FAMILIAR LETTERS OX CHEMISTRY 



might they not entertain hope of their growth? Such ideas could only be enter- 

 tained when nothing was known of the atmosphere, and its participation with tho 

 earth, in administering to the vital processes of plants and animals. Modern 

 chemistry, indeed, produces the elements of water, and combining them, forms 

 water anew ; but it does not create those elements it derives them from water ; the 

 new-formed artificial water has been water before. 



Many of our farmers are like the alchemists of old they are searching for the 

 miraculous seed the means, which, without any further supply of nourishment to 

 a soil scarcely rich enough to be sprinkled with indigenous plants, shall produce 

 crops of grain a hundred fold. 



The experience of centuries, nay, of thousands of years, is insufficient to guard 

 men against these fallacies; our only security from these and similar absurdities 

 must be derived from a correct knowledge of scientific principles. 



In the first period of natural philosophy organic life was supposed to be derived 

 from water only ; afterward, it was admitted that certain elements derived from the 

 air must be superadded to the water ; but we now know that other elements must 

 be supplied by the earth, if plants are to thrive and multiply. 



The amount of materials contained in the atmosphere, suited to the nourishment 

 of plants, is limited ; but it must be abundantly sufficient to cover the whole surface 

 of the earth with a rich vegetation. Under the tropics, and in those parts of our 

 globe where the most genial conditions of fertility exist a suitable soil, a moist 

 atmosphere, and a high temperature vegetation is scarcely limited by space ; and, 

 where the soil is wanting, it is gradually supplied by the decaying leaves, bark, and 

 branches of plants. It is obvious there is no deficiency of atmospheric nourishment 

 for plants in those regions, nor are these wanting in our own cultivated fields : all 

 which plants require for their development is conveyed to them by the incessant 

 motions of the atrnoshere. The air between the tropics contains no more than that 

 of the arctic zones ; and yet how different is the amount of produce of an equal 

 surface of land in the two situations ! 



This is easily explicable. All the plants of tropical climates, the oil and wax 

 palms, the sugar cane, &c., contain only a small quantity of the elements of the 

 blood necessary to the nutrition of animals, as compared with our cultivated plants. 



The tubers of the potato in Chili, its native country, where the plant resembles a 

 shrub, if collected from an acre of land, would scarcely suffice to maintain an Irish 

 family for a single day (Darwin.) The result of cultivation in those plants which 

 serve as food is to produce in them those constituents of the blood. In the absence 

 of the elements essential to these in the soil, starch, sugar, and woody fibre, are, 

 perhaps, formed ; but no vegetable fibrine, albumen, or caseine. If we intend to 

 produce on a given surface of soil more of these latter matters than the plants can 

 obtain from the atmosphere, or receive from the soil of the same surface in its 

 uncultivated and normal state, we must create an artificial atmosphere, and add the 

 needed elements to the soil. 



The nourishment which must be supplied in a given time to different plants, in 

 order to admit a free and unimpeded growth, is very unequal. 



On pure sand, on calcareous soil, on naked rocks, only a few genera of plants 

 prosper, and these are, for the most part, perennial plants. They require, for their 

 slow growth, only such minute quantities of mineral substances as the soil can 

 furnish, which may be totally barren for other species. Annual, and especially 

 summer plants, grow and attain their perfection in a comparative short time ; they 

 therefore do not prosper on a soil which is poor in those mineral substances neces- 

 sary to their development. To attain a maximum in height in the short period of 

 their existence the nourishment contained in the atmosphere is not sufficient. If 

 the end of cultivation is to be obtained we must create in the soil an artificial 

 atmosphere of carbonic acid and ammonia ; and this surplus of nourishment, which 

 the leaves cannot appropriate from the air, must be taken up by the corresponding 

 organs, that is, the roots, from the soil. But the ammonia, together with the car- 

 bonic acid, are alone insufficient to become part of a plant destined to the nourish- 

 ment of animals. In the absence of the alkalies, the phosphates and other earthy 

 salts, no vegetable fibrine, no vegetable easeine, can be formed. The phosphoric acid 

 of the phosphate of lime, indispensable to the ceralia and other vegetables in the 

 formation of their seeds, is separated as an excrement, in great quantities, by the 

 rind and bark of ligneous plants. 



How different are the evergreen plants, the oleaginous plants, the mosses, the 

 ferns, and the pines, from our annual grasses, the ceralia and leguminous vegetables ! 

 The former, at every time of the day during winter and summer, obtain carbon 

 through their leaves by absorbing carbonic acid which is not furnished by the 

 barren soil on which they grow ; water is also absorbed and retained by their 



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