Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 519 



instituted in this direction have not hitherto given any decisive 

 result. 



Tt is also worthy of notice that certain solid bodies which are 

 capable of assuming allotropic states, as, for instance, the oxygen- 

 compounds of iron, evolve a considerable quantity of heat on 

 passing from a less to a more hard condition. Such facts, the 

 number of which will doubtless continually increase with time, 

 agree perfectly with the above principle, that diminution of 

 cohesion involves an expenditure of heat, and, on the other hand, 

 increase of cohesion a production of heat. 



Customary language, according to which gravity is called a 

 moving force and heat a substance, occasions, on the one hand, 

 the significance of an important natural object, falling-space, or 

 the space through which a body falls, to be kept as much as 

 possible out of sight, and, on the other hand, heat to be removed 

 to the greatest possible distance from the vis viva of motion. 

 The sciences are thus reduced to an artificial system, over whose 

 fissured surface we can advance in safety only by the powerful 

 aid of the higher analysis. 



Without doubt the fact that so simple and obvious a matter 

 as the connexion between heat and motion could remain unper- 

 ceived up to the most recent times must also be attributed to the 

 same defect. Nevertheless, as has been already pointed out, 

 the quantitative determination of chemical heating-effects and 

 of galvanic actions, as well as researches into vital phenomena, 

 instituted in the spirit of those of Liebig, must soon have led to 

 the law, not difficult to discover, of the equivalence of heat and 

 motion. 



In reality this law and its numerical expression, the mecha- 

 nical equivalent of heat, were published almost simultaneously 

 in Germany and in England. 



Starting from the fact that the amount of chemical as well as 

 of galvanic effect is dependent only and solely on the amount 

 of material expenditure, the celebrated English physicist Joule 

 was led to the principle that the phenomena of motion and of 

 heat rest essentially upon one and the same foundation, or, as he 

 expressed himself, in the same way as I have done, heat and 

 motion are transformable one into the other. 



Not only did this philosopher indisputably make an indepen- 

 dent discovery of the natural law in question, but to him belongs 

 the credit of having made numerous and important contributions 

 towards its further establishment and development. Joule has 

 shown that when motion is produced by means of electro-mag- 

 netism, the heating effect of the galvanic current is diminished 

 in a corresponding and fixed proportion. He has further ascer- 



