A Manual Of Photography

PRACTICE OF PHOTOGRAPHY. 192 nitric acid liberates a quantity of carbonaceous matter, which, acting by a function peculiarly its own, will at certain tempera- tures effect the revival of gold and silver, as proved by Dr. Schafheutl’s and Count Rumford’s experiments., Having been informed that the paper-makers are in the habit of bleaching their papers with sulphur and sulphites, 1 have submitted a considerable quantity of the browned papers to careful examination. In all cases where the paper has blackened, I have detected the presence of sulphur. Consequently, when the darkening goes on rapidly, and terminates in blackness, we may, I think, correctly attribute it to the formation of a sul- nhuret of silver. . ,, It is however, certain that the slow action of organic matter is sufficient, under certain circumstances, to set up a chemical change which, once started, progresses slowly, but certainly, until the compound is reduced to its most simple form. China clay—kaolin—has of late years been much used by the paper manufacturers, for the double purpose of giving weight to the paper, and of enabling them to produce a smooth surface upon ah the finer varieties of paper ; such as the ‘ enamelled satin post.” This compound of alumina and silica would not, it the finest varieties of clay were employed, be lively to do much mischief in the papers used for photography ; varieties of the Cornish clay are employed, and tins commonly contains iron and other metals in a state of very fine division; and these where they come to the surface, form little centres o action from which dark circles spread in rather a curious manner. In France there has been manufactured a paper for this especial process • it is very thin, and of a tolerably uniform texture. It said to answer exceedingly well with the modified forms of photographie manipulation employed in France, but it does not appear adapted, from some cause which is not clearly explained to tte English processes. Thin papers have been tried, and many varieties would answer exceedingly well but that nearly every variety is found penetrated with small holes, which, though of nunute dimensions, suffer light to pass freely and conscquently produce a spottiness on the resulting picture. Sir John 1lerschef found that åis evil could be remedied by fastening two pieces of such paper together ; but this method is troublesome and un- Returning to the consideration of size in the paper, the abe re- named authority—who employed the lead salts in some of his photographic processes—has the follow mg remar es . « The paper with a basis of lead turns yellow by keeping in the dark,andthe tint goes on gradually deepening to a dark brown.