204 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



tance to mankind than the two elementary gases which make up by far 

 the greater part of the volume and weight of the whole. This is vapor 

 of water, the result of the process of evaporation of those vast watery 

 surfaces which are always in contact with the lower strata of the air. 



Deprive the air of any one of these three main constituents arid 

 human life becomes impossible. 



Next in rank from the human point of view is carbon dioxide, or car- 

 bonic acid gas, which, though comparatively very small in amount, 

 exists throughout at least all the lower ranges of the atmosphere, and 

 has the same close and necessary relations with plant life as oxygen 

 has, or rather as food has, with the life -of animals. It presents on a 

 great scale an example of the wonderful law of gaseous diffusion; for, 

 though much heavier than air, in the proportion of about 2 to 1, it 

 diffuses under natural conditions nearly equably through every part, 

 whether the region of its origin be near or distant. 



Stated in tons, the following are the calculated weights of the chief 

 substances composing the whole atmosphere: 



Billions of tons. 



Oxygen , 1,233,010 



Nitrogen 3,994,593 



Carbon dioxide 5, 287 



Vapor 54,460 



In addition to the above, we find in the air a variable and very small 

 quantity of ammonia, chlorides, sulphates, sulphurous acid, nitric acid, 

 and carburetted hydrogen, but some of these depend, where detected, 

 to a great extent on manufacturing operations and on aggregations of 

 men and animals. 



Liquids and solids in great variety are also very important, widely 

 diffused, and constant ingredients in the atmosphere. The solids are 

 everywhere present in the condition of very minute microscopic or ultra- 

 microscopic motes or dust, composed chiefly of sea salt, or chloride of 

 sodium, sand, or fine silicious particles, various dusts derived from 

 volcanoes, factories, towns, and the remains of meteors set on fire in 

 their passage through the upper air. Some of the most beneficent 

 functions of these microscopic and invisible motes will be considered 

 later. Other solids present in the upper air over a large part of the 

 globe and in the lower strata, especially in the Arctic regions, are 

 small particles of ice, condensed either in clouds or in air which appears 

 nearly clear. Explorers in high latitudes relate that on fine cold days 

 the air is frequently sprinkled with shining crystals of ice which seem 

 to fall from a blue sky, and, on the other hand, in heavy gales and 

 stormy weather the lower air is filled with a fine icy dust, resulting 

 from the freezing of the spray torn from the sea waves. In temperate 

 climates very much of the rain which falls on the surface of the earth 

 has existed previously at high levels in the state of snow or ice particles. 

 The experience of mountaineers and balloon voyagers, and, in a moun- 

 tainous country, the sight of peaks covered with fresh snow after a 



