128

ANIMAL MANURES.

of sulphates decay, a portion of the hydrogen, whilst setat liberty, combines with some of the sulphur containedin the sulphates, and sulphuretted hydrogen gas is formed.

397. This gas is evolved in considerable quantity |during the decomposition of nearly all animal substances, jand likewise those vegetables, such as cabbage, &c., Iwhich are rich in nitrogen and sulphur; though its odour

is not unfrequently almost overpowered by the strongpungent smell of the ammonia evolved at the same time.

398. When such decaying substances are exposed to jthe air, the sulphuretted hydrogen gas formed is dissi- ipated, and carried away in the atmosphere; but at the !same time a large quantity of the ammonia also formed jis lost, and this is of course highly objectionable. The :•value of these kinds of manure depends mainly upon jthe quantity of nitrogen which they are able to supply |to the plants; and everything tending to assist in retaining jthat substance in the soil, and rendering it available to )the growth of plants, is advantageous; whilst, on the iother hand, everything increasing its loss, or dissipation ■in the air, lessens the value of the manure, and rendersits application of less service.

399. Animal manures, to a slight degree, modify themechanical texture of the soil; though from their greatproneness to decay, the effects of this kind which theyproduce are comparatively transitory. Their principalvalue consists in the ammonia and carbonic acid, ornitric acid, which they yield, and the earthy and salinecompounds which they contain. In addition to this,