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CHAP. III.] point. The temperature of the inside of a tree is said to be above that of the atmosphere if the latter be below 57° Fahr.; but if the temperature of the atmosphere rises above that point, that of the tree does not rise in the same proportion. Hunter found that when the bulb of a thermometer was introduced into the stomach of a carp, the mercury rose to 69°, although the temperature of the pond in which the fish swam was only 65½°. When the temperature of the air was 58°, that of a viper's stomach was found to be ten degrees higher; but when the viper was placed in a temperature of 108°, the heat of its stomach did not exceed 92½°. In what are called warm-blooded animals the difference between the temperature of the body and its surroundings is still more marked, the blood being sometimes nearly 100° above the temperature of the surrounding atmosphere. In the animal body, as long as there is life, the processes of nutrition, of which I have already spoken, are going on, and in the course of these processes heat is constantly being evolved by chemical changes going on in the blood, which is the nourishing fluid. The oxygen gas breathed into our lungs is absorbed into the blood, it there meets with various compounds of carbon derived from the food, and uniting chemically with these gives off heat and produces the carbonic acid which we breathe out, as previously described. Rather less carbonic acid is given off from the lungs than the oxygen absorbed, and it is supposed that the remainder of the oxygen enters