330 rOTASU AND SODA RENDER SILICA SOLUBLE, ETC. 



ash of plants (page 216 et seq.) how very important a substance silica is, 

 especially to the grasses and the stems of our various corn-bearing plants. 

 Tliis silica exists very frequently in the soil in a state in which it is insol- 

 uble in pure water, and yet is more or less readily taken up by water 

 containing carbonate of potash or carbonate of soda; and as there is eve- 

 ry reason to believe that nearly all the silica they contain is actually con- 

 veyed into the circulation of plants by the agency of potash and soda, (in 

 the state of silicates — see pp. 83 and 207,) it is not unlikely that a portion 

 of the beneficial action of these substances, especially upon the grass and 

 corn crops, may be due to the quantity of silica they are the means of 

 conveying into the interior of the growing plants. 



d. Another mode in which these substances act, more obscurely, per- 

 haps, though not less certainly, is by disposing the organic matters con- 

 tained in the sap of the plant to form such new combinations as may be 

 re(]ulred for the production of the several parts of the living vegetable. 1 

 have on a former occasion illustrated ( pp. 112-114,) to you the very re- 

 markable changes which starch may be made to undergo, without any 

 essential alteration in its chemical composition — how gum and sugar 

 may be successively produced from it, without either ^oss or gain in respect 

 of its original elementary constitution. We have seen also how the 

 presence of a comparatively minute quantity of diastase (p. 118) or of 

 sulphuric acid (p. 113) is capable of inducing such changes, first rendering 

 the starch soluble, and then converting it into gum and into sugar. Ana- 

 logous, though somewhat different changes, are induced by the presence 

 in certain solutions of small quantities of potash* or soda, as, for example, 

 in milk — the addition of carbonate of soda to which gradually causes (per- 

 suades?) the whole of the sugar it contains to be converted into the acid of 

 milk. Such changes also must be produced or facilitated by the presence 

 of acid and of alkaline substances in the sap of plants ; and though we 

 can as yet only guess at the precise nature of these changes, yet there 

 seems good ground for believing that to facilitate their production is one of 

 the many purposes served by the constant presence of inorganic substances 

 in the sap of plants, indeed so important is this function considered by 

 some writers upon the nourishment of plants, (see especially Hlubeck's 

 Erndkrung der Pjianzen und Statik desLandbaues,) that they are inclined 

 to ascribe to it, erroneously however, as I believe, the main influence upon 

 vegetation, of nearly all the inorganic substances which are found in the 

 ash of plants, and therefore are known to enter into their circulation. 



€. I only allude to one other way in which these substances may be sup- 

 posed to have an influence upon vegetation. We have already seen (Lee. 

 Vin, § 5, 6, 7, pp.159 to 167,) how important a part the nitric acid produ- 

 ced in the atmosphere or in the soil may be supposed to perform in the gen- 

 eral vegetation of the globe. This acid is observed to be more abundantly 

 — either fixed or actually produced in the soils or composts which contain 

 much potash or soda. It may be, therefore, that in adding either of these 

 to our fields, we give to the soil the means of bringing within the reach 

 of the roots of our crops a more ready supply of nitric acid, and l^ence 

 of nitrogen, so necessary a part of their daily food. 



3°. Sulphates of Potash and Soda. — It is nearly 100 years since Dr. 



• It is also shown ^. 112,) that, by means of potas^., woody fibre may be converted 

 into starch. 



