inly on the darker ground surrounding the spot covered by the
first drop.

"In this reaction, water plays virtually the part of a sympathetic
fluid, and tracing the characters with water on sized and calendered
paper, the writing will show perfectly plain when the paper is dried
and exposed to action of iodine vapor. The brownish violet shade on a
yellowish ground will evolve to a dark blue on a light blue ground
after wetting. These characters disappear immediately under the action
of sulphurous acid, but will reappear after the first discoloration
provided the paper has not been wet and the discoloration has been
effected by the use of sulphurous acid gas.

"The process, therefore, affords means for tracing characters which
become legible and can be caused to disappear, but at will to reappear
again, or which can be used for one time only and be canceled forever
afterwards.

"The usual method of verifying whether paper has been rubbed is to
examine it as to its transparency. If the erasure has been so great as
to remove a considerable portion of the paper, the erased surface is
of greater translucency; but if the erasure has been effected with
great care, examining same close to a light will disclose it; the
erased part being duller than the surrounding surface because of the
partial upheaval of the fibers.

"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 combina