A Manual Of Photography

128 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. with their appropriate neutral salts, and, when dry, applying the nitrate of silver as usual. The results, however, were in no wav striking, as regards sensitiveness, in any case hut in that of th/muriatic applications. In all cases where such applications were used, a paper was produced infinitely more sensitive than any I had at that time made. And I may here observe, that in this respect the muriate of strontia appeared to have decided advantage.” , .. u It would he tedious and useless to mention al the combina- tions of alkaline and earthy muriates which have been devised to vary the effect, or increase the sensitiveness, of the silver pre- parations : the very considerable differences produced throilgh the influence of these salts will afford peculiarly interesting results to anv inquirer and furnish him with a curious collection of photographic specimens. As a general rule the solutions of the muriate, and indeed al1 other salts, and of the silver washes should be made in the combining proportions of the mate used With a scale of chemical equivalents at hand, the photo graphic experimentalist need not err, taking care that a slight excess of pure nitrate of silver prevails. These changes being understood, we may proceed to the con- sideration of the preparation of the muriated papers employe for the ordinary processes. . n Muriated paper is formed by producing a ehloiide of silver on the paper. This is done by washing the paper in the first place with the solution of muriate of soda, and then, when the naner is dry, with the silver solution, which it is sometimes necessary to apply twice., In this process, which requires more care than may be at first conceived we often suffer from the annoyances which arise from unequal texture of the paper, and also from the want of uniformity in the distribution of the salts over the surface. It will not unfrequently be found that irregular patches, with sharply defined outlines, will appear on the paper, exhibiting a much lower degree of sensibility than the other parts of the sheet These patches have been attributed by Sir John Herschel and Mr. Talbot to ‘6 the assumption of definite and different chemical states of the silver within and without their area." A few experiments will prove this to be the case. Prepare a piece of the less sensitive paper, with only one wash of silver, and whilst wet expose it to the sunshine ; in a few minutes it will exhibit the influence of light, by becoming very irregularly darkened, assuming such an appearance as th.it given in fig. 17, the light part being a pale blue, and the shaded portions a deep brown. In pursuing our inquiry into the cause