34 THE CULTURE OF FARM CROPS. 



but the only difference between the water and the steam cr 

 vapor is that the particles of the water are separated widely 

 apart by the heat and they become invisible ; but they exist 

 still and can be brought together again and condensed into 

 a fluid and reappear as water by the action of cold or the loss 

 of the heat. These are mixtures. A chemical combination 

 is entirely different. If we take some sulphur and burn it, 

 it combines with oxygen from the atmosphere, and the solid 

 sulphur becomes a powerfully corrosive gas which when 

 mixed with water is known as sulphuric acid. This acid is 

 a combination not a mixture of the two elements, and it 

 cannot be separated into these without a complicated chem- 

 ical process. If a piece of copper is put in a quantity of 

 this acid, the copper disappears and the liquid becomes a 

 solution of sulphate of copper. If the water is evaporated 

 the copper sulphate remains in the form of clear blue crys- 

 tals which are commonly called blue vitriol. This is a 

 combination of the elements oxygen, sulphur and copper, 

 but is not a mixture. 



All organic vegetable and animal substance consists of the 

 four elements which have been previously described. The 

 peculiar characters or properties of organic matter by which 

 they are distinguished from inorganic matter and on which 

 their connection with the culture of farm crops depends, are 

 chiefly the following. 



They are all easily decomposed, or apparently destroyed 

 by heat. Starch, sugar, cotton fiber, straw or wood, when 

 subjected to heat or flame, turn black and take fire and are 

 consumed. This is true of all vegetable substances. But 

 clay, sand or stone cannot be thus decomposed. 



They putrefy and decompose in warm moist air, and after 

 a time almost wholly disappear. This is not the case with 

 inorganic substance which is not subject to putrefactive de- 

 composition. 



They consist almost wholly of two or more of the few 

 organic elements previously described. 



They cannot be formed by art. Many of the inorganic 

 compounds may be, and have been, produced in the chem- 



