ARABLE SOIL COMPARED TO ANIMAL CHARCOAL. 79 



respect of absorptive power for inorganic substances in 

 solution, as the woody fibre altered bv the action of 

 heat bears to organic substances in solution. 



It has been stated, that from a solution of carbonate 

 of potash or ammonia, or from a solution of phosphate 

 of lime in carbonic acid water, the arable soil will with- 

 draw the potash, ammonia, and phosphoric acid, with- 

 out any chemical interchange with the constituents of 

 the earth taking place. In this respect the action of 

 arable soil is absolutely like 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 

 aifected 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 de- 

 composing 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 mag- 

 nesia to decompose 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 thereby 

 made insoluble and the lime soluble. There is evident- 

 ly here some other attractive force at work, which alters 

 the effect of chemical affinity. If a solution of phos- 



