=Albumen=, when mixed with a solution of a bichromate, is not thereby coagulated, as occurs upon the addition of pure chromic acid. If poured upon glass and dried it also becomes insoluble in water after a sufficient exposure to light. This solution is used with great advantage in photo-lithography, as the unexposed parts are readily soluble in cold water. The older method, which consisted of using gelatine in conjunction with bichromate, rendered hot water necessary in the development of the fatty ink image upon the transfer paper, and as a consequence of the rubbing at times necessary, the ink, already softened by the warm water, became smeared, the resulting image considerably impaired, and the transfer thereby rendered altogether useless. In collotype a bichromated albumen substratum is still used by many operators, this coating when dry being exposed _through_ the glass plate, the latter placed coated side downwards upon a flat black support, preferably of velvet. After this exposure the gelatine layer—from which the prints are produced—is applied. This method was first published by Albert, who originated it. The chromated albumen solution will not keep long even in the dark,[B] as the albumen coagulates, and on this account photo-lithographic papers coated with this solution must be used within a few hours of their preparation. Such papers are now conveniently prepared with the albumen,[C] and sensitised as required by floating upon an aqueous solution of one of the bichromates, to which has been added twenty-five per cent. of methylated spirit. [B] I have found this solution keep a considerable time—a month at least.—TRANS. [C] I believe Husnik has now discontinued to prepare his photo-litho transfer paper with an albumen coating. Recent issues of his paper have been simply coated with gelatine.—TRANS. The chromated gelatine used in collotype may be kept as a jelly in the dark room, but is better prepared fresh. Of carbon hydrates only gum chromate is of interest to the collotypist, its employment being restricted to the “dusting on” process, and the occasional addition of titles, &c., to the otherwise finished plate. As with all mixtures of the bichromates and organic substances, it is only sensitive to light when in a dry condition, and solutions of it may, without harm, be exposed for some time to diffused daylight. Aqueous solutions of gum arabic soon become sour and mouldy, and in this condition are useless for the “dusting on,” or, in fact, any other photographic process. As a preventive a few grains of sulphate of quinine may be added, which will preserve it effectually, and is quite inert in photographic operations.