ANIMAL HEAT. 189 



by drinking a cup of hot tea. The tea excites the perspira- 

 tion, which creates the necessity of evaporation ; and this is 

 done very much at the expense of the heat of the body, which 

 is thereby cooled. All the insensible, and most of the sensi- 

 ble perspiration is converted, on the skin, into vapor ; and, 

 by this conversion of liquid into vapor, a large portion of the 

 excess of heat is carried out of the body, and the standard 

 of the internal temperature is preserved. 



442. This evaporation was very rapid in Sir C. Blagden's 

 experiment, ( 402, p. 172 ;) consequently, the temperature 

 of his body was kept down to about its usual standard, which 

 was 162 below that of the surrounding air which he 

 breathed. He received dry air into his lungs heated to 260, 

 but when it went out, it was cooled down nearly to 98 ; and, 

 when he breathed this air upon his skin r it felt cold, instead 

 of warm, as it usually does. 



443. By the beautiful adaptation of Nature's supply to her 

 wants, the animal body is kept cool in the summer and warm 

 in the winter. The greater appetite and greater desire for 

 animal food, supply more carbon and hydrogen, and the 

 density of the air supplies more oxygen, and consequently 

 a greater fire is maintained, in the cold season, and in cold 

 climates, than in warm seasons, and in hot climates, when 

 and where the appetite craves, and the stomach digests, vege- 

 table diet, which gives less fuel, and the atmosphere affords 

 less oxygen for the support of the combustion. 



444. It is this greater supply of internal heat, and the 

 lesser cooling by evaporation from the surface, that give us 

 what is called the winter constitution ; and the diminished 

 internal fire, and increased evaporation of the perspired 

 fluids, give us the summer constitution. By these means, the 

 body is able to endure a greater degree of cold in the 

 winter, and in the climate of the polar regions, than in 

 the summer, and in the tropical countries; and we can 

 bear a greater degree of heat in the summer and in warm 

 climates, than in the winter and in cold climates. That 

 degree of cold which we bear without discomfort in January, 



