A Manual Of Photography

IODIDE OE SILVER. 209 Less Sensitive Paper for Copies of Engravings—Botanical or Entomological Specimens. Muriate of soda, twenty-five grains to an ounce of water. Nitrate of silver, ninety-nine grains to an ounce of distilled water. Applied as above directed. Common Sensitive Paper, for Copying Lace-work, Feathers, Patterns of Watch-work, ^e. Muriate of soda, twenty grains to an ounce of water. Nitrate of silver, sixty grains to an ounce of distilled water. Applied as above directed. This paper keeps tolerably well, and, if carefully prepared, may always be depended upon for darkening equally. Section III.—Iodide of Silver. This salt was employed very early by Talbot, Herschel, and others, and it enters as the principal agent into Mr. Talbot's calotype paper. Paper is washed with a solution of the iodide of potassium, and then with nitrate of silver. By this means papers may be prepared whicli are exquisitely sensi- tive to luminous influence, provided the right proportions are hit; but, at the same time, nothing can be more insensible to the same agency than the pure iodide of silver. A singular dif- ference in precipitates to all appearance the same, led to the belief that more than one definite compound of iodine and silver existed ; but it is now proved that pure iodide of silver will not change colour in the sunshine, and that the quantity of nitrate of silver in excess regulates the degree of sensibility. Experi- ment has proved that the blackening of one variety of iodidated paper, and the preservation of another, depends on the simple admixture of a very minute excess of the nitrate of silver. The papers prepared with the iodide of silver have all the peculiari- ties of those prepared with the chloride, and although, in some instances, they seem to exhibit a much higher order of sensi- tiveness, they cannot be recommended for general purposes with that confidence which experience has given to the chloride. It may, however, be proper to state the best proportions in which the iodidated papers can be prepared, and the most approved method of applying the solutions. p