CARBON AND OXYGEN. 71 



by its own powder. When found in the earth, 

 they are crystalHzed, but are usually rough, 

 having lost the angles of their crystals by attrition. 

 They may be cleaved or split, and are then cut 

 with facets for jewellery. They are of various 

 colours. The diamond was long thought to be an 

 incombustible body, but it is now known to be 

 capable of being burnt ; and by its union with oxy- 

 gen, it forms carbonic acid. But although we know 

 that diamond is only carbon, no attempts to crys- 

 tallize carbon, and thus to make diamonds, have 

 succeeded. Art cannot always imitate the pro- 

 cesses of nature, even when the materials she has 

 used are known. 



Carbon and Oxygen. 



Carbon unites to oxygen in two proportions. We 

 shall first consider the most common one : 



Carbonic Acid. — If charcoal be burnt, it com- 

 bines with the oxygen of the atmosphere, and 

 thus forms an acid, which, however, cannot be 

 condensed into the liquid form, but is always 

 aerial. Carbonic acid exists in great abundance 

 in nature, combined with mineral bodies, chiefly 

 lime. All limestones are formed of carbonic acid 

 in a fixed state, united to lime. Hence this gas 

 was at first called jixed air, which name is still 

 sometimes used. 



It may be procured from limestone or marble, 

 in the following manner : put a quantity of broken 

 pieces of marble or chalk into a retort, and add to 

 it some sulphuric acid, diluted with six times its 

 weight of water: a brisk effervescence will ensuei, 



F 4 



