For dyeing the hair metallic salts are chiefly employed. Owing to their poisonous action the use of _lead salts_ for the purpose is prohibited in some countries, for instance, in Germany and Austria. _Silver_ is used in the form of _nitrate of silver_ (lunar caustic). In the presence of organic substances, as well as under the influence of light, this combination is reduced, metallic silver in small black grains being separated. Silver salts also give a black precipitate of silver sulphide with sulphuretted hydrogen. By simply moistening the hair with silver solution they become brown to brown-black, the coloration appearing more rapidly by previously treating the hair with pyrogallic acid, or, after the application of the silver solution, with sulphydrate of sodium or potassium. The colorations produced with nitrate of silver are very durable, but, if not dyed again for some time, the hair acquires a greenish or reddish color, this being especially the case if they were not sufficiently freed from fat before dyeing. For freeing the hair from fat, wash the hair with a mixture of 1 part spirit of sal ammonia in 10 parts of brandy, and dry carefully. _Copper salts_ with certain substances, such as potassium ferrocyanide solution, potassium sulphydrate, calcium sulphydrate, and pyrogallic acid give dark-brown colorations. Of the copper salts, the sulphate in ammoniacal solution is most frequently employed, though occasionally also the chloride. These salts give a beautiful brown color to the hair. Small quantities of copper salt are also frequently added to the actual black dyes; the hair by this means acquiring a brown-black, instead of a deep black color. All the _iron salts_, with the exception of the chloride, may be used for dyeing the hair. Soluble iron salts by themselves make the hair somewhat darker, but this slow, natural process is not relied on, and a second substance forming dark colored combinations with the iron, such as sulphur, tannin, or pyrogallic acid, is, as a rule, employed. To this class belongs a _Turkish hair dye_, which, according to X. Landerer, is prepared as follows: Gall-nuts converted to a fine powder are mixed with oil and roasted in a pan until no more empyreumatic vapors and odor are evolved. The black powder thus obtained is made into a paste with water, and into it is stirred a finely pulverized mass prepared from ferrous oxide, copper, and antimony, the result being a deep black mass. This dye is called _Rastikopetra_ (eyebrow-stone). It is one of the Oriental cosmetics used by both men and women. _Potassium permanganate solution_ is reduced by organic substances, peroxide of manganese being formed. A concentrated solution of this salt imparts to the hair and skin an intense brown color, which, however, is not very durable, and requires frequent renewing.