A Manual Of Photography

BROMIDE OF SILVER. 131 falls but slowly to the bottom of the vessel, it will be found that it is sensitive to the weakest solar radiations. There does not appear to be any chemical difference between the iodides thus obtained; but there are some remarkable physical peculiarities, and it is believed that attention to these will be found eventually to be of the utmost importance. Section IV.—Bromide of Silver. In many of the works on chemistry, it is stated that the chloride is the most sensitive to light of all the salts of silver; and, when they are exposed in a perfectly formed and pure state to solar influence, it will be found that this is nearly correct. Modern discovery has, however, shown that these salts may exist in peculiar conditions, in which the affinities are so delicately balanced as to be disturbed by the faintest gleam; and it is singular that, as it regards the chloride, iodide, and bromide of silver, when in this condition, the order of sensibility is reversed, and the most decided action is evident on the bromide before the eye can detect any change in the chloride. The slight additional expense of the bromides is not worthy consideration, particularly as their use may be confined to papers for the camera obscura, the pictures on which are of course of the negative character, and the positive photographs can be formed by transfer on the chloridated papers of a highly sensitive kind. Since there has been some question as to the use of the iodide of silver without an infringement of patent, it is not a little surprising that the bromide has not been more generally employed. It will be found that the bromide and iodide are much alike in the singular want of sensibility which they sometimes exhibit under the circumstances already alluded to, which are not easy of explanation. If a paper first washed with a solution 205m of nitrate of silver has bromide of potas- sium applied to it in different propor- tions, say 20 grains, 15 grains, and 2Sgrs 10 grains each, in two drachms of water, and, when dry, be again washed over with the silver solution, it will be found, 50^74 unless, as is occasionally the case, some organic combination interferes, that the order of sensitiveness will begin with the 18. weakest solution, the strongest being the least influenced by