V i T 



tilicially imitated ; and in moit of the chemical changes that 

 take place when vegetable compounds arc expofed to air, 

 oxygen or vital air is abforbed, and carbonic acid formed or 

 evolved. Much advantage may be taken of this in the fow- 

 ing of different kinds of grain and feeds, and in the tillage 

 cultivation of different forts of land, as well as in different 

 other practices and proceffes ; the former not being done 

 too deeply in any cafe, nor the latter too lightly in ftiff 

 tenacious foils. See Tillage. 



When the roots and leaves of the infant plant are formed, 

 the cells and tubes throughout its ftruflure become, it is 

 faid, filled with fluid, which is ufually fupplied from the 

 foil of the land, and the fuuftion of nourifhment is per- 

 formed by the adlion of its organs upon the external ele- 

 ments. The conftituent parts of the air are fubfervient to 

 this procefs ; but, as might be expefted, they aft differently 

 under different circumftances, it is thought. When a grow- 

 ing plant, the roots of which are fupplied with a proper 

 nourifhment, is expofed in the prefence of folar light to a 

 given quantity of atmofpherical air, containing its due pro- 

 portion of carbonic acid, the carbonic acid after a certain 

 time is deftroyed, and a certain quantity ot vital air or 

 oxygen is found in its place. If new quantities of car- 

 bonic acid gas be fupplied, the fame refult occurs ; fo 

 that cai-bon.is added to plants from the air by the pro- 

 cefs of vegetation in fun-fliine ; and vital air or oxygen is 

 added to the atmofphere, as proved by the experiments 

 of Prieftley, Ingenhoufz, and many others more lately. 

 The abforption of carbonic acid gas, and the produftion 

 of vital air or oxygen, are performed by the leaf; and 

 leaves recently feparated from the tree or plaiit ctfeft the 

 change, when confined in portions of air containing car- 

 bonic acid ; and abforb the fame acid, and produce vital air 

 or oxygen, even when immerfcd in water liolding carbonic 

 acid in folulion. It is fuppofed that this acid is probably 

 abforbed by the fluids in the cells of the green or parenchy- 

 matous part of the leaf ; and that it is from this part that 

 vital air or oxygen gas is produced during the prefence of 

 light. M. Sennebier, it is faid, found that the leaf, from 

 wiich the epidermis was flripped off, continued to produce 

 vital air or oxygen when placed in water containing car- 

 bonic acid gas, and tliat the globules of air rofe from the 

 denuded parenchyma ; and it is fliewn, by the experiments of 

 the fame writer as well as thofc of Woodhoufe, that tlie 

 leaves moll abundant in parenchymatous parts produced 

 mofl vital air or oxygen in water impregnated with carbonic 

 acid. Some few plants, it is faid, will vegetate in an arti- 

 ficial atmofphere, conlilting principally of carbonic acid ; 

 and many will grow for fome time in air containing from 

 one-half to one-third ; but they arc not fo healthy as 

 when fupplied with fmaller quantities of this elallic fub- 

 ftance. Plants expofed to light have been found to produce 

 vital air or oxygen gas in an elaflic medium, and in water 

 containing no carbonic acid gas ; but in quantities much 

 fmaller than when that acid gas wa^ prefent. In the dark, 

 no vital air or oxygen gas is produced by plants, whatever 

 be the eladic medium to which they are expofed ; and no 

 carbonic Jcid abforbed. In mod cafes, on the contrary, 

 vital air or oxygen gas, if it be prefent, is abforbed, and 

 carbonic acid gas is produced. In the changes that take 

 place in the compolition of the organized parts, it is fup- 

 pofed probable that faccharine compounds are principally 

 formed during the abfence of light ; gum, woody fibre, 

 oils, and refins during its prefence ; and that the evolution 

 of carbonic acid gas, or its formation during the night, may 

 be neceffary to give greater folubility to certain compounds 

 io the plant. It was once fufpocted t!wt all the carbonic 



V I T 



acid gas produced by plants in the night, or in fhade, might 

 be owing to the decay of fome part of the leaf, or epi- 

 dermis ; but the late experiments of Mr. D. EUis are op- 

 pofed to this notion ; and it was found that a perfeftly 

 healthy plant of celery, placed in a given portion of air for 

 a few hours only, occafioned a produftion of carbonic acid 

 gas, and an abforption of vital air or oxygen. 



It has been fuppofed by fome, it is faid, that plants ex- 

 pofed in the free atmofpliere to the vicifTitudes of fun-fliinc 

 and fhade, light and darknefs, confume more vital air or 

 oxygen than they produce, and that their permanent agency 

 upon air is fimilar to that of animals ; and this opinion is 

 countenanced by the inquiries on vegetation of the writer 

 jufl noticed. But the whole of the experiments brought 

 forward in favour of this notion, and particularly thofe of 

 this writer, have, it is faid, been made under unfavourable 

 circuruflances to the accuracy of refult. The plants have 

 been confined and fupplied with food in an unnatural man- 

 ner ; and the influence of light upon them has been very 

 much diminiflied by the nature of the media through which 

 it paffed. Plants confined in limited portions of atmo- 

 fpheric air foon become difeafed ; their leaves decay, and 

 by their decompofition they rapidly deflroy the vital air or 

 oxygen of the air. In fome of the early experiments of 

 Priellley, before he was acquainted with the agency of light 

 upon leaves, air, it is faid, that had fupported combuflion 

 and refpiration, was found purified by the growth of plants 

 when they were expofed in it for fucceffive days and nights; 

 and his trials are the more unexceptionable, it is thought, 

 as the plants, in many of them, grew in their natural flates ; 

 and fhoots, or branches from them, only were introduced 

 through water into the confined atmofphere. And fome 

 further refearches on this fubjeft made by the able writer of 

 the work on agricultural chcmiilry noticed above, furniili 

 fafts which confirm the popular opinion, that when the 

 leaves of vegetables perform their healthy funftions, they 

 tend to purify the atmofphere in the common variations of 

 weather, and changes from light to darknefs. 



In germination, and at the time of the decay of the leaf, 

 vital air or oxygen mutl, it is faid, be abforbed ; but when 

 it is confidered how large a part of the furface of the earth 

 is clothed with perennial graffes, and that half of the globe 

 is always expofed to the folar liglit, it appears by far the 

 moll probable opinion, that more vital air or oxygen is pro- 

 duced than confumed during the procefs of vegetation ; 

 and that it is this circumflance which is the principal caufc 

 of the uniformity of the conflitution of the atmofphere. 

 Animals produce no vital air or oxygen gas during the ex- 

 ercife of any of their tiinftions, and they are conflaiitly con- 

 fuming it ; but tlie extent of the animal, compared to that 

 of the vegetable kingdom, is, it it faid, very fmall ; and 

 the quantity of carbonic acid gas produced in refpiration, 

 and in various proceffes of combullion and fermentalioa, 

 bears a proportion extremely minute to the whole volume of 

 the atmofphere : if every plant during the progrefs of its 

 life makes a very fmall addition of vital air or oxygen to 

 the common air, and occafions a very fmall conlumption of 

 carbonic acid, tlie elfcft may, it is fuppofed, be conceived 

 adequate to the wants of nature. 



It is fuppofed that it may occur as an objeftion to thcfe 

 views, that if the leaves of plants purify tlie atmofpjicre, 

 towards the end of autumn, and through the winter and 

 early tpring, the air in our climates mull become iaipure, 

 the vital air or oxygen in it diminilh, and the carbonic acid 

 gas increafe, which is not the cafe : but there is a very 

 latisfaftory anfwer, it is faid, to thia objeftion ; the dilTercnt 

 parts of the atmofphere are conlUuitly mixed together by 



winds, 



