INCOME AND EXPENDITURE OF THE BODY 657 



temperature of warm-blooded animals to a future page, let us 

 consider now the mechanism by which the loss of heat is adjusted 

 to its production, so that upon the whole the one balances the other. 



SECTION II. INCOME AND EXPENDITURE OF THE BODY IN TERMS 



OF ENERGY. 



Heat-Loss. Heat is lost (i) from the surfaces of the body by 

 radiation, conduction, and convection; (2) as latent heat in the 

 watery vapour given off by tjie skin and lungs; and (3) in the 

 excreta. Even in the bulky excrement of herbivora a compara- 

 tively trifling part of the total heat is lost. The second channel 

 of elimination is much more important; the first is in general the 

 most important of all. 



The loss of heat by direct radiation from a portion of the skin or 

 clothes, or from hair, fur, or feathers covering the skin, may be measured 

 by means of a thermopile or a resistance radiometer (bolometer). The 

 latter instrument is similar in principle and allied in construction to the 

 resistance thermometer used in measuring superficial temperatures, 

 and already described (Fig. 210, p. 653). It may consist of a grating 

 of lead-paper or tinfoil fixed vertically in a small box which protects 

 it from draughts. The box has a sliding lid, which is kept closed till 

 the moment of the observation, when it is withdrawn and the portion 

 of skin applied to the opening at a fixed distance (5 to 10 cm.) from the 

 grating. The intensity of radiation depends on the excess 01 tempera- 

 ture of the radiating surface over that of the surroundings, as well as 

 on the nature of the surface. The uncovered parts of the skin (face 

 and hands in man) radiate more per unit of area than the clothes or 

 hair; and the warm forehead more than the comparatively cool lobe 

 t>f the ear or tip of the nose. When a man is sitting at rest in a still 

 atmosphere, pure radiation plays a greater, and conduction and con- 

 vection play a smaller, part in the total loss of heat from the skin than 

 when he is walking about or sitting in a draught. The more rapidly 

 the air in contact with the skin and clothes is renewed, the lower, other 

 things being equal, the temperature of the radiating surfaces is kept, 

 the greater is the loss of heat by conduction to the adjacent portions of 

 air, and the smaller the loss by radiation to the walls of the room, the 

 furniture, and other surrounding objects. It is probable that, under 

 the most favourable conditions, the amount of heat lost from the sur- 

 face by true radiation does not exceed the amount lost by conduction 

 and convection. 



The loss of heat by evaporation of water from the skin can be calcu- 

 lated if we know the quantity of water so given off. For a gramme of 

 water at the ordinary temperature (say 15 C.) needs 0-555 calorie to 

 convert it into aqueous vapour at the average temperature of the skin. 

 If we take the average quantity of water excreted as sweat in twenty- 

 four hours as 750 c.c., this will be equivalent to a heat-loss of 416*25 

 say, in round numbers, 400 large calories. 



The quantity of heat given off by the lungs may be also deduced 

 from calculation, the data being (i) the weight, temperature, and 

 specific heat of the expired air, and (2) the excess of water it contains 

 in the form of aqueous vapour over that contained in the inspired air. 

 Helmholtz calculated the quantity of heat needed to warm the air 



42 



