130 



Power of Soils to absorb Manure. 



in the product depend entirely upon those of the solutions 

 used.* 



The double silicate, which contains the smallest per centage of 

 silica and therefore the highest per centage of soda, would be 

 most important in an agricultural sense. It is best made in a 

 state of purity as follows : — From a solution of common alum 

 the alumina is precipitated by carbonate of soda ; and after being 

 washed with pure water, it is dissolved in caustic soda; a solution 

 of silicate of soda containing not more than one equivalent 

 of silica to one of the alkali, but in which any convenient excess 

 of soda may be present, is then added to the alkaline aluminous 

 liquid; the resulting precipitate is the double silicate required. 

 It may be washed with pure water till all the caustic soda is 

 removed, and dried at the temperature of boiling water. As thus 

 prepared it is a fine white powder containing water of combina- 

 tion ; but in the following composition I have excluded the water, 

 which is about 12 per cent., and calculated the proportions on the 

 anhydrous salt. 



It contains in 100 parts — 



Silica 52.41 



Alumina . . . . 29.68 

 iSoda 17.91 



100.00 



It is only very slightly soluble in pure water — an imperial 

 gallon having been found to dissolve 3.36 grains of soda. In- 

 deed with this, as with the silicates which are yet to be described, 

 it is hardly proper to speak of the compounds as being soluble, 

 since the salt does not dissolve as a whole, but is decomposed — 

 silicate of soda being separated whilst the silicate of alumina 

 remains undissolved. 



From this soda silicate the other compounds of the same class 

 may be readily prepared. 



Double Silicate of Alumina and Lime. — When the double 

 silicate of alumina and soda is digested in excess of lime-water, 

 or of any neutral salt of lime, an absorption of the lime takes 

 place, soda being at the same time dissolved, and the result is, 

 the production of the lime double silicate. It is found difficult 

 in practice to separate the whole soda, but in several cases the 

 substitution of lime for the former alkali has been almost com- 



^- There appear to be at least three definite silicates of soda in wliicli the silica 

 is to the soda in the proportion of 1 , 2, and 3 equivalents. I have succeeded several 

 times in forming a solution of silicate of soda or of potash with the highest pro- 

 poi-tion of silica, but of course the smaller quantity is much more easily dissolved. 

 If in making the double silicate alum be used, three equivalents of silica must 

 enter into the compound for each equivalent of alumina ; but vrhen made as de- 

 scribed in the text, the lowest possible proportion of silica is the result — that is to 

 say, one equivalent for each equivalent of base. 



