HAIR-DYE. A substance employed for chang ing the natural color of the hair to a pre ferred one, and for hiding the approaches of age as indicated by the presence of gray hairs. Usual methods consist in washing the hair with a solu tion of some metallic salt known to have the effect of darkening its color. These are the salts or oxides of silver, mercury, lead, and bismuth. Peroxide of hydrogen is used to produce a golden tint. The most perfect mode of dyeing the hair, however, 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 color will be. When thoroughly wetted the hair is allowed to dry partially; and while still damp it is to be again thoroughly wetted with a solution of nitrate of silver, also proportioned in strength by the same rule as in the case of the solution first applied. This makes a very permanent dye, which only requires renewing as the new growth of hair becomes conspicuous. The fashion of dye ing the hair is very ancient, and belongs as much to savage as to civilized nations, but in the case of the former vegetable dyes have been chiefly used; the ladies of China and other Eastern coun tries also resort to the same, the juice of the petals of Hibiscus Trionum and probably other species of hibiscus being in general use with them.
Many hair-dyes cause disease; some kill the hair. and it assumes the appearance of mouse-fur. The practice is rarely justifiable, and almost in variably results in a loss of the esteem and re spect of others.
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 hy• drochloric acid or chlorine water, when the result ing chloride of silver may be dissolved out with a solution of ammonia, and submitted to the ordinary tests for silver (q.v.).