But the nitrate of silver also modifies the colour and vigour by its specific reaction with the fibre and size of the paper. Cotton and linen fibre have both an affinity for many metallic oxides, alumina, oxide of iron, oxide of tin, &c., and among others for the oxide of silver ; this affinity is sufficient to cause a decomposition of nitrate of silver in the light, though the light, unaided, has no power to decompose that salt, either in the solid state or in solution. The same solution of nitrate of silver which would of itself remain clear and bright for an indefinite period in the sun, will be very- quickly reduced by light when in contact with a piece of Swedish filtering, paper or pure cotton wool. The oxide of silver combines with the fibre, and the nitric acid is set at liberty as may be perceived by its smell. The warm brown oxide therefore mixes, and perhaps combines with the subchloride, to intensify the deposit, and at the same time render the colour less cold and slatey. The organic matter with which the paper is sized is still niore efficient in these respects, and so are the albumen, serum, and other substances with which papers are often purposely imbued. The principal classes of organic matt,er are, the lignines, the starches, the gums, the sugars, the gelatines, the proteiniferous bodies, the neutral and acid principles of fats, essential oils and liquid hydro-carbons, the resins and camphors, and the vegetable acids. These include an almost infinite variety of sub stances all varying in their behaviour in photographic processes. In sun-printing their action does not appear to depend upon the chlorine, combining with their elements in the light, but entirely on their power of forming compounds with the elements of nitrate of silver. All appear to possess this faculty in some measure or other. Generally they form soluble compounds with the alkalis, and insoluble ones with lime, baryta, and silver. Those which do not combine with oxide of silver to form an insoluble organic salt, do not produce so marked an effect in printing as those which do. Lignine, therefore, and starch, and even gelatine, are not so efficacious in giving warm, vigorous images as those bodies which contain protein, such as albumen, casein, and emulsine, but they require less after treatment to produce the blacks and purples, which are in general request. For this reason the French papers, prepared with starch, are by many preferred to • the English, sized with gelatine and resin ; and for the same cause, prints on albumenized paper are very red and difficult to tone. Sun-printing is thus a double reduction, where a violet subchloride and a red organic subsalt of silver are formed simul taneously, the resulting picture partaking of the colour and proper ties of both. That the red salt is a salt of the suboxide, as the violet is a subchloride, is shown by the fact that when citrate of silver is sub jected to the action of hydrogen at the temperature of 212°, the pro toxide contained in that salt loses half its oxygen, and a red brown salt is formed, just as when the citrate is decomposed by exposure to light. In both cases the hydrogen present removes half the oxygen. Many other organic salts of silver, according to Professor Graham, comport themselves in the same way when heated in hydrogen. Another proof is found in the fact, that the subsalt formed by hydrogen is soluble in water to some extent, giving a brownish solution ; so are the impressions on our albumenized and other papers weakened by the solvent action of water, especially when warm. It might at first sight he supposed, that since these organic salts of silver are thus easily decomposed by light, while the corresponding salts of lead and other metals are not so affected, that the silver was in some way the element acted upon by the light, and that the decomposition was not determined by the hydrogen and oxygen attracting each other. But the difference between lead and silver in these cases is probably found in the different condition of their oxides, the lead having no tendency to part with half its oxygen, as the silver does. Other metals which have two oxides, chlorides, &c. behave many of them like silver in this respect ; for example, the ammonio citrate of iron, salts of the sesquioxide of uranium, the ferrosesquicyanide of potas sium, the bichloride of mercury, and others. It may be that the second atom of chlorine or oxygen exists in these compounds in its allotropic form, and that therefore its removal by light is possible, whereas the removal of the second atom by the same means is impossible. • The chloride of silver is the only silver salt employed in sun-print ing on paper. The bromide and iodide cannot be so used, on accountof important differences in their chemical relations. If a paper, prepared as the chlorided papers in all respects, except the substitu tion of iodine for chlorine, be exposed to light, it is found impossible to get beyond a feeble grey or drab-coloured impression. The che mistry of this change is more obscure than that which oc,curs with chlmided surfaces. The following is a plausible explanation. Pure iodide of silver, exposed in the dry state to the action of the atmo sphere and light, remains unchanged, and is not reduced to the con dition of subiodide, as might be expected. Calotype papers, which have be,en prepared either by what is called the double or single process, may be thus exposed without change, for any period, to sun shine, but similar chlorided papers are immediately discoloured. Well-washed iodized collodion plates are equally incapable of re ceiving any visible impression; and, so far as any actual decomposi tion is concerned, pure moist iodide of silver may be pronounced insensitive to light. This seems to confnm very strongly what has been said respecting the mode in which the removal of half the chlorine of chloride of silver is brought about ; for if the light acted immediately on the elements of the compound to separate them, it ought is fortiori to decompose iddide of silver where the connecting affinity is weaker. But if it be allowed that an atom of water must
intervene, then it is plain why iodide of silver is not affected by the solar rays. Iodine dissolved in water and exposed to light, has not the power to overcome the more powerful affinity of hydrogen for oxygen, but on the contrary, a solution of hydriodic acid exposed. to the air gives up the hydrogen of the acid to the oxygen of the atmosphere and sets the iodine at liberty. The great difference, therefore, between chlorine and iodine lies in the fact that the for mer has a power of liberating oxygen from water in the lig,ht, of which iodine is destitute.
When, therefore, papers prepared with iodide of silver and nitrate of silver darken in the sunshine, an idea might be entertained that the darkening proceeded from the nitrate only ; but this would not be correct, because papers imbued with both salts darken more quickly than if the nitrate alone is present, and also because iodide of silver, precipitated from an excess of the nitrate in a test tube, will be found to change colour under the sun's action, while nitrate in such circumstances is not darkened except the light is assisted by organic matter. Some other reaction must consequently occur in this instance between the iodide and the nitrate in solution. It is found on trial, -that whatever change takes place there is no libera tion of iodine when iodide of silver and solution of nitrate are acted on by light, nor of any of the elements, except nitric acid, which re mains in the solution ; whatever change takes place is in their ar rangement only, for in the dark the yellow colour is restored, except organic matter interfere to prevent : the yellow salt may be thus darkened, and restored many times in succession. A similar change oe,curs when a brown aqueous solution of iodine is exposed to light : no oxygen is given off, as has be,en already observed, yet very slowly the brown colour disappears, but is gradually reproduced in the dark. When the colourless solution is examined, it is found that the iodine, not being able to displace oxygen, has c,om billed with both elements of the water in such a way as to form hydriodic acid with the hydrogen, and iodic acid with the oxygen : 5 H 0 + 6 I=I 05 + 5 H I. These acids form colourless solu tions. But in the ordinary course of chemistry, hydriodic acid is decomposed very quickly by iodic acid, water being formed and iodine set fre,e. This sufficiently explains the reappearance in the dark of the brown colour which the light destroyed. When free iodine is added to a solution of nitrate of silver, if iodide of silver only were formed, then, besides nitric acid, an atom of oxygen would be liberated ; for A g. 0 +N 0, +I=Ag. I +N 05 + 0 ; but here also, iodine not having the power of liberating oxygen, forms with it iodic acid, and so iodate of silver : 6 Ag. 0 N 05 + 6 I=Ag. 0 I 05 + 5 Ag. I+ 6 N 05. The reason, therefore, why light has not suffi cient power to decompose iodide of silver in the presence of water only, and yet can so decompose it when nitrate of silver is also pre sent, is tolerably obvious. In addition to the tendency which iodide of silver has in the light to give up iodine to form hydriodic acid and iodic acid, another force is introduced viz., the attraction which the oxide of silver in t,he nitrate has for those two acids ; and it is easy to suppose, that though either force separately might be insuffi cient, yet the two in union might be able to complete the change. The formula of the decomposition would be 6 Ag. I+ 6 Ag. 0 N 05 =Ag. 0 I 05 + 5 Ag. I+ 6 N 05 + 6 A g., if we suppose the silver is reduced to the metallic state ; or if we suppose it to become re duced to subiodide only, which seems more probable, the 6 atoms of metallic silver which are shown in the formula will be united to 6 atoms of undecomposed iodide. In the dark, if no organic matter prevent, this arrangement of the elements is altered and nitrate and iodide of silver reformed. When organic matt,er, such as paper, is present, the reaction will be different if an organic salt of silver take part in it instead of the nitrate. This organic matter may have such an affinity for the atom of oxygen, the non-liberation of which causes the above formulce to be so complex as to cause an entirely different change, or it may by its affinity for the subiodide simply maintain the arrangement of the salts which light has induced. The former is probably its action in developed prints, the latter in sun prints. We cannot wonder if this be the correct view of the dark ening of iodide of silver, that it should be carried to a much less degree than in the case of chloride, where not a fresh arrangement of the salts merely but actual release of one element occurs. The light is barely sufficient in the iodized paper to overcome the incli nation which the salts have to return to their first condition, and this inclination increases as the iodate and the free nitric acid ac cumulate ; the reduced subiodide is of a dingy colour, with very little force ; and the quantity of nitrate of silver must be small, because strong nitrate of silver has the property of dissolving iodide to such an extent that, if allowed by evaporation to become concentrated, the whole surface of the paper would be whitened and its sensitiveness destroyed ; and the organic matter has no influence directly on the iodide, any more than it has on the chloride. For these reasons the chloride is universally employed in sun-printing.