THE AIR AXD LIFE. 541 



IV. — SECONDARY INGREDIENTS ' OF THE ATMOSPHERE, 



We have now examiued the relationship of animate life with the 

 chemical composition of air, with its pressnre and its movements. It 

 remains for ns to consider the relationship with air contents. There 

 are many accidental, secondary, inconstant elements in air. Some are 

 gaseous, of natural or artificial origin, such as carbonic oxide, carburetted 

 hydrogen, and hundieds of others. We shall say nothing of these here, 

 for after all every substance known in chemistry may, according to 

 locality and circumstances, be found in air, and their presence is always 

 accidental. Those of which we pro]>()se to speak are regular although 

 unessential elements of air. Among these we shall especially exam- 

 ine vajjorized water or moisture and certain solid substances, animate 

 and inanimate, not mentioning for the present pulverized minerals cast 

 forth by volcanoes, produced by industries, or drawn from the earth. 



Moisture is at all times disseminated through the atmosphere in the 

 shape of clouds or fogs and also in the form of invisible vapor. It is 

 concerning the latter that we propose especially to speak. It has a 

 dual origin. One part is supplied through evaporation by the Avater 

 contained in the seas, the rivers, and the ground. This evaporation is 

 determined by both the temperature of the air and the quantity of 

 moisture already contained in it. Another part comes from living 

 beings, from transpiration through the pulmonary and cutaneous sur- 

 faces of animals and from evajjoration normally going on from the 

 leaves of plants. This production of watery vapor by living beings is 

 very variable and is considerably modified by external conditions. An 

 animal or man breathing in very dry air will produce a much greater 

 (luautity of vapor, for the breath when exhaled is saturated with it; but 

 if very moist air be inhaled the production is very small and merely 

 restores to the atmosphere the moisture taken from it. All mankind 

 produces about 15,000,000,000 kilograms of water in twenty-four hours, 

 but this is much more a restitution than a creation. In like manner 

 j)lants add but little to the supply of moisture in the air if it already 

 abounds; but in dry air they emit enormous quantities. It has been 

 l^ossible to estimate, for instance, that a grove of oOO full-grown, healthy 

 trees emits nearly 4,000 tons of moisture in twelve hours of daylight. 

 Vegetal transpiration is less by night and barely equals one-fifth of the 

 evaporation by day. This single illustration will suffice to show how 

 great is the iiroduction of moisture by vegetation. N^ow, if one stops to 

 consider that in the United States, for instance, the surface area of 

 foliage is, according to Mr. J. M. Anders, at least four times as great 

 as that of the laud, one will realize how important a part is filled by 

 vegetation in connection with the question we are now studying, and 

 ^ill not wonder that certain physicists have estimated the quantity of 

 water in the form of vapor contained in the atmosphere at 72 thousand 

 billions of tous or cubic meters. 



This moisture which is diffused in air in greatly variable ]:)r<)portions, 



