524 HISTORY OF SCIENCE. 



Stephen Montgolfier, the celebrated inventor of the fire-balloon, ap- 

 pears to have conceived the idea of a certain equivalence between heat 

 and mechanical work. His nephew, MARC SEGUIN, in a book on 

 railways published in 1839, developed the uncle's ideas. But in 1842 

 the relation between heat and work was for the first time reduced to a 

 quantitative form by Dr. J. R. MAYER, a physician of Heilbronn in 

 Germany. Mayer was the first to use the expression " equivalent of 

 heat," the exact meaning of which we shall presently have to show. 

 His first paper lays down the general principle of the indestructibility 

 of force, and the mutual convertibility of the different forms of force. 

 Mr. (now Justice) GROVE was, in the same year, independently enun- 

 ciating in England similar views on the relations of all the various 

 forces of nature. These views were afterwards extended, and em- 

 bodied in Mr. Grove's treatise on " The Correlation of the Physical 

 Forces." The teaching of both Mayer and Grove did not, however, 

 rest upon that solid basis of experimental demonstration which is 

 needed to establish definitely a great principle like that of the Con- 

 servation of Force. Mayer had, indeed, calculated the value of the 

 " equivalent of heat " from the difference between the specific heat of 

 gases when the volume is maintained unchanged, and the specific heat 

 when the gases are allowed to expand under a uniform pressure. The 

 extended basis of experimental truth, upon which alone the Conser- 

 vation of Force could be upheld as a demonstrated law of nature, 

 instead of a mere speculation, was supplied by Mr. JAMES PRESCOTT 

 JOULE, of Manchester, in the course of six years of laborious research. 

 Mr. Joule's investigations were conducted altogether independently, 

 and though the results were published from time to time, they were at 

 first coldly received, and for some years attracted little attention. In 

 the Report of the British Association for the meeting at Cork in 1843, 

 there appears in a dozen lines a brief abstract of Mr. Joule's first paper 

 on the subject. 



While Joule was working at his experiments, Mayer was boldly ap- 

 plying his theory to the explanation of the mechanical force exhibited 

 by animals, and to other phenomena. He published in 1845 a paper 

 on Organic Motion, and in 1848 an essay on Celestial Dynamics, in 

 which last work he propounded the celebrated meteoric theory of the 

 sun's heat. This theory supposes that the intense heat of the sun is 

 maintained by the impact of meteoric stones drawn into it by its gra- 

 vitative attraction. Sir William Thompson and others have since fully 

 developed this theory. 



The prominent positions which must be assigned to both Mayer 

 and Joule in connection with the subject we are now considering have 

 been thus discriminated by Professor Tyndall : " It is not his experi- 

 ments alone, but the spirit which they incorporate, and the applica- 

 tions which their author made of them, that entitle Mr. Joule to a 

 place in the foremost rank of physical philosophers. Mayer's labours 



