Book III. THE ATMOSPHERE. 355 



that it would be most abundant in the lower regions of the atmosphere; but unless it lias 

 been immediately produced at the surface of the earth in some chemical process, this does 

 not seem to be the case ; elastic fluids of different specific gravities have a tendency to 

 equable mixture by a species of attraction, and the different parts of the atmosphere are 

 constantly agitated" and blended together by winds or other causes. De Saussure found 

 lime-water precipitated on Mount Blanc, the highest point of land in Europe ; and car- 

 bonic acid gas has been always found, apparently in due proportion, in the air brought 

 down from great heights in the atmosphere by aeronautic adventurers. 



2339. The principal consumption of the carbonic acid in the atmosphere seems to be in 

 affording nourishment to plants ; and some of them appear to be supplied with carbon 

 chiefly from this source. 



2340 The formation of carbonic acid gas takes place during fermentation, combustion, 

 putrefaction, respiration, and a number of operations taking place upon the surface of the 

 earth ; and there is no other extensive operation known in nature, by which it can be 

 destroyed but by vegetation. 



2341. Oxygen and azote are the remaining constituents of the atmosphere. After a 

 given portion of common air has been deprived of aqueous vapour and carbonic acid gas, 

 it appears little altered in its properties ; it remains a compound of oxygen and azote, 

 which supports combustion and animal life. There are many modes of separating these 

 two gases from each other. A simple one is by burning phosphorus in a confined 

 volume of air ; this absorbs the oxygen and leaves the azote ; and 100 parts in volume 

 of air, in which phosphorus has been burnt, yield "9 parts of azote ; and by mixing this 

 azote with 21 parts of fresh oxygen gas artificially procured, a substance having the 

 original characters of air is produced. To procure pure oxygen from air, quicksilver may 

 be kept heated in it, at about 600°, till it becomes a red powder ; tliis powder, when 

 ignited, will be restored to the state of quicksilver by giving off oxygen. 



2342. Oxygen is necessary to some functions of vegetables ; but its great importance in 

 nature is its relation to the economy of animals It is absolutely necessary to their life. 

 Atmospheric air taken into the lungs of animals, or passed in solution in water through 

 the gills of fishes, loses oxygen ; and for the oxygen lost, about an equal volume of car- 

 bonic acid appears. 



2343. The effects of azote in vegetation are not distinctly known. As it is found in some 

 of the products of vegetation, it may be absorbed by certain plants from the atmosphere. 

 It prevents the action of oxygen from being too energetic, and serves as a medium 

 in w hich the more essential parts of the air act ; nor is this circumstance unconformable 

 to the analogy of nature ; for the elements most abundant on the solid surface of the 

 globe are not 'those which are the most essential to the existence of the living beings 

 belonging to it. _ . 



2344. The action of the atmosphere on plants differs at different periods of their growth, 

 and varies with the various stages of the developement and decay of their organs.^ If a 

 healthy seed be moistened and exposed to air at a temperature not below^ 45°, it soon 

 •Terminates, and shoots forth a plume, which rises upwards, and a radicle which descends. 

 If the air be confined, it is found that in the process of germination the oxygen, or a part 

 of it, is absorbed. The azote remains unaltered ; no carbonic acid is taken away from 

 the air ; on the contrary, some is added. Seeds are incapable of germinating, except 

 when oxygen is present. In the exhausted receiver of the air-pump, in pure azote, or 

 in pure carbonic acid, when moistened they swell, but do not vegetate ; and if kept in 

 these gases, lose their living powers, and undergo putrefaction. If a seed be examined 

 before germination, it will be found more or less insipid, at least not sweet; but after 

 germination it is always sweet. Its coagulated mucilage, or starch, is converted into 

 sugar in the process ; a substance difficult of solution is changed into one easily soluble ;^ 

 and the sugar carried through the cells or vessels of the cotyledons is the nourishment of 

 the infant plant. The absorption of oxygen by the seed in germination has been com- 

 pared to its absorption in producing the evolution of foetal life in the egg ; but this 

 analogy is only remote. All animals, from the most to the least perfect classes, require 

 a supply of oxygen. From the moment the heart begins to pulsate till it ceases to beat, 

 the aeration of' the blood is constant, and the function of respiration invariable : carbonic 

 acid is given off in the process; but the chemical change produced in the blood is 

 unknown ; nor is there any reason to suppose the formation of any substance similar to 

 sugar. It is evident, that in all cases of semination, the seeds should be sown so as to be 

 fully exposed to the influence of the air ; and one cause of the unproductiveness of cold 

 clayey adhesive soils is, that the seed is coated with matter impermeable to air. In sandy 

 soils "the earth is always sufficiently penetrable by the atmosphere; but in clayey soils 

 there can scarcely be too great a mechanical division of parts. Any seed not fully 

 supplied with air, alwavs produces a weak and diseased plant. We have already seen 

 that carbon is added to 'plants from the air by the process of vegetation in sunshine ; and 

 oxvgen is added to the atmosphere at the same time. It is worthy of remark that the 



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