PHOTOGRAPHIC CHErIISTRY.—The number of chemical changes that take place in pho tographic operations is so great that it will only be possible to mention the most important. The process that is now almost universally adopted is the gelatino-bromide dry plate process. This has taken the place of a large number of processes and improvements, starting with the Daguerre otype process. In this method of obtaining photographic images a sensitive plate was prepared by exposing a silvered tablet to the action of vapor of iodine, which combines with the metallic silver to form silver iodide, thus : Ag, + I, = 2AgI.
Subsequently bromine vapor was used with iodine. On exposing the plate thus prepared in the camera a latent image was formed, which was made to appear by the vapor of mercury. Sodium hyposulphite was afterwards discovered as a fixing agent.
After the Daguerreotype came the colotype or Talbotype process, named after the inven tor, Fox-Talbot. In this paper was used as the support for the image. It was prepared by first floating on a solution of silver iodide, and then washing slightly. When required for use this paper was excited with gallic acid and aceto-nitrate of silver brushed over it. It was then exposed in the camera, and afterwards developed with the same substances.
Then came the collodion process of Scott Archer, still employed for special work, as in photo-mechanical printing. In this process glass plates were first coated with collodion (a solu tion of gun-cotton dissolved in ether and alcohol) containing an iodide salt,* and then sensitized by immersion in a bath containing a silver nitrate solution. The iodide combining with the silver formed silver iodide, thus : AgNO, + CdI = AgI + CdNO„.
The sensitized plates, after exposure to the image in the camera, were developed with pyrogallic acid with either acetic or citric acids. The negatives thus obtained were fixed with potassium cyanide or sodium hypo-sulphite.
We now come to the modern dry plate process, so called to distinguish it from the collo dion process, in which the plates were exposed in the wet state. The material which forms the sensitive surface of the plate is silver bromide, in some cases mixed with chloride and iodide, and held in position by a gelatine.
Silver bromide is a heavy, almost insoluble, white precipitate, obtained by adding a solu tion of potassium or ammonium bromide to one of silver nitrate. The white flakes of silver bro mide thus formed fall to the bottom of the vessel, and in this form the compound is practically useless for photographic purposes. If, however, a fluid of a viscous nature is used, the silver bromide will not fall to the bottom, but remain in suspension, and in this form is termed an emul sion, which can be used for coating plates or paper. The usual method is to dissolve a quantity of gelatine in warm water, and mix with it the ammonium bromide. The silver nitrate is then gradually added, and a white silver bromide emulsion is produced. The change is thus repre sented : AgNO, + Br = AgBr + NO,.
Now, it must be noted that silver bromide is sensitive to the action of light, so that the addition above described in making the emulsion must, of course, be performed in a non-actinic light.
. When the bromide is added to the silver, a certain quantity will combine with it, and no more. A simple calculation tells us that according to the equation 119 parts by weight of potassium bromide will combine with 170 parts by weight of silver nitrate.* The sensitiveness of the emulsion is increased by boiling it. This has the effect of convert ing the insensitive very fine reddish silver bromide into a very sensitive coarse blue substance.
We have shown that in mixing the bromide with the silver, silver bromide and ammonium nitrate are formed. The next process is to wash the latter soluble compound from the emulsion. This is done by breaking the gelatine emulsion up into small shreds and washing well in several changes of water ; besides the potassium or ammonium nitrate, any soluble bromide is also washed away, leaving the insoluble sensitive silver bromide in the gelatine. It is then remelted, and glass plates coated with it and dried are termed gelatine dry plates.