290 CHEMICAL DISCOVERY AND INVENTION 



Petroleum is a mixture of compounds which consist of the 

 elements carbon and hydrogen only. The chemist speaks of 

 them as hydrocarbons. With these compounds are associated 

 in the natural oil minute quantities of compounds containing 

 nitrogen and sulphur, which, however, may be dismissed from 

 further discussion at this point. 



The American oils consist chiefly of hydrocarbons called 

 " paraffins " by the chemist. These are characterised by the 

 comparative indifference which they exhibit toward chemical 

 agents, such as strong sulphuric acid and bromine or 

 chlorine. 



By the process of distillation these are separated into succes- 

 sive fractions, the first portions collected in the condenser being 

 accompanied by a certain quantity of gas which has been held 

 in solution by the oil. These first and lightest portions of the 

 distillate constitute the liquids which are sold under the names 

 gasoline, rhigolene (petroleum ether), naphtha, and benzine 

 (petroleum spirit). Following these in order of volatility come 

 kerosene burning oil or mineral colza, and heavier oils used for 

 lubrication, and from which vaseline and solid paraffin are 

 extracted. After everything has been distilled off the residue 

 consists of coke. 



Some forty years ago it was found out that when solid paraffin 

 is heated pretty strongly it is converted into a mixture of liquid 

 products, while only a very little gas is given off. The liquid 

 contains one or more paraffins of simpler constitution than the 

 solid, together with a liquid having the composition of a paraffin 

 less two atoms of hydrogen, or, in chemical language, it belongs to 

 the series of olefines. The action of heat on the heavy paraffins, 

 therefore, is to break them up in such a way that each molecule 

 yields two smaller molecules, one being a paraffin, while the other 

 is an olefine. This is fundamentally the change which is effected 

 by the process of cracking, minor changes being brought about 

 at the same time. 



Russian oil yields a smaller proportion of the lighter and 

 more volatile portions of spirit, but a larger proportion of heavy 

 oils and solid paraffin. It is characterised by the presence of a 

 considerable quantity of compounds called naphthenes, which 

 present most of the characters of the paraffins, especially in their 

 insolubility in strong sulphuric acid. They are believed to 

 possess a constitution like that of benzene (see coal-tar, p. 308) 



