During an absence of several days, the inventor left a mantle of lanthanum oxide locked up in his laboratory. When he returned it had fallen to powder, having attracted from the atmosphere both moisture and carbon dioxide. Evidently this harmful attraction must be avoided by adding an ingredient to keep the mantle dry and preserve it from union with carbon dioxide. For this purpose magnesia was chosen; the resulting compound proved to be durable, and gave an agreeable light of moderate intensity. But, alas, after glowing about seventy hours, the mantle failed in its radiance, becoming of glassy and translucent texture. Thus impeded, the untiring inventor turned to mixtures having zirconium as a basis; these not only gave a steady beam, but extended to hundreds of hours the life of a mantle. Still bent on getting more light if he could, Dr. von Welsbach tested thorium oxide with gratifying results; yet, strange to say, when he had purified this material to the utmost, his light fell off in an unaccountable fashion. What could be the matter? Surely in the purifying process some invaluable element had been cast aside. This element, in the researches of an associate, Mr. Ludwig Haitinger, proved to be cerium in minute quantity. Here was a discovery of the highest moment; at the end of many experiments it was determined that one per cent. of cerium and ninety-nine per cent. of thorium oxide are the best proportions for a mantle such as we use to-day. Why these proportions are best nobody knows, any more than why one per cent. of carbon added to iron gives us a steel incomparably better than iron for many uses. A Welsbach mantle has good points apart from its economy of gas. Its combustion is thorough, so that it throws into the air a much lower percentage of injurious products than does an ordinary gas flame. It never smokes, and its light is so steady as to be available for work with the microscope and other exacting demands. It has one defect which may yet be removed: its light has a somewhat unpleasant tinge of green. In another chapter of this book, producer gas, much cheaper than common illuminating gas, is described. Dowson producer gas, with a Welsbach mantle, yields a light of 8 to 10 candle-power with a consumption of 4.5 to 4.8 cubic feet per hour. [Illustration: DR. CARL FREIHERR AUER VON WELSBACH OF VIENNA.] [Illustration: Boivin burner for alcohol, attachable to any lamp.]