rrections and writing are made with a _Prussian blue ink_
prepared by mixing the two following solutions:

A. Ferric chloride, 4 parts
dry
Water 350 parts
B. Potassium 15 parts
ferrocyanate
Water 250 parts

The precipitate being collected on a filter and washed until the water
commences to be tinged blue, is dissolved to the proper consistency in
about 400 parts of water. This ink does not corrode steel pens.

It has been stated that the cyanofer process keeps for years if preserved
from the combined action of dampness and the air. The writer found in his
practice that the ferric salts in presence of the organic matters (the
sizes) acts as does potassium bichromate and renders, in a certain period,
the cyanofer film insoluble even after a prolonged insulation. Paper
freshly prepared is always more sensitive and gives better whites and
generally finer results.(10)

The prints can be toned black in operating as in the cyonotype, but the
results are seldom good.

Captain Pizzighelli's formula is as follows: Prepare

A. Gum arabic 15 parts
Water 100 parts
B. Ammonia ferric 45 parts
citrate
Water 100 parts
C. Ferric chloride 45 parts
Water 100 parts

For sensitizing mix _in order_:

Solution A 100 parts
Solution B 40 parts
Solution C 20 parts

The mixture very much thickens at first, but becomes sufficiently fluid
for use in a few hours. It keeps well for two or three days. Leaving out
B and replacing it by rain water, this makes also a good solution for the
cyanotype.

THE BLACK OR INK PROCESS. (FERRO-TANNATE PROCESS.)

_This process gives black positive impressions on white ground from
positive cliches, and negative impressions from negative cliches._ It has
been attributed to Mr. Colas, but in reality it was invented by Mr.
Poitevin, who describes it as follows in his communication of May, 1860,
to the Societe Francaise de Photographie:

"I make a solution containing--"

Iron perchloride, cryst 10 parts
Tartaric acid 3 parts
Water 100 parts

"I apply the paper on this mixture and let it dry spontaneously in the
dark, and at the moment of using it I completely desiccate it at a gentle
heat. Thus prepared the paper is of a deep yellow color. Light decolors
it rapidly, and ten or twelve minutes' exposure through