138 



Power of Soils to absorb Manure, 



is immediately traceable to the greater solubility of the silicate of 

 ammonia in a saline solution. 



Hitherto we have spoken only of the power of the double sili- 

 cates to unite with ammonia, and separate it from solution. More 

 important, if possible, is the faculty which some of these salts 

 possess, of abstracting ammonia from the air. It has long been 

 known that soils acquire fertility by exposure to the influences of 

 the atmosphere — hence one of the uses of fallows. It has also 

 been generally understood that clay possessed a power of absorb- 

 ing ammonia from the air, but only through the influence of rain 

 or dews, to bring down the volatile carbonate. This latter con- 

 dition, however, is not at all necessary. I find that clay is so 

 greedy of ammonia, that if air charged with carbonate of ammonia, 

 so as to be highly pungent, is passed through a tube filled with 

 small fragments of dry clay, every particle of the gas is arrested. 

 In the same way, if into a bottle filled with air similarly impreg- 

 nated, a little ordinary dry soil is thrown, and the bottle is then 

 shaken once or twice, all ammoniacal smell is destroyed. The 

 double silicate of alumina and lime is in these cases also the 

 cause of the absorption. If, instead of clay, sand be placed in the 

 tube, no obstacle is presented to the passage of the gas ; but by 

 mixing with the sand a few grains of the lime silicate we can 

 immediately arrest it. The avidity of this silicate of lime and 

 alumina for carbonate of ammonia is most marked. A few grains 

 of the salt were spread upon a piece of paper, and covered with 

 a glass bell jar, some fragments of dry carbonate of ammonia in a 

 small dish being also covered by the jar ; in a few hours the 

 silicate was found to have absorbed between two and three per 

 cent, of ammonia, and the action will go on until the salt is 

 entirely saturated. The chemical change in this case is very 

 simple — the carbonic acid of the carbonate of ammonia attacks 

 the lime, forming carbonate of lime, whilst at the same time the 

 double silicate of alumina and ammonia is produced. It is re- 

 markable that the corresponding soda silicate does not absorb 

 carbonate of ammonia ; or, at all events, if it does so in an atmo- 

 sphere highly impregnated with the volatile alkali, it gives it off 

 again so soon as it is exposed to the air : in ordinary circum- 

 stances, therefore, it does not absorb ammonia from the air.* 



* This circtimstaiQce, which Avas not anticipated, is, however, of easy explana- 

 tion. When the lime silicate absorbs carbonate of ammonia, carbonate of lime is 

 formed, and this, being insoluble, does not re-act upon the silicate of ammonia ; 

 but, in the other case, carbonate of soda would be one of the products ; being a 

 soluble salt, it is enabled to re-act by degrees on the ammonia silicate, which is 

 also slightly soluble, and carbonate of ammonia is the result. In point of fact, it 

 is found that, when the double silicate of alumina and ammonia is mixed with 

 carbonate of soda, fumes of carbonate of ammonia at once come off ; whereas 

 carbonate of lime has no such action on the ammonia silicate. 



