Page:The American Cyclopædia (1879) Volume IV.djvu/782

766 COBALT and Pompeii, as specimens found in those places resemble the beads and ornaments of modern times. The first really authentic dis- covery of cobalt appears to have been made in 1733, by the Swedish chemist Brandt, who called it "cobalt king." Chemical analysis had not at that time attained sufficient pro- gress to enable any one to separate the constituents of ores with absolute certain- ty, and it was not till 1780 that the exist- ence of cobalt was confirmed by Bergman. Cobalt is one of the metals found in the at- mosphere of the sun and in meteorites. It usually occurs associated with nickel, arsenic, and sulphur, and is frequently an incidental product in the working of copper, bismuth, and nickel ores. The best known minerals are smaltine, called also speiss cobalt, cobaline, glance cobalt, cobalt bloom, and earthy cobalt. The fact that some of the minerals contain arsenic has led to the application of the name cobalt to the black arsenic sold as a fly pow- der. Metallic cobalt may be prepared from the oxide by heating two parts of the pure oxide of cobalt and one part of pure cream of tartar for six hours in a covered crucible lined with charcoal, and at a temperature sufficient to melt steel. The regulus obtained in this way is exceedingly hard and brittle, has the color of bismuth, is magnetic, and has a specific gravity of 8*43. By remelting in a clay cruci- ble, it can be freed from carbon, and it then has a silver-white color and a specific gravity of 8-754, is softer than steel, very elastic, does not oxidize in air nor after several days' im- mersion in water, and is as magnetic as iron. Becquerel found that by electrolysis a brilliant white metal goes to the negative electrode when the chloride of cobalt is first neutralized with ammonia. Prepared in this way, it is quite pure, and is malleable and magnetic. By treating an aqueous solution of the chlo- ride of cobalt with sodium amalgam, an amal- gam of cobalt is formed, from which the mer- cury can be expelled, and the cobalt obtained in the condition of a fine powder ; it can after- ward be fused to a pure regulus. The metal resembles steel with a slight red tinge, is very hard, and is said by Deville to be more tena- cious than iron. This latter property may hereafter give a value to wires made of cobalt, where it is required to attain great strength in a small compass. Arsenic and manganese render it brittle. Like pure iron, it requires a very high heat to melt it, and the temperature of fusion appears to be between that of iron and gold. Its specific heat is 0-1096, and its density ranges between 8-513 and 8'7. At a high temperature it burns with a red flame, yielding an oxide. Acids generally dissolve cobalt, nitric acid being especially adapted to this purpose. The metal decomposes water at a red heat, but not at ordinary temperatures. Plunged into fuming nitric acid, it is converted into the passive state, the same as iron, and the duration of this passive state is augmented by previously heating the metal. Antimony and cobalt fused together evolve heat and light, and afford an iron-gray alloy. The alloy of cobalt and iron is exceedingly hard. Gold and cobalt yield a yellow and very fragile al- loy. The alloy of platinum and cobalt is fu- sible. Cobalt amalgam is white, like silver, which is rendered brittle by it. Alloys of lead and cobalt and tin and cobalt have been made, but possess little interest. Many chemists suppose nickel to be an alloy of cobalt and some other metal. Fairbairn found that the tenacity of cast iron was greatly reduced by its admixture with nickel, and the same result is probable in the case of cobalt. The latter metal is said to reduce copper from solutions. Weiske found that cobalt was contained in nearly every brand of commercial iron exam- ined, sometimes to the extent of 7 grammes in 100 pounds. Finely divided metallic cobalt is soluble in a boiling solution of caustic potash, and yields a blue liquid, which is supposed to contain cobaltic acid. The finely divided co- balt for this purpose is prepared by heating an intimate mixture of pure oxide with 10 to 12 per cent, of starch meal, or by reducing the oxide with hydrogen. Cobalt contaminated with phosphorus has a different color from or- dinary metal, and loses its lustre in the air. The oxides and salts of cobalt are distinguished for their beautiful colors, red, blue, yellow, and green ; hence they were early used as pig- ments. If a little oxide of cobalt be added to melted glass, we obtain a mass which after cooling is intensely blue. "When this is ground to powder, it yields the well known smalt, which at one time was extensively employed by paper makers and in the laundry. The color is very fast, as it is not affected by the at- mosphere, or by acids or other liquids ; and this fact afforded a method of detecting adul- terations, as sand or pulverized glass which was simply immersed in some coloring liquid could easily be washed clean by acid. Since the extensive and cheap manufacture of arti- ficial ultramarine was established, the demand for smalt has greatly diminished. There is an- other blue color formed by the union of alumi- na and the oxide of cobalt, known as Th6nard's blue, which has long been applied in the arts, but, in consequence of its high price, cannot compete with ultramarine. It can be pre- pared by mixing 3 parts freshly precipitated moist phosphate or arsenate of cobalt with 12 to 15 parts, also freshly precipitated, of hy- drate of alumina, and exposing after drying to a red heat. Thus produced, it is a compact insoluble mass, which can be ground to a fine blue powder. Rinmann's green is a compound of the oxides of zinc and cobalt. It is a much prized green pigment. A beautiful yellow col- or is produced by mixing the nitrate of potas- sium with a solution of cobalt; a double ni- trate of cobalt and potash is produced, in the form of an insoluble yellow crystalline body, which is not only of value as a color, but of-