FIXING AMMONIA.

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add to the manure a small quantity of a weak solutionof any acid; but we may also fix it by the addition ofany salt containing an acid united to a base by a lessaffinity than the acid has for ammonia : when this isdone, the salt is decomposed, the base is set free, andthe acid combines with the ammonia.

412. Ammonia may be fixed by the addition of asmall quantity of sulphuric, nitric, muriatic, phosphoric,or any other acid. Sulphuric is, generally speaking, thecheapest, and therefore the most convenient to employ ;the quantity to be added of course depends on thequantity of ammonia in the manure; when enough acidlias been used, all smell (93,397 ) of ammonia disappears.A slight excess of acid does no harm, as it is certain tobecome neutralised by the bases always present in the soil.

413. It not unfrequently happens that from localcircumstances a large supply of some other acid liquorsmay be obtained, more particularly in the vicinity ofmanufactories; all such substances may be used withadvantage. The comparative value of the differentsalts of ammonia is as yet unknown; perhaps the bestacid, if it could be procured cheap enough, would be thephosphoric. Phosphate of ammonia forms a mostvaluable manure.

414. Various salts have been proposed for the purposeof fixing ammonia; amongst these the best appear to begypsum or sulphate of lime (166), and copperas orgreen vitriol, the sulphate of iron (210). When either ofthese salts is mixed with solutions containing ammonia