Page:Elementary Text-book of Physics (Anthony, 1897).djvu/270

256 time. Despretz and Dulong made similar experiments with more perfect apparatus, and found that the heat produced by the animal was about one-tenth greater than would have been produced by the formation by combustion of the carbonic acid and water exhaled.

These and similar experiments, although not taking into account all the chemical actions taking place in the body, leave no doubt that animal heat is due to atomic and molecular changes within the body.

The work performed by muscular action is also the transformed energy of food. Eumford, in 1798, saw this clearly; and he showed, in a paper of that date, that the amount of work done by a horse is much greater than would be obtained by using its food as fuel for a steam-engine.

Mayer, in 1845, held that an animal is a heat-engine, and that every motion of the animal is a transformation into work of the heat developed in the tissues.

Hirn, in 1858, executed a series of interesting experiments bearing upon this subject. In a closed box was placed a sort of treadmill, which a man could cause to revolve by stepping from step to step. He thus performed work which could be measured by suitable apparatus outside the box. The tread-wheel could also be made to revolve backward by a motor placed outside, when the man descended from step to step, and work was performed upon him.

Three distinct experiments were performed; and the amount of oxygen consumed by respiration, and the heat developed, were determined.

In the first experiment the man remained in repose; in the second he performed work by causing the wheel to revolve; in the third the wheel was made to revolve backward, and work was performed upon him. In the second experiment the amount of heat developed for a gram of oxygen consumed was much less, and in the third case much greater, than in the first; that is, in the first case, the heat developed was due to a chemical action, indicated by the absorption of oxygen; in the second, a portion of the chemical action went to perform the work, and hence a less amount of heat