444 



COMPARATIVE PHYSIOLOGY. 



If the amount of teat that a body will produce in its com- 

 bustion be known, then by the law of the conversion and equiv- 

 alence of energy the mechanical equivalent can be estimated in 

 that particular case. 



The heat-producing power of different substances can be 

 directly learned by ascertaining the extent to which, when fully 

 burned (to water and carbonic anhydride), they elevate the 

 temperature to a given volume of water ; and this can at once be 

 translated into its mechanical equivalent of work, so that we 

 may say that one gramme of dry proteid would give rise to a 

 certain number of gramme-degrees of heat or kilogramme- 

 metres of work. A few figures will now show the relative 

 values of certain food-stuffs : 



The reason of the subtraction has been explained above. 



Taking another diet in regard to which the estimates differ 

 somewhat from those given previously, but convenient now as 

 showing how equal weights of substances produce very dif- 

 ferent amounts of energy, we find that — 



In other words nearly a million kilogramme-metres of en- 

 ergy are available from the above diet for one day, provided it 

 be all oxidized in the body. 



Food-stufPs, then, with the oxygen of the air, are the body's 

 sources of energy. What are the forms in which its expendi- 

 ture appears ? We may answer at once heat and mechanical 

 work ; for it is assumed that internal movements as tliose of 

 the viscera, and all the friction of the body, all its molcular 

 motion, all secretive processes, are to be regarded as finally 



