280 CHEMICAL DISCOVERY AND INVENTION 



than 10,000 cubic feet of gas were obtained per ton of coal 

 heated, the yield is now commonly 13,000 cubic feet. A cheap 

 gas produced by the addition of water gas charged with vapours 

 from petroleum (see petrol) also helps to reduce the cost. The 

 testing of gas, for statutory purposes, now relates more to its 

 power as a source of heat than as a source of light. 



At the same time this improvement in gas illumination has 

 not been without its effect on the system of electric lighting. 

 The improved efficiency of the incandescent electric lamp by 

 the substitution of various metal filaments for the carbon 

 filament, exclusively used a few years ago, may not have been 

 due entirely to the increasing success and economy of the gas 

 mantle light, but invention has undoubtedly been stimulated by 

 these results. 



The mantle industry is only one of many examples wh ch 

 could be quoted of the ultimate practical application of the 

 results of purely scientific research to common industrial pur- 

 poses. A generation ago the salts of thorium and cerium, 

 lanthanum and didymium, and the rest were interesting only to 

 a few enthusiasts. The place of these elements was and continues 

 to be among the problems perplexing to the scientific chemist, 

 and they were only known to exist in a few comparatively 

 scarce minerals found .chiefly in Scandinavia, and all this was 

 implied in the name which for so long a time they bore, namely 

 " the rare earths." Now these elements are known to be widely 

 diffused and available in any required amount from mineral 

 deposits which are actually handled to the extent of thousands 

 of tons annually. 



CHAPTER XIX 



PETROL 



LESS than twenty years ago the arrival of any sort of motor 

 vehicle in a country place would have been sufficient to bring 

 together a crowd of wondering folk, and less than ten years ago 

 any street in London exhibited a collection of omnibuses, 

 carriages, wagons, carts, and other vehicles of which the majority 

 were drawn by horses. The proportions of motor to horse- 



