ACTIVITIES COMMON TO ALL ANIMALS. 29 
bottle with lime-water. The milky appearance of the 
water proves that the gas, carbon dioxide, is present 
in the bottle. How did the gas get in the bottle ? 
Experiment. Carbon Dioxide in the Breath. Breathe 
through a glass tube into a test-tube containing a little 
lime-water. What does the milky appearance of the 
lime-water prove ?) Whence came the carbon dioxide ? 
How was it produced ? 
Experiment. The Oxidation of Hydrogen in our 
Lodies. What collects when we breathe on a cold 
glass? Whence comes the water? The hydrogen 
enters the body with the food. How does the oxygen 
enter the body ? 
Substances Taken into the Body and Substances 
Excreted. It has been found that the greater part of 
the substances taken into the body are compounds of 
hydrogen, oxygen, nitrogen, and carbon. These 
enter the body as food. Oxygen also enters the 
body in breathing. It has been found that the sub- 
stances regularly excreted from the body by the lungs, 
by the skin, and by the kidneys are carbon dioxide, 
water or hydrogen oxide, and a compound containing 
nitrogen and hydrogen, called urea. In this way the 
four elements which enter the body as complex com- 
pounds are all finally excreted as very simple com- 
pounds. 
A Summary of Activities. All animals take food 
of some kind. As in our bodies, so in the bodies of 
all other animals, the food must be chemically changed 
to build up tissues and furnish material for oxidation. 
Although all other animals do not have lungs, skin, 
and kidneys like ours, they nevertheless must excrete 
the materials which result from the oxidations and 
other chemical changes in the body. All animals also 
reproduce. All are capable of movements different 
from the movements of inorganic things. All, too, are 
able in some way to establish communication with the 
outside world. For this purpose we are endowed with 
