"Valuable information is often obtainable by treating writing or other ink-marks with reagents. Some inks are affected much more rapidly than others, though the rate of change depends greatly on the age of the writing. Normal oxalic acid (63 grammes per litre), or hydrochloric acid of corresponding strength, should be applied to a part of the ink marked with a feather or camel-hair brush (or the writing may be traced over with a quill pen), and the action observed by means of a lens, the reagent being allowed to dry on the paper. Recent writing (one or two days old) in gallic inks is changed by one application of oxalic acid to a light gray, or by hydrochloric acid to yellow. Older stains resist longer, in proportion to their age, and a deeper color remains. Log-wood ink marks are mostly reddened by oxalic acid, and alizarin marks become bluish, but aniline inks are unaffected. With hydrochloric acid, logwood ink marks turn reddish or reddish-gray, alizarin marks greenish, and aniline ink marks reddish or brownish-gray. The treatment with acid should be followed by exposure to ammonia vapors, or blotting paper wet with ammonia may be applied. Thus treated, marks in logwood ink turn dark violet or violet-black. The age of ink marks very greatly affects the rate of their fading when treated with dilute ammonia, the old marks being more refractory. The behavior of ink marks when treated with solution of bleaching powder is often characteristic, the older writings resisting longer; but unless the reagent be extremely dilute, writings of all ages are removed almost simultaneously. Hydrogen peroxide acts more slowly than bleaching solution, but gives more definite results. After bleaching the mark by either reagent, the iron of the ink remains mordanted on the paper, and the mark may be restored by treatment with a dilute solution of galls, tannic acid, or acidulated ferro- cyanide. The same reagents may be used for restoring writing which has been faded from age alone. "When ink marks have been erased or discharged by chemical means, traces of the treatment are often recognizable. After effecting the erasure the spot is often rubbed over with a powdered alum or gum sandarac, or coated with gelatin or size. The bleaching agents most likely to have been used are oxalic, citric, or hydrochloric acid, bleaching powder solution, or acid sulphite of sodium. Moistened litmus paper will indicate the presence of a free acid, and in some cases treatment with ammonia fumes will restore the color. The presence of calcium, chlorides, or sulphates in the water in which the paper is soaked will afford some indication of bleaching powder or a sulphite having been used. Potassium ferro-cyanide will detect any iron remaining in the paper. Exposure to iodine vapor often affords evidence of chemical treatment, and other methods of examination readily suggest themselves."