No. VIII.] OF THE SOLUBLE SILICATES OF POTASH AND SODA. 



41 



The first series of comparative experiments-, therefore, would be as follo^rs: 



The ap2)lication may be frora 1 cwt. to IJ 

 cwt. per acre, laid on as a top-dressing in 

 moist weather early in the spring. Or it 

 may be mixed with a large quantity of wa- 

 ter, and applied with a water-cart. In either 



Silicate of 

 Soda. 



'^Silicate of, 

 P(jlash, ! 



% Silicate of I 

 Soda. 



ci- . r ^^ ^''f *'f °^ case it ought to be in the state of a fine powder, 

 bilicate of Potash, i> . i r i .1 i i- .• ^3 1 



Potash. }i Silicate of t>ut altliough the ibove applications produce a bene- 

 Soda. ficial effect upon the crops, it will not necessarily follow 



that the silica, which the silicates contain, has had any 



share in bringing about the good result. By mere expo- 

 sure to the air for a length of time tiie potash or soda of those silicates will absorb 

 carbonic acid from the atmosphere, and be converted into carbonates. The 

 same will take place more rapidly still in the soil, where carbonic acid abounds 

 This conversion of the alkali into carbonate will set free a large part of the 

 silica — in a state it is true in which it is in some degree soluble in water (page 

 206,) — but in whicn, nevertheless, it will find its way into the plant with 

 much more difficulty than if it had remained in the state of a soluble silicate. 



Now as the carbonates of potash and soda are known to promote vegetation 

 (page '>i28), — though even with these, sufficient trials have not yet been made 

 — it is possible, as 1 have remarked above, that a good eifect may follow the 

 application of the sihcates, and yet it maybe altogether due to the action of the 

 carbonates which are formed by their decomposition. It is of consequence to 

 ascertain if this really be the case, because the quantity of carbonates which 

 would be formed by the decomposition of the silicates could be laid on directly 

 at one lialf of the price at which the silicates can as yet be sold. 



The second series of comparative experiments, therefore, which it would be 

 interesting to try, would be such as the following ; — 



The quantities here indicated are bj^ the acre — thai 

 of carbonate of soda is given so great, because this salt 

 contains upwards of three-fifths its weight of water (see 

 p. 215.) 



Another consideration ought not here to be omitted. 

 Nature, as has been frequently illustrated in the text, 

 feeds her plants with a mixture of many different sub- 

 stances, and by the aid of such mixtures they always 

 thrive the best. I'he full benefit of the silicates, when 

 applied alone, will be experienced only when every oth- 

 er ingredient which the plant requires is already present in the soil, and in suf- 

 ficient abundance. But this can rarely be the case. Its success will be more 

 sure, therefore, if it be applied in a state of mixture with other saline substances 

 which are known to be more or less useful to vegetation, and which will not, 

 upon admixture, decompose these silicates. Such are common salt and the 

 sulphate and nitrate of soda. 



A third series of comparative experiments, therefore, might be made, in which 

 from 1 to 1 1 cwt. per acre of the following mixtures might be applied : — l'^. 

 Equal weights of common salt, of dri/ sulphate of soda, of nitrate of soda, and 

 of silicate o^ potash; 2°. Equal weights of the same substances, omitting the 

 silicate of potash ; 3°. Equal weights of common salt, oi dry sulphate of soda, 

 of nitrate of potash, and of silicate of soda; and 4'-\ Equal weights of the same 

 substances, omitting the silicate of soda, or substituting carbonate of soda in 

 its stead. 



The sulphate of magnesia (Epsom salts) or of lime (gypsum) can not be 

 safely used along with the silicates, as the magnesia or lime they contain may 

 decompose the silicates — forming sulphate of potash or soda and silicate of 

 magnesia or lime, in which the silica is insoluble, and could not, therefore, until 

 a further chemical change took place, find its way into the roots of the plant. 



Silicate of 

 Potash, 

 I cwt. 



Silicate of 

 Soda, 

 1 cwt. 



Crude 

 Potash or 

 Pearlash, 



75 lbs. 



Crystallized 



Carbonate 



of Soda, 



150 lbs. 



