when in an impalpable state of division, are soluble 

 in water ; but the extent of it is infinitesimal, so 

 that often with the most delicate reagents their pre- 

 sence can barely be detected. The fact, however, of 

 their solubility being established, and knowing that 

 two thousand grains of water have to pass through 

 the cells of plants to every grain of mineral matter 

 contained therein, we cannot, then, be surprised to 

 find in a plant one or two grains of a substance 

 which is considered, generally speaking, insoluble. 

 Silica, as mentioned before, is dissolved by 

 potash, and thus brought into the system of plants ; 

 but the Silica here referred to must be distinguished 

 from sand, which is Silica also, but which, as such, 

 is incapable of being used as food by plants ; for 

 sand cannot be dissolved and carried into the 

 vegetable system, by any common means at the 

 command of nature. (But sand has functions to 

 fulfil in the soil, as will be shown when the subject 

 of soils will be treated ; but they are merely 

 mechanical.) So, when speaking of Silica in con- 

 nection with the nourishment of plants and their 

 chemical constituents, I mean the Silica existing in 

 an impalpable state in combination with other bases, 

 as alumina, potash, iron, &c., which combinations 

 are called clays ; and any analysis of soils for agri- 

 cultural purposes should always show the amount 

 of this Silica as distinct from the Silica present as 

 sand in the uncombined state. 



