TRANSFORMATION OF PHYSICAL FORCES. 15 



nothing of their value, is justly considered the most remarkable 

 of modern times. 



Partly seen by Sadi-Carnot, clearly formulated by R. Mayer, 

 demonstrated brilliantly by the experiments of Joule, the 

 notion of the equivalence of forces is now admitted by all 

 physicists. Each day furnishes a fresh confirmation of this 

 doctrine, and leads to greater precision in the determination of 

 the mechanical equivalent of heat. The value now generally 

 admitted for that equivalent is 425, that is to say, that work 

 equal to 425 kilogramme tres must be transformed into heat 

 to obtain a unit, and inversely, that the heat capable of heat- 

 ing to one degree one kilogramme of water at zero, if it be 

 transformed into work, can, in its turn, lift a weight of 425 

 kilogrammes one metre."^ 



But one restriction must be placed upon the estimation 

 of thermo- dynamic transformations. Carnot suspected, and 

 Clausius had clearly established that in the case of heat being 

 employed to produce work, the heat cannot transform itself 

 altogether, and that the greater part remains still in the state 

 of heat ; while in the inverse operation the whole of the work 

 applied to that efiect may be transformed into heat. This 

 does not exclude the law of equivalence, of which we have 

 just spoken; for if it be true that, in a steam engine for 

 instance, there is only to be found under the form of work a 

 email quantity, about 12° of the heat produced by the fur- 

 nace, it is no less true that the quantity of heat which has 

 disappeared furnishes in work the exact number of kilo- 

 grammetres which corresponds to its mechanical equivalent. 



These notions had no sooner been introduced into science 

 than the physiologists endeavoured to use them for the 

 clearing up of the obscure question of heat and work pro- 

 duced by animals. The assimilation of living beings to 

 thermal machines was already in the state of vague con- 

 ception. We shall see what light has been thrown upon it 

 by the new theory. 



* Some experiments made by Regnault on tlie rapidity of sound, and 

 on the expansion of gases, give as the true value of the equivalent the 

 number 439. 



