B.— CHEMISTRY. 57 



obvious that the conditions which produce ' knocking,' and the reason why 

 certain substances are ' anti-knock ' compounds, and why the presence 

 of aromatic hydrocarbons prevent the phenomenon, must be made the 

 subject of systematic research. 



The question is also one of national importance, because in the case 

 of high-compression engines, such as those used in aeroplanes, it is essential 

 that a petrol should be used containing a high percentage of aromatic 

 hydrocarbons. In war-time these aromatic compounds will be required 

 for the manufacture of explosives, and it is quite certain that there will 

 not be enough for both purposes. 



Nevertheless, it must be remembered that it is only at the moment 

 that the low boiling fraction of petroleum is the chief marketable product. 

 It is probable that progress in the future will tend more and more to 

 produce a motor-car engine of the Diesel type, or one having a carburettor 

 capable of effectively vaporising the higher fractions of petroleum. In 

 these circumstances it may well be that the low fraction will become the 

 less important part of crude petroleum, and that, instead of having to 

 resort to ' cracking, ' a process of synthesis, by which the lower hydrocarbons 

 can be converted into higher ones, will have to be adopted. As a matter 

 of fact there are methods known by which this can be effected. Pure 

 i'soamylene can, for example, be converted into diamylene by interaction 

 with stannic or aluminium chloride, and the process can be carried 

 further, so that perfectly good lubricating oils can now be made by the 

 polymerisation of the lower unsaturated hydrocarbons. 



Polymerisation and depolymerisation are, therefore, the two operations 

 which the petroleum industry must investigate and establish on a firm 

 ■scientific basis by research, so that it may be in a position to supply the 

 public need for any particular form of engine which the engineer may 

 ■evolve. Especially is it desirable to ascertain under what conditions 

 polymerisation leads to the formation of aromatic and naphthenic hydro- 

 carbons. Considerable attention has been drawn within recent times to 

 what may be termed in general the Bergius processes for depolymerising 

 organic substances. The operation, which consists in heating the material 

 under high pressure in the presence of hydrogen, was introduced in the 

 first instance for the treatment of coal. There can be no question that 

 great and fundamental changes are brought about in organic substances 

 by the treatment whether a catalyst is present or not, and that a wide 

 field for research is opened up thereby, but it is doubtful if, at the moment, 

 general operations of this type can be regarded as commercial propositions. 

 The plant is exceeding costly and the conditions subject to wide variations 

 which are difl&cult to control. Actually it has been ascertained that in 

 the ' cracking ' of the kerosene fraction of petroleum hydrogen is un- 

 necessary, and can be replaced by nitrogen without affecting the character 

 of the final product. 



Little is known of the constituents of crude petroleum, or, indeed, 

 of the fractions into which it can be separated after purification and 

 distillation. Some of the simpler hydrocarbons of the pentane and 

 hexane type have been isolated and the presence of cyclic compounds has 

 been established. Many of them are classed under the head of ' naphthenes, ' 

 but these are of uncertain structure. No doubt many are present in the 



