a positive cliche
suffices to well impress it, that is, to reduce in the whites the iron
perchloride to the state of protochloride."

"To print, one is guided by the decoloration of the paper, and even for
more facility I add to the solution of iron perchloride and tartaric acid
a small quantity of a solution of potassium sulphocyanide for the purpose
of obtaining a red tint, which is more visible and disappears also under
the influence of light in proportion to the decomposition of the
perchloride. One obtains then after exposure a red design on the white
ground of the paper. This red color is not permanent. It even disappears
by keeping the proof in the dark."

"To develop and then to fix the design thus obtained I wash rapidly the
paper in ordinary water, or better, in water holding chalk in suspension.
The red coloration disappears, a part of the iron perchloride is washed
out, and in the parts which have not been acted on by light the
perchloride is transformed into sesquioxide. I replace then the water by
solution of gallic acid or of tannin and the image progressively appears
in ink-black. When I judge the image to be sufficiently intense I wash
the proof in rain water, in preference to ordinary water, which might
cause the gallic acid and tannin to turn brown. I sponge between sheets of
blotting paper and let the proof dry spontaneously."

"If in place of gallic acid I use a diluted solution of potassium
ferricyanide (red prussiate of potash), Prussian blue is formed in the
parts acted on by light. The preparation is even sensitive enough to
permit one to obtain an impression in the camera obscura in developing by
the ferricyanide."

"As to the proofs in gallate (or tannate) of iron, they can be transformed
into Prussian blue in a solution of potassium ferrocyanide (yellow
prussiate of potash) slightly acidified by sulphuric acid."

The paper most suitable for this process is that which has been previously
well sized with starch, as explained in a special paragraph of this
pamphlet. Paper prepared with a film of coagulated albumen gives also good
results. It may be prepared by brushing as well as by floating, but in
either case the paper should be wetted on the surface only and dried
rapidly at a temperature of about 115 deg. Fahr. (46 deg. C.) and kept in
a dry place. It does not keep for more than from ten to fifteen days,
owing to the hygroscopicity of the iron compound. Mr. Colas, who prepares