. Discovery. Science. DISCOVERY 231 experimenters shows that the mcrease in size and sensi- tiveness of the commercial crystals can be obtained by two methods, one being the heating of the emulsion to a considerable temperature, the other tlie digestion of the emulsion at far lower temperatures in tht presence of ammonia, which is a solvent, in a sHght degree, of the silver'halide. But what actuall}- happens to the crystals in the ripening process is still a mystery. One can obtain, for instance, large crystals of compara- tively low sensitiveness, and small crystals of very great sensitivenes
. Discovery. Science. DISCOVERY 231 experimenters shows that the mcrease in size and sensi- tiveness of the commercial crystals can be obtained by two methods, one being the heating of the emulsion to a considerable temperature, the other tlie digestion of the emulsion at far lower temperatures in tht presence of ammonia, which is a solvent, in a sHght degree, of the silver'halide. But what actuall}- happens to the crystals in the ripening process is still a mystery. One can obtain, for instance, large crystals of compara- tively low sensitiveness, and small crystals of very great sensitiveness compared with the usual run of small crystals. Slowing-up Fast Plates Examination of the sensitive crystals by Svedberg, Toy, Sheppard and TriveUi, and others has shown that on the surface of the crj^stals, and frequently at the edges, are " sensitive spots" of possibly adsorbed matter, and it is assumed that these local centres are in some way responsible for the increase in sensitiveness. Clark has recently found that by treating " fast " crystals with chromic acid, and thereby removing the sensitive spots, the^^ become greatly reduced in speed, and that crystals of different degrees of sensitiveness all appear to become reduced to one common level of speed when so treated. This would tend to indicate that ripened silver bromide, per se, is of a very low sensitiveness (about 6 H. and D., or probably much less), and that the speed of the plate is obtained by depositing on the crystals as many as possible of these reduction centres. \Miat they are is 0 0 ^ 8 '.^O" O o O o o. o <\. ^ 0 ⢠»^ o 9 « o â o ^ Fig. I.âcrystals of .\x kmulsion aftbr ripening to a spef,d OF 50 H. AND D. at present very uncertain. They must, of course, be derived from impurities in the water, the gelatin, or from the other raw materials used. The adsorbed matter is extremely small, and may itself be supremely sensi- tive and easily ionised by light, or it may beha
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