BooK ii. OF THE ATMOSPHERE. 255 



earth ; and there is no other process known in nature by which it can be destroyed but by 



vegetation. , , .. 



1224. Oxygen and azote are the remaining constituents of the atmosphere. Alter a given 

 portion of common air has been deprived of aqueous vapor and carbonic acid gas, it ap- 

 pears 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 m a confined volume of 

 air this absorbs the oxvgen 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 oxygene gas artificially procured, a substance having the original charac- 

 ters 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. 



1225. Oiysen is necessary to some functions of vegetables; but its great importance in na- 

 ture is in 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. 



1226. The effects of azote in vegelation 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 

 which the more essential parts of the air act ; nor is tbis 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 die existence of the living beings be- 

 longing to it. 



1227. 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. We have 

 seen (723.) that 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 vessels 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 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 sup- 

 plied with air, always produces a weak and diseased plant. We have already seen (756.) 

 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. 



1228. Those changes in the atmosjrfiere which constitute the most important meteorological 

 phenomena, may be classed under five distinct heads ; the alterations that occur in the 

 weight of the atmosphere ; those that take place in its temperature ; the changes produced 

 in its quantity by evaporation and rain ; the excessive agitation to which it is frequently 

 subject ; and the phenomena arising from electric and other causes, that at particular times 

 occasion or attend the precipitations and agitations alluded to. All the above phenomena 

 prove to demonstration that constant changes take place, the consequences of new com- 

 binations and decompositions rapidly following each other. 



1229. With respect to the changes in the weight of tlie atmosphere it is generally known 

 that the instrument called the barometer shows the weight of a body of air immediately 

 above it, extending to the extreme boundary of the atmosphere, and the base of which is 

 equal to that of the mercury contained within it. As the level of the sea is the lowest 



