Power of Soils to ahsorh Manure, 



127 



1000 grains of the same white clay, digested with different 

 solutions of nitrate of potash, absorbed — 



In one experiment . . . 4.366 grains of potash. 

 In a second experiment . . 4.980 



In these results it is plain that there is a decided negative to 

 the supposition, that the whole clay is active in absorbing the 

 ammonia or potash. We know that chemical combinations always 

 take place in certain definite proportions between the substances 

 combining. Supposing, then, that the clay, as a whole, acting 

 as a definite chemical compound, united with ammonia, we should 

 expect it to absorb at least 2 or 3 per 1 00 ; whereas it requires 

 1000 grains of clay to remove this quantity. 



I was, indeed, convinced, at a very early period of this inquiry, 

 that the absorptive property was due to a small quantity of some 

 definite chemical compound existing in the clay, and possibly 

 not constituting more than 4 or 5 per cent, of its whole weight. 

 I had every hope that, although I might not be able to separate 

 this substance from clay — for of that there was little prospect — 

 it might yet be possible to form it artificially from other sources 

 at the disposal of the chemist, and by producing a compound, or 

 compounds, having the same properties as those shown to be 

 possessed by clay, to prove their identity with the active prin- 

 ciples of clay itself, and thus indirectly establish its real nature. 

 I am satisfied that this point is gained, and I now proceed to 

 describe the nature of the result, and to give a very short history 

 of the steps by which that result was obtained. 



It will be remembered that, in the experiments described in 

 my first paper, a salt of lime was invariably found in the resulting 

 solution, and since many of the soils that were employed did not 

 yield to pure water any considerable quantity of lime, and there- 

 fore did not contain any soluble salt of this base ; as, further, they 

 did not give, when treated with acids, any indication of the 

 presence of carbonate of lime, it followed that the lime compound 

 in the soil could not be one of the ordinary salts of lime — not, for 

 instance, the sulphate, nitrate, or muriate, all of which are soluble 

 in water — nor, as has been said, the carbonate. 



That this active substance in the soil was really a salt of lime, 

 and not the free or caustic earth itself, was also evident, from the 

 facts, first, of want of solubility, as in the other cases; and, 

 secondly, that the retention of the ammonia and potash by the soil 

 could only be in the form of some insoluble salt of those alkalies, 

 and could not have occurred without the existence of some 

 similar salt of lime with which to interchange. What, then, was 

 the nature of that salt ? The large quantity of silica present in 

 soils, some of which was known to exist in the form of silicates 



