ANIMAL HEAT 497 



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 of 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 convection 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, is the temperature of the radiating surfaces 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 surface 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 

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

 gramme of water at the ordinary temperature (say, 15 C.) needs 

 555 millicalories 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 416,250 say, in round numbers, 400,000 

 millicalories. 



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 expired by a man in twenty-four hours from an initial temperature 

 of 20 to body temperature, at 70,000 small calories, and that required 

 to evapora'c-a the water given off by the lungs at 397,000, making the 

 total heat-loss by the lungs from 400,000 to 500,000 small calories. 

 By direct calorimetric observations it was found that a man of 70 

 kilos weight gave off in normal breathing, with an air temperature 

 of 12 to 15 C., from 350,000 to 450,000 small calories. Forced 

 respiration, as might be expected, increased the amount often to 

 double or even treble. A diagram of a respiration calorimeter is 

 shown in Fig. 140. (See Practical Exercises, p. 528.) 



The following table gives an analysis of the heat-loss of 

 an average man. It must be understood that the figures are 

 only approximate. In round numbers we may say that two- 

 thirds of the heat loss is due to radiation, conduction, and 

 convection, and one-third to the evaporation of water. 



Per cent. Millicalories. 



f Evaporation of water - - 15 



Skin < Radiation - - 30 



[Conduction (and convection) - 35 



Lungs 



Evaporation of water - 15 



Heating the expired air- - 2-5 



400,000 



80 750,000 



900,006 



ooo 



J400, 



\ 70,000 



Heating the excreta - - 2-5 70,000 



100 2,590,000 

 32 



