TRANSFORMATION OF PHYSICAL FORCES. 17 



it a certain quantity of heat ; the same thing takes place in the 

 boiler of a steam-engine, as well as in cutaneous evaporation. 



This complication in the measure of force among organized 

 beings shows what difficulties await those who are en- 

 deavouring to verify the principles of thermo- dynamics in 

 animals ; yet, nevertheless, it would be illogical to admit with- 

 out proof that, in living beings, the physical forces do not 

 obey natural laws. Several savants, firmly convinced of the 

 generality of the laws of thermo -dynamics, have attempted to 

 demonstrate them upon the animal organism. 



J. Beclard was the first who endeavoured to prove that in 

 the muscles of man heat may be substituted for mechanical 

 work, and vice versa. For this purpose he examined the 

 therm ometrical temperature of two muscles, both of which 

 contracted, but one worked, that is to say, raised weights, while 

 the other did not work. It might have been expected that less 

 heat would have been found in the first muscle, because a 

 portion of the heat produced during its contraction ought to 

 have been transformed into work. 



The idea which governed Beclard's experiments was 

 assuredly correct, but the means at his disposal for ascer- 

 taining the heating of the muscles were altogether insufficient. 

 A thermometer was applied to the skin at the level of 

 the muscle, in order to give the measure of the heat pro- 

 duced ; thus the variations of temperature obtained by Beclard 

 according as the muscle worked or not, were so slight that no 

 real value could be attached to them. 



Herdenheim obtained clearer results by operating upon 

 frogs' muscles, which he made to contract with or without the 

 production of work, ascertaining their temperature by means 

 of thermo-electric apparatus. 



Hirn was bolder in his experiments, for he sought to deter- 

 mine the equivalent of mechanical work in animated motors. 



In order to make Hirn's experiment comprehensible, let us 

 consider the simpler case of a mechanician desiring to establish 

 the thermal equivalent of the work of a steam engine, knowing 

 how much fuel it has burned, what heat has been given out, 

 and what quantity of work has been produced. 



First, he will estimate the heat which should correspond 



