the Mechanical Theory of Heat. 469 



haps to the influence of the current opinions of his time, he 

 speaks of gases as if they resulted from the combination of cer- 

 tain bases with caloric. But he always showed a reserve, no 

 trace of which is to be found in the writings of his disciples ; it 

 was not without scruple that he placed light and caloric at the 

 head of the list of elementary bodies. 



As to Laplace, his ideas changed very quickly, and in all that 

 he wrote after the period of his association with Lavoisier 

 he has shown himself the convinced defender of the theory of 

 the materiality of caloric. His imposing authority even 

 produced partisans of that theory long after it had been shown 

 to be without the least foundation. 



Towards the end of the eighteenth century, in 1798 and 1799, 

 two experiments were made which were sufficient to demonstrate 

 the inanity of the theory adopted by the author of the Mecanique 

 Celeste. These were the celebrated experiments of Rumford and 

 Davy on the heat disengaged by friction. Rumford had measured 

 in a precise manner the heat produced in the boring of a cannon 

 in the royal foundry at Munich ; and to leave no doubt as to the 

 origin of this heat, he determined the specific heat of the solid 

 bronze and of the rubbed-off portions of this metal. No sensible 

 difference appeared to exist between the two specific heats. The 

 only reasonable explanation that could be given of the pheno- 

 menon according to the material theory of heat was thus peremp- 

 torily refuted. It had indeed been admitted that in pulverized 

 bodies the specific heat was much less than in the same bodies 

 in a compact condition, and it would certainly follow from this 

 hypothesis that the pulverization of a body by friction ought to 

 liberate heat. But it was forgotton that friction liberates heat 

 even when there is no alteration of the rubbing surfaces. The 

 incorrectness of the hypothesis was moreover shown by the 

 experiment of Rumford. 



The experiment of Davy, later by a year than that of Rumford, 

 was, if possible, still more conclusive. Two pieces of ice rubbed 

 against each other by Davy were rapidly melted, and a liquid 

 was produced the specific heat of which is more than double 

 that of ice. Davy, moreover, had taken all care to prove that 

 the liberation of heat by friction was not compensated by any 

 sensible absorption of heat in any portion of the apparatus. 



Among the contemporaries of Rumford and Davy, Young was 

 the only man who comprehended the full bearing of their expe- 

 riments. In his ' Lectures on Natural Philosophy/ published in 

 1807, he has connected them with his immortal discoveries on 

 the nature of light, and he almost arrived at the true principle 

 of the mechanical theory of heat. He was the first to call in 

 question the principle, assumed by Lavoisier and Laplace, of 



