166 HISTOKY OF PHOTOGRAPHY, 



grows rapidly confused, especially if the quantity of liquid applied be more 

 than the paper can easily and completely absorb, or if the brush in applying it 

 be allowed to rest on or to be passed twice over any part. The effect then 

 becomes that of a coarse and ill-printed wood cut, all the strong shades being 

 run together, and a total absence prevailing of half lights. 



To prevent this confusion, gum arable may be added to the prussiated solu- 

 tion, by which it is hindered from spreading unmanageably within the pores of 

 the paper, and the precipitated Prussian blue allowed time to agglomerate and 

 fix itself on the fibers. By the use of this ingredient also a much thinner and 

 more equable film may be spread over the surface; and wJien 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 

 l)eauty 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 manipula- 

 tions of this process are, however, delicate, and complete success is compara- 

 tively rare. 



If sulphocyanate of potash be added to the ammonio-citrate or ammonio- 

 tartrate of iron, the peculiar red color 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 neu- 

 trality 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 afilnity, has, I confess, 

 inclined me to speculate on the possibility of their ferruginous base existing in 

 them, not in the ordinary form of peroxide, but in one isomeric with it. The 

 nonformation of Prussian blue, when their solutions are mixed with prussiate 

 of potash, and the formation in its place of a deep violet-colored liquid of 

 singular instability under the action of light, seem to favor this idea. Nor is 

 it altogether impossible that the peculiar "prepared" state superficially as- 

 sumed by iron under the influence of nitric acid, first noticed by Keir, and since 

 made the suljject of experiment by M. Sccihnbein 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 considered as the 

 radical of that peroxide which exists in the salts in question, and possibly also 

 of an isomeric protoxide. A combination 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 unplausible notion of the chemical nature of that 

 peculiar intermediate oxide to which the name of " Ferrosoferric " 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 sidcriuifi, the 

 oxide in question might be regarded as a sideriate of iron. Both phosphorus 

 and arsenic (bodies remarkable for sequi combinations) admit isomeric forms 

 in their oxides and acids. But to return from this digression. 



If to 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 discoloration begins to relax, and the paper when dry retains 

 a considerable degree of color, 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. .' 



