324 ASHES. 



coal, lime, phosphoric acid, and alkaline salts which 

 they contain. The ashes of organic substances, such 

 as the ashes of wood and vegetable matters, consist 

 principally of those substances which plants require. 

 Kelp, or the ashes of seaweed, consists of similar 

 saline compounds, generally containing more alka- 

 line salts than the ashes of common land plants. 

 Soap-maker's ash, w^hich consists of the insoluble 

 part of wood-ash, contains a considerable quantity of 

 phosphate of lime, and is consequently a valuable 

 manure (890). 



880. The ashes of turf, peat, and weeds, consist 

 of pretty nearly the same substances as the ashes of 

 trees and other plants ; they therefore constitute 

 valuable manures. The good effects which have been 

 produced by paring and burning, are in great part 

 due to the charcoal and saline matters w^hich are 

 spread over the soil in the form of ashes. The earth 

 which adheres to the roots and plants burnt in this 

 process acquires considerable power of condensing 

 ammonia from the air (162). This is because burnt 

 clay, or clay which has been strongly heated, has its 

 mechanical properties very greatly altered, and ac- 

 quires the property of absorbing ammonia in large 

 quantity. When dry burnt clay, which has been 

 exposed to the air for some time, is moistened or 

 breathed upon, it gives out a very perceptible smell 

 of ammonia. 



881. The ashes of coal are of less value as manure 

 than most other ashes j they do not contain alkali or 



