GASES. 



Providence, therefore, has ordained that this 

 should be ever ready to meet the demands of 

 vegetable life, and that its quantity should vary 

 with the temperature, increase with the warmth 

 when its pn-vsure is most needed by the plant, 

 and diminish in proportion as the air becomes 

 cooler. Thus, at a temperature of 50, suppos- 

 ing it to have a free communication with water, 

 the atmosphere contains about l-75th of 'ts 

 weight of vapour; but when its temperature is 

 increased to 100, then its proportion of water 

 leased to l-21st of its weight: and this 

 nl arrangement is the more important, 

 as Davy well observed, in the economy of na- 

 ture, because, in very intense heats, and when 

 the soil is dry, the life of plants is mainly, 

 if not entirely, preserved by this absorbent 

 power of their leaves and the earth in which 

 they grow; and, happily, this watery vapour is 

 most abundant in the atmosphere when it is 

 most needed for the purposes of life : when 

 other sources of its supply are cut off, this is 

 most copious. The amount, however, of the 

 atmospheric vapour varies with the kind of 

 wind. Those which have passed over warm 

 teas contain more than those which have tra- 

 versed extensive dry countries ; that which 

 crosses the hot, dry sands of Asia and northern 

 Africa is so dry that it scorches, as it were, all 

 the adjoining countries. It is the cause of the 

 sirocco of Malta being so noxious, and why the 

 1 -u farmer finds that an easterly wind, in 

 1 :id the driest of all winds, is the least 

 propitious to vegetation. He well knows, on 

 the other hand, that the westerly or south-west- 

 ern breezes, the' most watery of all winds in 

 Uiitain, which come to his fields surcharged 

 with all the vapours of the Atlantic, are pre- 

 cisely those which bring with them luxuriance 

 to his crops, and clothe his woods with ver- 

 dure. 



The cultivator will derive many advantages 

 from a careft. investigation of the support 

 yielded by the vapour of the atmosphere to his 

 plants. He will perceive that its unvaried pre- 

 sence affords an additional reason why the air 

 should be allowed to circulate freely through 

 the well-pulverized and loosened soil, to the 

 roots of all growing crops; and let him, above 

 all, avoid the very common erroneous conclu- 

 sion, that the atmosphere is ever dry, that it 

 no longer contains watery vapour ; for the real 

 fact is, he will find the very opposite to the 

 common vulgar conclusion. The chemist's 

 laborious investigations have clearly demon- 

 strated, that though the watery vapour varies 

 in amount, yet it is never absent from the at- 

 mosphere, but that it happily always the more 

 abounds where the cultivator's crops need its 

 ; it is then the most able to 

 furnish the roots of his grain crops with all the 

 re they require; and if it is unable to 

 penetrate to them, the fault is not in the wise 

 economy of nature, but in the carelessness of 

 the cultivator, who is either too inattentive to 

 see the advantages which he might thus freely 

 derive, or too indolent to loosen the case-hard- 

 ened soil, which prevents the entrance of the 

 requisite supply of moisture. One of the causes 

 01 the unproductiveness of cold, clayey, adhe- 

 sive soils, as Davy well remarked, is, that the 

 526 



GASES. 



seed is coated with matter impermeable to air. 

 The farmer can convince himself of these facts 

 by the simplest of all experiments. Let him 

 merely use his rake or his hoe on a portion of 

 a bed of wheat, of turnips, or lettuces, or any 

 other kind of crop, and let him, in the driest 

 weather, merely keep this portion of soil loose 

 by this gentle stirring, and he will find that, 

 instead of prejudicing his crop by letting out tlvt 

 nuns' a re, as is often ignorantly supposed, some- 

 thing is evidently let into the soil ; for the por- 

 tion thus tilled will be soon visibly increased 

 in luxuriance by the mere manual labour thus 

 bestowed ; and in this experiment, which I 

 have often tried, I am supposing that both the 

 portions of the ground are equally free from 

 weeds ; that in every other respect the treat- 

 ment of both the tilled and undisturbed portions 

 of the experimental plot is exactly the same. 

 To a very great extent, some of the best of the 

 English farmers have long found out these 

 facts, and have acted upon the discovery. The 

 horse-hoe of the east and south of England, in 

 the driest days of summer, may be seen at work 

 in the large sandy turnip-fields of Norfolk and 

 Suffolk, with unvaried regularity, not for the 

 mere destruction of weeds, for these are not 

 the abounding tenants of such skilful farmers' 

 lands, but for the chief and highly beneficial 

 purpose of increasing the circulation of the 

 gases and vapour of the air. " The longer I 

 keep stirring the soil between my turnip drills," 

 said Lord Leicester to me, some years since, 

 " in dry weather, the better the turnips grow." 



The same uniform presence of aqueous va- 

 pour which marks the atmosphere in all times 

 and seasons, in a still more remarkable degree 

 distinguishes its constituent gases, for these 

 never vary in amount in any times, or seasons, 

 or countries. The atmospheric air has been 

 analyzed, when obtained from the lowest val- 

 leys, and the tops of the highest mountains, in. 

 crowded cities, and in the open country, but its 

 composition was always found to be the same, 

 viz., nearly 21 per cent, of oxygen, and 79 of 

 nitrogen, and from 1 part in 500 to 1 part in. 

 800 of carbonic acid gas. 



Such, then, are the principal matters con- 

 tained in the atmosphere, or added to it by pu- 

 trefaction, which influence the progress of 

 vegetation. That there are other matters oc- 

 casionally present in the air, which are in all 

 probability grateful to vegetation, is very cer- 

 tain; our very senses tell us that there are 

 clouds of smoke, which is a mixture of carbu- 

 retted and sulphuretted hydrogen, soot, and 

 vapour, hourly hovering over all large towns 

 and cities, and which huge mass the winds 

 disperse over the country. Of these the soot, 

 and finely divided earthy matters with which 

 it is combined, are very speedily deposited ; it 

 is one reason why the lands near to populous 

 places are very commonly rich and fertile. 

 Ammonia has been detected in rain-water. 

 That other substances also exist in the air in 

 minute, yet active proportions, is very certain, 

 though they are too subtle to allow the chemist 

 to detect them: thus, to such finely divided 

 matters the physician attributes the progress 

 of contagion the chemical philosopher the 

 aroma of flowers, and of many other sub- 



