Hair-dyes. Various means have been adopted for changing the natural colour of the hair to a more favoured one, and for hiding the approaches of age, as indicated by the presence of gray hairs. These usually consist in washing the hair with a solution of some metallic salt known to have the effect of darkening its colour, such as salts of silver, mercury, lead, and bismuth. Pyrogallic acid is also employed to give a brown tint, while a solution of peroxide of hydrogen in water imparts a fine golden colour. The most perfect mode of dyeing the hair black is that of previously preparing it by a complete soaking with a solution of sulphide of potassium; the strength of this solution must depend on the depth of tint intended to be given; the stronger the solution the darker the colour will be. When thoroughly wetted, the hair is allowed to dry partially; and whilst still damp it is to be saturated with a solution of nitrate of silver, of a strength proportionate to the depth of colour desired. This makes a very permanent dye, which only requires renewing where the new growth of hair becomes conspicuous. The fashion of dyeing the hair is very ancient, and belongs as much to savage as to civilised nations; but in the case of the former vegetable dyes have been chiefly used. In China and other eastern countries the juice of the petals of Hibiscus Trionum, the Bladder-Ketmia, and probably other species of Hibiscus, is in general use.
The detection of stained hair is sometimes an object of medico-legal investigation. Lead may be detected by boiling the hair in dilute nitric acid, and then applying the tests for Lead (q.v.) to the acid solution; while the presence of silver may be shown by digesting the hair in dilute hydrochloric acid or chlorine water, when the resulting chloride of silver may be dissolved out with a solution of ammonia, and submitted to the ordinary tests for Silver (q.v.).