42 FAMILIAR LETTERS ON CHEMISTRY. 



pendent of a supply of carbonaceous manure, but it depends upon the presence of 

 certain elements of the soil which in themselves contain no carbon, together with 

 the existence of conditions under which their assimilation by plants can be effected. 

 We increase the produce of our cultivated fields, in carbon, by a supply of lime, 

 ashes, and marl, substances which cannot furnish carbon to the plants, and yet it 

 is indisputable being founded upon abundant experience that in these substances 

 we furnish to the fields elements which greatly increase the bulk of their produce, 

 and consequently the amount of carbon. 



If we admit these facts to be established, we can no longer doubt that a deficient 

 produce of carbon, or in other words, the barrenness of a field does not depend 

 upon carbonic acid, because we are able to increase the produce, to a certain degree, 

 by a supply of substances which do not contain any carbon. The same source, 

 when the meadow and the forest are furnished with carbon, is also open to our cul- 

 tivated plants. The great object of agriculture, therefore, is to discover the means 

 best adapted to enable these plants to assimilate the carbon of the atmosphere which 

 exists in it as carbonic acid. In furnishing plants, therefore, with mineral elements, 

 we give them the power to appropriate carbon from a source which is inexhaustible ; 

 while in the absence of these elements the most abundant supply of carbonic acid, 

 or of decaying vegetable matter, would not increase the produce of a field. 



With an adequate and equal supply of these essential mineral constituents in the 

 soil, the amount of carbonic acid absorbed by a plant from the atmosphere in a 

 given time is limited by the quantity which 4s brought into contact with its organs 

 of absorption. 



The withdrawal of carbonic acid from the atmosphere by the vegetable organism 

 takes place chiefly through its leaves ; this absorption requires the contact of the 

 carbonic acid with their surface, or with the part of the plant by which it is 

 absorbed. 



The quantity of carbonic acid absorbed in a given time is in direct proportion to 

 the surface of the leaves, and the amount of carbonic acid contained in the air ; 

 that is, two plants of the same kind, and the same extent of surface of absorp- 

 tion, in equal times and under equal conditions, absorb one and the same amount 

 of carbon. 



In an atmosphere containing a double proportion of carbonic acid, a plant 

 absorbs, under the same condition, twice the quantity of carbon. Boussingault 

 observed, that the leaves of the vine, enclosed in a vessel, withdrew all the carbonic 

 acid from a current of air which was passed through it, however great its velocity. 

 (Dumas Le9on, p. 23.) If, therefore, we supply double the quantity of carbonic acid 

 to one plant, the extent of the surface of which is only half that of another 

 living in ordinary atmospheric air, the former will obtain and appropriate as much 

 carbon as the latter. Hence results the effects of humus, and all decaying organic 

 substances, upon vegetation. If we suppose all the conditions for the absorption 

 of carbonic acid present, a young plant will increase in mass, in a limited time, 

 only in proportion to its absorbing surface ; but if we create in the soil a new source 

 of carbonic acid, by decaying vegetable substances, and the roots absorb in the same 

 time three times as much carbonic acid from the soil as the leaves derive from the 

 atmosphere, the plant will increase in weight fourfold. This fourfold increase 

 extends to the leaves, buds, stalks, &c., and in the increased extent of surface, the 

 plant acquires an increased power of absorbing nourishment from the air, which 

 continues in action far beyond the time when its derivation of carbonic acid through 

 the roots ceases. Humus, as a source of carbonic acid in cultivated lands, is not 

 only useful as a means of increasing the quantity of carbon an effect which in most 

 cases may be very indifferent for agricultural purposes but the mass of the plant 

 having increased rapidly in a short time, space is obtained for the assimilation of 

 the elements of the soil necessary for the formation of new leaves and branches. 



Water evaporates incessantly from the surface of the young plant ; its quantity 

 is in direct proportion to the temperature and the extent of the surface. The 

 numerous radical fibrilliae replace, like so many pumps, the evaporated water ; and 

 so long as the soil is moist, or penetrated with water, the indispensable elements 

 of the soil, dissolved in the water, are supplied to the plant. The water absorbed 

 by the plant evaporating in an aeriform state, leaves the saline and other mineral 

 constituents within it. The relative proportion of these elements taken up by a 

 plant, is greater the more extensive the surface and more abundant the supply of 

 water ; where these are limited, the plant soon reaches its full growth, while if their 

 supply is continued, a greater amount of elements, necessary to enable it to appro- 

 priate atmospheric nourishment being obtained, its development proceeds much 

 further. The quantity, or mass of seed produced, will correspond to the quantity 

 of mineral constituents present in the plant. That plant, therefore, containing the 



