In 1818 the attention of chemists was called to some samples of zinc that were sold for medicinal purposes; they gave, when in solution, a suspiciously yellow color with sulphuretted hydrogen, and hence were condemned as containing arsenic. A number of chemists were furnished with specimens for examination, and several of them detected evidences of a new metal at the same time. Friedrich Stromeyer, professor of chemistry at Gottingen, was the first to publish, in September, 1818, a full account of his investigations into the properties of the substance. He gave to the new metal the name of cadmium. Kar-sten simultaneously proposed to call it meli-nium, from the quince-yellow color of one of its compounds; Gilbert gave it the name of Juno-mium, from the planet Juno; and John christened it Klaprothium, after the chemist Klap-roth; but cadmium is the only name now recognized. The discovery of cadmium forms an era in the line of scientific research. It was the first metal found in a compound and not in an ore, and it could not have been detected until chemical analysis had reached an advanced state of accuracy. Traces of it were soon found in zinc ores, but it was not till 20 years from the time of Stromeyer's publication that an ore of cadmium was discovered.
Lord Greenock at that time described a mineral which had been picked up on his estate, and which proved to be a cadmium blende, analogous to zinc blende or to galena. The new ore was called Green-ockite, and since that time it has been found in various localities; it is, however, a very rare mineral. - For commercial purposes, the metal is obtained from zinc ores and furnace deposits. By subjecting zinc to downward distillation, the first portions that come over often contain cadmium. The pure metal is obtained by dissolving the regulus in sulphuric acid, and converting it into a sulphide, by means of sulphuretted hydrogen; then redis-solving and reprecipitating by carbonate of ammonia, and reducing with a proper flux. As thus obtained, it is a white, soft, malleable, ductile metal. It leaves a mark upon paper the same .as lead, and when bent gives out a creaking sound similar to that known as the "tin cry." It can be distilled the same as zinc; but unlike zinc, when it is set on fire and burns, it gives a brown oxide. It sometimes happens that zinc white is contaminated by this brown powder and rendered worthless as a paint.
Cadmium has a specific gravity of 8-6; fuses at 315° C. (600° F.); and boils at 860° C. (1,580° F.). Specific gravity of its gas, 3.9, or 56 times heavier than hydrogen. - When alloyed with other metals, cadmium causes them to fuse at a lower temperature; a very little of it renders copper perfectly brittle. A composition of 78 parts of cadmium and 22 of mercury was for a long time used for plugging teeth; but, as the amalgam oxidizes easily and turns yellow, and the mercury proves injurious to health, this application is nearly abandoned. Mr. Abel has proposed an alloy for jewellers' use, which is said to be very malleable and ductile, and to possess a fine color. It is composed of 750 parts of gold, 166 of silver, and 84 of cadmium. Wood's alloy, which fuses at 158° F., is composed of 2 parts of cadmium, 2 of tin, 4 of lead, and 8 of bismuth. It is as a yellow paint that cadmium compounds are most highly prized. By mixing a solution of gum arabic, chloride of cadmium, and hyposulphite of soda together, we obtain a fine yellow paint, which is one of the most durable known to artists.
There are other modes of making it, and the purity of color depends very much upon the absence of metals that turn black when mixed with sulphur, and upon the care with which it is dried. The very property that led to the condemnation of zinc white, and which ultimately brought about the discovery of cadmium, is the yellow color, now most frequently turned to valuable account. The keeping properties of the collodion, made sensitive by the iodide and bromide of cadmium, have made these salts great favorites with photographers, and a new use for cadmium has sprung up of late years in this direction. Manufacturers are becoming accustomed to save the furnace and flue dust of zinc works, and separate the cadmium from them, and in this way the supply of the metal. is increasing. The following mixture burns with a brilliant white flame, surrounded by a magnificent blue border: saltpetre, 20 parts; sulphur, 5; sulphide of cadmium, 4; lampblack, 1. This can be moistened, and made up into balls or candles, and ignited after the manner of a fuse. - The salts of cadmium are in general soluble and take crystallized forms. They have no color, but possess a nauseous taste and act as emetics.
The sulphate is obtained by dissolving the carbonate or the metal itself in dilute sulphuric acid, a little nitric acid being added. It is a salt of similar properties to those of sulphate of zinc, but much more powerful. It is used in medicine in the treatment of syphilis, rheumatism, and gout; in diseases of the eyes as an astringent and stimulant, and for the removal of specks and opacities of the cornea. There seems to be still some doubt as to the proper dose for internal use, since some authorities state that it is about equal in power to sulphate of zinc, while others estimate it as being ten times more active.