this done and without washing treated by a solution of aqueous
ammonia at 2 per 100 of water. In this the blue color disappears, being
changed into a red brownish tint, which indicates that the Turnbull's and
Prussian blues are transformed, the former into ferroso-ferric hydrate,
with formation of ferrocyanate, and the latter into ferric hydrate. It is
by the action of tannin (gallotannic acid) on the ferric oxides thus
formed that the black is produced, and by that of catechu-tannic acid
contained in the extract of catechu that one obtains a dark green, almost
black color.

To obtain the black tone it suffices to immerse the proof on its removal
from the ammoniacal in a solution of tannin at 5 per 100 of water, and
when toned, to wash it in a few changes of water.

The process to turn the blue color into a green was devised by Mr. Paul
Roy. It is as follows: Dissolve 7 parts of borax in 100 parts of water,
and acidify the solution with sulphuric acid added drop by drop until the
litmus paper becomes red; then, in the same manner, neutralize with
aqueous ammonia not in excess, but just enough to show an alkaline
reaction; this done dissolve 1 part of powdered catechu and filter. In
this the proof is immersed after development until the desired effect is
attained. Wash, etc.

To clear the lines, or to make additions, or to write on the blue margin
of the proof a solution of potassium oxalate is employed. It dissolves the
blue without leaving scarcely any trace of it. The solution can be
prepared by mixing the two solutions whose formula is given below:(9)

A. Oxalic acid 10 parts
Water 100 parts
B. Caustic potassa 121/2 parts
Water 100 parts

The blue prints are permanent. When drying they darken a little from
oxidation; exposed to sunshine for some hours, they bleach considerably;
but in the shade the faded pictures progressively absorb oxygen from the
air and assume their original intensity and color in a period so much the
longer as the insulation has been more prolonged; it may take weeks if the
picture were much bleached.

THE CYANOFER. (PELLET'S PROCESS.)

_This process gives blue impressions on a white ground from positive
cliches, and white impressions on a blue ground from negative cliches._
It is termed "positive ferrotype process."

The cyanofer is an application of one of the numerous and useful
inventions for which photography is indeb