2 THE CHEMISTRY OF THE FARM. 



If a branch of a tree is burnt, the greater part is con- 

 sumed and passes away in the form of gas, but there is 

 left behind a small quantity of white ash. The same 

 happens if any other part of a plant is burnt. The con- 

 stituents which form the dry matter of plants may be thus 

 conveniently divided into two classes the combustible, 

 and the incombustible. 



The combustible part of plants is made up of five 

 chemical elements carbon, oxygen, hydrogen, nitrogen 

 and sulphur; without these no plant is ever produced. 

 Carbon generally forms about one-half of the dry com- 

 bustible matter of plants. Nitrogen seldom exceeds 4 per 

 cent, of the dry matter, and is generally present in much 

 smaller amount. Sulphur is still smaller in quantity. The 

 remainder is oxygen and hydrogen. 



The carbon, hydrogen, and oxygen form the cellulose, 

 lignin, pectin, gummy matters, starch, dextrin, sugar, fat, 

 and vegetable acids which plants contain. The same ele- 

 ments united with nitrogen form the amides and alkaloids ; 

 and further united with sulphur the still more important 

 albuminoids, which are essential constituents of all plants. 



The incombustible, cr ash constituents, form generally 

 but a small part of the plant. The timber of freely- 

 growing trees contains but 0*2 0*4 of ash constituents in 

 100 of dry matter. In seeds free from husk the ash is 

 generally 2 5 per cent, of the dry matter. In the straw 

 of cereals 4 7 per cent. In roots and tubers 4 8 per cent. 

 In hay 5 9 per cent. It is in leaves, and especially old 

 leaves, that the greatest proportion of ash is found ; in the 

 leaves of root crops the ash will amount to 10 25 per cent, 

 of the dry matter. 



The incombustible ash always contains five chemical 



