ARABLE SOIL COMPARED TO ANIMAL CHARCOAL. 69 



It has been stated, tliat from a solution of carbonate of 

 potash or ammonia, or from a sohition of phosphate of 

 hme in carbonic acid water, the arable soil will withdi^aw 

 the potash, ammonia, and phosphoric acid, without any- 

 chemical interchange with the constituents of the earth 

 taldng place. In this respect the action of arable soil is 

 absolutely hke that of charcoal. But it goes farther, for it 

 is sufficiently powerful to sever the connection between 

 the potash or ammonia and the mineral acid, for which 

 they have the greatest affinity, the potash being absorbed 

 by the soil just as though it were not combined with 

 an acid. 



In this property arable soil is like animal charcoal, 

 which, by means of the phosphates of the alkaline earths 

 contained in it, decomposes many salts that are not 

 affected by charcoals free from such phosphates ; and, 

 without doubt, the lime and magnesia compounds in- 

 variably present in arable soil have a share in this 

 decomposing power which it possesses. 



We must suppose that the attractive force of the earthy 

 particles would not in itself be strong enough to separate, 

 for instance, potash from nitric acid, and that it requires 

 the additional attraction of the lime or magnesia to de- 

 compose the nitrate of potash. On the one side the soil 

 attracts the potash, on the other the lime or magnesia in 

 the earth attracts the nitric acid, and thus the combined 

 attraction effects, as in innumerable instances in chemistry, 

 a separation which could not have been brought about by 

 a simple one. 



The process of decomposition effected by arable soil 

 differs only in one respect from the ordinary chemical 

 processes, namely, that in the latter, as a general rule, no 

 soluble potash salt is decomposed by an insoluble lime 

 salt, in such a manner that the potash is thercljy made 



