144

ASHES.-BURNT CLAY.

class of manures. They are of value for the charcoal,lime, phosphoric acid, and alkaline salts which theycontain. The ashes of organic substances, such as theashes of wood and vegetable matters, consist principallyof those substances which plants require. Kelp, or theashes of sea-weed, consists of similar saline compounds,generally containing more alkaline salts than the com-mon ashes of land plants.

450. The ashes of turf, peat, and weeds consist ofpretty nearly the same substances as the ashes of trees andother plants, they therefore constitute valuable manures.The good effects which have been produced by paringand burning, are in great part due to the charcoal andsaline matters which are spread over the soil, in theform of ashes. The earth which adheres to the rootsand plants burnt in this process acquires considerablepower of condensing ammonia from the air (155).

451. Burnt clay or clay which has been stronglvheated,has its mechanical properties also greatly altered, andacquires the property of absorbing ammonia in largequantity. When dry burnt clay, which has beenexposed to the air for some time, is moistened orbreathed upon, it gives out a very perceptible smell ofammonia.

The ashes of coal are of less value as manure thanmost other ashes; they do not contain alkali or phos-phoric acid, but consist of silica, alumine, oxide of iron,and a small quantity of lime. Their chief value appearsto consist in the charcoal and lime which they contain.