FOOD OF PLANTS. 15 



But the water which nature furnishes to the vege- 

 table organs is never perfectly pure ; for, besides 

 containing air, in which there is constantly a cer- 

 tain proportion of carbonic acid gas, it has always 

 acquired by percolation through the soil, various 

 earthy and saline particles, together with materials 

 derived from decayed vegetable or animal remains. 

 Most of these substances are soluble, in however 

 minute a quantity, in water : and others, finely 

 pulverized, may be suspended in that fluid, and 

 carried along with it into the vegetable system. It 

 does not appear, however, that pure carbon is ever 

 admitted ; for Sir H. Davy, on mixing charcoal, 

 ground to an impalpable powder, with the water 

 into which the roots of mint were immersed, could 

 not discover that the smallest quantity of that sub- 

 stance had been, in any case, absorbed.* But in 

 the form of carbonic acid, this element is received 

 in great abundance, through the medium of water, 

 which readily absorbs it ; and a considerable quan- 

 tity of carbon is also introduced into the fluids of the 

 plant, derived from the decomposed animal and 

 vegetable materials which the water generally con- 

 tains. The peculiar fertility of each kind of soil 

 depends principally on the quantity of these or- 

 ganic products it contains in a state capable of being 

 absorbed by the plant, and of contributing to its 

 nourishment. 



The soil is also the source whence plants derive 

 their saline, earthy, and metallic ingredients. The 

 silica they often contain is, in like manner, conveyed 

 to them by the water, which it is now well ascer- 



* Elements of Agricultural Cliemistry, Lect. VI. p. 234. 



