334 THE POPULAR SCIENCE MONTHLY. 



assigned to them their true significance, and, reviewing much after 

 Bacon the existing state of experience upon the question, drew forci- 

 bly attention to the superficiality of the views of those who still 

 adopted the hypothesis of caloric. 



In 1810 Haldat performed an extended series of experiments upon 

 the heat produced by friction between various metallic surfaces. 1 The 

 results which he obtained were not, however, decidedly confirmatory 

 of either supposition, but especially serve to increase our admiration 

 for the acumen of Rumford in perceiving and stating the true law of 

 its excitation. 



The rubbing surfaces employed by him were similar in size and 

 shape ; the pressure between them was maintained nearly constant 

 in several different experiments ; but the power or energy was re- 

 ceived in measured quantities, and from an indefinite source, namely, 

 the pulley of a turning-lathe. 



The quantities of heat developed for the same number of revolu- 

 tions, or in proportionate times, were naturally, therefore, different 

 for different metals ; but as to the cause of this diversity he hazarded 

 no positive opinion, and indeed his recorded observations do not 

 seem susceptible of reduction to any particular theory. Had he 

 measured the energy absorbed, or the coefficient of friction between 

 the rubbing surfaces, he might possibly have been able to trace some 

 relation between them and the heat produced in the operation. As 

 it was, his observations as to difference of capacity, the influence of 

 density, etc., were equally confused with the results, which he ob- 

 tained on varying the pressure and substituting different metals ; and 

 although upon the whole his conclusions were adverse to the calorists, 

 they w r ere not definite enough to attract any notable attention. 



In tracing thus far the inception of mechanical-heat theory, we 

 have seen two important generalizations made : The one, fully 

 attested by experiment, referring to the transformation of work into 

 heat in a peculiar class of operations, and entirely independent of 

 hypothesis, namely, that " the heat generated by friction is exactly 

 proportional to the force with which the two surfaces are pressed 

 together, and to the rapidity of the friction." The other, more com- 

 prehensive, including in the spirit of its enunciation thermal phenom- 

 ena of every variety, and to a greater or less extent dependent on 

 molecular and other hypothesis. These early statements are quite 

 characteristic of, and may be used to illustrate, a subsequent division 

 of our subject necessitated by experimental difficulties of investigation 

 and verification. 



The proposition that the entire energy existing in the universe is 

 a magnitude as- definite and unchangeable as the quantity of matter 

 which it contains, is now considered one of the most fundamental and 

 far-reaching in natural philosophy. The experimental evidence pos- 



1 Journal dc Physique, vol. lxv., p. 213 ; Nicholson's Journal, vol. xxvi., p. 30. 



