404 PHYSIOLOGICAL CHEMISTRY. 



down to some particular external temperature limit is suffi- 

 cient for the heat demands of the body. Numerous experi- 

 ments have shown that for each animal species there is an 

 external temperature at which the general metabolism, as indi- 

 cated by carbon dioxide excretion or oxygen consumption, and 

 the resultant heat liberation, reach a minimum value. This 

 temperature limit has been called the critical temperature; 

 below it the metabolism and heat liberation increase, evidently 

 not in consequence of a call for more \vork but because of the 

 necessity for more heat. 



ISODYNAMIC RATIOS. 



In metabolism, fats, carbohydrates and proteins all yield 

 kinetic energy and to a large extent each one is capable of 

 replacing the others, a certain minimum of protein being al- 

 ways, of course, necessarily present. The proportions in 

 which they may replace each other may be found by a variety 

 of experimental methods, which yield fairly concordant results. 

 The foods may be burned in a calorimeter and the heats of 

 combustion noted, or their values may be compared through 

 the amounts of oxygen required by calculation to oxidize 

 them, or finally animal experiments in the respiration calorim- 

 eter may be resorted to to fix the relative values. The isody- 

 namic relations are given in this table calculated from results 

 of animal experiments. 



100 gm. of fat 



Lean meat, dry 243 gm. 



Cane sugar 234 gm. 



Glucose 256 gm. 



Starch 232 gm. 



For such substances the calculated and observed values, or 

 the combustion calorimeter and the respiration calorimeter, 

 give closely agreeing results, but it must be remembered that 

 many compounds which show considerable value as measured 

 by combustion are absolutely worthless as measured by nutri- 

 tion. Creatine and urea are illustrations; both are products 



