196 AIR AND LIFE. 



constituents of the atmosphere, and we may say that every substance 

 known to chemistry may at some time or place be accidently present in 

 the air. Only such bodies deserve notice as are normally present in 

 the whole atmosphere, although they may be of minor importance. 

 Under this head must be mentioned aqueous vapor and different solid 

 materials, inanimate or animate, excluding those which are of volcanic 

 origin, and dust, natural or artificial. 



Aqueous vapor is always present in the atmosphere under the form 

 of fog or clouds, and also in an invisible form. "We will especially 

 refer to the latter. It has a dual origin. The one part comes from 

 evaporation, under the influence of heat, of the water of oceans, rivers, 

 lakes, and moist soil. The amount of vapor produced depends upon 

 the amount of heat, and also upon the amount of vapor already con- 

 tained in the air. For each degree of temperature air can only contain 

 a quite definite amount of vapor. The other part comes from living 

 organisms, by transpiration through the skin and pulmonary surfaces 

 of animals, by the evaporation Avhich occurs from the leaves of plants. 

 This production of aqueous vapor by living beings is very variable, 

 and circumstances affect it greatly. An animal or man in dry air pro- 

 duces a large amount, since the expired air is quite saturated with it, 

 but in moist air hardly any is produced, and that which is expired 

 hardly does more than restore to the atmosphere the moisture taken 

 from it. The whole of mankind pours into the atmosphere a total 

 amount of some 15,000,000,000 kilograms of water per twenty-four 

 hours, but a large proportion of this is merely returned; it has not 

 been generated by man. Similarly, plants yield but a small amount of 

 moisture if the air is already nearly saturated; they yield a very large 

 amount if it is dry. It has been calculated, for example, that a wood 

 of 500 adult and vigorous trees yields nearly 4,000 tons of aqueous 

 vapor during the twelve hours of daylight. By night the amount is 

 less considerable, and is only about one-fifth of the diurnal evapora- 

 tion. This instance is enough to show that plants are most important 

 producers of vapor. And if one only considers that in the United 

 States, as an example, the total surface of plant leaves is at least four 

 times that of the soil surface, one perceives how important must be 

 the part of plants in the function we refer to. Physicists have esti- 

 mated the total quantity of aqueous vapor in the atmosphere at 

 72,000,000,000,000 tons or cubic meters of water. 



This vapor, which is very unequally diffused (since the maximum 

 amount depends upon the temperature of air), and which varies in quan- 

 tity according to the time, locality, and other circumstances, plays an 

 important biological part; Air, when too dry irritates the respiratory 

 organs; when too moist it impedes transpiration and its beneficent 

 effects: in medio virtus, and the best condition is that in which air is 

 neither very dry nor very moist. 



Another more important partis played by this aqueous vapor in that 



