The Photographic Image does not appear readily susceptible of oxidation unless it be previously darkened by the action of Sulphur, or placed in contact with acids or bodies which act as solvents of Oxide of Silver (p. 146). The materials often used in sizing papers, such as Alum and Resin, being of an acid nature, are directly injurious to the image; and the removal of the size, which may easily be effected by means of a dilute alkali or an alkaline carbonate, without injury to the tint, has the additional advantage of carrying out the last traces of Hyposulphite of Soda, and also the germs of _fungi_, which if allowed to remain would vegetate and produce a destructive mouldiness on exposure to damp (Chap. III. Part II.). The fact that acids facilitate oxidation of the image is likewise a hint that Photographic Prints should not be handled too frequently, or touched with the finger more than is necessary; the warm hand may leave behind a trace of acid[29] which would tend in time to produce a yellow mark. [29] The writer has seen blue litmus-paper immediately reddened by being laid upon the arm of a person suffering from acute Rheumatism. This acid is probably Lactic Acid! c. _Moisture as a cause of fading._--Although. Photographs properly printed are not readily injured by damp air (p. 153), yet as there are _impurities_ of various kinds constantly floating in the atmosphere, a state of comparative dryness may be said to be essential to the preservation of all Photographs. In collecting evidence upon the subject, "wet" and "damp" are frequently alleged as having been causes of fading--the prints were hung against a damp wall during frosty weather, in a room without a fire: or the rain had been allowed to penetrate the frame! No pictures will long survive such treatment, and Photographs, like engravings and water-colour paintings, require common care to be exercised in their preservation. d. _The modes of Mounting the Proof._--This subject has been alluded to in the abstract of the Author's papers at p. 155. All cements which are of an acid nature, or which are liable to become _sour_ by acetous fermentation, should be avoided. Flour paste is especially injurious, and many cases of fading have been traced to this cause. The addition of Bichloride of Mercury, which is often made to prevent the paste from becoming mouldy, would still more unfit it for Photographic use (p. 151). Starch is not much preferable. No substance appears better than Gelatine, which does not readily decompose, and shows no tendency to absorb atmospheric moisture. The _deliquescent_ nature of many bodies is a point which should be borne in mind in mounting Photographs, and hence the use of a salt like _Carbonate of Potash_, which the writer has known to be added to paste to prevent the formation of acid, would be unadvisable.