20 SCIENTIFIC AGRICULTURE. 



sel or burned in a covered heap, the black residue is 

 chiefly carbon. This element forms the greater por- 

 tion of woody substances and enters largely into the 

 composition of all organic matter. Sugar contains 

 42 per cent, of carbon, spirits of turpentine 88 per 

 cent. The black smoke of a candle or lamp is car- 

 bon in a finely divided state (12). 



32. Carbon dioxide, or carbonic acid gas, 

 is a compound of carbon and oxygen that exists in 

 small quantities in the air, and is always formed 

 when carbon burns. It is a heavy, poisonous gas 

 that is formed during respiration, combustion, fer- 

 mentation, and decay. While it poisons animals, it 

 is an important food for plants, as will be shown 

 hereafter. If a little hydrochloric acid be poured 

 on a piece of marble or limestone, the effervescence 

 produced is caused by the escape of this gas (13). 

 It will extinguish a burning taper. 



33. Silicon is an element found in common sand 

 and quartz. It is very abundant, forming about one- 

 fourth of the solid part of the earth, but very diffi- 

 cult to separate from the oxygen with which it is near- 

 ly always combined. Some chemists doubt whether 

 it is a necessary constituent of plants. It is certainly 

 found in the stems of grass, wheat, corn, and in vari- 

 ous kinds of vegetation. 



34. Sulphur is a well-known substance of yellow 

 color that burns with a pale-blue flame and suffocat- 

 ing odor. In burning, it unites with the oxygen of 

 the air to form sulphur dioxide, which is used to de- 



