276 SILICA LIME. [BOOK n. 



Such facts show that while Lime, Potash, and Soda are an 

 important constituent of Cabbages : they are of little moment 

 in Flax ; that Silica is a great ingredient in Corn, but of 

 small amount in Buckwheat, and so on. But the constant 

 presence in every case of nearly all the inorganic matters 

 mentioned in the foregoing table leads to the conclusion that 

 they are indispensable adjuncts to Water, carbonic acid, and 

 nitrogen in sustaining vegetable life. 



Silica. This substance is extremely abundant in some 

 plants, and comparatively rare in others. They no doubt 

 obtain it from the soluble silicates, especially that of potash, 

 which are found everywhere in soil. In the Bamboo it forms 

 the hard varnish that guards the whole surface, and is occa- 

 sionally excreted in its hollow stems in the form of the opaline 

 substance called Tabasheer (Silex 70, Potash and Lime 30 

 Vauqueliri). It is the varnish and skeleton of grasses and many 

 other plants, occupying sometimes a definite and invariable 

 position in their structure, as in Equisetum. According to 

 the Rev. J. B. Reade (Taylor's Magazine, vol. Ixvii. p. 414), 

 some vessels are actually composed of silica, no trace of vessel 

 remaining if that substance is removed. Goppert has also 

 shown that mineral matter artificially introduced into plants 

 will take entire possession of them, destroying or displacing 

 the vegetable matter, without altering their tissue or their 

 structure. (Comptes rendus, iii. 656). 



Lime. This matter may be regarded as always present 

 wherever water that has passed through soil has access. It 

 must therefore be universally in contact with the roots of 

 plants, to some of which it is doubtless constitutionally indis- 

 pensable ; and hence it is that it so constantly occurs in the 

 ashes of plants. Very copious details regarding its action 

 and importance will be found in Johnson's Lectures on Agri- 

 cultural Chemistry. 



Magnesia, although so often found in the ashes of plants, 

 has been sometimes looked upon as injurious to vegetation ; 

 but this is certainly not true, unless it is present in a caustic 

 state, and in great excess. Professor Giobert states that 

 native magnesia is found abundantly in the environs of 

 Castellamonte and of Baldissero, the soils of which exhibit 



