42 PRODUCTION OF HEAT. 



heat produced by each substance used as food when com- 

 pletely burnt. The physiological heat-value of any sub- 

 stance is the quantity of heat produced by the chemical 

 disintegration which it actually undergoes in the animal 

 organism. Those substances which yield in the living 

 body the same quantity of heat as by complete combus- 

 tion in the laboratory, have the same physiological as 

 physical heat-value ; but as regards substances containing 

 nitrogen, the result must be corrected by deducting from 

 it the heat produced by the combustion of the equivalent 

 weight of urea. Thus, albumin, which by complete com- 

 bustion yields 4998 units of heat, has a physiological 

 heat-value of only 4263 units. The results so obtained, 

 although only approximately correct in application, are 

 correct in principle ; for, however various may be the 

 chemical processes of animal life, their value as regards 

 the quantities of heat and work produced must be esti- 

 mated by the end-products. 



Of the heat produced in the body, it is estimated by 

 Helmholtz that about 7 per cent, is represented by ex- 

 ternal mechanical work, and that of the remainder about 

 four-fifths is discharged by radiation and evaporation 

 from the surface, and one-fifth by the lungs and excreta. 



The following Table exhibits the relation between the 

 production and discharge of heat in twenty-four hours in 

 the human organism at rest, estimated in kilogramme 

 units or calories : 



TABLE V. 



PRODUCTION OF HEAT. 



Consumption of Albumin . (loogrms.) . 100X4*263= 426 Cals. 

 Fat . . (loogrms.) . 100X9069= 907 ,, 



Carbohydrates (223grms.) . 223 X 230 = 1167 



2500 Cals. 



