“If an erasure is effected by means of bread crumbs instead of India rubber, and care is taken to erase in one direction the change escapes notice; and it is generally impossible to detect it, should the paper thus handled be written upon again.

“Iodine vapors, however, show all traces of these manipulations very plainly giving their location with perfect certainty.  The erased surfaces assume a yellow brown or brownish tint.  If, after being subjected to the action of the iodine, the paper on which an erasure has been made is wet, it becomes of a blue color the intensity of which is commensurate with the length of time to which it has been under the action of the iodine, and when the paper is again dried the erased portions are more or less darker than the remainder of the sheet.  On the other hand when the erasure has been so rough as to take off an important part of the material exposure to iodine, wetting, and drying result in less intensity to coloration on the parts erased, because the erasing in its mechanical action of carrying off parts of the paper removes also parts of the substance which in combination with iodine give birth to the blue tint.  Consequently the action of the iodine differs according to the extent of the erasure.

“When paper is partially erased and wet, as when letters are copied, the same result although not so striking follows upon exposing it to the iodine vapor after letting it dry thoroughly.

“Iodine affords in certain cases the means of detecting the nature of the substance used for erasing.  Bread crumbs or India rubber turn yellow or brownish yellow tints and these are distinguished by more intense coloration; erasure by means of bread crumbs causing the paper to take a violet shade of great uniformity.  These peculiarities are due to the upheaval of the fibers caused by rubbing.  In fact this upheaval creates a larger absorbing surface and consequently a larger proportion of iodine can cover the rubbed parts than it would if there had been no friction.

“When paper upon which writing has been traced with a glass rod, the tip of which is perfectly round and smooth, is exposed to iodine vapor, the characters appear brown on yellow ground which wetting turns to blue.  This change also occurs when the paper written upon has been run through a super-calender.  If the paper is not wet the characters can be made to appear or be blotted by the successive action of sulphurous and iodine vapor.

“Writing done by means of glass tips instead of pens will show very little, especially when traced between the lines written in ink.  The reaction, however, is of such sensitiveness that where characters have been traced on a piece of paper under others they appear very plainly, although physical examination would fail to reveal their existence, but a somewhat lengthy exposure to iodine vapors will suffice to show them.

“If the wrong side of the paper is exposed to the iodine vapor the characters are visible; but of course in their inverted position.