460 ANIMAL PHYSIOLOGY. 
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 of 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 : 
Gram.-deg. Kilomet. 
1 gramme proteids«:ccccce cass cine sence cedey ¢ 5,103 2,161 
d. gramme Urea. ssc cicens sre sas see) ame tO. cw 135 311 
Available energy of the proteid................ 4,368 1,850 
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— 
| Gram.-deg. | Kilomet. 
100 grammes proteid yield.................0008 -...| 486,800 185,000 
100 grammes fat yield........... s$agNeeasnse tase 906,900 384,100 
240 grammes starch yield.................00 0c ceee 938,880 397,680 
Totals isaseneswrenes wae dewedperiase sags Ace 2,281,580 966,780 
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-stufts, 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 those of 
the viscera, and all the friction of the body, all its molecular 
motion, all secretive processes, are to be regarded as finally 
augmenting the heat of the body. Heat is lost by the skin, 
lungs, urine, and feces. 
