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Show 188 WilY VEGETABLE MATTER DOES NOT [Ch. XII. The principal elements found in plants are hydrogen, carbon, and oxygen, so that water and the atmosphere contain all of them, either in their own composition or in solution ''f.' , The constant supply of these elements is maintained not only by the putrefaction of animal and vegetable substances, and the decay of rocks before mentioned, but also by the copious evolution of carbonic acid and other gases from volcanos and mineral springs, and by the effects of ordinary evaporation, whereby aqueous vapours are made to rise from the ocean and to circulate round the globe. It is well known that when two gases of different specific gravity are brought into contact, even though the heavier be the lowermost, they become uniformly diffused by mutual absorption through the whole space which they occupy. By virtue of this law, the heavy carbonic acid finds its way upwards through the lighter air, and conveys nourishment to the lichen which covers the mountain top. The fact, therefore, that the vegetable mould which covers the earth's surface does not decrease in thickness, will not altogether bear out the argument which was founded upon it by Playfair. This vegetable soil, he observes, consists partly of loose earthy materials easily removed, in the form of sand and gravel, partly of finer particles suspended in the waters, which tinge those of some rivers continually, and those of aU occasionally, when they are flooded. 'l'he soil, although continually diminished from this cause, " remains the same in quantity, or at least nearly the same, and must have done so ever since the earth was the receptacle of animal or vegetable life. The soil, ther~for~, is augmented from other causes, just as much, at an average, as it is diminished by that now mentioned; and this augmentation evidently can proceed from nothing but the constant and slow disintegration of the rockst.'' • See some good remarks on the Formation of Soils, Bakewell's Geology, chap. xviii, t lllust, of Hutt. Theory, 9 103. Ch. XII.] INCREASE ON TilE SURFACE OF TUB LAND. 189 . That the repai~ of the ea-rthy portion of the soil can only proceed, as P. Jayfan· sugOo' est s, f rom t 11 e d ecompos.m.o n of rocks, may.be admitted; but the vegetable matter may be supplied an. d IS actually furnished in a great deg ree, b y ab sorpti.O n f rom' th. e atmospher.e , as we before mentioned , so th a t m· 1e ve1 s·i tua-tiOns, s.u ch as m pla. tforms tha. t intervene bet ween va 11 eys wh ere the act10n of t·unmng water 1s very triflinO' th £ bl • • 0 , e ne vegeta e partiCles earned off by the rain may be perpetually restored, not by the waste of the rock below, but from the air above. If we supposed the quantity of food consumed by terrestrial ~nimals, together with the matter absorbed by them in breathmg, and the elements imbibed by the roots and leaves of I t b d . pans, to e enved entirely from that supply of hyd rogen, carb on, oxygen, azote, and other elements, given out into the atmosp1h ere and •t he waters by the putrescence of organic subst ances, t 1en .we might imagine that the vegetable mould would, after a s:ries of year~, nei.ther gain nor lose a single particle by the actiOn of orgamc bemgs. This conclusion is not far from the tn~th; but the. operation which renovates the vegetable and ammal mould Is by no means so simple as that here supposed. Thousands of carcasses of terrestrial animals are floated down e.very century into the sea, and, together with forests of drifttimber, are i.mbe~ded in subaqueous deposits, where their elements are Imprisoned in solid strata, and may there remain throug~out whole geological epochs before they again become subservient to the purposes of life. On the other hand, fresh supplies are derived by the atmosp~~ re, an~ by running water, as we before stated, from the dlSl~tegr~twn of rocks and their organic contents, and from the I~tenor of the earth, from whence all the elements beforem~ ntiOned, which enter principally into the composition of ~mmals and vegetables, are continually evolved. Even nitrogen las been recently found to be contained very generally in the waters of mineral springs*. tin:n~r. Daubeny has nscertnin~d this interesting fact in his late tour ou the con. |
