AGRICULTURAL CHEMISTRY. 173 



the more porous ingredients of the soil, namely, humus, oxyd of iron 

 and alumina; an agency which is exhibited most strikingly in case of 

 ammonia by charcoal, which, when freshly ignited, may absorb as 

 much as ninety times its bulk of this gas. The ammonia thus con- 

 densed is, however, easily removed. Water or exposure to moist 

 air at once displaces it, for it is only the absolutely dry charcoal that 

 absorbs ammonia. Common moist charcoal has no appreciable faculty 

 of this kind, its pores being already fully occupied, having satisfied 

 their absorptive power on vapor of water and the ingredients of the 

 atmosphere. 



Liebig, reasoning from these facts, asserted in his ' ' Chemistry 

 applied to Agriculture and Physiology, " that "the ammonia absorbed 

 by clay or ferruginous oxyds is separated by every shower of rain 

 and conveyed in solution to the soil." 



The chemical absorption consists in the fixation and retention in 

 the soil of volatile or dissolved matters, by their entering into com- 

 paratively insoluble combinations. This fixation is not, however,, 

 absolute, as we shall presently see. 



Thompson and Way of England, in 1850, (see Journal Royal Agri- 

 cultural Society of England for that year,) first began to develope the 

 interesting facts which relate to this subject. Since the date of their 

 investigations Liebig, Voelcker, Henneberg & Stohmann, Eichhorn, 

 and Brustlein, have occupied themselves with its study. 



The main facts are, briefly stated, as follows: 



Free ammonia and lime, and their carbonates, are absorbed and 

 chemically retained by the organic acids, (humic, crenic, <fec.,) the 

 ammonia in a non-volatile, but to some extent soluble form. Am- 

 monia is also absorbed by oxyd of iron and alumina, and held in a 

 non- volatile and very slightly soluble state. 



Salts of ammonia, namely, sulphate hydrochlorate and nitrate, are 

 at once decomposed by the soil when their dilute solutions are agitated 

 with or filtered through it; the ammonia being retained, the acid re- 

 maining in solution united to lime. 



The same salts of potash are likewise decomposed as above; the 

 potash being retained, the acids uniting with lime. 



Salts of lime, in general, are not absorbed, especially when added 

 alone to the soil, or when the soil is rich in lime; but in several of 

 Voelcker' s experiments the liquor from a dung-heap containing a con- 

 siderable quantity of sulphate of lime lost this ingredient nearly or 

 entirely by filtration through a sandy soil, and at the same time the 

 amount of carbonate of lime in the solution was diminished. 



Salts of soda and magnesia are also retained, though usually in a 

 less degree. 



When solutions of phosphates and silicates of the alkalies are em- 

 ployed in these experiments, we find that the acids are also retained; 

 and from the trials of Voelcker already referred to, we have evidence 

 that sulphuric and hydrochloric acids are also liable to absorption. 

 In no instance has a fixation of nitric acid been observed. 



According to Brustlein's late researches, the retention of the bases 

 when employed in saline combinations cannot occur except in presence 

 of carbonate of lime. This view is, however, erroneoiis. 



