2 THE CHEMISTRY OF THE FARM. 



If a branch of a tree is burnt, the gi*eater 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, 

 lignose, pectin, starch, sugar, fat, and vegetable acids which 

 plants contain. The same elements 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 or ash constituents form generally 

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

 ing trees contains but 0.2 — 0.4 of ash constituents in 100 

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

 rally 2 — 5 per cent. In the straw of cereals 4 — 7 per 

 cent. In farm roots 4 J — 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 



