348 SCIENCE OF AGRICULTURE. Part II. 



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

 has 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 carbonic acid gas has been always found, apparently in due proportion, in 

 the air brought down from great heights in the atmosphere by aerostatic adventurers. 



2272. 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. 



2273. 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 process known in nature, by which it can be destroyed but by 

 vegetation. 



2274. Oxygen and azote are the remaining constituents of the atmosphere. After a 

 given portion of common air has been deprived of aqueous vapor 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 vo- 

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

 air, in which phosphorus has been burnt, yield 79 parts of azote ; and by mixing this 

 azote with 21 parts of fresh oxygenegas artificially procured, a substance having the ori- 

 ginal 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 ; this powder, when 

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



2275. 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. 



2276. 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 atmo- 

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

 medium in which the more essential parts of the air act ; nor is this circumstance un- 

 conformable to the analogy of nature ; for the elements most abundant on the solid sur- 

 face of the globe, are not those which are the most essential to the existence of the living 

 beings belonging to it. 



2277. The action of the atmosphere on plaiits 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 germinates, 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 germin- 

 ation 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 yessels of the cotyledons, is the nou- 

 rishment of the infant plant. The absorption of oxygen by the seed in germination has 

 been compared 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, re- 

 quire 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 or?e 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 sup- 

 plied with air, always produces a weak and diseased plant. We have already seen ( 1 530. ) 

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

 oxygen is added to the atmosphere at the same time. 



