THE HYDROCARBONS AND THEIR DERIVATIVES. FLAME 345 



At some suitable stage, the residual oil is chilled, and a quan- 

 tity of the solid members of the series (C^H^ to C 2 8H 5 8) crystal- 

 lizes in flakes (solid paraffin) and is separated by filtration in 

 presses. The final residue is used for lubricants and for fuel. 

 The fractions are still mixtures, but contain mainly compounds 

 lying close together in the series. Some of the products are as 

 follows: 



Vaseline, C 2 2H 4 6 to C 2 3H 4 8, is separated in some refineries. 

 Solid paraffin is employed for waterproofing paper, as an ingredi- 

 ent in candles, and in making chewing gum. 



Asphalt, a natural mixture of the solid hydrocarbons, found 

 particularly in Trinidad, is used in road-making. 



Oil Shale. In Scotland, petroleum is also obtained by heating 

 shale. The shale is a clay deposit, which contains no oil as such, 

 but which when heated gives off fuel gas, ammonia (see p. 299), 

 petroleum oils, and many valuable hydrocarbon derivatives. 

 The richer shales yield from 30 to 40 gallons of oil per ton. The 

 oil usually contains a much larger percentage of unsaturated 

 hydrocarbons than well petroleum. The yield can be somewhat 

 increased, and the proportion of undesirable unsaturated hydro- 

 carbons diminished, by blowing superheated steam into the 

 retorts during the distillation. 



Natural Gas: Methane CH 4 . Natural gas is obtained from 

 wells, tapping strata close to those which contain petroleum, and 

 in the same localities. It often issues under very high pressure. 



