96 ' NON-METALS AND THEIR COMBINATIONS. 



an insufficient supply of air. The carbon dioxide formed in the lower 

 parts of the furnace is decomposed by the coal above. The blue 

 flames frequently playing over a coal fire are burning carbon mon- 

 oxide. This gas is formed also by the decomposition of oxalic acid 

 (and many other organic substances) by sulphuric acid : 



H 2 C 2 4 + H 2 S0 4 == H 2 SO 4 .H 2 O + CO 2 + CO. 

 Oxalic Sulphuric 



acid. acid. 



Carbon monoxide is now manufactured on a large scale by causing 

 the decomposition of steam by coal heated to red heat. The decom- 

 position takes place thus : 



H 2 O + C : : 2H -f CO. 



The gas mixture, thus obtained and known as water-gas, may be used 

 for heating purposes directly, but has to be mixed with hydrocarbons 

 when used as an illuminating agent, for reasons which will be pointed 

 out below when considering the nature of flames. 



Compounds of carbon and hydrogen. There are no other two 

 elements which are capable of forming so large a number of different 

 combinations as are carbon and hydrogen. Several hundred of these 

 hydrocarbons are known, and their consideration belongs to the 

 domain of organic chemistry. 



Two of these hydrocarbons, however, may be briefly mentioned, 

 as they are of importance in the consideration of common flames. 

 These compounds are : methane (marsh-gas, fire-damp), CH 4 ; and 

 eihene (olefiant gas), C 2 H 4 . 



Both compounds are colorless, almost odorless gases, and both are 

 products of the destructive distillation of organic substances. De- 

 structive distillation is the heating of non-volatile organic substances 

 in such a manner that the oxygen of the atmospheric air has no access, 

 and to such an extent that the molecules of the organic matter are 

 split up into simpler compounds. Among the gaseous products 

 formed by this operation, more or less of the two hydrocarbons 

 mentioned above is found. 



Marsh-gas is formed frequently by the decomposition of organic 

 matter in the presence of moisture (leaves, etc., in swamps) ; and dur- 

 ing the formation of coal in the interior of the earth the gas often 

 gives rise to explosion in coal mines. During these explosions of the 

 methane (mixed with air and other gases), called fire-damp by the 

 miners, carbon is converted into carbon dioxide, which the miners 

 speak of as choke-damp, or after-damp. 



