Occurs in monoclinic crystals or in crystalline masses; hardness 5.5; specific gravity 7.4; color dark-brown to black; streak nearly black; luster submetallic; opaque on thin edges. If in crystals the form will serve to distinguish this mineral from cassiterite and ilmenite, the two which it most resembles; but if it is massive the only sure way to decide is to put a piece in strong sulphuric acid; if it dissolves and throws down a yellow precipitate (tungstic acid) it is wolframite. Like the two other minerals mentioned above it occurs in veins in igneous rocks, being associated with high temperatures. As it is almost insoluble in water, like cassiterite and ilmenite, it is likely to occur with them in the sands which are the result of the disintegration of the rocks which carried the minerals; and so a large part of the supply today comes from placer deposits. It is found in Connecticut, North Carolina, Missouri, Colorado, and California. Radium, Uranium and Vanadium These three metals are all rare and occur together. Radium, discovered in 1898, is a heavy metal which has proved very useful because of its radio-activity, that is, its power of giving off or radiating tiny particles of matter known as _X-rays_, part of which are charged with positive electricity, and part of them with negative electricity. The ability of these rays to pass through other substances has made possible photographing the denser substances within those less dense, as the bones within the flesh, or metal within leather or wood, etc. The rays have proved of great value medicinally, and are also used to make objects luminous in the dark. These X-rays are also used in the study of the ultimate structure of matter, as it can be thus obtained in such small units. Uranium is another element which is radio-active and can be used for many of the same purposes as radium. Vanadium, the third of these associated metals, and the commonest of the group, is not radio-active. It is a silvery-white metal, mostly used as an alloy with steel to give it great hardness. Carnotite K₂O·2U₂O₃·V₂O₅·3H₂O Pl. 27 Occurs in earthy masses; color yellow. This mineral is included here, not because it is common, but because it is of such great interest. It is the chief source of supply in the United States of radium, uranium and vanadium. It is a lemon-yellow earth or powder, which looks a little like orpiment. It is however found in a sandstone, instead of where hot waters have deposed minerals. From a ton of this ore about 10 pounds of uranium oxide, 55 pounds of vanadium and ¹/₁₀₀₀th of a gram of radium are obtained. Carnotite is found in south-west Colorado and south-east Utah, and on Carrizo Mountain on the line between Arizona and New Mexico. Mercury