A Manual Of Photography

66 HISTORY OF PHOTOGRAPHY. admit isomeric forms in their oxides and acids. But to return from this digression. " ^^ ^o a mixture of ammonio-citrate of iron and sulphocyanate of potash, a small dose of nitric acid be added, the resulting red liquid, spread on paper, spontaneously whitens in the dark. If more acid be added till the point is attained when the discolora- tion begins to relax, and the paper when dry retains a conside- rable degree of colour, it is powerfully affected by light, and receives a positive picture with great rapidity, which appears at the back of the paper with even more distinctness than on its face The impression, however, is pallid, fades on keeping, nor am I acquainted at present with any mode of fixing it. ^ paper be washed with a mixture of the solutions of ammonio-citrate of iron and ferrosesquicyanate of potash, so as to contain the two salts in about equal proportions, and being then impressed with a picture, be thrown into water and dried, a negative blue image will be produced. This picture I have found to be susceptible of a very curious transformation, pre- ceded by total obliteration. To effect this it must be washed with solution of proto-nitrate of mercury, which in a little time entirely discharges it. The nitrate being thoroughly washed out and the picture dried, a smooth iron is to be passed over it, somewhat hotter than is used for ironing linen, but not sufficiently so to scorch or injure the paper. The obliterated picture immediately reappears, not blue, but brown. If kept for some weeks in this state between the leaves of a portfolio in complete darkness, it fades, and at length almost entirely dis- appears. But what is very singular, a fresh application of the heat revives and restores it to its full intensity. “This curious transformation is instructive in another way It is not operated by light, at least not by light alone. A certain temperature must be attained, and that temperature suffices in total darkness. INevertheless, I find that on exposing to a very concentrated spectrum (collected by a lens of short focus) a slip of paper duly prepared as above (that is to say, by washing with the mixed solutions, exposure to sunshine, washing, and dis- charging the uniform blue colour so induced as in the last article), its whiteness is changed to brown over the whole region of the red and orange rays, but not beyond the luminous spec- trum. Three conclusions seem unavoidable:—1st, that it is the heat of these rays, not their light, which operates the change; 2ndly, that this heat possesses a peculiar chemical quality which is not possessed by the purely calorific rays outside of the visible spectrum, though far more intense ; and, 3rdly, that the heat radiated from obscurely hot iron abounds especially in rays