HYDROCARBONS. 263 



of explosive gas mixtures in the coal mines, or escaping like 

 carbon dioxide through fissures to the surface of the earth, where 

 it may be ignited. 



The liquids formed are water and that mixture of hydro- 

 carbons known as crude coal-oil or petroleum, which finds its 

 way to the surface either from natural causes, or is lifted up by 

 suitable pumps. 



Petroleum, as has been stated, is chiefly a mixture of various 

 hydrocarbons, the boiling-points of which lie between and 

 300, or even higher. The crude oil has to be purified, which 

 is accomplished by a treatment with sulphuric acid, followed by 

 other processes of refining, and finally by fractional distillation, 

 in order to separate the members of low boiling-points from 

 those of higher boiling-points. 



The hydrocarbons of low boiling-points, chiefly a mixture of 

 C 5 H 12 and C 6 H 14 , are officinal, under the. name of petroleum-ether 

 or benzin, which name must not be confounded with benzene or 

 benzol, C 6 H 6 . 



Other similar liquids are sold in the market under the name 

 of rhigoline (B. P. about 21) and gasoline (B. P. about 75); 

 they are highly inflammable. 



The paraffines distilling above 150 constitute the common 

 illuminating oil, various kinds of which are sold as kerosene, 

 paraffine oil, astral-oil, mineral sperm-oil, etc. The danger which 

 arises in the use of coal-oil as an illuminating agent is caused 

 by the Use of oils which have not been sufficiently freed from 

 the more volatile members of the series, which, when but slightly 

 heated (or even at ordinary temperature), will vaporize, and, 

 upon mixing with atmospheric air, form explosive mixtures. 

 An oil to be safely used for illuminating purposes in common 

 lamps should not give off inflammable vapors (or flash) below 

 60, and should not burn at temperatures below 65. 



The residue left from the coal-oil, after the more volatile pro- 

 ducts have been distilled, is a mixture of substances, some of 

 which are used for lubricating purposes or are officinal under 

 the name of petrolatum, petroleum ointment, or vaseline. 



A mixture of the highest and solid members of the paraffine 

 series (and also left in the residue from the coal-oil distillation) 

 is known as paraffine, a white, crystalline, fusible substance used 

 for candles, etc. 



