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CORRELATION OF PHYSICAL

FORCES :
BEING THE SUBSTANCE OF A COURSE OF LECTURES
DELIVERED IN THE LONDON INSTITUTION,
IN THE YEAR 1843.
, ſº -: -
By W. R. GROVE, Esq., M. ... }' R.S.,
BAR.R. STER A C I,A §7. -
PRINTED AT THE REQUEST of THE PROPRIETORS
OF THE LONJJON INSTITUTION,
L () NI) O N :
SA MUEL H IG HLEY, 32, FLEET STREET.
1846.
PR E FA C E.
THE subjects of the Course of Lectures embraced by the
title of this sketch are much better adapted to oral than
to written explanation; they demand at every step experi-
mental illustration, and suggest questions which the Lecturer
has an opportunity of answering, but which the writer
cannot anticipate. .
Notwithstanding the manifold difficulties which surround
the task, I am anxious to comply as far as in me lies with
the wishes of the Proprietors of The London Institution,
and to furnish them with a short resumé of some of those
views of Physical Science which I have, in their Theatre,
endeavoured to inculcate.
Although these views have not been lost sight of in any
of my Courses of Lectures, yet they were more prominently
brought forward in the Course delivered in 1843, under
the same title as heads this Essay; and it is a short
exposition of the principles advanced in this Course, not
a report of the Lectures themselves, which I am here
attempting to set forth. As far as I am able, I shall give
expression in this abstract to such views only as I offered
at the time of the Lectures.
iv PR.EFACE.
In a Lecture which I gave in January, 1842, on the
- Progress of Physical Science (printed at the request of the
Proprietors), I have briefly enunciated the propositions which
were explained and illustrated by the Course of 1843, of
which Course a tolerably full report appears in the Literary
Gazette for January, 1844. As it would interrupt the
consecutive developement of my own ideas were I fre:
quently to quote, in the body of this paper, authorities
either opposing or coinciding with them, and as I might
also be charged with misconstruction of their views, I give,
at the conclusion, references to such passages of different
authors, published previously to my Lectures, as I have,
since their date, found in any way bearing upon the views
I have ventured to put forth : most of these were kindly
furnished me by Mr. Brayley.
My professional occupations having compelled me to
resign an office which I held for five years, I cannot let
this opportunity pass without expressing my grateful sense
of the uniform kindness I have received from the Proprietors
of The London Institution.
4, Hare Court,
Temple, AM/ust 31, 1846.
:
<d#
.
CORRELATION OF PHYSICAL
FORCES.
WHEN natural phenomena are for the first time observed, a
tendency immediately developes itself to refer them to some-
thing 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 already 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-ordinates 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 pos-
sibility, 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 facts. - -
The theory consequent upon new facts, whether it be a
co-ordination of them with known ones, or the more difficult
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 that
B
2 CORRELATION OF PHYSICAL FORCES.
period defers; others are not bold enough, or if they be so,
are not listened to. The earliest theories thus enunciated,
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 received solely
or mainly upon authority; there being no means of con-
tradiction, it becomes received, in the first instance perhaps,
with some degree of doubt, but as the time in which it
can fairly be investigated, 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 be-
comes, for want of a better, received as an established
truth; it is handed from father to son, and gradually takes its
place in education. Succeeding generations whose minds are
thus formed to an established view, are much less likely to
abandon it. They have adopted it, in the first instance,
upon authority, 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 undertake, 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 revolutions.
This necessity has its good, but the evil with regard
to the point we are considering 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 incorrect, as that which is formed at the
first flush of a new discovery, and though time exalts the
authority of those from whom it emanates, time can never
give to the illustrious dead such means of analysing and
correcting erroneous views as subsequent discoveries confer.
Take for instance the Ptolemaic System, which we may
CORRELATION OF PHYSICAL FORCES. 3
almost literally explain by the expression of Shakespear,
“He that is giddy thinks the world turns round;”—we now
see the error of this system, because we have all an imme-
diate opportunity of refuting it, but this identical error was
received as a truth for centuries, because, when first pro-
mulgated, the means of refuting it were not at hand, and
when the means of its refutation became attainable, man-
kind had been so educated to the supposed truth, that they
rejected the proof of its fallacy.
I have premised this for two reasons, first, to obtain a
fair hearing by requesting as far as possible that dismissal,
from the minds of my readers, of preconceived views, by
and in favour of which we are all liable to be prejudiced;
and secondly, to defend myself from 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
comparison with the giant.
The subject on which I am about to treat, viz., the
relation of the affections of matter, generally called the Im-
ponderables, to each other and to matter, peculiarly demands
an unprejudiced regard. The different ways in which these
agencies have been regarded; 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 experience, present difficulties
almost insurmountable. Without, however, dilating on
these difficulties, I will first shortly sketch what I conceive
to have been and to be, in some degree, the motions of
the public as to the power and aims of physical science,
and then proceed to the examination of the views which
B 2
4 CORRELATION OF PHYSICAL FORCES.
I have had the honor to expose to you in the Course of Lec-
tures alluded to in the Preface.
Not to waste your time with the 3 priori speculations
of the early philosophers, who, instead of examining phe-
momena, and collating instances, until they arrived at general
analogies and differences, assumed certain axioms or asserted
truisms, and then, by syllogistic deductions, carried them out
into complex, and, to us, somewhat unintelligible, conclusions;
I will pass to Bacon, the great remodeller of science, who
entertained the notion that, by experimentally 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 Platonic philosophy, but differently
applied. He appears to have understood by form—the
essence of quality, that in which, abstracting every thing
extraneous, a given quality consists, or that which superim-
duced on any body would give it its peculiar quality; thus
the form of transparency is that which constitutes transpa-
rency, or that by which, when discovered, transparency
could be produced or superinduced. To take a specific ex-
ample of what I may term the synthetic application of his
philosophy. “In gold there meet together yellowness,
gravity, malleability, fixedness in the fire, a particular
manner of flowing in the fire, a determinate way of solution,
which are the simple natures in gold; for he who under-
stands form, and the manner of superinducing this yellow-
ness, gravity, ductility, fixedness, faculty of fusion, solution,
etc., with their particular degrees and proportions, will con-
sider how to join them together in some body, so that a
transmutation 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 gene-
coRRELATION OF PHYSICAL FORCEs. 5
º
rated, from its first fluid matter or rudiments, up to a perfect
mineral,” is to be perceived by what Bacon calls the latent
process, or a search for “what in every generation or trans-
formation of bodies, flies off, what remains behind, what is
added, what separated, etc., also, in other alterations and
motions, 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, by which view one phenomenon
is supposed necessarily to hang on another, and that on
another, until at last we arrive at an essential cause, subject
immediately to the GreatPirstCause. This motion is generally
prevalent both on the Continent and in this country: nothing
is more familiar to you than the expression, “study the
effects 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 totally 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 permissible, it should be only with reference to the special
phenomena referred to, as it can never be generalised.
The misuse, or, rather, varied use, of the term Cause, has
been a source of great confusion in physical theories, and
philosophers are now by no means agreed as to their con-
ception of causation. The most received view of causation,
6 CORRELATION OF PHYSICAL FORCES.
that of Brown, refers it to invariable antecedence, i.e., we
call that a cause which invariably precedes, that an effect
which invariably succeeds. Physically speaking, it is diffi-
cult 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, because 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
proposition, and say, generally, that gravitation is the cause
of water flowing, as water may flow from other causes,
atmospheric pressure, for instance, which causes water to
flow into a vacuum,-and gravitation may, under certain
circumstances, arrest, instead of causing, the flow of water.
Upon neither view, then, can we get at anything like
abstract causation; if, with Brown, we regard causation as
invariable sequence, 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 causation as a force, we could say that water could
be caused to flow only by gravitation, we might say abstract-
edly that gravitation 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 particular case, and cannot be supported as an abstract
proposition,-yet this is constantly attempted.
COR,RELATION OF PHYSICAL FORCES. 7
Electricity and Magnetism afford us a very instructive
example of the belief in secondary causation. Subsequent
to the discovery by Oersted of Electro-Magnetism, and prior
to that by Faraday of Magneto-Electricity, Electricity and
Magnetism were believed by the highest authorities to stand
in the relation of cause and effect, i.e., electricity was re-
garded as the cause, and magnetism as the effect, and where
magnets existed without any apparent electrical currents to
cause their magnetism, hypothetical currents have been sup-
posed, for the purpose of carrying out the causative 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; again, if electricity cause
magnetism, and magnetism cause electricity, why then elec-
tricity causes electricity, which is absurd.
To take another instance, which may render these posi-
tions more intelligible. By heating two bars of Bismuth
and Antimony 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 abstractedly that heat is the cause of electri-
city, or that electricity is the cause of heat P Certainly not,
for if either be true, both must be so, and the effect then be-
comes the cause of the cause, or, in other words, a thing
causes itself. If you will put any other proposition on this
subject, you will find it involve similar difficulties, until, at
length, your minds will become convinced 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 imponderable agencies, or the affections
8 CORRELATION OF PHYSICAL FORCES.
of matter which constitute the main objects of experi-
mental physics, viz., Heat, Light, Electricity, Magnetism,
Chemical Affinity, and Motion are all Correlative, or have
a reciprocal dependence. That neither, taken abstractedly,
can be said to be the essential or proximate cause of the
others, but that either may, as a force, produce or be
convertible into the other; thus heat may mediately or
immediately produce electricity, electricity may produce
heat; and so of the rest.
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 five other affections of matter,
which I have above named, are, and will ultimately be
resolved into, modes of motion, it would be going too far,
at present, 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 induces its
various changes.
Let us begin with MOTION.—the most obvious, the most
distinctly conceived of all the affections of matter. It is a
proposition universally received since the time of Newton,
that a body in motion will continue so for ever, in the same
direction, and with the same velocity, unless impeded by
some other body, or affected by some other force than that
which originally impelled it; but it is also very generally
believed, that if the visible or palpable motion be arrested
by impact on another body, the motion ceases, and the
force which produced it is annihilated. On the other hand,
the view which I venture to submit to you in these lectures
is, that force cannot be annihilated, but is merely subdivided
or altered in direction or character.
First, as to direction. Wave your hand; the motion
p
CoRRELATION OF PHYSICAL FORCEs. 9
which has apparently ceased is taken up by the air, from
the air by the wall of the room, and so, by direct and re-
acting waves, continually comminuted, but never destroyed.
It is true that, at a certain point, we lose all means of de-
tecting the motion, from its minute subdivision, which defies
our most delicate means of appreciation, but we can inde-
finitely extend our power of detecting it, accordingly as we
confine its direction or increase the delicacy of our exami-
nation. Thus, if the hand be moved in unconfined air, the
motion of the air would not be sensible to a person 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 re-
strained, so as to make its 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; while the same puff of air which
would, in the air-gun, project a bullet a quarter of a mile ;
if allowed to escape without restraint, as steam does in the
candle bomb, would not be perceptible at a yard distance,
though the same absolute amount of motion is impressed
on the surrounding air.
It may, however, be asked, what becomes of force when mo-
tion 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 mode of force now ensues, the exponent of
which, instead of visible motion, is Heat. I venture to regard
the heat which results from friction or percussion, as a con-
tinuation of the force which was previously associated with
10 CORRELATION OF PHYSICAL FORCES.
the moving body, and which when this impinges on another
body, ceasing to exist as gross palpable motion, continues to
exist as heat. Thus, let two bodies, A and B, be supposed
moving in opposite directions (putting for the moment out
of the 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 respective 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 resulting heat is in direct proportion
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 com-
municated to A, then we have less heat 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, the heat produced is very trifling,
because the particles of the fluid themselves move, and con-
tinue the motion originally communicated to the moving
body; for every portion of motion 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
CORRELATION OF PHYSICAL FORCES. 11
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
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 motion of the bodies themselves, but less heat; if we
increase the resistance to motion, as by roughening the points
of contact, so that each particle strikes against and impedes
the motion of others, then we have diminished motion, but
increased heat. I cannot present to my mind any case of
heat resulting from friction which is not explicable by this
view ; friction, according to it, is simply impeded motion,
and the resulting heat is a continuation of indestructible
force, capable, as we shall presently see, of reproducing
palpable motion, or motion of definite masses.
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 these Lectures, I have not found such facts con-
firmed by my own experiments, and I am happy to see this
conclusion corroborated by some experiments of Professor
Erman, communicated to the last meeting of the British
Association, in which he finds no electricity to result from
the friction of perfectly homogeneous substances, as, for in-
stance, the ends of a broken bar. Such experiments as these
will, indeed, be seldom free from slight electrical currents,
12 CORRELATION OF PHYSICAL FORCES.
on account of the practical impossibility of fulfilling the con-
dition of perfect homogeneity in the substances themselves,
their size, their temperature, etc., but as the effects produced
vary in direction, the resultant effect is nought ; indeed it
would be difficult to conceive the contrary : how could we
possibly image to the mind or describe the direction of a
current from the same body to the same body, or give
instructions for a repetition of the experiment It would
be unintelligible to say, that in rubbing two pieces of
bismuth together, a current of electricity circulated from
bismuth to bismuth, or from iron to iron, or from glass to
glass, for the question immediately occurs, from which
bismuth to which does it circulate P And should this ques-
tion be answered, by calling one piece A, the other B, this
would only apply to the particular specimens employed, and
without heterogeneity, or a distinction in qualities, the
phenomenon is absolutely indescribable: we may say that it
circulates from rough glass to smooth, from cast iron to
wrought, for here there is not homogeneity. It is, in-
deed, conceivable that when the motion is continuous 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, parallel currents of posi-
tive and negative electricity may be conceived circulating
within the metals, and be described by reference to the
direction of their motion, but this would be a totally dis-
tinct phenomenon from those we are considering, and is,
experimentally, unknown.
We may say, then, that in our present state of know-
ledge, where, and in proportion as, homogeneity exists in
the mutually impinging bodies, heat and not electricity is
the result of friction and percussion; and, secondly, we may
say, that where, and in proportion as, the bodies impinging
CORRELATION OF PHYSICAL FORCES. 13
are heterogeneous, electricity results; never practically un-
accompanied by heat, because perfect heterogeneity is in-
conceivable, but greater in proportion as the bodies differ,
and as the heat is less; thus the friction of similar metals
gives little electricity, that of dissimilar metals more, and
that of a metal and glass still more, as in the common
electrical machine, the action of which is so greatly exalted
by the metal mercury. , ſº
It is highly probable that heat and electricity are
themselves modes of motion; that the gross or palpable
motion, which is arrested by the contact of another body, is
subdivided into molecular vibrations or undulations, which
vibrations are heat or electricity as the case may be. To
discuss this theory in full would lead me into a compa-
rative view of the corpuscular and dynamic theories of heat,
too long and too foreign to my present object to be
detailed here: it will be seen in the course of this essay
that I incline to the dynamic theory; my principal aim
here, however, is rather to shew the relation of forces, as
evinced by acknowledged facts, than to enter upon any
explanation of their specific modes of action.
I have said that either of the forces above mentioned 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 I will venture as an opinion, founded after much
consideration, that science is rapidly 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 maſſ-
netism, a force which is always developed by electrical cur-
14 CORRELATION OF PHYSICAL FORCES.
rents, at right angles to the direction of those currents:
Light also is readily produced 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 all the attributes of common light, its sole dif-
ference being, as far as I am aware, the position of the fixed
lines in its spectrum—a difference which ever obtains with
light emanating from different sources, or seen through
different media. In the decompositions and compositions
which the terminal points proceeding from the conductors of
an electrical machine, develope when immersed in different
chemical media, we get the production 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 divergence of the elec-
trometer, the revolution of the electrical wheel, the deflection
of the magnetic needle, are palpable movements reproduced
by the intermediate modes of force, which have themselves
been originated by motion.
If we now take HEAT as our starting point, we shall
find 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 resolve heat into motion, and view it mecha-
nically as a repulsive force, a force antagonist to attraction,
whether of cohesion or gravitation, and tending to move
the particles of all bodies, or to separate them from each
other: thus, if water be heated, its particles move asunder,
forming steam, and this molecular motion will instantly
produce the motion of masses, as in the steam engine;
whether we raise a piston by weight and pulley, or by
heating the air and vapour beneath it, motion becomes the
exponent of the amount of heat, as it does of the amount
CORRELATION OF PHYSICAL FORCES. 15
of gravitation: nor do we by any of our ordinary methods
test heat in any other way than by its purely dynamical
action—the various modifications of the thermometer and
pyrometer are all measurers 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 are moved.
The phenomena of what is termed latent heat, have been
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
diffidence 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 ab-
solute necessity, the more so, as these subtleties are apt to
be carried on to other natural phenomena, and so they add
to the hypothetical 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 doc-
trine of “invisible light,” and I do this meaning no disre-
spect to its distinguished author, any more than in discussing
the doctrine of latent heat can I be supposed, in the slightest
degree, to detract from the merits of the illustrious inves-
tigators of the facts which that doctrine seeks to explain-
Is not “invisible light” in fact 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
16 CORRELATION OF PHYSICAL FORCES.
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 :
if 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 an
erroneous momenclature which calls such an agent, light—
a deviation from the plain accepted meaning of words
which, when it takes place, is always injurious to that
precision of language, which is the safest guard to know-
ledge, and from the absence of which physical science has
materially suffered. -
Let us now shortly examine this question, and see
whether the phenomena cannot be as well, if not more
satisfactorily, explained without the hypothesis of latent
matter. Latent heat is supposed to be the matter of heat,
associated, in a masked or dormant state, with ordinary
matter, not capable 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 sen-
coRRELATION OF PHYSICAL FORCEs. 17
sible, 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 retain
it in the liquid state, which it renders latent, or retains asso-
ciated with itself, as 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
connected with a change of state, may be explained and
distinctly comprehended without recourse to the conception
of latent heat, though it requires some effort of the mind to
divest itself of this idea, and to view the phenomena simply
in their dynamical relations. Let us first parallel with
purely mechanical actions certain simple effects of heat,
where change of state, (I mean such change as from the
liquid to the solid, or gaseous to the liquid), is not con-
cerned. Thus, place a bladder within a receiver, 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 mechanically by an
exhausting pump, the bladder also expands; conversely
increase the repellent force without, 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, i. e., from the muscular force which
works the air-pump, or, it might be, from gravitation, as
in a water-wheel, reacting elasticity, as in a coiled spring,
or any similar force. 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, pro-
C
18 CORRELATION OF PHYSICAL FORCES.
duces a correlative contraction in the other; thus, a
bladder partly filled with cold air, and contained within
another filled with hot air, expands, while the external one
contracts, exhibiting a mere transfer of the same amount of
repulsive force, the mobility of the particles, or their mu-
tual attraction of cohesion, being the same in each body; in
other words, the repulsive force acts in the direction of least
resistance until equilibrium is produced, it them becomes
a static or balanced, instead of a dynamic or motive force.
Let us now consider the case where a solid is to be
changed to a liquid, Or, still more, a liquid to a gas;
here a much greater 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 re-
pellent power of the hot particles is transferred to the cold
particles, and separates them in their turn, but the anta-
gonist 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 different effect on
a body such as the common thermometer, the expanding
liquid of which does not undergo a similar change of state.
Thus, in the example above given, in the first mixture, the
hot and cold water and the mercury of the thermometer
* The recent experiments of Henry and Donny have shewn that the cohesion
of liquids, as far as their antagonism to rupture goes, is much greater than has
generally been believed; these experiments, however, make no difference in the
view I am putting forth, as, whatever be the character of the attraction, there is
a molecular attraction to be overcome, in changing bodies from the solid to the
liquid state, which must require and exhaust force.
CORRELATION OF PHYSICAL FORCEs. 19
being all 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 thermometer 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 equivalent of repellent force.
In the second instance, the substance we use as an indicator
does not undergo the same physical change as those whose
dynamical relations we are examining. The force, viewing
heat simply as mechanical force, which is employed in loosen-
ing or tearing asunder the particles of the solid, ice, is ab-
stracted from the liquid water, and from the liquid mercury
of the thermometer, and in proportion as this force 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 contraction.
Another very usual mode of regarding the subject may
embarrass you at first sight, but a little consideration will
shew you that it is explicable by the same doctrine. You
may have experimentally observed, or seen stated in treatises,
that water which has ice floating in it will give, when mea-
sured by the thermometer, the same temperature as the
ice; i.e., both the water and ice contract the mercury of the
thermometer to the point conventionally marked as 32°.
You may say, how is this reconcileable with the dynamical
doctrine, for, according to that, the solid should take from
the mercury of the thermometer more repulsive power than
the liquid, consequently, the ice should contract the mercury
more than the water?
My answer is, that in the proposition as usually stated,
the quantities of the water, ice, and mercury, are not taken
into consideration, and hence a necessary dynamical element
C 2
20 coRRELATION OF PHYSICAL FORCEs.
is neglected: if you take notice of the element of quantity,
this objection will not apply. Let the thermometer, for
instance, contain 13% oz. of Mercury, and stand at 100°; if
placed in contact 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 mer-
cury sinks also to 32°, not absolutely, perhaps, because,
however great the quantity of water or ice, it will be some-
what raised in temperature by the warmer mercury. This
elevation of temperature above 32° will be smaller, in pro-
portion as the quantity of water or ice is larger, than the
quantity of mercury; and, theoretically speaking, 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
liquefy the whole, provided 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 ele-
vation of temperature above the freezing point liquefies ice,
every portion should be liquefied. Practically speaking, how-
ever, in both cases, that of the 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°,
in the latter to 32°: apply to this the doctrine of repulsive
force, and we get a satisfactory explanation.
In the first case, the quantities both of ice and water
being indefinitely great in respect to the mercury, each
reduces it to its own temperature, viz., 32°, and the ice can.
not reduce the mercury below 32°, because if so, it would
receive back repulsive power from the newly formed water,
and this would become ice; in the second case, where the
CORRELATION OF PHYSICAL FORCES: 21
quantities are limited, the mercury does lose more repulsive
power by the ice than by the water, and the observations
I have made in reference to my first illustration, apply.
I do not here notice certain exceptional cases, as the
expansion of water and other liquids at a certain degree of
cold, for these are exceptions upon any view, and explicable
by other interfering dynamic causes, such, possibly, as crys-
talline polarization, leaving interstitial spaces.
The general phenomena of heat may, I believe, be all
explained upon a purely dynamical view, and without having
recourse to the hypothesis of latent matter.
Thus much for the correlation of heat with motion,
and if Heat be a force capable of producing motion, and
motion be capable of producing the other modes of force,
it necessarily follows, that heat is capable, mediately, of pro-
ducing them; we will, therefore, content ourselves with in-
quiring how far heat is capable of immediately producing
the other modes of force? It will immediately produce
electricity, as shewn in the beautiful experiments of Seebeck,
one of which I have already cited, which experiments proved,
that when dissimilar metals are made to touch, or are sol-
dered together, and heated at the point of contact, a current
of electricity flows through the metals, having a definite
direction according to the metals employed, which current
continues as long as the temperature continues increasing,
or rather, as long as an increasing temperature is gradually
pervading the metals, ceases when that temperature is sta-
tionary, and flows in the contrary direction with the decre-
ment of temperature.
To say that heat will produce light, is in common par-
lance to assert a fact familiar to every one, but there may
be some reason to doubt whether the expression to produce
light is correct in this particular application; the relation
22 CORRELATION OF PHYSICAL FORCES.
between heat and light is not analogous to the correla-
tion between these and the other four affections of matter.
Heat and light appear to be rather modifications of the
same force than distinct forces mutually dependent. The
modes of action of radiant heat and of light are so similar,
both being subject to the same laws of reflection, refraction,
double refraction and polarization, that their difference ap-
pears to exist more in the manner in which they affect our
senses, than in our mental conception of them.
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 magnetic effects, and also of forming and decom-
posing 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
produced by it, because so little of the heat is utilized or
converted into electricity, much being dissipated, without
change, in the form of heat.
Heat however directly affects and modifies both the
magnet and chemical compounds; the union of certain
chemical substances is induced by heat, and cannot take
place without it, as for instance, the formation of water by
the union of oxygen and hydrogen gases, in other, indeed,
in the greater number of cases, this union is facilitated by
heat, and in some instances, as in ammonia and its salts, it
is weakened or antagonized; in all these cases, however, it
must be admitted, that the force of heat seems more a de-
termining than a producing influence, yet to be this, it must
have an immediate relation with the force whose reaction
CORRELATION OF PHYSICAL FORCEs. 23.
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, re-
quires a force in direct relation with those which maintain
equilibrium. tº -
ELECTRICITY is that affection of matter or mode of force,
which most distinctly and beautifully relates other modes
or force, and exhibits, to a great extent, in a quantitative
form, its own relation with them, and their reciprocal rela-
tions with it, and with each other. From the manner
in which the peculiar force called Electricity, is apparently
transmitted through certain bodies, such as metallic wires,
the term current is commonly used to demote its progressive
direction. 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-re-
pulsive 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 attentively studied electrical phenomena, in view-
ing them as resulting not from the action of a fluid or
fluids, but as a molecular polarization 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; in the electrolysis of
water, for instance, a molecule of oxygen is supposed to be
displaced by the exalted attraction of the neighbouring elec-
24 CORRELATION OF PHYSICAL FORCES.
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 no-
thing else than this molecular transmission of chemical affi-
nity.—Again the electric spark, the brush, and similar phe-
nomena, the old theories regarded as actual emanations of
the matter or fluid, Electricity; I venture to regard them as
produced by an emission of the material itself, from whence
they issue, and a molecular action of the gas or intermedium,
through or across which they are transmitted.
The colour of the voltaic arc, or electric spark, is depen-
dent upon the substance of the metal, subject to such modi-
fications of the intermedium; thus, the electric spark or arc
from zinc is blue, from silver green, from iron red and scin-
tillating, 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 electrodes or terminals can be readily
collected, tested, or even weighed; it would thus appear
that the electrical discharge arises from an actual repulsion
and severance of the electrified matter itself, which flies off
at the points of least resistance.
A careful examination of the phenomena attending the
electric discharge, or voltaic arc, which is the electric dis-
charge acting on greater portions of matter, tends to modify
considerably our previous ideas of the nature of the electric
force of ignition, and also of combustion. The voltaic arc
is, perhaps, strictly speaking, neither ignition or combustion:
it is not ignition, because the matter of the terminals is not
merely brought to a state of incandescence, but is physically
separated and partially transferred from one electrode to
coRRELATION OF PHYSICAL FORCES. 25
another, much of it being dissipated in a vaporous state; it
is not combustion, for the phenomena will take place inde-
pendently 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 a close vessel, in vacuo
or nitrogen, the substance forming the electrodes is con-
densed, and precipitated upon the interior of the vessel,
chemically speaking in an unaltered state; thus, to take
a very striking example, if the voltaic discharge be taken
between zinc terminals in an exhausted receiver, a fine
black powder of zinc is deposited on the sides of the
receiver; this can be collected, 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 more
simple, as those who were present at these Lectures may
recollect. Iron is volatilised in nitrogen or vacuo, and
when a scarcely perceptible film has lined the receiver, this
is washed with an acid, which then gives, with ferrocyanide
of potassium, the prussian blue precipitate : in this case
we readily distil iron, a metal fusible only at a very high
temperature.
Another strong evidence that the voltaic discharge con-
sists of the material itself of which the terminals are com-
posed, 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.
26 coRRELATION OF PHYSICAL FORCES.
I have entered very fully into this theory in my Lectures
on Electricity delivered in the Theatre of this Institution,
and it would lead me into too long a digression from the
principal subject of this essay, were I further to dilate on it :
I will content myself with requesting my readers not to dis-
miss it as a hastily formed view, without applying it carefully
to all the recorded phenomena of electricity; and although,
at first sight, some of these may seem inconsistent with it,
yet, upon examination, they will, I feel convinced, find these
apparent inconsistencies disappear, or at all events they will
find the difficulties of other theories more insuperable. In-
deed, 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.
To commence, then, with Electricity as an initiating
force, we get motion directly produced by it in various
forms; for instance, 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. Electricity directly produces heat, as
shewn in the ignited wire, the electric spark, and the voltaic
arc, in the latter the most intense heat with which we are ac-
quainted, so intense, indeed, that it cannot be measured, every
sort of matter being dissipated by it. Electricity directly pro-
duces light in the same phenomena. It directly produces
ma/netism in all ferruginous bodies placed at right angles to its
line of direction, and, indeed, in the substances, of whatever
nature, traversed by the electrical current, in a direction at
right angles to that of the current; in this case giving us a
new character of force, viz., a force acting not in direct
CORRELATION OF PHYSICAL FORCES. 27
straight lines, but in a tangential or rather rectangular direc-
tion. This representation of transverse direction by magnet-
ism 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, remains to be proved.
Lastly, electricity directly produces chemical affinity, and
by its agency we are enabled to obtain effects of analysis or
synthesis, with which ordinary chemistry does not furnish us.
Of these effects we have examples in the brilliant disco-
veries by Davy of the alkaline metals, and in the peculiar
crystalline compounds made known by Crosse and Becquerel.
LIGHT is, perhaps, that mode of force the reciprocal
relations of which with the others has been the least traced
out. Until the discoveries of Daguerre and Talbot, very
little could be definitely predicated of the action of light
in producing other modes of force; and, even, since these
discoveries, it is doubted by many competent investigators,
whether the phenomena of photography are not mainly de-
pendant upon a separate agent accompanying light, rather
than upon light itself. It is, indeed, difficult not to believe
that a picture, taken in the focus of the camera obscura, and
which represents to the eye all the gradations of light and
shade shewn by the Original luminous image, is not an effect
of light; certain it is, however, that the different coloured
rays exercise different actions upon various chemical com-
pounds, and that the effects on many, perhaps 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 pronoun-
cing myself positively upon the question, hitherto so little
examined, I think it will be safer to regard the action on
28 CORRELATION OF PHYSICAL FORCES.
Photographic compounds as resulting from a function of
light: so viewing it, we get light as an initiating 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. Those who were present at these
Lectures will recollect the experiment in which I shewed
the production of all the other modes of force by light:-
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; between 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 Breguet's helix; the other
extremities of the galvanometer and helix are connected by
a wire, and the needles brought to zero. As soon as a beam
of either day-light or 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.
There are other apparently more direct agencies of light
in producing electricity and magnetism, such as those
observed by Morichini and others, as well as its effects
upon crystallization, but these results have hitherto been of
so indefinite a character, that they can only be regarded as
presenting fields for experiment, and not as proving the re-
lations of light to the other forces.
Light was regarded by what was termed the corpuscular
theory, as being in itself matter or a specific fluid, ema-
nating from luminous bodies, and producing the effects of
sensation by impinging on the retina; this theory gave way
CORRELATION OF PHYSICAL FORCES. 29
to the undulatory 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 permeate the pores of all bodies.
In a Lecture which I delivered in the Theatre of this
Institution, in January, 1842, I stated, that it appeared to
me more consistent with the facts, to regard light as re-
Sulting from a vibration of the molecules of matter itself,
and not from a specific ether pervading it; just as sound is
propagated by the vibrations of wood, or as waves are by
water.”
The transparency or opacity of a body appears to depend
entirely upon its molecular arrangement. Thus, if striae
occur in a lens or glass through which objects are viewed
the objects are distorted; increase the number of these striae,
and the distortion is so increased that the objects become
invisible, and the glass ceases to be transparent, though re-
maining 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 transparent, but agitate them to-
gether so as to form a froth or lather, and this, though con-
sisting of two transparent bodies, is opaque; and the re-
flexion of light from the surface of these bodies, when so
intermixed, is strikingly different from its reflection 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.
* I am not here speaking of the character of the vibrations of light, sound, or
water, which are 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.
30 CORRELATION OF PHYSICAL FORCES.
Crystalline bodies again affect light definitely according
to their structure. I might weary you with examples,
shewing that, in every case which we can trace, 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 assumption, to regard Ether, a purely hypothetical
creation, as changing its elasticity for each change of struc-
ture, to regard it as penetrating the pores of bodies of
whose porosity we have no proof, and which pores must
moreover have a definite and peculiar communication, also
assumed for the purpose of the theory. The advocates of
the etherial hypothesis certainly have this advantage, that
the ether, being hypothetical, can have its characters modi-
fied or changed without any possibility of disproof, either of
its existence or modifications. t
I was not aware at the time that I first adopted the
above view and submitted it to you, that the celebrated
Leonard Euler had published a somewhat similar theory,
and though it has been considered defective by a 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 still adhere to it; the fact itself of
the correlation of the different modes of force is to my mind
an irresistible argument in favour of their being produced
by a similar agency, and though Electricity, Magnetism and
Heat, might be viewed as produced by undulations of the
same ether as that by which light is produced, yet this
hypothesis offers greater difficulties with regard to the other
affections than with regard to light; thus conduction and
non-conduction are not explained 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
CORRELATION OF PHYSICAL FORCES. 3]
by the ether, is irreconcileable with such an hypothesis. The
phenomena exhibited by these forces afford, as I think, equally
strong evidence with those of light, of Ordinary matter acting
from particle to particle, and having no action at a distance;
the experiments of Faraday on electrical induction, shewing
it to be an action of contiguous particles, are strongly in
favour of this view, and many experiments which I have
made on the Voltaic arc, some of which I have already men-
tioned in this essay, are, to my mind, confirmatory of it.
One of the objections to which this view is open, is the
necessity involved in it of an universal plenum, for if Light,
Heat, Electricity, &c., be affections of ordinary matter, then
matter must be supposed to be everywhere where these
phenomena are apparent, and consequently there can be no
vacuum. The answer appears to me to be, first, that we
have no proof of a vacuum ever having been formed; the
Torricellian, the most perfect with which we are acquainted,
is filled with the vapour of mercury: Davy's experiments
on this point prove, at all events, the formation of a vacuum
to have been up to his time impracticable. Secondly : the
other two theories equally suppose the non-existence of a
vacuum ; according to the emissive or corpuscular theory,
the vacuum is filled by the matter itself, of light, heat, etc.;
according to the etherial, it is filled by the all-penetrating
ether. Of the existence of matter in the interplanetary
spaces, we have some evidence in the diminishing periods
of Comets, and where from its highly attenuated state, we
cannot test the character of the medium by which the
forces are conveyed, we may, if we please, call such medium
ether.
At the utmost our assumption, on the one hand, is, that
wherever light, heat, etc. exist, ordinary matter exists,
though it may be so attenuated that we cannot recognise
32 CORRELATION OF PHYSICAI, FORCES.
it by the tests of other forces, such as gravitation. On the
other hand, we must assume a specific matter without
weight, and of the existence of which we have no evidence
but in the phenomena, for the explanation of which we
assume its existence. To account for the phenomena, we
assume the ether; and to prove the existence of the ether,
we cite the phenomena. For these reasons and others above
given, I think that the assumption of the universality of
ordinary matter is the least gratuitous.
MAGNETISM, as was proved by the important discovery
of Faraday, will produce the other modes of force, but with
this peculiarity—that in itself it is static; and, therefore, to
produce 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,
perhaps, more difficult to form a definite conception of the
force of magnetism than of any other force. The following
illustration may give a rude idea of static polarity. Suppose
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
spindles, 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
irregular positions, themselves creating a slight breeze by
their return. 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
CORRELATION OF PHYSICAL FORCES. 33
a polar arrangement, which they will lose as soon as the
dynamic inducing 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 retain their position, and should a gentle breeze spring
up in another direction, it will not alter their position,
but will itself 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 their 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 perma-
ment magnets; they are polarised with greater difficulty,
but, when so polarised, they cannot be affected by a feeble
current of electricity; again, if the magnets themselves be
moved, they themselves originate a current of electricity, and,
lastly, the magnetic polarity and the electric current may be
both mutually affected, if the conditions of force 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. In
a manner, then, similar to this, magnetism acts on the other
dynamic forces, definitely directing them, but not initiating
them, except while its polarity is either increasing or de-
creasing in intensity.
Magnetism, under these conditions, i.e., when conjoined
with motion, being capable of producing electricity, it fol-
lows, from what I have already stated, that it is capable of
producing the other modes of force, since electricity is capable
of producing them. Thus it can, through the medium of
D
34 CORRELATION OF PHYSICAL FORCES.
electricity, produce heat and light. Motion it can directly
produce under the above conditions, i.e., a magnet being
itself moved or raised, will move other ferruginous bodies;
these will acquire a static condition of equilibrium, 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, however pow-
erful, 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.
I have no doubt that magnetism will also directly affect
the other forces—heat, light,” and chemical affinity—and
change their direction or mode of action, and will when in
motion directly produce them ; but these remain as facts to
be elicited by future experimentalists: there are, indeed,
experiments of the older philosophers to shew that chemical
affinity is directly affected by magnetism.t
According to the view I have here ventured to take,
magnetism may be produced by the other forces, just as the
vanes in the instance given are definitely deflected, but can-
not produce them except when in motion; motion therefore
is to be regarded in this case as the initiative force, and
there can scarcely be said to be a mutual productive power
between static magnetism and the other forces.
There is, however, what may be viewed as a dynamic
condition of magnetism, i.e., its condition at the com-
mencement and the termination, or during the increment
and decrement of its development.
* In the important discovery of Faraday, published since the delivery of these
Lectures, of the deflexion of the plane of the polarised light in solids and liquids
when magnetised, there is no reciprocal production of light or magnetism, the effect
is a change of direction; and the same with M. Wartmann's repetition of the experi-
ment, on polarised heat.
f Recently further carried out by Mr. Hunt. ,
CORRELATION OF PHYSICAL FORCES. 35
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 dy-
namic force; the molecules are as far as we can infer, in mo-
tion, and while in this state, 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 all the other forces,
they have no commencement that we can trace,—we must
ever refer them back to some antecedent force equal in
amount to that produced, and therefore the word initiative
cannot in strictness apply, but must only be taken as sig-
mifying the force selected as the first : this is another reason
why the idea of causation is inapplicable to physical pro-
duction. To this point I shall again advert.
The static condition of magnetism resembles the static con-
dition of other forces, such as the state of tension existing in
the beam and 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 presents 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 doubtful question,
as such absence would involve in this case perfect elasticity,
and in all other cases, a stability which in a long course of
time nature generally negatives, shewing, as I believe, an
inseparable connection of motion with matter, and an im-
possibility 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 pro-
portionate to its original resistance to assuming a polarised
condition. This, however, must be taken merely as a matter
D 2
36 CORREIATION OF PHYSICAL FORCES.
of opinion; we have in support of it, the general fact 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 fairly an opportunity of investi-
gating 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 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.
Those who can imagine perfectly static forces, regard
them as existing in a state of tension, i.e., tendency to mo-
tion; so that whenever we get the idea of force, we cannot
separate from it the idea of motion or resistance to motion,
and when we resolve into a theory the action of the more
mysterious forces, such as light, heat, and electricity, we refer
them to 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 explaining them; whether it be that it is in reality the
only móde in which all material force is rendered evident;
or whether it be that it is the only mode in which our minds,
as contradistinguished from our senses, are able to conceive
material agencies; certain it is that all hypotheses hitherto
framed to account for the varied phenomena of nature, have
resolved them into motion: in vain has the mind hitherto
sought to comprehend, or the tongue to explain, natural
agencies by other means than by motion. Take, for exam-
ple, the theories of light, to which I have before alluded:
one of these supposes light to be a highly rare matter, emit-
ted from, i.e., put in motion by, luminous bodies; a second
Supposes that the matter is not emitted from luminous
CORRELATION OF PHYSICAL FORCES. 37
bodies, but that it is put into a state of vibration or undu-
lation, i.e., motion, by them; and, thirdly, light may be re-
garded as an undulation or motion of ordinary matter, and
propagated by undulations of air, glass, etc., as I have before
stated. In all these hypotheses, matter and motion are the
only conceptions; 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.
CHEMICAL AFFINITY, or the force by which dissimilar
bodies tend to unite, and form compounds, differing, gene-
rally, 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
an expression of the facts, 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
contra-distinction from other material changes, and we thus
use, as contra-distinguished, the terms physical and chemi-
cal. The nearest approach, however, that we can form to a
comprehension of chemical action, is by regarding it vaguely
as a molecular 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 ac-
tion. 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 libera-
tion of other forces existing in a state of static equilibrium;
but, at all events, through the medium of electricity, chemical
affinity may be directly and quantitatively converted into the
38 CORRELATION OF PHYSICAL FORCES.
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, as 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, never-
theless, not chemically altered, or, at least, not proved or
believed to be chemically altered. gº
The discovery of Volta—probably the greatest ever made
in experimental science—was that which first enabled us defi-
nitely 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,
a d that peculiar mode of force called an electric current is
generated, which circulates from metal to metal, across the
liquid, and through the point of contact.
The quantity of this 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 ge-
nerated it; and its intensity or power of overcoming re-
sistance, is also proportionate to the intensity of chemical
affinity when a single voltaic pair is employed, or to the
number of reduplications, when the well-known instrument
called the voltaic battery is used.
As Heat, Light, Magnetism, or Motion, can be produced
by the requisite application of the electric current, and as
* Perhaps, from the essential dependence of the phenomena on three dissimilar
substances in mutual contact, voltaic trine would be a more expressive generic term
for such combinations, than voltaic circuit.
CORRELATION OF PHYSICAL FORCES. 39
this is definitely produced by chemical action, we get these
forces very definitely, though not immediately, produced by
chemical action; let us, however, here inquire, as we have
already done with motion and electricity, how far these other
modes of force may directly emanate from chemical affinity.
Heat is an immediate product of chemical affinity. I
know of no exception to the general proposition, that all
bodies in chemically combining produce heat, i.e. if solution
be not considered as chemical action, and even there, when
cold results, it is from a physical change, as from the solid
to the liquid state, and not from chemical action.
The definite thermic effects produced by chemical
changes, have been lately much studied by Mr. Graham,
M. Hess, Dr. Andrews, and Mr. Joule.
The results, excepting those of M. Hess and Dr. An-
drews, are complicated, and do not admit of enunciation in
such simple propositions, as would be intelligible without
much detail. M. Hess and Dr. Andrews have arrived at
contrary results, the former concluding, that in chemical
combinations, the quantity of heat is determined by the
acid, the latter by the basic ingredient.
Light is also 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 apparently 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 certain modes of phospho-
rescence, such as those of phosphorescent wood, putrescent
fish, etc., have been detected.
*40 CORRELATION OF PHYSICAL FORCES.
Chemical action produces magnetism whenever it is
thrown into a linear direction, as in the phenomenon of
electrolysis; I may adduce the gas voltaic battery, as pre-
senting a simple instance of the direct production of mag-
netism by chemical synthesis, oxygen and hydrogen in that
combination chemically 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 de-
flected, and places itself at right angles to the line; 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 irregular and confused,
there is no general resultant by which the magnet can be
affected.
What the exact mature of the transference of chemical
power across an electrolyte is, we at present know not, nor
can we form any definite idea of it, any more than we know
the exact nature of any mode of chemical action; for the
present we must leave it as an obscure action of force, of
which future researches may simplify our apprehension.
I have now gone through the six affections of matter,
for which distinct names have been given in our received
momenclature; that other forces may be discovered, differing
as much from these as these 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
£nter se and to these six as these six are to each other, I be-
lieve to be as far certain as certainty can be predicted of
any future event.
CORRELATION OF PHYSICAL FORCES. 41
It may in many cases be a difficult question to deter-
mine, what constitutes a distinct affection of matter or mode
of force. It is highly probable, that different lines of demar-
cation 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 distin-
guished 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 probably be
entertained of their character and relation.
Electricity again was named from the body 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, 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 matu-
ral 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 che-
mistry. I mention these facts, to shew that the distinction
in the name may frequently be much greater than the dis-
tinction in the thing which it represents, and vice versá;
not as at all objecting to the received momenclature 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 be found to apply to our new
terminology.
42 CORRELATION OF PHYSICAL FORCES.
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 these
suddenly departed from, or varied, according to individual
perceptions, the acquisition and transmission of knowledge
would cease. Undoubtedly, neology is more permissible 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,
“Continget; dabiturque licentia, sumpta pudenter.”
Even should the mind ever be led to dismiss the idea of
various forces, and regard them as the exertion of one force,
or resolve them definitely into motion, still, we could never
avoid the use of different conventional terms, for the differ-
ent modes of action of this one pervading force.
The term Correlation, which I selected as the title
of these Lectures, 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
offspring. It has not been much 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
cannot be depressed without the other being elevated,—
the finger cannot press the table without the table pressing
the finger.
coRRELATION OF PHYSICAL FORCEs. 43
The probability is that, if not all, the greater number
of physical phenomena, are, strictly speaking, correlative,
and that without a duality of conception, the mind cannot
form an idea of them, thus, motion cannot be perceived or
probably imagined without 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 conviction that he is moving, and not these
objects, it is by correcting 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
different parts of the body and the vessel inform him of its
alternating, though not of its progressive movement. So in
all physical phenomena, the effects produced by motion are
all in proportion to the relative motion; thus, whether the
rubber of an electrical machine be stationary, and the cylin-
der mobile, or the rubber mobile and the cylinder stationary,
or both mobile, the electrical effects are, cateris pariffus,
precisely the same, provided the relative motion is the same,
and so, without exception, of all other phenomena. The
question of whether there can be absolute motion, or, indeed,
any absolute isolated force, is purely the metaphysical ques-
tion of idealism or realism—a question for our purpose of
little import; sufficient for the purely physical enquirer, the
maxim “de non apparentibus et non emistențăus eaden esſ
ratio.”
44 CORRELATION OF PHYSICAL FORCEs.
The sense I have attached to the word correlation, in
treating of physical phenomena, will, I think, be evident,
from the previous parts of this essay, to be that of a recipro-
cal production or convertibility; in other words, that any
force capable of producing or being convertible into 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. º
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 party writes
to prove that electricity is the cause of every change in mat-
ter; another that chemical action is the cause of every thing;
another that heat is the universal cause,_and so on. If, as
I have submitted to you, the true expression of the fact is,
that each mode of force is capable of producing the other,
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 indiscriminately used
in both these senses.
Another confusion of terms has arisen, and has, indeed,
much embarrassed me in enunciating the propositions sought
to be proved in these pages, on account of the imperfection
of scientific 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 pro-
ducing, or the subjective idea of force or power, and of the
CORRELATION OF PHYSICAL FORCEs. 45
effect produced, or the objective phenomenon. The word
motion, indeed, is only applied to the effect, and not to the
force,—and chemical affinity is generally applied to the force,
and not to the effect; but the other four terms are applied
indiscriminately to both, for want of a distinct terminology.
I may possibly have occasionally used the same word, at
one time in a subjective, at another in an objective, sense;
all I can say is, that I cannot avoid this without a neology,
which I have not the presumption to introduce, or the au-
thority to enforce. The word force itself, and the idea it
aims at expressing, may be objected to by the purely phy-
sical philosopher, as representing a subtle mental conception,
and not a sensuous perception, or phenomenon; to avoid its
use however, if open to no other objection, would be so far
a departure from recognised views, as to render language
scarcely intelligible. 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
phenomena are but diversely modified effects.
Whether the imponderable agencies viewed as force, and
not as matter, ought to be regarded as distinct forces or
distinct modes of force, I do not think very material, for as
far as I am aware, the same result would follow either view,
and I have used the terms indiscriminately, as either hap-
pened to be the more expressive for the occasion.
The great problem which remains to be solved, in regard to
the correlation of physical forces, is the establishment of their
equivalent of power, or their measurable relation to a given
standard. Viewed in their static relations, or in the condi-
tions requisite for producing equilibrium or quantitative
equality of force, a remarkable relation between chemical
affinity and heat, is that discovered in many simple bodies
46 CORRELATION OF PHYSICAL FORCES.
by Dulong and Petit, and extended to compounds by Neu-
mann and Avogadro : their researches have shewn that the
specific heats of certain substances, when multiplied by their
chemical equivalents, give a constant quantity as product, or
in other words, that the combining weights of such substances
are those weights, which require equal accessions or ab-
stractions of heat, equally to raise or lower their temperature.
To put the proposition more in accordance with the view we
have taken of the nature of heat: each body has a power of
communicating or receiving molecular repulsive power,
exactly equal, weight for weight, to its chemical or com-
bining power; for instance, the equivalent of lead is 104,
of zinc 32, 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 quantity of an acid or substance com-
bining with it; but their power of communicating or ab-
stracting 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 quan-
tity of a third substance, say water.
Again, a great nnmber 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 affinities or tendencies of the molecules of dissimilar sub-
stances to combine, and Saturate each other, consequently,
here we have an inverse relation between these two modes
of force, gravitation, and affinity.
Were the above relations extended into an universal
law, we should have the same numerical expression for the
CORRELATION OF PHYSICAL FORCES. 47
three forces of heat, gravity, and affinity, and as electricity
and magnetism are quantitatively related to them, we should
have a similar expression for these forces; but, at present,
the bodies in which this parity of force has been discovered,
though in themselves numerous, are small compared with
the exceptions, and, therefore, this point can only be indi-
cated as promising a generalization, should subsequent re-
searches alter our knowledge as to the elements and equi-
valents 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 difficulty of
establishing 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, etc., it ought to follow, that
when we collect the dissipated and changed forces, and re-
convert them, the initial motion, affecting the same amount
of matter with the same velocity, should be reproduced, and
so of the changes in matter produced by the other forces;
but the difficulties of proving the truth of this by experi-
ment, 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. Electricity promises
us the best means of effecting this, but little has hitherto
been done in carrying it out.
In investigating the relations of the different forces, I
have in turn taken each one as the initial force or starting
point, and endeavoured to shew how the force, thus arbi-
trarily selected, could mediately or immediately produce or
be converted into the others; but it will be obvious to those
who have attentively considered the subject, and brought
their minds into a general accordance with the views I have
48 CORRELATION OF PHYSICAL FORCES.
submitted to them, that no force can, strictly speaking, be
initial, as there must be some anterior force which pro-
duced 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 produced by chemical affinity, i. e., the
affinity of the carbon of the coal for the oxygen of the air;
this carbon and this Oxygen 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 alternating effects of heat, light, chemical affi-
nity, etc.; thus tracing any force backwards to its ante-
cedents, 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 con-
ceivable by the mind, of motion without antecedent motion?
I cannot, without calling for the interposition of creative
power, any more than I can conceive the sudden appearance
of a mass of matter come from no where, and formed from
nothing. With matter, the impossibility of creation, humanly
speaking, has long been admitted, though, perhaps, its dis-
tinct reception in philosophy may be set down to the over-
throw of the doctrine of Phlogiston, and the reformation of
chemistry, at the time of Lavoisier. The reasons for the
admission of the non-annihilation of force, appear to be
CORRELATION OF PHYSICAL FORCES. 49
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
wheel of a carriage?—who can re-combine the particles 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, in-
deed, 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
correlates, in the strictest sense of the word; the conception
of the existence of the one, involves the conception of the
existence of the other; the quantity of matter again and the
degree of force involve conceptions of space and time. But
here I must conclude : —- were I to pursue this further, I
should be tempted into the alluring paths of metaphysical
speculation. *
I believe that the same principles and mode of reasoning
as I have adopted in this essay, might be applied to the
organic, as well as the inorganic, world; and that muscular
force, animal and vegetable heat, etc., might, and at some
time will, be shewn to have similar definite correlations;
but I have purposely avoided this subject, as pertaining to
a department of science to which I have not devoted my
attention.
My aim has been to give you a generalised view of what
I conceive to be the connected results already produced in
inorganic physical science, and, thence, to enable you to form
E
50 CORRELATION OF PHYSICAL FORCES.
some deduction as to the probable character of the results to
be anticipated. It is a great assistance in such investigations
to be intimately convinced that no physical phenomenon can
stand alone; each is inevitably connected with anterior
changes and as inevitably productive of consequential changes,
each with the other, and all 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 con-
nected and explained; explanation is, indeed, only relation
to something more familiar, not more known, i.e., known
as to causative or creative agencies: in all phenomena,
the more closely they are investigated, the more are we con-
vinced that, humanly speaking, neither matter nor force can
be created, and that an essential cause is unattainable.—
Causation, is the will, Crcation, the act, of GoD.
REFERENCES MENTIONED IN THE PREFACE.
BISHop BERKELEY—Principles of human knowledge. Sec. 107
et seq. - º A.D. 1784.
Count RUMFORD–On the Source of heat excited by Friction.
Philosophical Transactions. 1798.
SIR. H. DAvy—On the Conversion of Ice into water by Friction.
West of England Contributions, Page 16. - 1799.
Of Heat or Calorific repulsion. Elements of Chemical Philo-
sophy, Page 69. 1812.
DR. YoUNG—Cause of Solidity as distinguished from Liquidity.
Supp. Encyclopædia Britannica, Vol. vi., Pages 862—3. 1824.
SIR. J. F. W. HERscHEL–On the Phenomena of Force. Dis-
course on the Study of Natural Philosophy, Page 221. 1830.
and Treatise on Astronomy, Page 232. 1833.
On the absorption of light by coloured Media, viewed in
connexion with the undulatory theory. Philosophical Ma-
gazine, December, 1833.
Professor WHEATstone–On the Prismatic decomposition of
Electrical light. Notices of Communications to the 5th
Meeting of British Association, Page 11. 1835.
C. BABBAGE, Esq.-On the permanent impression of our words
and actions on the Globe we inhabit, Ninth Bridgewater
Treatise, Chap. 9. 1837.
REv. W. WHEWELL, B.D.—A Demonstration that all matter is
heavy. Cambridge Philosophical Transactions. 1841.
On the question, are cause and effect successive or simulta-
neous 2 Cambridge Philosophical Transactions. 1842.
52 REFERENCES MENTIONED IN THE PREFACE.
M. FARADAY, Esq.-Improbability of contact exciting force.
Eaperimental Researches, Series 17, Sec. 2071. 1840.
J. P. Joule, Esq.-On the Calorific effects of Magneto-Elec-
tricity and on the Mechanical value of heat, Philosophical
Magazine, October, 1843.
In the original Lectures I alluded very cursorily to gravitation and
Inertia, I have therefore abstained from noticing these forces in the
text—Inertia appears to me to be a static condition of the force of
gravitation, or in other words resistance to motion occasioned by the
force of gravitation—without gravitation I cannot conceive Inertia.
Whether this be so or not, as the phenomenal effects of gravitation
and Inertia, are, motion and resistance to motion, I have, in consider-
ing motion, necessarily included their relations to the other forces.
For the identity of gravitation with other modes of attractive force, see
Mosotti. Scientific Memoirs, 1837, Vol. 1, Page 448. See also
Franklin on Inertia. Miscellaneous Pieces, 1779, Page 479.
CHARLES SKIPPER AND EAST, ST. DUNSTAN’s HILL, LONDON.