214 THE POPULAR SCIENCE MONTHLY. 



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, which 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 prede- 

 cessors, by Descartes, by Bacon, by Hobbes, and in particular by 

 Boyle, while Hooke and Huygens 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 

 history 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 

 " Optics " 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 sev- 

 eral bignesses ? " And still again : " Is not the heat conveyed by 

 the vibrations of a much subtler 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 Huygens, 

 that apprehending before the latter, and equally clearly, the un- 

 dulatory 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, 

 Huygens would perhaps have decided so too, in the face of diffi- 

 culties, all of which have not been cleared up even to-day. 



