August 17, 1888.] 



SCIENCE 



75 



ing a consideralile truth in it, but one left out of sight by the 

 writers of books. 



At any rate, the actual moveincnt has been tortuous, or often 

 even retrograde, to a degree of which you will get no idea from the 

 account in the text-book or encyclop;edia. where, in the main, only 

 the resultant of all these vacillating motions is given. With rare 

 exxeptions, the backward steps — that is, the errors and mistakes, 

 which count in reality for nearly half, and sometimes for more than 

 half, the whole — are left out of scientific history ; and the reader, 

 while he knows that mistakes have been made, has no just idea 

 how intimately error and truth are mingled in a sort of chemical 

 union, even in the work of the great discoverers, and how it is the 

 test of time chiefly which enables us to say vvhich is progress when 

 the man himself could not. If this be a truism, it is one which is 

 often forgotten, and which we shall do well to here keep before 

 us. 



This is not the occasion to review the vague speculations of the 

 ancient natural philosophers from Aristotle to Zeno, or to give the 

 opinion of the schoolmen on our subject. We take it up with the 

 immediate predecessors of Newton, among whom we may have 

 been prepared to expect some obscure recognition of heat as a 

 mode of motion, but where it has been, to me at least, surprising, 

 on consulting their original works, to find how general and how 

 clear an anticipation of our modern doctrine may be fairly said to 

 exist. Whether this early recognition of the atomic and vibratory 

 theories be a legacy from the Lucretian philosophy, it is not neces- 

 sary to here consider. The interesting fact, however it came about, 

 is the extent to which seventeenth-century thought is found to be 

 occupied with views which we are apt to think very recent. 



Descartes, in 1664, commences his ' Le Monde ' by a treatise on 

 the propagation of light, and what we should now call radiant heat, 

 by vibrations, and further associates this view of heat as motion 

 with the distinct additional conception that in the cause of light 

 and radiant heat we may expect to find something quite different 

 from the sense of vision or of warmth ; and he expresses himself 

 with the aid of the same simile of sound employed by Draper over 

 two hundred years later. The writings of Boyle on the mechanical 

 production of heat contain illustrations (like that of the hammer 

 driving the nail, vvhich grows hot in proportion as its bodily motion 

 is arrested) which show a singularly complete apprehension of 

 views we are apt to think we have made our own ; and it seems to 

 me that any one who consults the originals will admit, that, though 

 its full consequences have not been wrought out till our own time, 

 yet the fundamental idea of heat as a mode of motion is so far from 

 being a modern one, that it was announced in varying forms by 

 Newton's immediate predecessors, by Descartes, by Bacon, by 

 Hobbes, and in particular by Boyle, while Hooke and Huyghens 

 merely continue their work, as at first does Newton himself. 



If, however, Newton found the doctrine of vibrations already, so 

 to speak, " in the air," we must, while recognizing that in the his- 

 tory of thought the new always has its root in the old. and that it 

 is not given even to a Newton to create an absolutely new light, still 

 admit that the full dawn of our subject properly begins with him, 

 and admit, too, that it is a bright one, when we read in the ' Op- 

 tics ' such passages as these : — 



" Do not all fixed bodies, when heated beyond a certain degree, 

 emit light and shine, and is not this emission performed by the 

 vibrating motions of their parts ? " And again : " Do not several 

 sorts of rays make vibrations of several bignesses .' " And still 

 again : " Is not the heat conveyed by the vibrations of a much sub- 

 tler medium than air? " 



Here is the undulatory theory ; here is the connection of the 

 ethereal vibrations with those of the material solid ; here is " heat 

 as a mode of motion ; " here is the identity of radiant heat and 

 light ; here is the idea of wave-lengths. What a step forward this 

 first one is ! And the second ? 



The second is, as we now know, backward. The second is the 

 rejection of this, and the adoption of the corpuscular hypothesis, 

 with which alone the name of Newton (a father of the undulatory 

 theory) is, in the minds of most, associated to-day. 



Do not let us forget, however, that it was on the balancing of 

 arguments from the facts then known that he decided, and that 

 perhaps it was rather an evidence of his superiority to Huyghens, 



that apprehending before the latter, and equally clearly, tne undu- 

 latory theory, he recognized also more clearly that this theory as 

 then understood failed utterly to account for several of the most 

 important phenomena. 



With an equally judicial mind, Huyghens would perhaps have 

 decided so too, in the face of diflicuUies, all of which have not been 

 cleared up even to-day. 



These two great men, then, each looked around in the then dark- 

 ness as far as his light carried him. All beyond that was chance 

 to each ; and fate willed that Newton, whose light shone farther 

 than his rival's, found it extend just far enough to show the en- 

 trance to the wrong way. He reaches the conclusion that we all 

 know ; and with the result on other men's thought, that, light being 

 conceded to be material, heat, if afniiated to light, must be re- 

 garded as material too, for we may see this strange conclusion 

 drawn from experiments of Herschel a century later. 



It would seem that the result of this unhappy corpuscular theory 

 was more far-reaching than we commonly suppose, and that it is 

 hardly too much to say that the whole promising movement of that 

 age toward the true doctrine of radiant energy is not only arrested 

 by it, but turned the other way ; so that in this respect the philos- 

 ophy of fifty years later is actually farther from the truth than that 

 of Newton's predecessors. 



The immense repute of Newton as a leader, on the whole so 

 rightly earned, here leads astray others than his conscious disciples, 

 and, it seems to me, affects men's opinions on topics which appear 

 at first far removed from those he discussed. The adoption of 

 phlogiston was, as we may reasonably infer, facilitated by it, and 

 remotely Newton is perhaps also responsible in part for the doc- 

 trine of caloric a hundred years later. After him, at any rate, 

 there is a great backward mov'ement. We have a distinct retro- 

 gression from the ideas of Bacon and Hobbes and Boyle. Night 

 settles in again on our subject almost as thick as in the days of the 

 schoolmen, and there seems to be hardly an important contribution 

 to our knowledge, in the first part of the eighteenth century, due to 

 a physicist. 



" Physics, beware of metaphysics," said Newton, — words which 

 physicists are apt so exclusively to quote, that it seems only due 10 

 candor to observe that the most important step, perhaps, in the 

 fifty years which followed the ' Optics,' came from Berkeley, who, 

 reasoning as a metaphysician, gave us during Newton's lifetime a 

 conception wonderfully in advance of his age. Yet the ' New 

 Theory of Vision ' was generally viewed by contemporary philos- 

 ophers as only an amusing paradox, while " coxcombs van- 

 quish [ed] Berkeley with a grin ; " and this contribution to science, 

 — an exceptional if not a unique instance of a great physical gen- 

 eralization reached by a priori reasoning, — though published in 

 1709, remains in advance of the popular knowledge even in these 

 closing years of the nineteenth century. 



In the mean time a new error had risen among men, — a new 

 truth, as it seemed to them, and a thing destined to have a strong 

 reflex action on the doctrine of radiant energy. It began with the 

 generalization of a large class of phenomena (which we now asso- 

 ciate with the action of oxygen, then of course unknown), — a gen- 

 eralization useful in itself, and accompanied by an explanation 

 which was not in its origin objectionable. Let us consider, in illus- 

 tration, any familiar instance of oxidation, and try to look first for 

 what was reasonable in the eighteenth-century views of the cause 

 of such phenomena. 



A piece of dry wood has in it the power of giving out heat and 

 light when set on fire ; but after it is consumed there is left of it 

 only inert ashes, which can give neither. Something, then, has left 

 the wood in the process of becoming ashes ; virtue has gone out of 

 it, or, as we should say, its potential energy has gone. 



This is, so far, an important observation, extending over a wide 

 range of phenomena, and. if it had presented itself to the predeces- 

 sors of Newton, it would probably have been allied to the vibratory 

 theories, and become proportionately fruitful. But to his disciples, 

 and to chemists and others, who, without being perhaps disciples, 

 were like all then, more or less consciously influenced by the ma- 

 teriality of the corpuscular theory, it appeared that this also was a 

 material emanation, that this energy was an actual ingredient of the 

 wood. — a crudeness of conception which seems most strange to 



