On the Silica Strata of the Lower Chalk. 



239 



silica in the proper proportions (to be mentioned shortly), we get 

 the same thing happening — the lime successively dissolves and 

 unites with the silica, till the combination of the whole is accom- 

 plished. Now, the silica of the Farnham rocks is happily in 

 this same active condition — it can be made to unite with lime in 

 several ways, and with the greatest ease ; and when formed with 

 the proper precautions, the silicate of lime is soluble in water, 

 not to a great extent, but sufficiently so, as we believe, for all 

 the purposes of a manure. 



We shall describe the various ways which we have tried in 

 making silicate of lime, in order that any person who wishes to 

 make this salt should do so in the manner which may be most 

 open to him : — 



1st. Slaked lime is to be mixed with the silica rock in fine 

 powder, in such proportion that the free soluble silica shall be 

 to the weight of lime before slaking as 5 to 1 ; consequently, if 

 the silica rock used contains 50 per cent, of soluble silica, 10 

 parts of it will be necessary for every 1 part of quick lime, and 

 so on. The mixture, being made into a puddle with water (in 

 such quantity that it will be thoroughly moist without being so 

 thin as to separate), is to be left for a few weeks or a month, 

 when combination will be in great part or altogether effected. 

 We cannot state exactly what time will be necessary for this 

 purpose, but we have reason to think that, for all ordinary pur- 

 poses, a sufficient amount of action would be developed in one 

 or two months. As the change goes on, the materials dry up, 

 from uniting, as plaster of Paris does, with the water. It is 

 desirable to keep them moist to a given extent; but, for a manu- 

 facturing process, the object should be to hit that quantity of 

 water which, by the time combination was effected, should have 

 been nearly all absorbed or dried off, so that the mixture might 

 be ready for use or for transportation to a distance. 



2nd. The same ingredients as the last, with the addition of a 

 little common carbonate of soda (about 2 per cent^ of the weight 

 of the lime and silica rock). This carbonate of soda is to be 

 dissolved in the water ; it soon becomes caustic, and, by dis- 

 solving the silica, which it afterwards gives up to the lime, and 

 then commencing again, it speedily converts the whole of the 

 lime into silicate ; its office, therefore, is as a carrier between 

 the two, and it greatly expedites the process. 



3rd. The materials as in the last cases, but the lime ground 

 finely, but not slaked. The slaking of the lime in the mixture 

 gives much heat, which materially assists the process. Still 

 better will it be if the water can be at the boiling temperature, 

 when the heat, in a large mass of materials, would be retained 

 for some time, and days would do the work of weeks. 



