ACCELERATORS. This name is given to any substances used in photographic processes, with a view to shorten the time of ..v posure, either in the camera or the printing-frame. Ir daguerreotype, bromine and chlorine, and especially bromine, exer cise a'very decided accelerating action, the time of exposure being reduced from minutes to seconds. They require dilution, and there fore must be held in solution, or retained by some solid boclies which easily give them off in vapour. Under whatever names the accelerators used in this process may pass, Woolcott solution, Hun garian solution, chloro-bromide of lime, &c., the active principle is the same in all. When bromine and chlorine are used in their com binations in the calotype and other processes by solutions, they have no such quickening power. It is probable that the pure iodide is not quickened by any addition.
The substances which have been recommended to increase sensi tiveness in these processes, may be divided into five classes. Taldng iodide of silver as the basis of all quick formulte, the first class includes the other insoluble salts of silver, such as the bromide, fluoride, &c., which have all in turns been recommended, but with out good reason. The fact that these salts give a visible image more quickly than the unmixed iodide, has been taken as a proof of greater sensitiveness ; but the truth is, the iodide in its best state, though the last to give out an image to the eye, is the first to receive one capable of development by the usual developers. The second class includes the easily decomposed soluble salts of silver, as the nitrate and acetate. The presence of one or more of these is essential to forcible development, and this is their principal use ; but they are also said to accelerate, in a slight' degree, the action of light itself. The third class is composed of alkalis and basic salts. There can be no doubt that acids, in proportion to their oxidizing properties, restrain or neutralize the chemical action of light in the sensitive film. But beyond neutralizing any acid present in the
film, alkalis do not appear to modify the active power but rather to exert their OW11 peculiar action in decomposing the sensitive layer itself. Fourth : organic matter of many kinds is said to accelerate, as honey, sugar of milk, &c. Twenty things have been spoken of, but the best experiments prove that, though they add, by their power of combining with reduced silver, to the visibility of the scarcely apparent picture, and to its final vigour, their action is really to retard, and not accelerate. Still, their presence, in proper propor tion, is essential in all negative processes, and in all cases, except the delicate silver film of the collodion positive, where intensity is not required and where organic matter gives drab tones. The last class consists of bodies having a strong desire for more oxygen, and which therefore are, or may be, used as developers. Gallic acid, used in the calotype and in several procesess, and iodide of iron, which, when the film is formed, becomes nitrate of iron, are supposed to act as accelerators; but the effect is more to cheat the operator, by carrying on the development simultaneously with the exposure, than to favour the real ictus of the solar ray.
The truth seems to be, that a certain finely divided, semitrans parent, pappy and neutral condition of iodide of silver is obtain able, which with a minute excess of nitrate, in our wet processes, gives the most sensitive surface yet discovered. There are certain mechanical conditions, and conditions of temperature, molecular arrangement, proportions, dryness, optical correctness, and easiness and smartness of manipulation, atmospheric pressure, and other in fluences, which all affect the impressibility of a chemically prepared layer ; but these will be better treated under the word " Sensi tiveness," q. V.