By the second day the metal plate had acquired a faint yellow hue, not easily seen except in certain positions; but the Positives were unaffected. At the expiration of three days the majority of the pictures exhibited no signs of change, but a few untoned prints of a pale red colour, some of which had been printed by development, and others by direct exposure to light, had perceptibly darkened. After the eighth day, the action, appearing to progress more slowly than at first, was stopped, and the prints removed. The general results obtained were as follows:-- The Daguerreotype plate had become strongly tarnished with a film of Sulphuret of Silver, which appeared yellowish-brown in some parts and steel-blue in others. The Positives were, as a rule, toned to a slightly colder shade, but many of them had scarcely changed. No obvious difference was observed between prints developed on paper prepared with Chloride of Silver, and others printed by direct exposure to light; but in all cases the prints obtained by those methods which give a very red image after fixing, were the first to show the change of colour due to sulphuration, the proofs submitted to the test having all been previously toned with Gold. Effect of Oxidizing Agents upon Positive Prints.--It appeared of importance to ascertain to what extent Photographic Prints are susceptible of oxidation; on account of the atmospheric influences to which they are necessarily exposed. In experimenting upon this subject the following results have been obtained. Powerful oxidizers destroy Positive Prints rapidly; the action usually commencing at the corners and edges of the paper, or at any isolated point, such as a metallic speck or particle of extraneous matter, which can serve as a centre of chemical action. This same fact is often noticed in the fading of Positives by long keeping, and therefore since other destructive actions (with the exception of that of Chlorine) do not appear to follow the same rule, it is an argument in addition to others which can be adduced, that Photographic Prints are frequently destroyed by oxidation. Air which has been _Ozonized_ by Phosphorus, and in which blue litmus-paper becomes reddened, quickly bleaches the Positive image. Oxygen gas, obtained by voltaic decomposition of acidified water and which should contain Ozone, did not appear to have an equal amount of effect, the action being comparatively slight, or altogether wanting. _Peroxide of Hydrogen_ obtained in solution, and in conjunction with Acetate of Baryta, by adding Peroxide of Barium to dilute Acetic Acid,[26] bleaches darkened Positive paper; but the effect is slow, and does not take place to a very perceptible extent if the liquid be kept alkaline to test-paper. [26] Hydrochloric Acid, which is usually recommended in place of Acetic Acid, cannot be employed in this experiment; it seems to cause a liberation of free Chlorine, which bleaches the print instantly.