S. P. Langley — The History of a Doctrine. 3 



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 which is progress, when the man him- 

 self 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 recogni- 

 tion 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 doc- 

 trine may be fairly said to exist. Whether this early recog- 

 nition be a legacy from the Lucretian philosophy, it is not 

 necessary 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 radi- 

 ant 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 some- 

 thing 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 f 



* " Me proposant de traiter ici de la lumiere, la premiere chose dont je veux vous 

 avertir est qu'il peut y avoir de la difference entre le sentiment que dous en avons 

 c'est-a-dire l'idee qui s'en forme en notre imagination par l'entremise de nos yeux, 

 et ce qui est dans les objets qui produit en nous ce sentiment, c'est-a-dire ce qui 

 est dans la flamme ou dans le soleil qui s'appelle du n'om de lumiere: la plupart 

 des philosophes assurent que le son n'est autre chose qu'un certain tremblement 

 d'air qui vient frapper nos oreilles ; en sorte que si le sens de l'ouie rapportoit a 

 notre pensee la vraie image de son objet, il faudroit, au lieu de nous faire concevoir 

 le son, qu'il nous fit concevoir le mouvement des parties de l'air qui tremble pour 

 lors contre nos oreilles. * * On passe doucement une plume sur les levres d'un 

 enfant qui s'endort. et il sent qu'on le chatouille : pensez-vous que l'idee du 

 chatouillement qu'il concoit ressemble k quelque chose de ce qui est en cette 

 plume ? 



_ f Detached passages from Bacon, Hobbes, and Locke, referring to the concep- 

 tion of heat, as a mode of motion, have been so often cited, that I will not repeat 

 them here ; but as these after all convey rather an impression of the acuteness 

 of their authors, as men before .their time, than the idea of a doctrine fully and 

 clearly apprehended at the time, I prefer to offer the following much less known 

 quotation, which I find in the works of Boyle, published circa 1670. 



I beg the attention of the reader to the remarkable passages which follow, and 

 which must have had, in Newton's day, all the currency which the eminent repu- 

 tation of the author (a founder of the Royal Society) could give. 



Extracts from the treatise on the " Mechanical Origin of Meat" by the Honorable 

 Robert Boyle. — " Heat will appear the more likely to be mechanically producible, 



