BONES,

137

or the excrement of animals, and oil and fat of all kindscontain no nitrogen at all. Bones consist of earthymatters (279), together with a quantity of glue andfat; the value of bones as a manure consists in theirmechanical effect on the texture of the soil, the earthysubstances which they supply (405), the ammoniagiven out during the decay of the glue or gelatine, andthe carbonic acid and hydrogen supplied by the fat.

424. The quantity of putrefiable matter in bones issmall in proportion to their whole weight, and its decayis retarded by the large quantity of earthy matter theycontain, hence bones form a good and lasting manure.Boiled bones form a better manure than raw or greenbones. This appears to be occasioned by the removal offat during boiling ; the fat which bone contains retardsthe decomposition of the gelatin, and probably alsorenders the phosphate of lime less soluble than it iswhen the oil is removed.

425. It is frequently the custom to mix a considerablequantity of earth or soil, with putrefying animal manure.This to some extent is a good practice; the earth addedprevents too rapid decay, and retains some of the pro-ducts of decomposition; a considerable quantity ofammonia, which would otherwise be lost, is absorbedby the soil (155). The addition of burnt clay orcharcoal is useful for a similar reason (195).

426. The use of lime, on the other hand, is decidedlyobjectionable ; it increases the rapidity of decay, andtends to the formation of a portion of nitric acid, but its