CH. XL.] 



PHYSIOLOGICAL HEAT VALUE. 



597 



Hydrogen 

 Carbon . 

 Urea . 

 Albumin 



345 

 8100 

 2205 



Fat . 

 Cane sugar 

 Starch 



9069 

 3348 

 3898 



It is, however, most important to remember that the " physio- 

 logical heat-value " of a food may be different from the " physical 

 heat-value," i.e., the amount of heat produced by combustion in 

 the body may be different from that produced when the same 

 amount of the same food is burnt in a calorimeter. This is 

 the case with the proteids, because they do not undergo complete 

 combustion in the body, for each gramme of proteid yields a 

 third of a gramme of urea, which has a considerable heat-value of 

 its own. Thus albumin, which, by complete combustion, yields 

 4998 heat-units, has a physiological heat-value = 4998 minus 

 one-third of the heat-value of urea (22O5) = 4998 735 = 4263. 

 Rubner has recently shown that this figure must be reduced to 

 4000, as some of the imperfectly burnt products of decomposition 

 of proteids escape as uric acid, creatinine, &c., in the urine, and 

 there is a small quantity of similar substances in the fseces. 



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

 that about 7 per cent, is represented by external mechanical 

 work, and that of the remainder about four-fifths are discharged 

 by radiation, conduction, and evaporation from the skin, and 

 the remaining fifth by the lungs and excreta. 



The following table exhibits the relation between the produc- 

 tion and discharge of heat in twenty-four hours in the human 

 organism at rest, estimated in calories.* The table conveniently 

 takes the form of a balance-sheet in which production and discharge 

 of heat are compared ; to keep the body-temperature normal these 

 must be equal. The basis of the table in the left-hand (income) 

 side is the same as Ranke's diet (see p. 585) : 



* The calorie we arc taking is sometimes called the small calorie; by 

 some the word calorie is used to denote the amount of heat necessary to 

 raise one kilogramme of water i C. 



