SOu'RCE OF MUSCLE-FORCE. 81 



within the body in order to afford this increase in the 

 amount of urea excreted may be calculated. Now, the 

 equivalent in warmth of albuminous bodies is known ; 

 that is, the amount of warmth produced by the com- 

 bustion of a definite weight of albuminous matter is 

 known. And, as the mechanical equivalent of warmth 

 is also known, the amount of work which could be 

 produced by these albuminous bodies under favourable 

 circumstances may, therefore, also be calculated. When 

 this value in work is compared with the amount of 

 work really accomplished, the figures found are always 

 far too low. From this it may safely be inferred that 

 the albuminous matter which undergoes combustion 

 within the body is not capable of affording the work 

 which is performed, and we must rather assume that 

 other substances also undergo combustion, and con- 

 tribute to the labour performed, contribute indeed even 

 the greater part of such labour. If, on the other hand, 

 the amount of carbonic acid excreted by a man during 

 rest is compared with that excreted during greater 

 labour, the increase is found to be very great indeed, 

 and on calculating the amount of labour which should 

 result from the combustion of a corresponding mass of 

 carbon, the amount found corresponds nearly enough 

 with that of the work really performed. 



This experiment, therefore, shows that the muscles 

 generate their work not so much at the expense of 

 albuminous bodies as by the combustion of non-nitro- 

 genous matter. The addition of matter required by the 

 body if it is to remain in a condition capable of labour 

 must, therefore, be regulated accordingly. Hence fol- 

 lows the conclusion, of the greatest importance with 

 reference to the question of diet, that men who have 

 5 



