-may be
used with safety.

During the development the ground takes a blue color which rapidly
intensifies, while the iron compound, not acted on and imparting a yellow
green tint to the design, is washed out from the white paper. If the
print has not been sufficiently exposed the ground remains pale blue, more
or less; the reason has been explained. In this case the development
should be done quickly, as the blue is always discharged by washing. On
the other hand, whenever the whites are tinted by excess of exposure, they
can be cleared partly or entirely by a prolonged immersion in water, but
the ground is also to some extent lightened.

When the proof is well developed and fixed, that is, when the soluble iron
salts are eliminated, the blue color can be brightened by adding to the
last but one washing water a small quantity of citric acid, or of
potassium bisulphate, or a little of a solution of hypochlorite of lime
(bleaching powder).

The action of light in this, as well as in the other photographic
processes with metallic salts described in this work, is one of
deoxidation, as shown by Herschel. The chemical changes which produce the
blue precipitate is quite complicated. It is evident that both the ferric
citrate and the ferric cyanate are partly reduced to ferrous salts under
the luminous influence, and react in presence of water with the unreduced
part of each of these compounds, the ferric citrate with the ferrous
cyanate forming Prussian blue (ferric-ferrocyanate), and the ferric
cyanate with the ferrous citrate giving rise to Turnbull's blue (ferrous
ferricyanate). The blue of the print is consequently a mixture in a
certain proportion of the two compounds; and as the color of Prussian blue
is quite different from that of Turnbull's, it follows that by varying in
a certain measure the percentage of the two ferric salts forming the
sensitizing solution, the color of the blue may be varied thereby. Hence
the difference in the formulas given by different authors.(8)

The blue color of the image can be changed into black or dark green. But
to that purpose the paper should be, although not exactly necessary, well
sized as before directed, and sensitized with extra care to prevent the
imbibition of the iron solution into the paper. After exposure the proof
should necessarily be thoroughly washed to eliminate the soluble iron
salts, then immersed for a moment in water acidified with nitric acid,
1:100, and