234 MAiTUAL OP oattle-fb:eding. 



Evaporation of Water. — ^An important regulator of 

 tlie temperature of the body is the evaporation of water, 

 especially from the skin. 



In the conversion of any liquid into vapor, a rery con- 

 siderable amount of heat is absorbed, and becomes latent 

 in the vapor. Tiiis absorption of heat during vaporization 

 may be rendered evident to the senses by wetting the hand 

 with some volatile liquid, such as alcohol or ether, and 

 moving it through the air to hasten evaporation. In the 

 same way, the evaporation of water from the skin, which 

 is constantly going on, cools the latter ; and though the 

 effect is less noticeable than with a more volatile liquid, on 

 account of the greater slowness of the evaporation, the 

 total amount of heat thus abstracted from the body is very 

 considerable, amounting, according to Ilenneberg (p. 231), 

 in the case of sheep, to nearly 27 per cent, of the total loss 

 of heat. 



The conversion of one gramme of water at the tempera- 

 ture of the body into vapor of the same temperature 

 requires 580 heat units, an amoimt equal to that produced 

 by the combustion of 0.148 grammes of organic matter 

 having the composition of starch, and corresponding to an 

 excretion of 0.241 grammes of carbonic acid. In the ex- 

 periments by Henneberg just mentioned, the average daily 

 excretion of water from lungs and skin was 881.7 grammes, 

 which, according to the above figures, required for its 

 evaporation as much heat as would be produced by the 

 combustion of 130.5 grammes of starch, while the average 

 amount of carbhydrates digested per day was 464.3 

 grammes. Consequently, if the loss of heat by evapora- 

 tion was supplied by the combustion of these substances, 

 about 28 per cent, of them w^as thus consumed. 



Such results as this, of which many might be cited, show 



