2i8 THE POPULAR SCIENCE MONTHLY. 



Thompson (Count Rumf ord) not only weighed " caloric " literally 

 in the balance and found it wanting, but made that memorable 

 experiment in the Munich foundries which showed that heat 

 was perpetually and without limit created from motion. 



It was in the last years of the century, too, that he provided 

 for the medal called by his name, and which, though to be given 

 for researches in heat and light, has, I believe, been allotted in 

 nearly every instance to men who, like Leslie, Mains, Davy, 

 Brewster, Fresnel, Melloni, Faraday, Arago, Stokes, Maxwell, and 

 Tyndall, have contributed toward the subject of radiant energy 

 in particular. 



We observe that till Rumf ord's time the scientific literature of 

 the century scarcely considers the idea even of radiant heat, still 

 less of radiant energy ; so that we have been obliged here to dis- 

 cuss the views of its physicists about heat in general, heat and 

 light in most eighteenth-century minds being distinct entities. 

 We must remember, then, to his greater honor, that the idea of 

 radiant heat as a separate study has before Rumford scarcely 

 an existence ; all the ways for pilgrims to this special shrine of 

 truth being barred, like those in Bunyan's allegory, by two un- 

 friendly monsters who are called Phlogiston and Caloric, so that 

 there are few scientific j^ilgrims who do not pay them toll. 



The doctrine of caloric is, however, even then recognized as a 

 chemical hypothesis rather than one acceptable to physicists, 

 some of whom still stand out for vibratory theories even through 

 the darkest years of the century ; and, further, we may find, on 

 strict search, that the old idea of heat as a mode of motion has 

 not so utterly died that it does not appear here and there during 

 the last century, not only among philosophers, but even in a popu- 

 lar form. 



In an old English translation of Father Regnault's compilation 

 on physics, dated about 1730, I find the most explicit statement of 

 the doctrine of heat as a mode of motion. Here heat is defined 

 (with the aid of a simile due, I believe, to Boyle) as " any Agita- 

 tion whatever of the insensible parts. Thus a Nail which is drove 

 into the Wood by the stroke of a Hammer does not appear to be 

 hot, because its immediate parts have but one common Move- 

 ment. But should the Nail cease to drive, it would acquire a 

 sensible Heat, because its insensible Parts which receive the Mo- 

 tion of the Hammer now acquire an agitation every way rapid." 

 We certainly must admit that the user of this illustration had 

 just and clear ideas ; and the interesting point here appears to be, 

 that as Father Regnault's was not an original work, but a mere 

 compendium or popular scientific treatise of the period, we see, if 

 only from this instance, that the doctrine of heat as a mode of 

 motion was not confined to the great men of an earlier or a later 



