346 SMITH'S INTERMEDIATE CHEMISTRY 



It owes its combustibility to its chief component (over 90 per cent), 

 methane CH 4 . It is largely used as a fuel in the regions in which 

 it is found and, in the United States, the annual value of the gas 

 so consumed is nearly $160,000,000 (1919). The same gas issues 

 from many coal seams (" fire-damp "), and forms explosive 

 mixtures with the air of mines. It rises to the surface when stag- 

 nant pools containing decomposing vegetable matter are stirred 

 (" marsh-gas"). 



The formation of methane by direct union of carbon and hydro- 

 gen has already been discussed (p. 331). 



Chemical Properties of the Hydrocarbons. The hydro- 

 carbons, whether pure or in solution, show no conductivity for 

 electricity. They have none of the chemical properties of acids, 

 bases, or salts, and therefore do not enter into double decom- 

 positions with substances of these classes. The saturated hydro- 

 carbons are in fact quite indifferent to the presence of most chem- 

 ical reagents. 



All the hydrocarbons burn with oxygen or air to form carbon 

 dioxide and water: 



CH 4 + 2O 2 -> CO 2 + 2H 2 0. 

 C 7 Hi 6 + H0 2 - 7CO 2 + 8H 2 0. 



The water can be shown by its condensation on a cold vessel 

 held over the flame. The carbon dioxide gives a precipitate 

 of calcium carbonate (p. 336) when the gases rising from the flame 

 are drawn through lime-water. 



All the hydrocarbons, when heated strongly (air excluded), 

 decompose or crack. They usually lose a part of their hydrogen 

 and become unsaturated. These of high molecular weight break 

 up to give a mixture of hydrocarbons of low molecular weight. 

 Ethylene C 2 H 4 , for example, is produced in large amounts by 

 heating the higher members of the series to a red heat. On the 

 other hand, the lower members of the series, when heated, often 



