164 HISTORY OF PHOTOGRAPHY. 



a greater or less degree, pervades all nature and connects itself intimately with 

 the mechanism by which chemical combination and decomposition is operated. 

 The general instability of organic combinations might lead us to expect the 

 occurrence of numerous an(i remarkable cases of this affection among bodies 

 of that class, but among metallic and other elements inorganically arranged 

 instances enough have already appeared and more are daily presenting them- 

 selves to justify its extension to all cases in which chemical elements may be 

 supposed combined with a certain degree of laxity and, so to speak, in a 

 tottering cquilibriiini. There can be no doubt that the process in a great 

 majority, if not in all, cases which have been noticed among inorganic sub- 

 stances is a deoxidizing one so far as the more refrangible rays are concerned. 

 It is obviously so in the cases of gold and silver. In that of the bichromate of 

 potash it is most probable that an atom of oxygen is parted with, and so of 

 many others. A beautiful example of such deoxidizing action on a nonargen- 

 tine compound has lately occurred to me in the examination of that interesting 

 salt, the ferrosesquicyanuret of potassium, described by Mr. Smee in the Philo- 

 sophical Magazine, No. 109, September, 1840, and which he has shown how to 

 manufacture in abundance and purity by voltaic action on the common or 

 yellow ferrocyanuret. In this process nascent oxygen is absorbed, hydrogen 

 given off. and the characters of the resulting compound in respect of the oxides 

 of iron forming, as it does, Prussian blue with protosalts of that metal, but pro- 

 ducing no precipitate with its persalts, indicate an excess of electro-negative 

 energy, a disposition to part with oxygen, or, which is the same thing, to absorb 

 hydrogen (in the presence of moisture) and thereby to I'eturn to its pristine 

 state under circumstances of moderate solicitation, such as the affinity of pro- 

 toxide of iron, for instance, for an additional dose of oxygen, etc. 



Paper simply washed with a solution of this salt is highly sensitive to the 

 action of light. Prussian blue is deposited (the base being necessarily supplied 

 by the destruction of one portion of the acid and the acid by decomposition of 

 another). After half an hour or an hour's exposure to sunshine a very beauti- 

 ful negative photograph is the result, to fix which all that is necessary is to 

 soak it in water in which a little sulphate of soda is dissolved to insure the 

 Gxity of the Prussian blue deposited. While dry the impression is dove color 

 or lavender blue, which has a curious and striking effect on the greenish-yellow 

 ground of the paper, produced by the saline solution. After washing the 

 ground color disappears and the photograph becomes bright blue on a white 

 ground. If too long exposed it gets " over sunned " and the tint has a brownish 

 or yellowish tendency, which, however, is removed in fixing, but no increase 

 of intensity beyond a certain point is obtained by continuance of exposure. 



If paper be washed with a solution of ammonio-citrate of iron and dried, and 

 then a wash passed over it of the yellow ferrocyanuret of potassium, there is 

 no immediate formation of true Prussian blue, but the paper rapidly acquires 

 a violet-purple color, which deepens after a few minutes, as it dries, to almost 

 absolute blackness. In this state it is a positive photographic paper of high 

 sensibility and gives pictures of great depth and sharpness, but with this pe- 

 culiarity, that they darken again spontaneously on exposure to the air in dark- 

 ness and are soon obliterated. The paper, however, remains susceptible to 

 light and capable of receiving other pictures, which in their turn fade, without 

 any possibility (so far as I can see) of arresting them, which is to be regretted, 

 as they are very beautiful and the paper of such easy preparation. If washed 

 with ammonia or its carbonate, they are for a few moments entirely obliter- 

 ated, hut presentlii rcnppcar with reversed lights and shades. In this state 

 they are fixed, and the ammonia, with all that it will dissolve, being removed 



