DEVELOPMENT PRINTING.
I. Chemistry of Sun Printing.—Printing on paper prepared with chloride of silver may be taken as an example. This paper is first prepared with a soluble chloride 'of sodium, barium or ammonium ; it is then brushed over with a solution of nitrate of silver strong enough to decompose the whole of the chloride and leave an excess of nitrate. Na. el.+ Ago. No5 Ag. Cl.+Nao. N o5+ Ago. No5. The paper now contains (1) chloride of silver, (2) moisture, (3) nitrate of silver, (4) vegetable fibre, (5) starch or gelatine in the form of sizing, and lastly, nitrate of soda, which, however, produces no effect appreciable in the results. As to their separate influence, it may be said in general terms, that the chloride decomposed by light and mois ture gives a feeble violet image, that the nitrate of silver strengthens it and somewhat darkens the tone, and that the vegetable fibre and size, but especially the size, give a iedder and warmer tone to the colour, with more vigour and greater transparency. What is the chemistry of these effects ? First, as to the chloride. Some have supposed that the chloride is reduced to the metallic state by the entire removal of the chlorine ; others, that the silver loses all its chlorine, but takes oxygen ; others, that it loses half its chlorine and takes oxygen to supply its place, becoming a mixture of sub-chloride and sub-oxide ; and others again, that it loses half its chlorine without taking oxygen. It seems to be a sufficient objection to the idea that in the reaction the silver is oxidized, that oxide of silver, in the presence of water, is reduced to the metallic state by light, and that the violet colour is not at all in dicative of the presence of oxide or sub-oxide, for of these oxides one is brown, and the other black. The doubt, therefore, must lie between the reduction of chloride to the metallic state, or to the state of sub chloride. The chloride of silver reduced to the metallic state by hydrogen in the ordinary methods, is of a dull grey colour, in no degree resembling the violet compound produced by the sun's rays acting on it, but, on the other hand, the sub-chloride of silver which is formed by immersing silver plate into a solution of per-chlo ride of copper or iron is of a deep violet tint, which, when another atom of chlorine is given to it by an aqueous solution of that gas, becomes white. The affinity of chlorine for silver is about equal to its affinity for hydrogen, if we may judge by experiments on them in a heated state ; for red-hot silver will decompose hydrochloric acid, and red-hot chloride of silver will give up its chlorine to hydrogen passing over it. When, therefore, chloride of silver and water act on each other in the light, if the silver be reduced to the metallic state, the light must be powerful enough to decompose water, to prevent the oxidation of the silver, and to cause the hydrogen to remove the chlorine entirely from the salt, by the superior affinity for it in those circum stances. But this is scarcely in accordance with the known power
of light in other similar reactions. And, as the metals which have two degrees of chloridation are known to part withone atom of chlorinemuch more easily than with both, we conclude the reduction to be to the state of sub-chloride only. 2 Ag. CI. Ho. + H. C1.1- 0. It is doubtful whether the atom of oxygen is immediately liberated and escapes into the air, for experiments with the iodine test for chlorine indicate its presence ; it would therefore seem that, as in the electrolysis of water, the liberated oxygen often unites with the water to form peroxide of hydrogen, so in photolysis, the atom of chlorine sometimes remains attached to an atom of water as oxychloride of hydrogen, until set free by other causes.
The action of nitrate of silver is next to be considered. Its own decomposition by the organic matter of the paper will be examined in the next paragraph, with the paper itself ; here we are concerned with the darkness which it adds to decomposing chloride of silver when no such matter is present. It will be found, if chloride of silver is precipitated from an excess of nitrate and exposed to the sun, that it darkens more than if there be no excess of nitrate in the solution. Pure chloride of silver itself is so perfectly opaque, that after very prolonged insolation it is only superficially decomposed, and immediately beneath the surface remains white : the colour is therefore too feeble to produce an effective photograph. This is the case when organic matter, however active, is present, and occurs with all kinds of papers, albumen as well as others, when there is no free nitrate. Organic matter seems to liave no action on the chloride in this case ; and the reason of this is doubtless to be found in the solid and intractable state in which they coexist, for in processes by de velopment where the active organic matter is in solution its influence is very perceptible. The reason why nitrate of silver, added to the simple chloride, increases its darkening power, would appear to be that the hydrochloric acid formed by the light, precipitates from the nitrate new chloride continually upon the surfac,e, which again suffers partial decomposition and renewal. In this manner a larger quantity of the violet salt vvill be formed than if no nitrate assisted in its accumulation.