A Manual Of Photography

SIR J. HERSCHEL’S CYANOTYPE. 55 manageably within the pores of the paper, and the precipitated Prussian blue allowed time to agglomerate and fix itself on the fibres. By the use of this ingredient also, a much thinner and more equable film may be spread over the surface ; and when perfectly dry, if not sufficiently developed, the application may be repeated. By operating thus I have occasionally (though rarely) succeeded in producing pictures of great beauty and richness of effect, which they retain (if not thrown into water) between the leaves of a portfolio, and have even a certain degree of fixity— fading in a strong light, and recovering their tone in the dark. The manipulations of this process are, however, delicate, and complete success is comparatively rare. “If sulphocyanate of potash be added to the ammonio-citrate or ammonio-tartrate of iron, the peculiar red colour which that test induces on persalts of the metal is not produced, but it appears at once on adding a drop or two of dilute sulphuric or nitric acid. This circumstance, joined to the perfect neutrality of these salts, and their power, in such neutral solution, of enduring, undecomposed, a boiling heat, contrary to the usual habitudes of the peroxide of iron, together with their singular transformation by the action of light to proto-salts, in apparent opposition to a very strong affinity, has, I confess, inclined me to speculate on tire possibility of their ferruginous base existing in them, not in the ordinary form of peroxide, but in one isomeric with it. The non-formation of Prussian blue, when their solutions are mixed with prussiate of potash, and tire formation in its place of a deep violet-coloured liquid of singular instability under the action of light, seem to favour this idea. Nor is it altogether impossible that the peculiar 1 prepared ’ state super- ficially assumed by iron under the influence of nitric acid, first noticed by Keir, and since made the subject of experiment b} M. Schönbein and myself, may depend on a change superficially operated on the iron itself into a new metallic body isomeric with iron, unoxidable by nitric acid, and which may be consi- dered as the radical of that peroxide which exists in the salts in question, and possibly also of an isomeric protoxide. A combi- nation of the common protoxide with the isomeric peroxide, rather than with the same metal in a simply higher stage of oxidation, would afford a not implausible notion of the chemical nature of that peculiar intermediate oxide to whieli the name of ‘Ferroso-ferric’ has been given by Berzelius. If (to render my meaning more clear) we for a moment consent to designate such an isomeric form of iron by the name siderium, the oxide in question might be regarded as a sideriate of iron. Botli phos- phorus and arsenic (bodies remarkable for sesqui-combinations)