128 



Power of Soils to ahsorh Manure. 



of lime, and other alkaline silicates, seemed to point to the salts 

 of this acid as most probably the true cause of the absorptive 

 property ; but so little was, and is even now, known of the 

 silicates, except as they are met with in the different igneous rocks, 

 that it became necessary to institute a distinct inquiry into the 

 nature of these compounds ; and the result of that inquiry has 

 been to extend very largely our acquaintance with them, and to 

 show the existence of some salts of the class hitherto unknown. 



It is not my intention, in this place, to enter into any detailed 

 account of these experiments, which are necessarily of an abstract 

 character ; and I shall content myself with reporting so much of 

 the results as may serve to show the agricultural bearings of this 

 inquiry. When a solution of silicate of soda or potash is added 

 to a neutral solution of a salt of lime, or to lime water itself, a 

 gelatinous precipitate is obtained, which is silicate of lime ; this 

 compound may be washed in distilled water, in which it is very 

 slightly soluble. Its composition varies according to the relative 

 proportions of soda and silica in the liquid from which it is 

 formed, but it is possible to obtain it of definite composition. 

 The silicate of lime thus formed was digested in solutions of 

 muriate of ammonia, but without success ; it did not absorb am- 

 monia, and is not therefore the substance to which the absorptive 

 property of soils is due. The silicate of lime is the type of 

 simple silicates of the same class, which would be quite unlikely 

 to act otherwise than it did with salts of ammonia. The class of 

 simple silicates was therefore abandoned, and attention was turned 

 to the possibility that the absorptive property might be due to 

 some of the compound silicates present in clay, and derived from 

 the granitic rocks to which clay owes its origin. Fragments of 

 such rocks are found still to be present in clay, and the most com- 

 monly known are felspar, the double silicate of alumina and pot- 

 ash, and albite, which is a soda felspar, or double silicate of 

 alumina and soda. There is also a similar double silicate of 

 alumina and lime. These different natural silicates, finely pow- 

 dered, were digested in a solution of sal ammoniac, but none of 

 them possessed the power of combining with its ammonia. It is 

 not, therefore, to the undecomposed remains of the granitic rocks 

 that the absorptive power of clay is to be referred. It was still 

 possible, however, that these double silicates, when formed arti- 

 ficially by precipitation, might be capable of effecting that which 

 in the mineral state they were unable to accomplish, because it 

 is a well known fact in chemical science, that substances recently 

 formed, and in the highly divided state resulting from precipita- 

 tion, may be much more active to produce or undergo chemical 

 change, than after they have, as in the case of the granitic rocks^ 

 been subject to the agency of heat. Accordingly, the next attempt 



