ANIMAL MANURES. 293 



powered by the strong pungent smell of the ammonia 

 evolved at the same time. 



776. When such decomposing substances are ex- 

 posed to the air, the sulphuretted hydrogen gas formed 

 is dissipated, and carried away in the atmosphere ; 

 but at the same time a large quantity of the ammonia 

 also formed is lost, and this is of course highly ob- 

 jectionable. The value of these kinds of manure 

 depends in great part on the quantity of nitrogen 

 which they are able to supply to the plants; and 

 everything tending to assist in retaining that sub- 

 stance in the soil, and rendering it available to the 

 growth of plants, is advantageous; whilst, on the 

 other hand, everything increasing its loss, or dissi- 

 pation in the air, lessens the value of the manure, and 

 renders its application of less service. 



777. Animal manures, to a slight degree, modify 

 the mechanical texture of the soil ; though from their 

 great proneness to decompose, the effects of this kind 

 which they produce are comparatively transitory. 

 Their principal value consists in the ammonia and 

 carbonic acid, or nitric acid, which they yield, and 

 the earthy and saline compounds which they contain. 

 In addition to this, they are also of value by facili- 

 tating the decomposition of vegetable substances 

 employed as manure (767). 



778. As perhaps the most useful part of these kinds 

 of manure is the volatile substances formed during 

 their putrefaction, great care should be taken to suffer 



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