26 ANIMAL ACTIVITIES. 
oxidizes in a furnace and produces energy or power to 
work, so the various materials of which our bodies are 
made oxidize and set free the energy by which we 
perform the varied movements of our bodies. In a 
very true sense, then, our bodies may be called chemi- 
cal engines. 
Waste and Repair. The furnace which furnishes 
power for any kind of machinery must constantly 
receive new material and give out waste products. 
Without a constant supply of coal and air, the fire goes 
out and work stops. The chimney must be kept clean 
in order to allow the gases produced by the fire to pass 
out, and the ashes must be raked away as fast as they 
are formed, to make space for new fuel. In the same 
way every living animal takes into its body substances 
corresponding to the fuel of a furnace, and it as con- 
stantly gives out the waste products which would soon 
cause death if they should remain. In order that we 
may know these substances better, a few simple experi- 
ments may be considered. 
Experiment. Water. Using the apparatus shown 
in Fig. 35, pour water and a 
little sulphuric acid into the U 
tube, and place test-tubes filled 
with water over the ends of the 
wires. When the circuit is 
closed notice that bubbles of 
gas arise from the wires and 
collect in the upper part of the 
test-tubes. Note the fact that 
water is separated by a current 
saree of electricity into two invisible 
sa Spee oe ae & gases, and that, after a time, 
Drawn by A. E. San. One tube contains twice as much 
ford. gas as the other. The larger 
amount of gas is hydrogen, and 
the smaller, oxygen. Light the hydrogen, noting the 
fact that it burns very readily. Into the top of the 
