It is evident from these figures that the permanency of a print does not depend so much on the wishing it receives as on its thorough fixing; and we are strongly of opinion that the best way of removing all the silver is not by using two baths of hypo in succession, but by washing for about five minutes after the first bath, and then soaking in a second fresh bath of hypo.
In what form the silver occurs in the paper, whether as the insoluble or the soluble held in a sort of a " loose combination " with the films of the paper, we are unable to state. However, the fact remains that, after a more thorough washing and fixing than prints generally get, .246 of a grain of silver remains in each 22 X 17 in. sheet of paper, and this, which is probably in the form of hyposulphite in the presence of light and organic matter, is reduced to silver sulphide, and will easily account for the yellowing of the whites of prints after a time." In order to test whether the hypo has been thoroughly eliminated from the prints, several methods have been devised. The simplest is the permanganate test. In a pint of distilled water one grain of permanganate and ten grains of potassium carbonate are dissolved. This will give a pink solution. A little of the water in which the prints have been last soaking is now poured into a clean white glass bottle, and to it is added four or five drops of the permanganate solution.
If the water be quite pure the addition of the solution will give it a faint pink tinge, but if any trace of sodium hyposulphite be present this color will be of a light green. After the addition of the permanganate solution the bottle should be well shaken, allowed to stand for about ten min utes or so, and then carefully examined against a white surface.
Another method commonly employed is that known as the "starch iodide test." A small piece of starch, about the size of a pea, is boiled in a quarter of an ounce of water until a clear solution is obtained. To this is then added one drop of tincture of iodine (iodine dissolved in alcohol), which will give it a dark blue color. Two test tubes are now required. Into one is poured some of the last washing water to be tested, and into the other some plain distilled water. One drop of the test solution is now added to each tube, and both well shaken. The next point is to examine the two tubes side by side in front of a piece of white paper. Hypo will cause the blue color to disappear, so that if the washing water does not present as blue an appearance as the other tube containing the distilled water only, it may be decided that hypo is still present.
Dr. Bannon recommended* silver nitrate as a very delicate test for hyposulphite of soda. A print should be removed from the washing apparatus, and the water drained from it into a test tube. This is then heated, and a few drops of a silver nitrate solution added to it. It is stated that even if so small a quantity as the one ten-thousandth part of hypo be present a black pre cipitate will be formed, while a smaller amount will produce a precipitate of a yellow color.
WASTE.—All waste material, papers, solutions, etc., containing gold, silver or platinum should be saved for the extraction of the precious metals. See Residues.