ANIMAL HEAT 



the moot 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 Iarg3 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 

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

 20 to body-temperature, at 70 calories, and that required to evaporate 

 the water given off by the lungs at 397, making the total heat-loss by 

 the lungs in these processes from 400 to 500 calories. A certain amount 

 of heat is also absorbed in connection with the escape of the carbon 

 dioxide. The reason why a great deal more water and therefore more 

 he.'it is not given off by the lungs with their enormous surface, and the 

 high degree of imbibition (p. 426) of the epithelium of the alveoli, is 



Fig. 224. Calorimeter for 

 Measuring Heat given off in 

 Respiration. B, copper tube 

 with mouthpiece, connected with 

 the thin brass capsule 4; 4 is 

 connected with a similar capsule 

 3 by a short tube, which passes 

 out from it at the side opposite 

 to that at which B enters; 2 and 

 i are similar capsules. From i 

 an outlet tube C passes off. 

 The whole is set in a copper 

 cylinder A filled with water. 

 A piece is supposed to be cut out 

 of A in order to show the cap- 

 sules. A is placed in another 

 wider copper cylinder. 



that the air is already saturated with aqueous vapour, or nearly so, 

 before it reaches the alveoli. 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 to 450 calories. 

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

 to double or even treble. A diagram of a calorimeter for measuring 

 the heat given off in respiration is shown in Fig. 224. (Se Practical 

 Exercises, p. 721.) 



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. 



