222 THE ATMOSPHERE: ITS COMPONENT PARTS. 



mainly composed of Ditrogen and oxygen gases, in the proportion of 

 about 4 to 1 by volume — not chemically united, but each equally ditfused, 

 as it would exist if alone. The former has comparatively few afiinities, 

 and is not a common element in plants, while the latter is invariably 

 found in all vegetable and animal organizations — is essential to com- 

 bustion and to life, and as an oxide of metallic bases and a constituent 

 of air and water, it is the most abundant element known. 



There is also always present a perceptible amount of carbonic-acid 

 gas, and a variable amount of aqueous vapor. All of these, obey the 

 laws common to all gases, expanding when heated, and contracting when 

 cooled. Expansion, by diminishing the weight of a given volume, gives 

 it a tendency to rise, when displaced by other volumes of greater den- 

 sity. The unequal warming of portions of the earth's surface by solar 

 heat, the earth's rotation, the approach and passage of areas of high 

 or low barometer, and other influences, cause winds, which become the 

 means of carrying over the land the vapor raised from the sea. Wher- 

 ever these vapors, whether raised from the sea or other bodies of water, 

 or from the earth, condense in sufficient falls of rain, we generally find 

 the earth's surface in a state of nature covered with forests ; but where 

 the winds are deprived of their moisture by passing over mountains, or 

 from other causes, no rains fall, and we have arid deserts. 



Carbonic-acid gas was probably a more abundant element in the at- 

 mosphere in the early geological ages than at present, as vast quanti- 

 ties are found fixed by chemical affinities in the mineral carbonates, 

 more particularly of lime, (forming immense masses of limestone, chalk, 

 and marbles), and in beds of mineral coal. In the former of these the car- 

 bonic acid has been largely fixed through the agency of animal life; in 

 the latter it is the result of a luxuriant vegetable growth, at a geolog- 

 ical period when the temperature of the earth was much greater, and 

 the amount of carbonic-acid gas relatively much more abundant than 

 at present. This proportion in the present atmosphere, is variously es- 

 timated at from one-thousandth to four ten-thousandths part of the air 

 by weight; in other words, there are from 18.8 to 47 tons of this gas 

 over every acre of the earth's surface. There is no evidence that this 

 proportion has sensibly changed since man was created, and for all 

 jiractical purposes it may be regarded as uniform from age to age. 

 From this source, the carbon of wood is largely derived, and in the 

 chemical operation of plant-growth, and under the action of light, it is 

 being taken into the tissues of vegetables through the growing season — 

 while from the respiration of animals, the vital processes of plants, 

 combustion, decay, and other operations going on in nature, it is again 

 given back to the atmosphere, thus maintaining its due proportions with- 

 out sensible variation, as we compare observations made in different re- 

 gions of the earth or at wide intervals of time. 



The proportion of vapor in the atmosphere has an important influ- 

 ence upon vegetable growth, and every cause tending to increase or 

 diminish the amount, will have a favorable or adverse effect upon it. 



We know that water, when exposed to the open air, will under ordi- 

 nary conditions, slowly evaporate, the rate chiefly depending upon the 

 temperature, the amount of moisture already present in the air, and the 

 movement of the lower strata of air in contact with the evaporat- 

 ing surface. It is known by experiment, as well as by calculation, 

 that in a given volume, and under a uniform barometric pressure, 

 dry air is slightly/ heavier than air saturated with moisture, and 

 that it decreases a little less rapidly in weight with an increase of tern- 



