ted to A. Poitevin. In 1863 he
discovered that certain organic substances were rendered insoluble by
ferric chloride, and that they again became soluble; when under the
influence of light the ferric chloride has been reduced to a ferrous salt.
This curious phenomenon is the base of the process now to be described.
As usual the process has been modified by compounding the sensitive
solution in various ways and by minor details in the manner operating.
But although these modifications have rendered the process easier to work
with, there is not a great difference in the results obtained. We give
two formulas. Aside from the addition of gum arabic, which was suggested
by Mr. Pellet, and which constitutes the capital improvement of the
process, the formula is substantially that devised by Mr. Poitevin.

Prepare three solutions as follows:

A. Gum arabic, best 50 parts
quality
Water 170 parts
B. Tartaric acid 12 parts
Water 80 parts
C. Ferric chloride 35 parts in volume
solution at 45
deg. Baume

Mix gradually B to C, then C, by small quantities, in agitating briskly.
It is important to prepare the solution as directed, for by adding the
ferric chloride before tartaric acid, the gum arabic would be at once
coagulated. When the ferric chloride is mixed, the solution at first
thickens, but becomes sufficiently fluid for use in a certain period. It
does not keep, and should be employed the day it is made if possible.

The paper, which should be well sized and calendered, and which, when not
giving good results by too much absorbing the sensitive solution, must be
starched as before directed, is coated either by brushing or by floating.
By the first method a roll of paper five yards long can be prepared
without great trouble, and give, perhaps, better results than if prepared
by floating; but the latter method is by far the the most convenient: one
does not generally prepare by brushing sheets of paper larger than about
30x40 inches.

For brushing, the paper is pinned on a board, then, with a large badger
brush dipped in the sensitive solution, the latter is applied as evenly as
possible; after which, by lightly passing the brush over, the striae are
removed, the coating well equalized, and the paper hung up to dry. The
coating should not be very thin, and, above all, not too thick, for then
it would require an unusually long exposure to allow the li