ORGANIC REMAINS. 51 



and thirdly, of the water and organic matter which 

 they contain. The latter constitutes the principal 

 nourishment of plants derived from the soil, while 

 the salts are necessary to prepare that nourishment 

 to be taken up and assimilated by their delicate 

 vessels. The earth serves as a basis of support for 

 the plant, as a receptacle for the nourishment, and 

 probably also, in connection with the roots, as a gal- 

 vanic combination for the development of those 

 electrical agencies by which the food of plants is 

 taken up and converted into vegetable matter. 

 Soils, to be fertile, should contain silica, allumina, 

 and lime, and should be in a good state of sub- 

 division, as this has much influence on the reten- 

 tion of moisture. The proportion of earthy ma- 

 terials is not of so much importance, if they are 

 only present, because the amount consumed is so 

 small compared with the humic acid, or organic 

 remains. Dr. Dana aptly says that the earths are 

 the plates, the salts the seasoning, and the geine (or 

 humic acid) the food of plants." Cultivation does 

 not materially alter the natural composition of the 

 earths, but it modifies its salts and humic acids. 

 Plants cannot flourish without this latter element. 

 Alluvial and sandy diluvial contain the least of it, 

 and should therefore be supplied with manure ; the 

 crops then gathered may be even larger than those 

 from a soil in which it naturally exists, because 

 such lands, being porous, the atmosphere is able to 



