296 ELEMENTARY GENERAL vSCIENCE CHAP. 



chloric acid. By ascertaining the weight per cent, of carbon dioxide 

 which is given off by the chalk, we can prove that chalk is really a 

 compound of lime and carbon dioxide. 



Carbon Dioxide is a heavy gas which does not burn nor support 

 combustion. It is slightly soluble in water, the solution acting as 

 a weak acid which turns blue litmus to a port wine colour. This 

 solution may be regarded as carbonic acid, H 2 CO 3 . The solubility of 

 carbon dioxide in water is increased by pressure. By cold and pressure 

 it may be liquefied and also solidified to a soft, white substance, which 

 when mixed with ether forms a powerful freezing mixture. 



Carbon dioxide is always found in the atmosphere, being produced 

 by the oxidation of animal and vegetable tissues. This carbon di- 

 oxide is decomposed by the green colouring matter of plants in the 

 presence of sunlight. It is very common in caverns in limestone 

 regions. Its formula is C0 2 . 



Explanation of Hardness of Natural Waters. Temporary hard- 

 ness is due either to chalk (calcium carbonate) or to magnesium 

 carbonate which is dissolved in the water. The solution of these 

 compounds is possible because the carbon dioxide in the water first 

 converts them into soluble hydrocarbonates. When the water is 

 boiled these compounds are converted into insoluble carbonates. 



Permanent hardness is generally due to calcium or magnesium 

 sulphate which is dissolved in the water. They cannot be got rid of 

 by boiling because they are both soluble in water. 



Carbon Monoxide contains less oxygen than the dioxide. It can be 

 prepared from the dioxide by abstracting some oxygen. This can 

 be effected by passing it over heated iron : 



Carbon monoxide burns with a blue flame, forming carbon dioxide 

 again : 2CO + 2 =: 2C0 2 . It is a colourless gas which is very poisonous. 



Compounds of Carbon with Hydrogen. Acetylene, C 2 H 2 , is formed 

 by the direct union of carbon and hydrogen. It burns with a highly 

 luminous flame, and is present to a small extent in the products of 

 the incomplete combustion of coal gas. It is now largely prepared 

 by the action of water on calcium carbide. 



Ethylene, C 2 H 4 , is a product of the destructive distillation of coal, 

 wood, &c. , and is hence found in coal gas. It can be readily prepared 

 by heating a mixture of alcohol and sulphuric acid. It burns with 

 a luminous flame less bright than that of acetylene. Liquid ethylene 

 has been obtained. 



Marsh Gas, CH 4 , is also known as methane. Since it is one of the 

 products of the decomposition of vegetable matter it is often found 

 rising to the surface of swamps, hence the name marsh gas. It 

 occurs in coal mines as fire-damp. It is generally prepared in the 

 laboratory by heating a mixture of sodium acetate and caustic soda, 

 when it is seen to be a colourless and odourless gas which burns with 

 a feebly luminous flame. 



Coal Gas consists of a mixture of hydrogen, carbon monoxide, 

 marsh gas, and other hydrocarbons. In its ordinary combustion 

 carbon is at first separated and is a cause of the luminosity of the 

 flame. 



