by its elastic force.
Do we know more of the phenomena, viewed without refer-
ence to other phenomena, by saying it is produced by force?
Certainly not. All we know or see is the effect ; we do not
see force — we see motion or moving matter.

If now we take a piece of caoutchouc and stretch it, when
released it returns to its original length. Here, though the
subject-matter is very different, we see some analogy in the
effect or phenomenon to that of the strung bow. If again
we suspend an apple by a string, cut the string, the apple falls.
Here, though it is less striking, there is still an analogy to
the strung bow and the caoutchouc.

Now when the word force is employed as ^comprehending
these three different phenomena we find some use in the term,
not by its explaining or rendering more intelligible the modus
agendi of matter, but as conveying to the mind something
which is alike in the three phenomena, however distinct they
may be in other respects : the word becomes an abstract or
generalised expression, and regarded in this light is of high
utility. Although I have given only three examples, it is
obvious that the term would equally apply to 300 or 3,000 ex-
amples.

But it will be said, the term force is used not as express-
ing the effect, but as that which produces the effect. This is

INTRODrCTORY KEMAEKS. 21

true, and in this its ordinary sense I shall use it in these pages.
But though the term has a potential meaning, to depart from
which would render language unintelligible, we must guard
against supposing that we know essentially more of the phe-
nomena by saying they are produced by something, which
something is only a word derived from the constancy and
similarity of the phenomena we seek to explain by it. The
relations of the phenomena to which the terms force or forces
are applied give us real knowledge ; these relations may be
called relations of forces ; our knowledge of them is not there-
by lessened, and the convenience of expression is greatly in-
creased, but the separate phenomena are not more intimately
known ; no further insight into why the apple falls is acquired
by saying it is forced to faU, or it falls by the force of gravita-
tion ; by the latter expression we are enabled to relate it
most usefully to other phenomena, but we still know no more
of the particular phenomenon than that imder certain circum-
stances the apple does fall.

In the above illustrations, force has been treated as the
producer of motion, in which case the evidence of the force is
the motion produced ; thus we estimate the force used to pro-
ject a cannon ball in terms of the mass of matter, and the
velocity with which it is projected. The evidence of force
when the term is applied to resistance to motion is of a some-
what different character ; the matter resisting is molecularly
affected, and has its structure more or less changed ; thus a
strip of caoutchouc to which a weight is suspended is elonga-
ted, and its molecules are displaced as compared with their
position when unaffected by the gravitating force. So a piece
of glass bent by an appended weight has its whole structure
changed ; this internal change is made evident by transmit-
ting through it a beam of polarised light : a relation thus
becomes established between the molecular state of bodies
and the external forces or motion of masses. Eveiy particle
the caoutchouc or glass must be acting and contributing to

22 OOKEELATION OF PHYSICAL FORCES.

resist or arrest the motion of the mass of matter appended
to it.

It is difficult, in such cases, not to recognise a reality in
force. "We need some word to express this state of tension ;
we know that it produces an eflfect, though the effect be nega-
tive in character : although in this effort of inanimate matter
we can no more trace the mode of action to its ultimate ele-
ments than we can follow out the connection of our own
muscles with the volition which calls them into action, we
are experimentally convinced that matter changes its state
by the agency of other matter, and this agency we call
force.

In placing the weight on the glass, we have moved the
former to an extent equivalent to that which it would again
describe if the resistance were removed, and this motion of
the mass becomes an exponent or measure of the force exert-
ed on the glass ; while this is in the state of tension, the
force is ever existing, capable of reproducing the original
motion, and while in a state of abeyance as to actual motion,
it is really acting on the glass. The motion is suspended,
but the force is not annihilated.

But it may be objected, if tension or static force be thus
motion in abeyance, there is at all times a large amount of
dynamical action subtracted from the universe. Every stone
upon a hill, every spring that is bent, and has required force
to upraise or bend it, has for a time, and possibly for ever,
withdrawn this force, and annihilated it. Not so ; what
takes place when we raise a weight and leave it at the point
to which it has been elevated? we have changed the centre
of gravity of the earth, and consequently the earth's position
with reference to the sun, planets, and stars ; the effort we
have made pervades and shakes the universe ; nor can we
present to the mind any exercise of force, which is thus not
permanent in its dynamical effects. If, instead of one weight
being raised, we raise two weights, each placed at a point

INTEODIJCTOEY EEMAKKS. 23

diametrically opposite the other, it would be said, here you
have compensation, a balance, no change in the centre of
gravity of the earth ; but we have increased the mean diame-
ter of the earth, and a perturbation of our planet, and of all
other celestial bodies necessarily ensues.

The force may be said to be in abeyance with reference
to the effect it would have produced, if not arrested, or
placed in a state of tension ; but in the act of imposing this
state, the relations of equilibrium with other bodies have
been changed, and these move in their turn, so that motion
of the same amount would seem to be ever affecting matter
conceived in its totality.

Press the hands violently together ; the first notion may
be that this is power locked up, and that no change ensues.
Not so ; the blood courses more quickly, respiration is accele-
rated, changes which we may not be able to trace, take place
in the muscles and nei*ves, transpiration is increased ; we
have given off force in various ways, and must, if the effort
be prolonged, replenish our sources of power, by fresh chemi-
cal action in the stomach.

In books which treat of statics and dynamics, it is com-
mon and perhaps necessary to isolate the subjects of consid-
eration ; to suppose, for instance, two bodies gravitating, and
to ignore the rest of the universe. But no such isolation ex-
ists in reality, nor could we predict the result if it did exist.
Would two bodies gravitate towards each other in empty
space, if space can be empty? the notion that they would is
founded on the theory of attraction, which Newton himself
repudiated, further than as a convenient means of regard-
ing the subject. For purposes of instruction or argument it
may be convenient to assume isolated matter : many con-
clusions so arrived at may be true, but many will be
I^H erroneous.
^P If, in producing effects of tension or of static force, the
effort made pervades the universe, it may be said, when the

24 COEEELATION OF PHYSICAL FORCES.

bent spring is freed, when the raised weight falls, a converse
series of motions must be effected, and this theory would lead
to a mere reciprocation, which would be equally unproduc-
tive of permanent change with the annihilation of force. If
raising the weight has changed the centre of gravity of the
earth, and thence of the universe, the fall of the weight, it
will be said, restores the original centre of gravity, and every-
thing comes back to its original status. In this argument we
again, in thought, isolate our experiment; we neglect sur-
rounding circumstances. Between the time of the raising
and falling of the weight, be the interval never so small,
nay, more, during the rising and during the fall, the earth
has been going on revolving round its axis and round the
sun, to say nothing of other changes, such as temperature,
cosmical magnetism, &c., which we may call accidental, but
which, if we knew all, would probably be found to be as
necessary and as reducible to law as the motion of the earth.
A change having taken place, the fall of the weight does
not bring back the status quo, but other changes supervene,
and so on. Nothing repeats Jtself, because nothing can
be placed again in the same condition: the past is irre-
vocable.

II.— MOTION.

MOTION — which has been taken as the main exponent
of force in the above examples — is the most obvious,
the most distinctly conceived of all the affections of matter.
Visible motion, or relative change of position in space, is a
phenomenon so obvious to simple apprehension, that to at-
tempt to define it would be to render it more obscure ; but
with motion, as with all physical appearances, there are cer-
tain vanishing gradations or undefined limits, at which the
obvious mode of action fades away ; to detect the continu-
ing existence of the phenomena we are obliged to have re-
course to other than ordinary methods of investigation, and
we frequently apply other and different names to the effects
so recognised.

Thus sound is motion ; and although in the earlier pe-
riods of philosophy the identity of sound and motion was not
traced out, and they were considered distinct affections of
matter — indeed, at the close of the last century a theory was
advanced that sound was transmitted by the vibrations of an
ether — ^we now so readily resolve sound into motion, that to
those who are familiar with acoustics, the phenomena of
sound immediately present to the mind the idea of motion,
i. e. motion of ordinary matter.

Again, with regard to light : no doubt now exists that
light moves or is accompanied by motion. Here the phe-
2

26 OOERELATION OF PHYSICAL FOEOES.

nomena of motion are not made evident by the ordinary sen-
suous perception, as for instance the motion of a visibly mov-
ing projectile would be, but by an inverse deduction from
known relations of motion to time and space : as all observa-
tion teaches us that bodies in'moving from one point in space
to another occupy time, we conclude that, wherever a con-
tinuing phenomenon is rendered evident in two different
points of space at different times, there is motion, though we
cannot see the progression. A similar deduction convinces
us of the motion of electricity.

As we in common parlance speak of sound moving,
although sound is motion, it requires no great stretch of
imagination to conceive light and electricity as motions, and
not as things moving. If one end of a long bar of metal be
struck, a sound is soon per^jeptible at the other end. This
we now know to be a vibration of the bar ; sound is but a word
expressive of the mode of motion impressed on the bar ; so
one end of a column of air or glass subjected to a luminous im-
pulse gives a perceptible effect of light at the other end : this
can equally be conceived to be a vibration or transmitted
motion of particles in the transparent column : this question
will, however, be further discussed hereafter ; for the present
we will confine ourselves to motion within the limits to which
the term is usually restricted.

With the perceptible phenomena of motion the mental
conception has been invariably associated to which I have
before aUuded, and to which the term force is given —
the which conception, when we analyse it, refers us to
some antecedent motion. If we except the production of
motion by heat, light, &c., which will be considered in the
sequel, T^hen we see a body moving we look to motion hav-
ing been communicated to it by matter which has previously
moved.

Of absolute rest Nature gives us no evidence : all matter, ,
as far as we can ascertain, is ever in movement, not merely

MOTION.

27

in masses, as with the planetary spheres, but also mole-
cularly, or throughout its most intimate structure : thus every
alteration of temperature produces a molecular change
throughout the whole substance heated or cooled ; slow
chemical or electrical actions, actions of light or invisible
radiant forces, are always at play, so that as a fact we can-
not predicate of any portion of matter that it is absolutely at
rest. Supposing, however, that motion is not an indispensa-
ble function of matter, but that matter can be at rest, matter
at rest would never of itself cease to be at rest ; it would not
move unless impelled to such motion by some other moving
body, or body which has moved. This proposition applies
not merely to impulsive motion, as when a ball at rest is
struck by a moving body, or pressed by a spring which has
previously been moved, but to motion caused by attractions
such as magnetism or gravitation. Suppose a piece of iron
at rest in contact with a magnet at rest ; if it be desired to
moxQ the iron by the attraction of the magnet, the magnet or
the iron must first be moved ; so before a body falls it must
first be raised. A body at rest would therefore continue so
for ever, and a body once in motion would continue so for
ever, in the same direction and with the same velocity, un-
less impeded by some other body, or affected by some other
force than that which originally impelled it. These propo-
sitions may seem somewhat arbitrary, and it has been doubted
whether they are necessary truths ; they have for a long time
been received as axioms, and there can at all events be no
harm in accepting them as postulates. It is however very
generally believed that if the visible or palpable motion of
one body be arrested by impact on another body, the mo-
tion ceases, and the force which produced it is annihi-
lated.

Now the view which I venture to submit is, that force
cannot be annihilated, but is merely subdivided or altered in
direction or character. First, as to direction. Wave your

28 COEEELATION OF PHYSICAL FORCES.

hand : the motion, which has apparently ceased, is taken up
by the air, from the air by the walls of the room, &c., and
so by direct and reacting waves, continually comminuted, but
never destroyed. It is true that, at a certain point, we lose
all means of detecting the motion, from its minute subdivi-
sion, which defies our most delicate means of appreciation,
but we can indefinitely extend our power of detecting it ac-
cording as we confine its direction, or increase the delicacy
of our examination. Thus, if the hand be moved in uncon-
fined air, the motion of the air would not be sensible to a per-
son at a few feet distance ; but if a piston of the same extent
of surface as the hand be moved with the same rapidity in a
tube, the blast of air may be distinctly felt at several yards
distance. There is no greater absolute amount of motion in
the air in the second than in the first case, but its direction
is restrained, so to make the means of detection more facile.
By carrying on this restraint, as in the air-gun, we get a
power of detecting the motion, and of moving other bodies at
far greater distances. The puff of air which would in the
air-gun project a bullet a quarter of a mile, if allowed to es-
cape without its direction being restrained, as by the bursting
of a bladder, would not be perceptible at a yard distance,
though the same absolute amount of motion be impressed on
the surrounding air.

It may, however, be asked, what becomes of force when
motion is arrested or impeded by the counter-motion of another
body ? This is generally believed to produce rest, or entire
destruction of motion, and consequent annihilation of force :
so indeed it may, as regards the motion of the masses, but a
new force, or new character of force, now ensues, the expo-
nent of which, instead of visible motion, is heat. I venture
to regard the heat which results from friction or percussion
as a continuation of the force which was previously associa-
ted with the moving body, and which, when this impinges on

MOTION. 29

another body, ceasing to exist as gross, palpable motion, con-
tinues to exist as heat.

Thus, let two bodies, A and B, be supposed to move in
opposite directions (putting for the moment out of question
all resistance, such as that of the air, &c.), if they pass each
other without contact each will move on for ever in its re-
spective direction with the same velocity, but if they touch
each other the velocity of the movement of each is reduced,
and each becomes heated : if this contact be slight, or such as
to occasion but a slight diminution of their velocity, as when
the surfaces of the bodies are oiled, then the heat is slight ;
but if the contact be such as to occasion a great diminution
of motion, as in percussion, or as when the surfaces are
roughened, then the heat is great, so that in all cases the re-
sulting heat is proportionate to the diminished velocity.
Where, instead of resisting and consequently impeding the
motion of the body A, the body B gives way, or itself takes
up the motion originally communicated to A, then we have
less beat in proportion to the motion of the body B, for here
the operation of the force continues in the form of palpable
motion : thus the heat resulting from friction in the axle of a
wheel is lessened by surrounding it by rollers ; these take up
the primary motion of the axle, and the less, by this means,
the initial motion is impeded, the less is the resulting heat.
Again, if a body move in a fluid, although some Jieat is pro-
duced, the heat is apparently trifling, because the particles of
the fluid themselves move, and continue the motion originally
communicated to the moving body : for every portion of mo-
tion communicated to them this loses an equivalent, and
where both lose, then an equivalent of heat results.

As the converse of this proposition, it should follow that
the more rigid the bodies impinging on each other the greater
should be the amount of heat developed by friction, and so
we find it. Flint, steel, hard stones, glass, and metals, are
those bodies which give the greatest amount of heat from

30 CORKELATION OF PHYSICAL FOECES.

friction or percussion ; while water, oil, &c., give little or no
heat, and from the ready mobility of their particles lessen its
developement when interposed between rigid moving bodies.
Thus, if we oil the axles of wheels, we have more rapid mo-
tion of the bodies themselves, but less heat ; if we increase
the resistance to motion, as by roughening the points of con-
tact, so that each particle strikes against and impedes the
motion of others, then we have diminished motion, but in-
creased heat ; or if the bodies be smooth, but instead of slid-
ing past each other be pressed closely together and then
rubbed, we shall in many cases evolve more heat than by the
rjoughened bodies, as we get a greater number of particles in
contact and a greater resistance to the initial motion. I can-
not present to my mind any case of heat resulting from fric-
tion which is not explicable by this view : friction, according
to it, is simply impeded motion. The greater the impedi-
ment, the more force is required to overcome it, and the
greater is the resulting heat ; this resulting heat being a con-
tinuation of indestructible force, capable, as we shall pres-
ently see, of reproducing palpable motion, or motion of defi-
nite masses.

Whatever be the nature of the bodies, rough or smooth,
solid or liquid, provided there be the same initial force, and
the whole motion be ultimately arrested, there should be the
same amount of heat developed, though where the motion is
carried on through a great number of points of matter we do
not so sensibly perceive the resulting heat from its greater
dissipation. The friction of fluids produces heat, an effect
first noticed I believe by Mayer. The total heat produced by
the friction of fluids should, therefore, it will be said, be
equal to that produced by the friction of solids ; for although
each particle produces little heat, the motion being readily
taken up by the neighbouring particles, yet by the time the
whole mass has attained a state of rest there has been the
same impeding of the initial motion as by the friction of sol-

MOTION. 81

ids if produced by the same initial force. If the heat be
viewed in the aggregate, and allowance be made for the spe
cific thermal capacity of the substances employed, it probably
is the same, though apparently less ; the heat in the case of
solids being manifested at certain defined points, while in
that of fluids it is dissipated, both the time and space during
and through which the motion is propagated differ in the two
cases, so that the heat in the latter case is more readily car-
ried off by surrounding bodies.

If the body be elastic, and by its reaction the motion im-
pressed on it by the initial force be continued, then the heat
is proportionately less ; and were a substance perfectly elas-
tic, and no resistance opposed to it by the air or other mat-
ter, then the movement once impressed would be perpetual,
and no heat would result. A ball of caoutchouc bandied
about for many minutes between a racket and a wall is not
perceptibly heated, while a leaden bullet projected by a gun
against a wall is rendered so hot as to be intolerable to the
touch : in the former case, the motion of the mass is contin-
ued by the reaction due to its elasticity ; in the latter, the
motion of the mass is extinguished, and heat ensues.

A pendulum started in the exliausted receiver of an air-
pump continues its oscillation for hours or even days ; the
friction at its point of suspension and the resistance of the
air is minimised, and the heat is imperceptible, but these tri-
fling resistances in the end arrest the motion of the mass, the
one giving it out as heat, the other conveying the force to the
receiver, and thence to surrounding bodies. Similar reason-
ing may be applied to the oscillation of a coiled spring and
balance wheel.

To wind up a clock a certain amount of force is expended
by the arm ; this force is given back by the descent of the
weight, the wheels move, the pendulum is kept oscillating,
heat is generated at each point of friction, and the surround-
ing air is set in motion, a part of which is made obvin«^ ^'

32 COBRELATION OF PHYSICAL FORCES.

US by the ticking sound. But it will be said, if instead of
allowing the weight to act upon the machinery, the cord by
which it is suspended be cut, the weight drops and the force
is at an end. By no means, for in this case the house is
shaken by the concussion, and thus the force and motion are
continued, while in the former Case the weight reaches the
ground quietly, and no evidence of force or motion is mani-
fested by its impact, the whole having been previously dissi-
pated.

If the initial motion, instead of being arrested by the im-
pact of other bodies, as in friction or percussion, is impeded
by confinement or compression, as where the dilatation of a
gas is prevented by mechanical means, heat equally results :
thus if a piston is used to compress air in a closed vessel, the
compressed air and, from it, the sides of the vessel will be
heated : the air being unable to take up and carry on the
original motion communicates molecular motion or expansion
to all bodies in contact with it ; and, conversely, if we ex-
pand air by mechanical motion, as by withdrawing the pis-
ton, cold is produced. So when a solid has its particles com-
pressed or brought nearer together, as when a bar of iron is
hammered, heat is produced beyond that which is due to per-
cussion alone. In this latter case we cannot very easily ef-
fect the converse result, or produce cold by the mechanical
dilatation of a solid, though the phenomena of solution,
where the particles of a solid are detached from each other,
or drawn more widely asunder, give us an approximation to
it : in the case of solution cold is produced.

We are from a very extensive range of observation and
experiment entitled to conclude that, with some curious ex-
ceptions to be presently noticed, whenever a body is com-
pressed or brought into smaller dimensions it is heated, i. e.
it expands neighbouring substances. Whenever it is dilated
or increased in volume it is cooled, or contracts neighbouring
substances.

MOTION. 33

Mr. Joule has made a great number of experiments for
the purpose of ascertaining what quantity of heat is produced
by a given mechanical action. His mode of experimenting
is as follows. An apparatus formed of floats or paddles of
brass or iron is made to rotate in a bath of water or mercu-
ry. The power which gives rise to this rotation is a weight
raised like a clock-weight to a certain height ; this by acting
during its fall on a spindle and pulley communicates motion
to the paddle-wheel, the water or mercury serving as a fric-
tion medium and calorimeter ; and the heat is measured by a
delicate mercurial thermometer. The results of his experi-
ments he considers prove that a fall of 772 lbs. through a
space of one foot is able to raise the temperature of one
pound of water through one degree of Fahrenheit's thermom-
eter. Mr. Joule's experiments are of extreme delicacy — ^he
tabulates to the thousandth part of a degree of Fahrenheit,
and a large number of his thermometric data are compre-
hended within the limits of a single degree. Other experi-
menters have given very different numerical results, but the
general opinion seems to be that the numbers given by Mr.
Joule are the nearest approximation to the truth yet obtained.

Hitherto I have taken no distinction as to the physical
character of the bodies impinging on each other ; but Nature
gives us a remarkable difference in the character or mode of
the force eliminated by friction, accordingly as the bodies
which impinge are homogeneous or heterogeneous : if the
former, heat alone is produced ; if the latter, electricity.

We find, indeed, instances given by authors, of electricity
resulting from the friction of homogeneous bodies ; but, as I
stated in my original Lectures, I have not found such facts
confirmed by my own experiments, and this conclusion has
been corroborated by some experiments of Professor Erman,
communicated to the meeting of the British Association in
the year 1845, in which he found that no electricity resulted
from the friction of perfectly homogeneous substances ; as,
2*

84 CORRELATION OF PHYSICAL FORCES.

for instance, the ends of a broken bar. Such experiments as
these will, indeed, be seldom free from slight electrical cur-
rents, on account of the practical difficulty of fulfilling the
condition of perfect homogeneity in the substances themselves,
their size, their temperature, &c. ; but the effects produced
are very trifling and vary in direction, and the resultant effect
is nought. Indeed, it would be difficult to conceive the con-
trary. How could we possibly image to the mind or de-
scribe the direction of a current from the same body to the
same body, or give instructions for a repetition of the exper-
iment? It would be unintelligible to say that in rubbing to
and fro two pieces of bismuth, iron, or glass, a current of
electricity circulated from bismuth to bismjith, or from iron
to iron, or from glass to glass ; for the question immediately
occurs — ^from which bismuth to which does it circulate?
And should this question be answered by calling one piece
A, and the other B, this would only apply to the particular
specimens employed, the distinctive appellation denoting a
distraction in fact, as otherwise A could be substituted for B,
and the bar to which the positive electricity flowed would in
turn become the bar to which the negative electricity flowed.
We may say that it circulates from rough glass to smooth,
from cast iron to wrought, for here there is not homogeneity.
It is moreover conceivable, that when the motion is contin-
uous in a definite direction, electricity may result from the
friction of homogeneous bodies. If A and B rub against
each other, revolving in opposite directions, concentric cur-
rents of positive and negative electricity may be conceived
circulating within the metals, and be described by reference
to the direction of their motion ; this indeed would be a dif-
ferent phenomenon from those we have been considering ; but
without some distinction between the two substances in qual-
ity or direction, the electrical effects are indescribable, if not
inconceivable.

When, however, homogeneous bodies are fractured of

'

MOTION. 35

even rubbed together, phenomena are observed to which the
term electricity is applied ; a flash or line of light appears at
the point of friction which by some is called electrical, by
others phosphorescent.

I have myself observed a remarkable case of the kind in
the caoutchouc fabric now commonly used for waterproof
clothing : if two folds of this substance be allowed to cohere
so as partly to unite and present a difficulty of separation,
then, on stripping the one from the other, or tearing them
asunder, a line of light will follow the line of separation.

If this class of phenomena be electrical, it is electricity
determined as it is generated ; there is no dual character im-
pressed on the matter acting, the flash is electrical as a spark
from the percussion of flint is electrical, or as the slow com-
bustion of phosphorus, or any other ease of the development
of heat and light. It seems to be better to class this phe-
nomenon under the categories of heat and light than under
that of electricity, the latter word being retained for those
cases where a dual or polar character of force is manifested.
In experiments which have been made by the friction of sim-
ilar substances where the one appears positively and the
other negatively electrical, there will be found some differ^
ence in the mode of rubbing by which the molecular state of
the bodies is in all probability changed, making one a dissim-
ilar substance from the other ; thus it is said by Bergmann,
that when two pieces of glass are rubbed so that all the parts
of one pass over one part of the other, the former is positive
and the latter negative. It is obvious that in this case the
rubbing in one is confined to a line, and that must be more
altered in molecular structure at the line of friction than the
one where the friction is spread over the whole surface : so
if a ribbon be drawn transversely over another ribbon, the
substances are not, qua the rubbing action, identical; so
again, in the rupture of crystals, we are dealing with sub-
stances having a polar arrangement of particles — ^the surfaces

86 COEKELATION OF PHYSICAL FORCES.

of the fragments cannot be assumed to be molecularly identi-
cal,^

The developement of electricity by the common electrical
machine arises, as far as I can understand it, from the sepa-
ration or rupture of contiguity between dissimilar bodies ; a
metallic surface, the amalgam of the cushion, is in contact
with glass ; these two bodies act upon each other by the force
of cohesion ; and when, by an external mechanical force,
this is ruptured, as it is at each moment of the motion of the
glass plate or cylinder, electricity is developed in each ; were
they similar bodies, heat only would be developed.

According to the experiments of Mr. Sullivan electricity
may be produced by vibration alone if the substance vibra-
ting be composed either of dissimilar metals, as a wire partly
of iron and partly of brass caused to emit a musical sound ;
or of the same metal, if its parts be not homogeneous, as a
piece of iron, one portion of which is hard and crystallised
and the other soft and fibrous ; the current resulting appears
to be due to the vibration, and not to heat engendered, as it
ceases immediately with the vibration.

We may say, then, that in our present state of knowledge,
where the mutually impinging bodies are homogeneous, heat
and not electricity is the result of friction and percussion ;
where the bodies impinging are heterogeneous, we may safely
state that electricity is always produced by friction or percus-
sion, although heat in a greater or less degree accompanies
it ; but when we come to the question of ratio in which fric-
tional electricity is produced, as determined by the different
characters of the substances employed, we find very complex
results. Bodies may differ in so many particulars which in-
fluence more or less the developement of electricity, such as
their chemical constitution, the state of their surfaces, their
state of aggregation, their transparency or opacity, their
power of conducting electricity, &c., that the normoe of their
action are very difficult of attainment. As a general rule, i1

MOTION. 37

may be said that the developement of electricity is greater
when the substances employed are broadly distinct in their
physical and chemical qualities, and more particularly in their
conducting powers ; but up to the present time the laws gov-
erning such developement have not been even approximately
determined.

I have said, in reference to the various forces or affections
of matter, that either of them may, mediately or immediately^
produce the others ; and this is all I can venture to predicate
of them in the present state of science ; but after much con-
sideration I incline strongly to the opinion that science is rap-
idly progressing towards the establishment of immediate or
direct relations between all these forces. "Where at present
no immediate relation is established between any of them,
electricity generally forms the intervening link or middle
term.

Motion, then, will directly produce heat and electricity^
and electricity, being produced by it, will produce magnetism
— a force which is always developed by electrical currents at
right angles to the direction of those currents, as will be sub-
sequently more fully explained. Light also is readily pro-
duced by motion, either directly, as when accompanying the
heat of friction, or mediately, by electricity resulting from
motion ; as in the electrical spark, which has most of the at-
tributes of solar light, differing from it only in those respects
in which light differs when emanating from different sources
or seen through different media ; for instance, in the position
of the fixed lines in the spectrum or in the ratios of the spaces
occupied by rays of different refrangibility. In the decom-
positions and compositions which the terminal points proceed-
ing from the conductors of an electrical machine develope
when immersed in different chemical media, we get the pro-
duction of chemical affinity by electricity, of which motion is
the initial source. Lastly, motion may be again reproduced
by the forces which have emanated from motion ; thus, the

38 CORRELATION OF PHYSICAL FORCES.

divergence of the electrometer, the revolution of the electri-
cal wheel, the deflection of the magnetic needle, are, when
resulting from frictional electricity, palpable movements re-
produced by the intermediate modes of force, which have
themselves been originated by motion.

III.— HEAT.

IF we now take Heat as our starting point, we shall j&nd
that the other modes of force may be readily produced by
it. To take motion first ; this is so generally, I think I may
say invariably, the immediate effect of heat, that we may almost,
if not entirely, resolve heat into motion, and view it as a
mechanically repulsive force, a force antagonist to attraction
of cohesion or aggregation, and tending to move the particles
of all bodies, or to separate them from each other.

It may be well here to premise, that in using the terms
* particles ' or ' molecules,' which will be frequently employed
in this Essay, I do not use them in the sense of the atomist,
or mean to assert that matter consists of indivisible particles
or atoms. The words will .be used for the necessary purpose
of contradistinguishing the action of the indefinitely minute phy-
sical elements of matter from that of masses having a sensi-
ble magnitude, much in the same way as the term ' lines ' or
' points * may be used, and with advantage in an abstract
sense ; though there does not exist, in fact, a thing which has
length and breadth without thickness, and though a thing with-
out parts or dimensions is nothing.

If we put aside the sensation which heat produces in our
own bodies, and regard heat simply as to its effects upon in-
organic matter, we find that, with a very few exceptions, which I

40 COEEELATION OF PHYSICAL FOBCES.

shall presently notice, the effects of what is called heat are
simply an expansion of the matter acted upon, and that the
matter so expanded has the power by its OAvn contraction of
communicating expansion to all*bodies in contiguity with it.
Thus, if the body be a solid, for instance, iron, a liquid, say
water, or a gas, say atmospheric air — each of these, when
heated, is expanded in every direction ; in the two former
cases, by increasing the heat to a certain point, we change
the physical character of the substance, the soHd becomes a
liquid, and the liquid becomes a gas ; these, however, are
still expansions, particularly the latter, when, at a certain
period, the expansion becomes rapidly and indefinitely greater.
But what is, in fact, commonly done in order to heat a sub-
stance, or to increase the heat of a substance? it is merely
approximated to some other heated, that is, to some other
expanded substance, which latter is cooled or contracted as
the former expands. Letus now divest the mind of the impres-
sion that heat is in itself anything substantive, and suppose
that these phenomena are regarded for the first time, and
without any preconceived notions on the subject ; let us in-
troduce no hypothesis, but merely express as simply as we
can the facts of which we have become cognisant ; to what
do they amount ? to this, that matter has pertaining to it a
molecular repulsive power, a power of dilatation, which is
communicable by contiguity or proximity.

Heat thus viewed, is motion, and this molecular motion
we may readily change into the motion of masses, or motion
in its most ordinary and palpable form : for example, in the
steam engine, the piston and all its concomitant masses of
matter are moved by the molecular dilatation of the vapour of
water.

To produce continuous motion there must be an alternate
action of heat and cold ; a given portion of air, for instance,
heated beyond the temperature of the circumambient air, is
expanded. If now it be made to act on a movable piston, it

HEAT. 41

moves this to a point at which the tension or elastic force of
the confined air equals that of the surrounding air. If the
confined air be kept at this point, the piston would remain
stationary ; but if it be cooled, the external air exercising
then a greater relative degree of pressure, the piston returns
towards its original position ; just as it will be seen, when we
come to the magnetic force, that a magnet placed in a partic-
ular position produces motion in iron near it, but to make
this motion continuous, or to obtain an available mechanical
power, the magnet must be demagnetised, or a stable equili-
brium is obtained.

In the case of the piston moved by heated air the motion
of the mass becomes the exponent of the amount of heat —
i. e. of the expansion or separation of the molecules ; nor do
we, by any of our ordinary methods, test heat in any other
way than by its purely dynamical action. The various modi-
fications of the thermometer and pyrometer are all measur-
ers of heat by motion : in these instruments liquid or solid
bodies are expanded and elongated, i. e. moved in a definite
direction, and, either by their own visible motion, or by the
motion of an attached index, communicate to our senses the
amount of the force by which they moved. There are, in-
deed, some delicate expe^riments which tend to prove that a
repulsive action between separate masses is produced by heat.
Fresnel found that mobile bodies heated in an exhausted re-
ceiver repelled each other to sensible distances ; and Baden
Powell found that the coloured rings usually called Newton's
rings change their breadth and position, when the glasses be-
tween which they appear are heated, in a manner which
showed that the glasses repelled each other. M. Faye's the-
ory of comets is based on some such repellent force. There
is, however, some difficulty in presenting these phenomena to
the mind in the same aspect as the molecular repulsive action
of heat.

The phenomena of what is termed latent heat have been

4:2 COEBELATION OF PHYSICAL FORCES.

generally considered as strongly in favour of that view which
regards heat either as actual matter, or, at all events, as a
substantive entity, and not a motion or affection of ordinary
matter.

The hypothesis of latent matter is, I venture with diffi-
dence to think, a dangerous one — it is something like the old
principle of Phlogiston, it is not tangible, visible, audible ;
it is, in fact, a mere subtle mental conception, and ought, I
submit, only to be received on the ground of absolute neces-
sity, the more so as these subtleties are apt to be carried on
to other natural phenomena, and so they add to the hypothe-
tical scaffolding which is seldom requisite, and should be
sparingly used, even in the early stages of discovery. As an
instance, I think a striking one, of the injurious effects of
this, I will mention the analogous doctrine of ' invisible light ; '
and I do this, meaning no disrespect to its distinguished au-
thor, any more than in discussing the doctrine of latent heat,
I can be supposed, in the slightest degree, to aim at detract-
ing from the merits of the illustrious investigators of the facts
which that doctrine seeks to explain. Is not ' invisible light,'
a contradiction in terms ? has not light ever been regarded as
that agent which affects our visual organs ? Invisible light,
then, is darkness, and if it exist, then is darkness light. I
know it may be said, that one eye can detect light where
another cannot ; that a cat may see where a man cannot ; that
an insect may see where a cat cannot ; but then it is not
invisible light to those who see it : the light, or rather the
object seen by the cat, may be invisible to the man, but it
is visible to the cat, and, therefore, cannot abstractedly be
said to be invisible. K we go further, and find an agent
which affects certain substances similarly to light, but does
not, as far as we are aware, affect the visual organs of any
animal, then is it not an erroneous nomenclature which calls
such an agent light? There are many cases in which a de-
viation from the once accepted meaning of words has so grad-

HEAT. 4:3

ually entered into common usage as to be unavoidable, but I
venture to think that additions to such cases should as far
as possible be avoided, as injurious to that precision of lan-
guage which is one of the safest guards to knowledge, and
from the absence of which physical science has materially
suffered.

Let us now shortly examine the question of latent heat,
and see "v^:hcther the phenomena cannot be as well, if not
more satisfactorily, explained without the hypothesis of la-
tent matter, an idea presenting many similar difficulties to
that of invisible light, though more sanctioned by usage.
Latent heat is supposed to be the matter of heat, associated,
in a masked or dormant state, with ordinary matter, not ca-
pable of being detected by any test so long as the matter with
which it is associated remains in the same physical state, but
communicated to or absorbed from other bodies, when the
matter with which it is associated changes its state. To
take a common example : a pound or given weight of water
at 172°, mixed with an equal weight of water at 32°, will
acquire a mean temperature, or 102° ; while water at 172°,
mixed with an equal weight of ice at 32°, will be*reduced to
32°. By the theory of latent heat this phenomenon is thus
explained : — In the first case, that of the mixture of water
with water, both the bodies being in the same physical state,
no latent heat is rendered sensible, or sensible heat latent ;
but in the second, the ice changing its condition from the solid
to the liquid state abstracts from the liquid as much heat as
it requires to maintain it in the liquid state, which it renders
latent, or retains associated with itself, so long as it remains
liquid, but of which heat no evidence can be afforded by any
thermoscopic test.

I believe this and similar phenomena, where heat is con-
nected with a change of state, may be explained and dis-
tinctly comprehended without recourse to the conception of
latent heat, though it requires some effort of the mind to di-

44 COKKELATION OF PHYSICAL FOECES.

vest itself of this idea, and to view the phenomena simply in
their dynamical relations. To assist us in so viewing them,
let us first parallel with purely mechanical actions, certain
simple effects of heat, where change of state (I mean such
change as from the solid to the liquid, or liquid to the gase-
ous state) is not concerned. Thus, place within a receiver a
bladder, and heat the air within to a higher temperature
than that without it, the bladder expands ; so, force the air
mechanically into it by the air-pump, the bladder expands ;
cool the air on the outside, or remove its pressure mechani-
cally by an exhausting pump, the bladder also expands ; con-
versely, increase the external repellent force, either by heat
or mechanical pressure, and the bladder contracts. In the
mechanical effects, the force which produced the distension
is derived from, and at the expense of, the mechanical power
employed, as from muscular force, from gravitation, from the
reacting elasticity of springs, or any similar force by which
the air-pump may be worked. In the heating effects, the
force is derived from the chemical action in the lamp or
source of heat employed.

Let us next consider the experiment so arranged that the
force, which produces expansion in the one case, produces a
correlative contraction in the other : thus, if two bladders,
with a connecting neck between them, be half-filled with air,
as the one is made to contract by pressure the other will di-
late, and vice versa ; so a bladder partly filled with cold air,
and contained within another filled with hot air, expands,
while the space between the bladders contracts, exhibiting a
mere transfer of the same amount of repulsive force, the
mobility of the particles, or their mutual attraction, being
the same in each body ; in other words, the repulsive force
acts in the direction of least resistance until equilibrium is
produced ; it then becomes a static or balanced, instead of a
dynamic or motive force.

Let us now consider the case where a solid is to bo

\

HEAT. 45

changed to a liquid, or a liquid to a gas ; here a much great-
er amount of heat or repulsive force is required, on account
of the cohesion of the particles to be separated. In order to
separate the particles of the solid, precisely as much force
must be parted with by the warmer liquid body as keeps an
equal quantity of it in its liquid state ; it is, indeed, only with
a more striking line of demarcation, the case of the hot and
cold bladder — a part of the repellent power of the hot parti-
cles is transferred to the cold particles, and separates them in
their turn, but the antagonist force of cohesion or aggregation
necessary to be overcome, being in this case much stronger,
requires and exhausts an exactly proportionate amount of
repellent force mechanically to overcome it ; hence the differ-
ent effect on a body such as the common thermometer, the
expanding liquid of wliich does not undergo a similar change
of state. Thus, in the example above given, of the mixture
of cold with hot water, the hot and cold water and the
mercury of the thermometer being aU in a liquid state before,
and remaining so after contact, the resulting temperature is
an exact mean ; the hot water contracts to a certain extent,
the cold water expands to the same extent, and the ther-
mometer either sinks or rises the same number of degrees,
accordingly as it had been previously immersed in the cold
or in the hot solution, its mercury gaining or losing an equiva-
lent of repellent force. In the second instance, viz. the mix-
ture of ice with hot water, the substance we use as an indi-
cator, i. e. mercury, does not undergo the same physical
change as those whose relations of volume we are examining.
The force — viewing heat simply as mechanical force — which
is employed in loosening or tearing asunder the particles of
the solid ice, is abstracted from the liquid water, and from
the liquid mercury of the thermometer, and in proportion as
this fotce meets with a greater resistance in separating
the particles of a solid than of a liquid, so the bodies
which yield the force suffer proportionately a greater con-
traction.

4:6 COERELATION OF PHYSICAL F0ECE8.

If we compare the action of heat on the two substances,
water and mercury, alone, and throw out of our consideration
the ice, we shall be able to apply the same view : thus, if a
given source of heat be applied to water containing a mercu-
rial thermometer, both the water and mercury gradually ex-
pand, but in different degrees ; at a certain point the attrac-
tive force of the molecules of the water is so far overcome
that the water becomes vapour. At this point, the heat or
force, meeting with much less resistance from the attraction
of the particles of steam than from those of the mercury, ex-
pends itself upon the former ; the mercury does not further
expand, or expands in an infinitesimally small degree, and
the steam expands greatly. As soon as this arrives at a
point where circumambient pressure causes its resistance to
further expansion to be equal to the resistance to expansion
in the mercury of the thermometer, the latter again rises,
and so both go on expanding in an inverse ratio to their
molecular attractive force. If the circumambient pressure be
increased, as by confining the water at the commencement
of the experiment within a less expansible body than itself,
such as a metallic chamber, then the mercury of the ther-
mometer continues to rise ; and if the experiment were con-
tinued, the water being confined and not the mercury, until
we have arrived at a degree of repulsive force which is able
to overcome the cohesive power of the mercury, so that this
expands into vapour, then we get the converse effect ; the
force expends itself upon the mercury, which expands in-
definitely, as the water did in the first case, and the water
does not expand at all.

Another very usual mode of regarding the subject may
embarras at first sight, but a little consideration will show
that it is explicable by the same doctrine. Water which has
ice floating in it will give, when measured by the thermo-
meter, the same temperature as the ice ; i. e. both the water
and ice contract the mercury of the thermometer to the point

HEAT. 47

conventionally marked as 32°. It may be said, how is this
reconcile able with the dynamical doctrine, for, according to
that, the solid should take from the mercury of the ther-
mometer more repulsive power than the liquid ; conse-
quently, the ice should contract the mercury more than the
water?

My answer is, that in the proposition as thus stated, the
quantities of the water, ice, and mercury are not taken into
consideration, and hence a necessary dynamical element is
neglected ; if the element of quantity be included, this objec-
tion will not apply. Let the thermometer, for instance, con-
tain 13 oz. of mercury, and stand at 100° ; if placed in con-
tact with an unlimited quantity of ice at 32°, the mercury
will sink to 32°. If the same thermometer be immersed in
an unlimited quantity of water at 32°, the mercury sinks also
to 32° ; not absolutely, perhaps, because, however great the
quantity of water or ice, it will be somewhat raised in tem-
perature by the warmer mercury. This elevation of tempera-
ture above 32° will be smaller in proportion as the quantity
of water or ice is larger than the quantity of mercury ; and,
as we know of no intermediate state between ice and water,
the contact of a thermometer at a temperature above the
freezing point with any quantity of ice exactly at the freezing
point would, theoretically speaking, liquefy the whole, pro-
vided it had sufficient time ; for as every portion of that ice
would in time have its temperature raised by the contact of
the warmer body, and as any elevation of temperature above
the freezing point liquefies ice, every portion should be lique-
fied. Practically speaking, however, in both cases, that of
le water and of the ice, when the quantity is indefinitely
■great the thermometer falls to 32°.

Now place the same thermometer at 100°, successively
in one oz. of water at 32°, and in one of ice at 32° ; we shall
find in the former case it will be lowered only to 54°, and in
the latter to 32° ; apply to this the doctrine of repulsive force,
and we get a satisfactory explanation.

4:8 CORRELATION OF PHYSICAL FORCES.

In the first case, the quantities both of ice and water be-
ing indefinitely great in respect to the mercury, each reduces
it to its own temperature, viz. 32°, and the ice cannot reduce
the mercury below 32°, because it would receive back repul-
sive power from the newly formed water, and this would be-
come ice ; in the second case, where the quantities are limited,
the mercury does lose more repulsive power by the ice than by
the water, and the observations made in reference to the first
illustration apply.

The above doctrine is beautifully instanced in the experi-
ment of Thilorier, by which carbonic acid is solidified. Car-
bonic acid gas, retained in a strong vessel under great pres-
sure, is allowed to escape from a small orifice ; the sudden
expansion requires so great a supply of force, that in furnish-
ing the demands of the expanding gas certain other portions
of the gas contract to such an extent as to solidify : thus, we
have reciprocal expansion and contraction going on in one
and the same substance, the time being too limited for the
whole to assume a uniform temperature, or in other words, a
uniform extent of expansion.

It has been observed with reference to heat thus viewed,
that it would be as correct to say, that heat is absorbed, or
cold produced by motion, as that heat is produced by it. This
difficulty ceases when the mind has been accustomed to re-
gard heat and cold as themselves, motion ; i. e. as correlative
expansions and contractions, each being evidenced by relation,
and being inconceivable as an abstraction.

For instance, if the piston of an air-pump be drawn down
by a weight, cold is produced in the receiver. It may be here
said that a mechanical force, and the motion consequent upon
it, produces cold ; but heat is produced on the opposite side
of the piston, if a receiver be adapted so as to retain the com-
pressed air. Assuming them to equivalise each other, the
force of the falling weight would be expressed by the heat of
friction of the piston against its tube, and by the tension or

HEAT. 49

power of reaction of the compressed against the dilated air.
If the heat due to compression be made to perform mechani-
cal work, it would 'pro tanto be consumed, and could not
restore the temperature to the dilated air ; but if it perform
no work, no heat is lost. Mr. Joule has experimentally
proved tliis proposition.

In commencing the subject of heat, I asked my reader to
put out of consideration the sensations which heat produces
in our own bodies ; I did this because these sensations are
likely to deceive, and have deceived many as to the natiu*e of
heat. These sensations are themselves occasioned by simi-
lar expansions to those which we have been considering ; the
liquids of the body are expanded, i. e, rendered less viscid by
heat, and from their more ready flow, we obtain the sensation
of agreeable warmth. By a greater degree of heat, their ex-
pansion becomes too great, giving rise to a sense of pain, and,
if pushed to extremity, as with the heat which produces a
burn, the liquids of the body are dissipated in vapour, and an
injury or destruction of the organic structure takes place. A
similar though converse effect may be produced by intense
cold ; the application of frozen mercury to the animal body
produces a burn similar to that produced by great heat, and
accompanied with a similar sensation.

I: Doubtless other actions than those above mentioned inter-
fere in producing the sensations of heat and cold ; but I think
it wiU be seen that these will not affect the arguments as to
the nature of heat. The phenomenal effects will be found
unaltered : heat will still be found to be expansion, cold to be
contraction ; and the expansion and contraction are, as with
the two bladders of air, correlative — i. e. we cannot expand
one body, a, without contracting some other body, b ; we
cannot contract a without expanding b, assuming that we view
the bodies with relation to heat alone, and suppose no other
force to be manifested.

I have said that there are few exceptions as to heat being
3

50 CORRELATION OF PHYSICAL FORCES.

always manifested by an expansion of matter. One class of
these exceptions is only apparent: moist clay, animal or
vegetable fibre, and other substances of a mixed nature,
which contain matter of different characters, some of which is
more and some less volatile, i. e. expansible, are contracted
on the application of heat ; this arises from the more volatile
matter being dissipated in the form of vapour or gas ; and the
interstices of the less volatUe being thus emptied, the latter
contracts by its own cohesive attraction, giving thus a prima
facie appearance of contraction by heat. The pyrometer of
Wedgwood is explicable on this principle.

The second class of exceptions, though much more limited
in extent, is less easily explained. "Water, fused bismuth,
and probably some other substances* (though the fact as to
them is not clearly established), expand as they approach
very near to the freezing or solidifying point. The most
probable explanation of these exceptions is, that at the point
of maximum density the molecules of these bodies assume a
polar or crystalline condition ; that by the particles being
thus arranged in linear directions like chevaux de frise,
interstitial spaces are lefk, containing matter of less den-
sity, so that the specific density of the whole mass is dimin-
ished.

Some recent experiments of Dr. Tyndall on the physical
properties of ice seem to favour this view. When a sun-
beam, concentrated by a lens, is allowed to fall on a piece
of apparently homogeneous ice the path of the rays is in-
stantly studded with numerous luminous spots like minute air
bubbles, and the planes of freezing are made manifest by
these and by small fissures. Stars or flower-like figures of
six petals appear parallel to the planes of freezing, and seem-
ingly spreading out from a central bubble. These flowers
are formed of water. When the ice is melted in warm water
no air is given off from the bubbles, so they seem to be va-
cuous ; it is, however, possible that extremely minute parti-

HEAT. 51

cles of air sufficient to form foci for the melting points of ice
might be dissolved by the water as soon as they came in con-
tact with it. Be this as it may, the existence of these points
throughout the ice, where it gives way to the heat of the solar
beam, if it does not prove actual vacuous or aeriform spaces
to exist in ice, proves that it is not homogeneous, that its
structure is probably definitely crystalline, and that the
matter composing it is in difierent degrees of aggregation, so
that its mean specific gravity might well be less than that of
water.

We cannot examine piecemeal the ultimate structure of
matter, but in addition to the fact that the bodies which
evince this peculiarity are bodies which, when solidified, ex-
hibit a very marked crystalline character, there are experi-
ments which show that water between the point of maximum
density and its point of solidification polarises light circularly ;
showing, if these experiments be correct, a structural altera-
tion in water, and one analogous to that possessed by certain
crystalline solids, and to that possessed by water itself, where
it is forcibly made to assume a polarised condition by the in-
fluence of magnetism.

The accuracy of these results has, however, been doubted,
and the experiments have not succeeded when repeated by
very experienced hands. Whether this be so or not, and
whether the above explanation of the exception to the other-
wise invariable effect of expansion by heat be or be not re-
garded as admissible, must be left to the judgment of each
individual who thinks upon the subject ; at all events, no
theory of heat yet proposed removes the difficulty, and there-
fore it equally opposes every other view of the phenom-
ena of. heat, as it does that which I have here consid-
ered, and which regards heat as communicable expansive
force.

As certain bodies expand in freezing, and indeed, under
some circumstances, before they arrive at the temperature at

62 COKKELATION OF PHYSICAL FORCES.

which they solidify, we get the apparent anomaly that the
motion or mechanical force generated by heat or change of
temperature is reversed in direction when we arrive at the
point of change from the solid to the liquid state. Thus a
piece of ice at the temperature of Zero, Fahrenheit, would
expand by heat, and produce a mechanical force by such ex-
pansion until it arrives at 32° ; but then by an increment of
heat it contracts, and if the first expansion had moved a pis-
ton upwards, the subsequent contraction would bring it back
to a certain extent, or move it downwards, an apparent nega-
tion of the force of heat.

Again with water above 40°, i. e, above its point of
maximum density, a progressive increment of cold or decre-
ment of heat would produce contraction to a certain point,
and then expansion or a mechanical force in an opposite direc-
tion. Thus not only heat or the expansive force given to
other bodies by a body cooling would be given out by water
freezing, but also the force due to the converse expansion in the
body itself, and force would thus seem to be got out of noth-
ing : but if water in a confined space be gradually cooled, the
expansion attendant on its cooling as it approaches the freez-
ing point would occasion pressure amongst its particles, and
thence tend to antagonise the force of dilatation produced in
them by cooling, or to resist their tendency to freeze ; or in
other words, the pressure would tend to liquefaction, and con-
versely to the usual effect of pressure, produce cold instead
of heat, and thus neutralise some of the heat yielded by the
cooling body. Hence we find that it requires a lower tem-
perature to freeze water under pressure than when exempt
from it, or that the freezing point is lowered as the pressure
increases for bodies which expand in freezing — an effect first
predicted by Mr. J. Thompson, and experimentally verified
by Mr. W. Thompson ; while as shown by M. Bunsen, the
converse effect takes place with bodies which contract in
freezing. Here the pressure cooperates with the effects of

HEAT. 53

cold, both tending to approximate the particles, and such sub-
stances solidify at a higher temperature in proportion as the
pressure is greater ; so that we might expect a body of this
class, which under the ordinary pressure of the air is at a
temperature just above its freezing point, to solidify by
being submitted to pressure alone, the temperature being
kept constant.

A similar class of exception to the general effect of
heat in expanding bodies is presented by vulcanised caout-
chouc. This has been observed by Mr. Gough, and, in-
deed, was pointed out to me many years ago by Mr.
Brockedon to be heated when stretched, and cooled when
unstretched.

Mr. Joule finds that its specific gravity is lower when
stretched than when unstretched, and that when heated
in its stretched state it shortens, presenting in this par-
ticular condition a similar series of converse relations to
those which are presented by water near or at its freezing
point.

"With the exception of this class of phenomena, which
offer difficulties to any theory which has been proposed, the
general phenomena of heat may, I believe, be explained upon
a purely dynamical view, and more satisfactorily than by
having recourse to the hypothesis of latent matter. Many,
however, of the phenomena of heat are involved in much
mystery, particularly those connected with specific heat or
that relative proportion of heat which equal weights of differ-
ent bodies require to raise them from a given temperature to
another given temperature, which appear to depend in some
way hitherto inexpHcable upon the molecular constitution of
different bodies.

The view of heat which I have taken, viz. to regard it
simply as a communicable molecular repulsive force, is sup-
ported by many of the phenomena to which the term specific
or relative heat is applied ; for example, bodies as they in-

54 COERELATION OF PHYSICAL FORCES.

crease in temperature increase in specific heat. The ratio
of this increase in specific heat is greater with solids than
with liquids, although the. latter are more dilatable ; an
effect probably depending upon the commencement of fusion.
Again, those metals whose rate of expansion increases most
rapidly when they are heated, increase most in specific heat ;
and their specific heat is reduced by percussion, which, by
approximating their particles, makes them specifically more
dense. When, however, we examine substances of very
different physical characters, we find that their specific heats
have no relation to their density or rate of expansion by
heat; their differences of specific heat must depend upon
their intimate molecular constitution in a manner accounted
for (as far as I am aware) by no theory of heat hitherto
proposed.

In the greater number, probably in all solids and liquids,
the expansion by heat is relatively greater as the temperature
is higher ; or, preserving the view of expansion and contrac-
tion, if two equal portions of the same substance be juxta-
posed at different temperatures, the hotter portion wiU con-
tract a little more than the colder will expand ; from this
fact, viz. that the coetficient of expansion increases in a given
body with the temperature, and from other considerations,
Dr. Wood has argued, with much apparent reason, that the
nearer the particles of bodies are to each other, the less they
require to move to produce a given expansion or contraction
in those of another body. His mode of reasoning, if I rightly
conceive it, may be concisely put as follows : —

As bodies contract by cold, it is clear that, in a given
body, the lower the temperature the nearer are the particles ;
and, as the coefficient of expansion increases with the tem-
perature, the lower the temperature of the substance be, the
less the particles require to move, or approach to or recede
from each other, so as to compensate the correlative recession
or approach of the particles in a hotter portion of the same

HEAT. 65

substance, that is, in another portion of the same substance
in which the particles are more distant from each other.
The amount of approximation or recession of the particles of
a body, in other words, its change of bulk by a given change
of temperature, being thus in a given substance an index of
the relative proximity of its particles, may it not be so of all
bodies ? The proposition is very ingeniously argued by Dr.
Wood, but the argument is based upon certain hypotheses as
to the sizes and distances of atoms, which must be admitted
as postulates by those who adopt his conclusions. Dr.
Wood seeks by means of this theory to explain the heat pro-
duced by chemical combination, and I shall endeavour to give
a sketch of his mode of reasoning when I arrive at that part
of my subject.

Although the comparative effects of specific heat may not
be satisfactorily expKcable by any known theory, the absolute
effect of heat upon each separate substance is simply expan-
sion, but when bodies differing in their physical characters
are used, the rate of expansion varies, if measured by the
correlative contractions exhibited by the substances produc-
ing it. Though I am obliged, in order to be intelligible, to
talk of heat as an entity, and of its conduction, radiation, &c.,
yet these expressions are, in fact, inconsistent with the dyna-
mic theory which regards heat as motion and nothing else ;
thus conduction would be simply a progressive dilatation or
motion of the particles of the conducting substance, radiation
an undulation or motion of the particles of the medium
through which the heat is said to be transmitted, &c. ; and it
is a strong argument in favour of this theory, that for every
diversity in the physical character of bodies, and for every
change in the structure and arrangement of particles of the
same body, a change is apparent in the thermal effects.
Thus gold conducts heat, or transmits the motion called heat,
more readily than copper, copper than iron, iron than lead,
and lead than porcelain, &c.

66 CORRELATION OF PHYSICAL FORCES.

So when the structure of a substance is not homogeneous,
we have a change in the conduction of different parts depend-
ent upon the structure. This is beautifully shown with
bodies whose structure is symmetrically arranged, as in crys-
tals. Senarmont has shown that crystals conduct heat differ-
ently in different directions with reference to the axis of
symmetry, but definitely in definite directions. His mode of
experimenting is as follows : — ^A plate of the crystal is cut in
a direction, for one set of experiments parallel, and for
another at right angles to the axis ; a tube of platinum is in-
serted through the centre of the plate, and bent at one
extremity, so as to be capable of being heated by a lamp
without the heat which radiates from the lamp affecting the
crystal ; the surfaces or bases of the plate of crystal are
covered with wax. When the platinum is heated, the direc-
tion of the heat conducted by the crystal is made known by
the melting of the wax, and a curved line is visible at the
juncture of the solid and liquid wax. This curve, with
homogeneous substances, as glass or zinc, is a circle ; it is
also a circle on plates of calc spar cut perpendicular to the
axis of symmetry ; but on plafes cut parallel to the axis of
symmetry, and having their plane perpendicular to one of the
faces of the primitive rhombohedron, the curves are well-
defined ellipses, having their longer axes in the direction of
the axis of symmetry, showing that this axis is a direction of
greater conductibility. From experiments of this character
the inference is drawn, that * in media constituted like crys-
tals of the rhombohedral system, the conducting power varies
in such a manner, that, supposing a centre of heat to exist
within them, and the medium to be indefinitely extended in
all directions, the isothermal surfaces are concentric eUipsoids
of revolution round the axis of symmetry, or at least surfaces
differing but little therefrom.'

Knoblauch has further shown, that radiant heat is absorb*
ed in different degrees, according as its direction is parallel
or perpendicular to the axis of a crystal.

HEAT. 57

If we select a substance of a different but also of a delfinite
Btructurer, such as wood, we find that heat progresses through
it with more or less rapidity, according to its direction with
reference to the fibre of the wood : thus Decandolle and De
la Rive found that the conduction was better in a direction
parallel to the fibre than in one transverse to it ; and Dr.
Tyndall has added the fact, that the conduction is better in a
direction transverse to the fibres and layers of the wood than
when transverse to the fibre but parallel to the layers, though
in both these directions the conduction is inferior to that fol-
lowing the direction of the fibre. Thus, in the three possible
directions in which the structure of wood may be contem-
plated, we have three different degrees of progression for
heat.

In the above examples we see, as we shall see farther on
with reference to all the so-called imponderables, that the
phenomena depend upon the molecular structure of the mat-
ter affected ; and although these facts are not absolutely in-
consistent with the theory which supposes them to be fluids
or entities, it will, I think, be found to be far more consistent
with that which views them as motion. Heat, which we are
at present considering, cannot be insulated: we cannot re-
move the heat from a substance and retain it as heat ; we
can only transmit it to another substance, either as heat or
as some other mode of force. We only know certain changes
of matter, for which changes heat is a generic name ; the
thing heat is unknown.

Heat having been shown to be a force capable of pro-
ducing motion, and motion to be capable of producing the
other modes of force, it necessarily follows that heat is capa-
ble, mediately, of producing them ; I will, therefore, content
myself with enquiring how far heat is capable of immediately
producing the other modes of force. It will immediately
produce electricity, as shown in the beautiful experiments of
Seebeck, one of which I have already cited, which experi-
3*

58 OOEKELATION OF PHYSICAL F0ECE8.

ments proved, that when dissimilar metals are made to touch,
or are soldered together and heated at the point of contact, a
current of electricity flows through the metals having a defi-
nite direction according to the metals employed, which cur-
rent continues as long as an increasing temperature is grad-
ually pervading the metals, ceases when the temperature is
stationary, and flows in the contrary direction with the decre-
ment of temperature.

Another class of phenomena which have been generally
attributed to the effects of radiant heat, and to which, from
this belief, the term thermography has been applied, may
also, in their turn, be made to exliibit electrical eff*ects — ef-
fects here of Franklinic or static electricity, as Se check's ex-
periments showed effects of voltaic or dynamic electricity.

If polished discs of dissimilar metals — say, zinc and cop-
per— ^be brought into close proximity, and kept there for
some time, and either of them has irregularities upon its sur-
face, a superficial outline of these irregularities is traceable
upon the other disc, and vice versa. Many theories have
been framed to account for this phenomenon, but whether it
be due or not to thermic radiations, -the relative temperature
of the discs, their relative capacities and conducting and
radiating powers for heat, undoubtedly influence the phe-
nomena.

Now, if two such discs in close proximity be connected
with a delicate electroscope, and then suddenly separated,
the electroscope is affected, showing that the reciprocal ra-
diation from surface to surface has produced electrical force.
I cite this experiment in treating of heat as an initial force,
because at present the probabilities are in favour of thermic
radiation producing the phenomenon. The origin of these
so-called thermographic effects is, however, a question open
to much doubt, and needs much further experiment. When
I first published the experiment which showed that the mere
approximation of metallic discs would give rise to electrical

HEAT. 59

effects, I mentioned that I considered the fact of the superfi-
cial change upon the surface of metals in proximity, and, d
fortiori^ in contact, would explain the developement of elec-
tricity in Volta's original contact experiment, without having
recourse to the contact theory, i, e. a theory which supposes
a force to be produced by mere contact of dissimilar metals
without any molecular or chemical change. I have seen
nothing to alter this view. Mr. Gassiot has repeated and
verified my experiment with more delicate apparatus and
under more unexceptionable circumstances ; and without say-
ing that radiant heat is the initial force in this case, we have
evidence, by the superficial change which takes place in
bodies closely approximated, that some molecular change is
taking place, some force is called into action by their proxim-
ity, which produces changes in matter as it expends, or
rather transmits itself; and, therefore, is not a force without
molecular change, as the supposed contact force would be.
The force in this, as in all other cases, is not created, but de-
veloped by the action of matter on matter, and not annihi-
lated, as it is shown by this experiment to be convertible into
another mode of force.

To say that heat will produce lights is to assert a fact ap-
parently familiar to every one, but there may be some rea-
son to doubt whether the expression to produce light is cor-
rect in this particular application ; the relation between heat
and light is not analogous to the correlation between these
and the other four afiections of matter. Heat and light ap-
pear to be rather modifications of the same force than dis-
tinct forces mutually dependent. The modes of action of ra-
diant heat and of light are so similar, both being subject to
the same laws of reflection, refraction, and double refraction,
and polarisation, that their difference appears to exist more
in the manner in which they affect our senses than in our
mental conception of them.

The experiments of Melloni, which have mainly contrib-

60 CORRELATION OF PHYSICAL FORCES.

uted to demonstrate this close analogy of heat and light, af-
ford a beautiful instance of the assistance which the progress
of one branch of physical science renders to that of another.
The discoveries of Oersted and Seebeck led to the construc-
tion of an instrument for measuring temperature, incompara-
bly more delicate than any previously known. To distin-
guish it from the ordinary thermometer, this instrument is
called the thermomultijplier. It consists of a series of small
bars of bismuth and antimony, forming one zigzag chain of
alternations arranged parallel to each other, in the shape of
a cylinder or prism ; so that the points of junction, which are
soldered, shall be all exposed at the bases of the cylinder :
the two extremities of this series ai;e united to a galvano-
meter— that is, a flat coil of wire surrounding a freely-sus-
pended magnetic needle, the direction of which is parallel to
the convolutions of the wire. When radiant heat impinges
upon the soldered ends of the multiplier, a thermo-electric
current is induced in each pair ; and, as all these currents
tend to circulate in the same direction, the energy of the
whole is increased by the cooperating forces : this current,
traversing the helix of the galvanometer, deflects the needle
from parallelism by virtue of the electro-magnetic tangential
force, and the degree of this deflection serves as the index
of the temperature.

Bodies examined by these means show a remarkable dif-
ference between their transcalescence, or power of transmit-
ting heat, and their transparency : thus, perfectly transparent
alum arrests more heat than quartz so dark coloured as to be
opaque ; and alum coupled with green glass Melloni found
was capable of transmitting a beam of brilliant light, while,
with the most delicate thermoscope, he could detect no indi-
cations of transmitted heat : on the other hand, rock-salt, the
most transcalescent body known, may be covered with soot
until perfectly opaque, and yet be found capable of transmit-
ting a considerable quantity of heat. Radiant heat, when

HEAT. 61

transmitted through a prism of rock-salt, is found to be une-
qually refracted, as is the case with light ; and the rays of
heat thus elongated into what is, for the sake of analogy,
called a spectrum, are found to possess similar properties to
the primary or coloured rays of light. Thus rock-salt is to
heat what colourless glass is to light ; it transmits heat of all
degrees of refrangibility : alum is to heat as red glass to
light ; it transmits the least, and stops the most refrangible
rays ; and rock-salt covered with soot represents blue glass,
transmitting the most, and stopping the least refrangible rays.

Certain bodies, again, reflect heat of different refrangi-
bility : thus paper, snow, and lime, although perfectly white
— that is, reflecting light of all degrees of refrangibility, re-
flect heat only of certain degrees ; while metals, which are
coloured bodies — that is, bodies which reflect light only of
certain degrees of refrangibility — reflect heat of all degrees.
Radiant heat incident upon substances which doubly refract
light is doubly refracted ; and the emergent rays are polar-
ised in planes at right angles to each other, as is the case
with light.

The relation of radiation to absorption also holds good
with light as with heat: with the latter it has been long
known that the radiating power of different substances is di
rectly proportional to their absorptive and inverse to their
reflective power ; or rather, that the sum of the heat radia
ted and reflected is a constant quantity. So, as has been
shown by Mr. Balfour Stewart, the absorption bears the
same relation to radiation for heat as to quality as well aa
quantity.

Light presents us with similar relations. Coloured glass,
when heated so as to be luminous, emits the same light
which at ordinary temperatures it absorbs: thus red glass
gives out or radiates a greenish light, and green glass a red
tint.

The flame of substances containing sodium yields a yel-

62 CORRELATION OF PHYSICAL FORCES.

low light of such purity that other colours exposed to it ap-
pear black — a phenomenon shown by the familiar experiment
of exposing a picture of bright colours, other than yeUow, to
the flame of spirits of wine with which common salt is mixed :
the picture loses its colours, and appears to be black and
white. When the prismatic spectrum of such a flame is ex-
amined, it is found to exhibit two bright yellow lines at a cer-
tain fixed position. K a source of light be employed which
gives no lines in its spectrum, and this, being at a higher
temperature, be made to pass through the sodium flame, two
dark lines will appear in the spectrum precisely coincident
in position with the yellow lines which were given by the so-
dium flame itself. The same relation of absorption to radia-
tion is therefore shown here : the substance absorbs that
light which it yields when it is itself the source of light.
The same is true of other substances, the spectra of which
exhibit respectively lines of peculiar colour and position.
Now, the solar prismatic spectrum is traversed by a great
number of dark lines ; and Kirchoff" has deduced from con-
siderations such as those which I have shortly stated, that
these dark lines in the solar spectrum are due to metals ex-
isting in an atmosphere around the sun, which absorb the
light from a central incandescent nucleus, each metal absorb-
ing that light which would appear as a bright line or lines in
its own spectrum.

By comparing the position of the bright lines in the spec-
tra of metals with that of the dark lines in the solar spec-
trum, several of them are found to be in identically the same
place : hence it is inferred, and the inference seems reason-
able, that the metals which show luminous lines in their spec-
tra, identical in position with dark lines in the solar spec-
trum, exist in the sun, and are diffused in a gaseous state in
its atmosphere. It does not seem to me necessary to this
conclusion to assume that the sun is a solid mass of incandes-
cent matter : it may well be that what we term the photo-

HEAT. 63

sphere or luminous envelope of the sun has surrounding it a
more diffuse atmosphere containing vaporised metals, and
that the mass of the sun itself may be in a different state,
and not necessarily at an incandescent temperature ; indeed,
the protuberances and red light seen at the period of total
eclipses afford some evidence of an atmosphere exterior to
the photosphere. It would, however, be out of place here to
speculate on these subjects : the point which concerns us is
the analogies of heat and light, which these discoveries illus-
trate. Kirchoff has carried the analogy farther by showing
that a plate of tourmaline absorbs the polarised ray which
when heated it radiates. Thus, the phenomena of light are
imitated closely by those of radiant heat ; and the same the-
ory which is considered most plausibly to account for the
phenomena of the one, will necessarily be applied to the other
agent, and in each case molecular change is accompanied by
a change in the phenomenal effects.

In certain cases heat appears to become partially con-
verted into light, by changing the matter affected by heat :
thus gas may be heated to a very high point without pro-
ducing light, or producing it to a very slight degree ; but the
introduction of solid matter — for instance, the metal platinum
into the highly-heated gas — instantly exhibits light. Whether
the heat is converted into light, or whether it is concentrated
and increased in intensity by the solid matter so as to become
visible, may be open to some doubt : the fact of solid matter,
when ignited by the oxyhydrogen jet decomposing water, as
will be presently explained, would seem to indicate that the
heat was rendered more intense by condensation in the solid
matter, as water is in this case decomposed by a heated body,
which body has itself been heated by the combining elements
of water. The apparent effect, however, of the introduction
of solid incombustible matter into heated gas, is a conversion
of heat into light.

There is another method by which heat would probably

64 CORRELATION OF PHYSICAL FORCES.

be made to produce luminous effects, though I am not aware
that the experiment has ever been made.

If we concentrate into a focus by a large lens a dim light,
we increase the intensity of the light. Now if a heated body
be taken, which, to the unassisted eye, has just ceased to be
visible, it seems probable that by collecting and condensing
by a lens the different rays which have so ceased to be visi-
ble, light would reappear at the focus. The experiment is,
for reasons obvious to those acquainted with optics, a difficult
one, and, to be conclusive, should be made on a large scale,
and with a very perfect lens of large diameter and short fo-
cus. I have obtained an approximation to the result in the
following manner : — In a dark room a platinum wire is
brought just to the point of visible ignition by a constant vol-
taic battery ; it is then viewed, at a short distance, through
an opera-glass of large aperture applied to one eye, the other
being kept open. The wire will be distinctly visible to that
eye which regards it through the opera-glass, and at the same
time totally invisible to the other and naked eye. It may be
said with some justice that such experiments prove little more
than the fact already known, viz. that by increasing the in-
tensity of heat, light is produced : they however exhibit this
effect in a more striking form, as bearing on the relations of
heat and light.

With regard to chemical affinity and magnetism, perhaps
the only method by which in strictness the force of heat may
be said to produce them is through the medium of electricity,
the thermo-electrical current, produced, as before described,
by heating dissimilar metals, being capable of deflecting the
magnet, of magnetising iron, and exhibiting the other mag-
netic effects, and also of forming and decomposing chemical
compounds, and this in proportion to the progression of heat :
this has not, indeed, as yet been proved to bear a measurable
quantitative relation to the other forces thus produced by it,
because so little of the heat is utilised or converted into elec-

HEAT. 65

tricity, much being dissipated, without change, in the form of
heat.

Heat, however, directly affects and modifies both the mag-
net and chemical compounds ; the union of certain chemical
substances is induced by heat, as, for instance, the formation
of water by the union of oxygen and hydrogen gases : in
other cases this union is facilitated by heat, and in many in-
stances, as in ammonia and its salts, it is weakened or antag-
onised. In many of these cases, however, the force of heat
seems more a determining than a producing influence ; yet to
be this, it must have an immediate relation with the force
whose reaction it determines: thus, although gunpowder,
touched with an ignited wire, subsequently carries on its own
combustion or chemical combination, independently of the
original source of heat, yet the chemical affinities of the first
portion touched must be exalted by, and at the cost of, the
heat of the wire ; for to disturb even an unstable equilibrium
requires a force in direct relation with those which maintain
equilibrium.

Since the first edition of this essay was published, I have
communicated to the Eoyal Society some experiments by
which an important exception to the general effect of heat on
chemical affinity is removed, and the results of which induce
a hope that a generalised relation will ultimately be estab-
lished between heat, chemical affinity, and physical attraction.
I find that if a substance capable of supporting an intense
heat, and incapable of being acted upon by water or either
of its elements — such, for instance, as platinum, or iridium —
be raised to a high point of ignition and then immersed in
water, bubbles of permanent gas ascend from it, which on
examination are found to consist of mixed oxygen and hydro-
gen in the proportions in which they form water. The tem-
perature at which this is effected is, according to Dr. Robin-
son, who has since written a valuable paper on the subject,
= 2386°. Now, when mixed oxygen and hydrogen are ex-

ba OOEBELATION OF PHYSICAL FORCES.

posed to a temperature of about 800°, they combine and form
water ; beat therefore appears to act differently upon these
elements according to its intensity, in one case producing
composition, in the other decomposition. No satisfactory
means of reconciling this apparent anomaly have been pointed
out : the best approximation to a theory which I can frame is
by assuming that the constituent molecules of water are, be-
low a certain temperature, in a state of stable equilibrium ;
that the molecules of mixed or oxyhydrogen gas are, above a
certain temperature, also in a state of stable equilibrium, but
of an opposite character ; while below this latter tempera-
ture the molecules of mixed gas are in a state of unstable
equilibrium, somewhat similar to that of the fulminates or
similar bodies, in which a slight derangement subverts the
nicely-balanced forces.

If, for instance, we suppose four molecules. A, B, C, D,
to be in a balanced state of equilibrium between attracting
and repeUing forces, the application of a repulsive force be-
tween B and C, though it may still farther separate B and C,
will approximate B to A and C to D, and may bring them
respectively within the range of attractive force ; or, sup-
posing the repulsive force to be in the centre of an indefinite
sphere of particles, all these, excepting those immediately
acted on by the force, will be approximated, and having from
attraction assumed a state of stable equilibrium, they will re-
tain this, because the repulsive force divided by the mass is
not capable of overcoming it. But if the repulsive force be
increased in quantity and of sufficient intensity, then the at-
tractive force of all the molecules may be overcome, and de-
composition ensue. Thus, water or steam below a certain
temperature, and mixed gas above a certain temperature,
may be supposed to be in a state of stable equilibrium, whilst
below this limiting temperature, the equilibrium of oxyhy-
drogen gas is unstable.

This, it must be confessed, is but a crude mode of explain*

HEAT. 67

ing the phenomena, and requires the assumption, that the
particles of a gas exercise an attraction for each other as do
the particles of a solid, though different in degree, perhaps in
kind. Whether this be so or not, there can be no doubt that
both gases and solids expand or contract according to the in-
verse contraction or expansion of other neighbouring bodies,
and so far resemble each other in their relations to heat and
cold. The extent to which such expansion or contraction
can be carried, seems to be limited only by the correlative
state of other bodies ; these again, by others, and so on, as
far as we may judge, throughout the universe.

Adopting the explanation above given of the decomposi-
tion of water by heat, heat would have the same relation to
chemical affinity as it has to physical attraction ; its imme-
diate tendency is antagonistic to both, and it is only by a sec-
ondary action that chemical affinity is apparently promoted
by heat. This view would explain how heat may promote
changes of the equilibrium of chemical affinity among mixed
compound substances, by decomposing certain compounds and
separating elementary constituents whose affinity is greater,
when they are brought within the sphere of attraction for the
substance with which they are mixed, than for those with
which they were originally chemically united : thus an intense
heat being applied to a mixture of chlorine and the vapour
of water, occasions the production of muriatic acid, libera-
ting oxygen.

Carrying out this view, it would appear that a sufficient
intensity of heat might yield indefinite powers of decomposi-
tion ; and there seems some probability of bodies now sup-
posed to be elementary, being decomposed or resolved into
further elements by the application of heat of sufficient inten-
sity ; or, reasoning conversely, it may fairly be anticipated
that bodies, which will not enter into combination at a certain
temperature, will enter into combination if their temperature
be lowered, and that thus new compounds may be formed by

68 COEEELATION OF PHYSICAL FORCES.

a proper disposition of their constituents when expose<3 to an
extremely low temperature, and the more so if compression
be also employed.

In considering the effect of heat as a mechanical force, it
would be expected, a priori, and independently of any theory
of heat which may be adopted, that a given amount of heat
acting on a given material must produce a given amount of
motive power ; and the next question which occurs to the
mind is, whether the same amount of heat would produce the
same amount of mechanical power, whatever be the material
acted on or affected by the heat. I will endeavour to reason
this out on the view of heat which I have advocated. Heat
has been considered in this essay as itself motion or mechan-
ical power, and quantity of heat as measured by motion.
Thus, if by a given contraction of a body (say mercury) air
within a cylinder having a moveable piston be expanded, the
piston moves, and in this case the expansion or motion of the
material (say iron) of the cylinder itself and of the air sur-
rounding it is commonly neglected. As the air dilates it be-
comes colder ; in other words, by undergoing expansion itself,
it loses its power of making neighbouring bodies expand ;
but if the piston be forcibly kept down, the expansive power
due to the mercury continues to communicate itself to the
iron and to the surrounding air, which become hotter than
they would if the piston had given way.

Now, in the above case, if the air be confined and its
volume unchanged, will the expansion of the iron, assuming
that it can be utilised, produce an exactly equivalent mechan-
ical effect to that which the expansion of the air would pro-
duce if the heat be entirely confined to it ?

Assuming that (with the exception of bodies which ex-
pand in freezing, where, through a limited range of tempera-
ture, the converse effects obtain) whenever a body is com-
pressed it is heated, i. e. it expands neighbouring substances ;
whenever it is dilated or increased in volume it is cooled, i. e.

HEAT. 69

it contracts neighbouring substances — ^the conclusion ap-
pears to me inevitable that the mechanical power produced
by heat will be definite, or the same for a given amount and
intensity of heat, whatever be the substance acted on.

Thus, let A be a definite source of heat, say a pound of
mercury at the temperature of 400° ; let B be another equal
and similar source of heat : suppose A be einployed to raise
a piston by the dilatation of air, and B to raise another pis-
ton by the dilatation of the vapour of water. Imagine the
pistons attached to a beam, so that they oppose each other's
action, and thus represent a sort of calorific balance. If A
being applied to air could conquer B, which is applied to
water, it would depress or throw back the piston of the latter,
and, by compressing the vapour, occasion an increase of
temperature ; this, in its turn, would raise the temperature
of the source of heat, so that we should have the anomaly
that a pound of mercury at 400° could heat another pound
of mercury at 400° to 401°, or to some point higher than its '
original temperature, and this without any adventitious aid :
it will be obvious that this is impossible, at least contradic-
tory to the whole range of our experience.

The above experiment is ideal, and stated for the object
of giving a more precise form to the reasoning ; to bring the
idea more prominently into relief, all statements as to quan-
tities, specific heats, &c., so as to yield comparative results
for given materials, are omitted. The argument may be
thus stated in another form, viz. that by no mechanical appli-
ance or difference of material acted on can a given source
of heat be made to produce more heat than it originally
possessed ; and that, if all be converted into mechanical
power, an excess cannot be supposed, for that could be con-
verted into a surplus of heat, and be a creation of force ; and
a deficit cannot be supposed, for that would be annihilation
of force. I cannot, however, see how the theoretical concep-
tion could be verified by experiment ; the enormous weights

TO COBKELATION OF PHYSICAL FOBCES.

and the complex mechanical contrivances requisite to give
the measure of power yielded by matter in its less dilatable
forms, would be far beyond our present experimental re-
ources. It would also be difficult to prevent the interference
of molecular attractions, inertia, &c., the overcoming of
which expends a part of the mechanical power generated,
but which could hardly be made to appear in the result.
We could not, for instance, practically realise the above con-
ception by the construction of a machine which should act by
the expansion and contraction of a bar of iron, and produce a
power equal to that of a steam engine, supplied with an equal
quantity of heat.

Carnot, who wrote in 1824 an essay on the motive power
of heat, regarded the mechanical power produced by heat as
resulting from a transfer of heat from one point to another,
without any ultimate loss of heat. Thus, in the action of an
ordinary steam engine, the heat from the furnace having ex-
panded the water of the boiler and raised the piston, a
mechanical motion is produced ; but this cannot be continued
without the removal of the heat, or the contraction of the ex-
panded water. This is done by the condenser, and the piston
descends. But then we have apparently transferred the heat
from the furnace to the condenser, and in the transfer effected
mechanical motion.

Should the mechanical motion produced by heat be con-
sidered as the effect of a simple transference of heat from one
point to another, or as the result of a conversion of heat into
the mechanical force of which this motion is the result ? This
question leads to the following : does the heat which generates
the mechanical power return to the thermal machine as heat,
or is it conveyed away by the work performed ?

If a definite quantity of air be heated it is expanded, and
by its expansion it cools or loses some of its power of com-
municating heat to neighbouring bodies. That which we
should have called heat if the expansion of the air had been

HEAT. 71

prevented, we call mechanical effect, or may view as converted
into mechanical effect ceasing to be heat ; but, throwing out
of the question nervous sensation, this expansion or mechani-
cal effect is all the evidence we have of heat, for if the air is
allowed to expand freely, this expansion becomes the index
of the heat ; if the air be confined, the expansion of the
matter of the vessel confining it, or of the mercury of a ther-
mometer in contact with it, &c., are the indices of the heat.

If, again, the air which has been expanded be, by mechani-
cal pressure or by other means, restored to its original bulk,
it is capable of heating or expanding other substances to a
degree to which it would not be equal, if it had remained in
its expanded state. To produce continuous motion, or the
up and down stroke of a piston, we must heat and cool, just
as with a magnetic machine we must magnetise and demagne-
tise in order to produce a continuous mechanical effect ; and
although, from the impossibility of insulating heat, some heat
is apparently lost in the process, the result may be said to be"
effected by the transfer of heat from the hot to the cold body,
from the furnace to the condenser. But we may equally well
say that the heat has been converted into mechanical force,
and the mechanical force back into heat ; the effects are
always correlative, as are the mechanical effects of an air
pump, with which, as we dilate the air on one side, we con
dense it on the other ; and as we cannot dilate without the
reciprocal condensation, so we cannot heat without the recip-
rocal cooling, or vice versa.

Hitherto the resistances of the piston or of any superim-
posed weight have been thrown out of consideration, or, what
amounts to the same thing, it has been assumed that the
weight raised by the piston has descended with it. The heat
has not merely been employed in dilating the air or vapour,
but in raising the piston with its weight. If, as the vapour
is cooled, the weight be permitted to descend, its mechanical
force restores the heat lost by. the dilatation ; but in this case

72 COKEELATION OF PHYSICAL FORCES.

no part of the power can be abstracted so as to be employed
for any practical purpose : this question then follows, what
takes place with regard to the initial heat, if, after the ascent
of the piston, the weight be removed so as not to help the pis-
ton in its descent, but to fall upon a lever or produce some
extraneous mechanical effect?

To answer this question, let us suppose a weight to rest
on a piston which confines air at a definite temperature, say
for example 50°, in a cylinder, the whole being assumed
to be absolutely non-conducting for heat. A part of the heat
of this confined air will be due to the pressure, since, as
we have seen, compression of an elastic fluid produces
heat.

Suppose, now, the confined air to be heated to 70°, the
piston with its superincumbent weight will ascend, and the
temperature, in consequence of the dilatation of the air, will
be somewhat lowered, say to 69° (we will assume, for the
sake of simplicity, that the heat engendered by the friction of
the piston compensates the force lost by friction).

The piston having reached its maximum of elevation, let
a cold body or condenser take away 20° from the temperature
of the confined air ; the piston will now descend, and by the
compression which the weight on it produces, will restore the
1° lost by dilatation, and when the piston reaches its original
position the temperature of the air will be restored to 50°.
Suppose this experiment repeated up to the rise of the piston ;
but when the piston is at its full elevation, and the cold body
applied, let the weight be removed, so as drop upon a wheel,
or to be used for other mechanical purposes. The descend-
ing piston will not now reach its original point without more
heat being abstracted ; in consequence of the removal of the
weight, there will not be the same force to restore the 1°, and
the temperature will be 49°, or some fraction short of the
original 50°. If this were otherwise, then, as the weight in
falling may be made to produce heat by friction, we should

HEAT. 73

have more heat than at first, or a creation of heat ont of noth-
ing— in other words, perpetual motion.

Let us now assume that this 20° supplied in the first in-
stance was yielded by a body at 90°, of such size and material
that its total capacity for heat is equal to that of the mass of
confined air : this body would be reduced in temperature to
70°, in other words, our furnace would have lost 20° of heat.
Let the cold body of the same size and material, used as a
condenser, be at 30°. In the first experiment, the body at
30° would bring back the piston to its original point ; but in
the second experiment, or that where the weight has been
removed, the body at 30° would not suffice to restore the pis-
ton : to efiect this, the cold body or condenser must be at a
lower temperature.

The question in Carnot's theory, which is not experi-
mentally resolved, and which presents extreme experimental
difficulty, is the following: Granted that a piston with a
superimposed weight be raised by the thermic expansion of
confined gas or vapour below it ; if the elastic medium be
restored to its original temperature by cooling, the weight in
depressing the piston will restore that portion of the heat
which has been lost by the expansion, and by the mechanical
effect consequent thereon ; but if the weight be removed when
at its maximum of elevation, and the piston be brought back
to its starting point by a necessarily cooler body than could
restore it if the weight were not removed, would the return of
the piston now restore the heat which had been lost by the
dilatation, or, in other words, would puUing the piston down
by cold restore the heat equally with the pressing it down by
mechanical force ? The argument from the impossibility of
perpetual motion would say no, for if all the heat were
restored, the mechanical effect produced by the fall of the
weight, or the heating effect which might be made to
result from this mechanical power, would be got from
nothing.

74: COEEELATION OF PHYSICAL FOECES.

Then follows another question, viz. whether, where an ex-
ternal or derived mechanical effect has been obtained, would
the return of the piston, effected without the weight or exter-
nal force to assist it, but solely by the colder body, give to
this latter the same number of thermometric degrees as had
been lost by the hot body in the first instance? Suppose, for
instance, the cold body in our experiment to be at 20° instead
of 30°, would this body gain 20°, and then reach the tempera-
ture of 40° when the piston is brought back, or would its
temperature be higher or lower than 40° ? The argument
from the impossibility of perpetual motion does not apply
here, for it does not necessarily follow that 20°, on the ther-
mometric scale from 20° to 40°, represents an equal amount
of force to 20° on the scale from 70° to 90°, and therefore it
is quite conceivable that we may lose 20° from the furnace,
and gain 20° in the condenser, and yet have obtained a cer-
tain amount of derived mechanical power. It will also follow,
upon a consideration of the above imaginary experiments,
that the greater the mechanical power required, the greater
should be the difference between the temperature of the
furnace and that of the condenser ; but the exact relation in
temperature between these, for a given mechanical effect, has
not, as far as I am aware, been satisfactorily established by
experiment, though it has been shown that steam at high
pressure produces, comparatively, a greater mechanical
effect for the same number of degrees than steam at low
pressure.

Camot, assuming the number of degrees of temperature
to be restored, but at a lower point of the thermometric scale,
termed this the fall (chute) of caloric. The mechanical effect
of heat, on this view, may be likened to that of a series of
cascades on water-wheels. The highest cascade turns a
wheel, and produces a given mechanical effect ; the water
which has produced this cannot again effect it at the same
level without being carried back to its original elevation, i. e.

HEAT. 75

without an extra force being employed equivalent to, or
rather a fraction more than the force of the descending
water ; but though its power is spent with reference to the
first wheel, the same water may, by faUing over a new
precipice upon a second wheel, again reproduce the same
mechanical effect (strictly speaking, rather more, for it has
approximated the centre of gravity), and so on, until no
lower faU can be attained. So with heat : it involves no
necessity of assuming perpetual motion to suppose that, after
a given mechanical effect, produced by a certain loss of heat,
the number of degrees lost from the original temperature
may be restored to the condenser, but at a lower point of the
the rmome trie scale.

If work has been done, i. e. if force has been parted
with, the original temperature itself cannot be restored, but
there is no a priori impossibility in the same number of
degi-ees of heat as have been converted into work being con-
veyed to a condensing body so cold that, when it receives this
heat, it will still be below the original temperature to which
the work-producing heat was added.

In the theory of the steam-engine, this subject possesses
a great practical interest. Watt supposed that a given
weight of water required the same quantity of what is
termed total heat (that is, the sensible added to the latent
heat) to keep it in the state of vapour, whatever was the
pressure to which it was subjected, and, consequently, how-
ever its expansive force varied. Clement Desormes was
also supposed to have experimentally verified this law. If
this were so, vapour raising a piston with a weight attached
would produce mechanical power ; and yet, the same heat
existing as at first, there would be no expenditure of the
initial force ; and if we suppose that the heat in the condens-
er was the real representative of the original heat, we
should get perpetual motion. Southern supposed that the
latent heat was constant, and that the heat of vapour under

76 CORRELATION OF PHYSICAL FORCES.

pressure increased as the sensible heat. M. Despretz, in
1832, made some experiments, which led him to the con-
clusion that the increase was not in the same ratio as the
sensible heat^ but that yet there was' an increase ; a result
confirmed and verified with great accuracy by M. Regnault,
in some recent and elaborate researches. What seems to
have occasioned the error in Watt and Clement Desormes'
experiments was, the idea involved in the term latent heat ;
by which, supposing the phenomenon of the disappearance of
sensible heat to be due to the absorption of a material sub-
stance, that substance, ' caloric,' was thought to be restored
when the vapour was condensed by water, even though the
water was not subjected to pressure ; but to estimate the
total heat of vapour under pressure the vapour should be
condensed while subjected to the same pressure as that under
which it is generated, as was done in M. Despretz and M.
Regnault's experiments.

M. Seguin, in 1839, controverted the position that derived
power could be got by the mere transfer of heat, and by
calculation from certain known data, such as the law of Mar-
iotte, viz. that the elastic force of gases and vapours increas-
ed directly with the pressure ; and assuming that for vapour
between 100° and 150° centigrade, each degree of elevation
of temperature was produced by a thermal unit, he deduced
the equivalent of mechanical work capable of being perform-
ed by a given decrement of heat ; and thus concluded that,
for ordinary pressures, about one granome of water losing
one degree centigrade would produce a force capable of rais-
ing a weight of 500 grammes through a space of one metre :
this estimate is a little beyond that given by the converse ex-
periments of Mr. Joule, already stated, in which the heat
produced by a given amount of mechanical action is estimat-
ed. I am not aware that the amount of mechanical work
wliich is produced by a given quantity of heat has been di-
rectly established by experiment, though some approximative

HEAT. 77

results in particular cases have been given. Theoretically it
should be the same — ^that is to say, if a fall of 772 lbs.
through a space of one foot will raise the temperature of 1 lb.
of vrater through one degree of Fahrenheit, then the fall in
the temperature of 1 lb. of water through one degree of Fah-
renheit should be able to raise 772 lbs. through a space of
one foot. The calculations of M. Seguin are not far from
this, but since the elaborate experiments of M. Regnault he
has expressed some doubt of the correctness of his former
estimate, as by these experiments it appears that, within cer-
tain limits, for elevating the temperature of compressed va-
pour by one degree, no more than about three-tenths of a de-
gree of total heat is required ; consequently, the equivalent
multiplied in this ratio would be 1,666 grammes, instead of
500. Other investigators have given numbers more or less
discordant ; so that, without giving any opinion on their dif-
ferent results, this question may be considered at present far
from settled. M. Regnault himself does not give the law by
which the ratio of heat varies with reference to the pressure,
and is still believed to be engaged in researches on the sub-
ject— one involving questions of which experiments on the
mechanical effects of elastic fluids seem to offer the most pro-
mising means of solution.

I have endeavoured to give a proof (by showing the
anomaly to which the contrary conclusion would lead) that,
whatever amount of mechanical power is produced by one
mode of application of heat, the same should, in theory, be
equally produced by any other mode. But in practice the
difference is immense ; and therefore it becomes a question
of great interest practically to ascertain what is the most
convenient medium on which to apply the heat employed, and
the best machinery for economising it. One great problem
to be solved is the saving of the heat which the steam in or-
dinary engines, after having done its work, carries into the
condenser, or, in the high-pressure engine, into the air. It

T8 COKKELATION OF PHYSICAL FORCES.

is argued you have a large amount of fuel consumed to raise
water to the boiling point, at which its efficiency as a motive
agent commences. After it has done a small portion of work,
and while it still retains a very large portion of the heat ori-
ginally communicated to it, you reject it, and have to start
again with a fresh portion of steam which has similarly ex-
hausted fuel — in other words, you throw away aU, and more
than all the heat which has been employed in raising the
water to the boiling point. Various plans have been devised
to remedy this. Using again the warm water of the conden-
ser to feed the boiler regains a part, but a very small part, of
the heat. Employing the steam first for a liigh pressure, and
then before its rejection or condensation using it for a low
pressure, cylinder, is a second mode ; a third is to use the
steam, after it has done its work on the piston, as a source of
heat or second furnace, to boil ether, or some liquid which
evaporates at a lower temperature than water. These plans
have certain advantages ; but the complexity of apparatus,
the danger from combustion of ether, and other reasons,
have hitherto precluded their general adoption. Under the
term regenerating engine various ingenious combinations
have lately been suggested, and some experimental engines
tried, with what success it is perhaps too early at present to
pronounce an opinion. The fundamental notion on which
this class of engine is based is that the vapour or air, when
it has performed a certain amount of work, as by raising a
piston, should, instead of being condensed or blown off, be
retained and again heated to its original high temperature,
and then used de novo; or that it should impart its heat to
some other substance, and the latter in turn impart it to the
fresh vapour about to act. The latter plan has been proposed
by Mr. Ericsson: he passes the air w^hich has done its
work through layers of wire gauze, whicb are heated by the
rejected air, and through which the next charge of air is
made to pass. M. Seguin and Mr. Siemens have construct-

HEAT.

79

ed machines upon the former principle, which are said to
have given good experimental results. There is, however,
a theoretical difficulty in aU these, not affecting their capabil-
ity of acting, but affecting the question of economy, which it
does not seem easy to escape from. Whether the heated air
or vapour be retained, or whether it yield its heat to a metal-
lic or other substance, this heat must exercise its usual repul-
sive force, and this must re-act either against the returning
piston or against the incoming vapour, and require a greater
pressure in that to neutralise it. Vapour raising a piston and
producing mechanical force effects this with decreasing power
in proportion as the piston is moved. At a certain point the
piston is arrested, or the stroke, as it is termed, is completed,
but there is still compressed vapour in the cylinder capable
of doing work, but so little that it is, and must in practice
be neglected ; if this compressed vapour be retained, the pis-
ton cannot be depressed without an extra force capable of
over coming the resistance of this, so to speak, semi-compress-
ed vapour, in addition to that which is requisite to produce the
normal work of the machine ; and in whatever way the resi-
dual force be retained, it must either be antagonised at a loss
of power for the initial force, or at most can only yield the
more feeble power which it would have originally given if it
had been allowed to act for a longer stroke on the piston. It
may be that a portion of this residual force may be econo-
mised ; indeed, this is done when the boiler is charged with
warm water from the condenser, instead of with cold water ;
but some, indeed a notable loss, seems inevitable.

Without farther discussing the various inventions and the-
ories on this subject, which are daily receiving increased de-
velopment, it may be well to point out how far nature dis-
tances art in its present state. According to some careftd es-
timates, the most economical of our furnaces consume from
ten to twenty times as much fuel to produce the same quantity
of heat as an animal produces ; and Matteucci found that.

80 COBKELATION OF PHYSICAL F0KCE8.

from a given consumption of zinc in a voltaic battery, a far
greater mechanical effect could be produced by making it act
on the limbs of a recently-killed frog, notwithstanding the
manifold defects of such an arrangement and its inferiority
to the action of the living animal, than when the same bat-
tery was made to produce mechanical power, by acting on an
electro-magnetic or other artificial motor apparatus. The ratio
in his experiments was nearly six to one. Thus in all our arti-
ficial combinations we can but apply natural forces, and with
far inferior mechanism to that which is perceptible in the
economy of nature.

Nature is made better by no mean ;
But nature makes that mean ; so o'er that art,
"Which you say adds to nature, is an ai*t
That nature makes.

A speculation has been thrown out by Mr. Thompson,
that, as a certain amount of heat results from mechanical ac-
tion, chemical action, &c., and this heat is radiated into space,
there must be a gradual diminution of temperature for the
earth, by which expenditure, however slow, being continuous,
it would ultimately be cooled to a degree incompatible with
the existence of animal and vegetable life — ^in short, that the
earth and the planets of our system are parting with more
heat than they receive, and are therefore progressively cool-
ing. Geological researches support to some extent this view,
as they show that the climate of many portions of the terres-
trial surface was at remote periods hotter than at the present
time : the animals whose fossilised remains are found in an-
cient strata have their organism adapted to what we should
now term a hot climate. There are, however, so many cir-
cumstances of difficulty attending cosmical speculations,
that but little reliance can be placed upon the most profound.
"We know not the original source of terrestrial heat ; still
less that of the solar heat ; we know not whether or not sys-

HEAT. 81

terns of planets may be so constituted as to conununicate
forces, inter se, so that forces which have hitherto escaped
detection may be in a continuous or recurring state of inter-
change.

The movements produced by mutual gravitation may be
the means of calling into existence molecular forces within
the substances of the planets themselves. As neither from
observation, nor from deduction, can we fix or conjecture any
boundary to the universe of stellar orbs, as each advance in
telescopic power gives us a new shell, so to speak, of
stars, we may regard our globe, in the limit, as surrounded by
a sphere of matter radiating heat, light, and possibly other,
forces.

Such stellar radiations would not, from the evidence we
have at present, appear sufficient to supply the loss of heat
by terrestrial radiations ; but it is quite conceivable that the
whole solar system may pass through portions of space hav-
ing different temperatures, as was suggested, I believe, by
Poisson ; that as we have a terrestrial simimer and winter,
so there may be a solar or systematic summer and winter, in
which case the heat lost during the latter period might be re-
stored during the former. The amount of the radiations of
the celestial bodies may again, from changes in their positions,
vary through epochs which are of enormous duration as re-
gards the existence of the human species.

The views of Mr. Thompson differ from those of Laplace,
recently enforced by M. Babinet, which suppose the planets
to have been formed by a gradual condensation of nebulous
matter. A modification of this view might, perhaps, be sug-
gested, viz. that worlds or systems, instead of being created
as wholes at definite periods, are gradually changing by at-
mospheric additions or subtractions, or by accretions or dim-
inutions arising from nebulous substance or from meteoric
bodies, so that no star or planet could at any time be said to
be created or destroyed, or to be in a state of absolute stabil-
4*

82 CORRELATION OF PHYSICAL FORCES.

ity, but that some may be increasing, others dwindling away,
and so throughout the universe, in the past as in the future.
When, however, questions relating to cosmogony, or to the
beginning or end of worlds, are contemplated from a physi-
cal point of view, the period of time over which our experi-
ence, in its most enlarged sense, extends, is so indefinitely
minute with reference to that which must be required for any
notable change, even in our own planet, that a variety of the-
ories may be framed equally incapable of proof or of dis-
proof. We have no means of ascertaining whether many
changes, which endure in the same direction for a term be-
yond the range of human experience, are really continuous or
only secular variations, which may be compensated for at
periods far beyond our ken, so that in such cases the ques-
tion of comparative stability or change can at best be only
answered as to a term which, though enormous with refer-
ference to our computations, sinks into nothing with reference
to cosmical time, if cosmical time be not eternity. Subjects
such as these, though of a kind on which the mind delights
to speculate, appear, with reference to any hope of attaining
reliable knowledge, far beyond the reach of any present or
immediately prospective capacity of man.

IV. ELECTKICITY.

ELECTRICITY is that affection of matter or mode of
force which most distinctly and beautifully relates other
modes of force, and exhibits, to a great extent in a quantita-
tive form, its own relation with them, and their reciprocal
relations with it and with each other. From the manner
in which the peculiar force called electricity is seemingly
transmitted through certain bodies, such as metallic wires,
the term cwrent is commonly used to denote its apparent
progress. It is very difficult to present to the mind any
theory which will give a definite conception of its modus
agendi: the early theories regard its phenomena as produced
either by a single fluid idio-repulsive, but attractive of all
matter, or else as produced by two fluids, each idio-repulsive
but attractive of the other. No substantive theory has been
proposed other than these two ; but although this is the case,
I think I shall not be unsupported by many who have atten-
tively studied electrical phenomena, in viewing them as re-
sulting, not from the action of a fluid or fluids, but as a mole-
cular polarisation of ordinary matter, or as matter acting by
attraction and repulsion in a definite direction. Thus, the
transmission of the voltaic current in liquids is viewed by
Grotthus as a series of chemical affinities acting in a definite
direction : for instance, in the electrolysis of water, i. e. its
decomposition when placed between the poles or electrodes

84 CORRELATION OF PHYSICAL FORCES.

of a voltaic battery, a molecule of oxygen is supposed to be
displaced by the exalted attraction of the neighbouring elec-
trode ; the hydrogen liberated by this displacement unites
with the oxygen of the contiguous molecule of water ; this in
turn liberates its hydrogen, and so on ; the current being
nothing els6 than this molecular transmission of chemical
affinity.

There is strong reason for believing that, with some ex-
ceptions, such as fused metals, liquids do not conduct elec-
tricity without undergoing decomposition ; for even in those
extreme cases where a trifling effect of conduction is appar-
ently produced without the usual elimination of substances at
.the electrodes, the latter when detached from the circuit
show, by the counter-current which they are capable of pro-
ducing when immersed in a fresh liquid, that their superficial
state has been changed, doubtless by the determination to
the surfaces of minute layers of substances having opposite
chemical characters. The question whether or not a minute
conduction in liquids can take place unaccompanied by chemi-
cal action, has however been much agitated, and may be re-
garded as inter apices of the science.

Assuming for the moment electrolysis to be the only
known electrical phenomenon, electricity would appear to con-
sist in transmitted chemical action. All the evidence we
have is, that a certain affection of matter or chemical change
takes place at certain distant points of space, the change at
one point having a definite relation to the change at the
other, and being capable of manifestation at any intermediate
points.

If, now, the electrical effect called induction be examined,
the phenomena will be found equally opposed to the theory of
a fluid, and consistent with that of molecular polarisation.
When an electrified conductor is brought near another which
is not electrified, the latter becomes electrified by influence or
induction, as it is termed, the nearest parts of each of these

ELECTRICITY. 85

two bodies exhibiting states of electricity of the contrary
denominations. Until this subject was investigated by Fara-
day, the intervening non-conducting body or dielectric was
supposed to be purely negative, and the effect was attributed
to the repulsion at a distance of the electrical fluid. Fara-
day showed that these effects differed greatly according to the
dielectric that was interposed. Thus they were more exalted
with sulphur than with shellac ; more with shellac than with
glass, &c, Matteucci, though differing from Faraday as to
the explanation he gave, added some experiments which prove
that the intervening dielectric is molecularly polarised. Thus
a number of thin plates of mica are superposed like a pack
of cards ; metallic plates are applied to the outer facings, and
one of them electrified, so that the apparatus is charged like
a Leyden phial. Upon separating the plates with insulating
handles, each plate is separately electrified, one side of it
being positive and the other negative, showing very neatly
and decisively a polarisation throughout the intervening sub-
stances by the effect of induction.

Indeed, chemical action or electrolysis may, as I have
shown, be transmitted by induction across a dielectric sub-
stance, such as glass, but apparently only while the glass is
being charged with electricity. A wire passing through and
hermetically sealed into a glass tube, a short portion only pro-
jecting, is made to dip into water contained in a Florence
flask ; the flask is immersed in water to an equal depth with
that within it ; the wire and another similar wire dipping
into the outer water are made to communicate metallically
with the powerful electrical machine known as Rhumkorf s
coil ; bubbles of gas instantly ascend from the exposed por-
tions of the wires, but cease after a certain time, and are
renewed when, after an interval of separation, the coil is
again connected with the wires.

The following interesting experiment by Mr. Karsten
goes a step farther in corroboration of the molecular changes

86 CORRELATION OF PHYSICAL FORCES.

consequent upon electrisation : A coin is placed on a pack of
thin plates of glass, and then electrified. On removing the
coin and breathing on the glass plate, an impression ^f the
coin is perceptible ; this shows a certain molecular change on
the surface of the glass opposed to the plate, or of the vapours
condensed on such surface. This effect might, and has been
interpreted as arising from a film of greasy deposit, supposed
to exist on the plate ; the impressions, however, have been
proved to penetrate to certain depths below the surface, and
not to be removed by polishing.

The following experiment, however, goes farther: On
separating carefully the glass plates, images of the coin can
be developed on each of the surfaces, showing that the mole-
cular change has been transmitted through the substance of
the glass ; and we may thence reasonably suppose that a
piece of glass, or other dielectric body, if it could be split up
while under the influence of electric induction, would exhibit
some molecular change at each side of each lamina, however
minutely subdivided. I have succeeded in farther extending
this experiment, and in permanently fixing the images thus
produced by electricity. Between two carefully-cleaned glass
plates is placed a word or device cut out of paper or tinfoil ;
sheets of tinfoil a little smaller than the glass plates are
placed on the outside of each plate, and these coatings are
brought into contact with the terminals of Rhumkorf s coil.
After electrisation for a few seconds, the glasses are sepa-
rated, and their interior surfaces exposed to the vapour of
hydrofluoric acid, which acts chemically on glass ; the por-
tions of the glass not protected by the paper device are cor-
roded, while those so protected are untouched or less
affected by the acid, so that a permanent etching is thus
produced, which nothing but disintegration of the glass will
efface.

Some farther experiments of mine on this subject bring
out in a still more striking manner these curious molecular

ELECTEICITY. 87

changes. One of the plates of glass having been electrified
in the manner just mentioned, is coated, on the side impressed
with the invisible electrical image, with a film of iodised
coUodion in the manner usually adopted for photographic
purposes ; it is then in a dark room immersed in a solution
of nitrate of silver ; then exposed to diffuse light for a few
seconds. On pouring over the collodion the usual solution
of pyrogallic acid, the invisible electrical image is brought
out as a dark device on a light ground, and can be permanently
fixed by hyposulphite of soda. The point worthy of obser-
vation in this experiment is, that this permanent image exists
in the collodion film, which can be stripped off" the glass, dried,
and placed on any other surface, so that the molecular change
consequent on electrisation has communicated, by contact or
close proximity, a change to the film of collodion corres-
ponding in form with that on the glass, but being undoubtedly
of a chemical nature. Electricity has, moreover, in this ex-
periment so modified the surface of glass, that it can, in its
tiirn, modify the structure of another substance so as to alter
the relation of the latter to light. It would require a curious
complication of hypothetic fluids to explain this ; but if elec-
tricity and light be supposed to be affections of ordinary pon-
derable matter, the difficulty is only one of detail.

If, again, we examine the. electricity of the atmosphere,
when, as is usually the case, it is positive with respect to that
of the earth, we find that each successive stratum is positive
to those below it and negative to those above it ; and the con-
verse is the case when the electricity of the atmosphere is
negative with respect to that of the earth.

If anotheii electrical phenomenon be selected, another sort
of change wiU be found to have taken place. The electric
spark, the brush, and similar phenomena, the old theories
regarded as actual emanations of the matter or fluid. Elec-
tricity ; I venture to regard them as produced by an emission
of the material itself from whence they issue, and a molecular

88 COKBELATION OF PHYSICAL FORCES.

action of the gas, or intermedium, through or across which
they are transmitted.

The colour of the electric spark, or of the voltaic arc (i.
e. the flame which plays between the terminal points of a
powerful voltaic battery) , is dependent upon the substance of
the metal, subject to certain modifications of the intermedium :
thus, the electric spark or arc from zinc is blue ; from silver,
green ; from iron, red and scintillating ; precisely the colours
afforded by these metals in their ordinary combustion. A
portion of the metal is also found to be actually transmitted
with every electric or voltaic discharge : in the latter case,
indeed, where the quantity of matter acted upon is greater
than in the former, the metallic particles emitted by the elec-
trodes or terminals can be readily collected, tested, or even
weighed. It would thus appear that the electrical discharge
arises, at least in part, from an actual repulsion and sever-
ance of the electrified matter itself, which flies off at the points
of least resistance.

A. careful examination of the phenomena attending the
electric spark or the voltaic arc, which latter is the electric
disruptive discharge acting on greater portions of matter,
tends to modify considerably our previous idea of the nature
of the electric force as a producer of ignition and combustion.
The voltaic arc is perhaps, strictly speaking, neither ignition
nor combustion. It is not simply ignition ; because the mat-
ter of the terminals is not merely brought to a state of incan-
descence, but is physically separated and partially transferred
from one electrode to another, much of it being dissipated in
a vaporous state. It is not combustion ; for the phenomena
will take place independently of atmospheric air, oxygen gas,
or any of the bodies usually called supporters of combustion,
combustion being in fact chemical union attended with heat
and light. In the voltaic arc we may have no chemical union ;
for if the experiment be performed in an exhausted receiver, or
in nitrogen, the substance forming the electrodes is condensed,

ELECTEICITY. 89

and precipitated upon the interior of tlie vessel in, chemically
speaking, an unaltered state. Thus, to take a very striking
example, if the voltaic discharge be taken between zinc ter-
minals in an exhausted receiver, a fine black powder of zinc
is deposited on the sides of the receiver ; this can be collect-
ed, and takes fire readily in the air by being touched with a
match, or ignited wire, instantly burning into white oxide of
zinc. To an ordinary observer, the zinc would appear to be
burned twice — first in the receiver, where the phenomenon
presents all the appearance of combustion, and secondly in
the real combustion in air. "With iron the experiment is
equally instructive. Iron is volatilised by the voltaic arc in
nitrogen or in an exhausted receiver ; and when a scarcely
perceptible film has lined the receiver, this is washed with an
acid, which then gives, with ferrocyanide of potassium, the prus-
sian-blue precipitate. In this case we readily distil iron, a
metal by ordinary means fusible only at a very high tempera-
ture.

Another strong evidence that the voltaic discharge con-
sists of the material itself of which the terminals are compos-
ed, is the peculiar rotation which is observed in the light
when iron is employed, the magnetic character of this metal
causing its molecules to rotate by the influence of the voltaic
current.

If we increase the number of reduplications in a voltaic
series, we increase the length of the arc, and also increase its
intensity or power of overcoming resistance. With a battery
consisting of a limited number, say 100 reduplications, the
discharge will not pass from one terminal to the other with-
out first bringing them into contact, but if we increase the
number of cells to 400 or 500, the discharge will pass from
one terminal to the other before they are brought into contact.
The difference between what is called Franklinic electricity, or
that produced by an ordinary electrical machine, and voltaic elec-
tricity, or that produced by the ordinary voltaic battery, is that

90 COREELATION OF PHYSICAL FORCES.

the former is of much greater intensity than the latter, or has a
greater power of overcoming resistance, but acts upon a much
smaller quantity of matter. If, then, a voltaic battery be
formed with a view to increase the intensity and lessen the
quantity, the character of the electrical phenomena approxi-
mate those of the electrical machine. In order to effect this,
the sizes of the plates of the battery and thence the quantity
of matter acted on in each cell, must be reduced, but the
number of reduplications increased. Thus if in a battery of 100
pairs of plates each plate be divided, and the battery be arranged
so as to form 200 pairs, each being half the original size, the
quantitative effects are diminished, and the effects of intensi-
ty increased. By carrying on this sub-division, diminishing
the sizes and increasing the number, as is the case in the vol-
taic piles of Deluc and Zamboni, effects are ultimately pro-
duced similar to those of Franklinic electricity, and we thus
gradually pass from the voltaic arc to the spark or electric
discharge.

This discharge, as I have already stated, has a colour de-
pending in part upon the nature of the terminals employed.
If these terminals be highly polished, a spot will be observed,
even in the case of a small electric spark, at the points from
which the discharge emanates. The matter of the terminals
is itself affected ; and a transmission of this matter across
the intervening space is detected by the deposition of minute
quantities of the metal or substance composing the one, upon the
other terminal.

If the gas or elastic medium between the terminals be
changed, a change takes place in the length or colour of the
discharge, showing an affection of the intervening matter.
If the gas be rarefied, the discharge gradually changes with
th^ degree of rarefaction, from a spark to a luminous glow or
diffuse light, differing in colour in different gases, and capable
of extending to a much greater distance than when it takes
place in air of the ordinary density. Thus, in highly attenu-

ELECTKICITY. 91

ated air a discharge may be made to pass across six or seven
feet of space, while in air of the ordinary density it would
not pass across an inch. An observer regarding the beauti-
ful phenomena exhibited by this electric discharge in attenua-
ted gas, which, from some degree of similarity in appear-
ance to the Aurora Borealis, has been called the electric Au-
rora, would have some difficulty in believing such effects
could be due to an action of ordinary matter. The amount
of gas present is extremely small ; and the terminals, to a
cursory examination, show no change after long experiment-
ing. It is therefore not to be wondered at that the first ob-
servers of this and similar phenomena, regarded electricity
as in itself something — as a specific existence or fluid. Even
in this extreme case, however, upon a more careful examination
we shall find that a change does take place, both as regards the gas
and as regards the terminals . Let one of these consist of a high-
ly-polished metal — a silver plate is one of the best materials for
the purpose — and let the discharges in attenuated atmospheric
air take place from a point, say a common sewing needle, to the
surface of the polished silver plate ; it will be found that this
is gradually changed in appearance opposite the point — it is ox-
idated, and gradually more and more corroded as the discharge
is continued.

J£ now the gas be changed, and highly-rarefied hydrogen
be substituted for the rarefied air, all other things remaining
the same, upon passing the discharges as before the oxide
will be cleared off the plate, and the polish to a great extent
restored — not entirely, because the silver has been disinte-
grated by the oxidation — and the portion which has been af-
fected by the discharge will present a somewhat different ap-
pearance from the remainder of the plate.

A question will probably here occur to the reader : — ^WJiat
will be the effect if there be not an oxidating medium pres-
ent, and the experiment be first performed in a rarefied gas,
which possesses no power of chemically acting on the plate ?

92 COEEELATION OF PHYSICAL FORCES.

In this case there will still be a molecular change or disinte-
gration of the plate ; the portion of it acted on by the dis-
charge will present a different appearance from that which is
beyond its reach, and a whitish film, somewhat similar to
that seen on the mercurialised portions of a daguerreotype,
will gradually appear on the portion of the plate affected by
the discharge. If the gas be a compound, as carbonic oxide,
or a mixture , as oxygen and hydrogen, and consequently contain
elements capable of producing oxidation and reduction, then
the effect upon the plate will depend upon whether it be pos-
itive or negative ; in the former case it will be oxidated, in
the latter the oxide, if existing, will be reduced. This effect
will also take place in atmospheric air, if it be highly rare-
fied, and can hardly be explained otherwise than by a mole-
cular polarisation of the compound gas. If, again, the metal
be reduced to a small point, and be of such material that the
gas cannot act chemically upon it, it can yet be shown to be
disintegrated by the electric spark. Thus, let a fine plati-
num wire be hermetically sealed in a glass tube, and the ex-
tremity of the tube and the wire ground to aflat surface, so as to
expose a section only of the wire ; after taking the discharge
from this for some time, it will be found that the platinum
wire is worn away, and that its termination is sensibly below
the level of the glass. If the discharges from such a plati-
num wire be taken in gas contained in a narrow tube, a cloud
or film consisting of a deposit of platinum will be seen on
the part of the tube surrounding the point.

Another curious effect which, in addition to the above, I
have detected in the electrical discharge in attenuated media,
is that when passing between terminals of a certain form, as
from a wire placed at right angles to a polished plate, the dis-
charge possesses certain phases or fits of an alternate character,
so that, instead of impressing an uniform mark on a polished
plate, a series of concentric rings is formed.

Priestley observed that, after the discharge of a Leyden

ELECTRICITY. 93

battery, rings consisting of fused globules of metal were
formed on the terminal plates ; in my experiments made in
attenuated media, alternate rings of oxidation and deoxida-
tion are formed. Thus, if the plate be polished, coloured
rings of oxide will alternate with rings of polished or unoxi-
dated surface ; and if the plate be previously coated with an
uniform film of oxide, the oxide will be removed in concen-
tric spaces, and increased in the alternate ones, showing a
lateral alternation of positive and negative electricity, or
electricity of opposite character in the same discharge.

It would be hasty to assert that in no case can the electri-
cal disruptive discharge take place without the terminals be-
ing affected. I have, however, seen no instance of such a re-
sult where the discharge has been sufficiently prolonged, and
the terminals in such a state as could be expected to render
manifest slight changes.

The next question which would occur in following out the
enquiry which has been indicated, would probably be. What
is the action upon the gas itself? is this changed in any man-
ner?

In answer to this, it must be admitted that, in the present
state of experimental knowledge on this subject, certain
gases only appear to leave permanent traces of their having
been changed by the discharge, while others, if affected by
it, which, as will be presently seen, there are reasons to be-
lieve they are, return to their normal state immediately after
the discharge.

In the former class we may place many compound gases,
as ammonia, olefiant gas, protoxide of nitrogen, deutoxide
of nitrogen, and others, which are decomposed by the action
of the discharge. Mixed gases are also chemically combined :
for instance, oxygen and hydrogen unite and form water ;
common air gives nitric acid ; chlorine and aqueous vapour
give oxygen, the chlorine uniting with the hydrogen of the
water.

94: CORRELATION OF PHYSICAL FORCES.

But, further than this, in the case of certain elementary
gases a permanent change is effected by the electrical dis-
charge. Thus, oxygen submitted to the discharge is par-
tially changed into the substance now considered to be an al-
lotropic condition of oxygen ; and there is reason to believe
that when the change takes place, there is a definite polar
condition of the gas, and that definite portions of it are affected
— that in a certain sense one portion of the oxygen bears
temporarily to the other the relation which hydrogen ordina-
rily does to oxygen.

If the discharge be passed through the vapour of phos-
phorus in the vacuum of a good air-pump, a deposit of allotro-
pic phosphorus soon coats the interior of the receiver, show-
ing an analogous change to that produced in oxygen ; and in
this case a series of transverse bands or stratifications appears
in the discharge, showing a most striking alteration in its
physical character, dependent on the medium across which it is
transmitted. These effects were first observed by me in
the year 1852. They have since been much examined by
continental philosophers, and much extended by Mr. Gassiot ;
but no satisfactory rationale of them has yet been given.

There are many gases which either do not show any per-
manent change, or (which is more probably the case) the
changes produced in them by the electrical discharge have
not yet been detected. Even with these gases, however, the
difference of colour, of length, or of the different position of
a certain dark space or spaces which appear in the discharge,
show that the discharge differs for different media. We nev-
er find that the discharge has itself added to or subtracted
from the total weight of the substances acted on : we find nb
evidence of a fluid but the visible phenomena themselves ;
and those we may account for by the change which takes
place in the matter affected.

I have here, as elsewhere, used words of common accep-
tation, such as ' matter affected by the discharge,' &c., though

ELECTRICITY. 95

Tipon the view I am suggesting, the discharge is itself this
affection of matter : and the writing these passages affords,
to me at least, a striking instance of how much ideas are
bound up in words, when, to express a view differing from
the received one, words involving the received one are neces-
sarily used.

Passing now to the effect of the transmission of electri-
city by the class of the best conducting bodies, such as the
metals and carbon, here, though we cannot at present give the
exact character of the motion impressed upon the particles,
there are yet many experiments which show that a change
takes place in such substances when they are affected by elec-
tricity.

Let discharges from a Ley den jar or battery be passed
through a platinum wire, too thick to be fused by the dis-
charges, and free from constraint, it will be found that the
wire is shortened ; it has undergone a molecular change, and
apparently been acted on by a force tranverse to its length.
If the discharges be continued, it gradually gathers up in
small irregular bends or convolutions. So with voltaic elec-
tricity : place a platinum wire in a trough of porcelain, so
that when fused it shall retain its position as a wire, and then
ignite it by a voltaic battery. As it reaches the point of fu-
sion it will snap asunder, showing a contraction in length, and
consequently a distension or increase in its transverse dimen-
sions. Perform the same experiment with a lead wire,
which can be more readily kept in a state of fusion, and fol-
low it, as it contracts, by the terminal wires of the battery ;
it will be seen to gather up in nodules, which press on each
other like a string of beads of a soft material which have
been longitudinally compressed.

As we increase the thickness of the wires in these exper-
iments with reference to the electrical force employed, we les-
sen the perceptible effect : but even in this case we shall be
enabled safely to infer that some molecular change accompa-

96 CORRELATION OF PHYSICAL FORCES.

nies the transmission of electricity: the wires are heated in a
degree decreasing as their thickness increases — ^but by in-
creasing the delicacy of our tests as the heating effects de-
crease in intensity, we may indefinitely detect the augmenta-
tion of temperature accompanying the passage of electri-
city— and wherever there is augmentation of temperature
there must be expansion or change of position of the mole-
cules.

Again, it has been observed that wires which have for a
long time transmitted electricity, such as those which have
served as conductors for atmospheric electricity, have their
texture changed, and are rendered brittle. In this observa-
tion, however, though made by a skillful electrician, M. Pel-
tier, the effects of exposure to the atmosphere, to changes of
temperature, &c., have not been sufficiently eliminated to
render it worthy of entire confidence. There are, however,
other experiments which show that the elasticity of metals is
changed by the passage through them of the electric current.

Thus M. Wertheim has, from an elaborate series of ex-
periments, arrived at the conclusion that there is a temporary
diminution in the coefficient of elasticity in wires while they
are transmitting the electric current, which is independent of
the heating effect of the current.

M. Dufour has made a considerable number of experi-
ments with the view of ascertaining if any permanent change
in metals is effected by electrisation. He arrives at the cu-
rious result that in a copper wire through which a feeble vol-
taic current has passed for several days, a notable diminution
in tenacity takes place ; while, in an iron wire, the tenacity
is increased ; and that these effects were more perceptible
when the wires had been electrised for a long time (nineteen
days) than for a short time (four days). The copper wire
was, in his experiment, not perfectly pure ; so that the effect,
or a portion of it, might be due to the state of alloy : in the
case of iron, the magnetic character of the metal would prob-

ELECTRICITY. 97

ably modify the effects, and might account for the opposite
character of the results with these two metals.

Matteucci has made experiments on the conduction of
electricity by bismuth in directions parallel or transverse to
the planes of principal cleavage, and he finds that bismuth
conducts electricity and heat better in the direction of the
cleavage planes than in that transverse to them.

Many other experiments have been made both on the pro-
duction of thermo-electric currents by two portions of the
same crystalline metal, but with the planes of crystallization
arranged in different directions relatively to each other, and
also on the differences in conduction of heat and electricity
according to the direction in which they are transmitted with
reference to the planes of crystallization.

It is found, moreover, that the slightest difference in ho-
mogeneity in the same metal enables it when heated to pro-
duce a thermo-electric current, and that metals in a state of
fusion, in which state they may be presumed to be homoge-
neous throughout, give no thermo-electric current : thus, hot
in contact with cold mercury has been shown by Matteucci
to give no thermo-electric current, and the same is the case
with portions of fused bismuth unequally heated.

The fact that the molecular structure or arrangement of a
body influences — indeed I may say determines — its conduct-
ing power, is by no means explained by the theory of a fluid ;
but if electricity be only a transmission of force or motion,
the influence of the molecular state is just what would be
expected. Carbon, in a transparent crystalline state, as dia-
mond, is as perfect a non-conductor as we know ; while in an
opaque amorphous state, as graphite or charcoal, it is one of
the best conductors : thus, in the one state, it transmits light
and stops electricity, in the other it transmits electricity and
stops light.

It is a circumstance worthy of remark, that the arrange-
ment of molecules, which renders a solid body capable of
5

98 COKEELATION OF PHYSICAL FOECES.

transmitting light, is most unfavourable to its transmission of
electricity, transparent solids being very imperfect conductors
of electricity; so all gases readily transmit light, but are
amongst the worst conductors of electricity, if, indeed, prop-
erly speaking, they can be said to conduct at all.

The conduction of electricity by different classes of bodies
has been generally regarded as a question of degree : thus
metals were viewed as perfect conductors, charcoal less so,
water and other liquids as imperfect conductors, &c. But,
in fact, though between one metal and another the mode of
transmission may be the same and the difference one of de-
gree, a different molecular effect obtains, when we contrast
metals with electrolytic liquids and these with gases.

Attenuated gases may be, in one sense, regarded as non-
conductors, in another, as conductors ; thus if gold-leaves be
made to diverge, by electrical repulsion, in air at ordinary
pressure, they in a short time collapse ; while in highly-rare-
fied air, or what is commonly termed a vacuum, they remain
divergent for days ; and yet electricity of a certain degree of
tension passes readily across attenuated air, and with diffi-
culty across air of ordinary density.

Again, where the electrical terminals are brought to a
state of visible ignition, there are symptoms of the transmis-
sion of electricity of low tension across gases ; but no such
effects have been detected at lower temperatures. All this
presents a strong argument in favour of the transmission of
electricity across gases being effected by the disruptive dis-
charge, and not by a conduction similar to that which takes
place with metals or with electrolytes.

The ordinary attractions and repulsions of electrified
bodies present no more difficulty when regarded as being pro-
duced by a change in the state or relations of the matter af-
fected, than do the attractions of the earth by the sun, or of
a leaden ball by the earth ; the hypothesis of a fluid is not
considered necessary for the latter, and need not be so for the

ELECTRICITY. 99

former class of phenomena. How the phenomena are pro-
duced to which the term attraction is applied is still a mys-
tery. Newton, speaking of it, says, ' What I call attraction
may be performed by impulse, or by some other means un-
known to me. I use that word here to signify only in gen-
eral any force by which bodies tend towards one another,
whatsoever be the cause.' If we suppose a fluid to act in at-
tractions and repulsions, the imponderable fluid must drag or
push the matter with it : thus when we feel a stream of
air rushing from an electrified metallic point, each molecule
of air contiguous to the point being repelled, another takes
its place, which is in its turn repelled : — ^how does a hypo-
thetic fluid assist us here ? K we say the electrical fluid re-
pels itself, or the same electricity repels itself, we must go
farther and assert, that it not only repels itself, but either
communicates its repulsive force to the particles of the air, or
carries with it the particle of air in its passage. Is it not
more easy to assume that the particle of air is in such a state
that the ordinary forces which keep it in equilibrium are dis-
turbed by the electrical force, or force in a definite direction
communicated to it, and that thus each particle in turn re-
cedes from the point? As this latter force is increased, not
only does the particle of air which was contiguous to the me-
tallic point recede, but the cohesion of the extreme particles
of metal may be overcome to such an extent that these are
detached, and the brush or spark may consist wholly or in
part of minute particles of the metal itself tlirown off. Of
this there is some evidence, though the point can hardly be
considered as proved. A similar effect undoubtedly takes
place with voltaic electricity, acting upon a terminal im-
mersed in a liquid ; thus if metallic terminals of a powerful
voltaic battery be immersed in water, metal, or the oxide of
metal, is forcibly detached, producing great heat at the point
of disruption.

K we apply ourselves to the effect of electricity in the

100 COERELATION OF PHYSICAL FOECES.

animal economy, we find that the first rationale given of the
convulsive effect produced by transmission through the living
or recently killed animal was, that electricity itself, something
substantive, passed rapidly through the body, and gave rise
to the contractions ; step by step we are now arriving at the
conviction that consecutive particles of the nerves and mus-
cles are affected. Thus the contractions which the prepared
leg of a frog undergoes at the moment it is submitted to a
voltaic current, cease after a time if the current be contin-
ued, and are renewed on breaking the circuit, i. e. at the mo-
ment when the current ceases to traverse it. The excitabil-
ity of a nerve, moreover, or its power of producing muscular
contraction, is weakened or destroyed by the transmission of
electricity in one direction, while the excitability is increased
by the transmission of electricity in the opposite direction ;
showing that the fibre or matter itself of the nerve is changed
by electrisation, and changed in a manner bearing a direct
relation to the other effects produced by electricity.

Portions of muscle and of nerve present different electri-
cal states with reference to other portions of the same muscle
or nerve ; thus the external part of a muscle bears the same
relation to the internal part as platinum does to zinc in the
voltaic battery ; and delicate galvanoscopes will show electri-
cal effects when interposed in a conducting circuit connecting
the surface of a nerve with its interior portions. Matteucci
has proved that a species of voltaic pile may be formed by a
series of slices of muscle, so arranged that the external part
of one slice may touch the internal part of the next, and
so on.

Lastly, the magnetic effects produced by electricity also
show a change in the molecular state of the magnetic sub-
stance affected ; as we shall see when the subject of magnet-
ism is discussed.

I have taken in succession all the known classes of elec-
trical phenomena ; and, as far as I am aware, there is not an

ELECTKICITY. 101

electrical effect, where, if a close investigation be instituted,
and the materials chosen in a state for exhibiting minute
changes, evidence of molecular change will not be detected ;
thus, excepting those cases where infinite simally small quan-
tities of matter are acted on, and our means of detection fail,
electrical effects are known to us only as changes of ordinary-
matter. It seems to me as easy to imagine these changes to
be effected by a force acting in definite directions, as by a
fluid which has no independent or sensible existence, and
which, it must be assumed, is associated with, or exerts a
force acting upon ordinary matter, or matter t)f a different
order from the supposed fluid. As the idea of the hypothetic
fluid is pursued, it gradually vanishes, and resolves itself into
the idea of force. The hypothesis of matter without weight
presents in itself, as I believe, fatal objections to the theories
of electrical fluids^ which are entirely removed by viewing
electricity as force, and not as matter.

If it be said that the effects we have been considering
may still be produced by a fluid, and that this fluid acts upon
ordinary matter in certain cases, polarising the matter af-
fected or arranging its particles in a definite direction, whilst
in others, by its attractive or repulsive force, it carries with
it portions of matter ; yet, if the fluid in itself be incapable
of recognition by any test, if it be only evidenced by the
changes which it operates in ponderable matter, the words
fluid and force become identical in meaning ; we may as well
say that the attraction of gravitation or weight is occasioned
by a fluid, as that electrical changes are so.

When, as is constantly done in conamon parlance, a house
is said to be struck^ windows broken, metals fused or dissiptx-
ted hy the electricaX fluid, are not the expressions used such
as, if not sanctioned by habit, would seem absurd ? In all
the cases of injury done by lightning there is no fluid per-
ceptible ; the so-called sulphurous odour is either ozone de-
veloped by the action of electricity on atmospheric air, or the

102 CORRELATION OF PHTSIOAIi FORCES.

vapour of some substance dissipated by the discharge ; on the
other hand, it seems more consonant with experience to re-
gard these effects as produced by force, as we have analogous
effects produced by admitted forces, in cases where no one
would invoke the aid of a hypothetic fluid for explanation.
For instance, glasses may be broken by electrical discharges ;
so may they by sonorous vibrations. Metals electrified or
magnetised will emit a sound ; so they will if struck, or if a
musical note with which they can vibrate in unison be sounded
near to them.

Even chemical decomposition, in cases of feeble affinity,
may be produced by purely mechanical effects. A number of
instances of this have been collected by M. Becquerel ; and
substances whose constituents are held together by feeble af-
finities, such as iodide of nitrogen and similar compounds, are
decomposed by the vibration occasioned by sound.

If, instead of being regarded as a fluid or imponderable
matter sui generis, electricity be regarded as the motion of an
ether, equal difficulties are encountered. Assuming ether to
pervade the pores of all bodies, is the ether a conductor or
non-conductor ? If the latter — ^that is, if the ether be incapa-
ble of transmitting the electrical wave — the ethereal hypothe-
sis of electricity necessarily falls ; but if the motion of the
ether constitute what we call conduction of electricity, then
the more porous bodies, or those most permeable by the
ether, should be the best conductors. But this is not the case.
If, again, the metal and the air surrounding it are both per-
vaded by ether, why should the electrical wave affect the
ether in the metal, and not stir that in the gas ? To support
an ethereal hypothesis of electricity, many additional and
hardly reconcilable hypotheses must be imported.

The fracture and comminution of a non-conducting body,
the fusion or dispersion of a metallic wire by the electrical
discharge, are effects equally difficult to conceive upon the
hypothesis of an ethereal vibration, as upon that of a fluid,

ELECTRICITY. * 103

but are necessary results of the sudden subversion of mole-
cular polarisation, or of a sudden or irregular vibratory move-
ment of the matter itself. We see similar effects produced
by sonorous vibrations, which might be called conduction
and non-conduction of sound. One body transmits sound ea-
sily, another stops or deadens it, as it is termed — i. e. dis-
perses the vibrations, instead of continuing them in the same
direction as the primary impulse ; and solid bodies may, as
has been above observed, be shivered by sudden impulses of
sound in those cases where all the parts of the body cannot
uniformly carry on the undulatory motion.

The progressive stages in the History of Physical Philoso-
phy will account in a great measure for the adoption by the
early electricians of the theories of fluids.

The ancients, when they witnessed a natural phenomenon,
removed from ordinary analogies, and unexplained by any
mechanical action known to them, referred it to a soul, a
spiritual or preternatural power : thus amber and the magnet
were supposed by Thales to have a soul ; the functions of
digestion, assimilation, &c., were supposed by Paracelsus to
be effected by a spirit (the Archseus). Air and gases were
also at first deemed spiritual, but subsequently became invest-
ed with a more material character ; and the word gas, from
geist^ a ghost or spirit, affords us an instance of the gradual
transmission of a spiritual into a physical conception.

The establishment by Torricelli of the ponderable charac-
ter of air and gas, showed that substances which had been
deemed spiritual and essentially different from ponderable
matter were possessed of its attributes. A less superstitious
mode of reasoning ensued, and now aeriform fluids were
shown to be analogous in many of their actions to liquids or
known fluids. A belief in the existence of other fluids, differ-
ing from air as this differed from water, grew up, and when
a new phenomenon presented itself, recourse was had to a
hypothetic fluid for explaining the phenomenon and coud^"^-

104 COEEELATION OF PHYSICAL FORCES.

ing it with others ; the mind once possessed of the idea of a
fluid, soon invested it with the necessary powers and proper-
ties, and grafted upon it a luxurious vegetation of imaginary
offshoots.

In what I am here throwing out, I wish to guard myself
from being supposed to state that the theory, historically
viewed, followed exactly the dates of the discoveries which
were effectual in changing its character ; sometimes a dis-
covery precedes, at other times it succeeds to a change in the
general course of thought; sometimes, and perhaps most
frequently, it does both — ^i. e. the discovery is the result of a
tendency of the age and of the continually improved methods
of observation, and when made, it strengthens and extends the
views which have led to it. I think the phases of thought
which physical philosophers have gone through, will be found
generally such as I have indicated, and that the gradual ac-
cumulation of discoveries which has taken place during the
more recent periods, by showing what effects can be produced
by dynamical causes alone, is rapidly tending to a general
dynamical theory into which that of the imponderable fluids
promises ultimately to merge.

Commencing with electricity as an initiating force, we
get motion directly produced by it in various forms ; for in-
stance, in the attraction and repulsion of bodies, evidenced by
mobile electrometers, such as that of Cuthbertson, where
large masses are acted on ; the rotation of the fly-wheel,
another form of electrical repulsion, and the deflection of
the galvanometer needle, are also modes of palpable, visible
motion.

It would foUoAv, from the reasoning in this essay, that
when electricity performs any mechanical work which does
not return to the machine, electrical power is lost. It would
be unsuitable to the scope of this work to give the mathemati-
cal labours of M. Clausius and others here ; but the follow-
ing experiment, which I devised for making the result evi-

ELECTRICITY. 105

dent to an audience at the Royal Institution, will form a
useful illustration : — A Leyden jar, of one square foot coated
surface, has its interior connected with a Cuthbertson*s elec-
trometer, between which and the outer coating of the jar
are a pair of discharging balls fixed at a certain distance
(about half an inch apart). Between the Leyden jar and
the prime conductor is inserted a small unit jar of nine inches
surface, the knobs of which are 0*2 inch apart.

The balance of the electrometer is now fixed by a stiff
wire inserted between the attracting knobs, and the Leyden
jar charged by discharges from the unit jar. After a certain
number of these, say twenty, the discharge of the large jar
takes place across the half-inch interval. This may be
viewed as the expression of electrical power received from
the unit jar. The experiment is now repeated, the wire
between the balls having been removed, and therefore the
' tip,* or the raising of the weight, is performed by the electri-
cal repulsion and attraction of the two pairs of balls. At
twenty discharges of the unit jar the balance is subverted,
and one attracting knob drops upon the other ; but no dis-
charge takes place:, shoAving that some electricity has been lost
or converted into the mechanical power which raised the
balance.

By another mode of expression, the electricity may be
supposed to be masked or analogous to latent heat, and it
would be restored if the ball were brought back without dis-
charge by extraneous force. K the discharge or other elec-
trical effects were the same in both cases, then, since the
raising of the ball or weight is an extra mechanical effort,
and since the weight is capable by its fall of producing elec-
tricity, heat, or other force, it would seem that force could be
got out of nothing, or perpetual motion obtained.

The above experiment is suggestive of others of a similar
character, which may be indefinitely varied. Thus I have
found that two balls made to diverge by electricity do not
5*

106 COEKELATION OF PHYSICAL FORCES.

give to an electrometer the same amount of electricity as they
do if, whilst similarly electrified, they are kept forcibly to-
gether. This experiment is the converse of the former one.
There is an advantage in electrical experiments of this class
as compared with those on heat, viz. that though there is no
perfect insulation for electricity, yet our means of insula-
tion are immeasurably superior to any attainable for heat.

Electricity directly produces lieat, as shown in the ignited
wire, the electric spark, and the voltaic arc : in the latter
the most intense heat with which we are acquainted — so in-
tense, indeed, that it cannot be measured, as every sort of
matter is dissipated by it.

In the phenomenon of electrical ignition, as shown by a
heated conjunctive wire, the relation of force and resistance,
and the correlative character of the two forces, electricity and
heat, are strikingly demonstrated. Let a thin wire of plati-
num join the terminals of a voltaic battery of suitable power,
the wire will be ignited, and a certain amount of chemical
action will take place in the cells of the battery — a definite
quantity of zinc being dissolved and of hydrogen eliminated
in a given time. If now the platinum wire be immersed in
water, the heat will, from the circulating currents of the
liquid, be more rapidly dissipated, and we shall instantly find
that the chemical action in the battery will be increased, more
zinc will be dissolved, and more hydrogen eliminated for the
same time ; the heat being conveyed away by the water,
more chemical action is required to generate it, just as more
fuel is required in proportion as evaporation is more
rapid.

Reverse the experiment, and instead of placing the wire
in water, place it in the flame of a spirit lamp, so that the
force of heat meets with greater resistance to its dissipation.
We now find that the chemical action is less than in the first
or normal experiment. If the wire be placed in other differ-
ent gaseous or liquid media, we shall find that the chemical

ELECTKICITY. 107

action of the battery will be proportioned to the facility with
which the heat is circulated or radiated by these media, and
we thus establish an alternating reciprocity of action between
these two forces : a similar reciprocity may be established
between electricity and motion, magnetism and motion, and
so of other forces. If it cannot be realised with all, it is
probably because we have not yet eliminated interfering ac-
tions. If we carefully think over the matter, we shall, unless
I am much mistaken, arrive at the conclusion that it cannot
be otherwise, unless it be supposed that a force can arise from
nothing — can exist without antecedent force.

In the phenomenon of the voltaic arc, the electric spark,
&c., to which I have already adverted, electricity directly
produces light of the greatest known intensity. It directly
produces magnetism^ as shown by Oersted, who first distinctly
proved the connection between electricity and magnetism.
These two forces act upon each other, not in straight lines,
as all other known forces do, but in a rectangular direction ;
that is, bodies affected by dynamic electricity, or the conduits
of an electric current, tend to place magnets at right angles
to them ; and, conversely, magnets tend to place bodies con-
ducting electricity at right angles to them. Thus an electric
current appears to have a magnetic action, in a direction
cutting its o^vvn at right angles ; or, supposing its section to
be a circle, tangential to it : if, then, we reverse the position,
and make the electric current form a series of tangents to an
imaginary cylinder, this cylinder should be a magnet. This
is effected in practice by coiling a wire as a helix or spiral,
and this, when conducting an electrical current, is to all in-
tents and purposes a magnet. A soft iron core placed within
such a helix has the property of concentrating its power, and
then we can, by connection or disconnection with the source
of electricity, instantly make or unmake a most powerful
magnet.

We may figure to the mind electrified and magnetised

t08 CORRELATION OF PHYSICAL FORCES.

matter, as lines of which the extremities repel each other in
a definite direction; thus, if a line A b represent a wire
affected by electricity, and superposed on c d a wire affected
by magnetism, the extreme points a and b will be repelled to
the farthest distances from the points c and d, and the line a
B be at right angles to the line c d ; and so, if the lines be
subdivided to any extent, each will have two extremities or
poles repulsive of those of the other. If the line of matter
affected by electricity be a liquid, and consequently have
^entire mobility of particles, a continuous movement will be
produced by magnetism, each particle successively tending,
as it were, to fly off at a tangent from the magnet : thus,
place a flat dish containing acidulated water on the poles of a
powerful magnet, immerse the terminals of a voltaic battery
in the liquid just above the magnetic poles, so that the lines
of electricity and of magnetism coincide ; the water will now
assume a movement at right angles to this line, flowing con-
tinously, as if blown by an equatorial wind, which may be
made east or west with reference to the magnetic poles by
altering the direction of the electrical current : a similar effect
may be produced with mercury. These cases afford an
additional argument to those previously mentioned of the
particles of matter being affected by the forces of electricity
and magnetism in a way irreconcilable with the fluid or
ethereal hypothesis.

The representation of transverse direction by magnetism
and electricity appears to have led Coleridge to parallel it by
the transverse expansion of matter, or length and breadth,
though he injured the parallel by adding galvanism as depth :
whether a third force exists which may bear this relation to
electricity and magnetism is a question upon which we have
no evidence.

The ratio which the attractive magnetic force produced
bears to the electric current producing it has been investigat-
ed by many experimentalists and mathematicians. The data

ELECTEICITY. 109

are so numerous and so variable, that it is difficult to arrive
at definite results. Thus the relative size of the coil and the
iron, the temper or degree of hardness of the latter, its shape,
or the proportions of length to diameter, the number of coils
surrounding it, the conducting power of the metal of which
the coils are formed, the size of the keeper or iron in which
magnetism is induced, the degree of constancy of the bat-
tery, &c., complicate the experiments.

The most trustworthy general relation which has been as-
certained is, that the magnetic attraction is as the square of
the electric force ; a result due to the researches of Lenz and
Jacobi, and also of Sir W. S. Harris.

Lastly, electricity produces chemical affinity ; and by its
agency we are enabled to obtain effects of analysis or synthe-
sis with which ordinary chemistry does not furnish us. Of
these effects we have examples in the brilliant discoveries, by
Davy, of the alkaline metals, and in the peculiar crystalline
compounds made known by Crosse and Becquerel.

Y.— LIGHT.

IN entering on the subject of Light, it will be well to de-
scribe briefly, and in a manner as far as may be inde-
pendent of theory, the effects to which the term polarisation
has been applied.

"When light is reflected from the surface of water, glass,
or many other media, it undergoes a change which disables it
from being again similarly reflected in a direction at right
angles to that at which it has been originally reflected.
Light so affected is said to be polarised ; it will always be
capable of being reflected in planes parallel to the plane in
which it has been first reflected, but incapable of being re-
flected in planes at right angles to that plane. At planes
having a direction intermediate between the original plane of
reflection, and a plane at right angles to it, the light will be
capable of being partially reflected, and more or less so ac-
cording as the direction of the second plane of reflection is
more or less coincident with the original plane. Light, again,
when passed through a crystal of Iceland spar, is what is
termed doubly refracted, i. e. split into two divisions or beams,
each having half the luminosity of the original incident light ;
each of these beams is polarised in planes at right angles to
each other ; and if they be intercepted by the mineral tour-
maline, one of them is absorbed, so that only one polarised
beam emerges. Similar effects may be produced by certain

LIGHT. Ill

other reflections or refractions. A ray of light once polaris-
ed in a certain plane continues so affected throughout its
whole subsequent course ; and at any indefinite distance from
the point where it originally underwent the change, the di-
rection of the plane will be the same, provided the media
through which it is transmitted be air, water, or certain other
transparent substances which need not be enumerated. If,
however, the polarised ray, instead of passing through water,
be made to pass through oil of turpentine, the definite direc-
tion in which it is polarised will be found to be changed ; and
the change of direction will be greater according to the
length of the column of interposed liquid. Instead of being
an uniform plane, it wiU have a curvilinear direction,
similar to that which a strip of card would have if forced
along two opposite grooves of a rifle-barrel. This curious
effect is produced in different degrees by different media.
The direction also varies ; the rotation, as it is termed, being
sometimes to the right hand and sometimes to the left, accord-
ing to the peculiar molecular character of the medium through
which the polarised ray is transmitted.

Light is, perhaps, that mode of force the reciprocal rela-
tions of which with the others have been the least traced
out. Until the discoveries of Niepce, Daguerre, and Talbot,
very little could be definitely predicated of the action of light
in producing other modes of force. Certain chemical com-
pounds, among which stand pre-eminent the salts of sUver,
have the property of suffering decomposition when exposed
to light. If, for instance, recently formed chloride of silver
be submitted to luminous rays, a partial decomposition en-
sues ; the chlorine is separated and expelled by the action of
light, and the silver is precipitated. By this decomposition
the colour of the substance changes from white to blue. If
now, paper be impregnated with chloride of silver, which can
be done by a simple chemical process, then partially covered
with an opaque substance, a leaf for example, and exposed to

112 CORRELATION OF PHYSICAL FORCES.

a strong light, the chloride will be decomposed in all those
parts of the paper where the light is not intercepted, and we
shall have, by the action of light, a white image of the leaf
on a purple ground. If similar paper be placed in the focus
of a lens in a camera-obscura, the objects there depicted will
decompose the chloride, just in the proportion in which they
are luminous ; and thus, as the most luminous parts of the im-
age will most darken the chloride, we shall have a picture of
the objects with reversed lights and shadows. The picture
thus produced would not be permanent, as subsequent expos-
ure would darken the light portion of the picture : to fix it,
the paper must be immersed in a solution which has the pro-
perty of dissolving chloride of silver, but not metallic silver.
Iodide of potassium will effect this ; and the paper being
washed and dried will then preserve a permanent image of
the depicted objects. This was the first and simple process
of Mr. Talbot ; but it is defective as to the purposes aimed
at, in many points. First, it is not sufficiently sensitive, re-
quiring a strong light and a long time to produce an image ;
secondly, the lights and shadows are reversed ; and thirdly,
the coarse structure of the finest paper does not admit of the
delicate traces of objects being distinctly impressed. These
defects have been to a great extent remedied by a process
subsequently discovered by Mr. Talbot, and which bears his
name, and which has led to the collodion process, and others
unnecessary to be detailed here.

The photographs of M. Daguerre, with which all are now
familiar, are produced by holding a plate of highly-polished
silver over iodine. A thin film of iodide of silver is thus
formed on the surface of the metal ; and when these iodized
plates are exposed in the camera, a chemical alteration takes
place. The portions of the plate on which the light has im-
pinged part with some of the iodine, or are otherwise changed
— for the theory is somewhat doubtful — soas to be capable
of ready amalgamation. When, therefore, the plate is placed

LIGHT. 113

over the vapour of heated mercury, the mercury attaches it-
self to the portions affected by light, and gives them a white
frosted appearance ; the intermediate tints are less affected,
and those parts where no light has fallen, by retaining
their original polish, appear dark ; the iodide of silver is
then washed off by hyposulphite of soda, which has the
property of dissolving it, and there remains a picture
in which the lights and shadows are as in nature, and the
molecular uniformity of the metallic surface enables the
most microscopic details to be depicted with perfect accu-
racy. By using chloride of iodine, or bromide of iodine,
instead of iodine, the equilibrium of chemical forces is ren-
dered still more unstable, so that images may be taken in an
indefinitely short period — a period practically instantaneous.

It would be foreign to the object of this essay to enter
upon the many beautiful details into which the science of
photography has branched out, and the many valuable discov-
eries and practical applications to which it has led. The
short statement I have given above is perhaps superfluous, as,
though they were new and surprising at the period when these
Lectures were delivered, photographic processes have now be-
come familiar, not only to the cultivator of science, but to
the artist and amateur ; the important point for consideration
here is that light will chemically or molecularly affect mat-
ter. Not only will the particular compounds above selected
as instances be changed by the action of light ; but a vast
number of substances, both elementary and compound, are
notably affected by this agent, even those apparently the most
unalterable in character, such as metals : so numerous, in-
deed, are the substances affected, that it has been supposed,
not without reason, that matter of every description is altered
by exposure to light.

The permanent impression stamped on the molecules of
matter by light can be made to repeat itself by the same
agency, but always with decreasing force. Thus a photo-

114 CORRELATION OF PHYSICAL FORCES.

graph placed opposite a camera containing a sensitive plate
will be reproduced, but if the size of the image be equal to
the picture, the second picture will be fainter than the first,
and so on. Thus again, a photograph taken on a dull day
cannot, by being placed in bright sunshine be made to repro-
duce a second photograph of the same size and more distinct-
ly marked than itself; I at least have never succeeded in
such reproduction, and I am not aware that others have : the
image loses in intensity as light itself does by each transmis-
sion. The surface of the metal or paper may give a brighter
image from its being exposed to a more intense light, but the
photographic details are limited to the intensity of the first
impression, or rather to something short of this. A question
of theoretical interest arises from the consideration of these
reproduced photographs. We know that the luminosity of
the image at the focus of a telescope is limited by the area
of the object-glass. The image of any given object cannot
be intensified by throwing upon it extraneous light ; it is in-
deed diminished in intensity, and when for certain purposes
astronomers illuminate the fields of their telescopes, they are
obliged to be contented with a loss of intensity in the telescopic
image.

Now, let us suppose that the minutest details in the image
of an object seen in a given telescope, and with a given pow-
er, are noted ; that then a photographic plate is placed in the
focus of the same telescope so as to obtain a permanent im-
pression of the image which has been viewed by the eye-glass.
Could the observer, by throwing a beam of condensed light
upon the photograph, enable himself to bring out fresh details ?
or in other words, could he use with advantage a higher pow-
er applied to the illuminated photograph ?

It is, perhaps, hardly safe to answer a priori this question ;
but the experiment of reproducing photographs would seem to
show that more than the initial light cannot be got, and that we
cannot expect to increase telescopic power by photography,

LIGHT. 115

though we may render observations more convenient ; may
by its means fix images seen on rare and favourable occasions,
and may preserve permanent and infallible records of the
past state of astronomical objects.

The effect of light on chemical compounds aifords us a
striking instance of the extent to which a force, ever active,
may be ignored through successive ages of philosophy. If
we suppose the walls of a large room covered with photo-
graphic apparatus, the small amount of light reflected from
the face of a person situated in its centre would simulta-
neously imprint his portrait on a multitude of recipient sur-
faces. Were the cameras absent, but the room coated with
photographic paper, a change would equally take place in
every portion of it, though not a reproduction of form and
figure. As other substances not commonly called photo-
graphic are known to be afifected by light, the list of which
might be indefinitely extended, it becomes a curious object of
contemplation to consider how far light is daily operating
changes in ponderable matter — ^how far a force, for a long
time recognised only in its visual effects, may be constantly
producing changes in the earth and atmosphere, in addition
to the changes it produces in organised structures which are
now beginning to be extensively studied. Thus every portion
of light may be supposed to write its own history by a change
more or less permanent in ponderable matter.

The late Mr. George Stephenson had a favourite idea,
which would now be recognised as more philosophical than it
was in his day, viz. that the light, which we nightly obtain
from coal or other fuel, was a reproduction of that which had
at one time been absorbed by vegetable structures from the
sun. The conviction that the transient gleam leaves its per-
manent impress on the world's history, also leads the mind
to ponder over the many possible agencies of which we of the
present day may be as ignorant as the ancients were of the
chemical action of light.

116 COKEELATION OF PHYSICAL FOECES.

I have used the term light, and affected by light, in speak-
ing of photographic effects ; but, though the phenomena de-
rived their name from light, it has been doubted by many
competent investigators whether the phenomena of photo-
graphy are not mainly dependent upon a separate agent ac-
companying light, rather than upon light itself. It is, indeed,
difficult not to believe that a picture, taken in the focus of a
camera-obscura, and which represents to the eye all the gra-
dations of light and shade shown by the original luminous
image, is not an effect of light; certain it is, however, that
the different coloured rays exercise different actions upon va-
rious chemical compounds, and that the effects on many, per-
haps on most of them, are not proportionate in intensity to
the effects upon the visual organs. Those effects, however,
appear to be more of degree than of specific difference ; and,
without pronouncing myself positively upon the question,
hitherto so little examined, I think it will be safer to regard
the action on photographic compounds as resulting from a
function of light. So viewing it, we get light as an initia-
ting force, capable of producing, mediately or immediately,
the other modes of force. Thus, it immediately produces
chemical action ; and having this, we at once acquire a means
of producing the others. At my Lectures in 1843, I showed
an experiment by which the production of all the other modes
of force by light is exhibited : I may here shortly describe it.
A prepared daguerreotype plate is enclosed in a box filled
with water, having a glass front with a shutter over it. Be-
tween this glass and the plate is a gridiron of silver wire ;
the plate is connected with one extremity of a galvanometer
coil, and the gridiron of wire with one extremity of a Bre-
guet's helix — an elegant instrument, formed by a coil of two
metals, the unequal expansion of which indicates slight
changes in temperature — the other extremities of the galva-
nometer and helix are connected by a wire, and the needles
brought to zero. As soon as a beam of either daylight or

LIGHT. 117

the oxyhydrogen light is, by raising the shutter, permitted to
impinge upon the plate, the needles are deflected. Thus,
light being the initiating force, we get chemical action on the
plate, electricity circulating through the wires, magnetism in
the coil, heat in the helix, and motion in the needles.

If two plates of platinum be . placed in acidulated water,
and connected with a delicate galvanometer, the needle of
this is always deflected, a result due to films of gas or other
matter on the surface of the platinum, which no cleaning can
remove. If, after the needle has returned to zero, which will
not be the case for some hours or even days, one of the plat-
inum surfaces be exposed to light, a fresh deflection of the
needle takes place, due, as far as I have been able to resolve
it, to an augmentation of the chemical action which had occa-
sioned the original deflection, for the deviation is in the same
direction. If, instead of white light, coloured light be per-
mitted to impinge on the plate, the deviation is greater with
blue than with red or yellow light, showing, in addition to
other tests, that the effect is not due to the heat of the sun's
rays, as the calorific effects of light are greater with red than
with blue light, while the chemical effects are the inverse.

There are other apparently more direct agencies of light
in producing electricity and magnetism, such as those ob-
served by Morichini and others, as well as its effects upon
crystallization ; but these results have hitherto been of so in-
definite a character, that they can only be regarded as pre-
senting fields for experiment, and not as proving the relations
of light to the other forces.

Light would seem directly to produce heat in the phenom-
ena of what is termed absorption of light : in these we find
that heat is developed in some proportion to the disappear-
ance of light. To take the old experiment of placing a se-
ries of different coloured pieces of cloth upon snow exposed
to sunshine, the black cloth absorbing the most light, and de-
veloping the most heat, sinks more deeply in the snow than

118 COEEELATION OF PHYSICAL FOECES.

any others ; the other colours or shades of colour sink the
more deeply in proportion as they absorb or cause to disap-
pear the more light, until we come to the white cloth, which
remains upon the surface. The heating powers of different
colours are, however, not by any means in exact proportion
to the intensity of their light as affecting the visual organs.
Thus red light, when produced by refraction from a prism of
glass, produces greater heating effect than yellow light in the
phenomena of absorption, as has been observed by Sir W.
Herschel. The red rays appear, however, to produce a dy-
namic effect greater than any of the others ; thus they pene-
trate water to a greater depth than the other colours ; but,
according to Dr. Seebeck, we get a further anomaly, viz.
that when light is refracted by a prism of water the yellow
rays produce the greater heating effect. The subject, there-
fore, requires much more experiment before we can ascertain
the rationale of the action of the forces of light and heat in
this class of phenomena.

In a former edition of this Essay, I suggested the follow-
ing experiment on this subject : — ^Let a beam of light be
passed through two plates of tourmaline, or similar sub-
stance, and the temperature of the second plate, or that on
which the light last impinges, be examined by a delicate ther-
moscope, first when it is in a position to transmit the polar-
ised beam coming from the first plate, and secondly when it
has been turned round through an arc of 90°, and the polar-
ised beam is absorbed. I expected that, if the experiment
were carefully performed, the temperature of the second plate
would be more raised in the second case than in the first, and
that it might afford interesting results when tried with light
of different colours. I met with difficulties in procuring a
suitable apparatus, and was endeavouring to overcome them
when I found that Knoblauch had, to some extent, realised
this result. He finds that, when a solar beam, polarised in a
certain plane, is transmitted perpendicularly to the axis of a

LIGHT. 119

crystal of brown quartz or tourmaline, the heat is transmit-
ted in a smaller proportion than when the beam passes along
the direction of the axis of the crystal.

It is generally — as far as I am aware, universally — ^true
that, while light continues as light, even though reflected or
transmitted by different media, little or no heat is developed :
and, as far as we can judge, it would appear that, if a me-
dium were perfectly transparent, or if a surface perfectly re-
flected light, not the slightest heating effect would take place ;
but, wherever light is absorbed, then heat takes its place, af-
fording us apparently an instance of the conversion of light
into heat, and of the fact that the force of light is not, in fact,
absorbed or annihilated, but merely changed in character,
becoming in this instance converted into heat by impinging
on solid matter, as in the instance mentioned in treating of
heat, this force was shown to be converted into light by im-
pinging on solid matter. As, however, I have before ob-
served, this correlation of light and heat is not so distinct, as
with the other affections of matter. One experiment, indeed,
of Melloni, already mentioned, would seem to show that
light may exist in a condition in which it does not produce
heat, which our instruments are able to detect ; but some
doubt has recently been thrown on the accuracy of this ex-
periment ; probably the substances themselves through which
the light is transmitted would be found to have been heated.

The recipient body, or that upon which light f/apinges,
seems to exercise as important an influence on ovjt percep-
tions of light as the emittent body, or that from which the
light first proceeds. The recent experiments o. Sir John
Herschel and Mr. Stokes show that radiant impulies, which,
falling on certain bodies, give no effect of light, become lu-
minous when falling on other bodies.

Thus, let ordinary solar light be refracted by t,. prism (the
best material for which is quartz), and the spectrum received
on a sheet of paper, or of white porcelain ; looking: un tho

120 CORRELATION OF PHYSICAL FORCES.

paper, the eye detects no light beyond the extreme violet
rays. If, therefore, an opaque body be interposed so as just
to cut off the whole visible spectrum, the paper would be
dark or invisible, with the exception of some slight illumina-
tion from light reflected by the air and surrounding bodies.
Substitute for that portion of the paper which was beyond
the visible spectrum a piece of glass tinged by the oxide of
uranium, and the glass is perfectly visible ; so with a bottle
of sulphate of quinine, or of the juice of horse-chestnuts, or
even paper soaked in these latter solutions. Other substances
exhibit this effect in different degrees ; and among the sub-
stances which have hitherto been considered perfectly analo-
gous as to their appearance when illuminated, notable differ-
ences are discovered. Thus it appears that emanations
which give no impression of light to the eye, when imping-
ing on certain bodies, become luminous when impinging on
others. "We might imagine a room so constructed that such
emanations alone are permitted to enter it, which would be
dark or light according to the substance with which the walls
were coated, though in full daylight the respective coatings
of the walls would appear equally white ; or, without alter-
ing the coating of the walls, the room exposed to one class
of rays, might be rendered dark by windows which would be
transparent to another class.

If, instead of solar light, the electrical light be employed
for similar experiments, an equally striking effect can actually
be produced. A design, drawn on white paper with a solu-
tion of sulphate of quinine and tartaric acid, is invisible by
ordinary light, but appears with beautiful distinctness when
illuminated by the electric light. Thus, in pronouncing upon
a luminous effect, regard must be had to the recipient as well
as to the emittent body. That which is, or becomes, light
when it fells upon one body is not light when it falls upon
another. Probably the retinas of the eyes of different per-
sons differ to some extent in a similar manner ; and the same

LIGHT. 121

substance, illuminated by the same spectrum, may present
different appearances to different persons, the spectrum ap-
pearing more elongated to the one than to the other, so that
what is light to the one is darkness to the other. A depend-
ence on the recipient body may also, to a great extent, be
predicated of heat. Let two vessels of water, the contents
of the one clear and transparent, of the other tinged by some
colouring matter, be suspended in a sunamer's sun ; in a very
short time a notable difference of temperature will be ob-
served, the coloured having become much hotter than the
clear liquid. If the first vessel be placed at a considerable
distance from the surface of the earth, and the second near
the surface, the difference is still more considerable. Carry-
ing on this experiment, and suspending the first over the top
of a high mountain, and the second in a valley, we may ob
tain so great a difference of temperature, that animals whose
organization is suited for the one temperature could not live
in the other, and yet both are exposed to the same limiinous
rays at the same time, and substantially at the same distance
from the emittent body — the substance nearer the sun is in
fact colder than the more remote. So, with regard to the
medium transmitting the influence : a green-house may have
its temperature considerably varied by changing the glass of
which its roof is made.

These effects have an important bearing on certain cos-
mical questions which have lately been much discussed, and
should induce the greatest caution in forming opinions on
such subjects as light and heat on the sun's surface, the tem-
perature of the planets, &c. This may depend as much upon
their physical constitution as upon their distance from the
sun. Indeed, the planet Mars gives us a highly probable ar-
gument for this ; for, notwithstanding that it is half as far
again from the sun as the earth is, the increase of the white
tracts at its poles during its winter, and their* diminution dur-
ing its summer, show that the temperature of the surface of
6

122 COEEELATION OF PHYSICAL FORCES.

this planet oscillates about that of the freezing point of water,
as do the analogous zones of our planet. It is true, in this
we assume that the substance thus changing its state is water,
but, considering the many close analogies of this planet with
the earth, and the identity in appearance of these very effects
with what takes place on the earth, it seems a highly proba-
ble assumption.

So it by no means necessarily follows, that because Venus
is nearer to the sun than the earth, that planet is hotter than
our globe. The force emitted by the sun may take a differ-
ent character at the surface of each different planet, and
require different organisms or senses for its appreciation.
Myriads of organised beings may exist imperceptible to our
vision, even if we were among them ; and we might be also
imperceptible to them !

However vain it may be, in the present state of science,
to speculate upon such existences, it is equally vain to assume
identity or close approximations to our own forms in those
beings which may people other worlds. From analogical
reasoning, or from final causation, if that be admitted, we
may feel convinced that the gorgeous globes of the universe
are not unpeopled deserts ; but whether the denizens of other
worlds are more or less powerful, more or less intelligent,
whether they have attributes of a higher or lower class than
ourselves, is at present an utterly hopeless guessing.

Specific gravity and intelligence have no necessary con-
nexion. On our own planet five senses, and a mean density
equal to that of water, are not invariably associated with in-
tellectual or moral greatness, and the many arguments wliich
have been used to prove that suns and planets other than the
earth are uninhabited, or not inhabited by intellectual beings,
might, mutatis mutandis^ equally be used by the denizens of
a sun or planet to prove that this world was uninhabited.

Men are too apt, because they are men, because their
existence is the one thing of all importance to themselves, to

LIGHT. 123

frame schemes of the universe as though it was formed for
man alone : painted by an artist of the sun, a man might not
represent so prominent an object of creation as he does
when represented by his own pencil.

Light was regarded, by what was termed the corpuscular
theory, as being in itself matter or a specific fluid emanating
from lumiQous bodies, and producing the effects of sensation
by impinging on the retina. This theory gave way to the un-
dulatory one, which is generally adopted in the present day,
And which regards light as resulting from the undulation of a
specific fluid to which the name of ether has been given,
which hypothetic fluid is supposed to pervade the universe,
and to penetrate the pores of all bodies.

In a Lecture delivered in January 1842, when I first
publicly advanced the views advocated in this Essay, I stated
that it appeared to me more consistent with known facts to
regard light as resulting from a vibration or motion of the
molecules of matter itself, rather than from a specific ether
pervading it ; jnst as sound is propagated by the vibrations
of wood, or as waves are by water. I am not here speaking
of the character of the vibrations of light, sound, or water,
which are doubtless very different from each other, but am
only comparing them so far as they illustrate the propagation
of force by motion in the matter itself.

I was not aware, at the time that I first adopted the
above view, and brought it forward in my Lectures, that the
celebrated Leonard Euler had published a somewhat similar
theory ; and, though I suggested it without knowing that it
had been previously advanced, I should have hesitated in
reproducing it had I not found that it was sanctioned by so
eminent a mathematician as Euler, who cannot be supposed
to have overlooked any irresistible argument against it — ^the
more so in a matter so much controverted and discussed as
the undulatory theory of light was in his time.

Although this theory has been considered defective by a

124 CORRELATION OP PHYSICAL FORCES.

philosopher of high repute, I cannot see the force of the
arguments by which it has been assailed ; and therefore, for
the present, though with diffidence, I stiU adhere to it. The
fact itself of the correlation of the different modes of force is
to my mind a very cogent argument in favour of their being
affections of the same matter ; and though electricity, magnet-
ism, and heat might be viewed as produced by undulations of
the same ether as that by means of which light is supposed to
be produced, yet this hypothesis offers greater difficulties with
regard to the other affections than with regard to light : many
of these difficulties I have already alluded to when treating of
electricity ; thus conduction and non-conduction are not ex-
plained by it ; the transmission of electricity through long
wires in preference to the air which surrounds them, and
which must be at least equally pervaded by the ether, i^
irreconcilable with such an hypothesis. The phenomena ex-
hibited by these forces afford, as I think, equally strong evi-
dence with those of light, of ordinary matter acting from par-
ticle to particle, and having no action at a distance. I have
already instanced the experiments of Faraday on electrical
induction, showing it to be an action of contiguous particles,
which are strongly in favour of this view, and many experi-
ments which I have made on the voltaic arc, some of which
I have mentioned in this Essay, are, to my mind, confirma-
tory of it.

If it be admitted that one of the so-caUed imponderables is
a mode of motion, then the fact of its being able to produce
the others, and be produced by them, renders it highly diffi-
cult to conceive some as molecular motions and others as
fluids or undulations of an ether. To the main objection of
Dr. Young, that all bodies would have the properties of solar
phosphorus if light consisted in the undulations of ordinary
matter, it may be answered that so many bodies have this
property, and with so great a variety in its duration, that
non constat all may not have it, though for a time so short

LIGHT. 125

that the eye cannot detect its duration. M. E. Becquerel
has made many experiments which support this view ; the
fact of the phosphorescence by insolation of a large number
of bodies, is in itself evidence of the matter of which they are
composed being thrown into a state of undulation, or at aU
events molecularly affected by the impact of light, and is
therefore an argument in support of the view to which objec-
tion is taken. Dr. Young admits that the phenomena of
solar phosphorus appear to resemble greatly the sympathetic
sounds of musical instruments, which are agitated by other
sounds conveyed to them through the air, and I am not aware
that he gives any explanation of these effects on the ethereal
hypothesis.

Some curious experiments of M. Niepce de St. Victor
seem also to present an analogy in luminous phenoniena to
sympathetic sounds. An engraving which has been kept for
some days in the dark is half covered by an opaque screen,
and then exposed to the sun ; it is then removed from the
light, the screen taken away, and the engraving placed oppo-
site, and at a short distance from, photographic paper : an
inverted image of that portion of the engraving which has
been exposed to the sun is produced on the photographic
paper, while the part which had been covered by the screen
is not reproduced. If the engraving, after exposure, is
allowed to remain in contact with white paper for some hours,
and the white paper is then placed upon photographic paper,
a* faint image of the exposed portion of the engraving is repro-
duced. Similar results are produced by mottled marble ex-
posed to the sun ; an invisible tracing on paper by a fluores-
cent body, sulphate of quinine, is, after insolation, reproduced
on the photographic paper. Insolated paper retains the power
of producing an impression for a very long period, if it is kept
in an opaque tube hermetically closed.

It is right to observe that these effects are supposed by
many to be due to chemical emanations proceeding from the

126 COKRELATION OF PHYSICAL FORCES.

substances exposed to the sun, and which are believed to have
undergone some chemical change by this exposure. It is
desirable to await further experiment before fonning a decid-
ed opinion.

The analogies in •the progression of sound and light are
very numerous : each proceed in straight lines, until inter-
rupted ; each is reflected in the same manner, the angles of
incidence and reflexion being equal ; each is alternately nulli-
fied and doubled in intensity by interference ; each is capable
of refraction when passing from media of different density :
this last effect of sound, long ago theoretically determined,
has been experimentally proved by Mr. Sondhauss, who con-
structed a lens of films of collodion, which, when filled with
carbonic acid, enabled him to hear the ticking of a watch
placed in one focus of the lens, the ear of the experimenter
being in the opposite focus. The ticking was not heard
when the watch was moved aside from the focal point, though
it remained at an equal distance from the ear. An experi-
ment of M. Dove seems, indeed, to show an effect of polari-
sation of sound.

The phenomena presented by heat, viewed according to
the dynamic theory, cannot be explained by the motion of an
imponderable ether, but involve the molecular actions of
ordinary ponderable matter. The doctrine of propagation by
undulations of ordinary matter is very generally admitted by
those who support the dynamical theory of heat ; but the
analogies of the phenomena presented by heat and light are
so close, that I cannot see how a theory applied to the one
agent should not be applicable to the other. When heat is
transmitted, reflected, refracted, or polarised, can we view
that as an affection of ordinary matter, and when the same
effects take place with light, view the phenomena as pro-
duced by an imponderable ether, and by that alone ?

An objection that immediately occurs to the mind in
reference to the ethereal hypothesis of light is, that the most

LIGHT. 127

porous bodies are opaque ; cork, charcoal, pumice stone, dried
and moist wood, &c., all very porous and very light, are all
opaque. This objection is not so superficial as it might seem
at first sight. The theory which assumes that light is an
undulation of an ethereal medium pervading gross matter,
assumes the distances between the molecules or atoms of
matter to be very great. Matter has been likened by Demo-
critus, and by many modem philosophers, to the starry firma-
ment, in which, though the individual monads are at immense
distances from each other, yet they have in the aggregate a
character of unity, and are firmly held by attraction in their
respective positions and at definite distances. Now, if matter
be built up of separate molecules, then, as far as our knowl-
edge extends, the lightest bodies would be those in which the
molecules are at the greatest distances, and those in which
any undulation of a pervading medium would be the least
interfered with by the separated particles — such bodies should
consequently be the most transparent.

If, again, the analogy of the starry firmament held good,
in this case an undulation or wave proportioned to the indivi-
dual monads would be broken up by the number of them, and
the very appearance of continuity which results, as in the
milky way, from each point of vision being occupied by one
of the monads, would show that at some portion of its pro-
gress the wave is interrupted by one of them, so that the
whole may be viewed in some respect as a sheet of ordinary
matter interposed in the ethereal expanse.

Even then, if it be admitted that a highly elastic medium
pervades the interspaces, the sejJarate masses as a whole must
exercise an important influence on the progress of the wave.

Sound or vibrations of air meeting with a screen, or, as
it were, sponge of diffused particles, would be broken up and
dispersed by them ; but if they be sufficiently continuous to
take up the vibration and propagate it themselves, the sound
continues comparatively unimpaired.

128 COEEELATION OF PHYSICAL F0KCE8.

With regard, however, to liquid and gaseous bodies, there
are very great difficulties in viewing them as consisting of
separate and distant molecules. If, for instance, we assume
with Young that the particles in water are at least as distant
from each other comparatively as 100 men would be if dis-
persed at equal distances over the surface of England, the dis-
tance of these particles, when the water is expanded into
steam, would be increased more than forty times, so that the
100 men would be reduced to two, and by further increasing
the temperature this distance may be indefinitely increased ;
adding to the effects of temperature rarefaction by the air-
pump, we may again increase the distance, so that, if w^ as-
sume any original distance, we ought, by expansion, to in-
crease it to a point at which the distance between molecule
and molecule should become measurable. But no extent of
rarefaction, whether by heat or the air-pump, or both, makes
the slightest change in the apparent continuity of matter ;
and gases, I find, retain their peculiar character, as far as a
judgment of it can be formed from its effect on the electric
spark, throughout any extent of rarefaction which can exper-
imentally be applied to them : thus the electric spark in prot-
oxide of nitrogen, however attenuated, presents a crimson
tint, that in carbonic oxide a greenish tint.

Without, however, entering on the metaphysical enquiry
as to the constitution of matter (or whether the atomic phil-
osophers or the followers of Boscovich are right), a question
which probably human appliances will never answer : and
even admitting that an ethereal medium, not absolutely im-
ponderable as asserted by many, but of extreme tenuity, per-
vades matter, stiU ordinary or non-ethereal matter itself must
exercise a most important action upon the transmission of
light ; and Dr. Young, who opposed the theory of Euler, that
light was transmitted by undulations of gross matter itself,
just as sound is, was afterwards obliged to call to his assis-
tance the vibrations of the ponderable matter of the refract-

LIGHT. 129

ing media, to explain why rays of all colours were not equal-
ly refracted, and other difficulties. One of his arguments in
support of the existence of a permeating ether was, " that a
medium resembling in many properties that which has been
denominated Ether does exist, is undeniably proved by the
phenomena of electricity." This seems to me, if I may ven-
ture to say so of anything proceeding from so eminent a man,
scarcely logical : i#is supporting one hypothesis by another,
and considering that to be proved which its most strenuous
advocates admit to be surrounded by very many difficulties.

If it be said that there is not sufficient elasticity in ordi-
nary matter for the transmission of undulations with such ve-
locity as light is known to travel, this may be so if the vibra-
tions be supposed exactly analogous to those of sound ; but
that molecular motion can travel with equal and even greater
velocity than light, is shown by the rapidity with which elec-
tricity traverses a metal wire where each particle of metal is
undoubtedly affected. It has, moreover, been shown by the
experiments of Mr. Latimer Clarke upon a length of wire
of 760 miles, that whatever be the intensity of electrical cur-
rents, they are propagated with the same velocity provided
the effects of lateral induction be the same — a striking anal-
ogy with one of the effects observed in the propagation of
light and sound. The effects observed by MM. Fizeau and
Foucault, of the slower progression of light in proportion as
the transmitting medium is more dense, seem to me in favour
of the view here advocated ; as a greater degree of heat
would be produced by light in proportion to the density of
the medium, force would be thus carried off, and the molecular
system disturbed so that the progress of the motion should be
more slow ; but so many considerations enter into this question,
and the phenomena are so extremely complex, that it would
be rash to hazard any positive opinion.

Dr. Young ultimately came to the conclusion that it was
simplest to consider the ethereal medium, together with the
6*

130 COEEELATION OF PHYSICAL FOECES.

material atoms of the substance, as constituting together a
compound medium denser than pure ether, but not more elas-
tic. Ether might thus be viewed as performing the functions
which oil does with tracing paper, giving continuity to the
particles of gross matter, and in the interplanetary spaces
forming itself the medium which transmits the undulations.

Since the period when Huyghens, Euler, and Young, the
fathers of the undulatory theory, applied their great minds to
this subject, a mass of experimental data has accumulated,
%11 tending to establish the propositions, that whenever matter
transmitting or reflecting light undergoes a structural change,
the light itself is affected, and that there is a connection or
parallelism between the change in the matter and the change
in the affection of light, and conversely that light will modify
or change the structure of matter and impress its molecules
with new characteristics.

Transparency, opacity, refraction, reflection, and colour
were phenomena known to the ancients, but suflicient attention
does not appear to have been paid by them to the molecular
states of the bodies producing these effects ; thus the trans-
parency or opacity of a body appears to depend entirely upon
its molecular arrangement. If striae occur in a lens or glass
through which objects are viewed, the objects are distorted :
increase the number of these striae, the distortion is so in-
creased that the objects become invisible, and the glass ceases
to be transparent, though remaining translucent ; but alter
completely the molecular structure, as by slow solidification,
and it becomes opaque. Take, again, an example of a liquid
and a gas : a solution of soap is transparent, air is transpar-
ent, but agitate them together so as to form a froth or lather,
and this, though consisting of two transparent bodies, is
opaque ; and the reflection of light from the surface of these
bodies, when so intermixed, is strikingly different from its re-
flection before mixture, in the one case giving to the eye a
mere general effect of whiteness, in the other the images of
objects in their proper shapes and colours.

LIGHT. 131

To take a more refined instance : nitrogen is perfectly
colourless, oxygen is perfectly colourless, but chemically uni-
ted in certain proportions they form nitrous acid, a gas which
has a deep orange brown colour. I know not how the col-
our of this gas, or of such gases as chlorine or vapour of
iodine, can be accounted for by the ethereal hypothesis, with-
out calling in aid molecular afiections of the matter of these

Colour in many instances depends upon the thickness of
the plate or film of transparent matter upon which light is in-
cident ; as in all those cases which are termed the colours of
thin plates, of which the soap bubble affords a beautiful in-
stance.

When we arrive at the more recent discoveries of double
refraction and polarisation, the effects of light are found to
trace out as it were the structure of the matter affected, and
the crystalline form of a body can be determined by the
effects which a minute portion of it exercises on a ray of
Hght.

Let a piece of good glass be placed in what is called a
polariscope, or instrument in which light that has undergone
polarisation is transmitted through the substance to be exam-
ined, and the emergent light is afterwards submitted to anoth-
er substance capable of polarising light, or, as it is termed, an
analyser ; no change in effect will be observed. Remove the
glass, heat it and suddenly or quickly cool it as to render it
unannealed, in which state its molecules are in a state of
tension or strain, and the glass highly brittle, on replacing it
in the polariscope, a beautiful series of colours is perceptible.
Instead of subjecting the glass to heat and sudden cooling,
let it be bent or strained by mechanical pressure, and the col-
ours will be equally visible, modified, according to the direc-
tion of the flexure, and indicating by their course the curves
where the molecular state has been changed by pressure. So
if tough glue be elongated and allowed to cool in a stretched

132 CORBEL ATION OF PHYSICAL FOECES.

state, it doubly refracts light, and the colours are shown as in
the instance of glass.

Submit a series of crystals to the same examination, and
different figures will be formed by different crystals, bearing
a constant and definite relation to the structure of the partic-
ular crystal examined, and to the direction in which, with
reference to crystalline form, the ray crosses the crystal.

In the crystallised salts of paratartaric acid, M. Pasteur
noticed two sets of crystals which were hemihedral in oppo-
site directions, i. e. the crystals of one set were to those of
the other as to their own image reflected in a mirror ; on
making a separate solution of each of these classes of crys-
tals, he found that the solution of the one class rotated the
plane of polarisation to the right, while that of the other
class rotated to the left, and that a mixture in proper propor-
tions of the two solutions produced no deviation in the plane
of polarisation. Yet all these three solutions are what is term-
ed isomeric, that is, have as far as can be discovered the same
chemical constitution.

In the above, and in innumerable other cases, it is seen
that an alteration in the structure of a transparent substance
alters the character and effects of the transmitted light. The
phenomena of photography prove that light alters the struc-
ture of matter submitted to it ; with regard even to vision it-
self, the persistence of images on the retina of the eye would
seem to show that its structure was changed by the impact
of light, the luminous impressions being as it were branded
on the retina, and the memory of the vision being the scar of
such brand. The science of photography has reference main-
ly to solid substances, yet there are many instances of liquid
and gaseous bodies being changed by the action of light : thus
hydrocyanic acid, a liquid, undergoes a chemical change and
deposits a solid carbonaceous compound by the action of
light. Chlorine and hydrogen gases, when mixed and pre-
served in darkness, do not unite, but when exposed to light
rapidly combine, forming hydrochloric acid.

LIGHT. 133

The above facts — and many others might have been given
— go far to connect light with motion of ordinary matter, and
to show that many of the evidences which our senses receive
of the existence of light result from changes in matter itself.
When the matter is in the solid state, these changes are more
or less permanent ; when in the liquid or gaseous state, they
are temporary in the greater number of instances, unless there
be some chemical change effected, which is, as it were, seized
upon during its occurrence, and a resulting compound formed,
which is more stable than the original compound or mix-
ture.

I might weary my reader with examples, * showing that,
in every case which we can trace out, the effects of light are
changed by any and every change of structure, and that light
has a definite connection with the structure of the . bodies
affected by it. I cannot but think that it is a strong assump-
tion to regard ether, a purely hypothetical creation, as chang-
ing its elasticity for each change of structure, and to regard
it as penetrating the pores of bodies of whose porosity we
have in many cases no proof; the which pores must, more-
over, have a definite and peculiar communication, also assumed
for the purpose of the theory.

Ether is a most convenient medium for hypothesis : thus,
if to account for a given phenomenon the hypothesis requires
that the ether be more elastic, it is said to be more elastic ;
if more dense, it is said to be more dense ; if it be required
by hypothesis to be less elastic, it is pronounced to be less
elastic ; and so on.

The advocates of the ethereal hypothesis certainly have
this advantage, that the ether, being hypothetical, can have
its characters modified or changed without any possibility of
disproof either of its existence or modifications.

It may be that the refined mathematical labours on
light, as on electricity, have given an undue and adventitious
strength to the hypotheses on which they are based.

134 COEKELATION OF PHYSICAL FORCES.

An objection to which the view I have been advocating is
open, and a formidable one, is, the necessity involved in it of
an universal plenum ; for if light, heat, electricity, &c., be affec-
tions of ordinary matter, then matter must be supposed to be
everywhere where these phenomena are apparent, and con-
sequently there can be no vacuum.

These forces are transmitted through what are called
vacua, or through the interplanetary spaces, where matter, if
it exist, must be in a highly attenuated state.

It may be safely stated that hitherto all attempts at pro-
curing a perfect vacuum have failed. The ordinary air-
pump gives us only highly rarefied air ; and, by the principle
of construction, even of the best, the operation depends upon
the indefinite expansion of the volume of air in the receiver ;
even in the vacuum which is formed in this, so great is the
tendency of matter to fill up space, that I have observed dis-
tilled water contained in a vessel within the exhausted receiv-
er of a good air-pump has a taste of tallow, derived from the
grease, or an essential oil contained in it, which is used to
form an air-tight junction between the edges of the receiver
and the pump-plate.

The Torricellian vacuum, or that of the ordinary baro-
meter, is filled with the vapour of mercury ; but it might be
worth the trouble to ascertain what would be the effect of a
good Torricellian vacuum, when the mercury in the tube is
frozen, which might, without much difficulty, be now effected
by the use of solid carbonic acid and ether ; the only proba-
ble difficulty would be the different rates of contraction of
mercury and glass, at such a degree of cold, and more par-
ticularly the contraction of mercury at the period of its
solidification. Davy, however, endeavoured to form a
vacuum, in a somewhat similar manner, over fused tin, with
but partial success ; he also made many other attempts to
obtain a perfect vacuum ; his main object being to ascertain
what would be the effect of electricity across empty space :

LIGHT. 135

he admits that he could not succeed in procuring a vacuum,
but found electricity much less readily conducted or trans-
mitted by the best vacuum he could procure than by the ordi-
nary Boylean vacuum.

Morgan found no conduction by a good Torricellian vac-
uum ; and, although Davy does not seem to place much reliance
on Morgan's experiments, there was one point in which they
were less liable to error than those of Davy. Morgan, whose
experiments seem to have been carefully conducted, operated
with hermetically-sealed glass tubes and by induced electricity,
while Davy sealed a platinum wire into the extremity of the
tube in which he sought to produce a vacuum. I have found
in very numerous experiments which I made to exclude air
from water, that platinum wires, most carefully sealed into
glass, allow liquids to pass between them and the glass ; and
this gives every reason to believe that gases may equally pass
through ; I have observed such effect in the gas battery when
it has been in action for a long period. Davy supposed that
the particles of bodies maybe detached, and so produce elec-
trical effects in a vacumm ; and such effects would more read-
ily take place in his experiments, where a wire projected
into the exhausted space, than in Morgan's, where the in-
duced electricity was diffused over the surface of the glass.

M. Masson found that the barometric vacuum does not
conduct a current of electricity, or even a discharge, unless
the tension is considerable and sufficient to detach particles
from the electrodes ; and by adopting a plan of Dr. An-
drews, viz. absorbing carbonic acid by potash, M. Gassiot
has recently succeeded in forming vacua across which
the powerful discharge from the Rhumkorf coil will not

The odour which many metals, such as iron, tin, and
zinc emit, and the so-called thermographic radiations, we
can hardly explain upon any other theory than the evapora-
tion of an infinitesimally small portion of the metal itself.

136 COEEELATION OF PHYSICAL FORCES.

So universal is the tendency of matter to diffuse itself
into space, that it gave rise to the old saying that nature
abhors a vacuum ; an aphorism which, though cavilled at and
ridiculed by the self-sufficiency of some modern philosophers,
contains in a terse, though somewhat metaphorical, form of
expression a comprehensive truth, and evinces a large extent
of observation in those who, with few of the advantages which
we possess, first generalised by this sentence the facts of
which they had become cognisant.

It has been argued that, if matter were capable of infinite
divisibility, the earth's atmosphere would have no limit, and
that consequently portions of it would exist at points of space
where the attraction of the sun and planets would be greater
than that of the earth, and whence it would fly off to those
bodies and form atmospheres around them. This was sup-
posed to be negatived by the argument of the well-known
paper of Dr. Wollaston ; in which, from the absence of nota-
ble refraction near the margin of the sun and of the planet
Jupiter, he considered himself entitled to conclude that the
expansion of the earth's atmosphere had a definite limit, and
was balanced at a certain point by gravitation : this deduc-
tion has been shown to be inconclusive by Dr. WheweU, and
has also been impugned upon others grounds by Dr. Wilson.
There is a point not adverted to in these papers, and which
Wollaston does not seem to have considered, viz. that there
is no evidence that the apparent discs of the sun and of Jupi-
ter show us their real discs or bodies. Sir. W. Herschel
regards the margin of the visible discs as that of clouds or a
peculiar state of atmosphere, and the rapidly changing char-
acter of the apparent surfaces render some such conclusion
necessary. If this be so, refraction of an occulted star could*
not be detected — at all events, in the denser portion of the
atmosphere.

Sir W. Herschel's observations go to prove that the
Bun and Jupiter have dense atmospheres, while Wollaston's

LIGHT. 137

were believed to prove that they have no appreciable atmos-
pheres.

If it be admitted, or considered proved, that the sun and
planets have atmospheres— and little doubt now exists on this
point — then the grounds upon which WoUaston founded his
arguments are untenable ; and there appears no reason why
the atmosphere of the different planets should not be, with
reference to each other, in a state of equilibrium. Ether, or
the highly-attenuated matter existing in the interplanetary
spaces, being an expansion of some or all of these atmos-
pheres, or of the more volatile portions of them, would thus
furnish matter for the transmission of the modes of motion
which we call light, heat, &c. ; and possibly minute portions
of these atmospheres may, by gradual changes, pass from
planet to planet, forming a link of material communication
between the distant monads of the universe.

The view given above would approximate the theory of
the transmission of light by the undulations of ordinary mat-
ter to the other two theories, which equally suppose the non-
existence of a vacuum ; for, according to the emissive or
corpuscular theory, the vacuum is filled by the matter itself,
of light, heat, &c. ; according to the ethereal, it is filled by
the all-penetrating ether. Of the existence of matter in the
interplanetary spaces we have some evidence in the diminish-
ing periods of comets ; and where, from its highly attenuated
state, the character of the medium by which the forces are
conveyed cannot be tested, the term ether is a most appropri-
ate generic name for such medium.

Newton has some curious passages on the subject matter
of light. In the ' Queries to the Optics ' he says : —
^ ^ Are not gross bodies and light convertible into one
another, and may not bodies receive much of their activity
from the particles of light which enter their composition?
* * * The changing of bodies into light and light into
bodies is very conformable to the course of nature, which

138 CORRELATION OF PHYSICAL FORCES.

seems delighted with transmutations. "Water, which is a
very fluid, tasteless salt, she changes by heat into vapour,
which is a sort of air, and by cold into ice, which is a hard,
pellucid, brittle, fusible stone, and this stone returns into
water by heat, and vapour returns into water by cold. * *
And, among such various and strange transmutations, why
may not nature change bodies into light, and light into
bodies ? '

Newton has here seemingly in his mind the emissive
theory of light ; but the passages might be applied to either
theory ; the analogy he saw in the change of state of matter,
as in ice, water, and vapour, with the hypothetic change into
light, is very striking, and would seem to show that he regard-
ed the change or transmutation of which he speaks as one
analogous to the known changes of state, or consistence, in
ordinary matter.

The difference between the view which 1 am advocating
and that of the ethereal theory as generally enunciated is,
that the matter which in the interplanetary spaces serves as
the means of transmitting by its undulations light and heat, I
should regard as possessing the qualities of ordinary, or as it
has sometimes been called gross, matter, and particularly
weight ; though, from its extreme rarefaction, it would mani-
fest these properties in an indefinitely small degree ; whilst, on
the surface of the earth, that matter attains a density cognisa-
ble by our means of experiment, and the dense matter is
itself, in great part, the conveyer of the undulations in which
these agents consist. Doubtless, in very many of the forms
which matter assumes it is porous, and pervaded by more
volatile essences, which may differ as much in kind as matter
does. In these cases a composite medium, such as that indi-
cated by Dr. Young, would result ; but even on such a suppo-
sition, the denser matter would probably exercise the more
important influence on the undulations. Returning to the
"somewhat strained hypothesis, that the particles of dense

LIGHT. 139

matter in a so-called solid are as distant as the stars in heaven,
still a certain depth or thickness of such solid would present
at every point of space a particle or rock in the successive
progress of a wave, which particles, to carry on the move-
ment, must vibrate in unison with it.

At the utmost, our assumption, on the one hand, is that
wherever light, heat, &c., exist, ordinary matter exists, though
it may be so attenuated that we cannot recognise it by the
tests of other forces, such as gravitation, and that to the ex-
pansibility of matter no limit can be assigned. On the other
hand, a specific matter without weight must be assumed, of
the existence of which there is no evidence, but in the phe-
nomena for the explanation of which its existence is supposed.
To account for the phenomena the ether is assumed, and to
prove the existence of the ether the phenomena are cited.
For these reasons, and others above given, I think that the
assumption of the universality of ordinary matter is the least
gratuitous.

OvSei' Ti Tov iravTOs K€vov ireA-ei ovSe irepKrcrov.

A question has often occurred to me and possibly to oth-
ers : Is the continuance of a luminous impulse in the inter-
planetary spaces perpetual, or does it after a certain distance
dissipate itself and become lost as Hght — I do not mean by
mere divergence directly as the squares of the distances it
travels, but does the physical impulse itself lose force as it
proceeds? Upon the view I have advocated, and indeed
upon any undulatory hypothesis, there must be some resist-
ance to its progress ; and unless the matter or ether in the
interplanetary spaces be infinitely elastic, .and there be no
lateral action of a ray of light, there must be some loss.
That it is exceedingly minute is proved by the distance light
travels. Stars whose parallax is ascertained are at such a
distance from the earth that their light, travelling at the rate
of 192,500 miles in a second, takes more than ten years to

140 COEEELATION OF PHYSICAL FOECES.

reach the earth ; so that we see them as they existed ten
years ago. The distance of most visible stars is probably far
greater than this, and yet their brilliance is great, and in-
creases when their rays are collected by the telescope in pro-
portion ceteris paribus to the area of the object-glass or spec-
ulum. There is, however, an argument of a somewhat spec-
ulative character, by which light would seem to be lost or
transformed into some other force in the interplanetary spaces.

Every increase of space-penetrating power in the tele-
scope gives us a new field of visible stars. If this expansion
of the stellar universe go on indefinitely and no light be lost,
then, assuming the fixed stars to be of an average equal
brightness with our sun, and no light lost other than by diver-
gence, the night ought to be equally luminous with the day ;
for though the light from each point diminishes in intensity
as the square of the distance, the number of luminous points
would fill up the whole space around us ; and if every point
of space is occupied by an equally brilliant point of light, the
distance of the points becomes immaterial. The loss of light
intercepted by stellar bodies would make no difference in the
total quantity of light, for each of these would yield from its
own self-luminosity at least as much light as it intercepted.
Light may, however, be intercepted by opaque bodies, such
as planets ; but, making every allowance for these, it is diffi-
cult to understand why we get so little light at night from the
stellar universe, without assuming that some light is lost in its
progress through space — ^not lost absolutely, for that would be
an annihilation of force — ^but converted into some other mode
of motion.

It may be objected that this hypothesis assumes the stel-
lar universe to be illimitable : if pushed to its extreme so as
to make the light of night equal that of day, provided no
stellar light be lost, it does make this assumption ; but even
this is a far more rational assumption to make than that the
stellar universe is limited. Our experience gives no indicar

LIGHT. 141

tion of a limit ; each improvement in telescopic power gives
us new realms of stars or of nebulae, which, if not stellar
clusters, are at all events self-luminous matter ; and if we as-
sume a limit, what is it? We cannot conceive a physical
boundary, for then immediately comes the question, what
bounds the boundary ? and to suppose the stellar universe to
be bounded by infinite space or by infinite chaos, that is to
say, to suppose a spot — ^for it would then become so— of mat-
ter in definite forms, with definite forces, and probably teem-
ing with definite organic beings, plunged in a universe of
nothing, is to my mind at least far more unphilosophical than
to suppose a boundless universe of matter existing in forms
and actions analogous to those which, as far as our examina-
tion goes, pervade space. But without speculating on topics
in which the mind loses itself, it may not unreasonably be
expected that a greater amount of light would reach us from
the surrounding self-luminous spheres were not some portion
lost as light, by its action on the medium which conveys the
impulses. What force this becomes, or what it effects, it
would be idle to speculate upon.

YI.— MAGNETISM.

MAGNETISM, as was proved by the important discov-
ery of Faraday, will produce electricity, but with this
peculiarity — that in itself it is static ; and, therefore, to pro-
duce a dynamic force, motion must be superadded to it : it is,
in fact, directive, not motive, altering the direction of other
forces, but not, in strictness, initiating them. . It is difficult
to convey a definite notion of the force of magnetism, and of
the mode in which it afiects other forces. The following il-
lustration may give a rude idea of magnetic polarity. Sup-
pose a number of wind-vanes, say of the shape of arrows,
with the spindles on which they revolve arranged in a row,
but the vanes pointing in various directions : a wind blowing
from the same point with an uniform velocity will instantly
arrange these vanes in a definite direction, the arrow-heads
or narrow parts pointing one way, the swallow-tails or broad
parts another. If they be delicately suspended on their spin-
dles, a very gentle breeze will so arrange them, and a very
gentle breeze will again deflect them ; or, if the wind cease,
and they have been originally subject to other forces, such as
gravity from unequal suspension, they will return to irregu-
lar positions, themselves creating a slight breeze by their re-
turn. Such a state of things will represent the state of the
molecules of soft iron ; electricity acting on them — ^not indeed
in straight lines, but in a definite direction — produces a polar

MAGNETISM. 143

arrangement, which they will lose as soon as the djmamic in-
ducing force is removed.

Let us now suppose the vanes, instead of turning easily,
to be more stiffly fixed to the axles, so as to be turned with
difficulty : it will require a stronger wind to move them and
arrange them definitely ; but when so arranged, they will re-
tain their position ; and should a gentle breeze spring up in
another direction, it will not alter their position, but will it-
self be definitely deflected. Should the conditions of force
and stability be intermediate, both the breeze and the vanes
will be slightly deflected ; or, if there be no breeze, and the
spindles be all moved in any direction, preserving their linear
relation, they will themselves create a breeze. Thus it is
with the molecules of hard iron or steel in permanent mag-
nets ; they are polarised with greater difficulty, but, when so
polarised, they cannot be affiscted by a feeble current of elec-
tricity. Again, if the magnets be moved, they themselves
originate a current of electricity; and, lastly, the magnetic
polarity and the electric current may be both mutually af-
fected, if the degrees of motion and stability be intermediate.

The above instance will, of course, be taken only as an
approximation, and not as binding me to any closer analogy
than is generally expected of a mechanical illustration. It
is difficult to convey by words a definite idea of the dual or
antithetic character of force involved in the term polarity.
The illustration I have employed' niay, I hope, somewhat aid
in elucidating the manner in which magnetism acts on the
other dynamic forces ; i. e., definitely directing them, but not
initiating them, except while in motion.

Magnets being moved in the direction of lines, joining
their poles, produce electrical currents in such neighbouring
bodies as are conductors of electricity, in directions trans-
verse to the line of motion ; and if the direction of motion
or the position of the magnetic poles be reversed, the current
of electricity flows in a reverse direction. So if the magnet

144 COEKELATION OF PHYSICAL FORCES.

be stationary, conducting bodies moved across any of the
lines of magnetic force, i. e. lines in the direction of which
the mutual action of the poles of the magnet would place
minute portions of iron, have currents of electricity devel-
oped in them, the direction of which is dependent upon that
of the motion of the substance with reference to the magnetic
poles. Thus, as bodies affected by an electrical current are
definitely moved by a magnet in proximity to them, so con-
versely bodies moved near a magnet have an electrical cur-
rent developed in them. Magnetism can, then, through the
medium of electricity, produce Jieat^ lights and chemical affin-
ity. Motion it can directly produce under the above condi-
tions ; i. e. a magnet being itself moved will move other fer-
reous bodies : these wiU acquire a static condition of equilib-
rium, and be again moved when the magnet is also moved.
By motion or arrested motion only, could the phenomena of
magnetism ever have become known to us. A magnet, how-
ever powerful, might rest for ever unnoticed and unknown,
unless it were moved near to iron, or iron moved near to it,
so as to come within the sphere of its attraction.

But even with other than either magnetic or electrified
substances, all bodies will be moved when placed near the
poles of very powerful magnets — some taking a position ax-
ially, or in the line from pole to pole of the magnet ; others
equatorially, or in a direction transverse to that line — ^the
former being attracted, the latter apparently repelled, by the
poles of the magnet. These effects, according to the views
of Faraday, show a generic difference between the two
classes of bodies, magnetics and diamagnetics ; according to
others, a difference of degree or a resultant of magnetic ac-
tion ; the less magnetic substance being forced into a trans-
verse position by the magnetisation of the more magnetic
medium which surrounds it.

According to the view given above, magnetism may be
produced by the other forces, just as the vanes in the instance

I

MAGNETISM. 146

given are definitely deflected, but cannot produce them except
when in motion : motion, therefore, is to be regarded in this
case as the initiative force. Magnetism will, however, di-
rectly affect the other forces — light, heat, and chemical affin-
ity, and change their direction or mode of action, or, at all
events, will so affect matter subjected to these forces, that
their direction is changed. Since these lectures were deliv-
ered, Faraday has discovered a remarkable effect of the mag-
netic force in occasioning the deflection of a ray of polarised
light.

If a ray of polarised light pass through water, or through
any transparent liquid or solid which does not alter or turn
aside the plane of polarisation, and the column, say of water,
through which it passes be subjected to the action of a pow-
erful magnet, the line of magnetic force, or that which would
unite the poles of the magnet, being in the same direction as
the ray of polarised light, the water acquires, with reference
to the light, similar, though not quite identical, properties to
oil of turpentine — the plane of polarisation is rotated, and
the direction of this rotation is changed by changing the di-
rection of the magnetic force : thus, if we suppose a polar-
ised ray to pass first in its course the north pole of the mag-
net, then between that and the south pole it will be deflected,
or curved to the right ; while if it meets the south pole first
in its course, it will, in its journey between that and the north
pole, be tm'ned to the left. If the substance through which
the ray is transmitted be of itself capable of deflecting the
plane of polarisation, as, for instance, oil of turpentine, then
the magnetic influence will increase or diminish this rotation,
according to its direction. A similar effect to this is observed
with polarised heat when the medium through which it is
transmitted is subjected to magnetic influence.

Whether this effect of magnetism is rightly termed an ef-
fect upon light and heat, or is a molecular change of the mat-
ter transmitting the light and heat, is a question the resolu-
7

146 CORRELATION OF PHYSICAL FORCES.

tion of which must be left to the future ; at present, the an-
swer to it would depend upon the theory we adopt. If the
view of light and heat which I have stated be adopted, then
we may fairly say that magnetism, in these experiments, di-
rectly affects the other forces ; for light and heat being, ac-
cording to that view, motions of ordinary matter, magnetism,
in affecting these movements, affects the forces which occa-
sion them. If, however, the other theories be adhered to, it
would be more consistent with the facts to view these results
as exhibiting an action upon the matter itself, and the heat
and light as secondarily affected.

When substances are undergoing chemical changes, and a
magnet is brought near them, the direction or lines of action
of the chemical force will be changed. There are many old
experiments which probably depended on this effect, but
which were erroneously considered to prove that permanent
magnetism could produce or increase chemical action : these
have recently been extended and explained by Mr. Hunt and
Mr. Wartmann, and are now better understood.

The above cases are applicable to the subject of the pres-
ent Essay, inasmuch as they show a relation to exist between
magnetic and the other forces, which relation is, in all proba-
bility, reciprocal ; but in these cases there is not a production
of light, heat, or chemical affinity, by magnetism, but a change
in their direction or mode of action.

There is, however, that which may be viewed as a dy-
namic condition of magnetism ; i. e. its condition at the com-
mencement and the termination, or during the increment or
decrement of its development. While iron or steel is being
rendered magnetic, and as it progresses from its non-magnetic
to its maximum magnetic state, or recedes from its maximum
to zero, it exhibits a dynamic force ; the molecules are, it
may be inferred, in motion. Similar effects can then be pro-
duced to those which are produced by a magnet whilst in mo-
tion.

MAGNETISM. • 147

An experiment which I published in 1845 tends, I think,
to illustrate this, and in some degree to show the character
of the motion impressed upon the molecules of a magnetic
metal at the period of magnetisation. A tube filled with the
liquid in which magnetic oxide of iron had been prepared,
and terminated at each end by plates of glass, is surrounded
by a coil of coated wire. To a spectator looking through this
tube a flash of light is perceptible whenever the coil is elec-
trised, and less light is transmitted when the electrical current
ceases, showing a symmetrical arrangement of the minute
particles of magnetic oxide while under the magnetic in-
fluence.

In this experiment it should be borne in mind, that the
particles of oxide of iron are not shaped by the hand of man,
as would be the case with iron filings, or similar minute por-
tions of magnetic matter, but being chemically precipitated,
are of the form given to them by nature.

While magnetism is in the state of change above described,
it will produce the other forces ; but it may be said, while
magnetism is thus progressive, some other force is acting on
it, and therefore it does not initiate : this is true, but the
same may be said of aU the other forces ; they have no com-
mencement that we can trace. "We must ever refer them
back to some antecedent force equal in amount to that pro-
duced, and therefore the word initiation cannot in strictness
apply, but must only be taken as signifying the force selected
as the first : this is another reason why the idea of abstract
causation is inapplicable to physical production. To this
point I shall again advert.

Electricity may thus be produced directly by magnetism,
either when the magnet as a mass is in motion, or when its
magnetism is commencing, increasing, decreasing, or ceasing ;
and heat may similarly be directly produced by magnetism.
I have, since the first edition of this Essay was published,
communicated to the Royal Society a paper by which I think

148 COKEELATION OF PHYSICAL FOKCES.

I have satisfactorily proved, that whenever any metal suscepti-
ble of magnetism is magnetised or demagnetised, its tempera-
ture is raised. This was shown, first, by subjecting a bar of
iron, nickel, or cobalt to the influence of a powerful electro-
magnet, which was rapidly magnetised and demagnetised in
reverse directions, the electro-magnet itself being kept cool by
cisterns of water, so that the magnetic metal subjected to the
influence of magnetism was raised to a higher temperature
than the electro-magnet itself, and could not, therefore, have
acquired its increased temperature by conduction or radiation
of heat from the electro- magnet ; and secondly, by rotating
a permanent steel magnet with its pole opposite to a bar
of iron, a thermo-electric pile being placed opposite the
latter.

Dr. Maggi covered a plate of homogeneous soft iron with
a thin coating of wax mixed with oil, a tube traversed the
centre through which the vapour of boiling water was passed.
The plate was made to rest on the poles of an electro-magnet,
with card interposed. When the iron is not magnetised, the
melted wax assumes a circular form, the tube occupying the
centre, but when the electro-magnet is put in action, the curve
marking the boundary of the melted substance changes its form
and becomes elongated in a direction transverse to the line
joining the poles, showing that the conducting power of the
iron for heat is changed by magnetisation.

Thus we get heat produced by magnetism and the conduc-
tion of heat altered by it in a direction having a definite rela-
tion to the direction of the magnetism. Is it necessary to
call in aid ethep or the substance ' caloric' to explain these
results ? is it not more rational to regard the calorific effects
as changes in the molecular arrangements of the matter sub-
jected to magnetism ?

There is every probability that magnetism, in the dyna-
mic state, either when the magnet is in motion, or when the
magnetic intensity is varying, Trill also directly produce chemi-

MAGNETISM. 149

cal affinity and llglit, though, up to the present time, such has
not been proved to be the case ; the reciprocal effect, also, of
the direct production of magnetism by light and heat has not
yet been experimentally established.

I have used, in contradistinction, the terms dynamic and
static to represent the different states of magnetism. The
applications I have made of these terms may be open to some
exception, but I know of no other vrords which will so nearly
express my meaning.

The static condition of magnetism resembles the static
condition of other forces : such as the state of tension exist-
ing in the beam and a cord of a balance, or in a charged
Leyden phial. The old definition of force was, that which
caused change in motion ; and yet even this definition pre-
sents a difficulty : in a case of static equilibrium, such, for
instance, as that which obtains in the two arms of a balance,
we get the idea of force without any palpable apparent motion :
whether there be really an absence of motion may be a doubt-
ful question, as such absence would involve in this case per-
fect elasticity, and, in all other cases, a stability which, in a
long course of time, nature generally negatives, showing, as
I believe, an inseparable connection of motion with matter,
and an impossibility of a perfectly immobile or durable state.
So with magnetism : I believe no magnet can exist in an
absolutely stable state, though the duration of its stability
will be proportionate to its original resistance to assuming
a polarised condition. This, however, must be taken merely
as a matter of opinion : we have, in support of it, the general
facts that magnets do deteriorate in the course of years ; and
we have the further general fact of the instability, or fluxional
state, of all nature, when we have an opportunity of fairly
investigating it at different and remote periods : in many
cases, however, the action is so slow that the changes escape
human observation, and, until this can be brought to bear
over a proportionate period of time, the proposition cannot be

150 COKKELATION OF PHYSICAL FOKCES.

said to be experimentally or inductively proved, but must be
left to the mental conviction of those who examine it by the
light of already acknowledged facts.

All cases of static force present the same difficulty : thus,
two springs pressing against each other would be said to be
exercising force ; and yet there is no resulting action, no heat,
no light, &c.

So if gas be compressed by a piston, at the time of com-
pression heat is given off; but when this is abstracted,
although the pressure continues, no further heat is eliminated.
Thus, by an equilibrium produced by opposing forces, motion
is locked up, or in abeyance, as it were, and may be again
developed when the forces are relieved from the tension.
But in the first instance, in producing the state of tension,
force has to be employed ; and as we have said in treating of
mechanical force, so with the other forces the original change
which disturbs equilibrium produces other changes which go
on without end. Thus, by the act of charging a Leyden
phial, the cylinder, the rubber, and the adjoining portions of
the electrical machine have each and all their states changed,
and thence produce changes in surrounding bodies ad infinir
turn ; when the jar is discharged, converse changes are again
produced.

As with heat, light, and electricity, the daily accumulating
observations tend to show that each change in the phenomena
to which these names are given is accompanied by a change
either temporary or permanent in the matter affected by them ;
so many recent experiments on magnetism have connected
magnetic phenomena with a molecular change in the subject
matter. Thus M. Wertheim has shown that the elasticity of
iron and steel is altered by magnetisation ; the co-efficient of
elasticity in iron being temporarily, in steel permanently
diminished.

He has also examined the effects of torsion upon magnet-
ised iron, and concludes, from his experiments, that in a bar

MAGNETISM. 161

of iron arrived at a state of magnetic equilibrium, temporary-
torsion diminishes the magnetism, and that the untwisting or
return to its primitive state restores the original degree of
magnetisation.

M. Guillemin observed that a bar slightly curved by its
own weight is straightened by being magnetised. Mr. Page
and Mr. Marrion discovered that a sound is emitted when
iron or steel is rapidly magnetised or demagnetised ; and Mr.
Joule found that a bar of iron is slightly elongated by mag-
netisation.

Again, with regard to diamagnetic bodies, M. Matteucci
found that the mechanical compression of glass altered the
rotatory power upon a ray of polarised Hght which it trans-
mitted. He further considered that a change took place in
the temper of portions of glass which he submitted to the in-
fluence of powerful magnets.

The same arguments which have been submitted to
the reader as to the other affections of matter being modes
of molecular motion, are therefore equally applicable to mag-
netism.

YII.— CHEMICAL AFFINITY.

CHEMICAL AFFINITY, or the force by which dissimi-
lar bodies tend to unite and form compounds diiFering
generally in character from their constituents, is that mode of
force of which the human mind has hitherto formed the least
definite idea. The word itself — affinity — ^is ill chosen, its
meaning, in this instance, bearing no analogy to its ordinary
sense ; and the mode of its action is conveyed by certain con-
ventional expressions, no dynamic theory of it worthy of
attention having been adopted. Its action so modifies and
alters the character of matter, that the changes it in-
duces have acquired, not perhaps very logically, a generic
contradistinction from other material changes, and we
thus use, as contradistinguished, the terms physical and
chemical.

The main distinction between chemical affinity and physi-
cal attraction or aggregation, is the diffi3rence of character of
the chemical compound from its components. This is, how-
ever, but a vague line of demarcation ; in many cases, which
would be classed by all as chemical actions, the change of
character is but slight ; in others, as in the effects of neutrali-
sation, the difference of character would be a result which
would equally follow from physical attraction of dissimilar
substances, the previous characters of the constituents depend-
ing upon this very attraction or affinity : thus an acid corrodes

CHEMICAL AFFCnTY. 153

because it tends to unite with another body ; when united,
its corrosive power, i. e. its tendency to unite, being satiated,
it cannot, so to speak, be further attracted, and it necessarily
loses its corrosive power. But there are other cases where
no such result could a priori be anticipated, as where the
attraction or combining tendency of the compound is higher
than that of its constituents : thus, who could, by physical
reasoning, anticipate a substance like nitric acid from the
combination of nitrogen and oxygen ?

The nearest approach, perhaps, that we can form to a
comprehension of chemical action, is by regarding it (vaguely
perhaps) as a molecular attraction or motion. It will
directly produce motion of definite masses, by the resultant
of the molecular changes it induces : thus, the projectile
effects of gunpowder may be cited as familiar instances of
motion produced by chemical action. It may be a question
whether, in this case, the force which occasions the motion
of the mass is a conversion of the force of chemical affinity,
or whether it is not, rather, a liberation of other forces exist-
ing in a state of static equilibrium, and having been brought
into such state by previous chemical actions ; but, at all
events, through the medium of electricity chemical affinity
may be directly and quantitatively converted into the other
modes of force. By chemical affinity, then, we can directly
produce electricity; this latter force was, indeed, said by
Davy to be chemical affinity acting on masses : it appears,
rather, to be chemical affinity acting in a definite direction
through a chain of particles ; but by no definition can the
exact relation of chemical affinity and electricity be expressed ;
for the latter, however closely related to the former, yet exists
where the former does not, as in a metallic wire, which when
electrified, or conducting electricity, is, nevertheless, not
chemically altered, or, at least, not known to be chemically
altered.

Volta, the antitype of Prometheus, first enabled us de-
7*

154 COEKELATION OF PHYSICAL FORCES.

finitely to relate the forces of chemistry and electricity.
When two dissimilar metals in contact are immersed in a
liquid belonging to a certain class, and capable of acting
chemically on one of them, what is termed a voltaic circuit is
formed, and, by the chemical action, that peculiar mode of
force called an electric current is generated, which circulates
from metal to metal, across the liquid, and through the points
of contact.

Let us take, as an instance of the conversion of chemical
force into electrical, the following, which I made known some
years ago. If gold be immersed in hydrochloric acid, no
chemical action takes place. If gold be immersed in nitric
acid, no chemical action takes place ; but mix the two acids,
and the immersed gold is chemically attacked and dissolved :
this an is ordinary chemical action, the result of a double chemi-
cal affinity. In hydrochloric acid, which is composed of
chlorine and hydrogen, the affinity of chlorine for gold being
less than its affinity for hydrogen, no change takes place ; but
when the nitric acid is added, this latter containing a great
quantity of oxygen in a state of feeble combination, the
affinity of oxygen for hydrogen opposes that of hydrogen for
chlorine, and then the affinity of the latter for gold is enabled
to act, the gold combines with the chlorine, and chloride of
gold remains in solution in the liquid. Now, in order to
exhibit this chemical force in the form of electrical force,
instead of mixing the liquids, place them in separate vessels
or compartments, but so that they may be in contact, which
may be effected by having a porous material, such as un-
glazed porcelain, amianthus, &c., between them. Immerse in
each of these liquids a strip or wire of gold : as long as these
pieces of gold remain separated, no chemical or electrical
effect takes place ; but the instant they are brought into
metallic contact, either immediately or by connecting each
with the same metallic wire, chemical action takes place —
the gold in the hydrochloric acid is dissolved, electrical action

CHEMICAL AFFINITY. 155

also takes place, the nitric acid is deoxidised by the trans-
ferred hydrogen, and a current of electricity may be detected
in the metals or connecting metal by the application of a gal-
vanometer or any instrument appropriate for detecting such
effect.

There are few, if any, chemical actions which cannot be
experimentally made to produce electricity : the oxidation of
metals, the burning of combustibles, the combination of oxy-
gen and hydrogen, &c., may all be made sources of elec-
tricity. The common mode in which the electricity of the
voltaic battery is generated is by the chemical action of water
upon zinc ; this action is increased by adding certain acids to
the water, which enable it to act more powerfully upon the
zinc, or in some cases act themselves upon it ; and one of the
most powerful chemical" actions known — that of nitric acid
upon oxidable metals — is that which produces the most pow-
erful voltaic battery, a combination which I made known in
the year 1839 : indeed, we may safely say, that when the
chemical force is utilised, or not wasted, but all converted into
electrical force, the more powerful the chemical action, the
more powerful is the electrical action which results.

If, instead of employing manufactured products or educts,
such as zinc and acids, we could realise as electricity the
whole of the chemical force which is active in the combustion
of cheap and abundant raw materials, such as coal, wood, fat,
&c., with air or water, we should obtain one of the greatest
practical desiderata, and have at our command a mechanical
power in every respect superior in its applicability to the
steam engine,

I have shown that the flame of the common blowpipe gives
rise to a very marked electrical current, capable not only of
affecting the galvanometer, but of producing chemical decom-
position : two plates or coils of platinum are placed, the one
in the portion of the flame near the orifice of the jet, or at
the points where combustion commences, the other in the full

156 COKEELATION OF PHYSICAL F0ECE8.

yellow flame "vvhere combustion is at its maximum ; this latter
should be kept cool, to enable a thermo-electric current, which
is produced by the different temperature of the platinum
plates, to co-operate with the flame current ; wires attached
to the plates of platinum form the terminals or poles. By a
row of jets a flame battery may be formed, yielding increased
effects ; but in these experiments, though theoretically inter-
esting, so small a fraction of the power, actually at work in
the combustion, has been thrown into an electrical form, that
there is no immediate promise of a practical result.

The quantity of the electrical current, as measured by the
quantity of matter it acts upon in its different phenomenal
effects, is proportionate to the quantity of chemical action
which generated it ; and its intensity, or power of overcoming
resistance, is also proportionate to the intensity of chemical
affinity when a single voltaic pair is employed, or to the num-
ber of reduplications when the well-known instrument called
the voltaic battery is used.

The mode in which the voltaic current is increased in in-
tensity by these reduplications, is in itself a striking instance
of the mutual relations and dynamic analogies of different
forces. Let a plate of zinc or other metal possessing a strong
affinity for oxygen, and another of platinum or other metal
possessing little or no affinity for oxygen, be partially im-
mersed in a vessel, A, containing dilute nitric acid, but not
in contact with each other ; let platinum wires touching each
of these plates have their extremities immersed in another
vessel, B, containing also dilute nitric acid : as the acid in
vessel A is decomposed, by the chemical affinity of the zinc
for the oxygen of the acid, the acid in vessel B is also decom-
posed, oxygen appearing at the extremity of the wire which
is connected with the platinum : the chemical power is con-
veyed or transferred through the wires, and, abstracting cer-
tain local effects, for every unit of oxygen which combines
with the zinc in the one vessel, a unit of oxygen is evolved

' CHEMICAL AFFINITY. 157

from the platinum wire in the other. The platinum wire is
thus thrown into a condition analogous to zinc, or has a pow
er given to it of determining the oxygen of the liquid to its
surface, though it cannot, as is the case with zinc, com-
bine with it under similar circumstances. If we now substi-
tute for the platinum wire which was connected with the
platinum plate, a zinc wire, we have in addition to the deter-
mining tendency by which the platinum was affected, the
chemical affinity of the oxygen in vessel B for the zinc wire :
thus we have, added to the force which was originally pro-
duced by the zinc of the combination in vessel A, a second
force, produced by the zinc in vessel B, co-operating with the
first ; two pairs of zinc and platinum thus connected produce,
therefore, a more intense effect than one pair ; and if we go
on adding to these alternations of zinc, platinum, and liquid,
we obtain an indefinite exaltation of chemical power, just as
in mechanics we obtain accelerated motion by adding fresh
impulses to motion akeady generated.

The same rule of proportion which holds good in chemi-
cal combinations also obtains in electrical effects, when these
are produced by chemical actions. Dalton and others proved
that the constituents of a vast number of compound substances
always bore a definite quantitative relation to each other :
thus, water, which consists of one part by weight of hydro-
gen united to eight parts of oxygen, cannot be formed by the
same elements in any other than these proportions ; you can
neither add to nor subtract from the normal ratio of the
elements, without entirely altering the nature of the com-
pound. Further, if any element be selected as unity, the
combining ratios of other elements will bear an invariable
quantitative relation to that and to each other : thus if hydro-
gen be chosen as 1, oxygen will be 8, chlorine wiU be 36 ;
that is, oxygen will unite with hydrogen in the proportion of
8 parts by weight to 1, while chlorine wiU unite with hydro-
gen in the proportion of 36 to 1, or with oxygen in the pro-

158 CORRELATION OF PHYSICAL FORCES.

portion of 36 to 8. Numbers expressing their combining
weights, which are thus relative, not absolute, may by a con-
ventional assent as to the point of unity, be fixed for all chemi-
cal reagents ; and, when so fixed, it will be found that bodies,
at least in inorganic compounds, generally unite in those pro-
portions, or in simple multiples of them : these proportions
are termed Equivalents.

< Now a voltaic battery, which consists usually of alterna-
tions of two metals, and a liquid capable of acting chemically
upon one of them, has, as we have seen, the power of pro-
ducing chemical action in a liquid connected with it by metals
upon which this liquid is incapable of acting : in such case the
constituents of the liquid will be eliminated at the surfaces of
the immersed metals, and at a distance one from the other.
For example, if the two platinum terminals of a voltaic
battery be immersed in water, oxygen will be evolved at one
and hydrogen at the other terminal, exactly in the propor-
tions in which they form water ; while, to the most minute
examination, no action is perceptible in the stratimi of
liquid. It was known before Faraday's time that, while this
chemical action was going on in the subjected liquid, a chemi-
cal action was going on in the cells of the voltaic battery ;
but it was scarcely if at all known that the amount of chemi-
cal action in the one bore a constant relation to the amount
of action in the other. Faraday proved that it bore a direct
equivalent relation : that is, supposing the battery to be
formed of zinc, platinum, and water, the amount of oxygen
which united with the zinc in each cell of the battery was
exactly equal to the amount evolved at the one platinum ter-
minal, while the hydrogen evolved from each platinum plate
of the battery was equal to the hydrogen evolved from the
other platinum terminal.

Supposing the battery to be charged with hydrochloric
acid, instead of water, while the terminals are separated by

water, then for every 36 parts by weight of chlorine which

I

OHEMIOAL AFFINITY. 159

united with each plate of zinc, eight parts of oxygen would
be evolved from one of the platinum terminals : that is, the
weights would be precisely in the same relation which Dalton
proved to exist in their chemical combining weights. This
may be extended to all liquids capable of being decomposed
by the voltaic force, thence called Electrolytes : and as no vol-
taic effect is produced by liquids incapable of being thus de-
composed, it follows that voltaic action is chemical action tak-
ing place at a distance, or transferred through a chain of
media, and that the chemical equivalent numbers are the ex-
ponents of the amount of voltaic action for corresponding
chemical substances.

As heat, light, magnetism, or motion, can be produced by
the requisite application of the electric current, and as this is
definitely produced by chemical action, we get these forces
very definitely, though not immediately, produced by chemi-
cal action. Let us, however, here enquire, as we have al-
ready done with respect to the other forces, how far other
forces may directly emanate firom chemical affinity.

Heat is an immediate product of chemical affinity. I
know of no exception to the general proposition that all bod-
ice in chemically combining produce heat; i. e. if solu-
tion be not considered as chemical action, and even in that
case, when cold results, it is from a change of consistence, as
from the solid to the liquid state, and not from chemical
action.

We shall find that the same view of the expenditure of
force which we have considered in treating of latent heat
holds good as to the expenditure of chemical force when re-
garded with reference to the amount of heat or repulsive
force which it engenders, the chemical force being here ex-
hausted by chemical expansion — that is, by heat. Thus, in
the chemical action of the ordinary combustion of coal and
oxygen, the expenditure of fuel will be in proportion to the
expansibility of the substances heated ; water passing freely

160 OOEEELATION OF PHYSICAL FOECES.

into the steam will consume more fuel than if it be confined
and kept at a temperature above its boiling point.

Why chemical action produces heat, or what is the action
of the molecules of matter when chemically uniting, is a
question upon which many theories have been proposed and
which may possibly be never more than approximately re-
solved.

Some authors explain it by the condensation which takes
place ; but this will not account for the many instances where,
from the liberation of gases, a great increase of volume en-
sues upon chemical combustion, as in the familiar instance
of the explosion of gunpowder : others explain it as resulting
from the union of atmospheres of positive and negative elec-
tricity which are assumed to surround the atoms of bodies ;
but this involves hypothesis upon hypothesis. Dr. Wood has
lately thrown out a view of the heat of chemical action which
is more in accordance with a dynamic theory of heat, and
as such demands some notice. Starting with his proposition,
which I have previously mentioned, ' that the nearer the par-
ticles of bodies are to each other the less they require to
move to produce a given motion in the particles of another
body,' his argument, if I rightly understand it, assumes some-
thing of this form.

In the mechanical approximation of the particles of a
homogeneous body heat results ; the particles a a of the body
A would, by their approximation, produce expansion in the
neighbouring body B, the more so in proportion as they them-
selves were previously nearer to each other. In chemically
combining, a a the particles of A are brought into very close
proximity with h h the particles of B ; heat should therefore
result, and the greater because the proximity may fairly be
assimied to be greater in the case of chemical combination
vthan in that of mechanical compression. In cases, then,
where there is no absolute diminution of bulk ensuing on
chemical combination, if the greater proximity of the com-

CHEMICAL APFINITY. 161

bining particles be such that the correlative expansion ought
to be greater (if there were no chemical combination) than
that occupied by the total volume of the new compound, an
extra expanding power is evolved, and heat or expansion
ought to be produced in surrounding bodies. In other words,
if a a could be brought by physical attraction as near each
other as they are by chemical attraction brought near to 6 5,
they would, from their increased proximity, produce an ex-
pansive power ultra the volume occupied by the actual chem-
ical compound A and B. The question, however, immedi-
ately occurs, why should the volume of the compound be lim-
ited and not occupy the full space equivalent to the expanding
power induced by the contraction or approximation of the
particles. As the distance of the particles is the resultant
of the contending contracting and expanding powers, this
result ought to express itself in terms of the actual volume
produced by the combination, which it certainly does not.

Though I see some difficulties in Dr. Wood's theory, and
perhaps have not rightly conceived it, his views have to my
mind great interest, his mode of regarding natural phenomena
being analogous to that which I have in this Essay, and for
many years, advocated, viz. to divest physical science as
much as possible of hypothetic fluids, ethers, latent entities,
occult qualities, &c. My own notion of the heat produced
by chemical combination, though I scarcely dare venture an
opinion upon a subject so controverted, is, that it is analogous
to the heat of friction, that the particles of matter in close
approximation and rapid motion inter se evolve heat as a con-
tinuation of the motion interrupted by the friction or intesti-
nal motion of the particles : heat would thus be produced,
whether the resulting compound were of greater or less bulk
than the sum of the components, though of course when the
compound is of greater bulk less heat would be apparent in
neighbouring bodies, the expansion taking place in one of the
substances themselves — I say in one of them, for it is stated

162 COERELATION OF PHYSICAL F0KCE8.

in books of authority that there is no instance of two or more
solids or liquids, or a solid and a liquid, combining and pro-
ducing a compound which is entirely gaseous at ordinary
temperatures and pressures. The substance gun-cotton, how-
ever, discovered by Dr. Schoenbein, very nearly realises this
proposition.

Dr. Andrews has arrived at the conclusion, after careful
experiment, that in chemical combinations where acids and
alkalies or analogous substances are employed, the amount of
heat produced is determined by the basic ingredient, and his
experiments have received general assent ; although it should
be stated that M. Hess arrived at contrary results, the acid
constituent according to his experiments furnishing the meas-
ure of the heat developed.

Light is directly produced by chemical action, as in the flash
of gunpowder, the burning of phosphorus in oxygen gas, and all
rapid combustions : indeed, wherever intense heat is developed,
light accompanies it. In many cases of slow combustion,
such as the phenomena of phosphorescence, the light is apparent-
ly much more intense than the heat ; the former being obvious,
the latter so difficult of detection that for a long time it was
a question whether any heat was eliminated ; and I am not
aware that at the present day, any thermic effects from cer-
tain modes of phosphorescence, such as those of phospho-
rescent wood, putrescent fish, &c., have been detected.

Chemical action produces magnetism whenever it is thrown
into a definite direction, as in the phenomenon of electrolysis.
I may adduce the gas voltaic battery, as presenting a simple
instance of the direct production of magnetism by chemical
synthesis. Oxygen and hydrogen in that combination chemi-
cally unite ; but instead of combining by intimate molecular
admixture, as in the ordinary cases, they act upon water, i. e.
combined oxygen and hydrogen, placed between them so as
to produce a line of chemical action ; and a magnet adjacent
to this line of action is deflected, and places itself at right

CHEMICAL AFFINITY. 163

angles to it. What a chain of molecules does here, there
can be no doubt all the molecules entering into combination
would produce in ordinary chemical actions ; but in such
cases, the direction of the lines of combination being irregu-
lar and confUsed, there is no general resultant by which the
magnet can be affected.

What the exact nature of the transference of chemical
power across an electrolyte is, we at present know not, nor
can we form any more definite idea of it than that given by
the theory of Grotthus. We have no knowledge as to the
exact nature of any mode of chemical action, and, for the
present must leave it as an obscure action of force, of which
future researches may simplify our apprehension.

We have seen that an equivalent or proportionate electri-
cal effect is produced by a given amount of chemical action ;
if we, in turn, produce heat and magnetism and motion by
the electricity resulting from chemical action, we shall be able
to measure these forces far more accurately than when they
are directly produced, and thus to deduce their equivalent re-
lation to the initial chemical action. Thus M. Favre, after
ascertaining the quantity of heat produced by the oxidation
of a quantity of zinc, and finding, as have others, that the
heat is the same when evolved from a voltaic battery by the
same consumption of zinc forming its positive element, makes
the following experiment.

A voltaic battery and electro-magnet are inamersed in cal-
orimeters, and the heat produced when the connection with
the magnet is effected is noted.

The electro-magnet is then made to raise a weight, and
thus perform mechanical work, and the heat produced is
again noted. It is found in the latter case that less heat is
evolved than in the former, a certain quantity of heat has
therefore been replaced by the mechanical work ; and by esti-
mating the amount of heat subtracted, and the amount of
work produced, he deduces the relative equivalent of work to

164: COEEELATION OF PHYSICAL FOECES.

heat. These experiments give a production of mechanical
work by chemical action, not, it is true, a direct production,
but, as the heat and work are in inverse ratios, and each has
its source in chemical action, they prove that they are definite
for a definite amount of chemical action, and as each is pro-
duced respectively by electricity and magnetism, these forces
must also bear a definite relation to the initial chemical force.

The doctrine of definite combining proportions, which so
beautifully serves to relate chemistry to voltaic electricity,
led to the atomic theory, which, though adopted in its univer-
sality by a large majority of chemists, presents great difficul-
ties when extended to all chemical combinations.

The equivalent ratios in which a great number of sub-
stances chemically combine, hold good in so many instances,
that the atomic doctrine is believed by many to be universally
applicable, and called a law ; and yet, when followed in the
combinations of substances whose natural chemical attractions
are very feeble, the relation fades away, and is sought to be
recovered by applying a separate and arbitrary multiplier to
the diflTerent constituents.

Thus, when it was found that a vast number of substances
combined in definite volumes and weights, and in definite vol-
umes and weights only, it was argued that their ultimate
molecules or atoms had a definite size, as otherwise there
was no apparent reason why this equivalent ratio should hold
good : why, for instance, water should only be formed of two
volumes or one unit by weight of hydrogen, and of one vol-
ume or eight units by weight of oxygen ; why, unless there
were some ultimate limits to the divisibility of its molecules,
should not water, or a fluid substance approximating to water
in character, be formed by a half, a third, or a tenth part of
hydrogen, with eight parts of oxygen ?

It was perfectly consistent with the atomic view that a
substance might be formed with one part combined with eight
parts, or with sixteen, or with twenty-four, for in such a sub-

CHEMICAL AFFINITY. 165

stance there would he no subdivision of the (supposed indivi-
sible) molecule ; and this held good with many compounds :
thus fourteen parts by weight, say grains of nitrogen, will
combine respectively with eight, sixteen, twenty-four, thirty-
two, and forty parts by weight, or grains, of oxygen.

So, again, twenty-seven grains of iron will combine with
eight grains of oxygen or with twenty-four grains, i. e. three
proportionals of oxygen. No compound is known in which
twenty-seven grains of iron will combine with two propor-
tionals or sixteen grains of oxygen ; but this does not much
affect the theory, as such a compound may be yet discovered,
or there may be reasons at present unknown why it cannot be
formed.

But now comes a difficulty : twenty-seven parts by weight
of iron will combine with twelve parts by weight of oxygen,
and twenty-seven parts of iron will also combine with ten
and two-third parts of oxygen. Thus if we retain the unit
of iron we must subdivide the unit of oxygen, or if we retain
the unit of oxygen we must subdivide the unit of iron, or we
must subdivide both by a different divisor. What then be-
comes of the notion of an atom or molecule physically indi-
visible ? ,

If iron were the only substance to which this difficulty
applied, it might be viewed as an unexplained exception, or
as a mixture of two oxides ; or recourse might be had to a
more minute subdivision to form the units or equivalents of
other substances ; but numerous other substances fall under
a similar category ; and in organic combinations, to preserve
the atomic nomenclature we must apply a separate multiplier
or divisor to far the greater number of the elementary con-
stituents, i. e. we must divide that which is, ex hypothesi, indi-
visible.

Thus, to take a more complex substance than any formed
by the combination of iron and oxygen, let us select the sub-
stance albumen, composed of carbon, hydrogen, nitrogen,

166 COREELATION OF PHYSICAL FORCES. ,

oxygen, phosphorus, and sulphur. In this case we must ei-
ther divide the atoms of phosphorus and sulphur so as to re-
duce them to small fractions, or multiply the atoms of the
other substances by extravagant numbers ; thus to preserve
the unit of one of the constituents of this substance, chemists
say it is composed of 400 atoms of carbon, 310 of hydrogen,
120 of oxygen, 50 of nitrogen, 2 of sulphur, and 1 of phos-
phorus. This is a somewhat extreme case, but similar diffi-
culties will be found in different degrees to prevail among or-
ganic compounds ; in very many no constituent can be taken
as a unit to which simple multiples of any of the others will
give their relative proportions. By the mode of notation
adopted, if any conceivable substance be selected, it could
whatever be the proportions of its constituents, be termed
atomic. A solution of an ounce of sugar in a pound of wa-
ter, in a pound and a half, in a pound and a quarter, in a
pound and a tenth, might be expressed in an atomic form, if
we select arbitrarily a multiplier or divisor.

It is true that in the case of solution, different proportions
can be united up to the point of saturation without any dif-
ference in the character of the compound, though the same
may be predicated to some extent of an acid and an alkali ;
but even where the steps are sudden, and compounds only
exist with definite proportions, they cannot, in a multitude of
cases, be reconciled with the true idea of an atomic combina-
tion, i. e. one to one, one to two, &c.

Although, therefore, nature presents us with facts which
show that there is some restrictive law of combination which
in numerous cases limits the ratios in which substances will
combine, nay, further, shows many instances of a proportion
between the combining weights of one compound and those
of another ; although she shows also a remarkable simplicity
in the combining volumes of numerous gases, she also gives
numerous cases to which the doctrine of atomic combinations
cannot fairly be applied.

CHEMICAL AFFnOTY. 167

That there must be something in the constitution of mat-
ter, or in the forces which act on it, to account for the per
saltum manner in which chemical combinations take place, is
inevitable ; but the idea of atoms does not seem satisfactorily
to account for it.

By selecting a separate multiplier or divisor, chemists
may denote every combination in terms derived from the
atomic theory ; but they have passed from the original law,
which contemplated only definite multiples, and the very hy-
pothetic expressions of atoms, which the apparently simple
relations of combining weights first led them to adopt, they
are obliged to vary and to contradict in terms, by dividing
that which their hypothesis and the expression of it assumed
to be indivisible.

While, therefore, I fully recognise a great natural truth
in the definite ratios presented by a vast number of chemical
combinations, and in the per saltum steps in which nearly all
take place, I cannot accept as an argument in favour of an
atomic theory, those combinations which are made to support
it by the application of an arbitrary notation.

A similar straining of theory seems gradually obtaining
in regard to the doctrine of compound radicals. The discov-
ery of cyanogen by Gay-Lussac was probably the first in-
ducement to the doctrine of compound radicals ; a doctrine
which is now generally, perhaps too generally, received in
organic chemistry. As, in the case of cyanogen, a body ob-
viously compound discharged in almost all its reactions the
functions of an element, so in many other cases it was found
that compound bodies in which a great number of elements
existed, might be regarded as binary combinations, by con-
sidering certain groups of these elements as a compound rad-
ical ; that is, as a simple body when treated of in relation to
the other complex substances of which it forms part, and
only as non-elementary when referred to its internal consti-
tution.

168 COEEELATION OF PHYSICAL FORCES.

Undoubtedly, by approximating in theory the reactions of
inorganic and of organic chemistry, by keeping the mind
within the limits of a beaten path, instead of allowing it to
wander through a maze of isolated facts, the doctrine of com-
pound radicals has been of service ; but, on the other hand,
the indefinite variety of changes which may be rung upon the
composition of an organic substance, by different associations
of its primary elements, makes the binary constituents vary
as the minds of the authors who treat of them, and makes
their grouping depend entirely upon the strength of the anal-
ogies presented to each individual mind. From this cause,
and from the extreme license which has been taken in theo-
retic groupings deduced from this doctrine, a serious question
arises whether it may not ultimately, unless carefully re-
stricted, produce confusion rather than simplicity, and be to
the student an embarrassment rather than an assistance.

I

yni.— OTHEK MODES OF FOKCE.

CATALYSIS, or the chemical action induced by the
mere presence of a foreign body, embraces a class of
facts which must considerably modify many of our notions of
chemical action : thus oxygen and hydrogen, when mixed in
a gaseous state, will remain unaltered for an indefinite pe-
riod ; but the introduction to them of a slip of clean plati-
num will cause more or less rapid combination, without being
in itself in any respect altered. On the other hand, oxygen-
ated water, which is a compound of one equivalent of hydro-
gen plus two of oxygen, will, when under a certain tempera-
ture, remain perfectly stable ; but touch it with platinum in
a state of minute division, and it is instantly decomposed,
one equivalent of oxygen being set free. Here, again, the
platinum is unaltered, and thus we have synthesis and analy-
sis effected apparently by the mere contact of a foreign body.
It is not improbable that the increased electrolytic power of
water by the addition of some acids, such as the sulphuric
and phosphoric, where the acids themselves are not decom-
posed, depends upon a catalytic effect of these acids ; but we
know too little of the nature and rationale of catalysis to ex-
press any confident opinion on its modes of action, and pos-
sibly we may comprehend very different molecular actions
under one and the same name. In no case does catalysis
yield us new power or force : it only determines or facilitates
8

170 COEEELATION OF PHYSICAL FORCES.

the action of chemical force, and, therefore, is no creation of
force by contact.

The force so developed by catalysis may be converted
into a voltaic form thus : in a single pair of the gas battery
above alluded to, one portion of a strip of platinum is im-
mersed in a tube of oxygen, the other in one of hydrogen,
both the gases and the extremities of the platinum being con-
nected by water or other electrolyte ; a voltaic combination
is thus formed, and electricity, heat, light, magnetism, and
motion, produced at the wiU of the experimenter.

In this combination we have a striking instance of cor-
relative expansions and contractions, analogous, though in a
much more refined form, to the expansions and contractions
by heat and cold detailed in the early part of this essay, and
illustrated by the alternations of two bladders partially filled
with air : thus, as by the efiect of chemical combination in
each pair of tubes of the gas battery the gases oxygen and
hydrogen lose their gaseous character and shrink into water,
^o at the platinum terminals of the battery, when immersed
in water, water is decomposed, and expands into oxygen and
hydrogen gases. The correlate of the force which changes
gas into liquid at one point of space, changes liquid into gas
at another, and the exact volume which disappears in the
one place reappears in the other ; so that it would appear to
an inexperienced eye as though the gases passed through
solid wires.

Gravitation, inertia, and aggregation, were but cursorily
alluded to in my original lectures ; their relation to the other
modes of force seemed to be less definitely traceable ; but the
phenomenal effects of gravitation and inertia, being motion
and resistance to motion, in considering motion I have in
some degree included their relations to the other forces.

To my mind gravitation would only produce other force
when the motion caused by it ceases. Thus, if we suppose
a meteor to be a mass rotating in an orbit round the earth,

OTHEB MODES OF FORCE. 171

and with no resisting medium, then, as long as that rotation
continues, the motion of the meteoric mass itself would be
the exponent of the force impelling it ; if there be a resist-
ing medium, part of this motion would be arrested and taken
up by the medium, either as motion, heat, electricity, or some
other mode of force ; if the meteor approach the earth suffi-
ciently to fall upon it, the perceptible motion of the meteor
is stopped, but is taken up by the earth which vibrates
through its mass ; part also reappears as heat in both earth
and meteor, and part in the change in the earth's position
consequent on its increase of gravity, and so on. Gravita-
tion is but the subjective idea, and its relation to other modes
of force seems to me to be identical with that of pressure or
motion. Thus, when arrested motion produces heat, it mat-
ters not whether the motion has been produced by a falling
body, i. e. by gravitation, or a body projected by an explo-
sive compound, &c. ; the heat will be the same, provided the
mass and velocity at the time of arrest be the same. In no
other sense can I conceive a relation between gravitation and
the other forces, and, with all diffidence, I cannot agree with
those who seek a more mysterious link,

Mosotti has mathematically treated of the identity of
gravitation with cohesive attraction, and Pliicker has recently
succeeded in showing that crystalline bodies are definitely af-
fected by magnetism, and take a position in relation to the
lines of magnetic force dependent upon their optical axis or
,axis of symmetry.

What is termed the optic axis is a fixed direction through
crystals, in which they do not doubly refract light, and which
direction, in those crystals which have one axis of figure, or
a line around which the figure is symmetrical, is parallel to
the axis of symmetry. When submitted to magnetic influ-
ence such crystals take up a position, so that their optic axis
points diamagnetically or transversely to the lines of magnetic
force ; and when, as is the case in some crystals, there ia

172 COEEELATION OF PHYSICAL FORCES.

more than one optic axis, the resultant of these axes points
diamagnetically. The mineral cyanite is injfluenced by mag-
netism in so marked a manner that when suspended it wiU
arrange itself definitely with reference to the direction of ter-
restrial magnetism, and may, according to Plucker, be used
as a compass-needle.

There is scarcely any doubt that the force which is con-
cerned in aggregation is the same which gives to matter its
crystalline form ; indeed, a vast number of inorganic bodies,
if not all, which appear amorphous are, when closely exam-
ined, found to be crystalline in their structure : we thus get a
reciprocity of action between the force which unites the mole-
cules of matter and the magnetic force, and through the me-
dium of the latter the correlation of the attraction of aggre-
gation with the other modes of force may be established.

I believe that the same principles and mode of reasoning
as have been adopted in this essay might be applied to the
organic as well as the inorganic world ; and that muscular
force, animal and vegetable heat, &c., might, and at some
time will, be shown to have similar definite correlations ; but I
have purposely avoided this subject, as pertaining to a depart-
ment of science to which I have not devoted my attention.
I ought, however, while alluding to this subject, shortly to
mention some experiments of Professor Matteucci, communi-
cated to the Royal Society in the year 1850, by which it ap-
pears that whatever mode of force it be which is propagated
along the nervous filaments, this mode of force is definitely*
affected by currents of electricity. His experiments show
that when a current of positive electricity traverses a portion
of the muscle of a living animal in the same direction as that
in which the nerves ramify— i. e. a direction from the brain
to the extremities — a muscular contraction is produced in the
limb experimented on, showing that the nerve of motion is
affected ; while, if the current, as it is termed, be made to
traverse the muscle in the reverse direction, or towards the

I

OTHER MODES OF FOECE. 173

nervous centres, the animal utters cries, and exhibits all the
indications of suffering pain, scarcely any muscular move-
ment being produced ; showing that in this case the nerves
of sensation are affected by the electric current, and therefore
that some definite polar condition exists, or is induced, in the
nerves, to which electricity is correlated, and that probably
this polar condition constitutes nervous agency. There are
other analogies given in the papers of M. Matteucci, and de-
rived from the action of the electrical organs of fishes, which
tend to corroborate and develope the same view.

By an application of the doctrine of the Correlation of
Forces, Dr. Carpenter has shown how a difiiculty arising
from the ordinary notions of the developement of an organised
being from its germ-cell may be lessened. It has been
thought by many physiologists that the nisus formativus, or
organising force of an animal or vegetable structure, lies dor-
mant in the primordial germ-cell. ' So that the organising
force required to build up an oak or a palm, an elephant or a
whale, is concentrated in a minute particle only discernible
by microscopic aid.'

Certain other views of nearly equal difficulty have been
propounded. Dr. Carpenter suggests the probability of ex-
traneous forces, as heat, light, and chemical affinity, contin-
uously operating upon the material germ ; so that all that is
required in this is a structure capable of receiving, directing,
and converting these forces into those which tend to the assim-
. ilation of extraneous matter and the definite developement of
the particular structure. In proof of this position he shows
how dependent the process of germ developement is upon the
presence and agency of external forces, particularly heat and
light, and how it is regulated by the measure of these forces
supplied to it.

It certainly is far less difficult so to conceive the supply
of force yielded to organised beings in their gradual process
of growth, than to suppose a store of dormant or latent force
pent up in a microscopic monad.

174: COEBELATION OF PHYSICAL FOECES. *

As by the artificial structure of a voltaic battery, chemi-
cal actions may be made to cooperate in a definite direction,
so, by the organism of a vegetable or animal, the mode of
motion which constitutes heat, light, &c., may, without extra-
vagance, be conceived to be appropriated and changed into
the forces which induce the absorption, and assimilation of
nutriment, and into nervous agency and muscular power.
Indications of similar thoughts may be detected in the writings
of Liebig.

Some difficulty in studying the correlations of vital with
inorganic physical forces arises from the efiects of sensation
and consciousness, presenting a similar confusion to that
alluded to, when, in treating of heat, I ventured to suggest,
that observers are too apt to confound the sensations with the
phenomena. Thus, to apply some of the considerations on
force, given in the introductory portion of this essay, to cases
where vitality or consciousness intervenes. When a weight
is raised by the hand, there should, according to the doctrine
of non-creation of force, have been somewhere an expenditure
equivalent to the amount of gravitation overcome in raising
the weight. That there is expenditure we can prove, though
in the present state of science we cannot measure it. Thus,
prolong the efibrt, raise weights for an hour or two, the vital
powers sink, food, i. e. fresh chemical force, is required to
supply the exhaustion. If this supply is withheld and the
exertion is continued, we see the consumptiofi of force in
the supervening weakness and emaciation of the body.

The consciousness of effort, which has formed a topic of
argument by some writers when treating of force, and is by
them believed to be that which has originated the idea of
force, may by the physical student be regarded as feeHng is
in the phenomena of heat and cold, viz. a sensation of the
struggle of opposing molecular motions in overcoming the
resistance of the masses to be moved. When we say we feel
hot, we feel cold, we feel that we are exerting ourselves, our

OTHER MODES OF FORCE. 176

expressions are intelligible to beings who are capable of ex-
periencing similar sensations ; but the physical changes
accompanying these sensations are not thereby explained.
Without pretending to know what probably we shall never
know, the actual modus agendi of the brain, nerves, muscles,
&c., we may study vital as we do inorganic phenomena, both by
observation and experiment. Thus, Sir Benjamin Brodie has
examined the effect of respiration on animal heat by inducing
artificial respiration after the spinal cord has been severed ;
in which case he finds the animal heat declines, notwith-
standing the continuance of the chemical action of respiration,
carbonic acid being formed as usual ; but he also finds that
under such circumstances the struggles or muscular actions
of the animal are very great, and sufficient probably to ac-
count for the force eliminated by the chemical action in
digestion and respiration ; and Liebig, by measuring the
amount of chemical action in digestion and respiration, and
comparing it with the labour performed, has to some extent
established their equivalent relations.

Mr. Helmholtz has found that the chemical changes which
take place in muscles are greater when these are made to
undergo contractions than when they are in repose ; and that,
as would be expected, the consumption of the matter of the
muscles, or, in other terms, the waste or excrementitious
matter thrown off, is greater in the former than in the latter
case.

M. Matteucci has ascertained that the muscles of recently
killed frogs absorb oxygen and exhale carbonic acid, and that
when they are thrown into a state of contraction, and still
more when they perform mechanical work, the absorption is
increased ; and he even calculates the equivalents of work so
performed.

M. Beclard finds that the quantity of heat produced by
voluntary muscular contraction in man is greater when that
contraction is what he tei-ms static, that is, when it produ^pa

176 CORRELATION OF PHYSICAL FORCES.

no external work, but is effort alone, than when that effort
and contraction are employed dynamically, so as to raise a
weight or produce mechanical work.

Thus, though we may see no present promise of being
able to resolve sensations into their ultimate elements, or to
trace, physically, the link which unites volition with exertion
or effort, in terms of our own consciousness of it, we may
hope to approximate the solution of these deeply interesting
questions.

In the same individual the chemical and physical state of
the secretions in the warm may be compared with those in
the cold parts of the body. The changes in digestion and
respiration, when the body is in a state of rest, may be com-
pared with those which obtain when it is in a state of activity.
The relations with external matter, maintaining, by the con-
stant play of natural forces, the vital nucleus, or the organi-
sation by means of which matter and force receive, for a
definite period, a definite incorporation and direction, may be
ascertained, while the more minute structural changes are
revealed to us by the ever-improving powers of the micro-
scope ; and thus step by step we may learn that which it is
given to us to learn, boundless in its range and infinite in its
progress, and therefore never giving a response to the ultimate
—Why?

As the first glimpse of a new star is caught by the eye of
the astronomer while directing his vision to a different point
of space, and disappears when steadfastly gazed at, only to
have its position and figure ultimately ascertained by the em-
ployment of more penetrative powers, so the first scintillations
of new natural phenomena frequently present themselves to
the eye of the observer, dimly seen when viewed askance,
and disappearing if directly looked for. When new powers
of thought and experiment have developed and corrected the
first notions, and given a character to the new image, proba-
bly very different from the first impression, fresh objects are

OTHER MODES OF FORCE. 177

again glanced at in the margin of the new field of vision,
which in their turn have to be verified, and again lead to
new extensions ; thus the effort to establish one observation
leads to the imperfect perception of new and wider fields of
research ; and, instead of approaching finality, the more
we discover, the more infinite appears the range of the undis-
covered I

% 8*

IX.— coisrcLUDiKa eemaeks.

I HAVE now gone through the affections of matter for
which distinct names have been given in our received
nomenclature : that other forces may be detected, differing as
much from them as they differ from each other, is highly
probable, and that when discovered, and their modes of
action fully traced out, they will be found to be related inter
se, and to these forces as these are to each other, I believe
to be as far certain as certainty can be predicted of any future
event.

It may in many cases be a difficult question to determine
what constitutes a distinct affection of matter or mode of
force. It is highly probable that different lines of demarca-
tion would have been drawn between the forces already
known, had they been discovered in a different manner, or
first observed at different points of the chain which connects
them. Thus, radiant heat and light are mainly distinguished
by the manner in which they affect our senses : were they
viewed according to the way in which they affect' inorganic
matter, very different notions would possibly be entertained
of their character and relation. Electricity, again, was
named from the substance in which, and magnetism from the
district where, it first happened to be observed, and a chain
of intermediate phenomena have so connected electricity with
galvanism that they are now regarded as the same force,

CONCLUDING REMAKKS. 179

differing only in the degree of its intensity and quantity,
though for a long time they were regarded as distinct.

The phenomenon of attraction and repulsion by amber,
which originated the term electricity ^ is as unlike that of the
decomposition of water by the voltaic pile, as any two natural
phenomena can well be. It is only because the historical
sequence of scientific discoveries has associated them by a
number of intermediate links, that they are classed under the
same category. What is called voltaic electricity might
equally, perhaps more appropriately, be called voltaic chemis-
try. I mention these facts to show that the distinction in the
name may frequently be much greater than the distinction of
the subject which it represents, and vice versa, not as at all
objecting to the received nomenclature on these points ; nor
do I say it would be advisable to depart from it : were we to
do so, inevitable confusion would result, and objections
equally forcible might be found to apply to our new termi-
nology.

Words, when established to a certain point, become a
part of the social mind ; its powers and very existence de-
pend upon the adoption of conventional symbols ; and were
these suddenly departed from, or varied, according to indivi-
dual apprehensions, the acquisition and transmission of knowl-
edge would cease. Undoubtedly, neology is more permissi-
ble in physical science than in any other branch of knowledge,
because it is more progressive ; new facts or new relations
require new names, but even here it should be used with great
caution.

Si forte necesse est
Indiciis monstrare recentibus abdita rerum,
Fingere cinctutis non exaudita Cethegis,
Oontinget ; dabiturque licentia, smnpta pudenter.

Even should the mind ever be led to dismiss the idea of
various forces, and regard them as the exertion of one force.

180 COEKELATION OF PHYSICAL FORCES.

oT' resolve them definitely into motion ; still we could never
avoid the use of difierent conventional terms for the different
modes of action of this one pervading force.

Reviewing the series of relations between the various
forces which we have been considering, it would appear that
in many cases where one of these is excited or exists, all the
others are also set in action : thus, when a substance, such as
sulphuret of antimony, is electrified, at the instant of electri-
sation it becomes inagnetic in directions at right angles to the
lines of electric force ; at the same time it becomes heated to
an extent greater or less according to the intensity of the
electric force. If this intensity be exalted to a certain point
the sulphuret becomes luminous, or light is produced ; it ex-
pands, consequently motion is produced ; and it is decomposed,
therefore chemical action is produced. If we take another
substance, say a metal, all these forces except the last are
developed; and although we can scarcely apply the term
mechanical action to a substance hitherto undecomposed, and
which, under the circumstances we are considering, enters
into no new combination, yet it undergoes that species of
polarisation which, as far as we can judge, is the first step
towards chemical action, and which, if the substance were
decomposable, would resolve it into its elements. Perhaps,
indeed, some hitherto undiscovered chemical action is pro-
duced in substances which we regard as undecomposable :
there are experiments to show that metals which have been
electrised are permanently changed in their molecular consti-
tution. Oxygen, we have seen, is changed by the electric
spark into ozone, and phosphorus into allotropic phosphorus,
both which changes were for a long time unknown to those
familiar with electrical science.

Thus, with some substances, when one mode of force is
produced all the others are simultaneously developed. "With
other substances, probably with all matter, some of the other
forces are developed, whenever one is excited, and all may be

CONCLUDING KEMAKKS. 181

SO were the matter in a suitable condition for their develope-
ment, or our means of detecting them sufficiently delicate.

This simultaneous production of several different forces
seems at first sight to be irreconcileable with their mutual and
necessary dependence, and it certainly presents a formidable
experimental difficulty in the way of establishing their equiv-
alent relations ; but when examined closely, it is not in fact
inconsistent with the views we have been considering, but is
indeed a strong argument in favour of the theory which re-
gards them as modes of motion.

Let us select one or two cases in which this form of ob-
jection may be prominently put forward. A voltaic battery
decomposing water in a voltameter, while the same current
is employed at the same time to make an electro-magnet,
gives nevertheless in the voltameter an equivalent of gas, or
decomposes an equivalent of an electrolyte for each equiva-
lent of chemical decomposition in the battery cells, and will
give the same ratios if the electro-magnet be removed. Here,
at first sight, it would appear that the magnetism was an ex-
tra force produced, and that thus more than the equivalent
power was obtained from the battery. In answer to this
objection it may be said, that in the circumstances under
which this experiment is ordinarily performed, several cells
of the battery are used, and so there is a far greater amount
of force generated in the cells than is indicated by the effect
in the voltameter. If, moreover, the magnet be not inter-
posed, stni the magnetic force is equally existent throughout
the whole current ; for instance, the wires joining the plates
will attract iron filings, deflect magnetic needles, &c., and
produce diamagnetic effects on surrounding matter. By the
iron core a small portion of the force is, indeed, absorbed
while it is being made a magnet, but this ceases to be ab-
sorbed when the magnet is made ; this has been proved by
the observation of Mr. Latimer Clarke, who has found that
along the wires of the electric telegraph the magnetic needles

182 COKRELATION OF PHYSICAL FORCES.

placed at different stations remained fixed after the connection
with the battery was made, and while the electric current
acted by induction on surrounding conducting matter, separa-
ted from the wires by their gutta percha coating, so that a
sort of Leyden phial was formed ; but as soon as this induc-
tion had produced its effect between each station, or, so to
speak, the phial was charged, the needles successively were
deflected : it is like the case of a pulley and weight, which lat-
ter exhausts force while it is being raised ; but when raised,
the force is free, and may be used for other purposes.

If a battery of one cell, just capable of decomposing water
and no more, be employed, this will cease to decompose while
making a magnet. There must, in every case, be prepon-
derating chemical affinity in the battery cells, either by the
nature of its elements or by the reduplication of series, to
effect decomposition in the voltameter ; and if the point is
just reached at which this is effected, and the power is then
reduced by any resistance, decomposition ceases : were it
otherwise, were the decomposition in the voltameter the
exponent of the entire force of the generating cells, and these
could independently produce magnetic force, this latter
force would be got from nothing, and perpetual motion be
obtained.

To take another and different example : A piece of zinc
dissolved in dilute sulphuric acid gives somewhat less heat than
when the zinc has a wire of platinum attached to it, and is
dissolved by the same quantity of acid. The argument is
deduced that, as there is more electricity in the second than
in the first case, there should be less heat ; but as, according
to our received theories, the heat is a product of the electric
current, and in consequence of the impurity of zinc electrici-
ty is generated in the first case molecularly, in what is called
local action, though not thrown into a general direction,
there should be more of both heat and electricity in the sec-
ond than in the first case, as the heat and electricity due to

I

CONCLUDING EEMAKKS. 183

the voltaic combination of zinc and platinum are added to
that excited on the surface of the zinc, and the zinc should be,
as in fact it is, more rapidly dissolved ; so that the extra heat
and electricity is produced by extra chemical force. Many
additional cases of a similar description might be suggested.
But although it is difficult, and perhaps impossible, to restrict
the action of any one force to the production of one other
force, and of one only — ^yet if the whole of one force, say
chemical action, be supposed to be employed in producing its
full equivalent of another force, say heat, then as this heat is
capable in its turn of reproducing chemical action, and in the
limit, a quantity equal or at least only infinitely short of the
initial force : if this could at the same time produce indepen-
dently another force, say magnetism, we could, by adding
the magnetism to the total heat, get more than the original
chemical action, and thus create force or obtain perpetual
motion.

The term Correlation, which I selected as the title of my
Lectures in 1843, strictly interpreted, means a necessary
mutual or reciprocal dependence of two ideas, inseparable
even in mental conception : thus, the idea of height cannot
exist without involving the idea of its correlate, depth ; the
idea of parent cannot exist without involving the idea of off-
spring. It has been scarcely, if at all, used by writers on
physics, but there are a vast variety of physical relations to
which, if it does not in its strictest original sense apply,
cannot certainly be so well expressed by any other term.
There are, for example, many facts, one of which cannot take
place without involving the other ; one arm of a lever can-
not be depressed without the other being elevated — the finger
cannot press the table without the table pressing the finger.
A body cannot be heated without another being cooled, or
some other force being exhausted in an equivalent ratio to
the production of heat ; a body cannot be positively elec-
trified without some other body being negatively electri-
fied, &c.

184 COKEELATION OF PHYSICAL F0ECE8.

The probability is, that, if not all, the greater number of
physical phenomena are correlative, and that, without a
duality of conception, the mind cannot form an idea of them :
thus motion cannot be perceived or probably imagined with-
out parallax or relative change of position. The world was
believed fixed, until by comparison with the celestial bodies,
it was found to change its place with regard to them : had
there been no perceptible matter external to the world, we
should never have discovered its motion. In sailing along a
river, the stationary vessels and objects on the banks seem
to move past the observer : if at last he arrives at the convic-
tion that he is moving, and not these objects, it is by correct-
ing his senses by reflection derived from a more extensive
previous use of them : even then he can only form a notion
of the motion of the vessel he is in, by its change of position
with regard to the objects it passes — that is, provided his
body partakes of the motion of the vessel, which it only does
when its course is perfectly smooth, otherwise the. relative
change of position of the diflferent parts of the body and the
vessel inform him of its alternating, though not of its pro-
gressive movement. So in all physical phenomena, the efiects
produced by motion are all in proportion to the relative mo-
tion : thus, whether the rubber of an electrical machine be
stationary, and the cylinder mobile, or the rubber mobile and
the cylinder stationary, or both mobile in different directions,
or in the same direction with different degrees of velocity,
the electrical effects are, cceteris paribus, precisely the same,
provided the relative motion is the same, and so, without ex-
ception, of aU other phenomena. The question of whether
there can be absolute motion, or, indeed, any absolute isolated
force, is purely the metaphysical question of idealism or real-
ism— a question for our purpose of little import ; sufficient
for the purely physical inquirer, the maxim ' de non ajoparenti-
hus et non existentihus eadem est ratio.'

The sense I have attached to the word correlation, in

CONCLUDING REMARKS. 185

treating of physical phenomena, will, I think, be evident from
the previous parts of this essay, to be that of a necessary
reciprocal production : in other words, that any force capable
of producing another may, in its turn, be produced by it —
nay, more, can be itself resisted by the force it produces, in
proportion to the energy of such production, as action is ever
accompanied and resisted by reaction : thus, the action of an
electro-magnetic machine is reacted upon by the magneto-
electricity developed by its action.

To many, however, of the cases we have been consider-
ing, the term correlation may be applied in a more strict
accordance with its original sense : thus, with regard to the
forces of electricity and magnetism in a dynamic state, we
cannot electrise a substance without magnetising it — we can-
not magnetise it without electrising it :— each molecule, the
instant it is affected by one of these forces, is affected by the
other ; but, in transverse directions, the forces are insepara-
ble and mutually dependent — correlative, but not identical.

The evolution of one force or mode of force into another
has induced many to regard all the different natural agencies
as reducible to unity, and as resulting from one force which
is the efficient cause of all the others : thus, one author writes
to prove that electricity is the cause of every change in
matter ; another, that chemical action is the cause of every-
thing ; another, that heat is the universal cause, and so on.
If, as I have stated it, the true expression of the fact is, that
each mode of force is capable of producing the others, and
that none of them can be produced but by some other as an
anterior force, then any view which regards either of them as
abstractedly the efficient cause of all the rest, is erroneous ;
the view has, I believe, arisen from a confusion between the
abstract or generalised meaning of the term cause, and its
concrete or special sense ; the word itself being indiscrimi-
nately used in both these senses.

Another confusion of terms has arisen, and has, indeed,

186 CORRELATION OF PHYSICAL FORCES.

much embarrassed me in enunciating the propositions put
forth in these pages, on account of the imperfection of scien-
tific language ; an imperfection in great measure unavoidable,
it is true, but not the less embarrassing. Thus, the words
light, heat, electricity, and magnetism, are constantly used in
two senses — ^viz. that of the force producing, or the subject-
ive idea of force or power, and of the effect produced, or the
objective phenomenon. The word motion, indeed, is only
applied to the effect, and not to the force, and the term chem-
ical affinity is generally applied to the force, and not to the
effect ; but the other four terms are, for want of a distinct
terminology, applied indiscriminately to both.

I may have occasionally used the same word at one time
in a subjective, at another in an objective sense ; all I can
say is, that this cannot be avoided without a neology, which
I have not the presumption to introduce, or the authority to
enforce. Again, the use of the term forces in the plural
might be objected to by those who do not attach to the term
force the notion of a specific agency, but of one universal
power associated with matter, of which its various phenom-
ena are but diversely modified effects.

Whether the imponderable agents, viewed as force, and
not as matter, ought to be regarded as distinct forces or as
distinct modes of force, is probably not very material, for, as
far as I am aware, the same result would follow either view ;
I have therefore used the terms indiscriminately, as either
happened to be the more expressive for the occasion.

Throughout this essay I have placed motion in the same
category as the other affections of matter. The course of
reasoning adopted in it, however, appears to me to lead inev-
itably to the conclusion that these affections of matter are
themselves modes of motion ; that, as in the case of friction,
the gross or palpable motion, which is arrested by the con-
tact of another body, is subdivided into molecular motions or
vibrations, which vibrations are heat or electricity, as the

I

CONCLIJDINa REMARKS. 187

case may be ; so the other affections are only matter moved
or molecularly agitated in certain definite directions. We
have abeady considered the hypothesis that the passage of
electricity and magnetism causes vibrations in an ether per-
meating the bodies through which the current is transmitted,
or the application of the same ethereal hypothesis to these
imponderables which had previously been applied to light ;
many, in speaking of some of the effects, admit that electri-
city and magnetism cause or produce by their passage vibra-
tions in the particles of matter, but regard the vibrations
produced as an occasional, though not always a necessary,
effect of the passage of electricity, or of the increment or
decrement of magnetism. The view which I have taken is,
that such vibrations, molecular polarisations, or motions of
some sort from particle to particle, are themselves electricity
or magnetism ; or, to express it in the converse, that dynamic
electricity and magnetism are themselves motion, and that
permanent magnetism, and Franklinic electricity, are static
conditions of force bearing a similar relation to motion which
tension or gravitation do.

This theory might well be discussed in greater detail
than has been used in this work ; but to do this and to anti-
cipate objections would lead into specialities foreign to my
present object, in the course of this essay my principal aim
having been rather to show the relation of forces as evinced
by acknowledged facts, than to enter upon any detailed ex-
planation of their specific modes of action.

Probably man will never know the ultimate structure of
matter or the minutiae of molecular actions ; indeed it is
scarcely conceivable that the mind can ever attain to this
knowledge ; the monad irresolvable by a given microscope
may be resolved by an increase in power. Much harm has
already been done by attempting hypothetically to dissect
matter and to discuss the shapes, sizes, and numbers of at-
oms, and their atmospheres of heat, ether, or electricity.

188 COEEELATION OF PHYSICAL FOECES.

Whether the regarding electricity, light, magnetism, &c.,
as simply motions of ordinary matter, be or be not admissi-
ble, certain it is, that all past theories have resolved, and all
existing theories do resolve, the actions of these forces into
motion. Whether it be that, on account of our familiarity
with motion, we refer other affections to it, as to a language
which is most easily construed and most capable of explain-
ing them ; whether it be that it is in reality the only mode in
which our minds, as contradistinguished from our senses, are
able to conceive material agencies ; certain it is, that since
the period at which the mystic notions of spiritual or preter-
natural powers were applied to account for physical phenom-
ena, all hypotheses framed to explain them have resolved
them into motion. Take, for example, the theories of light
to which I have before alluded : one of these supposes light
to be a highly rare matter, emitted from — i. e. put in motion
by — ^luminous bodies ; a second supposes that the matter is
not emitted from luminous bodies, but that it is put into a
state of vibration or undulation, i. e. motion^ by them ; and
thirdly, light may be regarded as an undulation or motion of
ordinary matter, and propagated by undulation of air, glass,
&c., as I have before stated. In all these hypotheses, matter
and motion are the only conceptions. Nor, if we accept
terms derived from our own sensations, the which sensations
themselves may be but modes of motion in the nervous fila-
ments, can we find words to describe phenomena other than
those expressive of matter and motion. We in vain struggle
to escape from these ideas ; if we ever do so, our mental
powers must undergo a change of which at present we see
no prospect.

If we apply to any other force the mode of reasoning
which we have applied to heat, we shall arrive at the same
conclusion, and see that a given source of power can, sup-
posing it to be fully utilised in each case, yield no more by
employing it as an exciter of one force than of another. Let

CONCLUDING BEMABKB. 189

US take electricity as an example. Suppose a pound of mer*
cury at 400° be employed to produce a thermo-electric cur-
rent, and the latter be in its turn employed to produce me-
chanical force ; if this latter force be greater than that which
the direct effect of heat would produce, then it could by com-
pression raise the temperature of the mercury itself, or of a
similar quantity equally heated, to a higher point than its
original temperature, the 400° to 401°, for example, which
is obviously Impossible ; nor, if we admit force to be inde-
structible, can it produce less than 400°, or cool the second
body except by some portion of it being converted into
another form or mode of force.

But as the mechanical effect here is produced through the
medium of electricity, and the mechanical efifect is definite,
so the quantity of electricity producing it must be definite
also, for unequal quantities of electricity could only produce
an equal mechanical effect by a loss or gain of their own
force into or out of nothing. The same reasoning will apply
to the other forces, and wiU lead, it appears to me, necessa-
rily and inevitably to the conclusion, that each force is defi-
nitely and equivalently convertible into any other, and that
where experiment does not give the full equivalent, it is be-
cause the initial force has been dissipated, not lost, by con-
version into other unrecognised forces. The equivalent is the
limit never practically reached.

The great problem which remains to be solved, in regard
to the correlation of physical forces, is this establishment of
their equivalents of power, or their measurable relation to a
given standard. The progress made in some of the branches
of this inquiry has been already noticed. Viewed in their
static relations, or in the conditions requisite for producing
equilibrium or quantitative equality of force, a remarkable
relation between chemical affinity and heat is that discovered
in many simple bodies by Dulong and Petit, and extended to
compounds by Neumann and Avogadro. Their researches

190 CORRELATION OF PHYSICAL FORCES.

have shown that the specific heats of certain substances,
when multiplied by their chemical equivalents, give a con-
stant quantity as product — or, in other words, that the com-
bining weights of such substances are those weights which
require equal accessions or abstractions of heat, equally to
raise or lower their temperature. To put the proposition
more in accordance with the view we have taken of the na-
ture of heat : each body has a power of communicating or
receiving molecular repulsive power, exactly equal, weight
for weight, to its chemical or combining power. For in-
stance, the equivalent of lead is 104, of zinc 33, or, in round
numbers, as 3 to 1 : these numbers are therefore inversely
the exponents of their chemical power, three times as much
lead as zinc being required to saturate the same quan-
tity of an acid or substance combining with it ; but their
power of communicating or abstracting heat or repulsive
power is precisely the same, for three times as much lead as
zinc is required to produce the same amount of expansion or
contraction in a given quantity of a third substance, such as
water.

Again, a great number of bodies chemically combine in
equal volumes, i. e. in the ratios of their specific gravities ;
but the specific gravities represent the attractive powers of
the substance, or are the numerical exponents of the forces
tending to produce motion in masses of matter towards each
other ; while the chemical equivalents are the exponents of
the afiinities or tendencies of the molecules of dissimilar sub-
stances to combine, and saturate each other ; consequently,
here we have to some extent an equivalent relation between
these two modes of force— gravitation and chemical attrac-
tion.

Were the above relations extended into an universal law,
we should have the same numerical expression for the three
forces of heat, gravity, and affinity ; and as electricity and
magnetism are quantitatively related to them, we should have

CONCLUDING EEMAEKS.^ 191

a similar expression for these forces": but at present the bod-
ies in which this parity of force has been discovered, though
in themselves numerous, are small compared with the excep-
tions, and, therefore, this point can only be indicated as prom-
ising a generalisation, should subsequent researches alter our
knowledge as to the elements and combining equivalents of
matter.

"With regard to what may be called dynamic equivalents,
i. e. the definite relation to time of the action of these varied
forces upon equivalents of matter, the difiiculty of establish-
ing them is still greater. If the proposition which I stated
at the commencement of this paper be correct, that motion
may be subdivided or changed in character, so as to become
heat, electricity, &c., it ought to follow that when we collect
the dissipated and changed forces, and reconvert them, the
initial motion, minus an infinitesimal quantity afiecting the
same amount of matter with the same velocity, should be re-
produced, and so of the changes in matter produced by the
other forces ; but the difficulties of proving the truth of this
by experiment will, in many cases, be all but insuperable ;
we cannot imprison motion as we can matter, though we may
to some extent restrain its direction.

The term perpetual motion, which I have not unfrequent-
ly employed in these pages, is itself equivocal. If the doc-
trines here advanced be founded, all motion is, in one sense,
perpetual. In masses whose motion is stopped by mutual
concussion, heat or motion of the particles is generated ; and
thus the motion continues, so that if we could venture to extend
such thoughts to the universe, we should assume the same
amount of motion affecting the same amount of matter forever.
Wliere force opposes force, as in cases of static equilibrium,
the balance of pre-existing equilibrium is affected, and fresh
motion is started equivalent to that which is withdrawn into
a state of abeyance.

But the term perpetual motion is applied, in ordinary par-

192 COERELATION OF PHYSICAL FOECES.

lance (and in such sense 1 have used it), to a perpetual recur-
rent motion, e.g. a weight which by its fall would turn a
wheel, which wheel would, in its turn, raise the initial weight,
and so on forever, or until the material of which the machine
is made be worn out. It is strange that to common appre-
hension the impossibility of this is not self-evident : if the in-
itial weight is to be raised by the force it has itself generated,
it must necessarily generate a force greater than that of its
own weight or centripetal attraction ; in other words, it must
be capable of raising a weight heavier than itself: so that,
setting aside the resistance of friction, &c., a weight, to pro-
duce perpetual recurrent motion, must be heavier than an
equal weight of matter, in short, heavier than itself.

Suppose two equal weights at each end of an equi-armed
lever, there is no motion ; cut off a fraction of one of them,
and it rises while the other falls. How, now, is the lesser
weight to bring back the greater without any extraneous ap-
plication of force ? If, as is obvious, it cannot do so in this
simple form of experiment, it is d fortiori more impossible if
machinery be added, for increased resistances have then to
be overcome. Can we again mend this by employing any
other force? Suppose we employ electricity, the initial
weight in descending turns a cylinder against a cushion, and
so generates electricity ; to make this force recurrent, the
electricity so generated must, in its turn, raise the initial
weight, or one heavier than it, i. e. the initial weight must,
through the medium of electricity, raise a weight heavier than
itself. The same problem, applied to any other forces, will
involve the same absurdity : and yet simple as the matter
seems, the world is hardly yet disabused of an idea little re-
moved from superstition.

But the importance of the deductions to be derived from
the negation of perpetual motion seems scarcely to have im-
pressed philosophers, and we only find here and there a scat-
tered hint of the consequences necessarily resulting from that

CONCLUDING EEMAKKS. 193

which to the thinking mind is a conviction. Some of these
I have ventured to put forward in the present essay, but
many remain, and will crowd upon the mind of those who
pursue the subject. ' Does not, for instance, the impossibility
of perpetual motion, when thought out, involve the demon-
stration of the impossibility, to which I have previously allud-
ed, of any event identically recurring ?

The pendulum in vacuo, at each beat leaves a portion of
the force which started it in the form of heat at its point of
suspension : this force, though ever existent, can never be re-
stored in its integrity to the ball of the pendulum, for in the
process of restoration it must affect other matter, and alter
the condition of the universe. To restore the initial force to its
integrity, everything as it existed at the moment of the first
beat of the pendulum must be restored in its integrity : but
how can this be — for while the force was escaping from the
pendulum by radiating heat from the point of suspension,
surrounding matter has not stood still; the very attraction
which caused the beat of the pendulum has changed in degree,
for the pendulum is nearer to or further from the sun, or
from some planet or fixed star.

It might be an interesting and not profitless peculation
to follow out these and other consequences ; it would, I be-
lieve, lead us to the conviction that the universe is ever
changing, and that notwithstanding secular recurrences which
would prima facie seem to replace matter in its original posi-
tion, nothing in fact ever returns or can return to a state of
existence identical with a previous state. But the field is too
illimitable for me to venture further.

The inevitable dissipation or throwing off a portion of
the initial force presents a great experimental difficulty in the
way of establishing the equivalents of the various natural
forces. In the steam-engine, for instance, the heat of the
furnace not only expands the water and thereby produces the
motion of the piston, but it also expands the iron of the boil-
9

194 COERELATION OF PHYSICAL FORCES.

er, of the cylinder and all surrounding bodies. The force ex-
pended in expanding this iron to a very small extent is equal
to that which expands the vapour to a very large extent : this
expansion of the iron is capable, in its turn, of producing a
great mechanical force, which is practically lost. Could all
the force be applied to the vapour, an enormous addition of
power would be gained for the same expenditure : and per-
haps even with our present means more might be done in
utilising the expansion of the iron.

Another great difficulty in experimentally ascertaining the
dynamic equivalents of different forces arises from the effects
of disruption, or the overcoming an existing force. Thus,
when a part of the initial force employed is engaged in twist-
ing or tearing asunder matter previously held together by
cohesive attraction, or in overcoming gravitation or inertia,
the same amount of heat or electricity would not be evolved
as if such obstacle were non-existent, and the initial force
were wholly employed in producing, not in opposing. There
is a difficulty apparently extreme in devising experiments
in which some portion of the force is not so employed.

The initial force, however, that has been employed for such
disruption is not lost, as at the moment of disruption the
bodies producing it fly off, and carry with them their force.
Thus, let two weights be attached to a cord placed across a
bar ; when their force is sufficient to break the cord or the
bar, the weights fall down and strike the earth, making it
vibrate, and so conveying away or continuing the force ex-
pressed by the cohesion of the bar or cord. If, instead of
breaking a cord, the weights be employed to bend a bar, their
gravitating force, instead of making the earth vibrate, pro-
duces heat in the bar, and so with whatever other force be
employed to produce effects of disruption, torsion, &c., so
that, though difficult in practice, the numerical problem of
the equivalent of the force is not theoretically irresolvable.

The voltaic battery affords us the best means of ascertain-

CONCLUDING EEMAEKS. 195

ing the dynamic equivalents of different forces, and it is probable
that by its aid the best theoretical and practical results will be
ultimately attained.

In investigating the relation of the different forces, I have
in .turn taken each one as the initial force or starting-point,
and endeavoured to show how the force thus arbitrarily se-
lected could mediately or immediately produce and be merged
into the others : but it will be obvious to those who have at-
tentively considered the subject, and brought their minds into
a general accordance with the views I have submitted to them,
that no force can, strictly speaking, be initial, as there must
be some anterior force which produced it : we cannot create
force or motion any more than we can create matter.

Thus, to take an example previously noticed, and recede
backwards ; the spark of light is produced by electricity,
electricity by motion, and motion is produced by something
else, say a steam-engine — that is, by heat. This heat is pro-
duced by chemical affinity, i.e. the affinity of the carbon of
the coal for the oxygen of the air : this carbon and this oxy-
gen have been previously eliminated by actions difficult to
trace, but of the pre-existence of which we cannot doubt,
and in which actions we should find the conjoint and al-
ternating effects of heat, light, chemical affinity, &c. Thus,
tracing any force backwards to its antecedents, we are merged
in an infinity of changing forms of force ; at some point we
lose it, not because it has been in fact created at any definite
point, but because it resolves itself into so many contributing
forces, that the evidence of it is lost to Our senses or powers
of detection ; just as in following it forward into the effect it
produces, it becomes, as I have before stated, so subdivided
and dissipated as to be equally lost to our means of detection.

Can we, indeed, suggest a proposition, definitely conceiv-
able by the mind, of force without antecedent force ? I can-
not, without calling for the interposition of created power,
any more than I can conceive the sudden appearance of a

196 COEBELATION OF PHYSICAL FORCES.

mass of matter come from nowhere, and formed from noth-
ing. The impossibility, humanly speaking, of creating or
annihilating matter, has long been admitted, though, perhaps,
its distinct reception in philosophy may be set down to the
overthrow of the doctrine of Phlogiston, and the reformation
of chemistry at the time of Lavoisier. The reasons for the
admission of a similar doctrine as to force appear to be equally
strong. "With regard to matter, there are many cases in
which we never practically prove its cessation of existence,
yet we do not the less believe in it: who, for instance, can
trace, so as to re- weigh, the particles of iron worn off
the tire of a carriage wheel ? who can re-combine the parti-
cles of wax dissipated and chemically changed in the burning
of a candle? By placing matter undergoing physical or
chemical changes under special limiting circumstances, we
may, indeed, acquire evidence of its continued existence,
weight for weight — and so we may in some instances of force,
as in definite electrolysis : indeed the evidence we acquire of the
continued existence of matter is by the continued exertion of
the force it exercises, as, when we weigh it, our evidence is
the force of attraction ; so, again, our evidence of force is
the matter it acts upon. Thus, matter and force are corre-
lates, in the strictest sense of the word ; the conception of
the existence of the one involves the conception of the exis-
tence of the other : the quantity of matter again, and the de-
gree of force, involve conceptions of space and time. But
to follow out these abstract relations would lead me too far
into the alluring paths of metaphysical speculation.

That the theoretical portions of this essay are open to ob-
jection I am fully conscious. I cannot, however, but think
that the fair way to test a theory is to compare it with other
theories, and to see whether upon the whole the balance of
probability is in its favour. Were a theory open to no ob-
jection it would cease to be a theory, and become a law ;
and were we not to theorise, or to take generalised views of

CONCLUDING REMARKS. 197

natural phenomena until those generalizations were sure and
unobjectionable — in other words, were laws — science would
be lost in a complex mass of unconnected observations,
which would probably never disentangle themselves. Excess
on either side is to be avoided ; although we may often err on
the side of hasty generalisation, we may equally err on the
side of mere elaborate collection of observations, which,
though sometimes leading to a valuable result, yet, when cu-
mulated without a connecting link, frequently occasion a cost-
ly waste of time, and leave the subject to which they refer in
greater obscurity than that in which it was involved at their
commencement.

Collections of facts differ in importance, as do theories :
the former, in many instances, derive their value from their
capability of generalisation ; while, conversely, theories are
valuable as methods of co-ordinating given series of facts,
and more valuable in proportion as they require fewer excep-
tions and fewer postulates. Facts may sometimes be as
well explained by one view as by another, but without a
theory they are unintelligible and incommunicable. Let us
use our utmost effort to communicate a fact without using the
language of theory, and we fail ; theory is involved in all
our expressions ; the knowledge of bygone times is imported
into succeeding times by terms involving theoretic conceptions.
As the knowledge of any particular science developes itself,
our views of it become more simple ; hypotheses, or the in-
troduction of supposititious views, are more and more dis-
pensed with ; words become applicable more directly to the
phenomena, and, losing the hypothetic meaning which they
necessarily possessed at their reception, acquire a secondary
sense, which brings more immediately to our minds the facts
of which they are indices. The scaffolding has served its
purpose. The hypothesis fades away, and a theory, or gen-
eralised view of phenomena, more independent of supposition,
but still fuU of gaps and difficulties, takes its place. This in

198 CORRELATION OF PHYSICAL FORCES.

its turn, should the science continue to progress, either gives place
to a more simple and wider generalisation, or becomes, by the re-
moval of objections, established as a law. Even in this more
advanced stage, words importing theory must be used, but
phenomena are now intelligible and connected, though express-
ed by varied forms of speech.

To think on nature is to theorise ; and difficult it is not
to be led on by the continuities of natural phenomena to the-
ories which appear forced and unintelligible to those who
have not pursued the same path of thought : which, more-
over, if allowed to gain an undue influence, seduce us from
that truth which is the sole object of our pursuit.

Where to draw the line — ^where to say thus far we may
go, and no farther, in any particular class of analogies or re-
lations which Nature presents to us ; how far to follow the
progressive indications of thought, and where to resist its al-
lurements— ^is a question of degree which must depend upon
the judgment of each individual or of each class of thinkers ;
yet it is consolatory that thought is seldom expended in vain.

I have throughout endeavoured to discard the hypotheses
of subtle or occult entities ; if in this endeavour some of my
views have been adopted upon insufficient data, I still hope
that this essay will not prove valueless.

The conviction that the so-called imponderables are modes
of motion, will, at aU events, lead the observer of natural
phenomena to look for changes in these affections, wherever
the intimate structure of matter is changed ; and, conversely,
to seek for changes in matter, either temporary or permanent,
whenever it is affected by these forces. I believe he will
seldom do this in vain. It was not until I had long reflected
on the subject, that I ventured to publish my views : their
publication may induce others to think on their subject-mat-
ter. They are not put forward with the same objects, nor do
they aim at the same elaboration of detail, as memoirs on
newly-discovered physical facts : they purport to be a method

coNCLUDma KEaiAKKs. 199

of mentally regarding known facts, some few of whicli I have
myself made known on other occasions, but the great mass
of which have been accumulated by the labours of others,
and are admitted as established truths. Every one has a right
to view these facts through any medium he thinks fit to em-
ploy, but some theory must exist in the minds of those who
reflect upon the many new phenomena which have recently,
and more particularly during the present century, been dis-
covered. It is by a generalised or connected view of past
acquisitions in natural knowledge that deductions can best be
drawn as to the probable character of the results to be antici-
pated. It is a great assistance in such investigations to be
intimately convinced that no physical phenomena can stand
alone : each is inevitably connected with anterior changes,
and as inevitably productive of consequential changes, each
with the other, and aU with time and space ; and, either in
tracing back these antecedents or following up their conse-
quents, many new phenomena will be discovered, and
many existing phenomena, hitherto believed distinct, will
be connected and explained : explanation is, indeed, only re-
lation to something more familiar, not more known — i.e.
known as to causative or creative agencies. In all pheno-
mena the more closely they are investigated the more are we
convinced that, humanly speaking, neither matter nor force
can be created or annihilated, and that an essential cause is
unattainable. — Causation is the will, Creation the act, of
God.

NOTES AND REFERENCES

PAGE

13. The reader who is curious as to the views of the ancients, regarding

the objects of science, will find clues to them in the second book of
Aristotle's Physics, and in the first three books of the Metaphy-
sics. See also the Timaeus of Plato, and Ritter's History of
Ancient Philosophy, where a sketch of the Philosophy of Leucippus
and Democritus will be found.

14. Bacon's Novum Organum, book ii. aph. 5 and 6.

16 Hume's Enquiry concerning Human Understanding, S. V, London,
1768.

Brown's Enquiry into the Relations of Cause and Effect, London,
1835.

The illustration I have used of floodgate has been objected to, as
being one to which the term cause would scarcely be applied, but
after some consideration I have retained it: if cause be viewed
only as sequence, it must be limited to sequence imder given condi-
tions or circumstances, and here, given the conditions, the sequence
is invariable. I see no difference quoad the argument, between
this illustration and that of Brown of a lighted match and gun-
powder (4th edit. p. 2*7), to which my reasoning would equally well
apply.

Herschel's Discourse on the Study of Natural Philosophy, pp. 88
and 149.

17. Quarterly Review, vol. Ixviii. p. 212.

"Whewell, On the Question ' Are Cause and Effect Successive or
Simultaneous ? (Cambridge Philosophical Transactions, vol. vii p.
319.)

18. Herschel's Discourse, p. 93.

Ampere, Theorie des Phenom&nes Electro-dynamiques, Mem(urs in

NOTES AND KEFEEENCES. 201

PAOB

the Ann, de Chimie et de Physique, and works from 1820 to 1826,

Paris.
23. Lamarck, ' Sur la Mati^re du Son ' (Journal dc Physique, vol. xlix.

p. S91).
25. D'Alembert, Traits de Dynamique, pp. 3 and 4, Paris, 1*796.
28. Babbage, On the Permanent Impression of our "Words and Actions on

the Globe we inhabit, 9th Bridgewater Treatise, ch. ix.
80. Mayer, Annalen der Pharmacie Leibig und Wohler, May 1852.
83. Joule on the Mechanical Equivalent of Heat (Phil. Trans. 1850,

p. 61.)
83. Erman, Influence of Friction upon Thermo-electricity (Reports of the

British Association, 1845.)
85. Becquerel, Pegagement de I'Electricit^ par Frottement, Traite do

l'Electricit6, torn. ii. p. 113 et seq.
36. Sullivan, Currents produced by the vibration of metals (Archiv. de

I'Electricite, t. 10, p. 480). Leroux, Vibrations arrested produce

heat (Cosmos, March 30, 1860).
87. Whbatstone on the Prismatic Decomposition of Electrical Light

(Notices of Communications to the British Association, p. 11,

1835).
39. Bacon, De Forma Calidi, Nov. Org. book 2, aph. 20.

RuMFORD, An Enquiry concerning the Source of Heat which is excited

by Friction (Phil. Trans, p. 80, 1198).
Davy, On the Conversion of Ice into Water by Friction (West of

England Contributions, p. 16).
Of Heat or Calorific Repulsion (Elements of Chemical Philosophy,

p. 69).

41. Baden Powell on the Repulsive Power of Heat (Phil. Trans. 1834,

p. 485).
Fresnel, AnnaJes de Chimie, tomi xxix. pp. 67 and 107.

42. Moser on Invisible Light (Taylor's Scientific Memoirs, vol. iil pp.

461 and 465).

43. Black on Latent Heat (Elements of Chemistry, p. 144 et passim,

1803).
45. The experiments of Henry and Donny have shown that the cohesion
of liquids, as far as their antagonism to rupture goes, is much
greater than has been generally believed. These experiments,
however, make no difierence in the view I have put forth, as, what-
ever be the character of the attraction, there is a molecular attrac-
tion to be overcome in changing bodies from the solid to the liquid
state, which must require and exhaust force.

202 NOTES AND KEFERENCE8.

DoNNY, Sur la Cohesion des Liquides (Memoires de I'Academie Roy-
ale de Bruxelles, 1843).
Henry, Proceedings of the American Philosophical Society, April

1844 (Silliman's Journal, vol. xlviiii. p. 215).
48. Thilorier, Solidification de I'Acide carbonique (Ann. de Ch, et de

Phys. torn. Ix. p. 432).
60. I. Wedgwood, Thermometer for measuring the Higher Degrees of

Heat (Phil. Trans. 1782, p. 305 ; and 1785, p. 390.
Tyndall, on the physical properties of Ice (Phil. Trans. 1858,

p. 211).
Despretz, Kecherches sur le Maximum de Density de I'Eau pure et

des Dissolutions aqueuses (Ann. de. Ch. et de Ph. tom. Ixx. p. 45,

and tom. Ixxiii. p. 295).

51. BiOT (Comptes rendus de I'Academie des Sciences, Paris 1850, p.

281). The experiments on circular polarisation by water were, I
believe, by Dr. Leeson.

52. I. Thompson, Trans. R. S. Edin. vol. xvi. p. 575.
W. Thompson, Phil. Mag. August 1850, p. 123.

BuNSEN, Pogg. Ann. vol. Ixxxi. p. 562 ; Ann. de Ch. et de Phys. voL
XXXV. p. 383. Effects of Pressure on the Freezing Point.

63. Joule, Phil. Trans. 1852, p. 99.

Although, taking the phenomena as they are known to exist, the
mechanical laws may be deduced, yet in any physical conception
of the nature of heat the expansion by cold has always been a
great stumbling-block to me, and I believe to many others.

DuLONG and Petit, and Regnault. See their Memoirs abstracted
and referred to in Gmelin's Handbook of Chemistry, translated by
Watts for the Cavendish Society, vol. i. p. 242 et seq.

64. Wood, Phil. Mag. 1851, 1852.

56. Senarmont, Conduction of Heat by Crystals (Gmelin's Handbook vol.

i. p. 222).
66. Knoblauch, Ann. de Ch. et de Ph. vol. xxxvi. p. 124.

Tyndall, Transmission of Heat through Organic Structures (PhiL
Trans, vol. cxliii. p. 217).
68. Grove, Electricity produced by approximating Metals : Report of
a Lecture at the London Institution (Literary Gazette, 1843,
p. 39). - .

Gassiot, Phil. Mag. October 1844.
Roget, On the Improbabihty of the Contact exciting Force : Treatise

on Galvanism (Library of Useful Knowledge, S. 113).
Faraday, Phil. Trans. 1840, p. 126.

NOTES. AND REFERENCES. 203

PAGE

60. Melloni, Sur la Polarisation de la Chaleur : Recherches sur plusieurs

Ph^nomenes calorifiques (Annales de Chimie et de Ph. torn. xlv.
pp. 5—68 ; torn. xli. pp. 375-410 ; torn, xlviii. pp. 198, 218).
Forbes, On the Refraction and Polarisation of Heat (Transactions of
the Royal Society of Edinburgh, vol. xiiL pp. 131, 168).

61. KiRCHOFP Trans. Belin Acad. 1861.

Balfour Stewart on the theory of Exchanges (Report British Asso-
ciation, 1861).

63. T. Wedgwood, On the Production of Light and Heat by different
Bodies (Phil. Trans, vol. Ixxxii. p. 272).

65. Grove, On the Decomposition of Water into its Constituent Gases by
Heat (Phil. Trans. 1847, p. 1).
Robinson, On the Effect of Heat in lessening the Affinities of the
Elements of Water (Transactions of the Royal Irish Academy, vol.
xxi. p. 2).

67. Grove, Water decomposed by Chlorine and Heat (Phil. Trans. 1847,
p. 20).

70. Carnot, Reflexions sur la Puissance motrice du Feu, Paris, 1824.

76. Seguin, Influence des Chemins de Fer, p. 378 et seq.

77. Rogers, Consumption of Coal for Man power (Cosmos, vol. ii. p. 66).

80. Mr. Waterston has suggested that solar heat may arise from the

mechanical action of meteoric stones falling into the sun, and Mr.
Thompson has written an elaborate paper on the subject (Trans.
Brit. Assoc. 1853). If a number of gravitating bodies exist in the
neighbourhood of the sun, and form, as is conjectured, the zodia-
cal light, it is difficult to conceive how comets as they approach
this region steer clear of such bodies, and are not even deflected
from their orbits.
For Mr. Thompson's various and valuable papers, see Phil. Mag. 1851
to 1854 inclusive.

81. PoissoN, Comptes rendus, Paris, January 30, 1837.

83. DuFAYE, Symmer, Watson, and Franklin, Theories of Electric Fluid

and Electric Fluids (Priestley's History of Electricity, pp. 429 —

441).
83. Grotthus, Sur la Decomposition de I'Eau et des Corps qu'elle tient

en dissolution k aide de l'Electricit6 galvanique (Ann. de Chimie,

tom. Iviii. p. 54).
Faraday, On the Question whether Electrolytes conduct without

Decomposition (Proceedings of the Weekly Meetings of the Royal

Institution, 1 8 55).
Grove (Comptes rendus, Paris, 1839).

204 NOTES AND BEFEEENOES.

PASK

84. Faraday, On Induction as an Action of contiguous Particles (Phil.

Trans. 1838, p. 30).

85. Matteucci, Plates of Mica polarised by Electricity (De la Rive's

Electricity, p. 140).
Grove, Electrolysis across Glass (Phil. Mag. Aug. 1860).
85. Karsten on Electrical Figures (Archiv. de I'Elec. vols. ii. iii. and iv).

87. Grove, Etching Electrical Figures and transferring them to Collo-

dion (Phil. Mag. January 1857).

88. FusiNiERi, Du Transport des Matiferes ponderable qui s'opfere dans

les D6charges 61ectriques (Archives de l'Electricit6 ; Supplement k
la Biblioth&que universelle de Geneve, tom. iii. p. 597).

88. Grove, On the Voltaic Arc (Report of Lecture at the Royal Insti-
tution, Lit. Gaz. and Athenaeum, Feb. 7, 1845 ; Phil. Trans. 1847,
p. 16).

90 to 94. Grove, On the Electro-chemical Polarity of Gases (Phil. Trans.
1852, p. 87).

94. Fremt and E. Becquerel, Oxygen changed to Ozone by the Electric
Spark (Ann. de Ch. et de Phys. 1852). This subject and the na-
ture of Ozone was first investigated by Dr. Schonbein. See also a
paper by Mr. Brodie On the Conditions of certain Elements at the
Moment of Chemical Change (Phil. Trans. 1850).
96, 96. Molecular Changes in Electrised Metals (Nairne, Phil. Trans.
1780, p. 334, and 1793, p. 223 ; Grove, Electrical Mag. vol. i. p.
120; Peltier, Archives de I'Electricite, vol. v. p. 182; Fusinieri,
id. p. 616).

96. Wertheim, Change in Elasticity of Metals by Electrisation (Ann. de

Ch. et de Phys. vol. xii. p. 623 ; Arch. Elec. vol. iv. p. 490).
DuFOUR, Alteration in Tenacity of Metals by Electrisation (Bibl. univ.
de Geneve, Fev. 1855, p. 156).

97. Matteucci, Conduction of Electricity by Crystals (Comptes rendus de

I'Acad., Paris, March, 6, 1855, p. 541).

98. E. Becquerel, Transmission of Electricity by heated Gases (Aim. de.

Ch. et de Phys. vol. xxxix. p. 355).
Grove, Proceedings of the Royal Inst. (1854, p. 361).
Becquerel, Divergence of Gold-Leaves in Vacuo (Traite d'Electricit6,

vol. V. ; part ii. p. 53).
Newton, Thirty-first Query to the Optics.

99. Grove, Particles of Metals and Metallic Oxids detached in Liquids by

Electricity (Elec. Mag. vol. i. p. 119).
100. Matteucci, Relations of Electricity and Nervous Force (Phil. Trans.

NOTES AND REFERENCES. 205

1845, p. ^85, 1846, p. 497 ; Phenom^nes physiques des Corps

vivants, p. 305 ; Lezioni di Fisica, p. 360).
Galvani Yolta Marianini et Nobili on Physiological Effects of

Electricity (Ann. de Ch. et de Phys. vols. 23, 25, 29, 88, 40, 48,

44, 56).
102. Becquerel, Chemical Changes by Friction (Traite de I'Elec. voL v.

part 1, p. 16).
106. De la Rive, Heat of the Voltaic Pile (Bibl. univ., vol. xiii. p. 889).
Davy, On the Properties of Electrified Bodies in their relations to

Conducting Powers and Temperature (Phil. Trans. 1821, p. 428).

106. Grove, On the Effects of surrounding Media on Voltaic Ignition (Phil.

Trans. 1849, p. 49).

107. Oersted, Experience sur I'Effet du Conflict 61ectrique sur 1' Aiguille

ajmant^e (Ann. de Ch. et de Phys., tom. xiv. p. 417).

108. Coleridge, Table Talk, vol. i. p. 65.

109. Lenz and Jacobi, Pogg. Ann. vol. xvlii. p. 403 ; Bulletin de I'Acad.

St. Petersburg, 1839 ; Harris, Magnetism, part 2, p. 63.

Davy, Decomposition of the fixed Alkahes (Phil. Trans. 1808, p. 1).

Becquerel Des Composes 61ectro-chimiques (Traite de l'Electricit4,
voL iii. c. 13).

Crosse, Transactions of the British Association, vol. v. p. 47 ; Pro-
ceedings of the Electrical Society, p. 320.

110. Malus, Polarisation of Light by Reflection (M^moires d'Arcueil, tom.

ii. p. 143).
Arago, Circular Polarisation by Solids (Memoires de I'Institut, 1811).

111. BiOT, Circular Polarisation by Liquids (Memoires de I'Institut, 1817).
111. NiEPCE and Daguerre, Historique et Description des Precedes du

Daguerreotype, Paris, 1839.

Talbot, Photogenic Drawing and Calotype (Phil. Mag. March. 1839,
and August 1841).
113. Hersckel, Chemical Action of the Solar Spectrum on various Sub-
stances (Phil. Trans. 1840, p. i. and 1842, p. 181).

Hunt, Researches on Light, London, 1844.

116. Grove, Other Forces produced by Light (Lit. Gaz. January 1844).

117. Grove, Influence of Light on the Polarised Electrode (PhiL Mag.

December 1858).

SoMERViLLE (Mrs.), On the Magnetising Power of the more Refrangi-
ble Solar Rays (Phil. Trans, 1862, p. 132).

MoRiCHiNi's experunents are given in Mrs. Somerville's paper.

118. Herschel, On the Absorption of Light in Coloured Media viewed

206 NOTES AND EEFERENCES.

in connection with the Undulatory Theory (Phil. Mag. Decem-
ber 1863).
Seebeck, Heat of Coloured Rays (Brewster's Optics, p. 90).

118. Knoblauch (Ann. de Ch. vol. xxxvi. p. 124, and Pogg. Ann. there

referred to).

119. Herschel, Epipolised Light (PhiL Trans, vol. cxxxv. pp. 143, 147).
Stokes, Change in Refran^bihty of Light (PhiL Trans, vols, cxlii.

cxliii.)

128. For the first enunciations of the Corpuscular and Undulatory Theories,
see Newton's Optics, Hooke's Micographia, and Huyghens' Trac-
tatus de Lumine. See also Brewster's Optics, p. 138.

124. Young, Lectures edited by KeUand, p. 358, et seq. ; Phil. Trans.
1800, p. 126; Herschel, Encyc. Metro, art. Light, pp. 450 and
T38 ; Newton's Optics, p. 822 ; Whewell's Hist. Indue. Sc. vol.
ii. p. 449 ; Foucault, Comptes rendus, Paris, 1850, p. 65 ; Harri-
son, Phil. Mag. November 1856 ; Camb. Phil. Trans.

126. SoNDHAUss, Refraction of Sound (Ann. de Ch. et de Phys. vol.
XXXV. p. 505) ; Dovk, Polarisation of Sound (Cosmos, May 13,
1859).

182. Pasteur, Rotation of Plane of Polarised Light by Solutions of
Hemihedral Crystals (Ann. de Ch. et de Phys. vol. xxiv. p. 442).

134 to 135. Wollaston, Phil. Trans. 1822, p. 89 ; Whewell, Phil, of
the Induct. Sc. vol. i. p. 419 ; Wilson, Trans, of the Roy. Soc. of
Edin. vol. xvi. p. 79 ; Sir W. Herschel, Phil. Trans. 1793, p. 201,
and 1801, p. 300 ; Morgan, Phil. Trans, vol. Ixxv. p. 272 ; Davy,
Phil. Trans. 1822, p. 64 ; Elements of Chemical Philosophy, p. 97 ;
Gassiot, Phil. Trans. 1859, p. 157.

187. Diminishing Periods of Comets (Herschel's Outlines of Astronomy,
p. 357).

140. Since writing the passage in the text, I find that Struve has been
led, from his astronomical researches, to the conclusion that some
light is lost in the interplanetary spaces. He gives as an approxi-
mation one per cent, as lost by the passage of light from a star of
the first magnitude, assuming a mean or average distance (Etudes
d' Astronomic Stellaire, 1847).
Newton, Thirtieth Query to the Optics.

142. Faraday, Evolution of Electricity from Magnetism (Phil. Trans.-
1882, p. 125).

144. Faraday, Magnetic Condition of all Matter (PhiL Trans. 1846, p. 21 ;
Phil. Mag. 1846, p. 249).
Becquerel, Ann. de Ch. et de Ph. torn, xxxvi. p. 337; Comptes
rendus, Paris, 1846, p. 147 ; and 1860, p. 201.

NOTES AND KEFEKENCES. 207

PAGE

145. Faraday, On the Magnetism of Light (Phil. Trans. 1846, p. 1).

145. Wartmann, Rotation of the Plane of Polarisation of Heat by Magnet-

ism (Journal de I'Institute, No. 644).
PROVOSTAYE and Dessaines, Ann. de Ch. et de Phys. October 1849.

146. Hunt, Influence of Magnetism on Molecular Arrangement (Phil. Mag.

1846, vol. xxviii. p. 1 ; Memoirs of the Geological Society, vol. i.
p. 433).
Wartmann, Phil. Mag. 1847, vol. xxx. p. 263.
14Y. Grove, Experiment on Molecular Motion of a Magnetic Substance
(Electrical Mag. 1845, vol. i. p. 601).

147. On the direct Production of Heat by Magnetism (Proceedings of the

Royal Society, 1849, p. 826).

After this paper was communicated and ordered to be prmted in the
Philosophical Transactions, I found that I had been anticipated by
Mr. Van Breda, who communicated, in 1845, a paper to the Insti-
tut on the subject : his paper appears in the Comptes rendus under
an erroneous title, which accounts for its having been overlooked :
he does not give thermometric measures of the heat he obtained,
nor did he produce heating effects by a permanent steel magnet, or
with other metals than iron. (Comptes rendus, October 27, 1845).
See also an earher experunent by Mr. Joule (Phil. Mag. 1843), to
which he called my attention after my paper was read.

148. 151. The Experiments on the effects of Magnetism on the Matter

magnetised, are collected by Mr. De la Rive in his recently-pub-
lished Treatise on Electricity, vol, i.

153. Davy, Electricity defined as Chemical aflSnity acting on Masses (Phil.

Trans. 1826, p. 389).
VoLTA, Electricity excited by the mere Contact of conducting Sub-
stances (Phil. Trans. 1800, p. 403).

154. Grove, Gold-Leaf Experiment (Comptes rendus, Paris, 1839, p. 667).

156. Grove, Voltaic Action of Sulphur, Phosphorus, and Hydrocarbons

(Phil. Trans. 1845, p. 351).
Grove, New Voltaic Combination (Phil. Mag. vol. xiv. p. 388 ; vol.
XV. p. 287).

155. Grove, Electricity of Blowpipe Flame (Proceedmgs of the Royal

Institution, February 1854), Phil. Mag.

157. Dalton, New System of Chemistry, London, 1810.

158. I have here and elsewhere used whole numbers, as sufficiently approxi-

mate for the argument, but without intending to express any opin-
ion as to the law of Prout.
168. Faraday, Definite Electrolysis (PhiL Trans. 1834, p. 77).

208 NOTES AlfD REFEEENCES.

PAGE

160. Wood, Heat disengaged in Chemical Combinations (Phil. Mag. 1852).

162. Andrews, Phil. Trans. 1844, p. 21.

Hess, PoggendoflTs Annalen, Bd. lii. p. 197.

163. Favbe, Ann. de Ch. et de Phys. vols. 39, 40 ; Comptes rendus, Paris,

v.ol. 45, p. 66, and vol. 46, p. 837.

169. Catalysis by Platinum (Dobereimer, Ann. de Ch. et de Phys. torn.

xxiv. p. 93 ; Dulong and Thenard, Ann. de Ch. et de Phys. torn,
xxiii. p. 440).

170. Grove, Gas Voltaic Battery (Phil. Mag. February 1839, and Decem-

ber 1842 ; Phil. Trans. 1843, p. 91).

171. MosoTTi, Forces which regulate the Internal Constitution of Bodies

(Taylor's Scientific Memoirs, vol. i. p. 448).

172. Plitcker, Repulsion of the Optic Axes of Crystals by the Poles of a

Magnet (Taylor's Scientific Memoirs, vol. v. p. 353).
Magnetic Action of Cyanite (Lit. Gaz. 1849, p. 431).

172. Matteucci, Correlation of Electric Current and Nervous Force (Phil.

Trans. 1850, p. 287).

173. Carpenter, On the Mutual Relations of the Vital and Physical Forces

(Phil. Trans. 1850, p. 751).

174. On Eflfort. See Brown, Cause and Efifect ; Herschel's Discourse ;

and Quarterly Review, June 1841.

175. Helmholtz, Muller's Archives, 1845 ; Matteucci, Comptes rendus,
Paris, 1856 ; Beclard, Archives de Medicine, 1861.

189. Dulong and Petit, Relation between Specific Heat and Chemical

Equivalents (Ann. de Ch. et de Phys. torn. x. p. 395).
189. Neumann, Poggendorffs Annalen, Bd. xxiii. p. 1.
AvoGADRO, Ann, de Ch. et de Phys. tom. Iv. p. 80.

ON THE

mTEKACTIOlS' OF JSTATUKAL FOECES.

By Peof. H. L. F. HELMHOLTZ.

Translated by JOHN TYNDALL, F.R.S.

Herman Ludwig Ferdinand Helmholtz was born at Pottsdam, August
31, 1821. He was first military physician, and afterwards assistant of the
Astronomical Museum in Berlin (1848), and subsequently Professor Extra-
ordinary of Physiology at the University of Konigsberg (1849 to 1852). He
became Professor of Physiology at the University of Bonn in 1855, and in
1858 accepted the physiological chair in the University of Heidelberg. The
lecture which follows was dehvered at Konigsberg in 1854. He is an emi-
nent investigator, and an able promoter of the recent philosophy of forces ;
but of his life we have fewer particulars than of his accomplished translator.

The ancestors of John Tyndall emigrated from England to the eastern
or Saxon border of Ireland about the middle of the last century. He was
bom at the village of Leighlin Bridge in 1820, where he received his early
education and acquired a taste for mathematics. In 1839 he left school
and joined the Ordnance Survey as a civil assistant, where he became in
turn draughtsman, computer, surveyor, and trigometrical observer. He was
five years connected with the survey, and for three years occupied as rail-
road engineer. In 1847 he became teacher in Queenswood College in Hamp-
shire, a school for agriculturists and engineers, where he was distinguished
for his mild but efl&cient discipline. Professor Frankland, the chemist, was
here joined with him in the work of instruction, and in 1848 the two friends
left the institution and went to the University of Marburg in Hesse Cassel,
to study with the eminent chemist, Bunsen. In 1851 Professor Tyndall
went to Berlin and worked at the subject of diamagnetism in the laboratory
of Professor Magnus. He returned to London the same year, and was
elected Fellow of the Royal Society in 1862.

Through the influence of Dr. Bence Jones, General Sabine, and Professor
Faraday, he was appointed Professor of Natural Philosophy in the Royal
Institution in 1853, an appointment which he now holds. In company with
his friend. Professor Huxley, he visited the Alps in 1856 ; and returning each
succeeding year, he accumulated the observations and adventures which are
80 graphically described in his "Glaciers of the Alps," published in 1860.
Professor Tyndall has worked with eminent success at various scientific
questions, but he is chiefly distinguished for his original and elaborate re-
searches on the relations of radiant heat to gaseous and vaporous matter.
These researches are given in his able work on " Heat as a mode of Motion,"
issued in 1863. As an experimenter. Professor Tyndall is marked for his
caution, accuracy, and tireless perseverance under difficulties ; as a writer,
for his clear, vivid, and vigorous style.

INTERACTION OF NATURAL FORCES.

ANEW conquest of very general interest has been recently
made by natural philosophy. In the following pages, I
wiU endeavour to give a notion of the nature of this conquest.
It has reference to a new and universal natural law, which
rules the action of natural forces in their mutual relations
towards each other, and is as influential on our theoretic
views of natural processes as it is important in their technical
applications.

Among the practical arts which owe their progress to the
development of the natural sciences, from the conclusion of
the middle ages downwards, practical mechanics, aided by
the mathematical science which bears the same name, was
one of the most prominent. The character of the art was, at
the time referred to, naturally very different from its present
one. Surprised and stimulated by its own success, it thought
no problem beyond its power, and immediately attacked some
of the most difficult and complicated. Thus it was attempted
to build automaton figures which should perform the functions
of men and animals. The wonder of the laet centmy was
Vaucanson's duck, which fed and digested its food ; the flute-
player of the same artist, which moved all its fingers cor-

212 INTERACTION OF NATURAL FORCES.

rectly ; the writing boy of the older, and the piano-forte play-
er of the younger Droz : which latter, when performing, fol-
lowed its hands with his eyes, and at the conclusion of the
piece bowed courteously to the audience. That men like
those ^nentioned, whose talent might bear comparison with
the most inventive heads of the present age, should spend so
much time in the construction of these figures, which we at
present regard as the merest trifles, would be incomprehensi-
ble, if they had not hoped in solemn earnest to solve a great
problem. The writing boy of the elder Droz was publicly
exhibited in Germany some years ago. Its wheel-work is so
complicated, that no ordinary head would be sufficient to
decipher its manner of action. When, however, we are in-
formed that this boy and its constructor, being suspected of the
black art, lay for a time in the Spanish Inquisition, and with
difficulty obtained their freedom, we may infer that in those
days even such a toy appeared great enough to excite doubts
as to its natural origin. And though these artists may not
have hoped to breathe into the creature of their ingenuity a
soul gifted with moral completeness, still there were many
who would be willing to dispense with the moral qualities of
their servants, if, at the same time, their immoral qualities
could also be got rid of ; and accept, instead of the mutability
of flesh and bones, services which should combine the regu-
larity of a machine with the durability of brass and steel.
The object, therefore, which the inventive genius of the past
century placed before it with the fullest earnestness, and not
as a piece of amusement merely, was boldly chosen, and was
followed up with an expenditure of sagacity which has contri-
buted not a little to enrich the mechanical experience which a
later time knew how to take advantage of. We no longer
seek to build machines which shall fulfil the thousand services
required of one, man, but desire, on the contrary, that a ma-
chine shall perform one service, but shall occupy in doing it
the place of a thousand men.

THE OLD MECHANICAL PROBLEM. 213

From these efforts to imitate living creatures, another idea,
also by a misunderstanding, seems to have developed itself,
which, as it were, formed the new philosopher's stone of the
seventeenth and eighteenth centuries. It was now the endeav-
our to construct a perpetual motion. Under this term was un-
derstood a machine, which, without being wound up, without
consuming in the working of it, falling water, wind, or any-
other natural force, should still continue in motion, the motive
power being perpetually supplied by the machine itself. Beasts
and human beings seemed to correspond to the idea of such an
apparatus, for they moved themselves energetically and inces-
santly as long as they lived, were never wound up, and nobody
set them in motion. A connection between the taking-in of
nourishment and the development of force did not make itself
apparent. The nourishment seemed only necessary to grease,
as it were, the wheelwork of the animal machine, to replace
what was used up, and to renew the old. The development
of force out of itself seemed to be the essential peculiarity, the
real quintessence of organic life. If, therefore, men were to
be constructed, a perpetual motion must first be found.

Another hope also seemed to take up incidentally the sec-
ond place, which, in our wiser age, would certainly have
claimed the first rank in the thoughts of men. The perpetual
motion was to produce work inexhaustibly without corre-
sponding consumption, that is to say, out of nothing. Work,
however, is money. Here, therefore, the practical problem
which the cunning heads of all centuries have followed in the
most diverse ways, namely, to fabricate money out of nothing,
invited solution. The similarity with the philosopher's stone
sought by the ancient chemists was complete. That also
was thought to contain the quintessence of organic life, and to
be capable of producing gold.

The spur which drove men to inquiry was sharp, and the
talent of some of the seekers must not be estimated as small.
The nature of the problem was quite calculated to entice por-

214 INTERACTION OF NATURAL FORCES.

ing brains, to lead them round a circle for years, deceiving
ever with new expectations, which vanished upon nearer ap-
proach, and finally reducing these dupes of hope to open in-
sanity. The phantom could not be grasped. It would be
impossible to give a history of these efforts, as the clearer
heads, among whom the elder Droz must be ranked, convinced
themselves of the futility of their experiments, and were
naturally not inclined to speak much about them. Bewildered
intellects, however, proclaimed often enough that they had
discovered the grand secret ; and as the incorrectness of their
proceedings was always speedily manifest, the matter fell into
bad repute, and the opinion strengthened itself more and more
that the problem was not capable of solution ; one difficulty
after another was brought under the dominion of mathemati-
cal mechanics, and finally a point was reached where it could
be proved, that, at least by the use of pure mechanical forces,
no perpetual motion could be generated.

We have here arrived at the idea of the driving force or
power of a machine, and shall have much to do with it in
future. I must, therefore, give an explanation of it. The
idea of work is evidently transferred to machines by compar-
ing their arrangements with those of men and animals to
replace which they were applied. We still reckon the work
of steam engines according to horse-power. The value of
manual labor is determined partly by the force which is ex-
pended in it (a strong laborer is valued more highly than a
weak one), partly however, by the skill which is brought into
action. A machine, on the contrary, which executes work
skiLftilly, can always be multiplied to any extent ; hence its
skiU has not the high value of human skill in domains where
the latter cannot be supplied by machines. Thus the idea of
the quantity of work in the case of machines has been limited
to the consideration of the expenditure of force ; this was the
more important, as indeed most machines are constructed for
the express purpose of exceeding, by the magnitude of their

^^zLbCTREMENT OF MECHANICAL POWER. 215

effects, the powers of men and animals. Hence, in a mechani-
cal sense, the idea of work is become identical with that of
the expenditm-e of force, and in this way I will apply it.

How, then, can we measure this expenditure, and compare
it in the case of different machines ?

I must here conduct you a portion of the way — as short a
portion as possible — over the uninviting field of mathematico-
mechanical ideas, in order to bring you to a point of view from
which a more rewarding prospect will open. And though the
example which I shall here choose, namely, that of a water-
mill with iron hammer, appears to be tolerably romantic, still,
alas, I must leave the dark forest valley, the spark-emitting
anvil, and the black Cyclops wholly out of sight, and beg a
moment's attention to the less poetic side of the question,
namely, the machinery. This is driven by a water-wheel which
in its turn is set in motion by the falling water. The axle of the
water-wheel has at certain places small projections, thumbs,
which, during the rotation, lift the heavy hammer and permit
it to fall again. The falling hammer belabors the mass of
metal, which is introduced beneath it. The work therefore
done by the machine consists, in this case, in the lifting of the
hammer, to do which the gravity of the latter must be over-
come. The expenditure of force will, in the first place, other
circumstances being equal, be proportioned to the weight of
the hammer ; it will, for example, be double when the weight
of the hammer is doubled. But the action of the hammer
depends not upon its weight alone, but also upon the height
from which it falls. If it falls through two feet, it will pro-
duce a greater effect than if it falls through only one foot. It
is, however, clear that if the machine, with a certain expendi-
ture of force, lifts the hammer a foot in height, the same
amount of force must be expended to raise it a second foot in
height. The work is therefore not only doubled when the
weight of the hammer is increased twofold, but also when the
space through which it falls is doubled. From this it is easy

216 INTEEACnON OFNATUBAL FORCES.

to see that the work must be measured by the product of the
weight into the space through which it ascends. And in this
way, indeed, do we measure in mechanics.

The unit of work is a foot-pound, that is, a pound weight
raised to the height of one foot.

While the work in this case consists in the raising of the
heavy hammer-head, the driving force which sets the latter in
motion, is generated by falling water. It is not necessary
that the water should fall vertically, it can also flow in a
moderately inclined bed ; but it must always, where it has
water-mills to set in motion, move from a higher to a lower
position. Experiment and theory coincide in teaching, that
when a hammer of a hundred weight is to be raised one foot,
to accomplish this at least a hundred weight of water must
faU through the space of one foot ; or what is equivalent to
this, twohimdred weight must fall full half afoot, or four hun-
dred weight a quarter of a foot, etc. In short, if we multiply
the weight of the falling water by the height through which it
falls, and regard, as before, the product as the measure of the
work, then the work performed by the machine in raising the
hammer, can, in the most favourable case, be only equal to the
number of foot-pounds of water which have fallen in the same
time. In practice, indeed, this ratio is by no means attained ;
a great portion of the work of the falling water escapes unused,
inasmuch as part of the force is willingly sacrificed for the
sake of obtaining greater speed.

I win further remark, that this relation remains unchanged
whether the hammer is driven immediately by the axle of the
wheel, or whether — ^by the intervention of wheel-work, end-
less screws, pulleys, ropes — ^the motion is transferred to the
hammer. We may, indeed, by such arrangements, succeed
in raising a hammer of ten hundred weight, when by the first
simple arrangement, the elevation of a hammer of one hundred
weight might alone be possible ; but either this heavier ham-
mer is raised to only one tenth of the height, or tenfold the

TEirE FUNCTION OF MACHINES. 217

time is required to raise it to the same height ; so that, how-
ever we may alter, by the interposition of machinery, the in-
tensity of the acting force, still in a certain time, during which
the mill-stream furnishes us with a definite quantity of water,
a certain definite quantity of work, and no more, can be per-
formed.

Our machinery, therefore, has, in the first place, done
nothing more than make use of the gravity of the falling wa-
ter in order to overpower the gravity of the hammer, and to
raise the latter. When it has lifted the haromer to the neces-
sary height, it again liberates it, and the hammer falls upon
the metal mass which is pushed beneath it. But why does
the falling hammer here exercise a greater force than when it
is permitted simply to press with its own weight on the mass
of metal ? "Why is its power greater as the height from which
it falls is increased ? "We find, in fact, that the work per-
formed by the hammer is determined by its velocity. In
other cases, also, the velocity of moving masses is a means of
producing great effects. I only remind you of the destructive
effects of musket-bullets, which, in a state of rest, are the most
harmless things in the world. I remind you of the wind-mill,
which derives its force from the moving air. It may appear
surprising that motion, which we are accustomed to regard as
a non-essential and transitory endowment of bodies, can pro-
duce such great effects. But the fact is, that motion appears
to us, under ordinary circumstances, transitory, because the
movement of all terrestrial bodies is resisted perpetually by
other forces, friction, resistance of the air, etc., so that motion
is incessantly weakened and finally neutralized. A body,
however, which is opposed by no resisting force, when once
set in motion, moves onward eternally with undiminished
velocity. Thus we know that the planetary bodies have
moved without change, through space, for thousands of years.
Only by resisting forces can motion be diminished or destroyed.
A moving body, such as the hammer or the musket-ball, when
10

218 INTEItACTION OP NATUEAL FORCES.

it Strikes against another, presses the latter together, or pene-
trates it, until the sum of the resisting fbrces which the body
struck presents to its pressure, or to the separation of its par-
ticles, is sufficiently great to destroy the motion of the ham-
mer or of the bullet. The motion of a mass regarded as
taking the place of working force is called the living force {vis
viva) of the mass. The word " living " has of course here
no reference whatever to living beings, but is intended to rep-
resent solely the force of the motion as distinguished from the
state of imchanged rest — from the gravity of a motionless
body, for example, which produces an incessant pressure
against the surface which supports it, but does not produce
any motion.

In the case before us, therefore, we had first power in the
form of a falling mass of water, then in the form of a lifted
hammer, and, thirdly, in the form of the living force of the
fallen hammer. We should transform the third form into the
second, if we, for example, permitted the hammer to fall upon
a highly elastic steel beam strong enough to resist the shock.
The hammer would rebound, and in the most favourable case
would reach a height equal to that from which it fell, but
would never rise higher. In this way its mass would ascend :
and at the moment when its highest point has been attained,
it would represent the same number of raised foot-pounds as
before it fell, never a greater number ; that is to say, living
force can generate the same amount of work as that ex-
pended in its production. It is therefore equivalent to this
quantity of work.

Our clocks are driven by means of sinking weights, and
our watches by means of the tension of springs. A weight
which lies on the ground, an elastic spring which is without
tension, can produce no effects ; to obtain such we must first
raise the weight or impart tension to the spring, which is
accomplished when we wind up our clocks and watches.
The man who winds the clock or watch communicates to the

RESERVOIR OF ACCUMULATED POWER. 219

weight or to the spring a certain amount of power, and ex-
actly so much as is Urns communicated is gradually given out
again during the following twenty-four hours, the original
force being thus slowly consumed to overcome the friction of
the wheels and the resistance which the pendulum encounters
from the air. The wheel-work of the clock therefore exhibits
no working force which was not previously communicated to
it, but simply distributes the force given to it uniformly over
a longer time.

Into the chamber of an air-gun we squeeze, by means of
a condensing air-pump, a great quantity of air. When we
afterwards open the cock of a gun and admit the compressed
air into the barrel, the ball is driven out of the latter with a
force similar to that exerted by ignited powder. Now we
may determine the work consumed in the pumping-in of the
air, and the living force which, upon firing, is communicated
to the ball, but we shall never find the latter greater than the
former. The compressed air has generated no working force,
but simply gives to the bullet that which has been previously
communicated to it. And while we have pumped for perhaps
a quarter of an hour to charge the gun, the force is expended
in a few seconds when the bullet is discharged ; but because
the action is compressed into so short a time, a much greater
velocity is imparted to the ball than would be possible to com-
municate to it by the unaided effort of the arm in throw-
ing it.

•From these examples you observe, and the mathematical
theory has corroborated this for all purely mechanical, that is
to say, for moving forces, that all our machinery and appara-
tus generate no force, but simply yield up the power com-
municated to them by natural forces, — falling water, moving
wind, or by the muscles of men and animals. After this law
had been established by the great mathematicians of the last
century, a perpetual motion, which should make only use of
pure mechanical forces, such as gravity, elasticity, pressure of

220 INTEEACTION OF NATUEAL FORCES.

liquids and gases, could only be sought after by bewildered
and ill-instructed people. But there are still other natural
forces which are not reckoned among the purely moving
forces, — ^heat, electricity, magnetism, light, chemical forces,
all of which nevertheless stand in manifold relation to me-
chanical processes. There is hardly a natural process to be
found which is not accompanied by mechanical actions, or
from which mechanical work may not be derived. Here the
question of a perpetual motion remained open ; the decision
of this question marks the progress of modern physics.

In the case of the air-gun, the work to be accomplished in
the propulsion of the baU was given by the arm of the man
who pumped in the air. In ordinary firearms, the condensed
mass of air which propels the bullet is obtained in a totally
different manner, namely, by the combustion of the powder.
Gunpowder is transformed by combustion for the most part
into gaseous products, which endeavor to occupy a much
larger space than that previously taken up by the volume of
the powder. Thus, you see, that, by the use of gunpowder,
the work which the human arm must accomplish in the case
of the air-gun is spared.

In the mightiest of our machines, the steam eagine, it is a
strongly compressed aeriform body, water vapour, which, by
its effort to expand, sets the machine in motion. Here also,
we do not condense the steam by means of an external
mechanical force, but by communicating heat to .a mass of
water in a closed boiler, we change this water into steam,
which, in consequence of the limits of the space, is developed
under strong pressure. In this case, therefore, it is the heat
communicated which generates the mechanical force. The
heat thus necessary for the machine we might obtain in many
ways ; the ordinary method is to procure it from the combus-
tion of coal.

Combustion is a chemical process. A particular constitu-
ent of our atmosphere, oxygen, possesses a strong force of

PRODUCTION OF FORCE BY COMBUSTION. 221

attraction, or, as it is named in chemistry, a strong affinity
for the constituents of the combustible body, which affinity,
however, in most cases, can only exert itself at high tempera-
tures. As soon as a portion of the combustible body, for ex-
ample the coal, is sufficiently heated, the carbon unites itself
with great violence to the oxygen of the atmosphere and forms
a peculiar gas, carbonic acid, the same which we see foaming
from beer and champagne . By this combination, light and heat
are generated ; heat is generally developed by any combination
of two bodies of strong affinity for each other ; and when the
heat is intense enough, light appears. Hence, in the steam
engine, it is chemical processes and chemical forces which pro-
duce the astonishing work of these machines. In like manner
the combustion of gunpowder is a chemical process, which,
in the barrel of the gun, communicates living force to the
bullet.

While now the steam engine develops for us mechanical
work out of heat, we can conversely generate heat by mechani-
cal forces. A skilful blacksmith can render an iron wedge red
hot by hammering. The axles of our carriages must be pro-
tected l)y careful greasing, from ignition through friction.
Even lately this property has been applied on a large scale. In
some factories, where a surplus of water power is at hand, this
surplus is applied to cause a strong iron plate to rotate swiftly
upon another, so that they become strongly heated by the fric-
tion. The heat so obtained warms the room, and thus a stove
without fuel is provided. Now, could not the heat generated
by the plates be applied to a small steam engine, which, in its
turn, should be able to keep the rubbing plates in motion ?
The perpetual motion would thus be at length found. This
question might be asked, and could not be decided by the
older mathematico-mechanical investigations. I will remark,
beforehand, that the general law which I will lay before you
answers the question in the negative.

By a similar plan, however, a speculative American set

222 INTEEACTION OF NATURAL FOECES.

some time ago the industrial world of Europe in excitement.
The magneto- electric machines often made use of in the case
of rheumatic disorders are well known to the public. By
imparting a swift rotation to the magnet of such a machine,
we obtain powerful currents of electricity, K those be con-
ducted through water, the latter will be reduced into its two
components, oxygen and hydrogen. By the combustion of
hydrogen, water is again generated. If this combustion takes
place, not in atmospheric air, of which oxygen only consti-
tutes a fifth part, but in pure oxygen, and if a bit of chalk be
placed in the flame, the chalk will be raised to a white heat,
and give us the sun-like Drummond's light. At the same
time, the flame develops a considerable quantity of heat.
Our American proposed to utilize in this Way the gases
obtained from electrolytic decomposition, and asserted that by
the combustion a sufficient amount of heat was generated to
keep a small steam engine in action, which again drove his
magneto-electric machine, decomposed the water, and thus
continually prepared its own fuel. This would certainly have
been the most splendid of all discoveries ; a perpetual motion
which, besides the force which kept it going, generated light
like the sun, and warmed all around it. The matter was by
no means badly cogitated. Each practical step in the afiair
was known to be possible ; but those which at that time were
acquainted with the physical investigations which bear upon
this subject could have afiirmed, on the first hearing the
report, that the .matter was to be numbered among the numer-
ous stories of the fable-rich America ; and indeed, a fable it
remained.

It is not necessary to multiply examples further. You
will infer from those given, in what immediate connection
heat, electricity, magnetism, light, and chemical affinity, stand
with mechanical forces.

Starting from each of these different manifestations of
natural forces, we can set every other in motion, for the most

STATEMENT OF DYNAMIC PROBLEM. 223

part not in one way merely, but in many ways. It is here as
with the weaver^s web, —

Where a step stirs a thousand threads,

The shuttles shoot from side to side,

The fibres flow unseen,

And one shock strikes a thousand combinations.

Now it is clear that if by any means we could succeed, as
the above American professed to have done, by mechanical
forces, to excite chemical, electrical, or other natural pro-
cesses, which, by any circuit whatever, and without altering
permanently the active masses in the machine, could produce
mechanical force in greater quantity than that at first applied,
a portion of the work thus gained might be made use of to
keep the machine in motion, while the rest of the work might
be applied to any other purpose whatever. The problem
was, to find in the complicated net of reciprocal actions, a
track through chemical, electrical, magnetical, and thermic
processes, back to mechanical actions, which might be followed
with a final gain of mechanical work ; thus would the perpet-
ual motion be found.

But, warned by the futility of former experiments, the
public had become wiser. On the whole, people did not seek
much after combinations which promised to furnish a perpetual
motion, but the question was inverted. It was no more
asked. How can I make use of the known and unknown rela-
tions of natural forces so as to construct a perpetual motion ?
but it was asked, If a perpetual motion be impossible, what
are the relations which must subsist between natural forces ?
Everything was gained by this inversion of the question.
The relations of natural forces rendered necessary by the
above assumption, might be easily and completely stated. It
was found that all known relations of force harmonize with
the consequences of that assumption, and a series of unknown
relations were discovered at the same time, the correctness of

224 INTERACTION OF NATURAL FORCES.

which remained to be proved. K a single one of them could
be proved false, then a perpetual motion would be possible.

The first who endeavoured to travel this way was a French-
man, named Camot, in the year 1824. In spite of a too
limited conception of his subject, and an incorrect view as to
the nature of heat, which led him to some erroneous conclu-
sions, his experiment was not quite unsuccessful. He dis-
covered a law which now bears his name, and to which I will
return further on.

His labors remained for a long time without notice, and it
was not till eighteen years afterwards, that is, in 1842, that
different investigators in different countries, and independent
of Carnot, laid hold of the same thought.

The first who saw truly the general law here referred to,
and expressed it correctly, was a German physician, J. R.
Mayer, of Heilbronn, in the year 1842. A little later, in
1843, a Dane, named Colding, presented a memoir to the
Academy of Copenhagen, in which the same law found utter-
ance, and some experiments were described for its further
corroboration. In England, Joule began about the same time
to make experiments having reference to the same subject.
We often find, in the case of questions to the solution of
which the development of science points, that several heads,
quite independent of each other, generate exactly the same
series of reflections.

I myself, without being acquainted with either Mayer or
Colding, and having first made the acquaintance of Joule's
experiments at the end of my investigation, followed the same
path. I endeavoured to ascertain all the relations between the
different natural processes, which followed from our regarding
them from the above point of view. My inquiry was made
public in 1847, in a small pamphlet bearing the title, '' On
the Conservation of Force."

Since that time the interest of the scientific public for this
subject has gradually augmented. A great number of the

PROGRESS OF THE INVESTIGATION. 225

essential consequences of the above manner of viewing the
subject, the proof of which was wanting when the first
theoretic notions were published, have since been confirmed
by experiment, particularly by those of Joule ; and during
the last year the most eminent physicist of France, Regnault,
has adopted the new mode regarding the question, and by
fresh investigations on the specific heat of gases has contri-
buted much to its support. For some important consequences
the experimental proof is still wanting, but the number of
confirmations is so predominant, that I have not deemed it
too early to bring the subject before even a non-scientific
audience.

How the question has been decided you may already infer
from what has been stated. In the series of natural processes
there is no circuit to be found, by which mechanical force can
be gained without a corresponding consumption. The per-
petual motion remains impossible. Our reflections, however,
gain thereby a higher interest.

"We have thus far regarded the development of force by
natural processes, only in its relation to its usefulness to man,
as mechanical force. You now see that we have arrived at a
general law, which holds good wholly independent of the
application which man makes of natural forces ; we must
therefore make the expression of our new law correspond to
this more general significance. It is in the first place clear,
that the work which, by any natural process whatever, is per-
formed under favourable conditions by a machine, and which
may be measured in' the way already indicated, may be used
as a measure of force common to all. Further, the impor-
tant question arises, " If the quantity of force cannot be aug-
mented except by corresponding consumption, can it be
diminished or lost ? For the purpose of our machines it cer-
tainly can, if we neglect the opportunity to convert natural
processes to use, but as investigation has proved, not for a.
nature as a whole."
10*

226 mxERACTnoN of natural forces.

In the collision and friction of bodies against each other ^
the mechanics of former years assumed simply that living
force was lost. But I have already stated that each collision
and each act of friction generates heat ; and, moreover, Joule
has established by experiment the important law, that for
every foot-pound of force which is lost, a definite quantity of
heat is always generated, and that when work is performed
by the consumption of heat, for each foot-pound thus gained a
definite quantity of heat disappears. The quantity of heat
necessary to raise the temperature of a pound of water a de-
gree of the centigrade thermometer, corresponds to a mechani-
cal force by which a pound weight would be raised to the
height of 1350 feet ; we name this quantity the mechanical
equivalent of heat. I may mention here that these facts con-
duct of necessity to the conclusion, that the heat is not, as
was formerly imagined, a fine imponderable substance, but
that, like light, it is a peculiar shivering motion of the ulti-
mate particles of bodies. In collision and friction, according
to this manner of viewing the subject, the motion of the mass
of a body which is apparently lost is converted into a motion
of the ultimate particles of the body ; and conversely, when
mechanical force is generated by heat, the motion of the ulti-
mate particles is converted into a motion of the mass.

Chemical combinations generate heat, and the quantity of
this heat is totally independent of the time and steps through
which the combination has been effected, provided that other
actions are not at the same time brought into play. If, however,
mechanical work is at the same time accomplished, as in the
case of the steam engine, we obtain as much less heat as is
equivalent to this work. The quantity of work produced by
chemical force is in general very great. A pound of the
purest coal gives, when burnt, sufficient heat to raise the tem-
perature of 8086 pounds of water one degree of the centi-
grade thermometer ; from this we can calculate that the mag-
nitude of the chemical force of attraction between the parti-

AMOUNT OF FORCE IN THE UNIVEK8E UNALTEKABLE. 227

cles of a pound of coal and the quantity of oxygen that corre-
sponds to it, is capable of lifting a weight of one hundred
pounds to a height of twenty miles. Unfortunately, in our
steam engines, we have hitherto been able to gain only the
smallest portion of this work ; the greater part is lost in the
shape of heat. The best expansive engines give back as
mechanical work only eighteen per cent, of the heat generated
by the fuel.

From a similar investigation of aU the other known physi-
cal and chemical processes, we arrive at the conclusion that
Nature as a whole possesses a store of force which cannot in
any way be either increased or diminished. And that, there-
fore, the quantity of force in nature is just as eternal and
unalterable as the quantity of matter. Expressed in this form,
I have named the general law " The Principle of the Conser-
vation of Force."

"We cannot create mechanical force, but we may help our-
selves from the general store-house of Nature. The brook
and the wind, which drive our mills, the forest and the coal-
bed, which supply our steam engines and warm our rooms,
are to us the bearers of a small portion of the great natural
supply which we draw upon for our purposes, and the actions
of which we can apply as we think fit. The possessor of a
mill claims the gravity of the descending rivulet, or the living
force of the moving wind, as his possession. These por-
tions of the store of Nature are what give his property its
chief value.

Further, from the fact that no portion of force can be
absolutely lost, it does not follow that a portion may not be
inapplicable to human purposes. In this respect the infer-
ences drawn by William Thomson from the law of Carnot
are of importance. This law, which was discovered by Car-
not during his endeavours to ascertain the relations between
heat and mechanical force, which, however, by no means
belongs to the necessary consequences of the conservation of

228 INTERACTION OF NATUEAL FORCES.

force, and which Clausius was the first to modify in such a
manner that it no longer contradicted the above general law,
expresses a certain relation between the compressibility, the
capacity for heat, and the expansion by heat of all bodies. It
is not yet considered as actually proved, buf some remarkable
deductions having been drawn from it, and afterwards proved
to be facts by experiment, it? has attained thereby a great
degree of probability. Besides the mathematical form in
which the law was first expressed by Carnot, we can give it
the following more general expression ; — " Only when heat
passes frqm a warmer to a colder body, and even then only
partially, can it be converted into mechanical work."

The heat of a body which we cannot cool further, cannot
be changed into another form of force ; into the electric or
chemical force, for example. Thus, in our steam engines,
we convert a portion of the heat of the glowing coal into
work, by permitting it to pass to the less warm water of the
boiler. If, however, all the bodies in nature had the same
temperature, it would be impossible to convert any portion of
their heat into mechanical work. According to this, we can
divide the total force store of the universe into two parts, one
of which is heat, and must continue to be such ; the other, to
which a portion of the heat of the warmer bodies, and the
total supply of chemical, mechanical, electrical, and magneti-
cal forces belong, is capable of the most varied changes of
form, and constitutes the whole wealth of change which takes
place in nature.

But the heat of the warmer bodies strives perpetually to
pass to bodies less warm by radition and conduction, and thus
to establish an equilibrium of temperature. At each motion
of a terrestrial body, a portion of mechanical force passes by
friction or collision into heat, of which only a part can be
converted back again into mechanical force. This is also
generally the case in every electrical and chemical process.
From this, it follows that the first portion of the store of force,

THE FORCES OF NATURE DISSIPATED IN HEAT. 229

the nnchangeable heat, is augmented by every natural pro-
cess, while the second portion, mechanical, electrical, and
chemical force, must be diminished ; so that if the universe
be delivered over to the undisturbed action of its physical pro-
cesses, all force will finally pass into the form of heat, and all
heat come into a state of equilibrium. Then all possibility of
a further change would be at an end, and the complete cessa-
tion of aU natural processes must set in. The life of men,
animals, and plants, could not of course continue if the sun
had lost its high temperature, and with it his light, — if all the
components of the earth's surface had closed tho^e combina-
tions which their affinities demand. In short, the universe
from that time forward would be condemned to a state of
eternal rest.

These consequences of the law of Carnot are, of course,
only valid, provided that the law, when sufficiently tested,
proves to be universally correct. In the mean time there is
little prospect of the law being proved incorrect. At all
events we must admire the sagacity of Thomson, who, in the
letters of a long known little mathematical formula, which
only speaks of the heat, volume, and pressure of bodies,
was able to discern consequences which threatened the uni-
verse, though certainly after an infinite period of time, with
eternal death.

I have already given you notice that our path lay through
a thorny and unrefreshing field of mathematico-mechanical
developments. We have now left this portion of our road
behind us. The general principle which I have sought to lay
before you has conducted us to a point from which our view
is a wide one, and, aided by this principle, we can now at
pleasure regard this or the other side of the surrounding
world, according as our interest in the matter leads us. A
glance into the narrow laboratory of the physicist, with its
small appliances and complicated abstractions, will not be so
attractive as a glance at the wide heaven above us, the clouds,

230 mTERA(5TI0N OF NATURAL FORCES.

the rivers, the woods, and the living beings around us. While
regarding the laws which have been deduced from the physi-
cal processes of terrestrial bodies, as applicable also to the
heavenly bodies, let me remind you that the same force which,
acting at the earth's surface, we call gravity (^Schwere), acts
as gravitation in the celestial spaces, and also manifests its
power in the motion of the immeasurably distant double stars
which are governed by exactly the same laws as those sub-
sisting between the earth and moon ; that, therefore, the
light and heat of terrestrial bodies do not in any way differ
essentially from those of the sun, or of the most distant fixed
star ; that the meteoric stones which sometimes fall from ex-
ternal space upon the earth are composed of exactly the same
simple chemical substances as those with which we are
acquainted. We need, therefore, feel no scruple in granting
that general laws to which all terrestrial natural processes
are subject, are also valid for other bodies than the earth.
We will, therefore, make use of our law to glance over the
household of the universe with respect to the store of force,
capable of action, which it possesses.

A number of singular peculiarities in the structure of our
planetary system indicate that it was once a connected mass
with a uniform motion of rotation. Without such an assump-
tion, it is impossible to explain why all the planets move in the
same direction round the sun, why they all rotate in the same
direction round their axes, why the planes of their orbits, and
those of their satellites and rings all nearly coincide, why all
their orbits differ but little from circles, and much besides.
From these remaining indications of a former state, astrono-
mers have shaped an hypothesis regarding the formation of
our planetary system, which, although from the nature of the
case it must ever remain an hypothesis, still in its special
traits is so well supported by analogy, that it certainly de-
serves our attention. It was Kant, who, feeling great inter-
est in the physical description of the earth and the planetary

ORIGIN OF THE NEBULAB HYPOTHESIS. 231

system, undertook the labour of studjdng the works of New-
ton, and as an evidence of the depth to which he had pene-
trated into the fundamental ideas of Newton, seized the notion
that the same attractive force of all ponderable matter which
now supports the motion of the planets, must also aforetime
have been able to form from matter loosely scattered in space
the planetary system. Afterwards, and independent of Kant,
Laplace, the great author of the Mecanique Celeste^ laid hold
of the same thought, and introduced it among astronomers.

The commencement of our planetary system, including
the sun, must, according to this, be regarded as an immense
nebulous mass which filled the portion of space which is now
occupied by our system, far beyond the limits of Neptune,
our most distant planet. Even now we perhaps see similar
masses in the distant regions of the firmament, as patches of
nebulae, and nebulous stars ; within our system also, comets,
the zodiacal light, the corona of the sun during a total eclipse,
exhibit remnants of a nebulous substance, which is so thin
that the light of the stars passes through it unenfeebled and
unrefracted. If we calculate the density of the mass of our
planetary system, according to the above assumption, for the
time when it was a nebulous sphere, which reached to the
path of the outmost planet, we should find that it would
require several cubic miles of such matter to weigh a single
grain.

The general attractive force of all matter must, however,
impel these masses to approach each other, and to condense,
so that the nebulous sphere became incessantly smaller, by
which, according to mechanical laws, a motion of rotation
originally slow, and the existence of which must be assumed,
would gradually become quicker and quicker. By the cen-
trifagal force which must act most energetically in the neigh-
bourhood of the equator of the nebulous sphere, masses
could from time to time be torn away, which afterwards would
continue their courses separate from the main mass, forming

232 INTERACTION OF NATURAL FORCES.

themselves into single planets, or, similar to the great origi-
nal sphere, into planets with satellites and rings, until finally
the principal mass condensed itself into the sun. With
regard to the origin of heat and light, this view gives us no
information.

When the nebulous chaos first separated itself from other
fixed star masses, it must not only have contained all kinds
of matter which was to constitute the future planetary sys-
tem, but also, in accordance with our new law, the whole
store of force whiqji at one time must unfold therein its wealth
of actions. Indeed in this respect an inmaense dower was
bestowed in the shape of the general attraction of aU the par-
ticles for each other. This force, which on the earth exerts
itself as gravity, acts in the heavenly spaces as gravitation.
As terrestrial gravity when it draws a weight downwards
performs work and generates vis viva, so also the heavenly
bodies do the same when they draw two portions of matter
from distant regions of space towards each other.

The chemical forces must have been also present, ready
to act ; but as these forces can only come into operation
by the most intimate contact of the different masses, con-
densation must have taken place before the play of chemical
forces began.

Whether a still further supply of force in the shape of
heat was present at the commencement we do not know. At
all events, by aid of the law of the equivalence of heat and
work, we find in the mechanical forces, existing at the time
to which we refer, such a rich source of heat and light, that
there is no necessity whatever to take refuge in the idea of a
store of these forces originally existing. When through con-
densation of the masses their particles came into collision,
and clung to each other, the vis viva of their motion would be
thereby annihilated, and must reappear as heat. Already in
old theories, it has been calculated, that cosmical masses must
generate heat by their collision, but it was far from any body's

HEAT DEVELOPED IN THE SOLAU SYSTEM. 233

thought, to make even a guess at the amount of heat to be
generated in this way. At present we can give definite
numerical values with certainty.

Let us make this addition to our assumption ; that, at the
commencement, the density of the nebulous matter was a van-
ishing quantity, as compared with the present density of the
sun and planets ; we can then calculate how much work has
been performed by the condensation ; we can further calcu-
late how much of this work still exists iii the form of mechani-
cal force, as attraction of the planets towards the sun, and as
vis viva of their motion, and find, by this, how much of the
force has been converted into heat.

The result of this calculation is, that only about the 454th
part of the original mechanical force remains as such, and
that the remainder, converted into heat, would be sufficient to
raise a mass of water equal to the sun and planets taken to-
gether, not less than twenty-eight millions of degrees of the
centigrade scale. For the sake of comparison, I will mention
that the highest temperature which we can produce by the
oxyhydrogen blowpipe, which is sufficient to fuse and vapor-
ize even platina, and which but few bodies can endure, is
estimated at about two thousand centigrade degrees. Of the
action of a temperature of twenty-eight millions of such de-
grees we can form no notion. If the mass of our entire sys-
tem were pure coal, by the combustion of the whole of it only
the 3500th part of the above quantity would be generated.
This is also clear, that such a development of heat must have
presented the greatest obstacle to the speedy union of the
masses, that the larger part of the heat must have been
diffused by radiation into space, before the masses could form
bodies possessing the present density of the sun and planets,
and that these bodies must once have been in a state of fiery
fluidity. This notion is corroborated by the geological phe-
nomena of our planet ; and with regard to the other planetary
bodies, the flattened form of the sphere, which is the form of

234 INTEBACTION OF NATURAL FOECES.

equilibrium of a fluid mass, is indicative of a former state of
fluidity. If I thus permit an immense quantity of heat
to disappear without compensation from our system, the
principle of the conservation of force is not thereby invaded.
Certainly for our planet it is lost, but not for the universe.
It has proceeded outwards, and daily proceeds outwards into
infinite space ; and we know not whether the medium which
transmits the undulations of light and heat possesses an end
where the rays must return, or whether they eternally pursue
their way through infinitude.

The store of force at present possessed by our system, is
also equivalent to immense quantities of heat. If our earth
were by a sudden shock brought to rest on her orbit — which
is not to be feared in the existing arrangements of our system
— ^by such a shock a quantity of heat would be generated
equal to that produced by the combustion of fourteen such
earths of solid coal. Making the most unfavourable assump-
tion as to its capacity for heat, that is, placing it equal to that
of water, the mass of the earth would thereby be heated 11,200
degrees ; it would therefore be quite fused and for the most
part reduced to vapour. If, then, the earth, after having been
thus brought to rest, should fall into the sun, which of course
would be the case, the quantity of heat developed by the shock
would be four hundred times greater.

Even now, from time to time, such a process is repeated
on a small scale. There can hardly be a doubt that meteors,
fire-balls, and meteoric stones, are masses which belong to
the universe, and before coming into the domain of our earth,
moved like the planets round the sun. Only when they enter
our atmosphere do they become visible and fall sometimes to
the earth. In order to explain the emission of light by
these bodies, and the fact that for some time after their
descent they are very hot, the friction was long ago thought
of which they experience in passing through the air. We
can now calculate that a velocity of 3000 feet a second,

THE LIGHT AOT) HEAT OF METEORS. 235

supposing iiiQ whole of the friction to be expended in heating
the solid mass, would raise a piece of meteoric iron 1000° C.
in temperature, or, in other words, to a vivid red heat. Now
the average velocity of the meteors seems to be thirty or forty
times the above amount. To compensate this, however, the
greater portion of the heat is, doubtless, carried away by the
condensed mass of air which the meteor drives before it. It
is known that bright meteors generally leave a luminous trail
behind them, which probably consists of several portions of
the red-hot surfaces. Meteoric masses which fall to the earth
often burst with a violent explosion, which may be regarded
as a result of the quick heating. The newly-fallen pieces
have been for the most part found hot, but not red-hot, which
is easily explainable by the circumstance, that during the
short time occupied by the meteor in passing through the
atmosphere, only a thin, superficial layer is heated to redness,
while but a small quantity of heat has been able to penetrate
to the interior of the mass. For this reason the red heat can
speedily disappear.

Thus has the falling of the meteoric stone, the minute
remnant of processes which seems to have played an impoi*-
tant part in the formation of the heavenly bodies, conducted
us to the present time, where we pass from the darkness of
hypothetical views to the brightness of knowledge. In what
we have said, however, all that is hypothetical is the assump-
tion of Kant and Laplace, that the masses of our system were
once distributed as nebulae in space.

On account of the rarity of the case, we will still further
remark, in what close coincidence the results of science here
stand with the earlier legends of the human family, and the
forebodings of poetic fancy. The cosmogony of ancient na-
tions generally commences with chaos and darkness.

Neither is the Mosaic tradition very divergent, particu-
larly when we remember that Hiat which Moses names heaven
is different from the blue dome above us, and is synonymous

236 INTEEACTION OF NATUEAL FORCES.

with space, and that the unformed earth, and the waters of
the great deep, which were afterwards divided into waters
above the firmament, and waters below the firmament, resem-
bled the chaotic components of the world.

Om' earth bears still the unmistakable traces of its old
fiery fluid condition. The granite formations of her moun-
tains exhibit a structure, which can only be produced by the
crystallization of fused masses. Investigation still shows
that the temperature in mines, and borings, increases as we
descend ; and if this increase is uniform, at the depth of fifty
miles, a heat exists sufficient to fuse all our minerals. Even
now our volcanoes project, from time to time, mighty masses
of fused rocks from their interior, as a testimony of the heat
which exists there. But the cooled crust of the earth has
already become so thick, that, as may be shown by calcula-
tions of its conductive power, the heat coming to the surface
from vdthin, in comparison with that reaching the earth from
the sun, is exceedingly small, and increases the temperature
of the surface only about one thirtieth of a degree centigrade ;
so that the remnant of the old store of force which is enclosed
as heat within the bowels of the earth, has a sensible influence
upon the processes at the earth's surface, only through the
instrumentality of volcanic phenomena. These processes owe
their 'power almost wholly to the action of other heavenly todies^
particularly to the light and heat of the sun, and partly also,
in the case of the tides, to the attraction of the sun and moon.

Most varied and numerous are the changes which we owe
to the light and heat of the sun. The sun heats our atmos-
phere irregularly, the warm rarefled air ascends, wliile fresh
cool air flows from the sides to supply its place : in this way
winds are generated. This action is most powerful at the
equator, the warm air of which incessantly flows in the upper
regions of the atmosphere towards the poles : while just as
persistently, at the earth's surTace, the trade wind carries new
and cool air to the equator. Without the heat of the sun all

bKju^JS. FORCE PEODUOES THE WATER CIRCULATIONS. 237

winds must, of necessity, cease. Similar currents are pro-
duced by the same cause in the waters of the sea. Their pow-
er may be inferred from the influence which in some cases
they exert upon climate. By them the warm water of the
Antnies is carried to the British Isles, and confers upon them
a mild, uniform warmth and rich moisture ; while, through
similar causes, the floating ice of the North Pole is carried to
the coast of Newfoundland, and produces cold. Further, by
the heat of the sun, a portion of the water is converted into
vapour which rises in the atmosphere, is condensed to clouds,
or falls in rain and snow upon the earth, collects in the form
of springs, brooks, and rivers, and finally reaches the sea
again, after having gnawed the rocks, carried away the light
earth, and thus performed its part in the geologic changes of
the earth ; perhaps, besides all this it has driven our water-
mill upon its way. J£ the heat of the sun were withdrawn,
there would remain only a single motion of water, namely,
the tides, which are produced by the attraction of the sun and
moon.

How is it, now, with the motions and the work of organic
beings. To the builders of the automata of the last century,
men and animals appeared as clockwork which was never
wound up, and created the force which they exerted out of
nothing. They did not know how to establish a connection
between the nutriment consumed and the work generated.
Since, however, we have learned to discern in the steam-en-
gine this origin of mechanical force, we must inquire whether
something similar does not hold good with regard to men. In-
deed, the continuation of life is dependent on the consumption
of nutritive materials : these are combustible substances, which,
after digestion and being passed into the blood, actually under-
go a slow combustion, and finally enter into almost the same
combinations with the oxygen of the atmosphere that are pro-
duced in an open fire. As the (][uantity of heat generated by
combustion is independent of the duration of the combustion and

INTEEACTION OF NATURAL FOECES.

the steps in which it occurs, we can calculate from the mass of
the consumed material how much heat, or its equivalent work
is thereby generated in an animal body. Unfortunately, the
difficulty of the experiments is still very great ; but within
those limits of accuracy which have been as yet attainable, the
experiments show that the heat generated in the animal body
corresponds to the amount which would be generated by the
chemical processes. The animal body therefore does not differ
from the steam-engine, as regards the manner in which it
obtains heat and force, but does differ from it in the man-
ner in which the force gained is to be made use of. The
body is, besides, more limited than the machine in the choice
of its fuel ; the latter could be heated with sugar, with starch-
flour, and butter, just as well as with coal or wood ; the ani-
mal body must dissolve its materials artificially, and distribute
them through its system ; it must, further, perpetually renew
the used-up materials of its organs, and as it cannot itself
create the matter necessary for this, the matter must come
from without. Liebig was the first to point out these various
uses of the consumed nutriment. As material for the perpet-
ual reinewal of the body, it seems that certain definite albumi-
nous substances which appear in plants, and form the chief
mass of the animal body, can alone be used. They form only
a portion of the mass of nutriment taken daily ; the remain-
der, sugar, starch, fat, are really only materials for warming,
and are perhaps not to be superseded by coal, simply because
the latter does not permit itself to be dissolved.

K, then, the processes in the animal body are not in this
respect to be distinguished from inorganic processes, the ques-
tion arises, whence comes the nutriment which constitutes
the source of the body's force ? The answer is, from the
vegetable kingdom ; for only the material of plants, or the
flesh of plant-eating animals, can be made use of for food.
The animals which live on plants occupy a mean position
between carnivorous animals, in which we reckon man, and

SOLAR OEIGIN OF ORGANIC FORCE. 239

vegetables, whicli the former could not make use of immedi-
ately as nutriment. In hay and grass the same nutritive sub-
stances are present as in meal and flour, but in less quantity.
As, however, the digestive organs of man are not in a condi-
tion to extract the small quantity of the useful from the great
excess of the insoluble, we submit, in the first place, these
substances to the powerful digestion of the ox, permit the
nourishment to store itself in the animal*s body, in order in
the end to gain it for ourselves in a more agreeable and use-
ful form. In answer to our question, therefore, we are re-
ferred to the vegetable world. Now when what plants take
in and what they give out are made the subjects of investiga-
tion, we find that the principal part of the former consists in
the products of combustion which are generated by the ani-
mal. They take the consumed carbon given off in respira-
tion, as carbonic acid, from the air, the consumed hydrogen
as water, the nitrogen in its simplest and closest combination
as ammonia ; and from these materials, with the assistance
of small ingredients which they take from the soil, they gen-
erate anew the compound combustible substances, albumen,
sugar, oU, on which the animal subsists. Here, therefore, is
a circuit which appears to be a perpetual store of force.
Plants prepare fuel and nutriment, animals consume these,
burn them slowly in their lungs, and from the products of
combustion the plants again derive their nutriment. The
latter is an eternal source of chemical, the former of mechan-
ical forces. Would not the combination of both organic king-
doms produce the perpetual motion ? We must not conclude
hastily : further inquiry shows, that plants are capable of pro-
ducing combustible substances only when they are under the
influence of the sun. A portion of the sun's rays exhibits a
remarkable relation to chemical forces, — it can produce and
destroy chemical combinations ; and these rays, which for the
most part are blue or violet, are called therefore chemical
rays. We make use of their action in the production of pho-

240 INTERACTION OF NATURAL FORCES.

tographs. Here compounds of silver are decomposed at the
place where the sWs rays strike them. The same rays over-
power in the green leaves of plants the strong chemical affinity
of the carbon of the carbonic acid for oxygen, give back the
latter free to the atmosphere, and accumulate the other, in
combination with other bodies, as woody fibre, starch, oU, or
resin. These chemically active rays of the sun disappear
completely as soon as they encounter the green portions of the
plants, and hence it is that in daguerrotype images the green
leaves of plants appear uniformly black. Inasmuch as the
light coming from them does not contain the chemical rays, it
is unable to act upon the silver compounds.

Hence a certain portion of force disappears from the sun-
light, while combustible substances are generated and accumu-
lated in plants ; and we can assume it as very probable, that
the former is the cause of the latter. I must indeed remark,
that we are in possession of no experiments from which we
might determine whether the vis viva of the sun's rays which
have disappeared, corresponds to the chemical forces accumu-
lated during the same time ; and as long as these experiments
are wanting, we cannot regard the stated relation as a cer-
tainty. If this view should prove correct, we derive from it
the flattering result, that all force, by means of which our bodies
live and move, finds its source in the purest sunlight ; and
hence we are all, in point of nobility, not behind the race of the
great monarch of China, who heretofore alone called himself
Son of the Sun. But it must also be conceded that our lower
fellow-beings, the frog and leech, share the same ethereal
origin, as also the whole vegetable world, and even the fuel
which comes to us from the ages past, as well as the youngest
offspring of the forest with which we heat our stoves and set
our machines in motion.

You see, then, that the immense wealth of ever-changing
meteorological, climatic, geological, and organic processes of
our earth are almost wholly preserved in action by the light

DYNAMICS OF SUNLIGHT. 241

and heat-giving rays of the sun ; and you see in this a re-
markable example, how Proteus-like the effects of a single
cause, under altered external conditions, may exhibit itself in
nature. Besides these, the earth experiences an action of
another kind from its central luminary, as well as from its
satellite the moon, which exhibits itself in the remarkable
phenomenon of the ebb and flow of the tide.

Each of these bodies excites, by its attraction upon the
waters of the sea, two gigantic waves, which flow in the same
direction round the world, as the attracting bodies themselves
apparently do. The two waves of the moon, on account of
her greater nearness, are about tliree and a half times as large
as those excited by the sun. One of these waves has its crest
on the quarter of the earth's surface which is turned towards
the moon, the other is at the opposite side. Both these quar-
ters possess the flow of the tide, while the regions which lie
between have the ebb. Although in the open sea the height
of the tide amounts to only about three feet, and only in cer-
tain narrow channels, where the moving water is squeezed
together, rises to thirty feet, the might of the phenomena is
nevertheless manifest from the calculation of Bessel, accord-
ing to which a quarter of the earth covered by the sea pos-
sesses, during the flow of the tide, about 25,000 cubic miles
of water more than during the ebb, and that therefore such a
mass of water must, in six and a quarter hours, flow from
one quarter of the earth to the other.

The phenomena of the ebb and flow, as already recognized
by Mayer, combined with the law of the conservation of force,
stand in remarkable connection with the question of the sta-
bility of our planetary system. The mechanical theory of the
planetary motions discovered by Newton teaches, that if a
solid body in absolute vacuo^ attracted by the sun, move
around him in the same manner as the planets, this motion
will endure unchanged through all eternity.

Now we have actually not only one, but several such
11

24:2 INTERACTION OF NATURAL FORCES.

planets, wliicli move around the sun, and by their mutual
attraction create little changes and disturbances in each other's
paths. Nevertheless Laplace, in his great work, the Mecau'
ique Celeste, has proved that in our planetary system all these
disturbances increase and diminish periodically, and can never
exceed certain limits, so that by this cause the eternal exist-
ence of the planetary system is unendangered.

But I have already named two assumptions which must be
made : first that the celestial spaces must be absolutely empty ;
and secondly, that the sun and planets must be solid bodies.
The first is at least the case as far as astronomical observa-
tions reach, for they have never been able to detect any retar-
dation of the planets, such as would occur if they moved in a
resisting medium. But on a body of less mass, the comet of
Encke, changes are observed of such a nature : this comet de-
scribes ellipses round the sun which are becoming gradually
smaller. If this kind of motion, which certainly corresponds
to that through a resisting medium, be actually due to the ex-
istence of such a medium, a time will come when the comet
will strike the sun ; and a similar end threatens all the planets,
although after a time, the length of which baffles our imagina-
tion to conceive of it. But even should the existence of a re-
sisting medium appear doubtful to us, there is no doubt that
the planets are not wholly composed of solid materials which
are inseparably bound together. Signs of the existence of an
atmosphere are observed on the Sun, on Venus, Mars, Jupi-
ter, and Saturn. Signs of water and ice upon Mars ; and
our earth has undoubtedly a fluid portion on its surface,
and perhaps a still greater portion of fluid within it. The
motions of the tides, however, produce friction, aU friction
destroys vis viva, and the loss in this case can only affect the
vis viva of the planetary system. We come thereby to the
unavoidable conclusion, that every tide, although with infinite
slowness, still with certainty, diminishes the store of mechani-
cal force of the system ; and as a consequence of this, the ro-

uSie'LUENCE OF TIDES UPON THE EABTH's ROTATION. 243

taticn of the planets in question round their axes must become
more slow ; they must therefore approach the sun, or their
satellites must approach them. What length of time must
pass before the length of our day is diminished one second by
the action of the tide cannot be calculated, until the height
and time of the tide in all portions of the ocean are known.
This alteration, however, takes place with extreme slowness,
as is known by the consequences which Laplace has deduced
from the observations of Hipparchus, according to which,
during a period of 2000 years, the duration of the day has
not been shortened by the one three hundredth part of a sec-
ond. The final consequence would be, but after millions of
years, if in the mean time the ocean did not become frozen,
that one side of the earth would be constantly turned towards
the sun, and enjoy a perpetual day, whereas the opposite side
would be involved in eternal night. Such a position we
observe in our moon with regard to the earth, and also in the
case of the satellites as regards their planets ; it is, perhaps,
due to the action of the mighty ebb and flow to which these
bodies, in the time of their fiery fluid condition, were sub-
jected.

I would not have brought forward these conclusions, which
again plunge us in the most distant future, if they were not
unavoidable. Physico-mechanical laws are, as it were, the
telescopes of our spiritual eye, which can penetrate into the
deepest night of time, past and to come.

Another essential question as regards the future of our
planetary system has reference to its future temperature and
illumination. As the internal heat of the earth has but little
influence on the temperature of the surface, the heat of the
sun is the only thing which essentially affects the question.
The quantity of heat falling from the sun during a given time
upon a given portion of the earth's surface may be measured,
and from this it can be calculated how much heat in a given
time is sent out from the entire sun. Such measurements

244 INTEBACTION OF NATUKAL FOECES.

have been made by the French physicist Po-uillet, and it has
been found that the sun gives out a quantity of heat per hour
equal to that which a layer of the densest coal ten feet thick
would give out by its combustion ; and hence in a year a
quantity equal to the combustion of a layer of seventeen miles.
If this heat were drawn uniformly from the entire mass of the
sun, its temperature would only be diminished thereby one and
one third of a degree centigrade per year, assuming its capa-
city for heat to be equal to that of water. These results can
give us an idea of the magnitude of the emission, in relation
to the surface and mass of the sun ; but they cannot inform
us whether the sun radiates heat as a glowing body, which
since its formation has its heat accumulated within it, or
whether a new generation of heat by chemical processes
takes place at the sun's surface. At all events the law of the
conservation of force teaches us that no process analogous to
those known at the surface of the earth, can supply for eternity
an inexhaustible amount of light and heat to the sun. But
the same law also teaches that the store of force at present
existing, as heat, or as what may become heat, is sufficient for
an immeasurable time. With regard to the store of chemical
force in the sun, we can form no conjecture, and the store of
heat there existing can only be determined by very uncertain
estimations. If, however, we adopt the very probable view,
that the remarkably small density of so large a body is caused
by its high temperature, and may become greater in time, it
may be calculated that if the diameter of the sun were dimin-
ished only the ten-thousandth part of its present length, by
this act a sufficient quantity of heat would be generated to
cover the total emission for 2100 years. Such a small change
besides it would be difficult to detect even by the finest astro-
nomical observations.

Indeed, from the commencement of the period during
which we possess historic accounts, that is, for a period of
about 4000 years, the temperature of the earth has not sensi-

bly diminished. From these old ages we have certainly no
thermometric observations, but we have information regard-
ing the distribution of certain cultivated plants, the vine, the
olive tree, which are very sensitive to changes of the mean
annual temperature, and we find that these plants at the pres-
ent moment have the same limits of distribution that they had
in the times of Abraham and Homer ; from which we may in-
fer backwards the constancy of the climate.

In opposition to this it has been urged, that here in Prussia
the German knights in former times cultivated the vine,
cellared their own wine and drank it, which is no longer pos-
sible. From this the conclusion has been drawn, that the
heat of our climate has diminished since the time referred to.
Against this, however. Dove has cited the reports of ancient
chroniclers, according to which, in some peculiarly hot years,
the Prussian grape possessed somewhat less than its usual
quantity of acid. The fact also speaks not so much for the
climate of the country as for the throats of the German
drinkers.

But even though the force store of our planetary system
is so immensely great, that by the incessant emission which
has occurred during the period of human history it has not
been sensibly diminished, even though the length of the time
which must flow by, before a sensible change in the state of
our planetary system occurs, is totally incapable of measure-
ment, still the inexorable laws of mechanics indicate that this
store of force, which can only sufier loss and not gain, must
be finally exhausted. Shall we terrify ourselves by this
thought ? Men are in the habit of measuring the greatness
and the wisdom of the universe by the duration and the profit
which it promises to their own race ; but the past history of
the earth already shows what an insignificant moment the
duration of the existence of our race upon it constitutes. A
Nineveh vessel, a Roman sword awakes in us the conception
of grey antiquity. What the museums of Europe show us of

246 mi^EBACTION OF NATURAL FOECES.

the remains of Egypt and Assyria we gaze upon with silent
astonishment, and despair of being able to carry our thoughts
back to a period so remote. Still must the human race have
existed for ages, and multiplied itself before the pyramids of
Nineveh could have been erected. We estimate the duration
of human history at 6000 years ; but immeasurable as this time
may appear to us, what is it in comparison with the time dur-
ing which the earth carried successive series of rank plants
and mighty animals, and no men ; during which in our neigh-
bourhood the amber-tree bloomed, and dropped its costly gum
on the earth and in the sea ; when in Siberia, Europe and
North America groves of tropical palms flourished ; where
gigantic lizards, and after them elephants, whose mighty re-
mains we still find buried in the earth, found a home ? Dif-
ferent geologists, proceeding from different premises, have
sought to estimate the duration of the above creative period,
and vary from a million to nine miUion years. And the time
during which the earth generated organic beings is again
small when we compare it with the ages during which the
world was a baU of fused rocks. For the duration of its cool-
ing from 2000° to 200° centigrade, the experiments of Bishop
upon basalt show that about 350 millions of years would be
necessary. And with regard to the time during which the first
nebulous mass condensed into our planetary system, our most
daring conjectures must cease. The history of man, there
fore, is but a short ripple in the ocean of time. For a much
longer series of years than that during which man has already
occupied this world, the existence of the present state of in-
organic nature favourable to the duration of man seems to be
secured, so that for ourselves and for long generations after
us, we have nothing to fear. But the same forces of air and
water, and of the volcanic interior, which produced former
geological revolutions, and buried one series of living forms
after another, act still upon the earth's crust. They more
probably will bring about the last day of the human race than

CULMINATION OF THE ARGUMENT. 24:7

those distant cosmical alterations of which we have spoken,
and perhaps force us to make way for new and more com-
plete living forms, as the lizards and the mammoth have
given place to us and our feUow-creatures which now exist.

Thus the thread which was spun in darkness by those
who sought a perpetual motion has conducted us to a univer-
sal law of nature, which radiates light into the distant nights
of the beginning and of the end of the history of the universe.
To our own race it permits a long but not an endless exist-
ence ; it threatens it with a day of judgment, the dawn of
which is still happily obscured. As each of us singly must
endure the thought of his death, the race must endure the
same. But above the forms of life gone by, the human race
has higher moral problems before it, the bearer of which it is,
and in the completion of which it fulfils its destiny.

I.

REMARKS ON

THE FOKOES OF INOKGAOTC NATURE.

Br Dr. J. R. MAYER.
Tbanslatbd bt J. O. FOSl'EE, B.A.

n.
ON CELESTIAL DYNAMICS. "

By Dr. J. R. MAYER.
Tbakslatbd by De. H. DEBUS, F.R.S.

m.

EEMAEK-S ON

THE MECHANICAL EQUIVALENT OF HEAT,

By Dr. J. R. MAYER.
Tbanslated bt J. 0. FOSTEE, B.A.

IV

Julius Robert Mater was bom at Heilbronn, November 25, 1814. He
received a medical education, and became first, comity wound-physician and
afterwards city physician of Heilbronn. But few particulars of his life have
been obtained. In 1840 he made a voyage on a Dutch freighter to Java,
and it was the accident of bleeding a feverish patient in this country, and
observing that the venous blood in the tropics was of a much brighter red
than in, colder latitudes, that led him to those investigations of natural
forces, the chief results of which are given in the following essays. Two
years after his attention was drawn to the subject — ^in 1842, he published
his first paper on the " Forces of Inorganic Nature." It was put together
briefly, and published in Liebig's journal to secure the public recognition of
. Ms claims. His second publication, " On Organic Motion and Nutrition "
(1845), an able essay of one hundred and twelve pages, is not yet translated.
His third paper, on "Celestial Dynamics," was published in 1848 ; and his
fourth, on the " Mechanical Equivalent of Heat," appeared in 1851.

These vast and rapid labors .were too much fbr his strength. His over-
tasked mind gave way, and he was taken to an insane asylum. He, how-
ever, fortunately recovered, and is now reported as occupied with the culti-
vation of the vine in Heilbronn.

I.

THE FORCES OF INORGANIC NATURE.

THE following pages are designed as an attempt to an-
swer the questions, What are we to understand by
"Forces" ? and how are different forces related to each other?
Whereas the term matter implies the possession, by the object
to which it is applied, of very definite properties, such as
weight and extension ; the term force conyeys for the most
part the idea of something unknown, unsearchable, and hypo-
thetical. An attempt to render the notion of force equally
exact with that of matter, and so to denote by it only objects
of actual investigation, is one which, with the consequences
that flow from it, ought not to be unwelcome to those who
desire that their views of nature may be clear and unencum-
bered by hypotheses.

Forces are causes: accordingly, we may in relation to
them make full application of the principle — causa oequat e/-
fectum. If the cause c has the effect e, then c=:e; if, in its
turn, e is the cause of a second effect/, we have e=/, and so
on: c=e=f,.,=c. In a chain of causes and effects, a
term or a part of a term can never, as plainly appears from
the nature of an equation, become equal to nothing. This
first property of all causes we call their indestructibility.

If the given cause c has produced an effect e equal to it-
self, it has in that very act ceased to be : c has become e ; if,
after the production of e, c still remained in whole or in part.

262 THE FORCES OF mOEGAOTC NATUEE.

there must be still further elffects corresponding to this re-
maining cause : the total effect of c would thus be > e, which
would be contrary to the supposition c=e. Accordingly,
since c becomes e, and e becomes/, &c., we must regard these
various magnitudes as different forms under which one and
the same object makes its appearance. This capability of
assuming various forms is the second essential property of all
causes. Taking both properties together, we may say, causes
are (quantitatively) indestructible and (qualitatively) convert-
ihle objects.

Two classes of causes occur in nature, which, so far as
experience goes, never pass one into another. The first class
consists of such causes as possess the properties of weight
and impenetrability ; these are kinds of Matter : the other
class is made up of causes which are wanting in the proper-
ties just mentioned, namely Forces, called also Impondera-
bles, from the negative property that has been indicated.
Forces are therefore indestructible y convertible^ imponderable
objects.

We will in the first instance take matter, to afford us an
example of causes and effects. Explosive gas, H+0, and
water, HO, are related to each other as cause and effect,
therefore H+0=HO. But if H+0 becomes HO, heat, cal.,
makes its appearance as well as water ; this heat must like-
wise have a cause, £c, and we have therefore H-j-0+a3=H0
-\-cal. It might, however, be asked whether H+0 is really
=H0, and x=caL, and not perhaps Il-\-0=caL, and aj=HO,
whence the above equation could equally be deduced ; and so
in many other cases. The phlogistic chemists recognized the
equation between caL and x, or Phlogiston as they called it,
and in so doing made a great step in advance ; but they in-
volved themselves again in a system of mistakes by putting — x
in place of O ; thus, for instance, they obtained H=HO+a;.

Chemistry, whose problem it is to set forth in equations
the causal connection existing between the different kinds of

MATTER AND FORCE AS CAUSES. 253

matter, teaches us that matter, as a cause, has matter for its
effect ; but we are equally justified in saying that to force as
cause, corresponds force as effect. Since c=e, and e=c, it
is unnatural to call one term of an equation a force, and the
other an effect of force or phenomenon, and to attach differ-
ent notions to the expressions Force and Phenomenon. In
brief, then, if the cause is matter, the effect is malj^er ; if the
cause is a force, the effect is also a force.

A cause which brings about the raising of a weight is a
force ; its effect (the raised weight) is, accordingly, equally a
force ; or, expressing this relation in a more general form,
separation in space of ponderable objects is a force ; since this
force causes the fall of bodies, we call it falling force. Fall-
ing force and fall, or, more generally still, falling force and
motion, are forces which are related to each other as cause
and effect — forces which are convertible one into the other —
two different forms of one and the same object. For exam-
ple, a weight resting on the ground is not a force : it is neither
the cause of motion, nor of the lifting of another weight ; it
becomes so, however, in proportion as it is raised above the
ground : the cause — ^the distance between a weight and the
earth — and the effect — ^the quantity of motion produced — ^beai
to each other, as we learn from mechanics, a constant rela-
tion.

Gravity being regarded as the cause of the falling of bod-
ies, a gravitating force is spoken of, and so the notions of
property and of force are confounded with each other ; pre-
cisely that which is the essential attribute of every force —
the union of indestructibility with convertibility — is wanting
in every property : between a property and a force, between
gravity and motion, it is therefore impossible to establish the
equation required for a rightly-conceived causal relation. If
gravity be called a force, a cause is supposed which produces
effects without itself diminishing, and incorrect conceptions
of the causal connections of things are thereby fostered. In

254 THE FORCES OF INOEGANIC NATURE.

order that a body may fall, it is no less necessary that it
should be lifted up, than that it should be heavy or possess
gravity ; the fall of bodies ought not therefore to be ascribed
to their gravity alone.

It is the problem of Mechanics to develop the equations
which subsist between falling force and motion, motion and
falling force, and between different motions : here we will call
to mind only one point. The magnitude of the falling force
V is directly proportional (the earth's radius being assumed =
00 ) to the magnitude of the mass m, and the height d to
which it is raised; that is, v=md. If the height cZ=l, to
which the mass m is raised, is transformed into the final ve-
locity 0=1 of thi»mass, we have also v=mc; but from the
known relations existing between d and c, it results that, for
other values of d or of c, the measure of the force v is mc^ ;
accordingly v='md=mc' : the law of the conservation of vis
viva is thus found to be based on the general law of the inde
structibility of causes.

In numberless cases we see motion cease without having
caused another motion or the lifting of a weight ; but a force
once in existence cannot be annihilated, it can only change its
form ; and the question therefore arises. What other forms is
force, which we have become acquainted with as falling force
and motion, capable of assuming? Experience alone can
lead us to a conclusion on this point. In order to experi-
ment with advantage, we must select implements which, be-
sides causing a real cessation of motion, are as little as possi-
ble altered by the objects to be examined. If, for example,
we rub together two metal plates, we see motion disappear,
and heat, on the other hand, make its appearance, and we
have now only to ask whether motion is the cause of heat.
In order to come to a decision on this point, we must discuss
the question whether, in the numberless cases in which the
expenditure of motion is accompanied by the appearance of
heat, the motion has not some other effect than the pro-

EFFECTS OF DESTROYED MOTION. 255

duction of heat, and the heat some other cause than the
motion.

An attempt to ascertain the effects of ceasing motion has
never yet been seriously made ; without, therefore, wishing to
exclude d priori the hypothesis which it may be possible to
set up, we observe only that, as a rule, this effect cannot be
supposed to be an alteration in the state of aggregation of the
moved (that is, rubbing, &c.) bodies. If we assume that a
certain quantity of motion v is expended in the conversion of
a rubbing substance m into n, we must then have m-{-v^=n,
and ?i=m+v; and when n is reconverted into m, v must ap-
pear again in some form or other. By the friction of two
metallic plates continued for a very long time, we can grad-
ually cause the cessation of an immense quantity of move-
ment; but would it ever occur to us to look for even the
smallest trace of the force which has disappeared in the me-
tallic dust that we could collect, and to try to regain it thence ?
We repeat, the motion cannot have been annihilated ; and
contrary, or positive and negative, motions cannot be regarded
as =0, any more than contrary motions can come out of
nothing, or a weight can raise itself.

Without the recognition of a causal connection between
motion and heat, it is just as difficult to explain the produc-
tion of heat as it is to give any account of the motion that
disappears. The heat cannot be derived from the diminution
of the volume of the rubbing substances. It is well known
that two pieces of ice may be melted by rubbing them to-
gether in vacuo ; but let any one try to convert ice into water
by pressure,* however enormous. Water undergoes, as was

* Since the original publication of this paper, Prof. W. Thomson has
shown that pressure has a sensible effect in liquefying ice ( Cojif. Phil. Mag.
S. 3, vol. xxxvii. p. 123) ; but the experiments of Bunsen and of Hopkins
have shown that the melting-points of bodies which expand on becoming
liquid are raised by pressure, which is all that Mayer's argument requires. —
G. C. F.

256 THE FORCES OF INOEGANIO NATUEE.

found by the author, a rise of temperature when violently
shaken. The water so heated (from 12° to 13° C.) has a
greater bulk after being shaken than it had before ; whence
now comes this quantity of heat, which by repeated shaking
may be called into existence in the same apparatus as often
as we please ? The vibratory hypothesis of heat is an ap-
proach toward the doctrine of heat being the effect of mo-
tion, but it does not favour the admission of this causal rela-
tion in its full generality ; it rather lays the chief stress on
uneasy oscillations (unhehagliche Schwingungen).

If it be now considered as established that in many cases
{excejptio confirmat regulam) no other effect of motion can be
traced except heat, and that no other cause than motion can
be found for the heat that is produced, we prefer the assump-
tion that heat proceeds from motion, to the assumption of a
cause without effect and of an effect without a cause — -just as
the chemist, instead of allowing oxygen and hydrogen to dis-
appear without further investigation, and water to be pro-
duced in some inexplicable manner, establishes a connection
between oxygen and hydrogen on the one hand and water on
the other.

The natural connection existing between falling force, mo-
tion, and heat may be conceived of as foUows : We know that
heat makes its appearance when the separate particles of a
body approach nearer to each other ; condensation produces
heat. And what applies to the smallest particles of matter,
and the smallest intervals between them, must also apply to
large masses and to measurable distances. The falling of a
weight is a diminution of the bulk of the earth, and must
therefore without doubt be related to the quantity of heat
thereby developed ; this quantity of heat must be proportional
to the greatness of the weight and its distance from the
ground. From this point of view we are very easily led to
the equations between falling force, motion, and heat, that
have already been discussed.

EQUIVALENCE OF HEAT AJSTD MOTION. 257

But just as little as the connection between falling force
and motion authorizes the conclusion that the essence of fall-
ing force is motion, can such a conclusion be adopted in the
case of heat. We are, on the contrary, rather inclined to
infer that, before it can become heat, motion — ^whether sim-
ple, or vibratory as in the case of light and radiant heat, &c.
— must cease to exist as motion.

If falling force and motion are equivalent to heat, heat
must also naturally be equivalent to motion and falling force.
Just as heat appears as an effect of the diminution of bulk and
of the cessation of motion, so also does heat disappear as a
cause when its effects are produced in the shape of motion,
expansion, or raising of weight.

In water-mills, the continual diminution in bulk which the
earth undergoes, owing to the fall of the water, gives rise to
motion, which afterwards disappears again, calling forth un-
ceasingly a great quantity of heat ; and inversely, the steam-
engine serves to decompose heat again into motion or the
raising of weights. A locomotive engine with its train may
be compared to a distilling apparatus ; the heat applied under
the boiler passes off as motion, and this is deposited again as
heat at the axles of the wheels.

We will close our disquisition, the propositions of which
have resulted as necessary consequences from the principle
" causa aequat effectum," and which are in accordance with
all the phenomena of Nature, with a practical deduction.
The solution of the equations subsisting between falling force
and motion requires that the space fallen through in a given
time, e. g. the first second, should be experimentally deter-
mined ; in like manner, the solution of the equations subsist-
ing between falling force and motion on the one hand and
heat on the other, requires an answer to the question. How
great is the quantity of heat which corresponds to a given
quantity of motion or falling force? For instance, we must
ascertain how high a given weight requires to be raised above

258 THE FORCES OF IKOEGANIC NATURE.

the ground in order that its falling force may be equivalent to
the raising of the temperature of an equal weight of water
from 0° to 1° C. The attempt to show that such an equa-
tion is the expression of a physical truth may be regarded as
the substance of the foregoing remarks.

By applying the principles that have been set forth to the
relations subsisting between the temperature and the volume
of gases, we find that the sinking of a mercury column by
which a gas is compressed is equivalent to the quantity of
heat set free by the compression ; and hence it foUows, the
ratio between the capacity for heat of air under constant pres-
sure and its capacity under constant volume being taken as
= 1*421, that the warming of a given weight of water from
0° to 1° C. corresponds to the fall of an equal weight from
the height of about 365 metres.* If we compare with this
result the working of our best steam-engines, we see how
small a part only of the heat applied under the boiler is really
transformed into motion or the raising of weights ; and this
may serve as justification for the attempts at the profitable
production of motion by some other method than the expendi-
ture of the chemical difference between carbon and oxygen —
more particularly by the transformation into motion of elec-
tricity obtained by chemical means.

* When the corrected specific heat of air is introduced into the calcu-
lation this number is increased, and agrees then with the experimental de-
terminations of Mr. Joule.

n.

CELESTIAL DYNAMICS

I.— INTRODUCTION.

EVERY incandescent and luminous body diminishes in
temperature and luminosity in the same degree as it
radiates light and heat, and at last, provided its loss be not
repaired from some other source of these agencies, becomes
cold and non-luminous.

For light, like sound, consists of vibrations which are
communicated by the luminous or sounding body to a sur-
rounding medium. It is perfectly clear that a body can only
excite such vibrations in another substance when its own par-
ticles undergo a similar movement ; for there is no cause for
imdulatory motion when a body is in a state of rest, or in a
state of equilibrium with the medium by which it is sur-
rounded. If a beU or a string is to be sounded, an external
force must be applied ; and this is the cause of the sound.

J£ the vibratory motion of a string could take place with-
out any resistance, it would vibrate for aU time ; but in this
case no sound could be produced, because sound is essentially
the propagation of motion ; and in the same degree as the

CELESTIAL DYNAMICS.

string communicates its vibrations to the surrounding and re-
sisting medium its own motion becomes weaker and weaker,
until at last it sinks into a state of rest.

The sun has often and appropriately been compared to an
incessantly sounding bell. But by what means is the power
of this body kept up in undiminished force so as to enable
him to send forth his rays into the universe in such a grand
and magnificent manner ? What are the causes which coun-
teract or prevent his exhaustion, and thus save the planetary
system from darkness and deadly cold ?

Some endeavoured to approach " the grand secret," as
Sir Wm. Herschel calls this question, by the assumption that
the rays of the sun, being themselves perfectly cold, merely
cause the " substance " of heat, supposed to be contained in
bodies, to pass from a state of rest into a state of motion, and
that in order to send forth such cold rays the sun need not be
a hot body, so that, in spite of the infinite development of
light, the cooling of the sun was a matter not to be thought of.

It is plain that nothing is gained by such an explanation ;
for, not to speak of the h3;Tpothetical " substance" of heat,
assumed to be at one time at rest and at another time in mo-
tion, now cold and then hot, it is a well-founded fact that the
sun does not radiate a cold phosphorescent light, but a light
capable of warming bodies intensely ; and to ascribe such
rays to a cold body is at once at variance with reason and
experience.

Of course such and similar hypotheses could not satisfy
the demands of exact science, and I will therefore try to ex-
plain in a more satisfactory manner than has been done up
to this time the connexion between the sun's radiation and its
effects. In doing so, I have to claim the indulgence of scien-
tific men, who are acquainted with the difficulties of my task.

SOUEC-ES OF HEAT. 261

II.— SOUKCES OF HEAT.

BEroRE we turn our attention to the special subject of
this paper, it will be necessary to consider the means by
which light and heat are produced. Heat may be obtained
from very different sources. Combustion, fermentation, pu-
trefaction, slaking of lime, the decomposition of chloride of
nitrogen and of gun-cotton, &c. &c., are all of them sources
of heat. The electric spark, the voltaic current, friction, per-
cussion, and the vital processes are also accompanied by the
evolution of this agent.

A general law of nature, which knows of no exception,
is the following : — In order to obtain heat, something must
be expended ; this something, however different it may be in
other respects, can always be referred to one of two catego-
ries : either it consists of some material expended in a chem-
ical process, or of some sort of mechanical work.

When substances endowed with considerable chemical af-
finity for each other combine chemically, much heat is devel-
oped during the process. We shall estimate the quantity of
heat thus set free by the number of kilogrammes of water
which it would heat 1° C. The quantity of heat necessary
to raise one kilogramme of, water one degree is called a unit
of heat.

It has been established by numerous experiments that the
combustion of one kilogramme of dry charcoal in oxygen, so
as to form carbonic acid, yields 7200 units of heat, which fact
may be briefly expressed by saying that charcoal furnishes-
7200° degrees of heat.

Superior coal yields 6000°, perfectly dry wood from 3300°
to 3900°, sulphur 2700, and hydrogen 34,600° of heat.

According to experience, the number of units of heat only
depends on the quantity of matter which is consumed, and

262 CELESTIAL DYNAMICS.

not on the conditions under which the burning takes place.
The same amount of heat is given out whether the combus-
tion proceeds slowly or quickly, in atmospheric air or in pure
oxygen gas. K in one case a metal be burnt in air and the
amount of heat directly measured, and in another instance
the same quantity of metal be oxidized in a galvanic battery,
the heat being developed in some other place — say, the wire
which conducts the current, — ^in both of these experiments
the same quantity of heat will be observed.

The same law also holds good for the production of heat
by mechanical means. The amount of heat obtained is only
dependent on the quantity of power consumed, and is quite
independent of the manner in which this power has been ex-
pended. If, therefore, the amount of heat which is produced
by certain mechanical work is known, the quantity which
will be obtained by any other amount of mechanical work
can easily be found by calculation. It is of no consequence
whether, this work consists in the compression, percussion, or
friction of bodies.

The amount of mechanical work done by a force may be
expressed by a weight, and the height to which this weight
would be raised by the same force. The mathematical ex-
pression for "work done," that is to say, a measure for this
work, is obtained by multiplying the height expressed in feet
or other units by the number of pounds or kilogrammes lifted
to this height.

"We shall take one kilogramme as the unit of weight, and
one metre as the unit of height, and we thus obtain the
weight of one kilogramme raised to the height of one metre
as a unit measure of mechanical work performed. This
measure we shall call a kilogrammetre, and adopt for it the
symbol Km.

Mechanical work may likewise be measured by the velo
city obtained by a given weight in passing from a state of rest
into that of motion. The work done is then expressed by

SOTIECES OF HEAT. 263

the product obtained by the multiplication of the weight by
the square of its velocity. The first method, however, be-
cause it is the more convenient, is the one usually adopted ;
and the numbers obtained therefrom may easily be expressed
in other units.

The product resulting from the multiplication of the num-
ber of units of weight and measures of height, or, as it is
called, the product of mass and height, as well as the pro-
duct of the mass and the square of its velocity, are called " vis
viva of motion," "mechanical effect," dynamical effect,"
" work done," " quantite de travail" &c. &c.

The amount of mechanical work necessary for the heating
of 1 kilogramme of water 1° C has been determined by ex-
periment to be = 367 Km ; therefore Km = 0*00273 units
of heat.*

A mass which has fallen through a height of 367 metres
possesses a velocity of 84*8 metres in one second ; a mass,
therefore, moving with this velocity originates 1° C. of heat
when its motion is lost by percussion, friction, &c. If the
velocity be two or three times as great, 4° or 9° of heat will
be developed. Generally speaking, when the velocity is c
metres, the corresponding development of heat wiU be ex-
pressed by the formula

* This essay was published in 1845. At that time de la Roche and
Berard's determination of the specific heat of air was generally accepted.
If the physical constants used by Mayer be corrected according to the re-
sults of more recent investigation, the mechanical equivalent of heat is
found to be 111-4: foot-pounds. Mr. Joule finds it =» 112 foot-pounds.—
Te.

264 CELESTIAL DYNAMICS.

III.— MEASURE OF THE SFN'S HEAT.

The actinometer is an instrument invented by Sir John
Herschel for the purpose of measuring the heating effiect
produced by the sun's rays. It is essentially a thermometer
with a large cylindrical bulb filled with a blue liquid, which
is acted upon by the sun's rays, and the expansion of which
is measured by a graduated scale.

From observations made with this instrument. Sir John
Herschel calculates the amount of heat received from the sun
to be sufficient to melt annually at the surface of the globe a
crust of ice 29*2 metres in thickness.

Pouillet has recently shown by some careful experiments
with the lens pyrheliometer, an instrument invented by him-
self, that every square centimetre of the surface of our globe
receives, on an average, in one minute an amount of solar
heat which wquld raise the temperature of one gramme of
water 0*4408°. Not much more than one-half of this quan-
tity of heat, however, reaches the solid surface of our globe,
since a considerable portion of it is absorbed by our atmo-
sphere. The layer of ice which, according to Pouillet, could
be melted by the solar heat which yearly reaches our globe
would have a thickness of 30*89 metres.

A square metre of our earth's surface receives, therefore,
according to Pouillet's results, which we shall adopt in the
following pages, on an average in one minute 4*408 units of
heat. The whole surface of the earth is = 9,260,500 geo-
graphical square miles* ; consequently the earth receives in
one minute 2247 billions of units of heat from the sun.

In order to obtain smaller numbers, we shall call the
quantity of heat necessary to raise a cubic mile of water 1°

* The geographical mile =» 7420 metres, and one English mile = 1608
metres.

265

C in temperature, a cubic mile of heat. Since one cubic
mile of water weighs 408*54: billions of kilogrammes, a cubic
mile of heat contains 408*54 billions of units of heat. The
effect produced by the rays of the sun on the surface of the
earth in one minute is therefore 5*5 cubic miles of heat.

Let us imagine the sun to be surrounded by a hollow
sphere whose radius is equal to the mean distance of the
earth from the sun, or 20,589,000 geographical miles ; the
surface of this sphere would be equal to 5326 billions of
square miles. The surface obtained by the intersection of
this hollow sphere and our globe, or the base of the cone of
solar light which reaches our earth, stands to the whole sur-

9 260 500

face of this hollow sphere as ' 4 : 5326 billions, or as 1 to
2300 millions. This is the ratio of the heat received by
our globe to the whole amount of heat sent forth from the
sun, which latter in one minute amounts to 12,650 millions
of cubic miles of heat.

This amazing radiation ought, unless the loss is by some
means made good, to cool considerably even a body of the
magnitude of the sun.

If we assume the sun to be endowed with the same capa-
city for heat as a mass of water of the same volume, and its
loss of heat by radiation to affect uniformly its whole mass,
the temperature of the sun ought to decrease 1°*8 C. yearly,
and for the historic time of 5000 years this loss would conse-
quently amount to 9000^ C.

A uniform cooling of the whole of the sun's huge mass
cannot, however, take place ; on the contrary, if the radiation
were to occur at the expense of a given store of heat or ra-
diant power, the sun would become covered in a short space
of time with a cold crust, whereby radiation would be brought
to an end. Considering the continued activity of the sun
through countless centuries, we may assimie with mathemati-
cal certainty the existence of some compensating influence to
make good its enormous loss.
12

266

CELESTIAL DYNAMICS.

Is tliis restoring agency a chemical process?

If such were the case, the most favourable assumption
would be to suppose the whole mass of the sun to be one
limip of coal, the combustion of every kilogramme of which
produces 6000 units of heat. Then the sun would only be
able to sustain for forty-six centuries its present expenditure
of light and heat, not to mention the oxygen necessary to
keep up such an immense combustion, and other unfavourable
circumstances.

The revolution of the sun on his axis has been suggested
as the cause of his radiating energy. A closer examination
proves this hypothesis also to be untenable.

Eapid rotation, without friction or resistance, cannot in
itself alone be regarded as a cause of light and heat, espe-
cially as the sun is in no way to be distinguished from the
other bodies of our system by velocity of axial rotation. The
sun turns on his axis in about twenty-five days, and his diam-
eter is nearly 112 times as great as that of the earth, from
which it follows that a point on the solar equator travels but
a little more than four times as quickly as a point on the
earth's equator. The largest planet of the solar system,
whose diameter is about 15th that of the sun, turns on its axis
in less than ten hours ; a point on its equator revolves about
six times quicker than one on the solar equator. The outer
ring of Saturn exceeds the sun's equator more than ten times
in velocity of rotation. Nevertheless no generation of light
or heat is observed on our globe, on Jupiter, or on the ring
of Saturn.

It might be thought that friction, though undeveloped in
the case of the other celestial bodies, might be engendered by
the sun's rotation, and that such friction might generate enor-
mous quantities of heat. But for the production of friction
two bodies, at least, are always necessary which are in imme-
diate contact with one another, and which move with differ-
ent velocities or in different directions. Friction, moreover,

26Y

has a tendency to produce equal motion of the two rubbing
bodies ; and when this is attained, the generation of heat
ceases. If now the sun be the one moving body, where is
the other ? and if the second body exist, what power prevents
it from assuming the same rotary motion as the sun ?

But could even these difficulties be disregarded, a weight-
ier and more formidable obstacle opposes this hypothesis.
The known volume and mass of the sun allow us to calculate
the vis viva which he possesses in consequence of his rotation.
Assuming his density to be uniform throughout his mass, and
his period of rotation twenty-five days, it is equal to 182,300
quintiUions of kilogrammetres (Km). But for one unit of
heat generated, 367 Km are consumed ; consequently the
whole rotation-effect of the sun could only cover the expendi-
ture of heat for the space of 183 years.

The space of our solar system is filled with a great num-
ber of ponderable objects, which have a tendency to move
towards the centre of gravity of the sun ; and in so doing,
their rate of motion is more and more accelerated.

A mass, without motion, placed within the sphere of the
sun*s attraction, will obey this attraction, and, if there be no
disturbing influences, wiU fall in a straight line into the sun.
In reality, however, such a rectilinear path can scarcely occur,
as may be shown by experiment.

Let a weight be suspended by a string so that it can only
touch the floor in one point. Lift the weight up to a certain
height, and at the same time stretch the string out to its full
length ; if the weight be now allowed to fall, it will be ob-
served, almost in every case, not to reach at once the point on
the floor towards which it tends to move, but to move round
this point for some time in a curved line.

The reason of this phenomenon is that the slightest devia-
tion of the weight from its shortest route towards the point
on the floor, caused by some disturbing influence such as the
resistance of the air against a not perfectly uniform surface,

CELESTIAL DYNAMICS.

will maintain itself as long as motion lasts. It is neverthe-
less possible for the weight to move at once to the point ; the
probability of its doing so, however, becomes the less as the
height from which it is allowed to drop increases, or the string,
by means of which it is suspended, is lengthened.

Similar laws influence the movements of bodies in the
space of the solar system. The height of the fall is here
represented by the original distance from the sun at which the
body begins to move ; the length of the string by the sun's
attraction, which increases when the distance decreases ; and
the small surface of contact on the floor by the area of the
section of the sun's sphere. If now a cosmical mass within
the physical limits of the sun's sphere of attraction begins its
fall towards that heavenly body, it will be disturbed in its
long path for many centuries, at first by the nearest fixed
stars, and afterwards by the bodies of the solar system.
Motion of such a mass in a straight line, or its perpendicular
fall into the sun, would, therefore, under such conditions, be
impossible. The observed movement of all planetary bodies
in closed curves agrees with this.

We shall now return to the example of the weight sus-
pended by a string and oscillating round a point towards
which it is attracted. The diameters of the orbits described
by this weight are observed to be nearly equal ; continued ob-
servation, however, shows that these diameters gradually di-
minish in length, so that the weight will by degrees approach
the point in which it can touch the floor. The weight, how-
ever, touches the floor not in a mathematical point, but in a
small surface ; as soon, therefore, as the diameter of the curve
in which the weight moves is equal to the diameter of this
surface, the weight will touch the floor. This final contact is
no accidental or improbable event, but a necessary phenome-
non caused by the resistance which the oscillating mass con-
stantly suffers from the air and friction. If all resistance
could be annihilated, the motion of the weight would of coursa
continue in equal oscillations.

MEASIJRE OF THE SUn's HEAT. 269

The same law holds good for celestial bodies.

The movements of celestial bodies in an absolute vacuum
would be as uniform as those of a mathematical pendulum,
whereas a resisting medium pervading all space would cause
the planets to move in shorter and shorter orbits, and at last
to fall into the sun.

Assuming such a resisting medium, these wandering ce-
lestial bodies must have on the periphery of the solar system
their cradle, and in its centre their grave ; and however long
the duration, and however great the number of their revolu-
tions may be, as many masses will on the average in a cer-
tain time arrive at the sun as formerly in a like period of
time came within liis sphere of attraction.

All these bodies plunge with a violent impetus into their
common grave. Since no cause exists without an effect, each
of these cosmical masses will, like a weight falling to the
earth, produce by its percussion an amount of heat propor-
tional to its vis viva.

From the idea of a sun whose attraction acts throughout
space, of ponderable bodies scattered throughout the universe,
and of a resisting aether, another idea necessarily follows —
that, namely, of a continual and inexhaustible generation of
heat on the central body of this cosmical system.

Whether such a conception be realized in our solar system
— whether, in other words, the wonderful and permanent evo-
lution of light and heat be caused by the uninterrupted fall
of cosmical matter into the sun — will now be more closely
examined.

The existence of matter in a primordial condition ( Urma-
terie), moving about in the universe, and assumed to follow
the attraction of the nearest stellar system, will scarcely be
denied by astronomers and physicists ; for the richness of
surrounding nature, as well as the aspect of the starry heav-
ens, prevents the belief that the wide space which separates
our solar system from the regions governed by the other fixed

2T0 CELESTIAL DYNAMICS.

Stars is a vacant solitude destitute of matter. We shall
leave, however, all suppositions concerning subjects so distant
from us both in time and space, and confine our attention ex-
clusively to what may be learnt from the observation of the
existing state of things.

Besides the fourteen known planets with their eighteen
satellites, a great many other cosmical masses move within
the space of the planetary system, of which the comets de-
serve to be mentioned first.

Kepler's celebrated statement that '' there are more com-
ets in the heavens than fish in the ocean," is founded on the
fact that, of all the comets belonging to our solar system,
comparatively few can be seen by the inhabitants of the earth,
and therefore the not inconsiderable number of actually ob-
served comets obliges us, according to the rules of the calcu-
lus of probabilities, to assume the existence of a great many
more beyond the sphere of our vision.

Besides planets, satellites, and comets, another class of
celestial bodies exists within our solar system. These are
masses which, on account of their smallness, may be consid-
ered as cosmical atoms, and which Arago has appropriately
called asteroids. They, like the planets and the comets, are
governed by gravity, and move in elliptical orbits round the
sun. When accident brings them into the immediate neigh-
bourhood of the earth, they produce the phenomena of shoot-
ing-stars and fireballs.

It has been shown by repeated observation, that on a
bright night twenty minutes seldom elapse without a shooting-
star being visible to an observer in any situation. At certain
times these meteors are observed in astonishingly great num-
bers ; during the meteoric shower at Boston, which lasted
nine hours, when they were said to fall " crowded together
like snow-flakes," they were estimated as at least 240,000.
On the whole, the number of asteroids which come near the
earth in the space of a year must be computed to be many

MEASURE OF THE SUn's HEAT. 271

thousands of millions. This, without doubt, is only a small
fraction of the number of asteroids that move round the sun,
which number, according to the rules of the calculus of prob-
abilities, approaches the infinite.

As has been already stated, on the existence of a resisting
aether it depends whether the celestial bodies, the planets, the
,comets, and the asteroids move at constant mean distances
round the sun, or whether they are constantly approaching
that central body.

Scientific men do not doubt the existence of such an cether.
Littrow, amongst others, expresses himself on this point as
follows : — " The assumption that the planets and the comets
move in an absolute vacuum can in no way be admitted.
Even if the space between celestial bodies contained no other
matter than that necessary for the existence of light (whether
light be considered as emission of matter or the undulations
of a -universal aether), this alone is sufficient to alter the mo-
tion of the planets in the course of time and the arrangement
of the whole system itself; the fall of all the planets and the
comets into the sun and the destruction of the present state
of the solar system must be the final results of this action."

A direct proof of the existence of such a resisting medium
has been furnished by the academician Encke. He found
that the comet named after him, which revolves roimd the
sun in the short space of 1207 days, shows a regular acceler-
ation of its motion, in consequence of which the time of each
revolution is shortened by about six hours.

From the great density and magnitude of the planets, the
shortening of the diameters of their orbits proceeds, as might
be expected, very slowly, and is up to the present time inap-
preciable. The smaller the cosmical masses are, on the con-
trary, other circumstances remaining the same, the faster they
move towards the sun ; it may therefore happen that in a
space of time wherein the mean distance of the earth from
the sun would diminish one metre, a small asteroid wo"i'^

272 CELESTIAL DYNAMICS.

travel more than one thousand miles towards the central
body.

As cosmical masses stream from all sides in immense
numbers towards the sun, it follows that they must become
more and more crowded together as they approach thereto.
The conjecture at once suggests itself that the zodiacal light,
the nebulous light of vast dimensions which surrounds the
sun, owes its origin to such closely-packed asteroids. How-
ever it may be, this much is certain, that this phenomenon is
caused by matter which moves according to the same laws as
the planets round the sun, and it consequently follows that
the whole mass which originates the zodiacal light is contin-
ually approaching the sun and falling into it.

This light does not surround the sun uniformly on all
sides ; that is to say, it has not the form of a sphere, but that
of a thin convex lens, the greater diameter of which is in the
plane of the solar equator, and accordingly it has to an ob-
server on our globe a pyramidal form. Such lenticular dis-
tribution of the masses in the universe is repeated in a re-
markable manner in the disposition of the planets and the
fixed stars.

From the great number of cometary masses and asteroids
and the zodiacal light on the one hand, and the existence of a
resisting aether on the other, it necessarily follows that pon-
derable matter must continually be arriving on the solar sur-
face. The effect produced by these masses evidently depends
on their final velocity ; and, in order to determine the latter, we
shall discuss some of the elements of the theory of gravitation.

The final velocity of a weight attracted by and moving
towards a celestial body will become greater as the height
through which the weight falls increases. This velocity, how-
ever, if it be only produced by the fall, cannot exceed a cer-
tain magnitude ; it has a maximum, the value of which de-
pends on the volume and mass of the attracting celestial body.

MEASURE OF THE SUN's HEAT. 273

Let r be the radius of a spherical and solid celestial body,
and g the velocity at the end of the first second of a weight
falling on the surface of this body ; then the greatest velocity
which this weight can obtain by its fall towards the celestial
body, or the velocity with which it will arrive at its surface
after a fall from an infinite height, is \/2gr in one second.
This number, wherein g and r are expressed in metres, we
shaU caU G.

For our globe the value of ^r is 9*8164 . . and that of r
6,369,800 ; and consequently on our earth

G=: |/(2x9-8164x6,369,800) = 11,183.

The solar radius is 112*05 times that of the earth, and the
velocity produced by gravity on the sun's surface is 28*36
times greater than the same velocity on the surface of our
globe ; the greatest velocity therefore which a body could ob-
tain in consequence of the solar attraction, or

G = |/(28*36 X 112*05) X 11,183 = 630,400 ;
that is, this maximum velocity is equal to 630,400 metres, or
85 geographical miles in one second.

By the help of this constant number, which may be called
the characteristic of the solar system, the velocity of a body
in central motion may easily be determined at any point of its
orbit. Let a be the mean distance of the planetary body from
the centre of gravity of the sun, or the greater semidiameter
of its orbit (the radius of the sun being taken as unity) ; and
let h be the distance of the same body at any point of its orbit
from the centre of gravity of the sun ; then the velocity,
expressed in metres, of the planet at the distance h is

/2a -/i

At the moment the planet comes in contact with the solar sur-
face, h is equal to 1, and its velocity is therefore

/2^^
12* ^XV-2^-

274 CELESTIAL DYNAMICS.

It follows from this formula that the smaller 2a (or the
major axis of the orbit of a planetary body) becomes, the
less will be its velocity when it reaches the sun. This velo-
city, like the major axis, has a minimum ; for so long as the
planet moves outside the sun, its major axis cannot be shorter
than the diameter of the sun, or, taking the solar radius as a
unit, the quantity 2 a can never be less than 2. The smallest
velocity with which we can imagine a cosmical body to arrive
on the surface of the sun is consequently

ax/l=

445,750,

or a velocity of 60 geographical miles in one second.

For this smallest value the orbit of the asteroid is circu-
lar ; for a larger value it becomes elliptical, until finally, with
increasing excentricity, when the value of 2a approaches in-
finity, the orbit becomes a parabola. In the last case the
velocity is

or, 85 geographical miles in one second.

If the value of the major axis become negative, or the
orbit assume the form of a hyperbola, the velocity may in-
crease without end. But this could only happen when cosmi-
cal masses enter the space of the solar system with a pro-
jected velocity, or when masses, having missed the sun's sm*-
face, move into the universe and never return ; hence a ve-
locity greater than G can only be regarded as a rare excep-
tion, and we shall therefore only consider velocities comprised
within the limits of 60 and 80 miles.*

The final velocity with which a weight moving in a

* The relative velocity also with which an asteroid reaches the solar
surface depends in some degree on the velocity of the sun's rotation. This,
however, as well as the rotatory effect of the asteroid, is without moment,
and may be neglected.

MEASURE OF THE SUN's HEAT. 275

Straight Kne towards the centre of the sun arrives at the solar
surface is expressed by the formula

-GrX

i/^-^.

wherein c expresses the final velocity in metres, and h the
original distance from the centre of the sun in terms of solar
radius. If this formula be compared with the foregoing, it
will be seen that a mass which, after moving in central mo-
tion, arrives at the sun's surface has the same velocity as it
would possess had it fallen perpendicularly into the sun from
a distance* equal to the major axis of its orbit ; whence it is
apparent that a planet, on arriving at the sun, moves at least
as quickly as a weight which falls freely towards the sun
from a distance as great as the solar radius, or 96,000 geo-
graphical miles.

What thermal effect corresponds to such velocities ? Is
the effect sufficiently great to play an important part in the
immense development of heat on the sun?

This crucial question may be easily answered by help of
the preceding considerations. According to the formula given
at the end of Chapter II., the degree of heat generated by
percussion is

= 0-000139° Xc%
where c denotes the velocity of the striking body expressed in
metres. The velocity of an asteroid when it strikes the sun
measures from 445,750 to 630,400 metres ; the caloric effect
of the percussion is consequently equal to from 27^ to 55 mil-
lions of degrees of heat-f.

An asteroid, therefore, by its fall into the sun developes

* This distance is to be counted from the centre of the sun.

f Throughout this memoir the degrees of heat are expressed in the
Centigrade scale. Unless stated to the contrary, the measures of length
are given in geographical miles. A geographical mile =— 7420 metres, and
an English mile == 1608 metres. — Tb.

276 CELESTIAL DYNAMICS.

from 4600 to 9200 times as much heat as would be generated
by the combustion of an equal mass of coal.

IV.— ORIGIN OF THE SUN'S HEAT.

The question why the planets move in curved orbits, one
of the grandest of problems, was solved by Newton in con-
sequence, it is believed, of his reflecting on the fall of an ap-
ple. This story is not improbable, for we are on the right
track for the discovery of truth when once we clearly recog-
nize that between great and small no qualitative but only a
quantitative difference exists — ^when we resist the suggestions
of an ever active imagination, and look for the same laws in
the greatest as well as in the smallest processes of nature.

This universal range is the essence of a law of nature,
and the touchstone of the correctness of human theories. We
observe the fall of an apple, and investigate the law which
governs this phenomenon ; for the earth we substitute the
sun, and for the apple a planet, and thus possess ourselves of
the key to the mechanics of the heavens.

As the same laws prevail in the greater as well as in the
smaller processes of nature, Newton's method may be used in
solving the problem of the origin of the sun's heat. We
know the connexion between the space through which a body
falls, the velocity, the vis viva, and the generation of heat on
the surface of this globe ; if we again substitute for the earth
the sun, with a mass 350,000 greater, and for a height of a
few metres celestial distances, we obtain a generation of heat
exceeding all terrestrial measures. And since we have suffi-
cient reason to assume the actual existence of such mechani-
cal processes in the heavens, we find therein the only tenable
explanation of the origin of the heat of the sun.

277

The fact that the development of heat by mechanicaJ
means on the surface of om* globe is, as a rule, not so great,
and cannot be so great as the generation of the same agent
by chemical means, as by combustion, follows from the laws
already discussed ; and this fact cannot be used as an argu-
ment against the assumption of a greater development of
heat by a greater expenditure of mechanical work. It has
been shown that the heat generated by a weight falling from
a height of 367 metres is only eooo^h part of the heat pro-
duced by the combustion of the same weight of coal ; just as
small is the amount of heat developed by a weight moving
with the not inconsiderable velocity of 85 metres in one sec-
ond. But, according to the laws of mechanics, the effect is
proportional to the square of the velocity ; if therefore the
weight move 100 times faster, or with a velocity of 8500
metres in one second, it will produce a greater effect than the
combustion of an equal quantity of coal.

It is true that so great a velocity cannot be obtained by
human means ; everyday experience, however, shows the de-
velopment of high degrees of temperature by mechanical
processes.

In the common flint and steel, the particles of steel which
are struck off are sufficiently heated to bum in air. A few
blows directed by a skilful blacksmith with a sledge-hammer
against a piece of cold metal may raise the temperature of
the metal at the points of collision to redness.

The new crank of a steamer, whilst being poHshed by
friction, becomes red-hot, several buckets of water being re-
quired to cool it down to its ordinary temperature.

When a railway train passes with even less than its ordi-
nary velocity along a very sharp curve of the line, sparks are
observed in consequence of the friction against the rails.

One of the grandest constructions for the production of
motion by human art is the channel in which the wood was
allowed to glide down from the steep and lofty sides of Mount

278 CELESTIAL DYNAMICS.

Pilatus, into the plain below. This wooden channel which
was built about thirty years ago by the engineer Rupp, was
9 English nules in length ; the largest trees were shot down
it from the top to the bottom of the mountain in about two
minutes and a half. The momentum possessed by the trees
on their escaping at their journey's end from the channel was
sufficiently great to bury their thicker ends in the ground to
the depth of from 6 to 8 metres. To prevent the wood get-
ting too hot and taking fire, water was conducted in many
places into the channel.

This stupendous mechanical process, when compared with
cosmical processes on the sun, appears infinitely small. In
the latter case it is the mass of the sun which attracts, and
in lieu of the height of Mount Pilatus we have distances of a
hundred thousand and more miles ; the amount of heat gene-
rated by cosmical falls is therefore at least 9 million times
greater than in our terrestrial example.

Rays of heat on passing through glass and other transpa-
rent bodies undergo partial absorption, which differs in de-
gree, however, according to the temperature of the source
from which the heat is derived. Heat radiated from sources
less warm than boiling water is almost completely stopped by
thin plates of glass. As the temperature of a source of heat
increases, its rays pass more copiously through diathermic
bodies. A plate of glass, for example, weakens the rays of
a red-hot substance, even when the latter is placed very close
to it, much more than it does those emanating at a much
greater distance from a white-hot body. If the quality of
the sun's rays be examined in this respect, their diathermic
energy is found to be far superior to that of all artificial
sources of heat. The temperature of the focus of a concave
metallic reflector in which the sun's light has been collected
is only diminished from one-seventh to one-eighth by the in-
terposition of a screen of glass. If the same experiment be

OEIGIN OF THE SUN's HEAT. 279

made with an artificial and luminous source of heat, it is
found that, though the focus be very hot when the screen is
away, the interposition of the latter cuts off nearly all the
heat ; moreover, the focus will not recover its former temper-
ature when reflector and screen are placed sufficiently near to
the source of heat to make the focus appear brighter than it
did in the former position without the glass screen.

The empirical law, that the diathermic energy of heat in-
creases with the temperature of the source from which the
heat is radiated, teaches us that the sun's surface must be
much hotter than the most powerM process of combustion
could render it.

Other methods fiimish the same conclusion. If we ima-
gine the sun to be surrounded by a hollow sphere, it is clear
that the inner surface of this sphere must receive all the heat
radiated from the sun. At the distance of our globe from the
sun, such a sphere would have a radius 215 times as great,
and an area 46,000 times as large as the sun himself; those
luminous and calorific rays, therefore, which meet this spheri-
cal surface at right angles retain only 46;55oth part of their
original intensity. J£ it be further considered that our at-
mosphere absorbs a part of the solar rays, it is clear that the
rays which reach the tropics of our earth at noonday can
only possess from i-^th. to ^m'^h. of the power with which
they started. These rays, when gathered from a surface of
firom 5 to 6 square metres, and concentrated in an area of
one square centimetre, would produce about the temperature
which exists on the sun, a temperature more than sufficient
to vaporize platinum, rhodium, and similar metals.

The radiation calculated in Chapter III. likewise proves
the enormous temperature of the solar surface. From the
determination mentioned therein, it follows that each square
centimetre of the sun's surface loses by radiation about 80
units of heat per minute — an immense quantity in compari-
son with terrestrial radiations.

280 CELESTIAL DYNAMICS.

A correct theory of the origin of the sun's heat must ex-
plain the cause of such enormous temperatures. This expla-
nation can be deduced from the foregoing statements. Ac-
cording to Pouillet, the temperature at which bodies appear
intensely white-hot is about 1500° C. The heat generated by
the combustion of one kilogramme of hydrogen is, as deter-
mined by Dulong, 34,500, and according to the more recent
experiments of Grassi, 34,666 units of heat. One part of
hydrogen combines with eight parts of oxygen to form water ;
hence one kilogramme of these two gases mixed in this ratio
would produce 3850°.

Let us now compare this heat with the amount of the
same agent generated by the fall of an asteroid into the sun.
"Without taking into account the low specific heat of such
masses when compared with that of water, we find the heat
developed by the asteroid to be from 7000 to 15,000 times
greater than that of the oxyhydrogen mixture. From data
like these, the extraordinary diathermic energy of the sun's
rays, the immense radiation from his surface, and the high
temperature in the focus of the reflector are easily accounted
for.

The facts above mentioned show that, unless we assume
on the sun the existence of matter with unheard of chemical
properties as a deus ex machind^ no chemical process could
maintain the present high radiation of the sun ; it also fol-
lows from the above results, that the chemical nature of bo-
dies which fall into the sun does not in the least afiect our
conclusions; the effect produced by the most inflammable
substance would not differ by one-thousandth part from that
resulting from the fall of matter possessing but feeble chemi-
cal affinities. As the brightest artificial light appears dark in
comparison with the sun's light, so the mechanical processes
of the heavens throw into the shade the most powerful chem-
ical actions.

The quality of the sun's rays, as dependent on his temper-

281

ature, is of the greatest importance to mankind. If the solar
heat were originated by a chemical process, and amounted
near its source to a temperature of a few thousand degrees,
it would be possible for the light to reach us, whilst the
greater part of the more important calorific rays would be ab-
sorbed by the higher strata of our atmosphere and then re-
turned to the universe.

In consequence of the high temperature of the sun, how-
ever, our atmosphere is highly diathermic to his rays, so that
the latter reach the surface of our earth and warm it. The
comparatively low temperature of the terrestrial surface is
the cause why the heat cannot easily radiate back through the
atmosphere into the universe. The atmosphere acts, there-
fore, like an envelope, which is easily pierced by the solar
rays, but which offers considerable resistance to the radiant
heat escaping from our earth ; its action resembles that of a
valve which allows liquid to pass freely in one, but stops the
flow in the opposite direction.

The action of the atmosphere is of the greatest impor-
tance as regards climate and meteorological processes. It
must raise the mean temperature of the earth's surface. Af-
ter the setting of the sun — in fact, in all places where his
rays do not reach the surface, the temperature of the earth
would soon be as low as that of the universe, if the atmos-
phere were removed, or if it did not exist. Even the power-
ful solar rays in the tropics would be unable to preserve wa-
ter in its liquid state.

Between the great cold which would reign at all times
and in aU places, and the moderate warmth which in reality
exists on our globe, intermediate temperatures may be ima-
gined ; and it is easily seen that the mean temperature would
decrease if the atmosphere were to become more and more
rare. Such a rarefaction of a valve-like acting atmosphere
actually takes place as we ascend higher and higher above

282 CELESTIAL DYNAMICS.

the level of the sea, and it is accordingly and necessarily ac-
companied by a corresponding diminution of temperature.

This weU-known fact of the lower mean temperature of
places of greater altitude has led to the strangest hypotheses.
The sun's rays were not supposed to contain all the conditions
for warming a body, but to set in motion the " substance"
of heat contained in the earth. This "substance" of heat,
cold when at rest, was attracted by the earth, and was there-
fore found in greater abundance near the centre of the globe.
This view, it was thought, explained why the warming power
of the sun was so much weaker at the top of a mountain than
at the bottom, and why, in spite of his immense radiation, he
retained his full powers.

This belief, which especially prevails amongst imperfectly
informed people, and which will scarcely succumb to correct
views, is directly contradicted by the excellent experiments
made by Pouillet at different altitudes with the p3nrheliometer.
These experiments show that, everything else being equal,
the generation of heat by the solar rays is more powerful in
higher altitudes than near the surface of our globe, and that
consequently a portion of these rays is absorbed on their pas-
sage through the atmosphere. Why, in spite of this partial
absorption, the mean temperature of low altitudes is never-
theless higher than it is in more elevated positions, is ex-
plained by the fact that the atmosphere stops to a far greater
degree the calorific rays emanating from the earth than it
does those from the sun.

v.— CONSTANCY OF THE SUN'S MASS.

Nevtton, as is well known, considered light to be the
emission of luminous particles from the sun. In the contin-
ued emission of light this great philosopher saw a cause tend-

CONSTANCY OF THE STJn's MASS.

ing to diminish the solar mass ; and he assumed, in order to
make good this loss, comets and other cosmical masses to be
continually falling into the central body.

If we express this view of Newton's in the language of
the undulatory theory, which is now universally accepted, we
obtain the results developed in the preceding pages. It is
true that our theory does not accept a peculiar " substance "
of light or of heat ; nevertheless, according to it, the radia-
tion of light and heat consists also in purely material pro-
cesses, in a sort of motion, in the vibrations of ponderable
resisting substances. Quiescence is darkness and death ; mo-
tion is light and life.

An undulating motion proceeding from a point or a plane
and excited in an unlimited medium, cannot be imagined apart
from another simultaneous motion, a translation of the parti-
cles themselves ;* it therefore follows, not only from the emis-
sion, but also from the undulatory theory, that radiation con-
tinually diminishes the mass of the sun. Why, nevertheless,
the mass of the sun does not really diminish has already been
stated.

The radiation of the sun is a centrifugal action equivalent
to a centripetal motion.

The caloric effect of the centrifugal action of the sun can
be found by direct observation ; it amounts, according to
Chapter HI., in one minute to 12,650 millions of cubic miles
of heat, or 5*17 quadrillions of units of heat. In Chapter
IV. it has been shown that one kilogramme of the mass of an
asteroid originates from 27*5 to 55 millions of units of heat ;
the quantity of cosmical masses, therefore, which falls every
minute into the sun amounts to from 94,000 to 188,000 bil-
lions of kilogrammes.

To obtain this remarkable result, we made use of a method

* This centrifugal motion is perhaps the cause of the repulsion of the
tails on comets when in the neighbourhood of the sun, as observed by

284: CELESTIAL DYNAMICS.

which is common in physical inquiries. Observation of the
moon's motion reveals to us the external form of the earth.
The physicist determines with the torsion-balance the weight
of a planet, just as the merchant finds the weight of a parcel
of goods, whilst the pendulum has become a magic power in
the hands of the geologist, enabling him to discover cavities
in the bowels of the earth. Our case is similar to these. By
observation and calculation of the velocity of sound in our
atmosphere, we obtain the ratio of the specific heat of air un-
der constant pressure and under constant volume, and by the
help of this number we determine the quantity of heat gene-
rated by mechanical work. The heat which arrives from the
sun in a given time on a small surface of our globe serves as
a basis for the calculation of the whole radiating effect of the
sun; and the result of a series of observations and well-
founded conclusions is the quantitative determination of those
cosmical masses which the sun receives from the space
through which he sends forth his rays.

Measured by terrestrial standards, the ascertained number
of so many billions of kilogrammes per minute appears in-
credible. This quantity, however, may be brought nearer to
our comprehension by comparison with other cosmical mag-
nitudes. The nearest celestial body to us (the moon) has a
mass of about 90,000 trillions of kilogrammes, and it would
therefore cover the expenditure of the sun for from one to
two years. The mass of the earth would afford nourishment
to the sun for a period of from 60 to 120 years.

To facilitate the appreciation of the masses and the dis-
tances occurring in the planetary system, Herschel draws the
following picture. Let the sun be represented by a globe 1
metre in diameter. The nearest planet (Mercury) will be
about as large as a pepper-corn, S^ millimetres in thickness,
at a distance of 40 metres. 78 and 107 metres distant from
the sun will move Venus and the Earth, each 9 millimetres in
diameter, or a little larger than a pea. Not much more than

285

a quarter of a metre from the Earth will be the Moon, the
size of a mustard seed, 2^ millimetres in diameter. Mars,
at a distance of 160 metres, will have about half the diame-
ter of the Earth ; and the smaller planets (Vesta, Hebe, As-
trea, Juno, Pallas, Ceres, &c.), at a distance of from 250 to
300 metres from the sun, will resemble particles of sand.
Jupiter and Saturn, 560 and 1000 metres distant from the cen-
tre, will be represented by oranges, 10 and 9 centimetres in
diameter. Uranus, of the size of a nut 4 centimetres across,
will be 2000 metres ; and Neptune, as large as an apple 6
centimetres in diameter, will be nearly twice as distant, or
about half a geographical mile away from the sun. From
Neptune to the nearest fixed star will be more than 2000 geo-
graphical miles.

To complete this picture, it is necessary to imagine finely-
divided matter grouped in a diversified manner, moving slowly
and gradually towards the large central globe, and on its ar-
rival attaching itself thereto ; this matter, when favourably
illuminated by the sun, represents itself to us as the zodiacal
light. This nebulous substance forms also an important part
of a creation in which nothing is by chance, but wherein all
IS arranged with Divine foresight and wisdom.

The surface of the sun measures 115,000 millions of
square miles, or 6^ trillions of square metres ; the mass of
matter which in the shape of asteroids falls into the sun every
minute is from 94,000 to 188,000 billions of kilogrammes ;
one square metre of solar surface, therefore, receives on an
average from 15 to 30 grammes of matter per minute.

To compare this process with a terrestrial phenomenon, a
gentle rain may be considered which sends down in one hour
a layer of water 1 millimetre in thickness (during a thunder-
storm the rainfall is often from ten to fifteen times this quan-
tity) ; this amounts on a square metre to 17 grammes per
minute.

CELESTIAL DYNAMICS.

The continual bombardment of the sun by these cosmical
masses ought to increase its volume as well as its mass, if
centripetal action only existed. The increase of volume,
could scarcely be appreciated by man ; for if the specific grav-
ity of these cosmical masses be assumed to be the same as
that of the sun, the enlargement of his apparent diameter to
the extent of one second, the smallest appreciable magnitude,
would require from 33,000 to 66,000 years.

Not quite so inappreciable would be the increase of the
mass of the sun. If this mass, or the weight of the sun,
were augmented, an acceleration of the motion of the planets
in their orbits would be the consequence, whereby their times
of revolution round the central body would be shortened.
The mass of the sun is 2*1 quintillions of kilogrammes ; and
the mass of the cosmical matter annually arriving at the sun
stands to the above as 1 to from 21 — 42 millions. Such an
augmentation of the weight of the sun ought to shorten the
sidereal year from 42,000,000th to 85,000,000th of its length, or from
f ths to f ths of a second.

The observations of astronomers do not agree with this
conclusion ; we must therefore fall back on the theory men-
tioned at the beginning of this chapter, which assumes that
the sun, like the ocean, is constantly losing and receiving
equal quantities of matter. This harmonizes with the suppo-
sition that the vis viva of the universe is a constant quantity.

VI.— THE SPOTS ON THE SUN'S DISC.

The solar disc presents, according to Sir John Herschel,
the following appearance. " When the sun is observed
through a powerful telescope provided with coloured glasses
in order to lessen the heat and brightness which would be

THE SPOTS ON THE SUN's DISC. 28Y

hurtful to the eyes, large dark spots are often seen surrounded
by edges which are not quite so dark as the spots themselves,
and which are called penumbras. These spots, however, are
neither permanent nor unchangeable. When observed from
day to day, or even from hour to hour, their form is seen to
change ; they expand or contract, and finally disappear ; on
other parts of the solar surface new spots spring into exist-
ence where none could be discovered before. When they dis-
appear, the darker part in the middle of the spot contracts to
a point and vanishes sooner than the edge. Sometimes they
break up into two or more parts that show all the signs of
mobility characteristic of a liquid, and the extraordinary
commotion which it seems only possible for gaseous matter to
possess. The magnitude of their motion is very great. An
arc of 1 second, as seen from our globe, corresponds to 465
English miles on the sun's disc ; a circle of this diameter,
which measures nearly 220,000 English square miles, is the
smallest area that can be seen on the solar surface. Spots,
however, more than 45,000 English miles in diameter, and,
if we may trust some statements, of even greater dimensions,
have been observed. For such a spot to disappear in the
course of six weeks (and they rarely last longer), the edges,
whilst approaching each other, must move through a space of
more than 1000 miles per diem.

" That portion of the solar disc wliich is free from spots
is by no means uniformly bright. Over it are scattered small
dark spots or pores, which are found by careful observation
to be in a state of continual change. The slow sinking of
some chemical precipitates in a transparent liquid, when
viewed from the upper surface and in a direction perpendicu-
lar thereto, resembles more accurately than any other phe-
nomenon the changes which the pores undergo. The similar-
ity is so striking, in fact, that one can scarcely resist the idea
that the appearances above described are owing to a luminous
medium moving about in a non-luminous atmosphere, either

288 CELESTIAL DYNAMICS.

like the clouds in our air, or in wide-spread planes and flame-
like colunms, or in rays like the aurora borealis.

" Near large spots, or extensive groups of them, large
spaces are observed to be covered with pecuharly marked
lines much brighter than the other parts of the surface ; these
lines are curved, or deviate in branches, and are called faculae.
Spots are often seen between these lines, or to originate there.
These are in all probability the crests of immense waves in
the luminous regions of the solar atmosphere, and bear wit-
ness to violent action in their immediate neighbourhood."

The changes on the solar surface evidently point to the
action of some external disturbing force ; for every moving
power resident in the sun itself ought to exhaust itself by its
own action. These changes, therefore, are no unimportant
confirmation of the theory explained in these pages.

At the same time it must be observed that our knowledge
of physical heliography is, from the nature of the subject,
very limited ; even the meteorological processes and other phe-
nomena of our own planet are still in many respects enigmat-
ical. For this reason no special information could be given
about the manner in which the solar surface is affected by
cosmical masses. However, I may be allowed to mention
some probable conjectures which offer themselves.

The extraordinarily high temperature which exists on the
sun almost precludes the possibility of its surface being solid ;
it doubtless consists of an uninterrupted ocean of fiery fluid
matter. This gaseous envelope becomes more rarefied in
those parts most distant from the sun's centre.

As most substances are able to assimie the gaseous state
of aggregation at high temperatures, the- height of the sun's
atmosphere cannot be inconsiderable. There are, however,
sound reasons for believing that the relative height of the so-
lar atmosphere is not very great.

Since the gravity is 28 times greater on the sun's surface
than it is on our earth, a column of air on the former must

289

cause a pressure 28 times greater than it would on our
globe. This great pressure compresses air as much as a tem-
perature of 8000° would expand it.

In a still greater degree than this increased gravity do the
qualities peculiar to gases affect the height of the solar atmo-
sphere. In consequence of these properties, the density of
our atmosphere rapidly diminishes as we ascend, and increases
as we descend. Generally speaking, rarefaction increases in
a geometrical progression when the heights are in an arith-
metical progression. K we ascend or descend 2^, 5, or 30
miles, we find our atmosphere 10, 100, or a billion times more
rarefied or more dense.

This law, although modified by the unequal temperatures
of the different layers of the photosphere, and the unknown
chemical nature of the substances of which it is composed,
must also hold good in some measure for the sun. As, how-
ever, the mean temperature of the solar atmosphere must
considerably exceed that of our atmosphere, the density of the
former will not vary so rapidly with the height as the latter
does. K we assume this increase and decrease on the sun to
be ten times slower than it is on our earth, it follows that at
the heights of 25, 50, and 300 miles, a rarefaction of 10,
100, and a billion times respectively would be observed. The
solar atmosphere, therefore, does not attain a height of 400
geographical miles, or it cannot be as much as ^^oth of the
sun's radius. For if we take the density of the lowest strata
of the sun's atmosphere to be 1000 times greater that that of
our own near the level of the sea, a density greater than that
of water, and necessarily too high, then at a height of 400
miles this atmosphere would be 10 biUion times less dense
than the earth's atmosphere ; that is to say, to human com-
prehension it has ceased to exist.

This discussion shows that the solar atmosphere, in com-
parison with the body of the sun, has only an insignificant
height ; at the same time it may be remarked that on the
13

290 CELESTIAL DYNAMICS.

sun's surface, in spite of the great heat, such substances as
water may possibly exist in the liquid state under a pressure
thousands of times greater than that of our atmosphere.

Since gases, when free from any solid particles, emit, even
at very high temperatures, a pale transparent light — ^the so-
called lumen philosophicum — it is probable that the intense
white light of the sun has its origin in the denser parts of his
surface. If such be assumed to be the case, the sun's spots
and faculae seem to be the disturbances of the fiery liquid
ocean, caused by most powerful meteoric processes, for which
all necessary materials are present, and partly to be caused
by the direct influence of streams of asteroids. The deeper
and less heated parts of this fiery ocean become thus exposed,
and perhaps appear to us as spots, whereas the elevations
form the so-called faculas.

According to the experiments made by Henry, an Ameri-
can physicist, the rays sent forth from the spots do not pro-
duce the same heating effect as those emitted by the brighter
parts.

We have to mention one more remarkable circumstance.
The spots appear to be confined to a zone which extends 30°
on each side of the sun's equator. The thought naturally
suggests itself that some connexion exists between those solar
processes which produce the spots and faculae, the velocity of
rotation of the sun, and the swarms of asteroids, and to de-
duce therefrom the limitation of the spots to the zone men-
tioned. It stiU remains enigmatical by what means nature
contrives to bring about the uniform radiation which pertains
alike to the polar and equatorial regions of the sun.

THE TIDAL WAVE. 291

VII— THE TIDAL WAVE.

In almost every case the forces and motions on the sur-
face of the earth may be traced back to the rays of the sun.
Some processes, however, form a remarkable exception.

One of these is the tides. Beautiful, and in some re-
spects exhaustive researches on this phenomenon have been
made by Newton, Laplace, and others. The tides are caused
by the attraction exercised by the sun and the moon on the
moveable parts of the earth's surface, and by the axial rota-
tion of our globe.

The alternate rising and falling of the level of the sea
may be compared to the ascent and descent of a pendulum
oscillating under the influence of the earth's attraction.

The continual resistance, however weak it may be, which
an instrument of this nature (a physical pendulum) suffers,
constantly shortens the amplitude of the oscillations which it
performs ; and if the pendulum be required to continue in
uniform motion, it must receive a constant supply of vis viva
corresponding to the resistance it has to overcome.

Clocks regulated by a pendulum obtain such a supply,
either from a raised weight or a bent spring. The power
consumed in raising the weight or in bending the spring,
which power is represented by the raised weight or the bent
spring, overcomes for a time the resistance, and thus secures
the uniform motion of the pendulum and clock. In doing so,
the weight sinks down or the spring uncoils, and therefore
force must be expended in winding the clock up again, or it
would stop moving.

Essentially the same holds good for the tidal wave. The
moving waters rub against each other, against the shore, and
against the atmosphere, and thus, meeting constantly with re-
sistance, would soon come to rest if a vis viva did not exist
competent to overcome these obstacles. This vis viva is the

CELESTIAL DYNAMICS.

rotation of the earth on its axis, and the diminution and final
exhaustion thereof will be a consequence of such an action.

The tidal wave causes a diminution of the velocity of the
rotation of the earth.

This important conclusion can be proved in different ways.

The attraction of the sun and the moon disturbs the equi-
librium of the moveable parts of the earth's surface, so as to
move the waters of the sea towards the point or meridian
above and below which the moon culminates. If the waters
could move without resistance, the elevated parts of the tidal
wave would exactly coincide with the moon's meridian, and
under such conditions no consumption of vis viva could take
place. In reality, however, the moving waters experience
resistance, in consequence of which the flow of the tidal
wave is delayed, and high water occurs in the open sea on
the average about 2^ hours after the transit of the moon
through the meridian of the place.

The waters of the ocean move from west and east towards
the meridian of the moon, and the more elevated wave is, for
the reason above stated, always to the east of the moon's me-
ridian ; hence the sea must press and flow more powerfully
from east to west than from west to east. The ebb and flow
of the tidal wave therefore consists not only in an alternate
rising and falling of the waters, but also in a slow progressive
motion from east to west. The tidal wave produces a gen-
eral western current in the ocean.

This current is opposite in direction to the earth's rota-
tion, and therefore its friction against and collision with the
bed and shores of the ocean must offer everywhere resistance
to the axial rotation of the earth, and diminish the vis viva of
its motion. The earth here plays the part of a fly-wheel.
The moveable parts of its surface adhere, so to speak, to the
relatively fixed moon, and are dragged in a direction opposite
to that of the earth's rotation, in consequence of which, ac-
tion takes place between the solid and liquid parts of this fly-

THE TroAL WAVE. 293

wheel, resistance is overcome, and the given rotatory effect
diminished.

Water-mills have been turned by the action of the tides ;
the effects produced by such an arrangement are distinguished
in a remarkable manner from those of a mill turned by a
mountain-stream. The one obtains the vis viva with which
it works from the earth's rotation, the other from the sun's
radiation.

Various causes combine to incessantly maintain, partly in
an undulatory, partly in a progressive motion, the waters of
the ocean. Besides the influence of the sun and the moon on
the rotating earth, mention must be made of the influence of
the movement of the lower strata of the atmosphere on the^
surface of the ocean, and of the different temperatures of the
sea in various climates ; the configuration of the shores and
the bed of the ocean likewise exercise a manifold influence on
the velocity, direction, and extent of the oceanic currents.

The motions in our atmosphere, as well as those of the
ocean, presuppose the existence and consumption of vis viva
to overcome the continual resistances, and to prevent a state
of rest or equilibrium. Generally speaking, the power neces-
sary for the production of aerial currents may be of threefold
origin. Either the radiation of the sun, the heat derived
from a store in the interior of the earth, or, lastly, the rota-
tory effect of the earth may be the source.

As far as quantity is concerned the sun is by far the most
important of the above. According to Pouillet's measure-
ments, a square metre of the earth's surface receives on the
average 4*408 units of heat from the sun per minute. Since
one unit of heat is equivalent to 367 Km, it follows that one
square metre of the surface of our. globe receives per minute
an addition of ms viva equal to 1620 Km, or the whole of the
earth's surface in the same time 825,000 billions of Km. A
power of 75 Km per second is called a horse-power. Ac-
cording to this, the effect of the solar radiation in mechanical

294 CELESTIAL DYNAMICS.

work on one square metre of the eartVs surface would be
equal to 0*36, and the total effect for the whole globe 180 bil-
lions of horse-powers. A not inconsiderable portion of this
enormous quantity of vis viva is consumed in the production
of atmospheric actions, in consequence of which numerous
motions are set up in the earth's atmosphere.

In spite of their great variety, the atmospheric currents
may be reduced to a single type. In consequence of the une-
qual heating of the earth in different degrees of latitude, the
colder and heavier air of the polar regions passes in an under
current towards the equator ; whereas the heated air of the
tropics ascends to the higher parts of the atmosphere, and
flows from thence towards the poles. In this manner the air
of each hemisphere performs a circuitous motion.

It is known that these currents are essentially modilSed by
the motion of the earth on its axis. The polar currents, with
their smaller rotatory velocity, receive a motion from east to
west contrary to the earth's rotation, and the equatorial cur-
rents one from west to east in advance of the axial rotation
of the earth. The former of these currents, the easterly
winds, must diminish the rotatory effect of the globe, the lat-
ter, the westerly winds, must increase the same power. The
final result of the action of these opposed influences is, as re-
gards the rotation of the earth, according to well-known me-
chanical principles, =0 ; for these currents counteract each
other, and therefore cannot exert the least influence on the
axial rotation of the earth. This important conclusion was
proved by Laplace.

The same law holds good for every imaginable action
which is caused either by the radiant heat of the sun, or by
the heat which reaches the surface from the earth's interior,
whether the action be in the air, in the water, or on the land.
The effect of every single motion produced by these means on
the rotation of the globe, is exactly compensated by the effect
of another motion in an opposite direction ; so that the result-

THE TIDAL WAVE. 295

ant of all these motions is, as far as the axial rotation of the
globe is concerned, = 0.

In those actions known as the tides, such compensation,
however, does not take place ; for the pressure or pull by
which they are produced is always stronger from east to west
than from west to east. The currents caused by this pull
may ebb and flow in different directions, but their motion pre-
dominates in that which is opposed to the earth's rotation.

The velocity of the currents caused by the tide of the at-
mosphere amounts, according to Laplace's calculation, to not
more than 75 millimetres in a second, or nearly a geographi-
cal mile in twenty-four hours ; it is clear that much more
powerful effects produced by the sim's heat would hide this
action from observation. The influence of these air-currents,
however, on the rotatory effect of the earth is, according to
the laws of mechanics, exactly the same as it would be were
the atmosphere undisturbed by the sun's radiant heat.

The combined motions of air and water are to be regarded
from the same point of view. If we imagine the influence
of the sun and that of the interior of our globe not to exist,
the motion of the air and ocean from east to west is still left
as an obstacle to the axial rotation of the earth.

The motion of the waters of the ocean from east to west
was long ago verified by observation, and it is certain that
the tides are the most effectual of the causes to which this
great westerly current is to be referred.

Besides the tidal wave, the lower air-currents moving in
the same direction, the trade-winds of the tropics especially,
may be assigned as causes of this general movement of the
waters. The westerly direction of the latter, however, is not
confined to the region of easterly winds ; it is met with in the
region of perpetual calms, where it possesses a velocity of
several miles a day ; it is observed far away from the tropics
both north and south, in regions where westerly winds pre-

296 CELESTIAL DYNAMICS.

vail, near the Cape of Good Hope, the Straits of Magellan,
the arctic regions, &c.

A third cause for the production of a general motion of
translation of the waters of the ocean is the unequal heating
of the sea in different zones. According to the laws of hy-
drostatics, the colder water of the higher degrees of latitude
is compelled to flow towards the equator, and the warmer
water of the tropics towards the poles, in consequence of
which, similar movements are produced in the ocean to those
in the atmosphere. This is the cause of the cold under cur-
rent from the poles to the equator, and of the warm surface-
current from the equator to the poles. The waters of the lat-
ter, by virtue of the greater velocity of rotation at the equa-
tor, assume in their onward progress a direction from west to
east. It is a striking proof of the preponderating influence
of the tidal wave that, in spite of this, the motion of the
ocean is on the whole in an opposite direction.

Theory and experience thus agree in the result that the
influence of the moon on the rotating earth causes a motion
of translation from east to west in both atmosphere and
ocean. This motion must continually diminish the rotatory
effect of the earth, for want of an opposite and compensating
influence.

The continual pressure of the tidal wave against the axial
rotation of the earth may also be deduced from statical laws.

The gravitation of the moon affects without exception all
parts of the globe. Let the earth be divided by the plane of
the meridian in which the moon happens to be into two hemi-
spheres, one to the east, the other to the west of this merid-
ian. It is clear that the moon, by its attraction of the east-
em hemisphere, tends to retard the motion of the earth, and
by its attraction of the western hemisphere, to accelerate the
same rotation.

Under certain conditions both these tendencies compensate
each other, and then the action of the moon on the earth's

THE TIDAL WAVE. 297

rotation becomes zero. This happens when both hemispheres
are arranged in a certain manner symmetrically, or when no
parts of the earth can change their relative position ; in the
latter case a sort of symmetry is produced by the rotation.

The form of the earth deviates from a perfectly symmet-
rical sphere on account of the three following causes : — (1)
the flattening of the poles, (2) the mountains on the surface,
and (3) the tidal wave. The first two causes do not change
the velocity of the earth's axial rotation. In order to com-
prehend clearly the effect of the tidal wave, we shall imagine
the earth to be a perfectly symmetrical sphere uniformly sur-
rounded by water. The attraction of the sun and the moon
disturbs the equilibriimi of this mass, and two flat mountains
of water are formed. The top of one of these is directed
towards the moon, and the summit of the other is turned
away from it. A straight line passing through the tops of
these two mountains is called the major axis of this earth-
spheroid.

In this state the earth may be imagined to be divided
into three parts — a smaller sphere, and two spherical seg-
ments attached to the opposite sides of the latter, and repre-
senting the elevations of the tidal wave. The attraction of
the moon on the small central sphere does not change the ro-
tation, and we have therefore only to consider the influence
of this attraction on the two tidal elevations. The upper ele-
vation or mountain, the one nearest the moon, is attracted
towards the west because its mass is principally situated to the
east of the moon, and the opposite mountain, which is to the
west of the moon, is attracted towards the east. The upper
tidal elevation is not only more powerfully attracted because
it is* nearer to the moon, but also because the angle under
which it is pulled aside is more favourable for lateral deflec-
tion than in the case of the opposite protuberance. The pres-
sure from east to west of the upper elevation preponderates
therefore over the pressure from west to east of the opposite
13*

298 CELESTIAL DYNAMICS.

mountain ; according to calculation, these quantities stand to
each other nearly as 14 to 13. From the relative position of
these two tidal protuberances and the moon, or the unchange-
able position of the major axis of the earth-spheroid towards
the centre of gravity of the moon, a pressure results, which
preponderates from east to west, and offers an obstacle to the
earth's rotation.

K gravitation were to be compared with magnetic attrac-
tion, the earth might be considered to be a large magnet, one
pole of which, being more powerfiilly attracted, would repre-
sent the upper, and the other pole the lower tidal elevation.
As the upper tidal wave tends to move towards the moon, the
earth would act like a galvanometer, whose needle has been
deflected from the magnetic meridian, and which, while tend-
ing to return thereto, exerts a constant lateral pressure.

The foregoing discussion may suffice to demonstrate the
influence of the moon on the earth's rotation. The retarding
pressure of the tidal wave may quantitatively be determined
in the same manner as that employed in computing the pre-
cession of the equinoxes and the nutation of the earth's axis.
The varied distribution of land and water, the unequal and
unknown depth of the ocean, and the as yet imperfectly ascer-
tained mean difference between the time of the moon's cul-
mination and that of high water in the open sea, enter, how-
ever, as elements into such a calculation, and render the de-
sired result an uncertain quantity.

In the mean time this retarding pressure, if imagined to
act at the equator, cannot be assumed to be less than 1000
millions of kilogrammes. In order to start with a definite
conception, we may be allowed to use this round number as a
basis for the following calculations.

The rotatory velocity of the earth at the equator is 464
metres, and the consumption of mechanical work, therefore,
for the maintenance of the tides 464,000 millions of Km, or
6000 millions of horse-powers per second. The effect of the

THE TIDAL WAVE. 299

tides may consequently be estimated at i5;^th of the effect re-
ceived by the earth from the sun.

The rotatory effect which the earth at present possesses,
may be calculated from its mass, volume, and velocity of rota-
tion. The volume of the earth is 2,650,686,000 cubic miles,
and its specific gravity, according to Reich, = 5*44. If, for
the sake of simplicity, we assume the density of the earth to
be uniform throughout its mass, we obtain from the above
premises, and the known velocity of rotation, 25,840 quadril-
lions of kilogrammetres as the rotatory effect of the » earth.
K, during every second in 2500 years, 464,000 millions of
Km of this effect were consumed by the ebb and flow of the
tidal wave, it would suffer a diminution of 36,600 trillions of
Km, or about ^mfiw^^ of i*^s quantity.

The velocities of rotation of a sphere stand to each other
in the same ratio as the square roots of the rotatory effects,
when the volume of the sphere remains constant. From this
it follows that, in the assumed time of 2500 years, the length
of a day has increased i;4oop)th ; or if a day be taken equal
to 86,400 seconds, it has lengthened ieth of a second, if the
volume of the earth has not changed. Whether this supposi-
tion be correct or not, depends on the temperature of our
planet, and wiQ be discussed in the next chapter.

The tides also react on the motion of the moon. The
stronger attraction of the elevation nearest to, and to the east
of the moon, increases with the tangential velocity of our sat-
ellite ; the mean distance of the earth and the moon, and the
time of revolution of the latter, are consequently augmented.
The effect of this action, however, is insignificant, and, ac-
cording to calculation, does not amount to more than a frac-
tion of a second in the course of centuries.

300 CELESTIAL DYNAMICS.

VIII.-.THE EARTH'S INTERIOR HEAT.

Without doubt there was once a time when our globe
had not assumed its present magnitude. According to this,
by aid of this simple assumption, the origin of our planet may
be reduced to the union of once separated masses.

To the mechanical combinations of masses of the second
order, with masses of the second and third order, &c., the
same laws as those enunciated for the sun apply. The collis-
ion of such masses must always generate an amount of heat
proportional to the squares of their velocities, or to their me-
chanical effect.

Although we are not in a position to affirm anything cer-
tain respecting the primordial conditions under which the
constituent parts of the earth existed, it is nevertheless of the
greatest interest to estimate the quantities of heat generated by
the collision and combination of these parts by a standard
based on the simplest assumptions.

Accordingly we shall consider for the present the earth to
have been formed by the union of two parts, which obtained
their relative motions by their mutual attraction only. Let
the whole mass of the present earth, expressed in kilo-
grammes, be T, and the masses of the two portions T— x and
oj. The ratio of these two quantities may be imagined to
assume various values. The two extreme cases are, when x
is considered infinitely small in comparison with T, and when
a: = T— a: = ^ T. These form the limits of all imaginable
ratios of the parts T— a; and a:, and will now be more closely
examined.

Terrestrial heights are of course excluded from the fol-
lowing consideration. In the first place, let cc, in comparison
with T— flj, be infinitely small. The final velocity with
which X arrives on the surface of the large mass, after having

301

passed through a great space in a straight line, or after pre-
vious central motion round it, is, according to the laws devel-
oped in relation to the sun in Chapter III., confined within
the limits of 7908 and 11,183 metres. The heat generated
by this process may amount to from 8685 X a; to 17,370 X a;
units, according to the value of the major axis of the orbit
of ic. This heat, however, vanishes by its distribution through
the greater mass, because x is, according to supposition, infi-
nitely small in comparison with T.

The quantity of heat generated increases with a;, and
amounts in the second case, when a5=!^T, to from 6000 XT
to 8685 xT units.

If we assume the earth to possess a very great capacity
for heat, equal in fact to that of its volume of water, which
when calculated for equal weights = 0*184, the above discus-
sion leads to the conclusion that the difierence of temperature
of the constituent parts, and of the earth after their union, or,
in other words, the heat generated by the collision of these
parts, may range, according to their relative magnitude, from
0° to 32,000°, or even to 47,000° !

With the number of parts which thus mechanically com-
bine, the quantity of heat developed increases. Far greater
still would have been the generation of heat if the constituent
parts had moved in separate orbits round the sun before their
union, and had accidentally approached and met each other.
For various reasons, however, this latter supposition is not
very probable.

Several facts indicate that our earth was once a fiery
liquid mass, which has since cooled gradually, down to a com-
paratively inconsiderable depth from the surface, to its pres-
ent temperature. The first proof of this is the form of the
earth. " The form of the earth is its history." According
to the most careful measurements, the flattening at the poles
is exactly such as a liquid mass rotating on its axis with the
velocity of the earth would possess ; from this we may con-

302 CELESTIAL DYNAMICS.

elude that the earth at the time it received its rotatory motion
was in a liquid state ; and, after much controversy, it may be
considered as settled that this liquid condition was not that of
an aqueous solution, but of a mass melted by a high tempera-
ture.

The temperature of the crust of the globe likewise fur-
nishes proof of the existence of a store of heat in its interior.
Many exact experiments and measurements show that the
temperature of the earth increases with the depth to which
we penetrate. In boring the artesian well at Grenelle, which
is 546 metres deep, it was observed that the temperature aug-
mented at the rate of 1° for every 30 metres. The same re-
sult was obtained by observations in the artesian well at Mon-
dorf in Luxembourg : this well is 671 metres in depth, and
its water 34° warm.

Thermal springs furnish a striking proof of the high tem-
perature existing in the interior of the earth. Scientific men
are agreed that the aqueous deposits from the atmosphere,
rain, hail, dew, and snow, are the sole causes of the forma-
tion of springs. The water obeying the laws of gravity, per-
colates through the earth wherever it can, and reappears at
the surface in places of a lower situation. When water sinks
to considerable depths through vertical crevices in the rocks,
it acquires the temperature of the surrounding strata, and
returns as a thermal spring to the surface.

Such waters are frequently distinguished from the water
of ordinary springs merely by their possessing a higher tem-
perature. If, however, the water in its course meets with
mineral or organic substances which it can dissolve and re-
tain, it then reappears as a mineral spring. Examples of
such are met with at Aachen, Carlsbad, &c.

In a far more decided manner than by the high tempera-
ture of the water of certain springs, the interior heat of our
globe is made manifest by those fiery fluid masses which
sometimes rise from considerable depths. The temperature

303

of the earth's crust increases at the rate of 1° for every 30
metres we descend from the surface towards the centre. Al-
though it is incredible that this augmentation can continue at
the same rate tiU the centre be reached, we may nevertheless
assume with certainty that it does continue to a considerable
depth. Calculation based on this assumption shows that at
a depth of a few miles a temperature must exist sufficiently
powerful to fuse most substances. Such molten masses pene-
trate the cold crust of the globe in many places, and make
their appearance as lava.

A distinguished scientific man has lately expressed him-
self on the origin of the interior heat of the earth as follows :
— " No one of course can explain the final causes of things.
This much, however, is clear to every thinking man, that
there is just as much reason that a body, like the earth, for
example, should be warm, warmer than ice or human blood,
as there is that it should be cold or colder than the latter. A
particular cause for this absolute heat is as little necessary as
a cause for motion or rest. Change — ^that is to say, transi-
tion from one state of things to another — alone requires and
admits of explanation."

It is evident that this reflection is not fitted to suppress the
desire for an explanation of the phenomenon in question. As
aU matter has the tendency to assume the same temperature
as that possessed by the substances by which it happens to be
surrounded, and to remain in a quiescent state as soon as
equilibrium has been established, we must conclude that,
whenever we meet with a body warmer than its neighbours,
such body must have received at a (relatively speaking) not
far distant time, a certain degree of heat, — a process which
certainly allows of, and requires explanation.

Newton's theory of gravitation, whilst it enables us to de-
termine, from its present form, the earth's state of aggrega-
tion in ages past, at the same time points out to us a source
of heat powerful enough to produce such a state of aggrega-

804 CELESTIAL DYNAMICS.

tion, powerful enough to melt worlds ; it teaches us to con-
sider the molten state of a planet as the result of the mechan-
ical union of cosmical masses, and thus derive the radiation
of the sun and the heat in the bowels of the earth from a
common origin.

The rotatory effect of the earth also may be readily ex-
plained by the collision of its constituent parts ; and we must
accordingly subtract the vis viva of the axial rotation from
the whole effect of the collision and mechanical combination,
in order to obtain the quantity of heat generated. The rota-
tory effect, however, is only a small quantity in comparison
with the interior heat of the earth. It amounts to about
4400 X T kilogrammetres, T being the weight of the earth in
kilogrammes, which is equivalent to 12 xT units of heat, if
we assume the density of the earth to be uniform throughout.

If we imagine the moon in the course of time, either in
consequence of the action of a resisting medium or from
some other cause, to unite herself with our earth, two princi-
pal effects are to be discerned. A result of the collision
would be, that the whole mass of the moon and the cold crust
of the earth would be raised some thousands of degrees in
temperature, and consequently the surface of the earth would
be converted into a fiery ocean. At the same time the velo-
city of the earth's axial rotation would be somewhat acceler-
ated, and the position of its axis with regard to the heavens,
and to its own surface, slightly altered. If the earth had
been a cold body without axial rotation, the process of its
combining with the moon would have imparted to it both
heat and rotation.

It is probable that such processes of combination between
different parts of our globe may have repeatedly happened
before the earth attained its present magnitude, and that lux-
uriant vegetation may have at different times been buried un-
der the fiery debris resulting from the conflict of these masses.

THE earth's INTEKIOK HEAT. 805

As long as the surface of our globe was in an incandes-
cent state, it must have lost heat at a very rapid rate ; grad-
ually this process became slower ; and although it has not yet
entirely ceased, the rate of cooling must have diminished to a
comparatively small magnitude.

Two phenomena are caused by the cooling of the earth,
which, on account of their common origin, are intimately re-
lated. The decrease of temperature, and consequent contrac-
tion of the earth's crust, must have caused frequent -distur-
bances and revolutions on its surface, accompanied by the
ejection of molten masses and the formation of protuberances ;
on the other hand, according to the laws of mechanics, the
velocity of rotation must have increased v\rith the diminution
of the volume of the sphere, or, in other words, the cooling
of the earth must have shortened the length of the day.

As the intensity of such disturbances and the velocity of
rotation are closely connected, it is clear that the youth of our
planet must have been distinguished by continual violent
transformations of its crust, and a perceptible acceleration of
the velocity of its axial rotation ; whilst in the present time
the metamorphoses of its surface are much slower, and the
acceleration of its axial revolution diminished to a very small
amount.

If we imagine the times when the Alps, the chain of the
Andes, and the Peak of Teneriffe were upheaved from the
deep, and compare vrith such changes the earthquakes and
volcanic eruptions of historic times, we perceive in these
modern transformations but weak images of the analogous
processes of bygone ages.

Whilst we are surrounded on every side by the monu-
ments of violent volcanic convulsions, we possess no record
of the velocity of the axial rotation of our planet in antedi-
luvian times. It is of the greatest importance that we should
have an exact knowledge of a change in this velocity, or in
the length of the day during historic tii^aes. The investiga-

306 - CELESTIAL DYNAMTOS.

tion of this subject by the great Laplace forms a bright mon-
ument in the department of exact science.

These calculations are essentially conducted in the follow-
ing manner : — In the first place, the time between two eclipses
of the sun, widely apart from each other, is as accurately
as possible expressed in days, and from this the ratio of the
time of the earth's rotation to the mean time of the moon's
revolution determined. If, now, the observations of ancient
astronomers be compared with those of our present time, the
least alteration in the absolute length of a day may be de-
tected by a change in this ratio, or in a disturbance in the
lunar revolution. The most perfect agreement of ancient rec-
ords on the movements of the moon and the planets, on the
eclipses of the sun, &c., revealed to Laplace the remarkable
fact that in the course of 25 centuries, the time in which our
earth revolves on its axis has not altered gjjjth part of a sexa-
gesimal second ; and the length of a day therefore may be
considered to have been constant during historic times.

This result, as important as it was convenient for astron-
omy, was nevertheless of a nature to create some difficulties
for the physicist. With apparently good reason it was con-
cluded that, if the velocity of rotation had remained constant,
the volume of the earth could have undergone no change.
The earth completes one revolution on its axis in 86,400 si-
dereal seconds ; it consequently appears, if this time has not
altered during 2500 years to the extent of g^oth of a second,
or 43,ooo,oooth part of a day, that during this long space of time
the radius of the earth also cannot have altered more than
this fraction of its length. The earth's radius measures
6,369,800 metres, and therefore its length ought not to have
diminished more than 15 centimetres in 25 centuries.

The diminution in volume, as a result of the cooling-pro-
cess, is, however, closely connected with the changes on the
earth's surface. When we consider that scarcely a day
passes without the occurrence of an earthquake or shock in

THE earth's interior HEAT. 307

one place or another, and that of the 300 active volcanos
some are always in action, it would appear that such a lively
reaction of the interior of the earth against the crust is in-
compatible with the constancy of its volume.

This apparent discrepancy between Cordier's theory of the
connexion between the cooling of the earth and the reaction
of the interior on the exterior parts, and Laplace's calcula-
tion showing the constancy of the length of the day, a calcu-
lation which is undoubtedly correct, has induced most scien-
tific men to abandon Cordier's theory, and thus to deprive
themselves of any tenable explanation of volcanic activity.

The continued cooling of the earth cannot be denied, for
it takes place according to the laws of nature ; in this respect
the earth cannot comport itself differently from any other
mass, however small it may be. In spite of the heat which
it receives from the sun, the earth will have a tendency to
cool so long as the temperature of its interior is higher than
the mean temperature of its surface. Between the tropics the
mean temperature produced by the sun is about 28°, and the
sun therefore is as little able to stop the cooling-tendency of
the earth as the moderate warmth of the air can prevent the
cooling of a red-hot ball suspended in a room.

Many phenomena, for instance the melting of the glaciers
near the bed on which they rest, show the uninterrupted
emission of heat from the interior towards the exterior of the
earth ; and the question is. Has the earth in 25 centuries
actually lost no more heat than that which is requisite to
shorten a radius of more than 6 millions of metres only 15
centimetres ?

In answering this question, three points enter into our
calculation ; — (1) the absolute amount of heat lost by the
earth in a certain time, say one day ; (2) the earth's capacity
for heat ; and (3) the coefficient of expansion of the mass of
the earth*.

As none of these quantities can be determined by direct

308 CELESTIAL DYNAMICS.

measurements, we are obliged to content ourselves with prob-
able estimates ; these estimates will carry the more weight
the less they are formed in favour of some preconceived opin-
ion.

Considering what is known about the expansion and con-
traction of solids and liquids by heat and cold, we arrive at
the conclusion that for a diminution of 1° in temperature,
the linear contraction of the earth cannot well be less than
ioojooo^h part, a number which we all the more readily adopt
because it has been used by Laplace, Arago, and others.

If we compare the capacity for heat of all solid and liquid
bodies which have been examined, we find that, both as re-
gards volume and weight, the capacity of water is the great-
est. Even the gases come under this rule ; hydrogen, how-
ever, forms an exception, it having the greatest capacity for
heat of all bodies when compared with an equal weight of
water. In order not to take the capacity for heat of the mass
of the earth too small, we shall consider it to be equal to that
of its volume of water, which, when calculated for equal
weights, amounts to 0*184,*

If we accept Laplace's result, that the length of a day has
remained constant during the last 2500 years, and conclude

* The capacity for heat, as well as the coefficient of expansion of mat-
ter, as a rule, increases at higher temperatures. As, however, these two
quantities act in opposite ways in our calculations, we may be allowed to
dispense with the influence which the high temperature of the interior of
the earth must exercise on these numbers. Even if, in consequence of
the high temperature of the interior, the earth's mass could have a capa-
city two or three times as great as that which it has from 0° to 100°, it
is to be considered, on the other hand, that the coefficient of expansion,
TTTTT.Vo o> o^y liolds good for soUds, and is even small for them, whilst in
the case of liquids we have to assume a much greater coefficient : for mer-
cury between 0° and 100°, it is about six times as great. Especially great
is the contraction and expansion of bodies when they change their state
of aggregation ; and this should be taken into account when considering
the formation of the earth's crust.

sod

that the earth's radius has not diminished 1^ decimetre in
consequence of cooling, we are obliged to assume, according
to the premises stated, that the mean temperature of our
planet cannot have decreased 4^5° in the same period of time.

The volmne of the earth amounts to 2650 millions of cu-
bic miles. A loss of heat sufficient to cool this mass 4^0°
would be equal to the heat given off when the temperature
of 6,150,000 cubic miles of water decreases 1° ; hence the
loss for one day would be equal to 6*74 cubic miles of heat.

Fourier has investigated the loss of heat sustained by the
earth. Taking the observation that the temperature of the
earth increases at the rate of 1° for every 30 metres as the
basis of his calculations, this celebrated mathematician finds
the heat which the globe loses by conduction through its crust
in the space of 100 years to be capable of melting a layer of
ice 3 metres in thickness and covering the whole surface of
the globe ; this corresponds in one day to 7*7 cubic miles of
heat, and in 2500 years to a decrease of 17 centimetres in
the length of the radius.

According to this, the cooling of the globe would be suffi-
ciently great to require attention when the earth's velocity of
rotation is considered.

At the same time it is clear that the method employed by
Fourier can only bring to our knowledge one part of the heat
which is annually lost by the earth ; for simple conduction
through terra firma is not the 'only way by which heat escapes
from our globe.

In the first place, we may make mention of the aqueous
deposits of our atmosphere, which, as far as they penetrate
our earth, wash away, so to speak, a portion of the heat, and
thus accelerate the cooling of the globe. The whole quantity
of water which falls ham. the atmosphere upon the land in
one day, however, cannot be assumed to be much more than
half a cubic mile in volume, hence the cooling effect produced
by this water may be neglected in our calculation. The heat

310 CELESTIAL DYNAMICS.

carried off by all the thermal springs in the world is very
small in comparison with the quantities which we have to
consider here.

Much more important is the effect produced by active vol-
canos. As the heat which accompanies the molten matter to
the surface is derived from the store in the interior of the
earth, their action must influence considerably the diminution
of the earth's heat. And we have not only tt) consider here
actual eruptions which take place in succession or simulta-
neously at different parts of the earth's surface, but also vol-
canos in a quiescent state, which continually radiate large
quantities of heat abstracted from the interior of the globe.
If we compare the earth to an animal body, we may regard
each volcano as a place where the epidermis has been torn
off, leaving the interior exposed, and thus opening a door for
the escape of heat.

Of the whole of the heat which passes away through
these numerous outlets, too low an estimate must not be
made. To have some basis for the estimation of this loss,
we have to recollect that in 1783 Skaptar-Jokul, a volcano in
Iceland, emitted sufficient lava in the space of six weeks to
cover 60 square miles of country to an average depth of 200
metres, or, in other words, about 1^ cubic miles of lava.
The amount of heat lost by this one eruption of one volcano
must, when the high temperature of the lava is considered,
be estimated to be more than 1000 cubic miles of heat ; and
the whole loss resulting from the action of all the volcanos
amounts, therefore, in aU probability, to thousands of cubic
miles of heat per annum. This latter number, when added
to Fourier's result, produces a sum which evidently does not
agree with the assumption that the volume of our earth has
remained unchanged.

In the investigation of the cooling of our globe, the influ-
ence of the water of the ocean has to be taken into account.
Fourier's calculations are based on the observations of the in-

311

crease of the temperature of the crust of our earth, from the
surface towards the centre. But two-thirds of the surface of
our globe are covered with water, and we cannot assume d
priori that this large area loses heat at the same rate as the
solid parts ; on the contrary, various circumstances indicate
that the cooling of our globe proceeds more quickly through
the waters of the ocean resting on it than from the solid parts
merely in contact with the atmosphere.

In the first place, we have to remark that the bottom of
the ocean is, generally speaking, nearer to the store of heat
in the interior of the earth than the dry land is, and hence
that the temperature increases most probably in a greater
ratio from the bottom of the sea towards the interior of the
globe, than it does in our observations on the land. Sec-
ondly, we have to consider that the whole bottom of the sea
is covered by a layer of ice-cold water, which moves con-
stantly from the poles to the equator, and which, in its pas-
sage over sand-banks, causes, as Humboldt aptly remarks,
the low temperatures which are generally observed in shallow
places. That the water near the bottom of the sea, on ac-
count of its great specific heat and its low temperature, is
better fitted than the atmosphere to withdraw the heat from
the earth, is a point which requires no further discussion.

We have plenty of observations which prove that the
earth suffers a great loss of heat through the waters of the
ocean. Many investigations have demonstrated the existence
of a large expanse of sea, much visited by whalers, situated
between Iceland, Greenland, Norway, and Spitzbergen, and
extending from lat. 76° to 80° N., and from long. 15° E. to
15° W. of Greenwich, where the temperature was observed
to be higher in the deeper water thati near the surface — an
experience which neither accords with the general rule, nor
agrees with the laws of hydrostatics. Franklin observed, in
lat. 77° N. and long. 12° E., that the temperature of the sea
near the surface was — 1°, and at a depth of 700 fathoms

312 CELESTIAL DYNAMICS.

-1-6°. Fisher, in lat. 80° N. and long. 11° E., noticed that
the surface-water had a temperature of 0°, whilst at a depth
of 140 fathoms it stood at-f 8^.

As sea-water, unlike pure water, does not possess a point
of greatest density at some distance above the freezing-point,
and as the water in lat. 80° N. is found at some depth to be
warmer than water at the same depth 10° southward, we can
only explain this remarkable phenomenon of an increase of
temperature with an increase of depth by the existence of a
source of heat at the bottom of the sea. The heat, however,
which is required to warm the water at the bottom of an ex-
panse of ocean more than 1000 square miles in extent to a
sensible degree, must amount, according to the lowest esti-
mate, to some cubic miles of heat a day.

The same phenomenon has been observed in other parts
of the world, such as the west coast of Australia, the Adri-
atic, the Lago Maggiore, &c. Especial mention should here
be made of an observation by Horner, according to whom the
lead, when hauled up from a depth varying from 80 to 100
fathoms in the mighty Grulf-stream off the coast of America,
used to be hotter than boiling water.

The facts above mentioned, and some others which might
be added, clearly show that the loss of heat suffered by our
globe during the last 2500 years is far too great to have been
without sensible effect on the velocity of the earth's rotation.
The reason why, in spite of this accelerating cause, the length
of a day has nevertheless remained constant since the most
ancient times, must be attributed to an opposite retarding ac-
tion. This consists in the attraction of the sun and moon on
the liquid parts of the earth's surface, as explained in the
last chapter.

According to the calculations of the last chapter, the re-
tarding pressure of the tides against the earth's rotation
would cause, during the lapse of 2500 years, a sidereal day
to be lengthened to the extent of fcth of a second ; as the

313

length of a day, however, has remained constant, the cooling
effect of the earth during the same period of time must have
shortened the day j^th of a second. A diminution of the
earth's radius to the amount of 4| metres in 2500 years, and
a daily loss of 200 cubic miles of heat, correspond to this
effect. Hence, in the course of the last 25 centuries, the
temperature of the whole mass of the earth must have de-
creased f4°.

The not inconsiderable contraction of the earth resulting
from such a loss of heat, agrees with the continual transfor-
mations of the earth's surface by earthquakes and volcanic
eruptions ; and we agree with Cordier, the industrious ob-
server of volcanic processes, in considering these phenomena
a necessary consequence of the continual cooling of an earth
which is still in a molten state in its interior.

When our earth was in its youth, its velocity of rotation
must have increased to a very sensible degree, on account of
the rapid cooling of its then very hot mass. This accelera-
ting cause gradually diminished, and as the retarding pressure
of the tidal wave remains nearly constant, the latter must
finally preponderate, and the velocity of rotation therefore
continually decrease. Between these two states we have a
period of equilibrium, a period when the influence of the
cooling and that of the tidal pressure counterbalance each
other ; the whole life of the earth therefore may be divided
into three periods — ^youth with increasing, middle age vsdth
uniform, and old age with decreasing velocity of rotation.

The time during which the two opposed influences on the
rotation of the earth are in equilibrium can, strictly speak-
ing, only be very short, inasmuch as in one moment the cool-
ing, and in the next moment the pressure of the tides must
prevail. In a physical sense, however, when measured by
human standards, the influence of the cooling, and still more
80 that of the tidal wave, may for ages be considered con-
stant, and there must consequently exist a period of many
14

314: CELESTIAL DYNAMICS.

thousand years* duration during which these counteracting
influences will appear to be equal. Within this period a si-
dereal day attains its shortest length, and the velocity of the
earth's rotation its maximum — circumstances which, accord-
ing to mathematical analysis, would tend to lengthen the du-
ration of this period of the earth's existence.

The historical times of mankind are, according to La-
place's calculation, to be placed in this period. Whether we
are at the present moment still near its commencement, its
middle, or are approaching its conclusion, is a question which
cannot be solved by our present data, and must be left to fu-
ture generations.

The continual cooling of the earth cannot be without an
influence on the temperature of its surface, and consequently
on the climate ; scientific men, led by Buffon, in fact, have
advanced the supposition that the loss of heat sustained by
our globe must at some time render it an unfit habitation for
organic life. Such an apprehension has evidently no founda-
tion, for the warmth of the earth's surface is even now much
more dependent on the rays of the sun than on the heat which
reaches us from the interior. According to Pouillet's meas-
urements, mentioned in Chapter III., the earth receives 8000
cubic miles of heat a day from the sun, whereas the heat
which reaches the surface from the earth's interior may be
estimated at 200 cubic miles per diem. The heat therefore
obtained from the latter source every day is but small in com-
parison to the diurnal heat received from the sun.

If we imagine the solar radiation to be constant, and the
heat we receive from the store in the interior of the earth to
be cut off", we should have as a consequence various changes
in the physical constitution of the surface of our globe. The
temperature of hot springs would gradually sink down to the
mean temperature of the earth's crust, volcanic eruptions
would cease, earthquakes would no longer be felt, and the
temperature of the water of the ocean would be sensibly al-

315

tered in many places — circumstances which would doubtless
affect the climate in many parts of the world. Especially it
may be presumed that Western Europe, with its present fa-
vourable climate, would become colder, and thus perhaps the
seat of the power and culture of our race transferred to the
milder parts of North America.

Be this as it may, for thousands of years to come we can
predict no diminution of the temperature of the surface of
our globe as a consequence of the cooling of its interior mass ;
and, as far as historic records teach, the climates, the tempe-
ratures of thermal springs, and the intensity and frequency
of volcanic eruptions are now the same as they were in the
far past.

It was different in prehistoric times, when for centuries
the earth's surface was heated by internal fire, when mam-
moths lived in the now uninhabitable polar regions, and when
the tree-ferns and the tropical shell-fish whose fossil remains
are now especially preserved in the coal-formation were at
home in all parts of the world.

m.

THE MECHANICAL EQUIVALENT OF HEAT.

THE vast and magnificent structure of the experimental
sciences has been erected on only a few pillars. His-
tory teaches us that the searching spirit of man requii'ed
thousands of years for the discovery of the fundamental prin-
ciples of the sciences, on which the superstructure was then
raised in a comparatively short time. But these very funda-
mental propositions are nevertheless so clear and simple, that
the discovery of them reminds us, in more than one respect,
of Columbus's egg.

But if, now that we are at last in possession of the truth,
we speak of a method by the application of which the most
essential fundamental laws might have been discovered with-
out waste of time, it is not that we would criticize in any light
spirit the efforts and achievements of our forerunners : it is
merely with the object of laying before the reader in an ad-
vantageous form one of the additions to our knowledge which
recent times have brought forth.

The most important — ^not to aay the only — rule for the
genuine investigation of nature is, to remain firm in the con-
viction that the problem before us is to learn to know phenom-
ena, before seeking for explanations or inquiring after higher

TRUE NATUEE OF SCIENTIFIC PROBLEMS. 317

causes. As soon as a fact is once known in all its relations,
it is therein explained, and the problem of science is at an
end.

Notwithstanding that some may pronounce this a trite
assertion, and no matter how many arguments others may
bring to oppose it, it remains none the less certain that this
primary rule has been too often disregarded even up to the
most modern times ; while all the speculative operations of
even the most highly gifted minds which, instead of taking
firm hold of facts as such, have striven to rise above them,
have as yet borne but barren fruit.

We shall not here discuss the modern naturalistic philoso-
phy (NaturpkilosopMe) further than to say that its character
is already sufficiently apparent from the ephemeral existence
of its offspring. But even the greatest and most meritorious
of the naturalists of antiquity, in order to explain, for exam-'
pie, the properties of the lever, took refuge in the assertion
that a circle is such a marvellous thing that no wonder if mo-
tions, taking place in a circle, offer also in their turn most
unusual phenomena. If Aristotle, instead of straining his
extraordinary powers in meditations upon the fixed point and
advancing line, as he calls the circle, had investigated the
numerical relations subsisting between the length of the arm
of the lever and the pressure exerted, he would have laid the
foundation of an important part of human knowledge.

Such mistakes, committed as they were, in accordance
with the spirit of those times, even by a man whose many
positive services constitute his everlasting memorial, may
serve to point us in the opposite road which leads us surely
to the goal. But if, even by the most correct method of in-
vestigation, nothing can be attained without toil and industry,
the cause is to be sought in that divine order of the world
according to which man is made to labour. But it is certain
that already immeasurably more means and more toil have
been sacrificed to error than were needed for the discovery of
ihQ truth.

318 THE MECHANICAL EQUIVALENT OF HEAT.

The rule which must be followed, in order to lay the foun-
dations of a knowledge of nature in the shortest conceivable
time, may be comprised in a few words. The natural phe-
nomena with which we come into most immediate contact,
and which are of most frequent occurrence, must be subjected
to a careful examination by means of the organs of sense,
and this examination must be continued until it results in
quantitative determinations which admit of being expressed
by numbers.

These numbers are the required foundations of an exact
investigation of nature.

Among all natural operations, the free fall of a weight is
the most frequent, the simplest, and — witness Newton's apple
— at the same time the most important. When this process
is analysed in the way that has been mentioned, we imme-
diately see that the weight strikes against the ground the
harder the greater the height from which it has fallen ; and
the problem now consists in the determination of the quanti-
tative relations subsisting between the height from which the
weight falls, the time occupied by it in its descent, and its
final velocity, and in expressing these relations by definite
numbers.

In carrying out this experimental investigation, various
difficulties have to be contended with; but these must and
can be overcome ; and then the truth is arrived at, that for
every body a faU of sixteen feet, or a time of descent of one
second, corresponds to a final velocity of thirty-two feet per
second.

A second phenomenon of daily occurrence, which is in
apparent contradiction to the laws of falling bodies, is the
ascent of liquids in tubes by suction. Here, again, the rule
applies, not to allow the maxim, velle rerum cognoscere causas^
to lead us into error through useless and therefore harmful
speculations concerning the qualities of the vacuum, and the
like ; on the contrary, we must again examine the phenoms-

THE PROBLEM OF FALLING BODIES. 319

non with attention and awakened senses ; and then we find,
as soon as we put a tube to the mouth to raise a liquid, that
the operation is at first quite easy, but that afterwards it re-
quires an amount of exertion which rapidly increases as the
column of liquid becomes higher. Is there, perchance, an
ascertainable limit to the action of suction ? As soon as we
once begin to experiment in this direction, it can no longer
escape us that there is a barometric height, and that it attains
to about thirty inches. This number is a second chief piUar
in the edifice of human knowledge.

Question now follows question, and answer, answer. We
have learned that the pressure exerted by a column of fluid is
proportional to its height and to the specific gravity of the
fluid ; we have thus determinied the specific gravity of the at-
mosphere, and by this investigation we are led to carry up
our measuring-instrument, the barometer, from the plain to
the mountains, and to express numerically the efiect produced
by elevation above the sea-level upon the height of the mer-
cury-column. Such experiments suggest the question. Whether
the laws of falling bodies, with which we have become
acquainted at the surface of the earth, do not likewise un-
dergo modification at greater distances from the ground.
And if, as d priori we cannot but expect, this should be really
the case, the further question arises. In what manner is the
number already found modified by distance from the earth?
We have thus come upon a problem the solution of which is
attended with many difficulties ; for what has now to be ac-
complished, is to make observations and carry out measure-
ments in places where no human foot can tread. History,
however, teaches that the same man who put the question
was also able to furnish the answer. Truly he could do so
only through a rich treasure of astronomical knowledge. But
how is this knowledge to be attained by us ?

Astronomy is, without question, even in its first principles,
the most difficult of all sciences. We have here to deal with

320 THE MECHAlflCAL EQUIVALENT OF HEAT.

objects and spaces which forbid all thought of experiment,
while at the same time the motions of the innumerable heavr
enly bodies are of so complicated a kind, that astronomical
science, in its stately unfolding, is rightly considered the high-
est triumph whereof human intellect here below is able to
boast.

In accordance with the natural rule that, both in particu-
lars and in general, man has to begin with that which is
easiest and then to advance step by step to what is more diffi-
cult, it might well be supposed that astronomy must have ar-
rived at a flourishing state of development later than any
other branch of human knowledge. But it is well known that
in reality the direct opposite was the case, inasmuch as it was
precisely in astronomy, and in no other branch, that the ear-
liest peoples attained to really sound knowledge, It may,
indeed, be asserted that the science of the heavenly bodies
had in antiquity reached as high a degree of perfection as the
complete want of all the auxiliary sciences rendered possible.

This early occurrence of a vigorous development of as-
tronomy, which, indeed, was a necessary forerunner of the
other sciences, since it alone furnished the necessary data for
the measurement of time, is observable among the most va-
rious races of mankind : the reason of it, moreover, lies in
the nature of things, and in the constitution of the human
mind. It furnishes a remarkable proof that a right method
is the most important condition for the successful prosecution
of scientific inquiry.

The explanation of this phenomenon lies in the fact that
the need which was felt at a very early period, of a common
standard for the computation of time, made it necessary to
institute observations such that their results required to be
expressed by definite numbers. There was a felt necessity of
determining the time in which the sun accomplishes his cir-
cuit through the heavens, as well as the time in which the
moon goes thi*ough her phases, and other similar questions.

FALLING BODIES AT GREAT HEIGHTS. 321

In order to meet this necessity, there was no temptation to
take up the Book of Nature, after the manner of expositors
and critics, merely to cover it with glosses :

" Mit eitler Rede wird hier nichts geschafft."

It was numbers that were sought, and numbers that were
found. The overpowering force of circumstances constrained
the spirit of inquiry into the right path, and therein led it at
once from success to success.

Now that after long-continued, accurate, and fortunate
observations the needful knowledge of the courses and dis-
ta,nces of the nearest heavenly bodies, as well as of the figure
and size of the earth, has been acquired, we are in a position
to treat the question, What is the numerical influence exerted
by increased distance from the earth upon the known laws of
falling bodies ? and we thus arrive at the pregnant discovery
that, at a height equal to the earth's semidiameter, the dis-
tance fallen through and the final velocity, for the first second,
is four times less than on the surface of the earth.

In order to pursue our inquiry, let us now return to the
objects which inunediately surround us. From the earliest
times, the phenomena of combustion must have claimed in an
especial degree the attention of mankind. In order to ea>
jjilain them, the ancients, in accordance with the method of
their naturalistic philosophy, put forward a peculiar upward-
striving element of Fire, which in conjunction with, and in
opposition to. Air, Water, and Earth, constituted all that ex-
isted. The necessary consequence of this theory, which they
discussed with the most acute sagacity, was, that in regard to
the phenomena in question and all that related to them, they
remained in complete ignorance.

Here, again, it is quantitative determinations, it is num-
bers alone, which put the Ariadne's clue in our hand. K we
want to know what goes on during the phenomena of com-
bustion, we must weigh the substances before and after they
14*

322 THE MECHAi^ICAL EQUIVALENT OF HEAT.

are burned ; and here the knowledge we have already ac-
quired of the weight of gaseous bodies comes to our aid.
We then find that, in every case of combustion, substances
which previously existed in a separate state enter into an inti-
mate union with each other, and that the total weight of the
substances remains the same both before and after the combi-
nation. "We thus come to know the different bodies in their
separate and in their combined states, and learn how to trans-
form them from one of these states into the other ; we learn,
for instance, that water is composed of two kinds of air which
combine with each other in the proportion of 1:8. An en-
trance into chemical science is thus opened to us, and the nu-
merical laws which regulate the combinations of matter (^die
Stochiometrie) hang like ripe fruit before us.

As we proceed further in our investigations, we find that
in all chemical operations — combinations as well as decompo-
sitions— changes of temperature occur, which, according to
the varying circumstances of different cases, are of all de-
grees of intensity, from the most violent heat downwards.
We have measured quantitatively the heat developed, or
counted the number of heat-units, and have so come into pos-
session of the law of the evolution of heat in chemical pro-
cesses.

We have long known, however, that in innumerable cases
heat makes its appearance where no chemical action is going
on ; for instance, whenever there is friction, when unelastic
bodies strike one another, and when aeriform bodies are com-

What then takes place when heat is evolved in such ways
as these?

We are taught by history that in this case also the most
sagacious hypotheses concerning the state and nature of a
peculiar *' matter " of heat, concerning a " thermal aether,"
whether at rest or in a state of vibration, concerning " ther-
mal atoms," supposed to exercise their functions in the inter-

CONVERTIBILITY OF HEAT AND MOTION. 323

stices between the material atoms, or other hypotheses of like
nature, have not availed to solve the problem. It is, notwith-
standing, of no less wonderfully simple a nature than the laws
of the lever, about which the founder of th® peripatetic phi-
losophy cudgelled his brains in vain.

After what has gone before, the reader cannot be in any
doubt about what is the course now to be pursued. "We must
again make quantitative determinations : we must measure
and count.

If we proceed in this direction and measure the quantity
of heat developed by mechanical agency, as well as the
amount of force used up in producing it, and compare these
quantities with each other, we at once find that they stand to
each other in the simplest conceivable relation — that is to say,
in an invariable direct proportion, and that the proportion also
holds when, inversely, mechanical force is again produced by
the aid of heat.

Putting these facts into brief and plain language, we may
say.

Heat and motion are transformable one into the other.

We cannot and ought not, however, to let this suffice us.
We require to know how much mechanical force is needed for
the production of a given amount of heat, and conversely.
In other words, the law of the invariable quantitative relation
between motion and heat must be expressed numerically.

When we appeal hereupon to experiment, we find that
raising the temperature of a given weight of water one degree
of the Centigrade scale corresponds to the elevation of an
equal weight to the height of about 1,200 [French] feet.

This number is the Mechanical Equivalent op Heat.

The production of heat by friction and other mechanical
operations is a fundamental fact of such constant occurrence,
that the importance of its establishment on a scientific basis
wiU be recognized by naturalists without any preliminary

324: THE MECHANICAL EQUIVALENT OF HEAT.

enumeration of its useful applications ; and, for the same
reason, a few historical remarks touching the circumstances
attending the discovery of the foregoing fundamental law,
will not be out of place here.

In the summer of 1840, on the occasion of bleeding Eu-
ropeans newly arrived in Java, I made the observation that
the blood drawn from the vein of the arm possessed, almost
without exception, a surprisingly bright red colour.

This phenomenon riveted my earnest attention. Starting
from Lavoisier's theory, according to which animal heat is
the result of a process of combustion, I regarded the twofold
change of colour which the blood undergoes in the capillaries
as a sensible sign — as the visible indication — of an oxidation
going on in the blood. In order that the human body may
be kept at a uniform temperature, the development of heat
within it must bear a quantitative relation to the heat which
it loses — a relation, that is, to the temperature of the sur-
rounding medium ; and hence both the production of heat
and the process of oxidation, as well as the difference in col-
our of tlie two kinds of blood, must be on the whole less in
the torrid zones than in colder regions.

In accordance with this theory, and having regard to the
known physiological facts which bear upon the question, the
blood must be regarded as a fermenting liquid undergoing
slow combustion, whose most important function — that is,
sustaining the process of combustion — is fulfilled without the
constituents of the blood (with the exception, that is, of the
products of decomposition) leaving the cavities of the blood-
vessels or coming into such relation with the organs that an
interchange of matter cai> take place. This may be thus
stated in other words : by far the greater part of the assimi-
lated food is burned in the cavities of the blood-vessels them-
selves, for the purpose of producing a physical effect, and a
comparatively small quantity only serves the less important
end of ultimately entering the substance of the organs them-

PROBLEM OF PHYSIOLOGICAL HEAT. 325

selves, so as to occasion growth and the renewal of the worn-
out solid parts.

If hence it follows that a general balance must be struck
in the organism between receipts and expenditure, or between
work done and wear and tear, it is unmistakably one of the
most important problems with which the physiologist has to
deal, to make himself as thoroughly acquainted as it is possi-
ble for him to be with the budget of the object of his exami-
nation. The wear and tear consists in the amount of matter
consumed ; the work done is the evolution of heat. This
latter effect, however, is of two kinds, inasmuch as the ani-
mal body evolves heat on the one hand directly in its own
interior, and distributes it by communication to the objects
immediately surrounding it ; while, on the other hand, it
possesses, through its organs of motion, the power of produc-
ing heat mechanically by friction or in similar ways, even at
distant points. "We now require to know

Whether the heat directly evolved is alone to he laid to the
account of the process of combustion, or whether it is the sum
of the heat evolved hoth directly and indirectly that it is to he
taken into calculation.

This is a question that touches the very foundations of sci-
ence ; and unless it receives a trustworthy answer, the healthy
development of the doctrine concerned is not possible. For
it has been already shown, by various examples, what are
the consequences of neglecting primary quantitative determi-
nations. No wit of man is able to furnish a substitute for
what nature offers.

The physiological theory of combustion starts fi*om the
fundamental proposition, that the quantity of heat which re-
sults from the combustion of a given substance is invariable —
that is, that its amount is uninfluenced by the circumstances
which accompany the combustion ; whence we infer, " in spe-
cie," that the chemical effect of combustible matter can un-
dergo no alteration in amount even by the vital process, or

THE MECHANICAL EQUIVALENT OF HEAT.

that the living organism, with all its riddles and marvels, can-
not create heat out of nothing.

But if we hold firm to this physiological axiom, the an-
swer to the question started above is already given. For,
imless we wish to attribute again to the organism the power
of creating heat which has just been denied to it, it cannot be
a,ssumed that the heat which it produces can ever amount to
more than the chemical action which takes place. On the
combustion-theory there is, then, no alternative, short of sa-
crificing the theory itself, but to admit that the total amount
of heat evolved by the organism, partly directly, and partly
indirectly by mechanical action, corresponds quantitatively,
or is equal to the amount of combustion.

Hence it follows, no less inevitably, that the heat ^produced
mechanically by the organism must bear an invariable quantita-
^tive relation to the ivorh expended in producing it.

For if, according to the varying construction of the me-
chanical arrangements which serve for the development of
the heat, the same amount of work, and hence the same
amount of organic combustion, could produce varying quanti-
ties of heat, the quantity of heat produced from one and the
same expenditure of material would come out smaller at one
time and larger at another, which is contrary to our assump-
tion. Further, inasmuch as there is no difference in kind be-
tween the mechanical performances of the animal body and
those of other inorganic sources of work, it follows that

AN INVAKIABLE QUANTITATIVE RELATION BETWEEN HEAT
AND WORK IS A POSTULATE OP THE PHYSIOLOGICAL THEORY
OF COMBUSTION.

While following in general the direction indicated, it was
accordingly needful for me in the end to fix my attention
chiefly on the physical connection subsisting between motion
and heat ; and it was thus impossible for the existence of the
mechanical equivalent of heat to remain hidden from me.
But, although I have to thank an accident for this discovery,

EELATION BETWEEN HEAT AND WORK. 327

it is none the less my own, and I do not hesitate to assert my
right of priority.

In order to ensure what had been thus discovered against
casualties, I put together the most important points in a short
paper which I sent in the spring of 1842 to Liebig, with a
request that he would insert it in the Annalen der Ghemie und
Fharmacie, in the forty-second volume of which, page 233, it
may be found under the title " Bemerkungen tiber die Krafte
der unbelebten Natur."

It was a fortunate circumstance for me that the reception
given to my unpretending work by this man, gifted with so
deep an insight, at once secured for it an entrance into one of
the first scientific organs, and I seize this opportunity of pub-
licly testifying to the great naturalist my gratitude and my
esteem.

Liebig himself, however, had about the same time already
pointed out, in more general but still unmistakable terms, the
connection subsisting between heat and work. In particular,
he asserts that the heat produced mechanically by a steam-
engine is to be attributed solely to the effect of combustion,
which can never receive any increase through the fact of its
producing mechanical effects, and, through these, again devel-
oping heat.

From these, and from similar expressions of other scien-
tific men, we may infer that science has recently entered upon
a direction in which the existence of the mechanical equiva-
lent of heat could not in any case have remained longer un-
perceived.

In the paper to which reference has been made, the nat-
ural law with which we are now concerned is referred back
to a few fundamental conceptions of the human mind. The
proposition that a magnitude, which does not spring from
nothing, cannot be annihilated, is so simple and clear that no
valid argument can be urged against its truth, any more than
against an axiom of geometry ; and until the contrary is

THE MECHANICAL EQITIVALENT OF HEAT.

proved by some fact established beyond a doubt, we may ac-
cept it as true.

Now we are taught by experience, that neither motion nor
heat ever takes its rise except at the expense of some meas-
urable object, and that in innumerable cases motion disap-
pears without any thing except heat making its appearance.
The axiom that we have established leads, then, now to the
conclusion that the motion that disappears becomes heat, or,
in other words, that both objects bear to each other an inva-
riable quantitative relation. The proof of this conclusion by
the method of experiment, the establishment of it in all its
details, the tracing of a complete harmony subsisting between
the laws of thought and the objective world, is the most inter-
esting, but at the same time the most comprehensive problem
that it is possible to find. What I, with feeble powers and
without any external support or encouragement, have effected
in this direction is truly little enough ; but — ultra posse nemo
ohligatus.

In the paper referred to (the first of Mayer's in the pres-
ent volume) I have thus expressed myself with regard to the
genetic connection of heat and moving force :

" If it be now considered as established that in many
^ cases {exceptio confirmat regulam) no other effect of motion
can be traced except heat, and that no other cause than motion
can be found for the heat that is produced, we prefer the as-
sumption that heat proceeds from motion, to the assumption
of a cause without effect and of an effect without a cause —
just as the chemist, instead of allowing oxygen and hydrogen
to disappear without further investigation, and water to be
produced in some inexplicable manner, establishes a connec-
tion between oxygen and hydrogen on the one hand and water
on the other."

From this point there is but one step to be made to the
goal. At page 257 it is said : " The solution of the equa-
tions subsisting between faUing-force [that is, the raising of

CONNECTION OF HEAT AND MOVINa FOECE. 329

weight] and motion requires that the space fallen through in
a given time, e. g. the first second, should be experimentally
determined; in like manner, the solution of the equations
subsisting between falling-force and motion on the one hand
and heat on the other, requires an answer to the question, How
great is the quantity of heat which corresponds to a given
quantity of motion or falling-force ? For instance, we must
ascertain how high a given weight requires to be raised above
the ground in order that its falling-force may be equivalent to
the raising of the temperature of an equal weight of water
from 0° to 1° C. The attempt to show that such an equation
is the expression of a physical truth may be regarded as the
substance of the foregoing remarks.

" By applying the principles that have been set forth to
the relations subsisting between the temperature and the vol-
ume of gases, we find that the sinking of a mercury column
by which a gas is compressed is equivalent to the quantity of
heat set fire by the compression ; and hence it follows, the
ratio between the capacity for heat of air under constant press-
ure and its capacity under constant volume being taken as
== 1*421, that the warming of a given weight of water from
0° to 1° C. corresponds to the fall of an equal weight from
the height of about 365 metres."

It is plain that the expression " equivalent" is here used
in quite a different sense from what it bears in chemistry.
The difference will be shown most distinctly by an example.
"When the same weight of potash is neutralized, first, with
sulphuric acid, then with nitric acid, the numbers which ex-
press the ratio which the absolute weights of these three sub-
stances bear to one another are called their equivalents ; but
there is no thought here either of the quantitative equality or
of the transformation of the bodies in question.

This peculiar signification which the word " equivalent "
has acquired in chemistry, is doubtless connected with the
fact that the chemist has been able to determine the object of

330 THE MECHANICAL EQUIVALENT OF HEAT.

his investigation by a common quantitative standard, their ab-
solute weights. Let us suppose, however, that we could de-
termine one body, for instance water,' only by weight, and
another, water-forming or explosive gas, only by volume, and
that we had agreed to choose one pound as the unit of weight,
and one cubic foot as the unit of volume ; we should then have
to ascertain how many cubic feet of explosive gas could be ob-
tained from one pound of water, and conversely. This num-
ber, without which neither the formation nor the decomposi-
tion of water could be made the subject of calculation, might
then be suitably called " the explosive-gas equivalent of wa-
ter."

In this latter sense a raised weight might, in accordance
with the known laws of mechanics, be called the " equiva-
lent " of the motion resulting from its fall. Now, in order to
compare these two objects, the raised and the moving weight,
which admit of no common measure, we require that. con-
stant number which is generally denoted by g. This number,
however, and the mechanical equivalent of heat, whereby the
relation subsisting between heat and motion is defined, belong
both of them to one and the same category of ideas.

In the paper that I have mentioned it is further shown
how we may arrive at such a conception of force as admits
of being consistently followed to its consequences and is sci-
entifically tenable ; and the importance of this subject induces
me to return to it again here.

The word " force " {Kraft) is used in the higher or scien-
tific mechanics in two distinct senses.

I. On the one hand, it denotes every push or pull, every
effort of an inert body to change its state of rest or of mo-
tion ; and this effort, when it is considered alone and apart
from the result produced, is called " pushing force," " pulling
force," or shortly " force," and also, in order to distinguish
between this and the following conception, " dead force " {vis
mortua).

MEAmNG OF THE TEEM "FORCE." 331

n. On the other hand, the product of the pressure into
the space through which it acts, or, again, the product — or
half-product — of the mass into the square of the velocity, is
named " force." In order that motion may actually occur, it
is in fact necessary that the mass, whatever it may be, should
under the influence of a pressure, and, in the direction of that
pressure, traverse a certain space, "the effective space"
( Wirhungsraum) : and in this case a magnitude which is pro-
portional to the " pushing force " and to the effective space,
likewise receives the name " force ; " but to distinguish it
from the mere pushing force, by which alone motion is never
actually brought about, it is also called[^ the " vis viva of mo-
tion," or " moving force."

With the generic conceiption of " force," the higher me-
chanics, as an essentially analytic science, is not concerned.
In order to arrive at it, we must, according to the general
rule, collect together the characters possessed in common by
the several species. As is well known, the definition so ob-
tained runs thus — " Force is every thing which brings about
or tends to bring about, alters or tends to alter motion."

This definition, however, it is easy to see, is tautological ;
for the last fourteen words of it might be omitted, and the
sense would be still the same.

This erroneous solution is occasioned by the nature of the
problem, which requires an impossibility. Mere pressure
(dead force) and the product of the pressure into the effective
space (living force) are magnitudes too thoroughly unlike to
be by possibility combined into a generic conception. Press-
ure or attraction is, in the theory of motion, what affinity is
in chemistry — an abstract conception : living force, like mat-
ter, is concrete ; and these two kinds of force, however closely
connected in the region of the association of ideas, are in
reality so widely separated that a frame which should take
them both in must be able to include the whole world.

There are several conceivable ways of escaping from the

332 THE MECHAKICAL EQUIVALENT OP HEAT.

difficulty. For instance, just as we speak of absolute weight,
specific weight, and combining weight, without its ever enter-
ing any one's bead to want to construct a generic idea out of
these distinct notions, so two or nxore meanings may be at-
tached to the word force. This is what is actually done in
the higher mechanics, and hence in this branch of science we
meet with no mention of a generic conception of " force."

There has been no lack of recommendations to carry, in
like manner, the notions of " dead " and " living force " as
distinct and separate through the other departments of sci-
ence ; it has, however, been found impossible to put in prac-
tice . such recommendations ; for the use of ambiguous ex-
pressions, which can in no case contribute any thing to clear-
ness, is altogether inadmissible if confusion can possibly arise.
It is true that the mathematician is in no danger of confound-
ing in his calculations a product with one of its factors ; but
in other departments of knowledge a systematic confusion of
ideas exists on this point ; and if any thing is to be done
toward clearing it up, the som'ce of the error must be stopped ;
for if we once recognize two meanings of the word " force,"
it would be the labour of Sisyphus to try to distinguish be-
tween them in each separate case. In order, then, to arrive
at any result, we must make up our minds to do without any
common denomination of the magnitudes mentioned above,
as I. and II., and either to give up the use of the word
" force " altogether, or to employ it for one only of these two
categories.

The notion of force was consistently employed in the lat-
ter sense by Newton. In solving his problems, he decom-
poses the product of the attraction into the effective space
into its two factors, and calls the former by the name " force."

As an objection to this mode of proceeding, it must, how-
ever, be remarked that in many cases it is not possible thus
to decompose the product in question. Let us take, for in-
stance, the following very simple case : a mass M, originally

MEANING OF THE TEEM "fOKCE." 333

at rest, is caused to move with the (uniform final) velocity c ;
from the knowledge of the magnitudes M and c it is certainly
possible to deduce the value of the product of the force (in
Newton's sense) into its effective space, but we are not thereby
enabled to conclude as to the magnitude of this force itself.

As a matter of fact, the necessity soon made itself felt of
treating and naming this product as a whole. It also has
been called " force," and the expressions " vis viva of motion,"
"moving force," " working force," ''horse-power" (or force),
" muscular force," &c., have been long naturalized in science.

However happy we may, in many respects, think the
choice of this word, there is still the objection that a new
meaning has been fixed upon an already existing technical
expression, without the old one having been called in from
circulation at the same time. This formal error has become
a Pandora's box, whence has sprung a Babylonian confusion
of tongues.

Under existing circumstances no choice is left us but to
withdraw the term " force " either from Newton's dead force
or from Leibnitz's living force ; but in either case we come
into conflict with prevailing usage. But if once we have
made up our minds to introduce into our science a logically
accurate use of terms, even at the cost of existing expressions
which have become easy and pleasant to us by long usage, we
cannot long hesitate in the choice we have to make between
the conceptions I. and II.

Let us consider the elementary case of a mass, originally
. at rest, which receives motion : this happens, as has been al-
ready said, by the mass being subjected to a certain push or
pull under the influence of which it traverses a certain space,
the effective space. Now, however, both the velocity and
also the intensity of the push (Newton's force) always vary
at every point of the effective space ; and in order to mul-
tiply these variable magnitudes into effective space, that is,
to deduce the quantity of motion from the intensity of the

334 THE MECHANICAL EQUIVALENT OF HEAT.

pushing force, we must call in the aid of the higher mathe-
matics.

But hence it follows that, except in statics, where the ef-
fective space is nought and the pressure constant, the New-
tonian conception of force is available only in the higher
branches of mechanics ; and it is plainly not advisable so to
choose our conception of " force " that it cannot be consist
ently employed in that branch (namely, the elementary parts
of the theory of motion) which of all others is chiefly con-
cerned with fundamental notions.

It is, however, a totally mistaken method to try to adapt
the -idea of a force, such as gravity, conceived in Newton's
sense, to the elementary parts of science, by leaving out of
consideration one of its most important properties, namely its
dependence on distance, and to make a " force" out of Gali-
leo's gravity thus inexactly and in some relations most incor-
rectly conceived. Some such ideal force (No. III.) seems
to hover before the minds of most writers on natural science
as the original type of a "force of nature."

Such quantitative determinations as hold good only ap-
proximately and under certain conditions ought never to be
employed to establish definitions. In a calculation, it is true,
we may correctly enough take an arc, which is sufficiently
small in comparison with the radius, as equal in size to the
sine or to the tangent ; but if we attempted to use such a rela-
tion in settling first principles, we should lay a foundation for
fallacies and errors.

The Newtonian idea of force, however, transplanted in ,
the manner that is commonly done into the region of element-
ary science, is no whit better than the notion of a straight
curve. Newton's force, or attraction, in sjpecie gravity, ^r, is
equal to the differential quotient of the velocity by the time ;

dc
that is, g=-T' This expression is quite exact, but in order to
ctt

understand and apply it a knowledge of the higher mathemat-

Newton's conception of force. 336

ics is required. On the other hand, it is quite true that, so
long as we have to do only with cases in which the space
fallen through is so small in comparison with the earth's semi-
diameter that it may be disregarded, the equation just given

c
may be abbreviated into the very convenient form 9=1. with-

out any considerable error ; but this expression can never be
mathematically exact so long as the space fallen through has
any calculable magnitude. But on the strength of an equa-
tion thus radically inaccurate, there are planted in the recep-
tive mind of youth such false notions as — ^that gravity is a
uniformly accelerating ( ?) force, a moving ( ?) force whose ac-
tion is proportional to the time ( ?) ; that force is directly pro-
portional ( ?) to the velocity produced ; and many other like
errors.

It would certainly be a great merit if authors of treatises
on physics would help to remedy this state of things, and in
framing their definitions would start only from thoroughly
exact determinations of magnitudes ; for elementary physics
in its present form, instead of being a well-grounded science,
is only a sort of half-knowledge, such that on passing to the
higher and strictly scientific departments the student must
try to forget its principles and theorems as quickly as he can.

If we have once convinced ourselves by unprejudiced
examination that the retention, under that name, of the con-
ception of force distinguished above by I. has nothing but its
origin to recommend it, but much to condemn it, the rest fol-
lows almost spontaneously. It accords with the laws of
thought, as well as with the common usage of language, to
connect every production of motion with an expenditure of
force. Hence " force " is —

Something which is expended in producing motion; and
this something which is expended is to be looked upon as a
cause equivalent to the effect, namely, to the motion pro-
duced.

336 THE MECHANICAL EQUIVALENT OF HEAT.

This definition not only corresponds perfectly with facts,
but it accords as far as possible with that which already ex-
ists ; for, as I shall show, it contains by implication the con-
ception of force as met with in the higher mechanics, and re-
ferred to above by II.

If a mass M, originally at rest, while traversi^g the effect-
ive space s, under the influence and in the direction of the
pressure _p, acquires the velocity c, we have j9s=Mc'*. Since,
however, every production of motion implies the existence of
a pressure (or of a pull) and an effective space, and also the
exhaustion of one at least of these factors, the effective space,
it follows that motion can never come into existence except
at the cost of this product, j9s=:Mc^ And this it is which
for shortness I call " force."

The connection between expenditure and performance (in
other words, the exhaustion of force in producing its effect)
presents itself in the simplest form in the phenomena of grav-
itation. The necessary condition of every falling motion is
that the centre of gravity of the two masses concerned in it
(that is, of the earth and of the falling weight) should ap-
proach each other. But in the case of the falling together of
the two masses, the approach of their centres of gravity
reaches its natural limit, and hence the production of a fall-
ing movement is thus bound up with an expenditure, namely,
with the exhaustion of the given falling-space, and thereby
also of the product of that space into the attraction. The
falling down of a weight upon the earth is a process of me-
chanical combination ; and just as in combustion the capacity
of performance (that is, the condition of the development of
heat) ceases when the act of combination comes to an end, so
also the production of motion ceases when the weight has
fallen to its lowest position. The weight, when lying on the
solid ground, is, like the carbonic acid formed in combustion,
nothing but a caput mortuum. The affinity, whether mechan-
ical or chemical, is still there after the union just as much as

HIGHEST VELOCITY OF A FALLING BODY. 337

before, and opposes a certain resistance to the reduction of
the compound ; but its power of performance (^Leistungs-
fdhigkeit) is at an end as soon as there is no further available
falling-space.

Whenever the attraction becomes indefinitely small, or
ceases altogether, space is no longer effective space ; and thus
it follows, from the diminution which gravity undergoes with
distance, that falling-space is limited in the centrifugal direc-
tion also, and hence that the cause of motion or " force " is,
under all circumstances, a finite magnitude which becomes
exhausted in producing its effect.

This fundamental physical truth will be most easily per-
ceived when applied to a special case and reduced to figures.
When a pound weight is lifted one foot from the ground, the
available force is, as every one knows, =:one foot-pound. K
the falling-height of this weight amounts to n feet, n not be-
ing a large number, the force may be taken as approximately
—n foot-pounds. But supposing n, or the original distance
of the weight from the earth, to be very considerable, or in-
deed infinite, the force (that is, the number of foot-pounds)
does not by any means thereby become infinite, but, according
to Newton's law of gravitation, it becomes at most =^r foot-
pounds, where r is the number of feet contained in the earth's
semidiameter. Thus how great soever the distance through
which a weight falls against the earth, or the time occupied
by its fall may be, it can acquire no higher final velocity than
34,450 Paris feet per second. On the other hand, were the
mass of the earth four times as great as it is, its bulk remain-
ing the same, the force would likewise become four times as
great, and the maximum Yclocity would be 68,900 feet.

It is one of the essentials of a good terminology that it

should put fundamental facts of this kind in a clear light ;

exactly the opposite, however, is done by the nomenclature at

present in use. A few expressions, employed by a very meri-

15

338 THE MECHANICAL EQDTV^ALENT OF HEAT.

torious naturalist in combating my views, may serve to sup-
port this assertion.

" Although," he says, " it is quite true that in nature no
motion can be annihilated, or that, as it is commonly ex-
pressed, the quantity of motion once in existence continues
unceasingly and without any lessening, and although in tliis
sense the character of indestructibility belongs to every prox-
imate cause even, every primary cause, that is, every true
physical force, possesses the additional characteristic of being
inexhaustible. These characteristics will best admit of being
unfolded by the closer consideration of gravity, the most
active and widely diffused of the natural forces (primary
causes) , which, as it were the soul of the world, indestructi-
bly and inexhaustibly upholds the life of those great masses
on whose motions depends the order of the universe, while
requiring no food from without to call forth its ever renewed
activity."

If these words are intended to contain a material contra-
diction of the views I have put forward, they must be meant
to imply that, by virtue of its being inexhaustible, the attract-
ive power of the earth must be capable of imparting to a fall-
ing weight, under certain conceivable circumstances, an infin-
ite velocity. But our author himself in several places lets us
see that he has a (quite well-founded) mistrust of any so de-
cided a conclusion : this is shown in the following, among
other passages : " If we follow up the chain of causes and
effects to its first beginnings, we come at length to the true
forces of nature, to those primary causes whose activity does
not require that they should be preceded by any others, which
ask for no nourishment, but which can ever call forth new
motions, as it were, out of an inexhaustible soil, and can
uphold and quicken those that are already in being."

Again : " If the moon every moment falls, at least vir-
tually, a certain distance toward the earth, what is the force
which the next moment puUs it away again, as it were, in

OBJECTIONS CONSIDERED. 339

order to give rise to a new falling force ? It is precisely its
indestructibility and inexhaustibility, its power at all times
and under all circumstances to bring about without ceasing,
at least virtually, the same eiFects, that is the essence of every
true force or primary cause."

This " as it were " and " at least virtually," which always
slips in at the critical moment, affords room for the suspicion
that our author is himself not quite confident of the power
of his " true natural causes" to give rise to an inexhaustible
amount of motion (of actual exertion of force) ; and the in-
definiteness of these expressions is quite characteristic of the
Protean part which the force of gravity plays in writings on
natural science. The most arbitrary explanations are given
of this word, and then, when facts no- longer admit of any
thing else, a retreat is sought in the Newtonian conception.

Gravity being called a force, and at the same time the
term force being connected, in accordance with the common
use of language, with the conception of an object capable of
producing motion, leads to the false assumption that a me-
chanical effect (the production of motion) can be produced
without a corresponding expenditure of a measurable object ;
and here is likewise plainly the reason why our author could
neither keep clear in his facts nor consistent in his reasoning.
If once the production of motion out of nothing is granted,
the annihilation of motion must also be admitted as a conse-
quence ; and the magnitude of motion must, in accordance
with this assumption, be simply proportional to the velocity,
or —Mc, and the " quantity of motion once in existence "
must be =-]rMc — Mc—O. But notwithstanding his "inex-
haustible forces," the writer referred to expressly declares
that motion is indestructible ; but, instead of stating his opin-
ion as to what becomes of motion which disappears by fric-
tion, he says in another place again that it remains " unde-
cided" whether the effect of a force (the amount of motion
produced by it) ia measured by the first or by the second

340 THE MECHANICAL EQUIVALENT OF HEAT.

power of the velocity (that is, whether it is or is not de-
structible) : he even appears, from repeated expressions, to
hold it possible that a given quantity of heat can produce mo-
tion ad infinitum I If such were the case, it would certainly
be useless to consider the convertibility of these magni-
tudes : the ground would rather have been won for the contact
theory.

The polemics of my respected critic, whom I have here
introduced as the representative and spokesman of prevailing
views, and to whom I feel that my sincere thanks are due for
his attentive examination of my first publication, appear to
me to be necessarily without result, inasmuch as the first
problem in combating my assertions, which all revolve about
the one point of an invariable quantitative relation between
heat and motion, must be to find out that this relation is va-
riable, and in what cases. Formal controversy without a
material basis is only beating the air ; and as to what relates
specially to the questions about force, the first point to con-
sider is, not what sort of thing a "force" is, but to what
thing we shall give the name " force." Backwards and for-
wards talk about gravity is fruitless, since all who understand
the matter are agreed as to its nature ; for gravity is and re-
mains a differential quotient of the velocity by the time, di-
rectly proportional to the attracting mass, and inversely pro-
portional to the square of the distance : on this point a final
decision was come to long ago. But whether it is expedient
to call this magnitude a force is quite another question.

Since, whenever an innovation of essential importance is
proposed, the public is so ready to misapprehend, I will here
state once more, as clearly as I can, my reasons for saving
that " the force of gravity " is an improper expression.

It is an unassailable truth that the production of every
falling motion is connected with a corresponding expenditure
of a measurable magnitude. This magnitude, if it is to be
made an object of scientific investigation (and why should it

APPLICATION OF THE TEEM " FORCE." 341

not?), must have a name given to it ; and in accordance with
the logical instinct of man, as manifested in the genius of lan-
guage, no other name can be here chosen than the word
" force." But since this expression is already used in a quite
different sense, we might be tempted to create for the concep-
tion which is as yet — in the fundamental parts of science at
least — ^unnamed an entirely new name. But before betaking
ourselves to this extreme course, which for reasons that are
not far to seek would be the one whereby we should be
brought most into conflict with existing usage, it is reasonable
to inquire whether the word " force," which' in itself answers
so well to the requirements of the case, is in its right place
where it was first put by the schools.

According to the common custom of speech, we under-
stand by " force " something moving — a cause of motion ; and
if, on the one hand, the expression "moving force" is for
this reason, strictly speaking, a pleonasm, the notion of a not
moving or " dead" force is, on the other hand, a contradidio
in adjecto. If it be said, for instance, that a load which
presses with its weight on the ground exerts thereby a force —
a force which, though never so great, is unable of itself to
bring about the smallest movement — the mode of conception
and of, expression is quite justified by scholastic usage, but it
is so far-fetched that it becomes the source of unnumbered
misapprehensions.

Between gravity and the force of gravity there is, so far
as I know, no difference ; and hence I consider the second
expression unscientific, inasmuch as it is tautological.

Let it not be objected that the " force " of pressure, the
" force " of gravity, cohesive " force," &c., are the higher
causes of pressure, gravity, and the like. The exact sciences
are concerned with phenomena and measurable quantities.
The first cause of things is Deity — a Being ever inscrutible
by the intellect of man ; while " higher causes," " supersen-
8U0US forces," and the rest, with all their consequences, be-

342 THE MECHANICAL EQUIVALENT OF HEAT.

long to the delusive middle region of naturalistic philosophy
and mysticism.

By a law that is universally true, waste and want go hand
in hand. If to the case before us, where this rule likewise
meets with confirmation, we apply an equalizing process, and
take away the word " force" from the connection in which it
is superfluous and hurtful, and bring it to where we are in
want of it, we get rid at one time of two important obstacles.
The higher mathematics at once cease to be required in order
to gain admittance into the theory of motion : nature presents
herself in simple beauty before the astonished eye, and even
the less gifted may now behold many things which hitherto
were concealed from the most learned philosophers.

Force and matter are indestructible objects. This law, to
which individual facts may most simply be referred, and
which therefore I might figuratively call the heliocentric stand-
point, constitutes a natural basis for physics, chemistry, phys-
iology, and philosophy.

Among the facts which, though known, have been hith-
erto only empirically established and have remained isolated,
but which can be easily referred to this natural law, is the one
that electric and magnetic srtlraction cannot be isolated any
more than gravity, or that the strength of this attraction
undergoes no alteration, so long as the distance remains the
same, by the intervening of indifferent substances (non-con-
ductors).

Among facts which have remained unknown up to the
most recent times, I will refer only to the influence which the
ebb and flow of the tide exerts, in accordance with the known
laws of mechanics, on the motion of the earth about its axis.
A fact of such importance, standing, as it does, in close rela-
tion with the fundamental law just stated, having been able
to escape the attention of naturalists, is of itself a proof that
the prevailing system has no exclusive title.

For the rest, it will not have escaped those who are ac-

MOTION AND FALLING-FOKCE. 34:3

quainted with the modern literature of science that a modifi-
cation of scientific language in the sense of my views is act-
ually beginning to take place. But in matters of this kind
the chief part of the work must be left to time.

Accordinoc to what has been said thus far, the vis viva of
motion must be called a force. But since the expression vis
viva denotes in mechanics, not only a magnitude which is
proportional to the mass and to the square of its velocity, but
also one which is proportional to the mass and to the height
from which it has fallen, force thus conceived naturally di-
vides itself into two very easily distinguished species, each of
which requires a distinct technical name, for which the words
motion (Bewegung) and falling-force (Fallkraft) seem to me
the most appropriate.*

Hence, according to this definition, " motion " is always
measured by the product of the moved mass into the square
of the velocity, never by the product of the mass into the
velocity.

By " falling-force" we understand a raised weight, or still
more generally, a distance in space between two ponderable

[* The distinction here drawn between " motion " and " falling-force "
is the same as that made by Helmholtz (^J)ie Eriialiung der Kraft^ 1847)
between " vis viva " {lebendige Kraft) and " tension " {Spankraft). The Eng-
lish expressions " dynamical energy " and " statical energy " were used by
Pro£ W. Thomson (Phil. Mag. S. vol. iv. p. 304:, 1852) in the same sense,
but were afterwards abandoned by him in favour of the terms " actual en-
ergy" and "potential energy " introduced by Prof. Rankine. More re-
cently (" Good Words " for October, 1862) Professors Thomson and Tait
have employed the expression "kinetic energy" in place of " actual ener-
gy." The German word Kraft in the text has been uniformly translated
force, to which term the ambiguity of the German original has thus been
transferred. This ambiguity, however, may be avoided in EngUsh by al-
lowing the word " force " to retain the meaning which it bears in common
language, that is, to denote all resistances which it requires the exertion of
a power to overcome (whence the expressions gravitating force, cohesive
force, &c.), and by using the word " energy " to denote force as defined by
Mayer.— G. C. R]

34:4 THE MECHAlflCAL EQUIVALENT OF HEAT.

bodies. In many cases falling-force is measured with suffi-
cient accuracy by the product of the raised weight into its
height ; and the expressions " foot-pound," " kilogramme-
metre," " horse-power," and many others, are conventional
units for the measurement of this force, which have of late
come into general use, especially in practical mechanics. But
in order to find the exact quantitative expression for the mag-
nitude in question, we must consider (at least) two masses
existing at a determinate distance from each other, which ac-
quire motion by mutually approaching ; and we must investi-
gate the relation which exists between the conditions of the
motion, namely, the magnitude of the masses and their orig-
inal and final distance, and the amount of motion produced.

It very remarkably happens that this relation is the sim-
plest conceivable ; for, according to Newton's law of gravita-
tion, the quantity of motion produced is directly proportional
to the masses and to the space through which they fall, but
inversely proportional to the distances of the centres of grav-
ity of the masses before and after the movement. That is,
if A and B are the two masses, c and c' the velocities which
they respectively acquire, and h and h! their original and final
distances apart, we have

or in words, the falling-force is equal to the product of the
masses into the space fallen through divided hy the two distances.
By help of this theorem, which, as will be easily seen, is
nothing but a more general and convenient expression of
Newton's law of gravitation,* the laws of the fall of bodies

* Newton's formula relates to the particular case in which the two dis-
tances (the initial and the final distance) are equal, so that their product
becomes a square. In this case, however, both the space fallen through
and the velocity become nought ; and hence, when this expression has to

OONVEBSIONS OF MOTION AND FALLINO-FORCE. 345

from cosmical elevations, and also the general laws of central
motions, can be developed without its being needful to employ
equations of more than the second degree.

Having now become acquainted with two species of force —
motion and falling-force — we can arrive at a conception of " a
force " in general, according to the well-known rule, by col-
lecting together the common characteristics of the two spe-
cies. To this end, we must consider the properties of these
objects somewhat more closely. Their most important prop-
erty depends on their mutual relation. Whenever a given
quantity of falling force disappears, motion is produced ; and
by the expenditure of this latter, the falling-force can be re-
produced in its original amount.

This constant proportion which exists between falling-
force and motion, and is knovm in the higher mechanics un-
der the name of " the principle of the conservation of vis
viva" may be shortly and fitly denoted by the term " trans-
formation " ( Umwandlung) . For instance, we may say that
a planet, in passing from its aphelion to its perihelion, trans-
forms a part of its falling-force into motion, and, as it moves
away from the sun again, changes a part of its motion into
falling-force. In using the word "transform" in this sense,
nothing else can or is intended to be expressed but a constant
numerical ratio.

But it follows from the axiom mentioned at page 326, that
the production of a definite quantity of motion from a given
quantity of falling-force, and vice versd, implies that neither
falling-force nor motion can be annihilated either totally or in
part. We thus obtain the following definition :

Forces are transformable, indestructible, and (in contradis-

be taken as the starting-point for the calculation of real velocities, mathe-
matical artifices become necessary which are inadmissible in the elementary
branches of science.

15*

346 THE MECHANICAL EQUIVALENT OF HEAT.

tinction from matter) imponderahle ohjeds, (Conf. paper al-
ready quoted, pp. 328, 329.)

It is easy to see that this definition embraces, among other
things, the fact that the motion which disappears in mechani-
cal processes of different kinds bears a constant relation to
the heat thereby produced, or that motion is convertible, as
an indestructible magnitude, into heat. Thus heat is, like mo-
tion, a force ; and motion, like heat, an imponderable.

I have characterized the relation which various forces
bear to one another by saying (Phil. Mag. S. 4, vol. xxiv. p.
252) that they are " different forms under which one and the
same object makes its appearance." At the same time I
have expressly guarded myself from making the certainly
plausible, but unproved, and, as it seems to me, hazardous de-
duction that thermal phenomena are to be regarded as merely
phenomena of motion. The following is what I said upon
this point (loc. cit,) p. 376 :

" But just as little as the connection between falling-force
and motion authorizes the conclusion that the essence of fall-
ing-force is motion, can such a conclusion be adopted in the
case of heat. We are, on the contrary, rather inclined to
infer that before it can become heat, motion — whether simple,
or vibratory as in the case of light and radiant heat, &c. —
must cease to exist as motion."

The relation which, as we have seen, subsists between
heat and motion has regard to quantity, not to quality ; for
(to bojrow the words of Euclid) things which are equal to
one another are not therefore similar. Let us beware of leav-
ing the solid ground of the objective, if we would not entan-
gle ourselves in difficulties of our own making.

In the mean time it at least results from the foregoing
considerations that the phenomena of heat, electricity, and
magnetism do not owe their existence to any particular fluids ;
and the immateriality of heat, asserted half a century ago by

VARIOUS KINDS OF HEAT. 347

Rumford, becomes, through the discovery of its mechanical
equivalent, a certainty.

The form of force denoted by the name "heat" is plainly
not single, but includes several distinct, though mutually
equivalent, objects, three principal forms of which are distin-
guished in common language : namely, I. Radiant Heat ; II.
Free (sensible) Heat, Specific Heat ; and III. Latent Heat.

There can be no doubt that radiant heat must be regarded
as a phenomenon of motion, especially since the recent detec-
tion of phenomena of interference in the radiation of heat.
But whether there really exists, as is commonly assumed, a
peculiar aether, of which the vibratory motion is perceived by
us as radiant heat, or whether the seat of this motion is the
particles of material bodies, is a question that is not yet made
out.

Still greater obscurity hangs about the essential nature of
specific heat, or what goes on in the interior of a heated body.
Not only does the unanswered question of the aether enter
again here, but, before we can be in a position to form any
clear ideas on this subject, we require to have an exact knowl-
edge of the internal constitution of matter. We are, how-
ever, still far from having reached this point ; for, in particu-
lar, we do not know whether such things as atoms exist — that
is, whether matter consists of such constituents as undergo no
further change of form in chemical processes.

But a span of that time which stretches both backwards
and forwards into eternity is meted out to man here on earth,
and the space which his foot can tread is narrowly bounded
above and below : so also his scientific knowledge finds nat-
ural limits in the direction of the infinitely small as well as of
the infinitely great. The question of atoms seems to me to
lead beyond these limits, and hence I consider it unpractical.
An atom in itself can no more become an object of our inves-
tigation than a differential, notwithstanding that the ratio
which such immensely small auxiliary magnitudes bear to

348 THE MECHANICAL EQUIVALENT OP HEAT.

one another may be represented by concrete numbers. In
every case, however, the conception of an atom must be re-
garded as merely relative, and must be considered in connec-
tion with some definite process ; for, as is well known, the
particles of an acid and base may play the part of atoms in
the formation and decomposition of a salt, while in another
process these atoms may themselves undergo further division.
But assuming that, in a chemical sense, atoms have a real
existence — an assumption which, among other things, the
laws of isomorphism certainly render probable — ^the further
question arises whether, by the continued division of matter,
we can at last arrive at molecules which are atoms in relation
to the phenomena of heat, such that heat cannot penetrate to
their interior, and such that, when the whole mass is heated,
they for their parts undergo no increase of bulk. But since
we are unable to grapple with such preliminary questions as

hese, we are forced to confess that, whether the existence of
«n aether and of atoms be admitted or not, we are, so far as

?gards the nature of specific heat, in a state of ignorance.
The expression "latent heat" has reference to its correctly

©cognized property of indestructibility. In all cases in which

iiermometrically sensible specific heat disappears, it must be
assumed that it eludes our perception only by taking on some
other state of existence, and that by an appropriate process
of inverse transformation the free heat can be reproduced in its
original amount. These are the facts on which the doctrine
of latent heat rests ; and hence, if we have regard to them
only, all the connected phenomena may be claimed as so many
confirmations of the principle of the transformation and con-
servation of force.

The conception of latent heat is accordingly nothing else
than the conception of something equivalent to free heat, and
thus the doctrine of free and specific heat embraces pretty
nearly the whole domain of physics. A few examples, cho-
sen from among the abundance of facts, may serve to show

HOW LATENT HEAT IS TO BE KEGAEDED. 34:9

how, according to my view, the phenomena wherein heat be-
comes latent are to be regarded.

If heat is communicated to a gas retained under constant
pressure, the free heat of the gas is increased, and at the
same time a calculable quantity of heat becomes latent; the
gas is thereby caused to expand, and there is consequently
produced an amount of vis viva proportional to the pressure
and to the space through which expansion takes place. There-
fore as soon as we know how much of the heat that has be-
come latent is to be attributed to the expansion of the gas, we
know also the amount of the remainder of the latent heat
corresponding to the vis viva produced. Now Gay-Lussac
has proved by experiment that the specific heat of a gas un-
dergoes no sensible alteration in flowing from a containing ves-
sel into a vacuum. Hence it follows that a gaseous body op-
poses no perceptible resistance to the separation of its parti-
cles, and that the rarefaction of a gas does not of itself (that
is, when it occurs without any evolution of force) cause any
heat to become latent. The total quantity of heat which be-
comes latent by the expansion of a gas is therefore to be taken
' as the equivalent of the vis viva produced.

It results from the principle of the indestructibility of
heat — a principle which no one calls in question — ^that the
quantity of heat which has thus become latent must again
become free when heat is in any way produced at the expense
of the acquired vis viva of motion. Motion is latent heat,
and heat is latent motion.

The celebrated law of Dulong, that the amount of heat
produced by the compression of a gas is dependent on the
amount of force expended, and not upon the chemical nature,
tension, or temperature of the gas, is a special application of
the above general principle. But in the communication so
often mentioned I have shown that this law of nature is capa-
ble of a very much wider application, and that the heat which
becomes latent in the expansion of a gas reappears again in

350 THE MECHANICAL EQUIVALENT OP HEAT.

every case, if the vis viva thereby produced is employed to
generate heat, whether by the compression of air, by friction,
or by the impact of nonelastic bodies ; and I have there
calculated the mechanical equivalent of heat upon principles
of which the accuracy cannot be disputed. I also measured
at that time, by way of control, the heat produced in the
manufacture of paper in Holland, and compared it with the
working force expended, and so found a sufficient degree of
concordance between the two quantities. I have recently,
moreover, succeeded in constructing, for the purpose of the
direct determination of the mechanical equivalent of heat, a
very simple thermal dynamometer on a small scale, with
which the truth of the principle in question can be demon-
strated ad oculos ; and I have reason to believe that the effi-
ciency of water-wheels and steam-engines might be easily and
advantageously measured by means of a similar calorimoto-
rial apparatus. It must, however, be left to the future judg-
ment of practical men to decide whether, and to what extent,
this method deserves to be preferred to Prony's.

Heat further becomes latent in certain changes of the state
of aggregation of bodies. Since it is a settled fact that both
solid and liquid bodies oppose a certain resistance to the sep-
aration of their parts, and since in general an expenditure of
vis viva is required for the overcoming of mechanical resist-
ances, we are led to conclude d priori that whenever the cohe-
sion of a body is diminished or done away with, force or heat
must become latent ; and this, as is well known, perfectly
accords with experience.

Starting from this point of view, the French physicist
Person has attempted to detect a direct quantitative relation
between the latent heat of metals, on which he has made a
great number of observations, and their cohesion ; but at pres-
ent determinations of this kind are beset with almost insur-
mountable difficulties.

The heat which becomes latent in the evaporation of water

HEAT AND CHANGES OF STATE. 351

has been considered from quite a similar point of view by
Holtzmann in his important memoir "On the Heat and Elas-
ticity of Gases and Vapours." Starting from the principle
that elevation of temperature is equivalent to the raising of a
weight, this philosopher has likewise calculated the mechani-
cal equivalent of heat from the quantity of heat which be-
comes latent by the expansion of a gas ; and he very rightly
conceives of the latent heat of steam as made up of two
parts, whereof one, the smaller, is expended in overcoming
the opposing pressure of the atmosphere, and can hence be
easily calculated by means of the mechanical equivalent of
heat, while the remaining part, the amount of which can also
be calculated, is what Holtzmann calls the heat required to
destroy the cohesion of the water. In all steam-engines this
latter portion is wasted, and Holtzmann calculates from these
data the superior efficiency of high-pressure compared with
low-pressure engines.*

If the view here taken of the latent heat of fusion and
evaporation is correct, heat must also become latent when
hard bodies are reduced to powder ; and when such substances
pass into the liquid condition from a state of fine division,
they must absorb a smaller quantity of heat than when they
are liquefied without previous comminution. A few experi-
ments that I have instituted in this direction have not hitherto
given any decisive result.

It is also worthy of notice that certain solid bodies which
are capable of assuming allotropic states, as, for instance, the
oxygen-compounds of iron, evolve a considerable quantity of
heat on passing from a less to a more hard condition. Such
facts, the number of which will doubtless continually increase
with time, agree perfectly with the above principle, that dim-
inution of cohesion involves an expenditure of heat, and, on
the other hand, increase of cohesion a production of heat.

* The engines which give the greatest useful effect must be those in
which the steam receives an addition of heat during its expansion.

352 THE MECHANICAL EQUIVALENT OF HEAT.

Customary language, according to which gravity is called
a moving force and heat a substance, occasions, on the one
hand, the significance of an important natural object, falling-
space, or the space through which a body falls, to be kept as
much as possible out of sight, and, on the other hand, heat to
be removed to the greatest possible distance from the vis viva
of motion. The sciences are thus reduced to an artificial sys-
tem, over whose fissured surface we can advance in safety
only by the powerful aid of the higher analysis.

Without doubt the fact that so simple and obvious a mat-
ter as the connection between heat and motion could remain
unperceived up to the most recent times must also be attrib-
uted to the same defect. Nevertheless, as has been already
pointed out, the quantitative determination of chemical heat-
ing-effects and of galvanic actions, as well as researches into
vital phenomena, instituted in the spirit of those of Liebig,
must soon have led to the law, not difficult to discover, of the
equivalence of heat and motion.

In reality this law and its numerical expression, the me-
chanical equivalent of heat, were published almost simulta-
neously in Germany and in England.

Starting from the fact that the amount of chemical as well
as of galvanic effect is dependent only and solely on the
amount of material expenditure, the celebrated English phys-
icist Joule was led to the principle that the phenomena of
motion and of heat rest essentially upon one and the same
foundation, or, as he expressed himself, in the same way as I
have done, heat and motion are transformable one into the
other.

Not only did this philosopher indisputably make an inde-
pendent discovery of the natural law in question, but to him
belongs the credit of having made numerous and important
contributions towards its further establishment and develop-
ment. Joule has shown that when motion is produced by
means of electro-magnetism, the heating effect of the galvanic

DISCOVEEIES OF JOULE. 353

current is diminished in a corresponding and fixed proportion.
He Las further ascertained that by reversing the poles of a
magnetic bar a quantity of heat is produced proportional to
the square of the magnetic tension — a fact which was also
discovered by myself, though at a later date. In particular,
Joule has likewise demonstrated, by means of numerous ex-
periments, that the heat evolved by friction under various cir-
cumstances stands in an unvarying proportion to the amount
of force expended. According to his most recent experiments
of this kind, he has fixed the mechanical equivalent of heat
at 423.*

Joule has likewise investigated experimentally, in relation
to this question, the thermal behaviour of elastic fluids when
expanded, and has thereby confirmed the earlier results of
other physicists.

The new subject soon began to excite the attention of
learned men ; but inasmuch as both at home and abroad the
subject has been exclusively treated as a foreign discovery, I
find myself compelled to make the claims to which priority en-
titles me ; for although the few investigations which I have
given to the public, and which have almost disappeared in the
flood of communications which every day sends forth without
leaving a trace behind, prove, by the very form of their pub-
lication, that I am not one who hankers after effect, it is not
therefore to be assumed that I am willing to be deprived of
intellectual property which documentary evidence proves to
be mine.

By help of the mechanical equivalent of heat many prob-
lems can be solved which, without it, could not be attacked at
all : among them, the calculation of the thermal effect of the
falling together of cosmical masses may be especially men-
tioned. It will not be out of place to indicate here briefly a
few results of such calculations.

* That is, 1 thermal unit =» 423 kilogrammetres.

354 THE MECHANICAL EQUIVALENT OF HEAT.

The following is one problem of this kind. It is assumed
that a cosmical body enters the atmosphere of our earth with
a velocity of four geographical miles per second, and that, in
consequence of the resistance which it here encounters, it
loses so much of its vis viva of motion that its remaining ve-
locity when it again quits the atmosphere amounts to three
miles : the question now arises. How great is the thermal
effect which accompanies this process ?

A simple calculation, based upon the mechanical equiva-
lent of heat, shows that the quantity of heat required is about
eight times as great as the heat of combustion of a mass of
coal of equal weight with the body in question, one kilo-
gramme of coal being taken as yielding 6,000 thermal units.
Hence it follows that the velocity of the motion of shooting-
stars and fire-balls, which, as is well-known, attains, accord-
ing to astronomical observations, to from four to eight miles,
is a cause fully sufficient to produce the most violent evolution
of heat, and an insight into the nature of these remarkable
phenomena is thereby afforded to us.*

The following is a problem of a similar kind : if two cos-
mical masses, moving in space about theii* common centre of
gravity, were by any cause whatever, for example by the re-
sistance of the surrounding medium, caused to fall together,
the question again arises, Ho'w great is the thermal effect cor-
responding to this process of mechanical combination ?

Even though the elements of the orbits (that is, their ex-
centricity) may be unknown, we can nevertheless calculate
from the given weight and volume of the masses in question
the maximum and the minimum of the required effect. Thus
let it be supposed, for the sake of an example, that our earth
had been divided into two equal globes which had united in

* The idea that the meteors here referred to owe their light to a me-
chanical process — whether friction, or the compression of the air — ^is not
new ; but without a knowledge of the mechanical equivalent of heat it
could have no scientific foundation.

COLLISION OF COSMICAL MASSES. 355

the manner described : calculation teaches us that the amount
of heat which would have been evolved in such a case would
considerably exceed that which an equal weight of matter
could furnish by the most intense process of chemical action.

It is more than probable that the earth has come into ex-
istence in some such way, and that in consequence our sun, as
seen from the distance of the fixed stars, exhibited at that
epoch a transient burst of light. But what took place in our
solar system perhaps millions of years ago, still goes on at
the present time here and there among the fixed stars ; and
the transient appearance of stars, which in some cases, like
the celebrated star of Tycho Brahe, have at first an extraor-
dinary degree of brilliance, may be satisfactorily explained
by assuming the falling together of previously invisible double
stars.

Contrasting with such explosive bursts of light is the
steady radiation, shown continuously through enormous pe-
riods, by the greater number of fixed stars, and among them
by our sun. Do these appearances, which in so special a
manner tempt to higher speculations, constitute a real excep-
tion to the exhaustion of a cause in producing its efiect, which,
in accordance with the foregoing considerations, we have re-
garded as an established law of Nature ? or does the small
sum of human knowledge authorize us in supposing that here
also there is an equivalence between performance and expen-
diture, and in searching for the conditions of that equivalent?

To enter further upon this subject would lead us beyond
the intended scope of this publication ; and I therefore close
in the hope that the reader will please to supplement by his
own reflection much that in this tract has been left unsaid.

I

SOME THOUGHTS

ON THE

CONSEEYATION^ OF FOECE.

By Dr. FARADAY.

Michael Faraday, son of a smith, was bom in London in lYOl. He
was taught reading, writing, and arithmetic at a day-school, and in all other
things educated himself. At thirteen he was apprenticed to a bookbinder,
choosing this vocation in order to be among books. He was early fond of ex-
periment, and averse to trade ; and being taken to hear some lectures of Sir
Hmnphrey Davy at the Royal Institution, he resolved to pursue science, and
wrote to Davy asking his assistance in obtaining a place. Davy favored his
application, and in 1813, at the age of twenty, he was appointed assistant in
the laboratory of the Royal Institution. In 1820 he discovered the chloride
of carbon, and in 1823 effected the condensation of chlorine and other gases.
On this account Davy became jealous of him, and discouraged the idea of
recommending him for election to the Royal Society, which, however, took
place in 1824. In 1820, Oersted announced his celebrated discovery of
electro-magnetism, and Faraday at once entered upon an investigation of the
relations of magnetism and electricity. In 1831 he commenced his cele-
brated series of Experimental Researches in Electricity, which extended to
three volumes, pubUshed in 1839, 1844, and 1855. In 1827 he published
his admirable work on "Chemical Manipulations," and, in 1830, a valuable
paper on " The Manufacture of Glass for Optical Purposes." In 1833 he
became Professor of Chemistry in the Royal Institution, and he has received
numerous honors from the learned societies of Europe. In 1835 he received
a pension of £300 a year, and in 1858 the Queen allotted him a residence in
Hampton Court. Dr. Faraday has talents of a high order, both as an orig-
inal investigator and as a lecturer. Advanced in years, he has now retired to
a considerable extent from active duty, but is still in the vigor of his powers,
as is shown by his recent lectures to juvenile audiences in the Royal Insti-
tution.

THE CONSERVATION OF FORCE.

YARIOUS circumstances induce me at the present mo-
ment to put forth a consideration regarding the con-
servation of force. I do not suppose that I can utter any
.truth respecting it that has not already presented itself to the
high and piercing intellects which move within the exalted
regions of science ; but the course of my own investigations
and views makes me think that the consideration may be of
service to those persevering labourers (amongst whom I en-
deavour to class myself) who, occupied in the comparison of
physical ideas with fundamental principles, and' continually
sustaining and aiding themselves by experiment and observa-
tion, delight to labour for the advance of natural knowledge,
and strive to follow it into undiscovered regions.

There is no question which lies closer to the root of all
physical knowledge than that which inquires whether force
can be destroyed or not. The progress of the strict science
of modem times has tended more and more to produce the
conviction that "force can neither be created nor destroyed ;"
and to render daily more manifest the value of the knowledge
of that truth in experimental research. To admit, indeed,
that force may be destructible or can altogether disappear,
would be to admit that matter could be uncreated ; for we
know matter only by its forces ; and though one of these is

360 THE CONSERVATION OF FORCE.

most commonly referred to, namely, gravity, to prove its
presence, it is not because gravity has any pretension, or any
exemption, amongst the forms of force as regards the princi-
ple of conservation, but simply that being, as far as we per-
ceive, inconvertible in its nature and unchangeable in its man-
ifestation, it offers an unchanging test of the matter which we
recognize by it.

Agreeing with those who admit the conservation of force
to be a principle in physics, as large and sure as that of the
indestructibility of matter, or the invariability of gravity, I
think that no particular idea of force has a right to unlimited
or unqualified acceptance that does not include assent to it ;
and also, to definite amount and definite disposition of the
force, either in one effect or another, for these are necessary
consequences ; therefore I urge, that the conservation of force
ought to be admitted as a physical principle in all our hypoth-
eses, whether partial or general, regarding the actions of mat-
ter. I have had doubts in my own mind whether the consid-
erations I am about to advance are not rather metaphysical
than physical. I am unable to define what is metaphysical in
physical science ; and am exceedingly adverse to the easy and
unconsidered admission of one supposition upon another, sug-
gested as they often are by very imperfect induction from a
small number of facts, or by a very imperfect observation of
the facts themselves ; but, on the other hand, I think the phi-
losopher may be bold in his application of principles which
have been developed by close inquiry, have stood through
much investigation, and continually increase in force. For
instance, time is growing up daily into importance as an ele-
ment in the exercise of force. The earth moves in its orbit
in time ; the crust of the earth moves in time ; light mo-s^s in
time ; an electro-magnet requires time for its charge by an
electric current ; to inquire, therefore, whether power, acting
either at sensible or insensible distances, always acts in time,
is not to be metaphysical ; if it acts in time and across space,

ACTION OF FORCES IN TIME. 361

it must act by physical lines of force ; and our view of the
nature of the force may be affected to the extremest degree
by the conclusions which experiment and observation on time
may supply ; being, perhaps, finally determinable only by
them. To inquire after the possible time in which gravita-
ting, magnetic, or electric force is exerted, is no more meta-
physical than to mark the times of the hands of a clock in
their progress ; or that of the temple of Serapis and its ascents
and descents ; or the periods of the occultations of Jupiter's
satellites ; or that in which the light from them comes to the
earth. Again, in some of the known cases of action in time,
something happens whilst the time is passing which did not
happen before, and does not continue after ; it is, therefore,
not metaphysical to expect an effect in every case, or to en-
deavour to discover its existence and determine its nature.
So in regard to the principle of the conservation of force ; I
do not think that to admit it, and its consequences, whatever
they may be, is to be metaphysical ; on the contrary, if that
word have any application to physics, then I think that any
hypothesis, whether of heat, or electricity, oi* gravitation, or
any other form of force, which either willingly or unwillingly
dispenses with the principle of conservation, is more liable to
the charge than those which, by including it, become so far
more strict and precise.

Supposing that the truth of the principle of the conserva-
tion of force is assented to, I come to its uses. No hypothesis
should be admitted, nor any assertion of a fact credited, that
denies the principle. No view should be inconsistent or in-
compatible with it. Many of our hypotheses in the present
state of science may not comprehend it, and may be unable
to suggest its consequences ; but none should oppose or con-
tradict it.

K the principle be admitted, we perceive at once that a
theory or definition, though it may not contradict the princi-
ple, cannot be accepted as sufiicient or complete unless the
16

THE CONSERVATION OF FORCE.

former be contained in it ; that however well or perfectly the
definition may include and represent the state of things com-
monly considered under it, that state or result is only partial,
and must not be accepted as exhausting the power or being
the full equivalent, and therefore cannot be considered as
representing its whole nature ; that, indeed, it may express only
a very small part of the whole, only a residual phenomenon,
and hence give us but little indication of the full natural truth.
Allowing the principle its force, we ought, in every hypothe-
sis, either to account for its consequences by saying what the
changes are when force of a given kind apparently disappears,
as when ice thaws, or else should leave space for the idea of
the conversion. If any hypothesis, more or less trustworthy
on other accounts, is insufficient in expressing it or incompat-
ible with it, the place of deficiency or opposition should be
marked as the most important for examination, for there lies
the hope of a discovery of new laws or a new condition of
force. The deficiency should never be accepted as satisfac-
tory, but be remembered and used as a stimulant to further
inquiry ; for conversions of force may here be hoped for.
Suppositions may be accepted for the time, provided they are
not in contradiction with the principle. Even an increased or
diminished capacity is better than nothing at all, because such
a supposition, if made, must be consistent with the nature of
the original hypothesis, and may, therefore, by the application
of experiment, be converted into a further test of probable
truth. The case of a force simply removed or suspended,
without a transferred exertion in some other direction, appears
to me to be absolutely impossible.

If the principle be accepted as true, we have a right to
pursue it to its consequences, no matter what they may be.
It is, indeed, a duty to do so. A theory may be perfection,
as far as it goes, but a consideration going beyond it, is not
for that reason to be shut out. We might as well accept our
limited horizon as the limits of the world. No magnitude,

RELATION OF GRAVITY.

either of the phenomena or of the results to be dealt with,
should stop our exertions to ascertain, by the use of the prin-
ciple, that something remains to be discovered, and to trace
in what direction that discovery may lie.

I will endeavour to illustrate some of the points which
have been urged, by reference, in the first instance, to a case
of power, which has long had great attractions for me, be-
cause of its extreme simplicity, its promising nature, its uni-
versal presence, and in its invariability under like circum-
stances ; on which, though I have experimented* and as yet
failed, I think experiment would be well bestowed, I mean the
force of gravitation. I believe I represent the received idea
of the gravitating force aright in saying that it is a simple at'
tractive force exerted between, any two or all the particles or
masses of matter., at every sensible distance, but with a strength
varying inversely as the square of the distance. The usual
idea of the force implies direct action at a distance ; and such
a view appears to present little difiiculty except to Newton,
and a few, including myself, who in that respect may be of
like mind with him.

This idea of gravity appears to me to ignore entirely the
principle of the conservation of force ; and by the terms of
its definition, if taken in an absolute sense, " varying inversely
as the square of the distance," to be in direct opposition to it,
and it becomes my duty now to point out where this contra-
diction occurs, and to use it in illustration of the principle of
conservation. Assume two particles of matter, A and B, in
free space, and a force in each or in both by which they gravi-
tate towards each other, the force being unalterable for an
unchanging distance, but varying inversely as the square of
the distance when the latter varies. Then, at the distance of
ten, the force may be estimated as one ; whilst at the distance
of one, that is, one-tenth of the former, the force will be one

* PhUosophical Transactions, 1851, p. 1.

364 THE CONSEEVATION OF FORCE.

hundred ; and if we suppose an elastic spring to be intro-
duced between the two as a measure of the attractive force,
the power compressing it will be a hundred times as much in
the latter case as in the former. But from whence can this
enormous increase of power come ? If we say that it is the
character of this force, and content ourselves with that as a
sufficient answer, then it appears to me we admit a creation
of power and that to an enormous amount ; yet by a change
of condition, so small and simple as to fail in leading the least
instructed mind to think that it can be a sufficient cause, we
should admit a result which would equal the highest act our
minds can appreciate of the working of infinite power upon
matter ; we should let loose the highest law in physical sci-
ence which our faculties permit us to perceive, namely, the
conservation of force. Suppose the two particles, A and B,
removed back to the greater distance of ten, then the force of
attraction would be only a hundredth part of that they pre-
viously possessed ; this, according to the statement that the
force varies inversely as the square of the distance, would
double the strangeness of the above results ; it would be an
annihilation of force — an effect equal in its infinity and its
consequences with creation, and only within the power of Him
who has created.

We have a right to view gravitation under every form that
either its definition or its effects can suggest to the mind ; it
is our privilege to do so with every force in nature ; and it is
only by so doing that we have succeeded, to a large extent, in
relating the various forms of power, so as to derive one from
another, and thereby obtain confirmatory evidence of the
great principle of the conservation of force. Then let us
consider the two particles, A and B, as attracting each other
by the force of gravitation, under another view. According
to the definition, the force depends upon both particles, and if
the particle A or B were by itself, it could not gravitate, that
is, it could have no attraction, no force of gravity. Suppos-

THE CASE OF GRAVITATING PARTICLES. 365

ing A to exist in that isolated state and without gravitating
force, and then B placed in relation to it, gravitation comes
on, as is supposed, on the part of both. Now, without try-
ing to imagine liow B, which had no gravitating force, can
raise up gravitating force in A ; and how A, equally without
force beforehand, can raise up force in B, still, to imagine it
as a fact done, is to admit a creation of force in both parti-
cles ; and so to bring ourselves within the impossible conse-
quences which have been already referred to.

It may be said we cannot have an idea of one particle by
itself, and so the reasoning fails. For my part I can compre-
hend a particle by itself just as easily as many particles ; and
though I cannot conceive the relation of a lone particle to
gravitation, according to the limited view which is at present
taken of that force, I can conceive its relation to something
which causes gravitation, and with which, whether the parti-
cle is alone, or one of a imiverse of other particles, it is al-
ways related. But the reasoning upon a lone particle does
not fail ; for as the particles can be separated, we can easily
conceive of the particle B being removed to an infinite dis-
tance from A, and then the power in A will be infinitely di-
minished. Such removal of B will be as if it were annihi-
lated in regard to A, and the force in A wiU be annihilated
at the same time ; so that the case of a lone particle and that
where different instances only are considered become one, be-
ing identical with each other in their consequences. And as
removal of B to an infinite distance is as regards A annihila-
tion of B, so removal to the smallest degree is, in principle,
the same thing with displacement through infinite space ; the
smallest increase in distance involves annihilation of power ;
the annihilation of the second particle, so as to have A alone,
involves no other consequence in relation to gravity ; there is
difference in degree, but no difference in the character of the
result.

It seems hardly necessary to observe, that the same line

366 THE CONSERVATION OP FORCE.

of thouglit grows up in the mind, if we consider the mutual
gravitating action of one particle and many. The particle A
will attract the particle B at the distance of a mile with a
certain degree of force ; it will attract a particle C at the
same distance of a mile with a power equal to that by which
it attracts B ; if mjnriads of like particles be placed at the
given distance of a mile, A will attract each with equal force ;
and if other particles be accumulated round it, within and
without the sphere of two miles diameter, it will attract them
all with a force varying inversely with the square of the dis-
tance. How are we to conceive of this force growing up in
A to a million-fold or more, and if the surrounding particles
be then removed, of its diminution in an equal degree ? Or,
how are we to look upon the power raised up in all these
outer particles by the action of A on them, or by their action
one on another, without admitting, according to the limited
definition of gravitation, the facile generation and annihilation
of force?

The assumption which we make for the time with regard
to the nature of a power (as gravity, heat, etc.), and the
form of words in which we express it, that is, its definition,
should be consistent with the fundamental principles of force
generally. The conservation of force is a fundamental prin-
ciple ; hence the assumption with regard to a particular form
of force ought to imply what becomes of the force when its
action is increased or diminished, or its direction changed ; or
else the assumption should admit that it is deficient on that
point, being only half competent to represent the force ; and,
in any case, should not be opposed to the principle of conser-
vation. The usual definition of gravity as an attractive force
between the particles of matter varying inversely as the square
of the distance, whilst it stands as a full definition of the
power, is inconsistent with the principle of the conservation
of force. If we accept the principle, such a definition must
be an imperfect account of the whole of the force, and is

GKAVITATION BUT PARTIALLY rNDEESTOOD. 367

probably only a description of one exercise of that power,
whatever the nature of the force itself may be. If the defi-
nition be accepted as tacitly including the conservation of
force, then it ought to admit that consequences must occur
during the suspended or diminished degree in its power as
gravitation, equal in importance to the power suspended or
hidden ; being in fact equivalent to that diminution. It ought
also to admit, that it is incompetent to suggest or deal with
any of the consequences of that changed part or condition of
the force, and cannot tell whether they depend on, or are re-
lated to, conditions external or internal to the gravitating par-
ticle ; and, as it appears to me, can say neither yes nor no to
any of the arguments or probabilities belonging to the subject.

If the definition denies the occurrence of such contingent
results, it seems to me to be unphilosophical ; if it simply ig-
nores them, I think it is imperfect and insufficient ; if it ad-
mits these things, or any part of them, then it prepares the
natural philosopher to look for effects and conditions as yet
unknown, and is open to any degree of development of the
consequences and relations of power ; by denying, it opposes
a dogmatic barrier to improvement ; by ignoring, it becomes
in many respects an inert thing, often much in the way ; by
admitting, it rises to the dignity of a stimulus to investigation,
a pilot to human science.

The principle of the conservation of force would lead us
to assume, that when A and B attract each other less, be-
cause of increasing distance, then some other exertion of
power, either within or without them, is proportionately grow-
ing up ; and again, that when their distance is diminished, as
firom ten to one, the power of attraction, now increased a
hundred-fold, has been produced out of some other form of
power which has been equivalently reduced. This enlarged
assumption of the nature of gravity is not more metaphysical
than the half assumption ; and is, I believe, more philosophi-
cal and more in accordance with all physical considerations.

368 THE CONSEEVATION OF FORCE.

The half assumption is, in my view of the matter, more dog-
matic and irrational than the whole, because it leaves it to be
understood that power can be created and destroyed almost at
pleasure.

When the equivalents of the various forms of force, as far
as they are known, are considered, their differences appear
very great ; thus, a grain of water is known to have electric
relations equivalent to a very powerful flash of lightning. It
may therefore be supposed that a very large apparent amount
of the force causing the phenomena of gravitation, may be
the equivalent of a very small change in some unknown con-
dition of the bodies, whose attraction is varying by change of
distance. For my own part, many considerations urge my
mind toward the idea of a cause of gravity, which is not res-
ident in the particles of matter merely, but constantly in
them, and all space. I have already put forth considerations
regarding gravity which partake of this idea,* and it seems
to have been unhesitatingly accepted by Newton.f

There is one wonderftd condition of matter, perhaps its
only true indication, namely, inertia ; but m relation to the
ordinary definition of gravity, it only adds to the difficulty.
For if we consider two particles of matter at a certain dis-
tance apart, attracting each other under the power of gravity,
and free to approach, they will approach ; and when at only
half the distance, each will have had stored up in it, because of
its inertia, a certain amount of mechanical force. This must

* Proceedings of the Royal Institution, 1855, vol. ii., p. 10, etc.

f " That gravity should be innate, inherent, and essential to matter, so
that one body may act upon another at a distance, through a vacuum, with-
out the mediation of any thing else, by and through which their action and
force may be conveyed from one to another, is to me so great an absurdity
that I believe no man who has in philosophical matters a competent fac-
ulty of thinking, can ever fall into it. Gravity must be caused by an agent,
acting constantly according to certain laws ; but whether this agent be ma-
terial or immaterial I have left to the consideration of my reader." — See
Newton^ s Third Letter to BefnUey.

INEETIA, GBAVrrY, AND CONSERVATION.

369

be due to the force exerted, and, if the conservation principle
be true, must have consumed an equivalent proportion of the
cause of attraction ; and yet, according to the definition of
gravity, the attractive force is not diminished thereby, but
increased four-fold, the force growing up within itself the
more rapidly, the more it is occupied in producing other force.
On the other hand, if mechanical force from without be used
to separate the particles to twice their distance, this force is
not stored up in momentum or by inertia, but disappears ; and
three-fourths of the attractive force at the first distance disap-
pears with it. How can this be ?

We know not the physical condition or action from which
inertia results ; but inertia is always a pure case of the con-
servation of force. It has a strict relation to gravity, as ap-
pears by the proportionate amount of the force which gravity
can communicate to the inert body ; but it appears to have
the same strict relation to other forces acting at a distance as
those of magnetism or electricity, when they are so applied
by the tangential balance as to act independent of the gTavi-
tating force. It has the like strict relation to force communi-
cated by impact, pull, or in any other way. It enables a
body to take up and conserve a given amount of force until
Ihat force is transferred to other bodies, or changed into an
equivalent 'of some other form ; that is all that we perceive
in it ; and we cannot find a more striking instance amongst
natural, or possible phenomena, of the necessity of the con-
servation of force as a law of nature ; or one more in con-
trast with the assumed variable condition of the gravitating
force supposed to reside in the particles of matter

Even gravity itself furnishes the strictest proof of the con-
servation of force in this, that its power is unchangeable for
the same distance ; and is by that in striking contrast with
the variation which we assume in regard to the cause of grav-
ity, to account for the results at different distances.

It will not be imagined for a moment that I am opposed
16*

370 THE CONSEEVATION OF FORCE.

to what may be called the law of gravitating action^ that is,
the law by which all the known effects of gravity are gov-
erned ; what I am considering is the definition of the force of
gravitation. That the result of one exercise of a power may
be inversely as the square of the distance, I believe and ad-
mit ; and I know that it is so in the case of gravity, and has
been verified to an extent that could hardly have been within
the conception even of Newton himself when he gave utter-
ance to the law ; but that the totality of a force can be em-
ployed according to that law I do not believe, either in rela-
tion to gravitation, or electricity, or magnetism, or any other
supposed form of power.

I might have drawn reasons for urging a continual recol-
lection of, and reference to, the principle of the conservation
of force from other forms of power than that of gravitation ;
but I think that when founded on gravitating phenomena,
they appear in their greatest simplicity ; and precisely for this
reason, that gravitation has not yet been connected by any
degree of convertibility with the other forms of force. If I
refer for a few minutes to these other forms, it is only to
point in their variations, to the proofs of the value of the
principle laid down, the consistency of the known phenomena
with it, and the suggestions of research and discovery which
arise from it. Heai, for instance, is a mighty form of power,
and its effects have been greatly developed ; therefore, assump-
tions regarding its nature become useful and necessary, and
philosophers try to define it. The most probable assumption
is, that it is a motion of the particles of matter ; but a view,
at one time very popular, is, that it consists of a particular
fluid of heat. Whether it be viewed in one way or the other,
the principle of conservation is admitted, I believe, with all its
force. When transferred from one portion to another portion
of like matter, the full amount of heat appears. When trans-
ferred to matter of another kind an apparent excess or defi-
ciency often results ; the word " capacity" is then introduced,

USES OF THE PEINCIPLE. 371

which, while it acknowledges the principle of conservation,
leaves space for research. When employed in changing the
state of bodies, the appearance and disappearance of the heat
is provided for consistently by the assumption of enlarged or
diminished motion, or else space is left by the term " capa-
city" for the partial views which remain to be developed.
When converted into mechanical force, in the steam or air
engine, and so brought into direct contact with gravity, being
then easily placed in relation to it, still the conservation of
force is fully respected and wonderfully sustained. The con-
stant amount of heat developed in the whole of a voltaic cur-
rent described by M. P. Favre,* and the present state of the
knowledge of thermo-electricity, are again fine, partial, or
subordinate illustrations of the principles of conservation.
Even when rendered radiant, and for the time giving no trace
or signs of ordinary heat action, the assumptions regarding
its nature have provided for the belief in the conservation of
force, by admitting either that it throws the ether into an
equivalent state, in sustaining which for the time the power
is engaged ; or else, that the motion of the particles of heat
is employed altogether in their OAvn transit from place to
place.

It is true that heat often becomes evident or insensible in
a manner unknown to us ; and we have a right to ask what
is happening when the heat disappears in one part, as of the
thermo-voltaic current, and appears in another ; or when it
enlarges or changes the state of bodies ; or what would hap-
pen, if the heat being presented, such changes were purposely
opposed. We have a right to ask these questions, but not to
ignore or deny the conservation of force ; and one of the
highest uses of the principle is to suggest such inquiries. Ex-
plications of similar points are continually produced, and will
be most abundant from the hands of those who, not desiring

* Comtes Rendus 1854, vol. xxxix., p. 1212.

872 THE COKSERVATION OF FORCE.

to ease their labour by forgetting the principle, are ready to
admit it, either tacitly, or, better still, effectively, being then
continually guided by it. Such philosophers believe that heat
must do its equivalent of work ; that if in doing work it
seem to disappear, it is still producing its equivalent effect,
though often in a manner partially or totally unknown ; and
that if it give rise to another form of force (as we imperfectly
express it), that force is equivalent in power to the heat which
has disappeared.

What is called chemical attraction affords equally instruc-
tive and suggestive considerations in relation to the principle
of the conservation of force. The indestructibility of indi-
vidual matter is one case, and a most important one, of the
conservation of chemical force. A molecule has been en-
dowed with powers which give rise in it to various qualities,
and these never change, either in their nature or amount. A
particle of oxygen is ever a particle of oxygen — nothing can
in the least wear it. If it enters into combination and disap-
pears as oxygen — ^if it pass through a thousand combinations,
animal, vegetable, mineral — if it lie hid for a thousand years
and then be evolved, it is oxygen with its first qualities, nei-
ther more nor less. It has all its original force, and only
that ; the amount of force which it disengaged when hiding
itself has again to be employed in a reverse direction when it
is set at liberty ; and if, hereafter, we should decompose oxy-
gen, and find it compounded of other particles, we should only
increase the strength of the proof of the conservation of
force, for we should have a right to say of these particles,
long as they have been hidden, all that we could say of the
oxygen itself.

Again, the body of facts included in the theory of definite
proportions, witnesses to the truth of the conservation of
force ; and though we know little of the cause of the change
of properties of the acting and produced bodies, or how the
forces of the former are hid amongst those of the latter, we

1

CHEMICAL ACTION AT A DISTANCE. 373

do not for an instant doubt the conservation, but are moved
to look for the manner in which the forces are, for the time,
disposed, or if they have taken up another form of force, to
search what that form may be.

Even chemical action at a distance, which is in such an-
tithetical contrast with the ordinary exertion of chemical affin-
ity, since it can produce effects miles away from the particles
on which they depend, and which are effectual only by forces
acting at insensible distances, still proves the same thing, the
conservation of force. Preparations can be made for a chem-
ical action in the simple voltaic circuit, but until the circuit
be complete that action does not occur ; yet in completing we
can so arrange the circuit, that a distant chemical action, the
perfect equivalent of the dominant chemical action, shall be
produced; and this result, whilst it establishes the electro-
chemical equivalent of power, establishes the principle of the
conservation of force also, and at the same time suggests
many collateral inquiries which have yet to be made and
answered, before all that concerns the conservation in this
case can be understood.

This and other instances of chemical action at a distance
carry our inquiring thoughts on from the facts to the physical
mode of the exertion of force ; for the qualities which seem
located and fixed to certain particles of matter appear at a
distance in connection with particles altogether different.
They also lead our thoughts to the conversion of one form of
power into another ; as, for instance, in the heat which the
elements of a voltaic pile may either show at the place where
thej act by their combustion or combination together, or in
the distance, where the electric spark may be rendered mani-
fest ; or in the wire of fluids of the different parts of the
circuit.

When we occupy ourselves with the dual forms of power,
electricity, and magnetism, we find great latitude of assump-
tion, and necessarily so, for the powers become more and

374: THE CONSEEVATION OF FORCE.

more complicated in their conditions. But still there is no
apparent desire to let loose the force of the principle of con-
servation, even in those cases where the appearance and dis-
appearance of force may seem most evident and striking.
Electricity appears when there is consumption of no other
force than that required for friction ; we do not know how,
but we search to know, not being willing to admit that the
electric force can arise out of nothing. The two electricities
are developed in equal proportions ; and having appeared, we
may dispose variously of the influence of one upon successive
portions of the other, causing many changes in relation, yet
never able to make the sum of the force of one kind in the
least degree exceed or come short of the sum of the other.
In that necessity of equality, we see another direct proof of
the conservation of force, in the midst of a thousand changes
that require to be developed in their principles before we
can consider this part of science as even moderately known
to us.

One assumption with regard to electricity is, that there is
an electric fluid rendered evident by excitement in plus and
minus proportions. Another assumption is, that there are
two fluids of electricity, each particle of each repelling all
particles like itself, and attracting all particles of the other
kind always, and with a force proportionate to the inverse
square of the distance, being so far analogous to the defini-
tion of gravity. This hypothesis is antagonistic to the law
of the conservation of force, and open to all the objections
that have been, or may be, made against the ordinary defini-
tion of gravity. Another assumption is, that each particle of
the two electricities has a given amount of power, and can
only attract contrary particles with the sum of that amount,
acting upon each of two with only half the power it could in
like circumstances exert upon one. But various as are the
assumptions, the conservation of force (though wanting in the
second) is, I think, intended to be included in all. I might

OPENING OF NEW PROBLEMS. 375

repeat the same observations nearly in regard to magnetism —
whether to be assumed as a fluid, or two fluids or electric cur-
rents— whether the external action be supposed to be action
at a distance, or dependent on an external condition and
lines of force — still, all are intended to admit the conserva-
tion of power as a principle to which the phenomena are
subject.

The principles of physical knowledge are now so far de-
veloped as to enable us not merely to define or describe the
hnown^ but to state reasonable expectations regarding the
unknown ; and I think the principle of the conservation of
force may greatly aid experimental philosophers in that duty
to science, which consists in the enunciation of problems to
be solved. It will lead us, in any case where the force re-
maining unchanged in form is altered in direction only, to
look for the new disposition of the force ; as in the cases of
magnetism, static electricity, and perhaps gravity, and to as-
certain that as a whole it remains unchanged in amount — or,
if the original force disappear, either altogether or in part, it
will lead us to look for the new condition or form of force
which should result, and to develop its equivalency to the
force that has disappeared. Likewise, when force is devel-
oped, it will cause us to consider the previously-existing equiv-
alent to the force so appearing ; and many such cases there
are in chemical action. When force disappears, as in the
electric or magnetic induction after more or less discharge, or
that of gravity with an increasing distance, it will suggest a
research as to whether the equivalent change is one within
the apparently acting bodies, or one external (in part) to
them. It will also raise up inquiry as to the nature of the
internal or external state, both before the change and after.
If supposed to be external, it will suggest the necessity of a
physical process, by which the power is communicated from
body to body ; and in the case of external action, will lead
to the inquiry whether, in any case, there can be truly action

376 THE CONSERVATION OF FOEOE.

at a distance, or whether the ether, or some other medium, is
not necessarily present.

"We are not permitted as yet to see the nature of the source
of physical power, but we are allowed to see much of the
consistency existing amongst the various forms in which it is
presented to us. Thus, if, in static electricity, we consider
an act of induction, we can perceive the consistency of all
other like acts of induction with it. If we then take an elec-
tric current, and compare it with this inductive effect, we see
their relation and consistency. In the same manner we have
arrived at a knowledge of the consistency of magnetism with
electricity, and also of chemical action and of heat with all
the former ; and if we see not the consistency between gravi-
tation with any of these forms of force, I am strongly of the
mind that it is because of our ignorance only. How imper-
fect would our idea of an electric current now be, if we were
to leave out of sight its origin, its static and dynamic induc-
tion, its magnetic influence, its chemical and heating effects ;
or our idea of any one of these results, if we left any of the
others unregarded ? That there should be a power of gravita-
tion existing by itself, having no relation to the other natural
powers, and no respect to the law of the conservation of force,
is as little likely as that there should be a principle of levity
as well as of gravity. Gravity may be only the residual part
of the other forces of nature, as Mositi has tried to show ;
but that it should fall out from the law of all other force, and
should be outside the reach either of further experiment or
philosophical conclusions, is not probable. So we must strive
"to learn more of this outstanding power, and endeavour to
avoid any definition of it which is incompatible with the prin-
ciples of force generally, for all the phenomena of nature lead
us to believe that the great and governing law is one. I
would much rather incline to believe that bodies affecting
each other by gravitation act by lines of force of definite
amount (somewhat in the manner of magnetic or electric in*

MENTAL QUALIFICATIONS FOR THE INQUIRY. 377

duction, though with polarity), or by an ether pervading all
parts of space, than admit that the conservation of force
could be dispensed with.

It may be supposed, that one who has little or no mathe-
matical knowledge should hardly assume a right to judge of
the generality and force of a principle such as that which
forms the subject of these remarks. My apology is this : I
do not perceive that a mathematical mind, simply as such, has
any advantage over an equally acute mind not mathematical,
in perceiving the nature and power of a natural principle of
action. It cannot of itself introduce • the knowledge of any
new principle. Dealing with any and every amount of static
electricity, the mathematical mind has balanced and adjusted
them with wonderful advantage, and has foretold results
which the experimentalist can do no more than verify. But
it could not discover dynamic-electricity, nor electro-magnet>
ism, nor magneto-electricity, or even suggest them ; though
when once discovered by the experimentalist, it can take them
up with extreme facility.

So in respect of the force of gravitation, it has calculated
the results of the power in such a wonderful manner as to
trace the known planets through their courses and perturba-
tions, and in so doing has discovered a planet before unknown ;
but there may be results of the gravitating force of other
kinds than attraction inversely as the square of the distance,
of which it knows nothing, can discover nothing, and can
neither assert nor deny their possibility or occurrence. Under
these circumstances, a principle which may be accepted as
equally strict with mathematical knowledge, comprehensible
without it, applicable by all in their philosophical logic, what-
ever form that may take, and above aU, suggestive, encour-
aging, and instructive to the mind of the experimentalist,
should be the more earnestly employed and the more fre-
quently resorted to when we are labouring either to discover
new regions of science, or to map out and develop those

378 THE CONSERVATION OF FORCE.

which are known into one harmonious whole ; and if in such
strivings, we, whilst applying the principle of conservation,
see but imperfectly, still we should endeavour to see, for even
an obscure and distorted vision is better than none. Let us,
if we can, discover a new thing in any shape ; the true ap-
pearance and character will be easily developed afterwards.

Some are much surprised that I should, as they think,
venture to oppose the conclusions of Newton ; but here there
is a mistake. I do not oppose Newton on any point ; it is
rather those who sustain the idea of action at a distance, that
contradict him. Doubtful as I ought to be of myself, I am
certainly very glad to feel that my convictions are in accord-
ance with his conclusions. At the same time, those who oc-
cupy themselves with such matters ought not to depend alto-
gether upon authority, but should find reason within them-
selves, after careful thought and consideration, to use and
abide by their own judgment. Newton himself, whilst refer-
ring to those who were judging his views, speaks of such as
are competent to form an opinion in such matters, and makes
a strong distinction between them and those who were incom-
petent for the case.

But after all, the principle of the conservation of force
may by some be denied. Well, then, if it be unfounded even
in its application to the smallest part of the science of force,
the proof must be within our reach, for all physical science
is so. In that case, discoveries as large or larger than any
yet made, may be anticipated. I do not resist the search for
them, for no one can do harm, but only good, who works
with an earnest and truthful spirit in such a direction. But
let us not admit the destruction or creation of force without
clear and constant proof. Just as the chemist owes all the
perfection of his science to his dependence on the certainty
of gravitation applied by the balance, so may the physical
philosopher expect to find the greatest security and the utmost
aid in the principle of the conservation of foice. All that

SUPPLEMENTARY CONSIDEEATIONS. 379

we have that is good and safe, as the steam-engine, the elec-
tric-telegraph, &c., witness to that principle — it would require
a perpetual motion, a fire without heat, heat without a source,
action without reaction, cause with effect, or effect without a
cause, to displace it from its rank as a law of nature.

During the year that has passed since the publication of
the foregoing views regarding gravitation, &c., I have come to
the knowledge of various observations upon them, some
adverse, others favourable : these have given me no reason to
change my own mode of viewing the subject ; but some of
them make me think that I have not stated the matter with
sufficient precision. The word " force" is understood by
many to mean simply " the tendency of a body to pass from
one place to another," which is equivalent, I suppose, to the
phrase " mechanical force ; " those who so restrain its mean-
ing must have found my argument very obscure. What I
mean by the word " force," is the cause of a physical action ;
the source or sources of all possible changes amongst the
particles or materials of the universe.

It seems to me that the idea of the conservation of force is
absolutely independent of any notion we may form of the
nature of force or its varieties, and is as sure and may be as
firmly held in the mind, as if we, instead of being very
ignorant, understood perfectly every point about the cause of
force and the varied effects it can produce. There may be
perfectly distinct and separate causes of what are called
chemical actions, or electrical actions, or gravitating actions,
constituting so many forces ; but if the " conservation of
force " is a good and true principle, each of these forces must
be subject to it : none can vary in its absolute amount ; each
must be definite at all times, whether for a particle, or for all
the particles in the universe ; and the sum also of the three

380 THE CONSERVATION OF FORCE.

forces must be equally unchangeable. Or, there may be but
one cause for these three sets of actions, and in place of three
forces we may really have but one, convertible in its manifes-
tations ; then the proportions between one set of actions and
another, as the chemical and the electrical, may become very
variable, so as to be utterly inconsistent with the idea of the
conservation of two separate forces (the electrical and
the. chemical), but perfectly consistent with the conservation
of a force, being the common cause of the two or more sets
of action.

It is perfectly true that we cannot always trace a force by
its actions, though we admit its conservation. Oxygen and
hydrogen may remain mixed for years without showing any
signs of chemical activity ; they may be made at any given
instant to exhibit active results, and then assume a new
state, in which again they appear as passive bodies. Now,
though we cannot clearly explain what the chemical force is
doing, that is to say, what are its effects during the three
periods before, at, and after the active combination,' and only
by very vague assumption can approach to a feeble concep-
tion of its respective states, yet we do not suppose the creation
of a new portion of force for the active moment of time, or
the less believe that the forces belonging to the oxygen and
hydrogen exist unchanged in their amount at all these periods,
though varying in their results. A part may at the active
moment be thrown off as mechanical force, a part as radiant
force, a part disposed of we know not how ; but believing, by
the principle of conservation, that it is not increased or
destroyed, our thoughts are directed to search out what
at all and every period it is doing, and how it is to be
recognized and measured. A problem, founded on the
physical truth of nature, is stated, and, being stated, is on the
way to its solution.

Those who admit the possibility of the common origin of
all physical force, and also acknowledge the principle of con-

SUPREMACY OF THE PEINCIPLE. 381

servation, apply that principle to the sum total of the force.
Though the amount of mechanical force (using habitual lan-
guage for convenience sake) may remain unchanged and
definite in its character for a long time, yet when, as in the
collision of two equal inelastic bodies, it appears to be lost,
they find it in the form of heat ; and whether they admit that
heat to be a continued mechanical action (as is most proba-
ble), or assume some other idea, as that of electricity, or
action of a heat-fluid, still they hold to the principle of con-
servation by admitting that the sum of force, i, e, of the
" cause of action," is the same, whatever character the effects
assume. "With them the convertibility of heat, electricity,
magnetism, chemical action and motion, is a familiar thought ;
neither can I perceive any reason why they should be led to
exclude, a priori, the cause of gravitation from association
with the cause of these other phaenomena respectively. All
that they are limited by in their various investigations,
whatever directions they may take, is the necessity of mak-
ing no assumption directly contradictory of the conservation
of force applied to the sum of all the forces concerned, and to
endeavour to discover the different directions in which the
various parts of the total force have been exerted.

Those who admit separate forces inter-unchangeable,
have to show that each of these forces is separately subject
to the principle of conservation. If gravitation be such a
separate force, and yet its power in the action of two par-
ticles be supposed to be diminished fourfold by doubling the
distance, surely some new action, having true gravitation
character, and that alone, ought to appear, for how else can
the totality of the force remain unchanged? To define the
force as " a simple attractive force exerted between any two
or all the particles of matter, with a strength varying in-
versely as the square of the distance," is not to answer the
question ; nor does it indicate or even assume what are the
other complementary results which occur ; or allow the sup-

382 THE CONSERVATION OF FORCE.

position that such are necessary : it is simply, as it appears
to me, to deny the conservation of force.

As to the gravitating force, I do not presume to say that I
have the least idea of what occurs in two particles when their
power of mutually approaching each other is changed by
their being placed at different distances ; but I have a strong
conviction, through the influence on my mind of the doctrine
of conservation, that there is a change ; and that the phe-
nomena resulting from the change will probably appear some
day as the result of careful research. If it be said that
" 'twere to consider too curiously to consider so," then I
must dissent ; to refrain to consider would be to ignore the
principle of the conservation of force, and to stop the inquiry
which it suggests — ^whereas to admit the proper logical force
of the principle in our hypotheses and considerations, and to
permit its guidance in a cautious yet courageous course of in-
vestigation, may give us power to enlarge the generalities we
already possess in respect of heat, motion, electricity, mag-
netism, &c., to associate gravity with them, and perhaps
enable us to know whether the essential force of gravitation
(and other attractions) is internal or external as respects the
attracted bodies.

Returning once more to the definition of the gravitating
power as " a simple attractive force exerted hetiveen any two or
all the particles or masses of matter at every sensible dist&nce^
hut with a strength yarying- inversely as the square of the
distance" I ought perhaps to suppose there are many who
accept this as a true and sufficient description of the force, and
who therefore , in relation to it, deny the principle of conser-
vation. If both are accepted and are thought to be consist-
ent with each other, it cannot be difficult to add words
which shall make "varying strength" and "conservation"
agree together. It cannot be said that the definition merely
applies to the effects of gravitation as far as we know them.
So understood, it would form no barrier to progress ; for,

UEFINinON OF THE GRAVITATmG POWER. 383

that particles at different distances are urged toward each
other with a power varying inversely as the square of the
distance, is a truth : but the definition has not that mean-
ing ; and what I object to is the pretence of knowledge
which the definition sets up when it assumes to describe, not
the partial effects of the force, but the nature of the force
as a whole.

OK

THE COIWECTIOlSr AND EQUIVALENCE
OF FOECES

By Prof. J. VON LIEBIG.

17

Justus von Liebig, born at Darmstadt in 1803, after spending ten ■
months in an apothecary's shop, entered the University of Bonn in 1819, f|
and afterwards graduated in medicine in Erlangen. In 1822 he went to
Paris, where he studied chemistry two years. In 1824 he read a paper on
the Fulminates before the French Institute, which attracted the attention of
Humboldt, by whose influence he was appointed adjunct Professor of Chem-
istry in the University of Geissen. He became professor of this institution
in 1826, and established here the first laboratory in Germany for teaching
practical chemistry. In 1840 he published his " Chemistry in its apphca-
tions to Agriculture and Physiology," in the form of a report to the British
Association. In 1842 he reported to the same body his work on "Animal
Chemistry." About the same time appeared his "Familiar Letters on
Chemistry," which has since been rewritten and much extended. He is the
author also of various other valuable works. He remained at Geissen till
1852, when he became professor and president of the laboratory in the
University of Munich. In 1854 his friends in Europe contributed and pre-
sented to him £1,000, and in 1860 he became President of the Academy of
Sciences in Munich. Professor Liebig is a bold and intrepid investigator,
and an ardent writer, who has made a profound impression upon his age.
While some of his views have not been accepted in the chemical world, and
indeed have been abandoned by himself, others have taken their place as
valuable additions to the body of scientific truth. The charge that some of
his doctrines have proved erroneous does not disturb him ; in the true scien-
tific spirit he replies, " Show me the man who makes no mistakes, and I will
show you a man who has done nothing."

THE CONNECTION AND EQUIVALENCE
OF FOKCES.

IT is well known that our machines create no power, but
only return what they have received. The motion of a
clock is produced by a weight or a spring ; but it is the power
of the human arm appL'ed to stretch the spring or elevate the
weight, which is expended in the movement of the wheels and
pendulum in twenty-four hours, or in eight or fourteen days.

A water-wheel sets in motion, in a mill, one or more mill-
stones ; in a foundry, one or more hammers ; in saltworks
or mines it pumps or raises weights to certain heights ; in
factories, it communicates movements to looms, spinning ma-
chines, and rollers. In all these instances, the work per-
formed by the water-Wheel is due to the force exerted by the
falling water on the buckets, which sets the wheel in motion ;
and this force must be greater than the resistance presented
by the different machines in operation. The performance of
the machine is measurable by this force.

The work of a steam-engine is executed by the movement
of a piston upwards and downwards by the pressure of steam,
just as a water-wheel is moved by the pressure of water.
The cause of this pressure is heat, which is derived from the
chemical process of combustion, and is absorbed by water.
By this heat, steam is produced, and the necessary expansion

388 THE CONNECTION AND EQUIVALENCE OF FORCES.

obtained for the movement of the piston. It is heat, in this
last form, which performs the mechanical work of the ma-
chine.

Every force acts by producing pressure either from or tow-
ards the centre of motion. In every machine in operation
the amount of power is always measurable by the resistance
overcome ; and this again can be expressed by corresponding
weights, which that power is capable of raising to a certain
height. If one man raises by a pump, in one minute, 150 lbs.
of water, and another 200 lbs. ; or if one horse draws to a
certain distance a load of 20 cwt., and another a load of 30
cwt., it is evident that these numbers express the relative
working power of these two men or horses. In mechanics,
the working power of every machine is expressed in horse
power, that is, a force capable of elevating in each second 75
kilogrammes (=2^ lbs. avoirdupois) to a height of one meter
(39-37 inches).

The whole power commimicated to a machine is not actu-
ally available, but is in part lost by friction. For if two
machines possess the same power, it is found that the greater
quantity of work wiU be executed by the one which has to
overcome the smaller amount of friction. In mechanics,
friction is always regarded as acting in direct opposition to
motion in every machine. It was believed that the working
power of a machine could be absolutely annihilated by it.

As the proximate cause of the cessation of motion, friction
was a palpable fact, and could as such be taken into account ;
but a fatal error was committed in giving a theoretical view
of its mode of action. For if a power could be annihilated,
or, in other words, have nothing as its eftect, then there would
be no contradiction involved in the belief, that out of nothing
also power could be created. To this erroneous idea we may
partly trace the belief, held for centuries by most able men, in
the possibility of discovering a machine which should renew
within itself its own power as it was expended, and thus ever

389

continue in motion, without the necessity for any external
motive force. The discovery of such a perpetual motion was,
indeed, worthy of every effort. It would be as valuable as
the bird which lays the golden eggs ; for by its means labour
would be performed, and money made in abundance without
any expenditure.

A mass of facts, hitherto unintelligible, have had much
light thrown upon them by a more correct view of natural
forces, for which we are indebted to a physician, Dr. Mayer,
of Heilbronn, and which, by the investigations of the most
eminent natural philosophers and mathematicians, has attained
a significance and importance scarcely to be foreseen.

According to Dr. Mayer, forces are causes, in which full
application of the axiom must be found, that every cause
must produce an effect which corresponds and is equal to the
cause. Causa cequat effectum. Thus if a cause C produces
an effect E, then C = E. Should the effect E become the
cause of another effect e, then also E = e = C. In such a
chain of causes and effects no link, or part of a link, can ever
become nothing = nothing. Should a given cause C have pro
duced its corresponding effect E, then C ceases to exist, for
it has been converted into E. Consequently, as C passes into
E, and the latter into e, it follows that all these causes, as far
as relates to their quantity^ possess the property of indestructv-
hility, and to their quality that of convertibility. In number-
less cases we see a motion cease, without its usual effects
being produced, such as lifting a weight or load ; but as the
force which has caused the motion cannot be reduced to
nothing, the question arises, what form has it assumed. Ex-
perience gives the answer, by showing that wherever motion
is arrested by friction, a blow, or concussion, heat is the re-
sult. The motion is the cause of the heat.

The rapid friction of two plates of metal can raise their
t^emperature to redness, and cause the ebullition of water if
the friction takes place below its surface. In like manne.r.

390 THE CONNECTION AND EQUIVALENCE OF FORCES.

by rapid motion tiie iron tires of carriage wheels become fre-
quently so hot that they cannot be touched. In grinding
needle-points the steel is heated to redness, and the detached
particles bum with sparks. The wooden breaks of railway
carriages become frequently so hot by friction that their sur-
face emits an empyreumatic odour. By the friction of an
iron grater, particles of white sugar can be melted and heated
so far as to acquire the taste of burnt sugar (Caramel). The
heat evolved by the friction of two pieces of ice is sufficient to
melt them.

In the English steel-foundries a bar of steel 10 or 12
inches long, by being heated at one end in a forge, is welded
by hammering to another slender bar, 10 or 12 feet long,
without the necessity of further direct application of heat — a
point of great importance for the preservation of the good
quality of the steel. Every spot on which the powerful blows
of the hammer rapidly descend, becomes red hot, and to the
spectators the red glow of heat appears to run up and down
the bar. This glow is produced by the blows of the hammer,
and corresponds to an amount of heat sufficient to raise many
pounds of water to the boiling point ; whilst the end of the
bar heated in the fire would scarcely by itself raise to the
same temperature as many ounces of water.

According to the preceding views a precise connection
exists between the blows of the hammer (the cause) and the
heat produced (the effect) ; and natural philosophers have
devised the most ingenious experiments to show this relation.
The working power is in this case converted into heat. If
the view of Mayer be correct, then should we by this amount
of heat thus obtained, be able to reproduce the same amount
of work, viz., the same number of blows of the hammer. But
a closer \dew of the point shows that we require to elevate
the hammer, and that therefore its working power was not
inherent, but only lent to it. The hammer was elevated by a
water-wheel set in motion by a certain weight of water falling

MECHANICAL POWEE CHANGED TO HEAT. 391

on its buckets. Thus to raise a hammer wefghing ten pounds
to the height of one foot requires at least the fall of ten pounds
weight of water from a height of a foot. It was then, properly
speaking, this weight of falling water which produced the
heat through means of the hammer. By simply altering the
arrangement of the machinery, the same force would have
caused a mill-stone to revolve with great rapidity on its axis,
or raised hy friction two iron disks to a red heat.

From experiments instituted to elucidate this point, it has
been established that 13,500 blows of a hammer, weighing 10
pounds, falling on a bar of iron from a height of one foot, pro-
duce an amount of heat sufficient to raise one pound of water
from the freezing point to that of ebullition. This fact may
be represented in another way by saying, that 1,350 cwts. of
water, falling from a height of one foot, will raise the temper-
ature of 1 lb. of water from freezing to the boiling point ; or
1,350 lbs. of water falling from the same height will raise one
pound of water one degree in temperature, or in other words,
that this amount of heat corresponds to a working power,
capable of elevating IS^ cwt. to the height of one foot.

Wherever motion is lost in a machine by friction or by
concussion, there is always produced a corresponding amount
of heat. When, on the other hand, a certain quantity of
work is performed by heat, there disappears, with the me-
chanical effect obtained, a certain amount of heat, which is
expressed by saying that the heat lost by one pound of water
in falling one degree in temperature, is equal to the elevation
of 13^ cwt. to the height of one foot. This quantity of heat
becomes then the equivalent or value of the working power
expressed by the above numbers.

This constant relation between heat and mechanical move-
ment has been confirmed in the most varied manner. A rod
of metal is extended by a weight, and on its removal resumes
its original length, provided certain limits be not exceeded.
The same effect is produced by heat ; and it is evident that

.892 THE CONNECTIOK AND EQUIVALENCE OP FOEOES.

an equal force must be exerted by the rod in its extension
as in its contraction. Now, experiment has shown that the
relation expressed by the numbers above given, must exist
between a given extension of a bar of iron, and the heat or
weight which has caused that extension, viz., that a quantity
of heat sufficient to raise a pound of water one degree in
temperature, wiU, when communicated to a bar of iron, en-
able it to elevate a weight of 1,350 lbs. to the height of one
foot.

An interesting application of this fact was long ago made
in the Conservatoire des Arts et Metiers, in Paris. In this
building, which was formerly a convent, the nave of the church
was converted into a museum for industrial products, machines,
and implements. In its arch, traversing its length, appeared
a crack, which gradually increased to the width of several
inches, and permitted the passage of rain and snow. The
opening could easily have been closed by stone and lime, but
the yielding of the side walls would not have been prevented
by these means. The whole building was on the point of
being puUed down, when a natural philosopher proposed the
following plan, by which the object was accomplished. A
number of strong iron rods were firmly fixed at one end to a
.side wall of the nave, and after passing through the opposite
wall were provided on the outside with large nuts, which
were screwed up tightly to the wall. By applying burning
straw to the rods, they extended in length. The nuts by this
extension being now removed several inches from the wall,
were again screwed tight to it. The rods on cooling con-
tracted with enormous force, and made the side walls ap-
proach each other. By repeating the operation the crack
entirely disappeared. This building with its retaining rods
is still in existence.

The working power of a machine, set in motion by elec-
tricity, can be expressed by numbers, in the same way as the
mechanical efiect of heat. An electrical current is generated

ELECTEICITY CONVERTED DJTO HEAT. 393

by a rotating magnet or by solution of zinc in the galvanic
battery. Such a current, in circulating through a thick or
thin wire, exhibits the same deportment as a fluid flowing
through a wide or narrow tube. As a given quantity of fluid
requires more time or greater pressure to pass through a nar-
row tube than through a large, so a thin wire offers a greater
resistance than a thick one to the passage of a current of elec-
tricity. The current is thus retarded and diminished, one
portion only passing through the conductor, the other being
converted into heat. According to the amount of heat thus
produced by the conversion of the electricity, a conducting
wire of platinum can be fused, one of gold fused and con-
verted into vapour, and a considerable quantity of water
brought into violent ebullition by passing the current through
a thin platinum wire wound round a glass tube in a spiral
form.

If the electrical current circulates through a wire wound
spirally round a bar of iron, the latter is converted into a
powerful magnet capable of attracting and carrying several
hundred weights of iron. The electrical is converted into the
magnetic force, by which a machine may be set in motion.
The power of attraction communicated to the iron bar is in
exact proportion to the amount of electricity circulating in
the surrounding wire, and this current is again dependent on
the property of the conductor. That portion of electricity
which in the conductor is converted into heat, produces no
power of attraction in the iron bar. It follows, from the
foregoing, that the quantity of electricity which circulates, of
that which produces heat, and the amount of magnetic power
convertible into working power, stand in the same relation to
each other, as the working power produced in a machine by
the pressure of falling water to the heat generated by friction
and concussion in the same machine. The same amount of
electricity which, when converted into heat by the resistance
of the conductor, raises by one degree the temperature of one
11*

394 THE CONNECTION AND EQUIVALENCE OF FOKCES.

pound of water, generates a magnetic force capable of ele-
vating a weight of 13J cwt. to the height of one foot.

If the metallic wire through which the electricity is cir-
culating, be cut, and both ends immersed in water, a chemical
decomposition of the water into hydrogen and oxygen takes
place. The circulating electricity is converted into chemical
affinity, and into a power of attraction which causes the sep-
aration of the elements of water. With the evolution of the
hydrogen and oxygen all traces of the electrical current dis-
appear. The power to produce heat and magnetic force, the
usual effects of the electrical current, is apparently in this
case annihilated, and in its place we obtain two gases, one of
which, hydrogen, when burned in oxygen, reproduces water
and evolves heat. Now it has been proved, by careful ex-
periments, that an electrical current of a given strength,
which, when converted into heat in a conductor, is capable
of raising the temperature of a pound of water by one degree,
will produce by the decomposition of water a quantity of hy-
drogen, by the combustion of which one pound of water can
also be elevated one degree in temperature.^

The heat and power of attraction which were apparently
lost by the decomposition of the water, had only become
latent, so to speak, in the elements of water. This heat is
again set free on the reunion of these elements, and if con-
verted into working power, would produce the same result
(viz., raising a given weight a foot high) as would have been
effected by a magnetic power generated by a quantity of elec-
tricity circulating round a bar of iron, equal to that which
was originally employed in the decomposition of the water.

The electrical current is the consequence of a chemical
action, and the amount of electricity which circulates can
therefore be measured by the quantity of zinc which is dis-
solved. The chemical force (affinity) is converted by the
solution of the zinc into a corresponding quantitj^- of elec-
tricity ; and this again in the conductor into its equivalent of

THE MOTIVE-FOECE OP PLANTS. 395

heat, or into magnetic force, or, as in the case of the decom-
position of water, into chemical force. In no case is there a
diminution or increase of force. If, according to the materi-
alist, matter is indestructible, the same holds good with regard
to force. It is not extinguished ; its apparent annihilation,
its disappearance, is only a conversion into some other form.
We know now the origin of the heat and light which
warm and illuminate our dwellings, of the heat and power
generated in our bodies by the vital process. Plants are the
one source of all materials used for the production of heat
and light, and of that nourishment which must be daily taken
to maintain the phenomena of vitality. The elements of
plants are earthy in their nature, and are obtained from
water, earth, and air. In plants, certain inorganic com-
pounds— carbonic acid, water, and anunonia-^are decom-
posed. The carbon of the carbonic acid, the hydrogen of
the water, and the nitrogen of the ammonia, are retained as
constituents of their organs, but the oxygen of the carbonic
acid and of the water are returned as gas to the air. With
out light, however, plants cannot grow.

The vital process in plants exhibits itself as directly oppo-
site in its character to the chemical process in the formation
of salts. ,

Carbonic acid^ water ^ and zinc, when brought together pro-
duce a certain effect on each other. In virtue of chemical
affinity there is formed a white powdery compound, contain-
ing carbonic acid, zinc, and oxygen from the water, and
hydrogen is at the same time evolved.

In plants, the living bud or part of the plant takes the
place of the zinc. By their growth are formed, from carbonic
acid and water, compounds containing carbon and hydrogen,
or carbon and the elements of water, and oxygen is at the
same time evolved. Sunlight acts in living plants like elec-
tricity, which arrests the natural attraction of the elements of
water, and separates them from each other.

396 THE COHNEcnON Am> EQUIVALENCE OF FORCES*

Without the light of the sun plants cannot grow. The
living germ, the green leaf, owe to the sun their power of
transforming earthy elements into living, vigorous structures.
The germ may, indeed, be evolved under ground without the
action of light, but only when it breaks through the surface
of the soil does it first acquire the power, by the sun's rays,
of converting inorganic elements into its own structure. The
illuminating and heating rays of the sun, in thus bestowing
life, lose their own light and heat. Their power now becomes
latent in the new products of the frame, which have been
produced under their influence from carbonic acid, water, and
ammonia. The light and heat with which our dwellings are
illuminated and warmed are but those bestowed by the sun.

The food of men and animals consists of two classes of
materials, which difier totally in their nature. One class is
destined to the production of blood and the maiutenance of
the structure of the body.; the other is similar in composition
to ordinary materials for combustion. Sugar, starch, the
gum of bread, may be regarded as transformed woody fibre,
for we can prepare them from this substance. Fat, in its
amount of carbon, resembles closely mineral coal. We heat
our bodies as we do stoves, by combustibles which possess the
same elements as wood and coal, but which differ essentially
from them by being soluble in the fluids of the body.

The elements of nutrition from which the temperature of
the body is derived, evidently produce no mechanical power ;
because power is but converted heat, and the heat which
maintains and elevates the temperature of the body does not
produce any other effect than that of warmth.

All those mechanical operations constantly taking place in
the living body, in the movement of organs and limbs, are
dependent on an accompanying change in the composition
and properties of those highly complex sulphur and nitrogen
constituents of the muscles, which, though furnished by the
blood, are in the first instance derived directly from the food

DERIVATION OF ANIMAL POWEE. 397

of man. The change of position in the elements of these
complex bodies, attendant on their rearrangement into new
and simpler compounds, necessarily gives rise to motion ; and
the molecular movement of the particles in a state of change
is transferred to the muscular mass. Chemical action is thus
evidently the source of mechanical power in bodies.

The elements of the food of men and animals which give
rise to power and heat, are produced in living plants only by
the action of sunlight. The rays of the sun become latent,
so to speak, in them in the same way as the current of elec-
tricity becomes latent in the hydrogen by decomposition of
water.

Man, by food, not only maintains the perfect structure of
his body, but he daily lays in a store of power and heat, de-
rived in the first instance from the sun. This power and
heat, latent for a time, reappears and again becomes active
when the living structures are resoLved by the vital processes
into their original elements.

The rays of the sun add daily to the store of indestructi-
ble forces of our terrestrial body, maintaining life and rtiotion.
Thus, from beyond the limits of our earth, the body, the more
earthly vessel, derives all that may be called good in it, and
of this not a single particle is ever lost.

ON

THE CORRELATION

OF THE

PHYSICAL AXD YITAL FOKCES,

By Dr. WILLIAM B. CARPENTER.

William Benjamin Carpenter, the eminent English physiologist, was
bom in the early part of this century, and graduated as doctor of medicine
in Edinburgh in 1839. He commenced practice in Bristol, but has been
chiefly occupied as a lecturer and author. His most important works are
the "Principles ot General and Comparative Physiology" (1839), the
" Principles of Human Physiology" (1846), which reached a fifth edition in
1855, and "The Microscope, its Kevelations and Uses" (1856). He is the
author of various minor, but valuable works, and of many elaborate papers
in the cyclopedias and scientific periodicals. For many years he was editor
of the " British and Foreign Medico-Chirurgical Review." He was elected
a member of the Royal Society in 1844, and is now Professor of Medical
Jurisprudence in University College, London ; Lecturer on General Anatomy
and Physiology at the London Hospital and School of Medicine, and Ex-
aminer in Physiology and Comparative Anatomy in the University of Lon-
don. In 1850 he pubHshed an able paper in the Transactions of the Royal
Society on the " Mutual Relations of the Vital and Physical Forces," and
has published upon the same general subject, the essays which close this vol-
ume, in the new Quarterly Journal of Science for the present year. Dr. Car-
penter is an able and original thinker, as well as a voluminous writer, and has
made many valuable contributions to the progress of physiological science.

ON THE COKEELATION OF THE PHYS-
ICAL AND VITAL FOECES.

L—RELATIONS OF LIGHT AND HEAT TO THE VITAL FORCES
OF PLANTS.

IN every period of the history of Physiology, attempts
have been made to identify all the forces acting in the
Living Body with those operating in the Inorganic universe.
Because muscular force, when brought to bear on the bones,
moves them according to the mechanical laws of lever action,
and because the propulsive power of the heart drives the
blood through the vessels according to the rules of hydraulics,
it has been imagined that the movements of living bodies may
be explained on physical principles ; the most important con-
sideration of all, namely, the source of that contractile power
which the living muscle possesses, but which the dead muscle
(though having the same chemical composition) is utterly in-
capable of exerting, being altogether left out of view. So,
again, because the digestive process, whereby food is reduced
to a fit state for absorption, as well as the formation of va-
rious products of the decomposition that is continually taking
place in the living body, may be initiated in the laboratory
of the chemist ; it has been supposed that the appropriation

402 CORRELATION OF PHYSICAL AND VITAL FORCES.

of the nutriment to the production of the living organized
tissues of which the several parts of the body are composed,
is to be regarded as a chemical action — as if any combination
of albumen and gelatine, fat and starch, salt and bone-earth,
could make a living Man without the constructive agency in-
herent in the germ from which his bodily fabric is evolved.

Another class of reasoners have cut the knot which they
could not untie, by attributing all the actions of living bodies
for which Physics and Chemistry cannot account, to a hypo-
thetical " Vital principle ;" a shadowy agency that does every
tiling in its own way, but refuses to be made the subject of
scientific examination ; like the " od-force " or the " spiritual
power " to which the lovers of the marvellous are so fond of
attributing the mysterious movements of turning and tilting
tables.

A more scientific spirit, however, prevails among the best
Physiologists of the present day ; who, whilst fully recogniz-
ing the fact that many of the phenomena of living bodies can
be accounted for by the agencies whose operation they trace
in the world around, separate into a distinct category — that
of vital actions — such as appear to differ altogether in kind
from the phenomena of Physics and Chemistry, and seek to
determine, from the study of the conditions under which these
present themselves, the laws of their occurrence.

In the prosecution of this inquiry, the Physiologist will
find it greatly to his advantage to adopt the method of philos-
ophizing which distinguishes the Physical science of the pres-
ent from that of the past generation ; that, namely, which,
whilst fully accepting the logical definition of the cause of any
phenomenon, as " the antecedent, or the concurrence of ante-
cedents on which it is invariably and unconditionally conse-
quent" (Mill), draws a distinction between the dynamical
and the material conditions ; the former supplying the power
which does the work, whilst the latter affords the instrumental
means through which that power operates.

DYNAMICAL Al^TD MATERIAL CONDITIONS. 4:03

Thus if we inspect a Cotton Factory in full action, we find
it to contain a vast number of machines, many of them but
repetitions of one another, but many, too, presenting the most
marked diversities in construction, in operation, and in result-
ant products. We see, for example, that one is supplied with
the raw material, which it cleans and dresses ; that another
receives the cotton thus prepared, and '' cards" it so as to lay
its fibres in such an arrangement as may admit of its being
spun ; that another series, taking up the product supplied by
the carding machine, twists and draws it out into threads of
various degrees of fineness ; and that this thread, carried into
a fourth set of machines, is woven into a fabric which may
be either plain or variously figured according to the construc-
tion of the loom. In every one of these dissimilar operations
the force which is immediately concerned in bringing about the
results is one and the same ; and the variety of its products is
dependent solely upon the diversity of the material instru-
ments through which it operates. Yet these arrangements,
however skilfully devised, are utterly valueless without the
force which brings them into play. All the elaborate mechan-
ism, the triumph of human ingenuity in devising, and of skill
in constructing, is as powerless as a corpse without the vis
viva which alone can animate it. The giant stroke of the
steam-engine, or the majestic revolution of the water-wheel
gives the required impulse ; and the vast apparatus which
was the moment previously in a state of death-like inactivity,
is aroused to all the energy of its wondrous life — every part
of its complex organization taking upon itself its peculiar
mode of activity, and evolving its own special product, in vir-
tue of the share it receives of the one general force distrib-
uted through the entire aggregate of machinery.

But if we carry back our investigation a stage further, and
inquire into the origin of the force supplied by the steam-
engine or the water-wheel, we soon meet with a new and
most significant fact. At our first stage, it is true, we find

404: COBKELATION OF PHYSICAL AND VITAL FORCES.

only the same mechanical force acting through a different
kind of instrumentality; the strokes of the piston of the
steam-engine being dependent upon the elastic force of the
vapour of water, whilst the revolution of the water-wheel is
maintained by the downward impetus of water en masse.
But to what antecedent dynamical agency can we trace these
forces ? That agency in each case is Heat ; a force altogether
dissimilar in its ordinary manifestations to the force which
produces sensible motion, yet capable of being in turn con-
verted into it and generated by it. For it is from the Heat
applied beneath the boiler of the steam-engine that the non-
elastic liquid contained in it derives all that potency as elastic
vapour, which enables it to overcome the vast mechanical re-
sistance that is set in opposition to it. And, in like manner,
it is the heat of the solar rays which pumps up terrestrial
waters in the shape of vapour, and thus supplies to Man a
perrenial source of new power in their descent by the force
of gravity to the level from which they have been raised.*

The power of the steam-engine, indeed, is itself derived
more remotely from those same rays ; for the Heat applied
to its boilers is but the expression of the chemical change in-
volved in combustion ; that combustion is sustained either by
the wood which is the product of the vegetative activity of
the present day, or by the coal which represents the vegeta-
tive life of a remote geological epoch ; and that vegetative
activity, whether present or past, represents an equivalent
amount of Solar Light and Heat, used up in the decomposi-
tion of the carbonic acid of the atmosphere, by the instru-
mentality of the growing plant.f Thus in either case we
come, directly or indirectly, to Solar Radiation as the main-

* See on this subject the recent admirable address of Sir William Arm-
strong at the Meeting of the British Association at Newcastle.

•j- This was discerned by the genius of George Stephenson, before the
general doctrine of the Correlation of Forces had been given to the world
by Mayer and Grove.

ESTABLISHMENT OF THE GENEEAL PRINCIPLE. 405

spring of our mecliaiiical power ; the vis viva of our whole
microcosm. Modern physical inquiry ventures even one step
further, and seeks the source of Light and Heat of the Sun
itself. Are these, as formerly supposed, the result of com-
bustion, or are they, as surmised by Mayer and Thomson,
the expression of the motive power continually generated in
the fall of aerolites towards the Sun, and as continually anni-
hilated by their impact on its surface ? Leaving the discus-
sion of this question to Physical Philosophers, I proceed now
to my own proper subject.

It is now about twenty years since Dr. Mayer first broadly
announced, in all its generality, the great principle now known
as that of " Conservation of Force ;" as a necessary deduc-
tion from two axioms or essential truths — ex nihilo nil fit, and
nil fit ad nihilum — the validity of which no true philosopher
would ever have theoretically questioned, but of which he
was (in my judgment) the first to appreciate the full practical
bearing. Thanks to the labours of Faraday, G-rove, Joule,
Thomson, and Tyndall, to say nothing of those of Helmholtz
and other distinguished Continental savans, the great doctrine
expressed by the term " Conservation of Force " is now
amongst the best-established generalizations of Physical Sci-
ence ; and every thoughtful Physiologist must desire to see
the same course of inquiry thoroughly pursued in regard to
the phenomena of living bodies. This ground was first broken
by Dr. Mayer in his remarkable treatise, ^' Die Organische
Bewegung in ihrem Zusammenhange nit dem Sloffwechsel "
(" On Organic Movement in its relation to Material Changes,"
Heilbronn, 1845) ; in which he distinctly set forth the princi-
ple that the source of all changes in the living Organism,
animal as well as vegetable, lies in the forces acting upon it
from without; whilst the changes in its own composition
i)rought about by these agencies, he considers to be the imme-
diate source of the forces which are generated by it.

In treating of these forces, however, he dwells chiefly on

406 COERELATIOK OF PHYSICAL AND VITAL FORCES.

the production of Motion, Heat, Light, and Electricity by-
living bodies ; touching more slightly upon the phenomena of
Growth and Development, which constitute, in the eye of the
physiologist, the distinct province of vitality. In a memoir
of my own, " On the Mutual Relations of the Vital and Phys-
ical Forces," published in the Philosophical Transactions for
1850,* I aimed to show that the general doctrine of the " Cor-
relation of the Physical Forces " propounded by Mr. Grove,
was equally applicable to those Vital forces which must be
assumed as the moving powers in the production of purely
physiological phenomena ; these forces being generated in
living bodies by the transformation of the Light, Heat, and
Chemical Action supplied by the world around, and being
given back to it again, either during their life, or after its
cessation, chiefly in Motion and Heat, but also to a less de-
gree in Light and Electricity. This memoir attracted but
little attention at the time, being regarded, I believe, as too
speculative ; but I have since had abundant evidence that the
minds of thoughtful Physiologists, as well as Physicists, are
moving in the same direction ; and as the progress of science
since the publication of my former memoir, would lead me
to present some parts of my scheme of doctrine in a different
form,t I venture to bring it again before the public in the
form of a sketch (I claim for it no other title), of the aspect
in which the application of the principle of the " Conserva-
tion of Force " to Physiology now presents itself to my mind.

* At this date the labours of Dr. Mayer were not known either to my-
self or (so far as I am aware) to any one else in this country, save the late
Dr. Baly, who a few months after the pubheation of my Memoir, placed in
my hands the pamphlet, " Die Organische Bewegung ; " to which I took the
earliest opportimity in my power of drawing pubUc attention in " The Brit-
ish and Foreign Medico-Chirurgical Review " for July, 1851, p. 237.

f I have especially profited by a Memoir on the Correlation of Physical,
Chemical, and Vital Force, and the Conservation of Force in Vital Phenom-
ena, by Prof. Le Conte (of South Carohna College), in Silliman's American
Journal for Nov., 1859, reprinted in the Philosophical Magazine for 1860.

CHAEACTERICnCS OF VITAL ACTIVITY. 407

If, in the first place, we inquire what it is that essentially
distinguishes Vital from every kind of Physical activity, we
find this distinction most characteristically expressed in the
fact that a germ endowed with Life, developes itself into an
organism of a type resembling that of its parent ; that this
organism is the subject of incessant changes, which all tend
in the first place to the evolution of its typical form, and sub-
sequently to its maintenance in that form, notwithstanding the
antagonism of Chemical and Physical agencies, which are
continually tending to produce its disintegration ; but that, as
its term of existence is prolonged, its conservative power de-
clines so as to become less and less able to resist these disinte-
grating forces, to which it finally succumbs, leaving the organ-
ism to be resolved by their agency into the components from
which its materials were originally drawn. The history of a
living organism, then, is one of incessant change; and the
conditions of this change are to be found partly in the organ-
ism itself, and partly in the external agencies to which it is
subjected. That condition wliich is inherent in the organism,
being derived hereditarily from its progenitors, may be con-
veniently termed its germinal capacity ; its parallel in the in-
organic world being that fundamental difference in properties
which constitutes the distinction between one substance,
whether elementary or compound, and another ; in virtue of
which each " behaves" in its own characteristic manner when
subjected to new conditions.

Thus, although there may be nothing in the aspect or sen-
sible properties of the germ of a Polype, to distinguish from
that of a Man, we find that each develops itself, if the requi-
site conditions be supplied, into its typical form, and no other;
if the developmental conditions required by either be not sup-
plied we do not find a different type evolved, but no evolution
at all takes place.*

* It is quite true that among certain of the lower tribes, both of Plants

408 CORRELATION OF PHYSICAL AND VITAL FORCES.

Now the difference between a being of high and a being
of low organization essentially consists in this : that in the
latter the constituent parts of the fabric evolved by the pro-
cess of growth from the original germ, are similar to each
other in structure and endowments, whilst in the former they
are progressively differentiated with the advance of develop-
ment, so that the fabric comes at last to consist of a number
of organs^ or instruments, more or les^ dissimilar in struc-
ture, composition, and endowments. Thus in the lowest
forms of Vegetable life, the primordial germ multiplies itself
by duplicative subdivision into an apparently unlimited num-
ber of cells, each of them similar to every other, and capa-
ble of maintaining its existence independently of them. And
in that lowest Rhizopod type of Animal life, the knowledge
of which is among the most remarkable fruits of modern bio-
logical research, " the Physiologist has a case in which those
vital operations which he is elsewhere accustomed to see car-
ried on by an elaborate apparatus, are performed without any
special instruments whatever ; a little particle of apparently
homogeneous jelly changing itself into a greater variety of
forms than the fabled Proteus, laying hold of its food without
members, swallowing it without a mouth, digesting it without
a stomach, appropriating its nutritious material without absorb-
ent vessels or a circulating system, moving from place to place
without muscles, feeling (if it has any power to do so) with-
out nerves, propagating itself without genital apparatus, and

and Animals, especially the Fungi and Entozoa — similar germs may devel-
op themselves into very dissimilar forms, according to the conditions un-
der which they are evolved ; but such diversities are only the same kind as
those which manifest themselves among individuals in the higher Plants and
Animals, and only show that in the types in question there is a less close
conformity to one pattern. Neither in these groups, nor in that group of
ForamipAfera^ in which I have been led to regard the range of variation
as pecuHarly great, does any tendency ever show itself to the assumption
of the characters of any group fundamentally dissimilar.

THE LOWEST GRADE OF LIFE. 409

not only this, but in many instances forming shelly coverings
of a symmetry and complexity not surpassed by those of any
testaceous animals,"* whilst the mere separation of a frag-
ment of this jelly is sufficient to originate a new and inde-
pendent organism, so that any number of these beings may
be produced by the successive detachment of such particles
from a single Rhizopod, each of them retaining (so far as we
have at present the means of knowing) the characteristic en-
dowments of the stock from which it was an offset.

When, on the other hand, we watch the evolution of any
of the higher types of Organization, whether vegetable or
animal, we observe that although in the first instance the pri-
mordial cell multiplies itself by duplicative subdivision into
an aggregation of cells, which are apparently but repetitions
of itself and of each other, this homogeneous extension has
in each case a definite limit, speedily giving place to a struc-
tural differentiation, which becomes more and more decided
with the progress of development, until in that most hetero-
geneous of all types — the Human Organism — no two parts are
precisely identical, except those which correspond to each
other on the opposite sides of the body. With this structural
differentiation is associated a corresponding differentiation of
function ; for whilst in the life of the most highly developed
and complex organism we witness no act which is not fore-
shadowed, however vaguely, in that of the lowest and sim-
plest, yet we observe in it that same " division of labour "
which constitutes the essential characteristic of the highest
grade of civilization. For, in what may be termed the ele-
mentary form of Human Society, in which every individual
relies upon himself alone for the supply of all his wants, no
greater result can be obtained by the aggregate action of the
entire community than its mere maintenance ; but as each in-

* See the Author's Introduction to the Study of the Foraminifera, pub-
lished by the Ray Society, 1862 : Preface, p. vii.
18

410 COEEELATION OF PHYSICAL AND VITAL FOECES.

dividual selects a special mode of activity for himself, and
aims at improvement in that specialty, he finds himself attain-
ing a higher and still higher degree of aptitude for it ; and
this speciaHzation tends to increase as opportunities arise for
new modes of activity, until that complex fabric is evolved
which constitutes the most developed form of the Social State
wherein every individual finds the work — ^mental or bodily —
for which he is best fitted, and in which he may reach the
highest attainable perfection ; while the mutual dependence
of the whole (which is the necessary result of this specializa-
tion of parts) is such that every individual works for the
benefit of all his fellows, as well as for his own. As it is
only in such a state of society that the greatest triumphs of
human ability become possible, so is it only in the most dif-
ferentiated types of Organization that Vital Activity can pre-
sent its highest manifestations. In the one case, as in the
other, does the result depend upon a process of gradual devel-
opment, in which, under the influence of agencies whose na-
ture constitutes a proper object of scientific inquiry, that most
general form in which the fabric — whether corporeal or social
— originates, evolves itself into that most special in which its
development culminates.

But notwithstanding the wonderful diversity of structure
and of endowments which we meet with in the study of any
complex Organism, we encounter a harmonious unity or coor-
dination in its entire aggregate of actions, which is yet more
wonderful. It is in this harmony or coordination, whose ten-
dency is to the conservation of the organism, that the state of
Health or Normal life essentially consists. And the more
profound our investigations of its conditions, the more definite
becomes the conclusion to which we are led by the study of
them — ^that it is fundamentally based on the coromon origin
of all these diversified parts in the same germ, the vital en-
dowments of which, equally diffused throughout the entire
fabric in those lowest forms of organization in which every

THE FUNDAMENTAL CHAKACTEEISTIC OF LIFE. 411

part is but a repetition of every other, are differentiated in
the highest amongst a variety of organs, acquiring in virtue
of this differentiation a much greater intensity.

Thus, then, we may take that mode of Vital Activity
which manifests itself in the evolution of the germ into the
complete organism repeating the type of its parent, and the
subsequent maintenance of that organism in its integrity, in
the one case as in the other, at the expense of materials de-
rived from external sources — as the most universal and most
fundamental characteristic of Life ; and we have now to con-
sider the nature and source of the Force or Power by which
that evolution is brought about. The prevalent opinion has
until lately been, that this power is inherent in the germ ;
which has been supposed to derive from its parent not merely
its material substance, but a nisus formativus^ hildungstrieh,
or germ force, in virtue of which it builds itself up into the
likeness of its parent, and maintains itself in that likeness
until the force is exhausted, at the same time imparting a
fraction of it to each of its progeny. In this mode of view-
ing the subject, all the organizing force required to build up
an Oak or a Palm, an Elephant or a Whale, must be concen-
trated in a minute particle, only discernible by microscopic
aid, and the aggregate of all the germ-forces appertaining to
the descendants, however numerous, of a common parentage,
must have existed in their original progenitors. Thus in the
case of the successive viviparous broods of Aphides, a germ-
force capable of organizing a mass of living, structure, which
would amount (it has been calculated)* in the tenth brood
to the bulk of five hundred millions of stout men, must have
been shut up in the single individual, weighing perhaps the
1-lOOOth of a grain, from which the first brood was evolved.
And in like manner, the germ-force which has organized the

* See Pro£ Huxley on the " Agamic Reproduction of Aphis," in Linn.
Trans., vol. xxii., p. 215.

4:12 CX)ERELATION OF PHYSICAL AND VITAL FORCES.

bodies of all the individual men that have lived from Adam
to the present day, must have been concentrated in the body
of their common ancestor. A more complete reductio ad ah*
surdum can scarcely be brought against any hypothesis ; and
we may consider it proved that in some way or other, fresh
organizing force is constantly being supplied from without
during the whole period of the exercise of its activity.

When we look carefully into the question, however, we
find that what the germ really suppHes is not the force, but
the directive agency ; thus rather resembling the control exer-
cised by the superintendent builder, who is charged with
working out the design of the architect, than the bodily force
of the workmen who labour under his guidance in the con-
struction of the fabric. The actual constructive force, as we
learn from an extensive survey of the phenomena of life, is
supplied by Heat, the influence of which upon the rate of
growth and development, both animal and vegetable, is so
marked as to have universally attracted the attention of Phys-
iologists, who, however, have for the most part only recognized
in it a vital stimulus that calls forth the latent power of the
germ, instead of looking upon it as itself furnishing the power
that does the work. It has been from the narrow limitation
of the area over which physiological research has been com-
monly prosecuted, that the intimacy of this relationship be-
tween Heat and the Organizing force has not sooner become
apparent. Whilst the vital phenomena of Warm-blooded ani-
mals, which possess within themselves the means of main-
taining a constant temperature, were made the sole, or at any
rate the chief objects of study, it was not likely that the in-
quirer would recognize the full influence of external heat in
accelerating, or of cold in retarding their functional activity.
It is only when the survey is extended to Cold-blooded ani-
mals and to Plants, that the immediate and direct relation be-
tween Heat and Vital Activity, as manifested in the rate of
growth and development, or of other changes peculiar to the

DYNAMICS OF GERMINATION. 413

living body, is unmistakably manifested. To some of those
phenomena, which afford the best illustrations of the mode
in which Heat acts upon the living organism, attention will
now be directed.

Commencing with the Vegetable kingdom, we find that the
operation of Heat as the motive power or dynamical agency,
to which the phenomena of growth and development are to be
referred, is peculiarly well seen in the process of Germina-
tion. The seed consists of an embryo which has already ad-
vanced to a certain stage of development, and of a store of
nutriment laid up as the material for its further evolution ;
and in the fact that this evolution is carried on at the expense
of organic compounds already prepared by extrinsic agency,
until (the store of these being exhausted) the young plant is
sufTiciently far advanced in its development to be able to elab-
orate them for itself, the condition of the germinating embryo
resembles that of an Animal. Now, the seed may remain
(under favourable circumstances) in a state of absolute inac-
tion during an unlimited period. If secluded from the free
access of air and moisture, and kept at a low temperature, it
is removed from all influences that would on the one hand
occasion its disintegration, or on the other, would call it into
active life. But when again exposed to air and moisture, and
subjected to a higher temperature, it either germinates or de-
cays, according as the embryo it contains has or has not pre-
served its vital endowments — a question which only experi-
ment can resolve. The process of germination is by no means
a simple one. The nutriment stored up in the seed is in great
part in the condition of insoluble starch ; and this must be
brought into a soluble form before it can be appropriated by
the embryo. The metamorphosis is effected by the agency of
a ferment termed diastase^ which is laid up in the immediate
neighbourhood of the embryo, and which, when brought to
act on starch, converts it in the first instance into soluble dex-
trine, and then (if its action be continued) into sugar. The

414 COEEELATION OF PHYSICAL AND VITAL FOECES.

dextrine and sugar, combined with the albuminous and oily
compounds also stored up in the seed, form the "protoplasm,"
which is the substance immediately supplied to the young
plant as the material of its tissues ; and the conversion of this
protoplasm into various forms of organized tissue, which be-
come more and more differentiated as development advances,
is obviously referable to the vital activity of the germ. Now
it can be very easily shown experimentally that the rate of
growth in the germinating embryo is so closely related (within
certain limits) to the amount of heat supplied, as to place its
dependence on that agency beyond reasonable question : so
that we seem fully entitled to say that Heat, acting through
the germ, supplies the constructive force or power by which the
Vegetable fabric is built up.* But there appears to be another
source of that power in the seed itself. In the conversion of
the insoluble starch of the seed into sugar, and probably also
in a further metamorphosis of a part of that sugar, a large
quantity of carbon is eliminated from the seed by combining
with the oxygen of the air, so as to form carbonic acid ; this
combination is necessarily attended with a disengagement of
heat, which becomes very sensible when (as in molting) a
large number of germinating seeds are aggregated together ;
and it cannot but be regarded as probable that the heat thus
evolved within the seed concurs with that derived from with-
out, in supplying to the germ the force that promotes its evo-
lution.

* The efifect of Heat is doubtless manifested very differently by differ-
ent seeds ; such variations being partly specific, partly individical. But
these are no greater than we see in the inorganic world : the increment of
temperature and the augmentation of bulk exhibited by different substances
when subjected to the same absolute measure of heat, being as diverse as
the substances themselves. The whole process of " Malting," it may be
remarked, is based on the regularity with which the seeds of a particular
species may be at any time forced to a definite rate of germination by a
definite increment of temperature.

DYNAMIC FUNCTIONS OF PLANTS. 415

The condition of the Plant which has attained a more ad-
vanced stage of its development, differs from that of the ger-
minating embryo essentially in this particular, that the organic
compounds which it requires as the materials of the extension
of the fabric are formed by itself, instead of being supplied to
it from without. The tissues of the coloured surfaces of the
leaves and stems, when acted on by light, have the peculiar
power of generating — at the expense of carbonic acid, water,
and ammonia — various ternary and quaternary organic com-
pounds, such as chlorophyll, starch, oil, and albumen ; and of
the compounds thus generated, some are appropriated by the
constructive force of the plant (derived from the heat with
which it is supplied) to the formation of new tissues ; whilst
others are stored up in the cavities of those tissues, where
they ultimately serve either for the evolution of parts subse-
quently developed, or for the nutrition of animals which em-
ploy them as food. Of the source of those peculiar affinities
by which the components of the starch, albumen, &c., are
brought together, we have no right to speak confidently ; but
looking to the fact that these compounds are not produced in
any case by the direct union of their elements, and that a de-
composition of binary compounds seems to be a necessary
antecedent of their formation, it is scarcely improbable that,
as suggested by Prof. Le Conte (op. ci^.), that source is to be
found in the chemical forces set free in the preliminary act of
decomposition, in which the elements would be liberated in
that " nascent condition" which is well known to be one of
peculiar energy.

The influence of Light, then, upon Vegetable organism ap-
pears to be essentially exerted in bringing about what may be
considered a higher mode of chemical combination between
oxygen, hydrogen, and carbon, with the addition of nitrogen
in certain cases ; and there is no evidence that it extends be-
yond this. That the appropriation of the materials thus pre-
pared, and their conversion into organized tissue in the opera-

416 COEEELATION OF PHYSICAL AXD VITAL F0KCE3.

tions of growth and development, are dependent on the agency
of Heat, is just as evident in the stage of maturity as in that
of germination. And there is reason to believe, further, that
an additional source of organizing force is to be found in the
retrograde metamorphosis of organic compounds that goes on
during the whole life of the plant ; of which metamorphosis
the expression is furnished by the production of carbonic acid.
This is peculiarly remarkable in the case of the Fungi, which,
being incapable of forming new compounds under the influ-
ence of light, are entirely supported by the organic matters
they absord, and which in this respect correspond on the one
hand with the germinating embryo, and on the other with
Animals. Such a decomposition of a portion of the absorbed
material is the only conceivable source of the large quantity
of carbonic acid they are constantly giving out ; and it would
not seem unlikely that the force supplied by this retrograde
metamorphosis of the superfluous components of their food,
which fall down (so to speak) from the elevated plane of " prox-
imate principles," to the lower level of comparatively simple
binary compounds, supplies a force which raises another por-
tion to the rank of living tissue, thus accounting in some de-
gree for the very rapid growth for which this tribe of Plants
is so remarkable. This exhalation of carbonic acid, however,
is not peculiar to Fungi and germinating embryos, for it takes
place during the whole life of flowering plants, both by day
and by night, in sunshine and in shade, and from their green
as well as from their dark surfaces ; and it is not improbable
that, as in the case of the Fungi, its source lies partly in the
organic matters absolved, recent investigations * having ren-
dered it probable that Plants really take up and assimilate
soluble humus, which, being a more highly carbonized sub-
stance than starch, dextrine, or cellulose, can only be con-

* See the Memoir of M. lUsler, " On the Absorption of Humus," in
Uie " Biblioth^que Universelle," N. S., 1858, torn, i., p. 305.

PLAJ^T-PEODUCTS STORES OF FOECE. 417

verted into compounds of the latter kind by parting with some
of its carbon. But it may also take place at the expense of
compounds previously generated by the plant itself, and stored
up in its tissues ; of which we seem to have an example in
the unusual production of carbonic acid which takes place at
the period of flowering, especially in such plants as have a
fleshy disk, or receptacle containing a large quantity of starch ;
and thus, it may be surmised, an extra supply of force is pro-
vided for the maturation of those generative products whose
preparation seems to be the highest expression of the vital
power of the vegetable organism.

The entire aggregate of organic compounds contained in
the vegetable tissues, then, may be considered as the expres-
sion, not merely of a certain amount of the material elements,
oxygen, hydrogen, carbon, and nitrogen, derived (directly or
indirectly) from the water, carbonic acid, and ammonia of the
atmosphere, but also of a certain amount of force which has
been exerted in raising these from the lower plane of simple
binary compounds, to the higher level of complex, " proxi-
mate principles ;" whilst the portion of these actually converted
into organized tissue may be considered as the expression of
a further measure of force, which, acting under the directive
agency of the germ, has served to build up the fabric in its
characteristic type. This constructive action goes on during
the whole Life of the Plant, which essentially manifests itself
either in the extension of the original fabric (to which in
many instances there seems no determinate limit), or in the
production of the germs of new and independent organisms.
It is interesting to remark that the development of the more
permanent parts involves the successional decay and renewal
of parts whose existence is temporary. The " fall of the
leaf" is the effect, not the cause, of the cessation of that pe-
culiar functional activity of its tissues, which consists in the
elaboration of the nutritive material required for the produc-
tion of wood. And it would seem as if the duration of their
18*

4:18 CORRELATION OF PHYSICAL AND VITAL FORCES.

existence stands in an inverse ratio to the energy of their ac-
tion ; the leaves of "evergreens," which are not cast off until
the appearance of a new succession, effecting their functional
changes at a much less rapid rate than do those of " deci-
duous " trees, whose term of life is far more brief.

Thus, the final cause or purpose of the whole Vital Activ-
ity of the Plant, so far as the individual is concerned, is to
produce an indefinite extension of the dense, woody, almost
inert, but permanent portions of the fabric, by the successional
development, decay, and renewal of the soft, active, and tran-
sitory cellular parenchyma ; and according to the principles
already stated, the descent of a portion of the materials of the
latter to the condition of binary compounds, which is mani-
fested in the largely-increased exhalation of carbonic acid
that takes place from the leaves in the later parf of the sea-
son, comes to the aid of external Heat in supplying the force
by which another portion of those materials is raised to the
condition of organized tissue. The vital activity of the
Plant, however, is further manifested in the provision made
for the propagation of its race, by the production of the
germs of new individuals ; and here, again, we observe that
whilst a higher temperature is usually required for the devel-
opment of the flower, and the maturation of the seed, than
that which suffices to sustain the ordinary processes of vege-
tation, a special provision appears to be made in some in-
stances for the evolution of force in the sexual apparatus it-
self, by the retrograde metamorphosis of a portion of the or-
ganic compounds prepared by the previous nutritive opera-
tions. This seems the nearest approach presented in the
Vegetable organism, to what we shall find to be an ordinary
mode of activity in the Animal. That the performance of
the generative act involves an extraordinary expenditure of
vital force appears from this remarkable fact, that blossoms
which wither and die as soon as the ovules have been fertil-
ized, may be kept fresh for a long period if fertilization be
prevented.

RECONVERSION OF ORGANIC FORCES. 419

The decay which is continually going on during the life
of a Plant restores to the inorganic world, in the form of car-
bonic acid, water, and ammonia, a part of the materials drawn
from it in the act of vegetation ; and a reservation being made
of those vegetable products which are consumed as food by
Animals, or which are preserved (like timber, flax, cotton,
&c.) in a state of permanence, the various forms of decom-
position which take place after death complete that restora-
tion. But in returning, however slowly, to the condition of
water, carbonic acid, ammonia, &c., the constituents of
Plants give forth an amount of heat equivalent to that which
they would generate by the process of ordinary combustion ;
and thus they restore to the inorganic world, not only the ma-
terials^ but the forces^ at the expense of which the vegetable
fabric was constructed. It is for the most part only in the
humblest Plants, and in a particular phase of their lives, that
such a restoration takes place in the form of motion, this mo-
tion being, like growth and development, an expression of the
vital activity of the " Zoospores " of Algoe, and being ob-
viously intended for their dispersion.

Hence we seem justified in affirming that the Correlation
between heat and the organizing force of Plants is not less
intimate than that which exists between heat and motion.
The special attribute of the vegetable germ is its power of
utilizing, after its own particular fashion, the heat which it
receives, and of applying it as a constructive power to the
building-up of its fabric after its characteristic type.

420 COEEELATION OF PHYSICAL AND VITAL FORCES.

n.— RELATIONS OF LIGHT AND HEAT TO THE VITAL FORCES
OF ANIMALS.

Those of our readers who accompanied us through the
first part of our inquiry are aware that it was our object to
show, that as force is never lost in the inorganic world, so
force is never created in the organic ; but that those various
operations of vegetable life which are sometimes vaguely
attributed to the agency of an occult " vital principle," and
are referred by more exact thinkers to certain Vital Forces
inherent in the organism of the plant, are really sustained by
solar light and heat. These, we have argued, supply to
each germ the whole jpower by which it builds itself up, at the
expense of the materials it draws from the Inorganic Uni-
verse, into the complete organism ; while the mode in which
that power is exerted (generally as vital force, specially as
the determining cause of the form peculiar to each type) de-
pends upon the "germinal capacity" or directive agency inher-
ent in each particular germ. The first stage in this construc-
tive operation consists in the production of certain organic
compounds of a purely chemical nature — such as gum, starch,
sugar, chlorophyll, oil, and albumen — at the expense of the
oxygen, hydrogen, carbon, and nitrogen derived from the
water, carbonic acid, and ammonia of the atmosphere ;
whilst the second consists in the further elevation of a portion
of these organic compounds to the rank of organized tissue
possessing attributes distinctively vital. Of the whole amount
of organic compounds generated by the plant, it is but a
comparatively small part (a) that undergoes this progressive
metamorphosis into living tissue. Another small proportion
(b) undergoes a retrograde metamorphosis, by which the orig-
inal binary components are reproduced ; and in this descent
of organic compounds to the lower plane, the power con-

GRADES OF ORGANIC ASCENT. 421

sumed in their elevation is given forth in the form of heat
and organizing force (as is specially seen in germination),
which help to raise the portion a to a higher level. But by
far the larger part (c) of the organic compounds generated
by plants remains stored up in their fabric, without undergo-
ing any further elevation ; and it is at the expense of these,
rather than of the actual tissues of plants, that the life of
animals is sustained.

When, instead of yielding up any portion of its substance
for the sustenance of animals, the entire vegetable organism
undergoes retrograde metamorphosis, it not only gives back
to the inorganic world the binary compounds from which it
derived its own constituents, but in the descent of the several
components of its fabric to that simple condition — whether by
ordinary combustion (as in the burning of coal) or by slow
decay — it gives out the equivalents of the light and heat by
which they were elevated in the first instance.

In applying these views to the interpretation of the phe
nomena of animal life, we find ourselves, at the commence-
ment of our inquiry, on a higher platform (so to speak) than
that from which we had to ascend in watching the construc-
tive processes of the plant. For, whilst the plant had first
to prepare the pahulum for its developmental operations, the
animal has this already provided for it, not only at the ear-
liest phase of its development, but during the whole period
of its existence ; and all its manifestations of vital activity
are dependent upon a constant and adequate supply of the
same pabulum. The first of these manifestations is, as in the
plant, the building-up of the organism by the appropriation
of material supplied from external sources under the directive
agency of the germ. The ovum of the animal, like the seed
of the plant, contains a store of appropriate nutriment pre-
viously elaborated by the parent ; and this store suffices for
the development of the embryo, up to the period at which it
can obtain and digest alimentary materials for itself. That

422 COEKELATION OF PHYSICAL AND VITAL FOECES.

period occurs, in the different tribes of animals, at very dis*
similar stages of the entire developmental process. In many
of the lower classes, the embryo comes forth from the egg^
and commences its independent existence, in a condition
which, as compared with the adult form, would be as if a
human embryo were to be thrown upon the world to obtain
its own subsistence only a few weeks after conception ; and its
whole subsequent growth and development takes place at the
expense of the nutriment which it ingests for itself.

We have examples of this in the class of insects, many of
which come forth from the egg in the state of extremely simple
and minute worms, having scarcely any power of movement,
but an extraordinary voracity. The eggs having been depos-
ited in situations fitted to afford an ample supply of appropriate
nutriment (those of the flesh-fly, for example, being laid in
carcases, and those of the cabbage-butterfly upon a cabbage-
leaf), each larva on its emersion is as well provided with ali-
mentary material as if it had been furnished with a large sup-
plemental yolk of its own ; and by availing itself of this, it
speedily grows to many hundred or even many thousand
times its original size, without making any considerable ad-
vance in development. But having thus laid up in its tissues a
large additional store of material, it passes into a state which,
so far as the external manifestations of life are concerned,
is one of torpor, but which is really one of great develop-
mental activity : for it is during the pupa state that those new
parts are evolved, which are characteristic of the perfect in-
sect, and of which scarcely a trace was discoverable in the
larva ; so that the assumption of this state may be likened in
many respects to a reentrance of the larva into the ovum.
On its termination, the imago or perfect insect comes forth
complete in all its parts, and soon manifests the locomotive
and sensorial powers by which it is specially distinguished,
and of which the extraordinary predominance seems to jus-
tify our regarding insects as the types of purely animal life.

DEVELOPMENT OF INSECTS. 423

There are some insects whose imago-life has but a very short
duration, the performance of the generative act being appar-
ently the only object of this state of their existence : and such
for the most part take no food whatever after their final emer-
sion, their vital activity being maintained, for the short period
it endures, by the material assimilated during their larva
state.* But those whose period of activity is prolonged, and
upon whose energy there are extraordinary demands, are
scarcely less voracious in their imago than in their larva-
condition ; the food they consume not being applied to the
increase of their bodies, which grow very little after the as-
sumption of the imago-state, but chiefly to their maintenance ;
no inconsiderable portion of it, however, being appropriated
in the female to the production of ova, the entire mass of
which deposited by a single individual is sometimes enormous.
That the performance of the generative act involves not
merely a consumption of material, but a special expenditure
of force, appears from a fact to be presently stated, corre-
sponding to that already noticed in regard to plants.

Now if we look for the source of the various forms of
vital force — which may be distinguished as constructive,
sensori-motor, and generative — that are manifested in the dif-
ferent stages of the life of an insect, we find them to lie, on
the one hand, in the heat with which the organism is sup-
plied from external sources, and, on the other, in the food
provided for it. The agency of heat, as the moving power
of the constructive operations, is even more distinctly shown
in the development of the larva within the Qgg^ and in the
development of the imago within its pupa-case, than it is in

* It is not a little curious that in the tribe of Rotifera, or Wheel-ani-
malcules, all the males yet discovered are entirely destitute of digestive
apparatus, and are thus incapable of taking any food whatever ; so that not
only the whole of their development within the egg, but the whole of their
active life after their emersion from it, is carried on at the expense of the
store of yolk provided by the parent.

424: COEEELATION OF PHYSICAL AND VITAL FORCES.

the germinating seed ; the rate of each of these processes
being strictly regulated by the temperature to wliich the
organism is subjected. Thus ova which are ordinarily not
hatched until the leaves suitable for the food of their larvse
have been put forth, may be made, by artificial heat, to pro-
duce a brood in the winter ; whilst, on the other hand, if they
be kept at a low temperature, their hatching may be retarded
almost indefinitely without the destruction of their vitality.
The same is true of the pupa-state ; and it it remarkable that
during the latter part of that state, in which the developmental
process goes on with extraordinary rapidity, there is in cer-
tain insects a special provision for an elevation of the tem-
perature of the embryo by a process resembling incubation.
Whether, in addition to the heat imparted from without, there
is any addition of force developed within (as in the germinat-
ing seed) by the return of a part of the organic constituents
of the food to the condition of binary compounds, cannot at
present be stated with confidence : the probability is, how-
ever, that such a retrograde metamorphosis does take place,
adequate evidence of its occurrence during the incubation of
the bird's egg being afforded by the liberation of carbonic
acid, which is there found to be an essential condition of the
developmental process. During the larva-state there is very
little power of maintaining an independent temperature, so
that the sustenance of vital activity is still mainly due to
the heat supplied from without. But in the active state of the
perfect insect there is a production of heat quite comparable
to that of warm-blooded animals ; and this is effected by the
retrograde metamorphosis of certain organic constituents of
the food, of which we find the expression in the exhalation of
carbonic acid and water. Thus the food of animals becomes
an internal source of heat, which may render them independ-
ent of external temperature.

Further, a like retrograde metamorphosis of certain con-
stituents of the food is the source of that sensori-motor power

VITAL FORCES OF INSECTS. 425

which is the peculiar characteristic of the animal organism ;
for on the one hand the demand for food, on the other the
amount of metamorphosis indicated by the quantity of car-
bonic acid exhaled, bear a very close relation to the quantity
of that power which is put forth. This relation is peculiarly
manifest in insects, since their conditions of activity and re-
pose present a greater contrast in their respective rates of
metamorphosis, than do those of any other animals. Of the
exercise of generative force we have no similar measure ; but
that it is only a special modification of ordinary vital activity
appears from this circumstance, that the life of those insects
which ordinarily die very soon after sexual congress and the
deposition of the ova, may be considerably prolonged if the
sexes be kept apart so that congress cannot take place.
Moreover, it has been shown by recent inquiries into the
agamic reproduction of insects and other animals, that the
process of generation differs far less from those reproductive
acts which must be referred to the category of the ordinary
nutritive processes, than had been previously supposed.

Thus, then, we find that in the animal organism the de-
mand for food has reference not merely to its use as a mate-
rial for the construction of the fabric ; food serves also as a
generator of force ; and this force may be of various kinds —
heat and motor-power being the principal but by no means
the only modes under which it manifests itself. We shall
now inquire what there is peculiar in the sources of the vital
force which animates the organisms of the higher animals at
different stages of life.

That the developmental force which occasions the evolu-
tion of the germ in the higher vertebrata is really supplied
by the heat to which the ovum is subjected, may be regarded
as a fact established beyond all question. In frogs and
other amphibia, which have no special means of imparting a
high temperature to their eggs, the rate of development (which
in the early stages can be readily determined with great exact-

426 COREELATION OF PHYSICAL AND VITAL FORCES.

ness) is entirely governed by the degree of warmth to which
the ovum is subjected. But in serpents there is a peculiar
provision for supplying heat ; the female performing a kind
of incubation upon her eggs, and generating in her own body
a temperature much above that of the surrounding air.* In
birds, the developmental process can only be maintained by
the steady application of external warmth, and this to a de-
gree much higher than that which is needed in the case of
cold-blooded animals ; and we may notice two results of this
application as very significant of the dynamical relation be-
tween heat and developmental force — first, that the period
required for the evolution of the germ into the mature embryo
is nearly constant, each species having a definite period of
incubation — and second, that the grade of development at-
tained by the embryo before its emersion is relatively much
higher than it is in cold-blooded vertebrata generally; the
only instances in which any thing like the same stage is at-
tained without a special incubation, being those in which (as
in the turtle and crocodile) the eggs are hatched under the
influence of a high external temperature. This higher devel-
opment is attained at the expense of a much greater consump-
tion of nutrient material ; the store laid up in the " food yolk"
and " albumen" of the bird's egg, being many times greater
in proportion to the size of the animal which laid it, than that
contained in the whole egg of a frog or a fish. There is
evidence in that liberation of carbonic acid which has been
ascertained to go on in the egg (as in the germinating seed)
during the whole of the developmental process, th&t the return
of a portion of the organic substances provided for the suste-

* In the Viper the eggs are usually retained within the oviduct until
they are hatched. In the Python, which recently went through the process
of incubation in the Zoological Gardens, the eggs were imbedded in the
coils of the body ; the temperature to which they were subjected (as ascer-
tained by a thermometer placed in the midst of them) averaging 90° F.,
whilst that of the cage averaged 60° F.

DYNAMICS OF EMBRYONIC DELELOPMENT. 427

nance of the embryo, to the condition of simple binary com-
pounds, is an essential condition of the process ; and since it
can scarcely be supposed that the object of this metamor-
phosis can be to furnish heat (an ample supply of that force
being afforded by the body of the parent) , it seems not un-
likely that its purpose is to supply a force that concurs with
the heat received from without in maintaining the process of
organization.

The development of the embryo within the body, in the
mammalia, imparts to it a steady temperature equivalent to
that of the parent itself; and in all save the imijlacental or-
ders of this class, that development is carried still further
than in birds, the new-born mammal being yet more com-
plete in all its parts, and its size bearing a larger proportion
to that of its parent, than even in birds. It is doubtless ow-
ing in great part to the constancy of the temperature to which
the embryo is subjected, that its rate of development (as
shown by the fixed term of utero-gestation) is so uniform.
The supply of organizable material here afforded by the ovum
itself is very small, and suffices only for the very earliest
stage of the constructive process ; but a special provision is
very soon made for the nutrition of the embryo by materials
directly supplied by the parent ; and the imbibition of these
takes the place, during the whole remainder of foetal life, of
the appropriation of the materials supplied in the bird's egg
by the " food yolk" and " albumen." To what extent a retro-
grade metamorphosis of nutrient material takes place in the
foetal mammal, we have no precise means of determining;
since the products of that metamorphosis are probably for the
most part imparted (through the placental circulation) to the
blood of the mother, and got rid of through her excretory
apparatus. But sufficient evidence of such a metamorphosis
is afforded by the presence of urea in the amniotic fluid and
of biliary matter in the intestines, to make it probable that it
takes place not less actively (to say the least) in the foetal

428 COKEELATION OF PHYSICAL AND VITAL FORCES.

mammal than it does in the chick in ovo. Indeed, it is im-
possible to study the growth of any of the higher organisms —
which not merely consists in the formation of new parts, but
also involves a vast amount of interstitial change — without
perceiving that in the remodelling which is incessantly going
on, the parts first formed must be removed to make way for
those which have to take their place. And such removal can
scarcely be accomplished without a retrograde metamorphosis,
which, as in the numerous cases already referred to, may be
considered with great probability as setting free constructive
force to be applied in the production of new tissue.

If, now, we pass on from the intra-uterine life of the
mammalian organism to that period of its existence which
intervenes between birth and maturity, we see that a tempo-
rary provision is made in the acts of lactation and nursing
for afibrding both food and warmth to the young creature,
which is at first incapable of adequately providing itself with
aliment, or of resisting external cold without fostering aid.
And we notice that the offspring of man remains longer de-
pendent upon parental care than that of any other mammal,
in accordance with the higher grade of development to be
ultimately attained. But when the period of infancy has
passed, the child is adequately supplied with food, and is pro-
tected by the clothing which makes up for the deficiency of
other tegumentary covering, ought to be able to maintain its
own heat, save in an extremely depressed temperature ; and
this it does by the metamorphosis of organic substances, partly
derived from its own fabric, and partly supplied directly by
the food, into binary compounds. During the whole period
of growth and development, we find the producing power at
its highest point ; the circulation of blood being more rapid,
and the amount of carbonic acid generated and thrown off
being much greater in proportion to the bulk of the body,
than at any subsequent period of life. We find, too, in the
large amount of other excretions, the evidence of a rapid

CONSTEXJCTIVE FORCES OF ANIMALS. 429

metamorpliosis of tissue ; and it can hardly be questioned (if
our general doctrines be well founded) that the constructive
force that operates in the completion of the fabric will be de-
rived in part from the heat so largely generated by chemical
change, and in part from the descent which a portion of the
fabric itself is continually making from the higher plane of
organized tissue to the lower plane of dead matter. This
high measure of vital activity can only be sustained by an
ample supply of food ; which thus supplies both material for
the construction of the organism, and the force by whose
agency that construction is accomplished.

How completely dependent the constructive process still is
upon heat, is shown by the phenomena of reparation in cold-
blooded animals ; since not only can the rate at which they
take place be experimentally shown to bear a direct relation
to the temperature to which these animals are subjected, but
it has been ascertained that any extraordinary act of repara-
tion (such as the reproduction of a limb in the salamander)
will only be performed under the influence of a temperature
much higher than that required for the maintenance of the
ordinary vital activity. After the maturity of the organism
has been attained, there is no longer any call for a larger
measure of constructive force than is required for the mainten-
ance of its integrity ; but there seems evidence that even then
the required force has to be supplied by a retrograde meta-
morphosis of a portion of the constituents of the food, over and
above that which serves to generate animal heat. For it has
been experimentally found that, in the ordinary life of an adult
mammal, the quantity of food necessary to keep the body in
its normal condition is nearly twice that which would be re-
quired to supply the " waste " of the organism, as measured
by the total amount of excreta when food is withheld ; and
hence it seems almost certain that the descent of a portion of
the organic constituents of this food to the lower level of sim-
ple binary compounds is a necessary condition of the ele-

430 CORRELATION OF PHYSICAL AND VITAL FORCES,

vation of another portion to the state of living organized
tissue.

The conditions of animal existence, moreover, involve a
constant expenditure of motor force through the instrumental-
ity of the nervo-muscular apparatus ; and the exercise of the
purely psychical powers, through the instrumentality of the
brain, constitutes a further expenditure of force, even when
no bodily exertion is made as its result. We have now to
consider the conditions under which these forces are devel-
oped, and the sources from which they are derived.

The doctrine at present commonly received among physi-
ologists upon these points may be stated as follows : — ^The
functional activity of the nervous and muscular apparatuses
involves, as its necessary condition, the disintegration of their
tissues ; the components of which, uniting with the oxygen
of the blood, enter into new and simpler combinations, which
are ultimately eliminated from the body by the excretory oper-
ations. In such a retrograde metamorphosis of tissue, we
have two sources of the liberation of force : — first, its descent
from the condition of living, to that of dead matter, involving
a liberation of that force which was originally concerned in'
its organization ;* — and second, the further descent of its
complex organic components to the lower plane of simple
binary compounds. If we trace back these forces to their
proximate source, we find both of them in the food at the

* It was by Liebig ("Animal Chemistry," 1842) that the doctrine was
first distinctly promulgated which had been already more vaguely affirmed
by various Physiologists, that every production of motion by an Animal
involves a proportional disintegration of muscular substance. But he seems
to have regarded the motor force produced as the expression only of the
vital force by which the tissue was previously animated ; and to have looked
upon its disintegration by oxygenation as simply a consequence of its death.
The doctrine of the " Correlation of Forces " being at that time undevel-
oped, he was not prepared to recognize a source of motor power in the ulte-
rior chemical changes which the substance of the muscle undergoes ; but
seems to have regarded them as only concerned in the production of heat.

TEST OF ANIMAL MOTOE FORCE. 431

expense of which the animal organism is constructed ; for
besides supplying the material of the tissues, a portion of
that food (as already shown) becomes the source, in its retro-
grade metamorphosis, of the production of the heat which
supplies the constructive power, whilst another portion may
afford, by a like descent, a yet more direct supply of organ-
izing force. And thus we find in the action of solar light
and heat upon plants — whereby they are enabled not merely
to extend themselves almost without limit, but also to accu-
mulate in their substance a store of organic compounds for
the consumption of animals — ^the ultimate source not only of
the materials required by animals for their nutrition, but also
of the forces of various kinds which these exert.

Recent investigations have rendered it doubtful, however,
whether the doctrine that every exertion of the functional
power of the nervo-muscular apparatus involves the disinte-
gration of a certain equivalent amount of tissue, really ex-
presses the whole truth. It has been maintained, on the basis
of carefully-conducted experiments, in the first place, that the
amount of work done by an animal may be greater than can
be accounted for by the ultimate metamorphosis of the azo-
tized constituents of its food, their mechanical equivalent
being estimated by the heat producible by the combustion of
the carbon and oxygen which they contain ;* and secondly,
that whilst there is not a constant relation (as affirmed by
Liebig) between the amount of motor force produced and the
amount of disintegration of muscular tissue represented by
the appearance of urea in the urine, such a constant relation
does exist between the development of motor force and the
increase of carbonic acid in the expired air, as shows that
between these two phenomena there is a most intimate rela-

* This view has been expressed to the author by two very high author-
ities, Prof. Hehnholtz and Prof. William Thomson, independently of each
other, as an ahuost necessary inference from the data furnished by the
experiments of Dr. Joule.

432 CORRELATION OF PHYSICAL AND VITAL FORCES.

tionship.* And the concurrence of these independent indica-
tions seems to justify the inference that motor force may be
developed, like heat, by the metamorphosis of constituents
of food which are not converted into living tissue ; — an infer-
ence which so fully harmonizes with the doctrine of the direct
convertibility of these two forces, now established as one of
the surest results of physical investigation, as to have in itself
no inherent improbability. Of the conditions which deter-
mine the generation of motor force, on the one hand, from the
disintegration of muscular tissue, on the other from the meta-
morphosis of the components of the food, nothing definite can
at present be stated ; but we seem to have a typical example
of the former in the parturient action of the uterus, whose
muscular substance, built up for this one effort, forthwith
undergoes a rapid retrograde metamorphosis ; whilst it can
scarcely be regarded as improbable that the constant activity
of the heart and of the respiratory muscles, wliich gives
them no opportunity of renovation by rest, is sustained not so
much by the continual renewal of their substance (of which
renewal there is no histological evidence whatever) as by a
metamorphosis of matters external to themselves, supplying
a force which is manifested through their instrumentality.

To sum up : The life of man, or of any of the higher
animals, essentially consists in the manifestation of forces
of various kinds, of which the organism is the instrument ;
and these forces are developed by the retrograde metamor-
phosis of the organic compounds generated by the instru-
mentality of the plant, whereby they ultimately return to the
simple binary forms (water, carbonic acid, and ammonia),
which serve as the essential food of vegetables. Of these
organic compounds, one portion (a) is converted into the

* On these last points reference is especially made to the recent experi-
ments of Dr. Edward Smith.

SUMMAEY OF THE ARGUMENT. 433

substance of the living body, by a constructive force which
(in so far as it is not supplied by the direct agency of external
heat) is developed by the retrograde metamorphosis of another
portion (&) of the food. And whilst the ultimate descent of
the first-named portion (a) to the simple condition from which
it was originally drawn, becomes one source of the peculiarly
animal powers — ^the 'psychical and the motor — exerted by the
organism, another source of these may be found in a like
metamorphosis of a further portion (c) of the food which has
never been converted into living tissue.

Thus, during the whole life of the animal, the organism
is restoring to the world around both the materials and the
forces which it draws from it ; and after its death this resto-
ration is completed, as in plants, by the final decomposition
of its substance. But there is this marked contrast between
the two kingdoms of organic nature in their material and
dynamical relations to the inorganic world — ^that whilst the
vegetable is constantly engaged (so to speak) in raising its
component materials from a lower plane to the higher, by
means of the power which it draws from the solar rays, the
animal, whilst raising one portion of these to a stiQ higher
level by the descent of another portion to a lower, ultimately
lets down the whole of what the plant had raised ; in so
doing, however, giving back to the universe, in the form of
heat and motion, the equivalent of the light and heat which
the plant had taken from it.

19

INDEX

Air-jjan, action of the, 219, 220.
Ancient method of inquiry, 317.
Ancients, philosophic aims of the, 13.

— their mode of explaining phenomena,

103.
Andrews, Dr., 135, 162.
Animal force, derivation of, 423.

— heat, source of, 324:.

— nutrition, 421.

— system, early conception of, 212.
Anomaly in expansion and contraction, 52.
Aphides, multiplication of, 411.
Aristotle, 317.

Astronomy, tendency of its progress, xi,

— difficulties of; 319.

— early development of, 320.
Atmosphere, limit of, 186.

— pressure of, 319.

Atomic theory, objections to, 164.
Atoms, question of existence of, 347.
Authority, value of, 11.
Automatic machines, 211.

Bacon, Francis, philosophy of, 14.

Becquerel, M., 102, 125.

Beclard, M., 175.

Bergmann on electrical effects, 35.

Bessel, 241.

Brodie, Bir B., 175.

Buffon, 814.

Causation, Grove on, 15.
— secondary, 18.
Cause and effect, 251.

simultaneity of, 17.

Cause, nature of, 402.
Caoutchouc, anomaly of, 53.

Carbonic acid the index of animal power,

431.
Carnot, 224.

— on the transfer of heat, 70.
Carpenter, Dr., xxxi., xxxiv., 173.

— biographical notice of, 400.
Catalysis, 0, 169.

Chemical action, what is it ? 153.

a cause of light, 102.

produces magnetism, 162.

-T the source of mechanical power, 89T.

— affinity as a cause of motion, 152.
a cause of heat, 159.

— force converted into electrical, 154.
Chemistry of plants, 895.

Clarke, Mr. Latimer, 129, 181.

Clausius, Prof., xxviii., 104, 228.

Cohesive attraction, 171.

Cold regarded dynamically, 48.

Colding, Prof, 224.

Coleridge, 108.

Collision, effects of, 29.

Combustion, key to the phenomena of; 321.

Comets, Mayer on, 270.

Compression, heat resulting from, 32.

Conservation of force, importance of the

problem, 350.
Constancy of the sun's mass, 282.
Constructive action of plants, 417.
Cooling of the earth, 80.
Cordier, M., 307, 313.
Correlation of physical forces, meaning of

the phrase, 3, 383.

the problem to be solved, 189.

difficulties of the investigation,

194.
Cosmogony, Grove on, 81.
Cotton factory as a distributor of force, 403.

Daguerre, M., 112.
Dalton, 157, 159.

INDEX.

435

Darkness converted into light, 119,

Davy, Sir H., xxvii., 4, 134, 245.

Day, variation in length of the, 303.

De Candolle, 57.

Decay restores heat to the universe, 419.

De la Rive, 57.

Despritz, M., 76.

Democritus, 127.

Descartes, xi.

Desormes, Clement, 75.

Development of the embryo ; derivation

of its force, 427.
Diathermancy, 278.
Differentiation in animal development,

409.
Diminution of the eai-th's volume, 303.
Discovery, conditions of, 104.
Distinctions among the forces imperfect,

178.
Dove, M., 126.
Dufour, M., 96.
Dulong and Petit, 189.
Dynamic function of the germ, 412.

Earth, age of, 245.

— form of, 297.

Earth's interior heat, 236, 300.

proofs of, 302.

reasonableness of, 303.

EflFect of electrical discharge upon gases,

93. .
Electrical induction, 84.
Electricity, limitation of the term, 35.

— from caoutchouc, 35.

— the link among the other forces, 87.

— as initiating other forces, 88.

— what is it ? 83.

— produces chemical decomposition, 84.

— atmospheric, 87.

— intensity of, 89.

— effect upon the terminals, 90.

— molecular changes of conductors, 95.

— hypothesis of u fluid unnecessary, 98,

101.

— animal, 99.

— in what it consists, 101.

— initiates motion, 104.

— and heat, 108.

— and light, 107.

— produced by blowpipe flame, 155.
Electro-chemical equivalence, 894.

equivalents of power, 373.

Electro-magnetic equivalence, 893.
Emotions, correlations of, xxxiv.
Ethereal medium, 123, 242, 271, 347.
objections to, 133.

Equivalent, meaning applied to the term,
329.

Ericsson, IVIr., 78.

Erman on friction of homogeneous sub-
stances, 38.

Essences, the search for, 14.

Euler, Leonard, 128.

Events can never be repeated, 193

Expansion. Inequality of, 54.

Falling bodies, highest velocity of, 337.
phenomena of, 318.

— force, 258, 348.

Faraday, Dr., discoveries of, 6, 85, 144,
145.

— biographical notice of, 858.
Favre, M., 871.

Faye's theory of comets, 41.
Flower figures in ice, 50.
Fizeau, M,, 129.
Forbes, Jas. D., xix.
Force, indestructibility of, xii.

— use of the doctrine, xiii.

— importance of new views o^ xxx.

— Grove's definition of, 19.

— imparted to machines, 218.

— what is it? 251.

— definition of, 885.

— meaning of the term, 330, 379.

— living and dead, 333.

— animal, derived from decomposition of

food, 482.
Foucault, M., 129.
Fourier, M., 809.
Franklin, Dr„ xvii.
Fresnel on heat repellancy, 41.
Friction, definition of, 30.

— producing electricity, 33.

— of homogeneous substances, 83.

— heat of, 889.

G

Gassiot, lilr., 59, 135.
Gay Lussac, M., 167.
Gei-m -force, 411.
Germination, dynamics of, 413.
Generative force In insects, 425.
Gravity implies a plenum, 368.

— in relation to other forces, 170, 253.

— is it a force ? 839, 840, 841.

— but half imderstood, 366.

— relation to conservation of force, 363.
Grove, aim of his essay, 19.

— biographical notice of, 2.

— claims of, 5.

— electrical images on glass, 86.

— discoveries on the relations of heat to

the gases of water, 65.
Grotthus, 163.

Growth in the mammalia, 428.
Guillemin, M,, 151.

H

Heat, material hypothesis of, annihilated
by Kuraford, xxiii., xxiv.

— definition of, 89.

— dynamical effects o^ 40.

— latent, 41, 348.

— influence of structure over the con-

dition of, 57.

— produces the various forces, 57.

— reflection of, 61.

— converted into light, 63.

436

INDEX.

Heat {continued).

— chemical and magnetic influence of, 64.

— as producing motive-power, 68.

— practical application of, T7.

— produced by chemical combination,

cause of, 161.

— definition of, 226.

— force only available in the cooling pro-

cess, 228.
-7 universal law of, 261.

— sources of, 261.

— solar, its chemical origin, 266.

— developed by friction, 27T.

— upon high mountains, 282.

— lost by volcanoes, 310.

— lost by the ocean, 311.

— and motion, convertibility of, 323.

— and work, 326.

— converted into organizing force, 412.
Helmholtz, 175.

— biographical notice of, 210.

— claims of, 224.
Henry, Prof., xxviii., 290.
Herschel, Sir John, 119, 264.
Herschel, Sir W., 118, 136.

— his hypothesis of the sun's action, 260.
Hipparchus, 243.

History of scientific discovery, xv.
Holtzmann, 351.
Horner, M., 312.
Hume on causation, 16.
Huxley, Prof., 411.
Hydraulic ram, 4.

Indestructibility of matter, xii.
Inertia, relation to forces, 369.
Intellectual and physical forces, xxxv.
Intensity of the sun's heat, 270.
Interaction of natural forces, 211.
Insects, development o^ 422.

Joule, Dr. J. P., xxiv., 8, 83, 151, 224, 312.

Kant, 23.

Karsten, 85.

KirchoflTs researches, 62.

Knoblauch, M., 56, 118.

Knowledge, slow growth of, 12.

Laplace, 231, 242, 243, 291.
Latent heat, 41, 348.
Lavoisier, xiL
Lecoute, Prof., xxviii.
Liebig, vi, 174, 175, 238, 480.

— biographical notice of, 386,
Light, polarization of, 110.

— chemical action of. 111.

— affects all forms of matter, 115.

Light (conUnned).

— produces the other forces, 116.

— influence of the recipient surface upon,

120.

— and sound, analogies of, 126.

— and molecular changes, 130.

— a motion of ordinary matter, 133.

— its loss or absorption, 139.

— function of, in organization, 415.

Life of the higher animals, in what it con-
sists, 432.
Living force, 218,
Littrow, 271.

M

Machines compared with the living sys-
tem, 79.

— driving force of, 214, 387.
Muggi, Dr., 148.

Magnetism a directive force, 142.

— influence of, upon light and heat, 145.

— as an initiating force, 147.

— static and dynamic, 149.
Magneto-electric machines, 222.
Marrion, Mr., 151.

Masson, M., 135.

Mathematics, function of, in investigation,

377.
Matter, ultimate structure of may never

be known, 187.
Matteucci, 79, 84, 97, 151, 172, 175.
Mayer, Dr., 3.

— Tyndall's estimate of, xxx.

— biographical sketch of, 250.

— physiological commencement of his

inquiries, 324.

— secures his discovery against rival

claimants, 224.

— various references to, xxix., 241, 82T,

350, 389, 465.
Measure of the sun's heat, 264.
Mechanical problem, the modem, 212.
■^ force and heat, 262.

— origin of solar heat, 276.

— equivalent of heat, 316, 391.

applications of, 363.

Melloni, researches of, 59.
Mental and vital forces, xxxii.
Metamorphosis retrograde in animals, 421.
Meteorites, 270.

— heat generated by, 234.
Metaphysics in science, 860.

Modern science, alleged materializing ten-
dency of, xi.

Molecules, how the term is employed by
Grove, 89.

Moon, efi"ect of collision with the earth,
304.

Mongolfier, M., 4.

Moral forces, correlation of, xxxvii.

Morgan, Mr., 135.

Morfchini, 117.

Mossoti, 171.

Motion, as an afiection of matter, 25.

— never destroyed, 27.

— produces heat, 28.

— produces various forces, 37.

mDEX.

43T

Motion {contimied).

— an affection of matter, 186.

— vast effects of, 217.
Mountain tops, heat of, 282.

N

Nebular hypothesis, 230.
Nerve-power, source of, 430.
New views, public acceptance of, 9.
New doctrine of forces, how far accepted,

xiv.
Newton, xiii., xvlii., 241, 276, 282, 368, 378.

— speculations on light, 137.
Niepce de St. Victor, M., 125.
Numbers the highest aim of investigation,

Oersted's discoveries, 5.
Optic axis of crystals, 171.
Organic beings, source of their motions,
237.

— kingdoms of nature, relations to each

other, 433.

— co-ordination, 410.

— correlations, Grove on, 172.
Organization, distinction between high and

low grades of, 408.
Origin of the sun's heat, 276.
Origin of terrestrial power, 236, 340.

Page, Mr., 151.

Particles, in what sense the term is used
by Grove, 39.

Pasteur, M., 132.

Peltier, M., 96.

Perpetual motion, 191, 192, 212, 219,221,
223, 388.

Persistence of force, xxxix.

Photography, chemistry of. 111.

Physical science of the present distin-
guished from that of the past, 402,

Planetary atmospheres, 137.

— motions, 267.

— system, structure of, 230.
Planets, temperatures of, 121.
Plants, chemistry of, 420.
Plenum, a universal, 134.
Plucker, 171.

Plurality of worlds, 122.

Poisson, M., 81.

Pouillet, M., 244, 264, 280.

Powell, Baden, on Newton's rings, 41.

Practical arts after the middle ages, 211.

Ptolemaic system, 11.

Qualities of matter, transference of, 15.

E

Kanktne, Mr., xxviii.
Eegnault, M., 225.

Eespiration in cold-blooded animals, 429.

Eoget, Dr., 4.

Eotation of the earth, diminution of the,

299.
Eoyal Institution, foundation of the, xxx.
Eule for the investigation of nature, 316,

318.
Eumford, Count, 4.

— summary of his claims, xv.

— sketch of, xvii.

— researches of, xx., 847.

Seebeck, Dr., 118.

Seguin, M., 4, 76.

Sensations of heat disturb the judgment,
49,

Sensations, misleading, 174.

Science, tendency of, toward immaterial-
ism, xii.

— the great event in the general progres*

of, xvi.

— the true scope of, xxxi.

— discovery of its fundamental principle,

816.
Simultaneous discovery, examples of, xv.
Social forces, correlation of, xxxvi.
Solar radiation, effects of, 286.
origin of organic power, 240.

— heat, amount of, 243, 264.

— spots, 286.

— atmosphere, 288.

— radiation, dynamical effects of, 403, 404.
Sondhauss, Mr., 126.

Sound and light, 259.

Specific heat, dynamic view of, 58.

Spectrum, analysis of, 62.

Spencer, Herbert, on the persistence of

force, xxxix.
Steam engine, action of, 220.
Stephenson, George, 115.
Stewart, Balfour, 61.
Store of force in the universe, 282.
in the planetary system at present,

Structure of bodies as affecting heat-
motion, 55.
Sun, cooling of, 265.

Talbot, Mr., 112.
Tension, a static force, 22.
Theories, immature, 110.

— new, how they are to be judged, 196.

— the test of, 276.
Theorizing, what is it ? 198.
Thermography, 58.

Thilorier's experiment on carbonic acid, 48.
Thompson, Prof., 52, 227, 229.
Tidal wave, 241, 291.

Time as an element of dynamic changes,
860.

— an element in, the sequence of phe-

nomena, 17.
— • necessity of its early measurement, 320.

488

INDEX.

Torricelli, 108.

Tainsmutation of force chemically illus-
trated, 872.
Tyndall, xxx., 67.

U
Units of heat produced in combustion, 261.

Vacuum, not yet obtained, 184.
Vital and physical forces, correlation ofi
xxxii.

— eflfects, crude explanations of, 401.
>— activity, characteristics of, 407.

of plants, purpose of, 418^

— principle, 402, 420.
Vis viva, 218, 263.
Volcanoes, loss of heat by, 810.
Volta, 153.

Voltaic arc, cause of light oi; 88.

W

Wartmann, Mr,, 146.

Water, expansion of, as it approaches

freezing, 50.
"Water-power, force produced by, 216.
Watt, James, 75.
Wortheim, 96, 150.
Whewell, Dr., xxvi., 136.
WoUaston, Dr., 136.
Wilson, Dr., 136.

Words, misguiding influence of, 94.
— their social import, 180.
Work performed by chemical force, 226.
Wood, Dr., 54, 160.

Young, Dr. Thomas, xxvii., 124, 125, 128,
129,13a

Zodiacal light, 272.

THE END.

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