On the Silica Strata of the Lower Chalk. 



241 



vegetable nutrition to believe that the least soluble salts (so 

 that they be soluble in some measure) are the most adapted for 

 the supplying the mineral matters of plants. The quantity of 

 water passing through a plant, and expaled by its leaves, during 

 the period of its growth, is very great. From some experiments 

 made by Mr. Lawes on this subject, we may infer that a wheat 

 crop exhales in its growth as much as 230 times its weight of 

 water ; and it is plain that this water must be charged with the 

 soluble matters of the soil. 



Silicate of lime, like many other slightly soluble salts, is more 

 soluble in solutions of other salts than in pure water. For instance, 

 in a weak solution of common salt it dissolves to twice the extent 

 that it does in plain water: it is also much more soluble in 

 water containing carbonic acid. But, calculating the quantity 

 dissolved by water only, we should find that, during the growth 

 of a crop of wheat or barley, enough silica might be introduced 

 in the form of silicate of lime for the necessities of the plant. 



We believe, therefore, that such silicate of lime will be found 

 to be sufficiently soluble for agricultural use. At the same time 

 its value as a manure can only be ascertained by practical ex- 

 periment. 



It is notorious that the growth of wheat is chiefly uncertain on 

 light sandy or peaty soils ; the difficulty is not so much the produc- 

 tion of the plant as that of bringing it to maturity, and prevent- 

 ing it from being lodged or laid before harvest. This condition of 

 the plant is usually attributed to a deficiency of silica in the 

 straw ; and although there is no very good evidence to prove that 

 the quantity of silica is actually less in such straw than in that 

 which is strong and bright, it seems very reasonable to suppose 

 that a want of silica, or indeed of any other of the necessary in- 

 gredients of the plant, would lead to an unhealthy and immature 

 state, and indirectly we should therefore be right in referring the 

 evil to such a cause. 



We have observed that silicate of lime has the peculiar pro- 

 perty of decomposing ammoniacal salts. When mixed, for instance, 

 with sulphate of ammonia it causes an instant disengagement of 

 ammonia, in the same way, though in a much less degree, than 

 caustic lime itself would. The change that occurs here is the 

 iormation of sulphate of lime and silicate of ammonia ; but the 

 latter, being a very weak salt, breaks up quickly into free am- 

 monia, which escapes into the air, and silica. If the solution of 

 silicate of ammonia is very weak, however, this decomposition 

 of the silicate of ammonia does not appear to occur. 



It is very probable that silicate of lime may in this way serve 

 an important purpose in light soils. If, as ]\ir. Way has urged 

 in former papers, crude ammoniacal salts are positively injurious 



