Of prints obtained by the ordinary process of direct exposure to light, those on plain paper are the first to fade, the oxidizing action being most seen upon the _half-tones_. The use of _Albumen_ gives a great advantage. Developed prints on Albumen stand far better than the same upon plain paper; and even the Albuminized sun prints are less injured by the Permanganate than the best of the Negative prints prepared without Albumen. Caseine has the same effect, but to a less extent; and as Serum of Milk almost invariably contains uncoagulated Caseine, its efficacy is thus explained. The manner of toning the print is a point of importance; previous sulphuration in an old Hyposulphite Bath always facilitating the oxidizing action. _Action of Chlorine upon Positive Prints._--Aqueous solution of Chlorine destroys the Photographic image, changing it first to a violet tint (probably Subchloride), and subsequently obliterating it by conversion into white Chloride of Silver. The impression, although invisible, remains in the paper, and may be developed in the form of yellow or brown Sulphuret of Silver by the action of Sulphuretted Hydrogen. It also becomes visible on exposure to light, and assumes considerable intensity if the paper be previously brushed with free Nitrate of Silver. Sulphate of Iron produces no effect upon the invisible image of Chloride of Silver; but Gallic or Pyrogallic Acid, rendered alkaline by Potash, converts it into a black deposit. The Action of Chlorine water usually commences at the edges and corners of the print, in the same manner as that of oxidizing agents. The proofs upon Albumen are the least readily injured, and next, those developed on Iodide of Silver. _Hydrochloric Acid._--The liquid acid of sp. gr. ·116, even when free from Chlorine, acts immediately upon the half-tones of a positive print, and destroys the full shadows in the course of a few hours; a slight residual colour however usually remains in the darkest parts. The prints developed on Iodide of Silver are the most permanent. _Sulphuric, Acetic Acids, etc._--Acids of all kinds appear to exert an injurious influence upon Positive prints, and especially so upon the half-tones of the image, the effect varying with the strength of the acid and the degree of dilution with water. Even a vegetable acid like Acetic gradually darkens the colour and destroys partially or entirely the faint outlines of the picture.