SCIENCE. 
184 
same instant of time. It is clear that the unity of effect 
which is achieved out of this immense variety of movements 
is a unity which lies altogether behind the mere unity of 
material, and is traceable to some one order of arrangement 
under which the original impulses are conveyed. We know 
that in respect to the waves of Sound, the production of 
perfect harmonies among them can onlv be attained by a 
skillful adjustment of the instruments, whose vibrations 
are the cause and the measure of the aerial waves which, in 
their combination, constitute perfect music. And so, in 
like manner, we may be sure that the harmonies of Heat. 
Light, and Chemical Action, effected as they are amongst an 
infinite number and variety of motions, very easily capable 
of separation and disturbance, must be the result of some 
close adjustment between the constituent element of the 
conveying medium and the constituent elements of the 
luminous bodies, whose complex, but joint, vibrations con- 
stitute that embodied harmony which we know as Light. 
Moreover, as this adjustment must be close and intimate 
between the properties of the ether and the nature of the 
bodies whose vibrations it repeats, so also must the same 
adjustment be equally close between these vibrations and 
the properties of Matter on which they exert such a power- 
ful influence. And when we consider the number and the 
nature of the things which this adjustment must include, 
we can, perhaps, form some idea what a bond and bridge it 
is between the most stupendous phenomena of the heavens 
and the minutest phenomena of earth. For this adjust- 
ment must be perfect between these several things — first, 
the flaming elements in the sun which communicate the dif- 
ferent vibrations in definite proportion ; next, the constitu- 
tion of the medium, which is capable of conveying them 
without division, confusion, or obstruction ; next, the con- 
stitution of our own atmosphere, so that neither shall it dis- 
tort, nor confuse, nor quench the waves ; and, lastly, the 
constitution of those forms of Matter upon earth which 
respond, each after its own laws, to the stimulus it is so made 
as to receive from the heating, lighting, and actinic waves. 
In contemplating this vast system of adjustment, it is 
important to analyze and define, so far as we can, the im- 
pression of unity which it makes upon us, because the real 
scope and source of this impression may very easily be mis- 
taken. It has been already pointed out that we can only see 
likeness by first seeing difference, and that the full perception 
of that in which things are unlike is essential to an accurate 
appieciation of that in which they are the same. The classi- 
fying instinct must be strong in the human mind, from the 
delight it finds in reducing diverse things to someone com- 
mon definition. And this instinct is founded on the power 
of setting differences aside, and of fixing our attention on 
some selected conditions of resemblance. But we must 
remember that it depends on our width and depth of vision 
whether the unities which we thus select in Nature are the 
smallest and the most incidental, or whether they are the 
largest and the most significant. And, indeed, for some 
temporary purposes — as, for example, to make clear to our 
minds the exact nature of the facts which science may have 
ascertained — it may be necessary to classify together, as 
coming under one and the same category, things as differ- 
ent from each other as light from darkness. Nor is this 
any extreme or imaginary case. It is a case actually exem- 
plified in a lecture by Professor Tyndall, which is entitled 
•‘The Identity of Light and Heat.” Yet those who have 
attended the expositions of that eminent physical philo- 
sopher must be familiar with the beautiful experiments 
which show how distinct in another aspect are Light and 
Heat ; how easily and how perfectly they can be separated 
from each other; how certain substances obstruct the one 
and let through the other ; and how the fiercest heat can be 
raging in the profoundest darkness. Nevertheless, there is 
more than one mental aspect, there is more than one 
method of conception, in terms of which these two separa- 
ble powers can be brought under one description. Light 
and Heat, however different in their effects — however distinct 
and separable from each other — can both be regarded as 
“ forms of motion ” among the particles of Matter. More- 
over, it can be shown that both are conveyed or caused by 
waves, or undulatory vibrations in one and the same ethe- 
real medium. And the same definition applies to the 
chemical rays, which again are separable and distinct from 
the rays both of Light and Heat. 
But although this definition may be correct as far it goes, 
it is a definition nevertheless which slurs over and keeps 
out of sight distinctions of a fundamental character. In 
the first place, it takes no notice of the absolute distinction 
between Light or Heat considered as sensations of our or- 
ganism, or as states of consciousness, and Light or Heat 
considered as the external agencies which produce these 
sensations in us. Sir W. Grove has expressed a doubt 
whether it is legitimate to apply the word “ Light ” at all to 
any rays which do not excite the sense of vision. This, 
however, is not the distinction to which I now refer. If it 
be an ascertained fact, or if it be the only view consistent 
with our present knowledge, that the ethereal pulsations 
which do, and those which do not, excite in us the sense of 
vision, are pulsations exactly of the same kind and in ex- 
actly the same medium, and that they differ in nothing but 
in periods of time or length of wave, so that our seeing of 
them or our not seeing of them depends on nothing but the 
focusing, as it were, of our eyes, then the inclusion of them 
under the same word Light involves no confusion of thought. 
We should confound no distinction of importance, for ex- 
ample, by applying the same name to grains of sand which 
are large enough to be visible, and to those which are so 
minute as to be wholly invisible even to the microscope. 
And if a distinction of this nature- — a mere distinction of 
size, or of velocity, or of form of motion, were the only 
distinction between light and heat — it might be legitimate 
to consider them as identical, and to call them by the same 
name. But the truth is there are distinctions between 
them of quite another kind. Light, in the abstract con- 
ception of it, consists in undulatory vibrations in the pure 
ether, and m these alone. They may or may not be visible — 
that is to say, they may or may not be within the range of 
our organs of vision, just as a sound may or may not be 
too faint and low, or too fine and high, to be audible to our 
ears. But the word “heat” carries quite a different mean- 
ing, and the conception it conveys could not be covered 
under the same definition as that which covers light. Heat 
is inseparably associated in our minds with, and does 
essentially consist in, certain motions, not of pure ether, 
but of the molecules of solid or ponderable matter. These 
motions in solid or ponderable matter are not in any sense 
identical with the undulatory motions of pure ether which 
constitute light ; consequently when physicists find them- 
selves under the necessity of defining more closely what 
they meant by the identity of heat and light, they are 
obliged to separate between two different kinds of heat — 
that is to say, between two wholly different things, both 
covered under the common name of heat — one of which 
is really identical in kind with light, and the other of 
which is not. “Radiant” heat is the kind, and the only 
kind of heat, which comes under the common definition. 
“Radiant” heat consists in the undulatory vibrations of 
pure ether which are set up or caused by those other 
vibrations in solid substances or ponderable matter, which 
are heat more properly so called. Hot bodies communicate 
to the surrounding ethereal medium vibrations of the same 
kind with light, some of these being, and others not being, 
luminous to our eyes. Thus we see that the unity or close 
relationship which exists between heat and light is not a 
unity of sameness or identity, but a unity which depends 
upon and consists in correspondences between things in 
themselves different. It has been suggested that the facts 
of nature would be much more clearly represented in lan- 
guage if the old word “ Caloric ” were revived, in order to 
distinguish one of the two very different things which are 
now confounded under the common term “Heat” — tha 
is to say, heat considered as molecular vibration in solid or 
ponderable matter, and heat considered as the undulatory 
vibrations of pure ether which constitute the “heat” 
called “radiant.” Adopting this suggestion, the relations 
between light and heat, as these relations are now known 
to science, may be thrown into the following propositions, 
which are framed for the purpose of exhibiting distinctions 
not commonly kept in view : 
I. Certain undulatory vibrations in pure ether alone are 
light ether (1) visible, or (2) invisible. 
II. These undulatory vibrations in pure ether alone not 
Caloric. 
III. No motions of any kind in pure ether alone are 
Caloric. 
