INCOME AND EXPENDITURE OF HEAT. 445 



action of cold. (5.) The permeability for air is of importance, but does not stand 

 in relation with the heat-conducting capacity. The following substances are 

 arranged in order from the most to the least permeable flannel, buckskin, linen, 

 silk, leather, waxcloth. 



215. Income and Expenditure of Heat- 

 Balance of Heat. 



As the temperature of the body is maintained within narrow limits, 

 the amount of heat taken in must balance the heat given off, i.e., exactly 

 the same amount of potential energy must be transformed in a given 

 time into heat, as heat is given off from the body. 



An adult produces as much heat in half an hour as will raise the 

 temperature of his body 1C. If no heat was given off, the body 

 would become very hot in a short time ; it would reach the boiling- 

 point in 36 hours, supposing the production of heat continued 

 uni nterruptedly. 



The following are the most important calculations on this subject : 



A. According to Helmholtz. 



Helmholtz was the first to estimate numerically the amount of heat produced by 

 a man. 



(1.) Heat-income. () A healthy adult, weighing 82 kilos., 

 expires in 24 hours, 878 '4 grms. C0 2 (Scharling). The 

 combustion of the C therein into COg produces . . 1,730,760 Cal. 



(b) But he takes in more O than reappears in the COg ; the 

 excess is used in oxidation-processes, e.g., for the forma- 

 tion of H 2 O, by union with H, so that 13,615 grms. H 

 will be oxidised by the excess of 0, which gives . . 318,600 ,, 



2,049,360 Cal. 

 (c) About 25 per cent, of the heat must be referred to sources 



other than combustion (Dulong), so that the total = 2,732,000 Cal. 



2,732,000 calories are actually sufficient to raise the 

 temperature of an adult weighing 80-90 kilos., from 10 to 

 38-39C., i.e., to a normal temperature. 



(2.) Heat-expenditure. (a) Heating the food 

 and drink, which have a mean temperature 

 of!2C. . .' . . . . 70, 157 Cal. = 2 '6 per cent. 



(b) Heating the air respired = 16,400 grm., with an 



initial temperature of 20C. . . 70,032 = 2 '6 ,, 



( When the temperature of the air is 0, 140,064 Cal. =5 '2 per cent.) 



(c) Evaporation of 656 grm. water by the lungs, 397,536 Cal. = 147 per cent. 



(d) The remainder given off by radiation and 



evaporation of water by the skin, (77 '5 per cent, to) = 80*1 per cent. 



