238 THE ATMOSPHERE, AND 



The atmosphere is composed principally of two invisi- 

 ble gases, termed oxygen^ (sometimes vital air, being 

 indispensable to animal life,) and azote or nitrogen, in the 

 proportion of about four fifths of the latter to one fifth of 

 the former. This proportion is found to exist, with tri- 

 fling modifications, in all latitudes and at all elevations. 

 Although these elements are invisible in the atmosphere, 

 they both assmne liquid and sohd forms under many and 

 various circumstances. 



Nitrogen abounds in animals, but seldom to a great 

 extent in plants. It is however found in wheat, in what 

 is denominated gluten, and it is this which gives to that 

 grain its prominent value. It abounds in the urine, but 

 seldom, or but partially, in the dung of animals. " It is 

 the base of ammonia and nitric acid, (aquafortis,) and ap- 

 pears to be the substance which Nature employs in con- 

 verting vegetable into animal substances." — Fourcroy. 

 Its principal office seems to be, to neutralize, in some 

 measure, the properties of oxygen, and to render it fit for 

 respiration and combustion. 



Oxygen enters more or less into all animal and vegeta- 

 ble matters. It constitutes 88 parts in 100 of water, — 

 forms from 40 to 70 per cent, of all vegetable acids, — 

 more than 40 per cent, of the wood of the oak and beech, 

 — about 50 per cent, of starch, the nutritious property, 

 next in value to gluten, of grain, pulse, and roots, and 64 

 per cent, in sugar. It is essential to animal and vegetable 

 life ; it is necessary to fermentation, to combustion, to the 

 germination of seeds, and to the growth and maturity of 

 plants ; and combining with the carbon of the blood, it 

 produces the greatest proportion of animal heat. It also 

 combines with metals and forms oxydes, or, in common 

 language, rust. 



Nitrogen and oxygen are called simple bodies, because 

 they are supposed to be incapable of division or decom- 

 position. 



Carbonic acid gas, also, is found to constitute about one 

 thousandth part of the atmosphere ; and in winter, it has 

 been found to amount to one five hundredth part. This 

 is a compound substance, composed of two parts of o.xy- 



