A Manual Of Photography

CHLORIDE OF SILVER. 129 of this singularity, it will be found that over the light parts a pure chloride of silver, or a chloride with the slight excess of the muriate of soda, is diffused; but over the dark parts the chloride of silver is united with an excess of the nitrate of silver. Where the rates of imbibition are different, this defect must follow, as a natural con- sequence, in very many cases ; but it is found to occur frequently where we cannot detect any sufficient cause for the annoyance. Although we are acquainted with the proxi- mate causes of the differences produced, yet the ultimate ones are involved in doubt. It is a remarkable fact, that the same ir- 17. regular patelles are formed in the dark on papers which have been kept a long time. Sir John Herschel suggested, as a means of preventing these troublesome occurrences, that the saline wash used should, prior to its application, be made to dissolve as much as possible of the chloride of silver, which it does to a considerable extent ; and that the last wash of the nitrate of silver should be diluted with an equal quantity of water, and ap- plied twice, instead of in one application. There can be no doubt but this evil is almost entirely overcome by operating in this way, but it is unfortunate that the process is somewhat inju- rious to the sensibility of the paper. Of the salts already named, some few of the effects should be noted. Muriate of Soda.—This is usually employed, and tire chloride ot silver precipitated by it darkens to a fine chocolate brown, and by regulating the quantity of salt in solution, relatively to the strength of the nitrate of silver, a moderately high degree of sensibility may be obtained. Muriate of Lime.—Not particularly sensitive, deepening to a brick-red in full sunshine, but is less liable to change in the fixing processes than almost any other preparation. Muriate of Potasii is scarcely in any respect different from the muriate of soda. The nitrate of potash, however, which is formed in the paper, is less liable to be affected by a humid atmosphere than the nitrate of soda. Muriate of Ammonia, used in the proportion of two scruples to four ounces of water, and the silver solution in the proportion of sixty grains of the nitrate to one ounce of water, forms aver/ beautiful paper, equalling in sensibility the best kind prepared with the muriate of soda, at nearly one-half its expense. It darkens to a fine chocolate brown.