COBALT, in chemistry, a metal, when pure, of a white colour, inclining to bluish or steel grey. At the common tempera ture its specific gravity is more than 8.5. It is attracted by the magnetic needle, and is itself capable of polarity. For fusion it requires nearly the same intensi ty of heat as cast iron. Ina state of oxide, it tinges the saline vitreous fluxes of a deep blue colour. It is soluble in nitro muriatic acid, and the diluted solution forms a blue sympathetic ink. Cobalt occurs in nature alloyed with other me tals, and mineralized by oxygen, and by arsenic acid. The white cobalt ore is an alloy of cobalt and arsenic, with a little sulphur, and in some specimens a little iron, the two latter being proba bly accidental. One variety, analyzed by Klaproth, gave 44 of cobalt, 55.5 of arse nic, and 0.5 of sulphur. Its colour is tin white, liable, however, to tarnish, and thus to assume a grey or reddish tinge : its lustre is weakly shining and metallic.
The grey cobalt ore, as it has been named, is an alloy of cobalt with arsenic and iron sometimes, also, as has been affirmed, with small portions of nickel and bismuth. Its colour is light grey, but very liable to tarnish ; its lustre weakly shining and metallic. Exposed to the flame of the blow-pipe, it gives an arsenical odour and smoke, but with out melting : to borax it gives a blue colour, and is reduced to a metallic globule. The native oxide of cobalt oc curs in a powdery form, or of various degrees of induration, but always dull, and earthy in its fracture, soft, and easily broken. It is also of different colours, from the intermixture of oxide of iron and perhaps other metallic oxides : whence even species have been formed and dis tinguished by the names of black cobalt ochre, brown cobalt ochre, and yellow co balt ochre. Of these the black appears to be the oxide of cobalt in its purest state. They all give a blue colour to glass, or to borax, when fused with it by the blow-pipe. Sometimes also they ex hale an arsenical odour. The last species is that in which cobalt is mineralized by arsenic acid, the :irineipal variety of which has been named peach-bloom co balt ore: This name it derives from its colour, which is a beautiful red, simi lar to that of the peach blossom, passing, however, into other shades of' red, and from decomposition into other colours. The ores of cobalt are easily distinguish ed from all others, by their property of communicating to borax or to glass, when fused with them, a deep blue colour ; and by their solution in nitro.muriatic acid, being a sympathetic ink, lines traced with it on paper not being visible when cold, but becoming visible on exposure to a moderate heat.
On a large scale, cobalt is extracted from its ores only in the state of an oxide, without being reduced to the metallic form, not only as this reduction is dif ficult, but also as the metal is not appli ed to any use. The ore is roasted, by which the sulphur and arsenic are expelled, and any fusible metal mixed with it is melted out. The cobalt remains in the state of
an impure oxide, named zaffre. The zaffre of commerce is always mixed with silicious earth ; hence, if exposed to a strong heat, it vitrifies ; a glass of a dark blue colour is thus formed, named smolt, which is used on account of its colour in various arts. It is from the zaffre of commerce that the chemist obtains co balt ; to obtain it pure, however, is ex tremely difficult. The common process is, to mix the zaffre with three times its weight of black flux, a small quantity of oil, and a little sea salt, and expose the mixture in a crucible to a strong white heat for some hours. A metallic button is thus obtained, on cooling, at the bot tom of the crucible ; but the cobalt pro cured is generally alloyed with arsenic and nickel, and sometimes with other metals, particularly iron.
A number of the acids omdise cobalt, and combine with its oxide. The con ce ntratedsul ph u ric acid on it in the cold, but when boiled on the metal, sulphurous acid gas is disengaged, and a saline matter is obtained, which, when lixiviated, forms a solution of sulphate of cobalt. Nitric acid is decomposed by cobalt, and the metal is oxydized and dissolved. The solution is of a red colour, and by t entle evaporation affords minute prismatic crystals of the same colour, which are deliquescent and decomposed by heat. Muriatic acid does not act on cobalt, but with the assistance of beat ; a small portion of the metal is then dis solved. The solution of muriate of co balt affords a celebrated sympathetic ink. When much diluted, if letters are traced with it on paper, and allowed to dry, they are invisible ; but when the paper is exposed to a moderate heat, they ap pear of a lively green : they disappear again when cold, and the experiment may be repeated for any number of times, ta king care only to avoid too strong a heat, by which they are rendered permanent. The cause of this phenomenon has been ascribed to the muriate of cobalt fixed upon the paper attracting, when cold, moisture from the atmosphere, by which it is, as it were, dissolved, and rendered invisible : when heated, this moisture is evarorated, and the green colour of the salt appears. This explanation appears to be confirmed by the fact, that the characters are rendered visible by con fining the paper in a vessel with quick lime, or sulphuric acid, either of which attracts humidity powerfully. The green colour cannot, however, be ascribed en tirely to the concentration, but is owing to the temperature ; for the solution itself becomes green when moderately heated in a close phial, and loses this green co lour as it cools ; nor is it easy to explain bow the temperature does produce this change of colour.
Cobalt combines with many of the me tals. Its alloys are generally brittle, and none of them has been applied to any use; nor have they been much examined. The principal, or, indeed, almost all the sole use of cobalt, is in communicating a blue colour to glass, enamel, and por celain.