DIFFERENCE BETWEEN COMPOUNDS AND MIXTURES. 37 



nhporbed with water by the roots of crops ; but by far the 

 largest quantity of these elements enter into plants in the 

 form of simple or complex combinations, or chemical com- 

 pounds of a distinct character, and these being absorbed by 

 the plants, are separated 'or decomposed; the plants then 

 selecting what they require for their subsistence, and re- 

 jecting the rest. It is then necessary for the farmer to study 

 not only the nature of these compounds, but the laws by 

 which their combinations are regulated and the manner in 

 which they are effected. 



The difference between a mixture and a compound has 

 been already explained, and it should always be borne in 

 mind in considering this part of our subject. Combination 

 is not a mechanical but a chemical action, and results ift a 

 permanent change in some or all of the substances employed ; 

 decomposition is also a chemical action, and results also in 

 permanent change of a combined substance or compound. 

 When sulphur, a solid substance, is burned in the air, it is 

 converted into a gas ; which is a compound of the sulphur 

 with oxygen from the air. When limestone is burned in a 

 kiln it is changed into lime, which is quite different from 

 the original stone. This is a case of chemical decomposition 

 for the limestone is a compound of lime and carbonic acid, 

 and the acid is driven off by the heat, and the lime is left 

 remaining. The limestone is quite neutral and inert; it has 

 no action upon water, nor any taste ; but the lime is strongly 

 alkaline and will effervesce strongly if put in vinegar or any 

 other acid ; if put into water, it will combine with it and 

 produce great heat, and it will destroy any organic matter 

 brought into contact with it. 



If hydrogen gas is burned in the air and a piece of cold 

 glass is held over the flame, the vapor produced by the un- 

 ion of the two gases, is condensed into drops of water on the 

 glass. It is thus seen by these examples which could be 

 extended almost infinitely ; how important changes in matter 

 are produced by chemical combinations and decompositions ; 

 gases are combined and form liquids or even solids ; liquids 

 tmd solids are changed to gases; mild substances become 



