THE SUBSTANCES OF THE EARTH. 21 



It forms an essential part of plants and animals, and 

 is of great importance for agricultural purposes. Its 

 force of attraction for other elements is very weak. This 

 fact increases the difficulty of retaining it in permanent 

 forms. It is the most expensive part of fertilizers. 

 With hydrogen it forms ammonia (NH 3 ) ; with hydrogen 

 and oxygen, nitric acid (HN0 3 ). These are the two 

 forms in which nitrogen most commonly becomes a 

 source of fertility, and of importance in agriculture. 



Carbon is found in nature in three forms : as charcoal 

 and similar substances ; as graphite, which is used in 

 making lead pencils ; and as diamonds, which are simply 

 crystallized carbon. 



It forms a large part of all animal and vegetable sub- 

 stances. Coal, wood, and woody substances are largely 

 composed of it. It is the element which gives value to 

 substances used as fuel. 



Charcoal, which is largely composed of free carbon, is 

 produced by burning wood in a partially smothered fire. 

 The process releases the carbon from other elements 

 with which it is combined, and retains it by excluding 

 the oxygen of the air, with which it would otherwise 

 unite and escape as carbonic acid gas. 



If a piece of wood is placed in sulphuric acid and al- 

 lowed to remain for some time it becomes black. The 

 acid removes the other elements of the wood and leaves 

 the carbon. 



If a piece of glass is held over the flame of a candle it 

 becomes " smoked," or covered with minute particles of 

 carbon, which escape faster than they can be consumed. 



All plants contain a large proportion of carbon. In 

 cotton fiber it is almost pure. Sugar, which is a product 

 of plants, is forty-two per cent, carbon, by weight. 



