APPENDIX 325 
APPENDIX. 
Composition of the Atmosphere.—The atmosphere 
—an invisible elastic ‘fluid surrounding the earth—is a 
mechanical mixture of watery vapour and a number of 
substances principally gases, the chief of which are oxygen, 
nitrogen, and carbon-dioxide. The proportion of these per 
100 volumes is approximately 
Oxygen ... £4 &; 20°80 
Nitrogen : 
Argon \ 18 
Carbon-dioxide _... bo “04 
100-00 
In addition to these there are present, as essential con- 
stituents, ammonia, ozone, and nitric acid. Other substances 
—accidental impurities as they are called—are sometimes 
present, the chief of which is sulphuric acid. 
Composition of Water.—Water is a chemical com- 
pound of two gases, oxygen and hydrogen, in the pro- 
portion of one volume of oxygen to two volumes of 
hydrogen. 
Since oxygen is approximately sixteen times heavier 
than hydrogen, it follows that these gases must be present 
in water, inthe proportion of sixteen parts by weight of 
oxygen to two parts by weight of hydrogen, 7.¢. in the 
proportion of 8 to 1. 
For a more detailed account of these substances, and for 
methods of determining the proportion in which the gases 
enter into the composition of air and water, the reader is 
referred to some treatise on chemistry. In the upper 
classes it will probably be necessary to discover these pro- 
portions by experiment, as also to investigate the properties 
of the gases. 
