78 SCIENCE IN THE LAST HALF CENTURY. 



fiuite body could coutain au iiifiuite quautity of another body ; but it 

 was uotuutil 1843, that clear aud unquestionable experimental proof was 

 given of the fact that there is a definite relation between mechanical 

 work and heat ; that so much work always gives rise, under the same 

 conditions, to so much heat, and so much heat to so much mechanical 

 work. Thus originated the mechanical theory of heat, which became 

 the starting point of the modern doctrine of the conservation of energy. 

 Molar motion had appeared to be destroyed by friction. It was proved 

 that no destruction took place, but that an exact equivalent of the 

 energy of the lost molar motion appears as that of the molecular motion, 

 or motion of the smallest particles of a body, which constitutes heat. 

 The loss of the masses is the gain of their particles. 



Before 1843, however, the doctrine of the conservation of energy had 

 been approached. Bacon's chief contribution to positive science is the 

 happy guess (for the context shows that it was little more) that heat 

 may be a mode of motion ; Descartes affirmed the quantity of motion in 

 the world to be constant; Newton nearly gave expression to the' com- 

 plete theorem, while Eumford's and Davy's experiments suggested, 

 though they did not prove, the equivalency of mechanical and thermal 

 energy. Again, the discovery of voltaic electricity, and the marvellous 

 development of knowledge in that field, effected by such men as Dav^', 

 Faraday, Oersted, Ampere, and Melloni, had brought to light a num- 

 ber of facts which tended to show that the so-called " forces " at work 

 in light, heat, electricit}', and magnetism, in chemical and in mechani- 

 cal operations, were intimately, and in various cases, quantitatively 

 related. It was demonstrated that any one could be obtained at the 

 expense of any other ; and apparatus was devised which exhibited the 

 evolution of all these kinds of action from one source of energy. Hence 

 the idea of the '• correlation of forces" which was the immediate fore- 

 runner of the doctrine of the conservation of energy. 



It is a remarkable evidence of the greatness of the progress in this 

 direction which has been effected in our time, that even the second edi- 

 tion of the " History of the Inductive Sciences," which was published in 

 1846, contains no allusion either to the general view of the " Correlation 

 of Forces" published in England in 1842, or to the publication in 1843 

 of the first of the series of exi)eriments by which themechanical equiva- 

 lent of heat was correctly ascertained.* Such a failure on the part of a 



* This is the more curious, as Ampere's hypothesis that Tibratious of molecnles, 

 causing and caused by vibrations of the aether constitute heat, is discussed. See vol. 

 ii, p. 5S7, 2d ed. In the Philosophy of the Inductive Sciences, 2d ed., 1847, p. 239, 

 Whewell remarks dj)roj>os of Bacon's definition of heat, "that it is an expansive, re- 

 strained mption, modified in certain ways, and exerted in the smaller particles of 

 the body; " that " although the exact nature of heat is still an obscure and contro- 

 verted matter, the science of heat now consists of many important truths; and that 

 to none of these truths is there any approximation in Bacon's essay." In point of 

 fact, Bacon's statement, however much open to criticism, does contain a distinct ap- 

 proximation to the most important of all the truths respecting heat which had been 

 discovered when Whewell wrote. 



