CAUSES OF ITS BENEFICIAL INFLUENCE. 171 



the formation of the wood takes its origin, acting 

 like a grain of sand around which the first crystals 

 form in a solution of a salt which is in the act of 

 crystallizing. Silicic acid appears to perform the 

 function of woody fibre in the Equisetacece and bam- 

 boos,* just as the crystalline salt, oxalate of lime, 

 does in many of the lichens. 



When we grow in the same soil for several years 

 in succession different plants, the first of which 

 leaves behind that which the second, and the second 

 that which the third may require, the soil will be a 

 fruitful one for all the three kinds of produce. If 

 the first plant, for example, be wheat, which con- 

 sumes the greatest part of the silicate of potash in a 

 soil, whilst the plants which succeed it are of such 

 a kind as require only small quantities of potash, as 

 is the case with Leguminosce, turnips, potatoes, &c., 

 the wheat may be again sowed with advantage after 

 the fourth year; for during the interval of three 

 years the soil will, by the action of the atmosphere, 

 be rendered capable of again yielding silicate of pot- 

 ash in sufficient quantity for the young plants. 



The same precautions must be observed with re- 

 gard to the other inorganic constituents, when it is 

 desired to grow different plants in succession on the 

 same soil: for a successive growth of plants which 

 extract the same components parts, must gradually 

 render it incapable of producing them. Each of 

 these plants during its growth returns to the soil a 

 certain quantity of substances containing carbon, 

 which are gradually converted into humus, and are 

 for the most part equivalent to as much carbon as 

 the plants had formerly extracted from the soil in a 

 state of carbonic acid. But although this is sufficient 

 to bring many plants to maturity, it is not enough 

 to furnish their different organs with the greatest 

 possible supply of nourishment. Now the object of 



* Silica is found in the joints of bamboos, in the form of small round 

 globules, which have received the name of Tabasheer, and are dis- 

 tinguished by their remarkable optical properties. — Ed. 



