Early in the day of gas-making it was noticed that gases of various kinds differed much in light-giving quality. It was presently shown that their light depended on the carbon brought to incandescence in a flame; in the absence of that carbon, as when a jet of pure hydrogen was consumed, extreme heat was accompanied by no light whatever. Then came a capital discovery, namely, that lime introduced within a burning jet of hydrogen became intensely luminous while itself but slowly consumed. Adopting lime for the core of his apparatus, Captain Thomas Drummond, of the Royal Engineers, in 1835 devised the lime light. Upon a block of pure, compressed quick lime, he directed a jet of burning gas, obtaining a beam of great vividness still employed in stereopticons and in theatres. For modern types of the Drummond lamp a twin jet of hydrogen and oxygen is used. Lime has many sister substances having light-giving quality when highly heated, and among them are many rare earths, oxides of uncommon elements. These strange substances were destined to play a prominent part in the battle between gas and electricity as illuminants. When Edison in 1878 perfected his incandescent bulb, it seemed as if electricity were soon to be the sole illuminator of houses. But the gas engineers were to be rejoiced by the invention of a mantle which quadrupled the brillancy of a gas flame, withstanding the rivalry of electricity in a notable degree. This mantle was invented by Dr. Auer von Welsbach, a chemist of Vienna, who virtually adopted the principle of the Drummond light. His efforts give us an admirable example of an inventor passing from a hint to a test, day after day meeting new difficulties with unfailing courage and resourcefulness. In 1880 Dr. von Welsbach took up the study of rare earths, mainly with a view to ascertaining their value as illuminants. As he brought one specimen after another to melting heat on bits of platinum wire, he found that the little beads formed were unfavorable in shape to the production of light. Then came into his mind an idea of that golden quality which occurs only to the man who earns it: Why not soak cotton with solutions of salts of rare earths, burn the cotton and leave behind an earthy skeleton of slight thickness and much surface? Experiment proved that the idea had promise, but the skeletons crumbled to dust with the least tremor. For success a fair degree of cohesion was imperative, but to secure that cohesion demanded skill, resource, and patience. After a long series of trials a mantle was made with lanthanum oxide; immersed in flame its beam was particularly bright, now for the first time suggesting that the rare earths might yield light on a large scale. But trouble was at hand, to be overcome only at the end of much toil.