SALTS OF ZINC.
210. WHITE VITRIOL, or SULPHAT OF ZINC, is a whitish, yellowish, or greenish white salt, formed by a combination of zinc ([241]) with sulphuric acid ([24]).
Although the white vitriol that is used in commerce is chiefly an artificial preparation, this salt sometimes occurs in a natural state, in mineral repositories that contain blende ([241]); and it appears to be formed by a decomposition of that ore. It is found at Holywell, in Flintshire, and in some parts of Cornwall.
When white vitriol is artificially prepared, the blende is roasted, and thrown, while red hot, into a vessel filled with water; in which it is allowed to remain about eighteen hours. This process is repeated several times; and, after the solution has become clear, it is removed into leaden vessels, and the water is evaporated by means of heat. On cooling, it crystallizes. After this the crystals are melted in a copper vessel, and the surface of the solution is skimmed with a hair sieve. It is then poured into a wooden vessel, and stirred till it becomes cool, and acquires a sufficient degree of consistence, when it is formed into loaves for sale. In this state it has the appearance and colour of refined sugar. White vitriol is chiefly manufactured in Germany.
It is used in medicine; and is employed in great quantities by varnishers, to make their oil varnishes dry more readily than they otherwise would. A fine white colour, called zinc-white, which is more durable than white lead, is prepared from it. Dyers use a considerable quantity of white vitriol to render deeper the colours produced by madder, cochineal, and other substances.
A pleasing experiment is made by mixing in a phial a small quantity of solution of white vitriol with a little liquid ammonia. Though each of the fluids is transparent when separate, yet the zinc will now be immediately precipitated in a white mass; and, what is peculiarly deserving of remark, if then shaken, it will almost as instantly be re-dissolved.