142 CHEMISTRY FOR AGRICULTURAL STUDENTS 



It was found (p. 104) that when wood is distilled 

 inflammable gases are obtained, one of which is the hydro- 

 carbon marsh gas. Similarly, when coal is distilled, the product 

 is the mixture of hydrocarbons and other gases known as coal 

 gas. When the carboniferous shale of the coal measures is 

 distilled the chief product is a mixture of hydrocarbons called 

 paraffin. American petroleum has a composition similar to 

 paraffin, and is perhaps a product of the natural distillation of 

 carbonaceous substances. It is obtained in Pennsylvania and 

 other districts from borings through the strata overlying the 

 sand or gravel in which the petroleum exists. Analyses and 

 determinations of the vapour density show that both petroleum 

 and paraffin are mixtures of hydrocarbons differing in their 

 proportion of carbon to hydrogen and in molecular weight. 

 Marsh gas or methane is the simplest of these hydrocarbons, 

 and that which contains the largest proportion of hydrogen. 

 Of the other hydrocarbons in the mixture a few are gases, but 

 the more complex are liquids and the most complex are 

 solids. The relation between the physical properties and 

 chemical composition is shown in the following table : — 



Gaseous Hydrocarbons. 



Liquid Hydrocarbons. 



By fractiofial distillation petroleum can be separated into :- 

 (1) A gas used for heating and lighting. 



