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MAGNETISM.—ELECTRICITY. 73 y the weight of the lid and cup filled with water, w the weight oi the lid and empty cup. Then, approximately, G _ x ~ w _ _ y — w Or, u heing the weight of the air contained in the cup, x + u — w y + u — w 26 k Where extreme precision is required, the exact tempera ture of the air and water should be recorded, and the expansibility °i the body weighed, relatively to that of water, ascertained. Specific gravity is an important distinctive character of minerals, and has been observed with much exactness ; but as it requires a delicate, and not easily portable apparatus, it cannot he ascertained with sufficient readiness and facility to render it useful in the immediate determination of mineral species. No reliance can be placed upon the very rude estimate °t this character when measured only by the hand; and the re sults of weighing by Nicholson’s areometer, even as improved Mobs, must be regarded as little more than a rough ap proximation to the true differences of weight. MAGNETISM. 265. This property is used in mineralogy only to distinguish some of the ores of iron. A feebly magnetized needle may be employed to ascertain the existence, and the degree and kind, of magnetism in a mineral; and also whether it has polarity, which may he known by its attracting one pole of the needle when one part of the specimen is held towards it, and repelling the same pole when another part is presented to it. Cobalt and nickel are slightly magnetic. The following substances, called ‘ magnetic,’ are attracted ty either pole of a magnet:—Iron, nickel, cobalt, manganese, chromium, cerium, titanium, palladium, platinum, osmium. The following, called ‘ diamagnetic,’ are repelled by either Pole of a magnet:—Eismuth, phosphorus, antimony, zinc, tm, cadmium, sodium, mercury, lead, silver, copper, gold, arsenic, uranium, rhodium, iridium, scheeliuin. . The substances are arranged nearly in the order of the intensity of the power by which they are attracted in the former case, and repelled in the latter. ELECTRICITY. 266. This is interesting rather as a physical property than a character of much use in either descriptive or determinative E