1 84 



SCIENCE. 



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 only 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 some one 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 

 temporaiy 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 maybe 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 in 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 undulatorj vibrations in pure ether alone are 

 light ether (i) visible, or (2) invisible. 



II. These undulatory vibrations in pure ether a'one not 

 ( 'aloric. 



III. No motions of any kind in pure ether alone are 

 < '.1I1 )i ic. 



