~Uses.~ Copper is extensively used for electrical purposes, for roofs and cornices, for sheathing the bottom of ships, and for making alloys. In the following table the composition of some of these alloys is indicated: COMPOSITION OF ALLOYS OF COPPER IN PERCENTAGES Aluminium bronze copper (90 to 97%), aluminium (3 to 10%). Brass copper (63 to 73%), zinc (27 to 37%). Bronze copper (70 to 95%), zinc (1 to 25%), tin (1 to 18%). German silver copper (56 to 60%), zinc (20%), nickel (20 to 25%). Gold coin copper (10%), gold (90%). Gun metal copper (90%), tin (10%). Nickel coin copper (75%), nickel (25%) Silver coin copper (10%), silver (90%). ~Electrotyping.~ Matter is often printed from electrotype plates which are prepared as follows. The matter is set up in type and wax is firmly pressed down upon the face of it until a clear impression is obtained. The impressed side of the wax is coated with graphite and the impression is made the cathode in an electrolytic cell containing a copper salt in solution. When connected with a current the copper is deposited as a thin sheet upon the letters in wax, and when detached is a perfect copy of the type, the under part of the letters being hollow. The sheet is strengthened by pouring on the under surface a suitable amount of molten metal (commercial lead is used). The sheet so strengthened is then used in printing. ~Two series of copper compounds.~ Copper, like iron, forms two series of compounds: in the cuprous compounds it is univalent; in the cupric it is divalent. The cupric salts are much the more common of the two, since the cuprous salts pass readily into cupric by oxidation. ~Cuprous compounds.~ The most important cuprous compound is the oxide (Cu_{2}O), which occurs in nature as ruby copper or cuprite. It is a bright red substance and can easily be prepared by heating copper to a high temperature in a limited supply of air. It is used for imparting a ruby color to glass. By treating cuprous oxide with different acids a number of cuprous salts can be made. Many of these are insoluble in water, the chloride (CuCl) being the best known. When suspended in dilute hydrochloric acid it is changed into cupric chloride, the oxygen taking part in the reaction being absorbed from the air: 2CuCl + 2HCl + O = 2CuCl_{2} + H_{2}O. ~Cupric compounds.~ Cupric salts are easily made by dissolving cupric oxide in acids, or, when insoluble, by precipitation. Most of them are blue or green in color, and the soluble ones crystallize well. Since they are so much more familiar than the cuprous salts, they are frequently called merely copper salts. ~Cupric oxide~ (CuO). This is a black insoluble substance obtained by heating copper in excess of air, or by igniting the hydroxide or nitrate. It is used as an oxidizing agent.