But there are often valuable or rare works which have sustained interior injury, and which it is desired to restore to a clean condition. The best method is to take the book apart as the first step. When separated into sheets, those leaves which are merely dirty should be placed in a bath composed of about four ounces of chloride of lime, dissolved in a quart of water. They should soak until all stains are removed, and the paper is restored to its proper color. Then the pages should be washed in cold water--running water is preferable--and allowed to soak about six hours. This removes all traces of the lime, which would otherwise tend to rot or injure the book. After this, the sheets are to be "sized," _i. e._, dipped in a bath of size and water, and laid out to dry. This process gives firmness and consistency to the paper, which would otherwise remain too soft to handle. The sheets should be pressed a few hours between glazed paste-boards, as used in printing offices. A cheap and simple size for this purpose may be made by mixing white gelatine with water, and this may be kept in a bottle, so as to be always conveniently at hand. The art of restoring and rendering fit for handling books and rare early pamphlets by sizing all the leaves is in constant use in Europe. By this means, and by piecing out margins, the most rotten paper, ready to drop apart in turning the leaves, may be restored to use, if not quite to its pristine condition. Ink-spots or mildew stains may be wholly removed, when freshly made, by applying a solution of oxalic or citric acid, and then washing the leaf with a wet sponge. It is more effectual to follow the bath of oxalic acid by applying a solution of one part hydrochloric acid to six parts of water, after which bathe in cold water, and dry slowly. Or an infusion of hypochlorite of potash in twice its volume of water may be used instead of the preceding. If a leather-bound book has grease on its cover, it can be removed by scraping French chalk or magnesia over the place, and ironing with a warm (not hot) iron. A simpler method is to apply benzine to the grease spots, (which dissolves the fatty material) and then dry the spot quickly with a fine cloth. This operation may be repeated, if not effectual at the first trial. The same method of applying benzine to oily spots upon plates or engravings, will remove the stains. Ink-stains may also be taken off from the leather covers of books bound in calf or morocco by the use of oxalic acid. Care should be taken first to try the acid on a piece of similar leather or on a discarded book of the same color. If the leather is discolored after removing the black spot, one may apply, after taking out the traces of oxalic acid by some alkali, a coloring matter similar to the tint of the leather.