82 NON-METALS AND THEIR COMBINATIONS. 



coal burns with an insufficient supply of air. The carbon di- 

 oxide 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 monoxide. This gas is also formed by 

 the decomposition of oxalic acid (and many other organic sub- 

 stances) by sulphuric acid : 



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



Oxalic Sulphuric 



acid. acid 



Compounds of carbon and hydrogen. There are no other two 

 elements which are capable of forming so large a number ot 

 diiferent combinations as carbon and hydrogen. Several hun- 

 dred of these hydro-carbons are known, and their consideration 

 belongs to the domain of organic chemistry. 



Two of these hydro-carbons, however, may be briefly men- 

 tioned, as they are of importance in the consideration of com- 

 mon flames. These compounds are: methane (marsh-gas, fire- 

 damp), CH 4 ; and ethene (olefiant gas), C 2 H 4 . 



Both compounds are colorless, almost odorless gases, and 

 both are products of the destructive distillation of organic sub- 

 stances. Destructive distillation is the heating of non-volatile 

 organic substances in such a manner that the oxygen of the at- 

 mospheric air has no access, and to such an extent that the 

 molecules of the organic matter are split up into simpler com- 

 pounds. Among the gaseous products formed by this operation, 

 more or less of the two hydro-carbons mentioned above is 

 found. 



Marsh-gas is also frequently formed by the decomposition of 

 organic matter in the presence of moisture (leaves, etc., in 

 swamps), and during the formation of coal in the interior of the 

 earth ; the gas there liberated giving rise to the explosions in 

 coal mines. 



Flame is a gas in the act of combustion. Of combustible gases, 

 have been mentioned: hydrogen, carbon monoxide, marsh-gas, 

 and olefiant gas. These four gases are actually those gases 

 which are chiefly found in any of the common flames produced 

 by the combustion of organic matter, such as paper, wood, oil, 

 wax, or illuminating gas itself. 



