In recent years radium has been applied to other important purposes, especially in luminous paint for watches, clocks, compasses and other instruments; and this use has so greatly increased in recent years, especially for military purposes, that it now consumes more radium than is used in medicine. Radium salts are more or less luminous when seen in a darkened room, and this quality is often increased by the admixture of certain other materials, notably zinc sulphide. Hence their value in luminous paints. Radium salts also cause certain minerals to fluoresce, notably the zinc minerals willemite and sphalerite. In Germany, where radium during the war became scarce on account of the shortage of the ores from which it is extracted, radium salts are said to have been preserved for medical purposes, and mesothorium and other radioactive substances used in making luminous paints. =Uses of Uranium.=--Uranium is a heavy white metal, which slowly tarnishes on exposure to the air. The chief use of uranium today is as a source of radium. For many years before the discovery of radium, however, uranium compounds were used in a small way in coloring glass and porcelain, in photography, in reagents for chemical analysis, in mordants for dyeing and for other minor purposes. The use of uranium metal in small quantities in steel manufacture has been tried with some degree of success. ORES OF RADIUM AND URANIUM =General Statement.=--The principal uranium minerals at present known in nature, which are therefore the principal sources of both uranium and radium, are carnotite and uraninite, with the impure amorphous form of uraninite known as pitchblende. Torbernite, autunite and some of the rarer uranium minerals have produced a little radium and uranium. Carnotite and uraninite or pitchblende as mined for ores are generally more or less mixed with other materials and are rarely found pure. The uranium in the ores is usually stated commercially, for convenience, in the form of the uranium oxides represented by the formula UO₂ + 2UO₃, briefly expressed as U₃O₈. Most carnotite ore varies from 1 per cent. to 3 per cent. of U₃O₈; a 5 to 10 per cent. ore is considered high grade; a 20 to 40 per cent. ore is remarkably rich. Uraninite and pitchblende ordinarily contain more uranium than carnotite contains, and even in the impure forms in which they are mined as ores, they often show this greater uranium content. The ordinary uraninite and pitchblende ores carry from 2 to 3 per cent. to 8 or 10 per cent. U₃O₈, and a 20 per cent. ore is very high grade, though some ore runs 60 or 70 per cent.