240 



On the Silica Strata of the Lower Chalk. 



4tb. The materials, lime and silica rock, in the same propor- 

 tions, and with water, may be made to combine in one or two 

 hours at or near the boiling point. There is no need of ebulli- 

 tion. A mixture — thicker than gruel — might be heated by 

 steam-pipes in vats, and the process would be simple and in- 

 expensive. It is advisable to have as little water as can be made 

 to serve the purpose, because there is then less trouble in drying 

 the product. 



This is the process which we prefer, as giving the best results, 

 with a certainty of none of the silicate of lime becoming insoluble, 

 as may happen in the processes now to be described. 



5th. Silica rock and carbonate of lime (chalk) heated in the 

 dry state. The silica rock must here be in such proportion to 

 the chalk that there may be three times as much soluble silica 

 as chalk. Consequently, if the rock contains 50 per cent, of 

 silica, we must use six parts of it to one part of chalk. These, 

 being ground together and well mixed, are to be heated to a 

 gentle dull-red heat in a reverberatory furnace. Combination 

 readily takes place ; and the silicate of lime, which, when it 

 comes out, will crush down to a fine powder under the fingers, 

 is admirably adapted for drilling or broadcasting. The great 

 point here is temperature : it must be sufficient to change the 

 carbonate into silicate ; but, if it goes far beyond this, the sili- 

 cate becomes insoluble in water, and is then useless, or compara- 

 tively so, for the purposes we have in view. We believe that 

 there is plenty of margin between these points for a careful 

 manufacturer, and there can be no doubt that he would prefer 

 this under most circumstances to the methods before described, 

 but, if adopted, the greatest caution must be used in its 

 employment. 



A 6th process consists in employing quick lime instead of 

 carbonate (chalk) in the operation. The heat required is still 

 lower, and there is therefore less fear of overheating. The ex- 

 pense will be rather more, of course, but would hardly mate- 

 rially increase the cost of the product. 



Having described rather minutely the methods of making sili- 

 cate of lime from this silica rock, we pass on to mention its 

 properties and probable uses in agriculture. Silicate of lime is 

 slightly soluble in water. The result of several experiments 

 which we have made is, that the solubility reaches about 20 

 grains in the imperial gallon. Of this quantity, three-fourths, 

 or 15 grains, is silica. Compared with other salts, even with 

 those which we are in the habit of considering as insoluble salts, 

 this is certainly a very slight degree of solubility ; but there is 

 <every reason to believe that it is sufficient for the requirements 

 of vegetation ; and it is consistent with what we now know of 



