FIXING AMMONIA. 299 



when this is done, the salt is decomposed, the base is 

 set free, and the acid combines with the ammonia. 



792. Ammonia may be fixed by the addition of a 

 small quantity of sulphuric, nitric, muriatic, phos- 

 phoric, or any other acid. Sulphuric is, generally 

 speaking, the cheapest, and therefore the most con- 

 venient to employ; the quantity to be added of course 

 depends on the quantity of ammonia in the manure ; 

 when enough acid has been used, all smell (153, 155) 

 of ammonia disappears. A slight excess of acid 

 does no harm, as it is certain to become neutralized 

 by the bases always present in the soil. 



793. It not unfrequently happens that, from local 

 circumstances, a large supply of some other acid 

 liquors may be obtained, more particularly in the 

 vicinity of manufactories ; all such substances may 

 be used with advantage. The comparative value of 

 the different salts of ammonia is as yet unknown; 

 perhaps the best acid, if it could be procured cheap 

 enough, would be the phosphoric. Phosphate of 

 ammonia forms a most valuable manure. 



794. Various salts have been proposed for the pur- 

 pose of fixing ammonia ; amongst these the best 

 appear to be gypsum, or sulphate of lime (237), and 

 copperas or green vitriol, the sulphate of iron (289). 

 When either of these salts is mixed with solutions 

 containing ammonia in the state of a carbonate, 

 both are decomposed, sulphate of ammonia is formed 

 (159), and carbonate of lime or oxide of iron is left, 

 as an insoluble powder. For several reasons, acid is 



