II. Development-Printing. — By development is meant the rendering visible an invisible actinically formed image, or the intensifying a visible one, by means of chemical reagents acting by their ordinary chemical affinities independent of the light. Both visible and invisible impressions of light may be developed. A body whose presence in a certain preparation is essential to its modification by light, is called a sensitizer ; if not essential to the actinic influence taking effect, but only favouring the production of the effect in a shorter time, it is an accelerator ; if itself decom posed in such a way by the substance which has already been modified or decomposed by light as to attach itself to it, or in any other way render the effect perceptible or plainer than before, it is a developer. The same compound may fill all three offices, but its use is different in the three cases. It is important to remember that the developer is always a compound, either decomposing spontaneously, or suffering decomposition by the substance produced by light in the sensitive surface. In the usual processes of photographic printing by development, the developer is always a mixture of solutions which are gradually letting fall a precipitate by their action on each other. The principle of development is therefore altogether different from sun-printing, and the chemistry proceeds on altogether a different principle. The power which is continually acting in sun-printing is the power of light in causing certain elements to unite : the force resorted to in development is simply the attraction of cohesion, by which atoms which are homogeneous tend to cling together. So entirely is this true, that after a print has been developed, washed, altered by a fixing solution, and washed again, after it has been dried and knocking about for months, the process of development may be continued. When the image has, therefore, once been formed in the camera, or the pressure frame, all that is done further is to pour over them a mixture from which silver, in some form, is being very slowly precipitated, so that at the moment when the precipitate is being liberated from the solution it finds itself in contact with the particle to which it has a tendency to adhere, just as when chloride of silver is being precipitated from the nitrate, the particles which are within each other's sphere of attraction, cohere to form a flocculent mass instead of going separately to the bottom. All that is necessary pre vious to development is, that centres of attraction shall be formed by the change which light has caused, having this affinity for the particles which are to be precipitated upon them. What are the particles which are thrown down by the usual developers ? These developers are nitrate of silver mixed with either sulphate of iron, gallic acid, or pyrogenic acid.
When nitrate of silver and sulphate of iron are mixed in solution, the iron takes the oxygen of the silver and the silver is precipitated in the metallic state. When gallic or pyrogallic acid is used, the precipitated silver is found to be combined with a small quantity of organic matter, which has not been examined. The image differs therefore in the three cases, hut until a proper analysis has been made, we cannot do more than point out the more truly metallic nature of the image formed when a protosalt of iron is employed, than when recourse is had to an organic deoxidizer.
Both chloride and iodide of silver have been extensively used for printing by development. The same objection does not lie against the iodide, in this mode of producing pictures, that was found in sun printing. Though giving a very feeble direct print, it is found that the incipient image is more quickly formed, and the reason may be easily drawn from what has been already remarked. A rearrange ment of the elements, and a production of suboxide sufficient to develope upon, is very speedily effected, and that long before any impression is perceptible to the eye, and when the developer is applied, having A strong affinity for oxygen, the atom of oxygen which prevented the formation of iodide without iodate is removed and the development proceeds rapidly. But in the case of chloride, the de composition, because more complete, requires a stronger and more pro longed action of the light, and the image must be quite visible before the development commences.
Two effects are observed in printing by development on iodide of silver which do not occur on the chloride. These are such a reversal of the usual manner of impression by the light that, on development, the precipitating silver is adherent on the parts which have had the least intense light upon them, instead of those which have been most vividly illuminated ; and the other is a reddening of the image in the parts which have been struck by the highest lights. They are pro bably, both of them, to be accounted for by changes produced in the sensitive surface, by light of a certain intensity and duration, in which the elements return to their unimpressed state, or some element is actually eliminated. The iodide of silyer with the nitrate is subject, as has been shown in the paragraphs on sun-printing, to hover between two states, either of which it may be made to assume at pleasure. When acted upon by light, iodate, iodide, and subiodide of silver are formed, and nitric acid liberated. This liberation of nitric acid, against its affinity for oxide of silver, must have a retard ing effect upon the gradual ehange operated by light. That the state of the substance,s after insolation is somewhat forced and strained when judged of by ordinary chemical affinities, is shovrn by the fact, that darkened iodide of silver, in the absence of organic matter, will return to the yellow form ; and therefore, as the nitric acid increases in quantity and in power, with the intensity and duration of the illumi nation, a time is likely to arrive when it would suddenly upset the constrained relations of the parts, and reproduce the original arrange ment. In this case, the most strongly lighted parts would receive little or no deposit in the development. This view of the matt,er is confirmed by the circumstance, that it is when a bath of nitrate of silver, containining much nitric acid is used, and when little or no organic matter is present that this reversal takes place. It has been produced and remedied many times successively, and with absolute uniformity, by alternately using a large comparative quantity of nitric acid, and of acetic acid, nor is it observed except when pictures of a leaden metallic appearance are being developed, the colour and feebleness of which are indicative of nitric acid and metallic silver, free from organic admixtures. It is also sometimes purposely produced by exposing the plate for an inst,ant to the light at the very commencement of the development. This momentary exposure perhaps acts just as the intenser light in the more exposed parts of the image, to which we have adverted. If this explanation be correct, a true alternation ought to take place, in the effect, since when the particles are again placed in the status quo ante the farther impact of light would operate as at first. Some experi menters have said that this regular alternation of effects really occurs. A similar phenomenon is observed when hydrochloric acid is subject ed to electric sparks : the hydrogen and chlorine are separated until a c,ertain point is reached, and then another spark causes their instant and explosive recombination. The effect seen in the reddening of the skies of landscapes and of the strongest lights in portraits may be the same thing, prevented by organic matter from fully taking effect, or more probably be only the consequence of more violent action upon the developer.
These observations show that if the use of iodide of silver be at tended by the great advantage of sensitiveness, the sensitiveness is itself the cause of disadvantages which it requires skill to obviate. On chloride of silver the sun-printing and development may be mixed in any proportion, and to the experimenter's taste without entailing any of these inconveniences.
Space will not allow any detailed explanation here of the che mistry of the fancy photographic processes, or even of the number less interesting photo-chemical experiments on particular salts which have been recorded ; they are briefly noted in their own places. The principles above laid down are the main clue to the interpreta tion of all.