Nitric Acid applied in a concentrated form acts immediately upon the darkened surface, bleaching every part of the print with the exception of the bronzed shadows, which usually retain a slight residual colour. A solution of Chromic Acid is still more active. This liquid may usefully be applied to distinguish prints toned by Sulphur from others toned by Gold; the presence of metallic Gold protecting the shadows of the picture in some measure from the action of the acid. The solution should be prepared as follows:-- Bichromate of Potash 6 grains. Strong Sulphuric Acid 4 minims. Water 12 ounces. A solution of Permanganate of Potash is an energetic destroyer of paper positives; and, as it is a neutral substance, may conveniently be employed in testing the relative capability of withstanding oxidation possessed by different Photographic Prints. The solution should be dilute, of a pale pink hue, and the Positives must be moved occasionally, as the first effect is to decolorize a great portion of the liquid, the Permanganate oxidizing the size and organic tissue of the paper. After an immersion of twenty minutes to half an hour, varying with the degree of dilution, the half-tones of the picture begin to die out, and the full shadows become darker in colour; the bronzed portions of the print withstand the action longer, but at length the whole is changed to a yellow image much resembling in appearance the Photograph faded by Sulphur. _Comparative permanence of Photographs treated with Permanganate of Potash._--Developed prints prepared by a Negative process withstand the action better than others. But to this rule there are exceptions; much depending upon the time of exposure to light, and the extent to which the development is carried. Those prints which, being exposed for a short time, and afterwards strongly developed, become dark in colour and vigorous in outline, are more permanent than others which having been over-exposed and under-developed, lose their dark colour and become red and comparatively faint in the Hyposulphite fixing Bath. Positives developed upon a surface of _Chloride_ of Silver on plain paper do not resist the oxidizing action so perfectly as those on Iodide of Silver. Prints developed upon paper prepared with Serum of Milk containing Caseine stand better than those on plain paper.