56 SECTIONAL ADDRESSES. 



motor-car,' and, be this as it may, it is evident that to meet such a colossal 

 consumption means have to be found to utilise the higher boiling fractions, 

 and, indeed, even the residues from the distillation processes. This 

 ' cracking ' operation is now carried out on an enormous scale by numerous 

 processes, all subject to patents, but differing from one another only 

 slightly on the question of principle. All depend on the well-established 

 fact tlaat hydrocarbons of high molecular weight will break down into 

 those of lower molecular weight if they are subjected to the requisite 

 degree of temperature. Pressure appears to play an important part in 

 the character of the product, as does also the surface action of the container 

 or material used in the container to promote surface action. All are 

 wasteful, because little or no research has been carried out on the true 

 chemical nature of the cracking operation. Much permanent gas is 

 always produced, consisting for the most part of ethylene and propylene. 

 In the States the ethylene is allowed to go free, because its obvious 

 utilisation in the form of ethyl alcohol is attended with difficulties, but 

 the propylene is usually absorbed in sulphuric acid, and thus converted 

 into isopropyl alcohol, useful as a solvent. The production of these two 

 unsaturated hydrocarbons provides a clue to the mechanism of the 

 cracking process which is of some significance. If you break a long chain- 

 saturated hydrocarbon one of your products must be an unsaturated 

 hydrocarbon, and it is evident that cracked spirit contains a considerable 

 proportion of such unsaturated bodies. Moreover, the cracking processes 

 at present in use do not produce aromatic hydrocarbons, and it is on the 

 presence of a proportion of these aromatic hydrocarbons that certain 

 special properties of petrol depend. For example, the tendency at the 

 present time is to produce for motor-cars internal-combustion engines of 

 increased compression ratio, in order mainly to diminish the petrol con- 

 sumption and thus increase mileage per gallon consumed. For some 

 reason, which research has not yet ascertained, the use of petrol which 

 does not contain the right quantity of aromatic hydrocarbons of the 

 benzene type leads to ' detonation,' ' knocking,' or ' pinking ' when ignited 

 in cylinders giving more than a small compression ratio. This detriment 

 diminishes the value of cracked spirit as such for any but low-compression 

 engines, and many have been the devices suggested in order to overcome this 

 difficulty. A vast number of substances, selected more or less at random, 

 have been tried as ' anti-knock ' materials, and as an outcome it has been 

 found that one, namely lead tetraethyl, possesses the property, when 

 present in exceedingly small quantities, of preventing the ' detonation ' 

 of the explosion mixture in the cylinder. For a time lead tetraethyl 

 (ethyl gas) fell under a ban in the States owing to a fatal accident which 

 attended the spilling of a certain amount in one of the American factories, 

 but it is understood that further investigation has led to a revision of 

 the view first formed, and that considerable quantities of ' ethyl gas ' 

 are now being used. The writer remembers visiting Wilmington in 1924, 

 when some 500 gallons of lead tetraethyl were being made daily. Although 

 there was naturally a strong smell of the material in the factory building, 

 and he remained for some hours there, no ill-efiects were noticed. It is 



s Cars registered on January 1, 1925, were: U.S.A., 17,591,981; Canada, 638,794; 

 Great Britain, 1,094,534. 



