r authorities,
the double ammonio-ferric oxalate is rightly recommended instead of the
simple oxalate. Not only is the preparation more sensitive to the
luminous action, but better half tones are obtained. As usual, it is
advantageous to size the paper with starch.

The ammonio-ferric oxalate is prepared by precipitating ferric chloride or
sulphate by aqueous ammonia, then washing the precipitate collected on a
filter until the washing water be neutral or does not evolve the smell of
ammonia. The precipitate is then placed in an evaporating dish, and by
small quantity is added a hot solution of ammonium oxalate until it is
nearly (not entirely) dissolved, when the solution is set aside for a few
hours, then filtered and evaporated to crystallization. For use, the
crystals of ammonio-ferric oxalate are dissolved in the proportion of 1
for 5 of distilled water. The solution as well as the crystals should be
kept in the dark.

If one object to the trouble of crystallizing, the solution can be
prepared by dissolving the ferric oxide in a hot solution of 30 parts of
ammonium oxalate and 25 parts of oxalic acid in 180 parts of water
observing that the oxide must be in excess.(42)

The following sensitizing solution gives also excellent results:

Ammonio-ferric oxalate 10 parts
Ammonio-ferric lactate 4 parts
Water 100 parts

After exposure, which varies from five to ten minutes, according to the
intensity of the light and the printing quality of the negatives, the
picture appears negative from formation of ferrous oxalate. It may be
developed in a great many ways: by a solution of silver nitrate at 2 or 3
per cent. of water acidified slightly by an organic acid--citric acid, for
example--or a diluted solution of ammonio-nitrate of silver, which most
likely constitutes the best developer; the image is black and consists of
metallic silver and ferric oxide, with formation of silver oxalate, which
dissolve in the ammonia. If the print be treated by a weak solution of
aqueous ammonia, the image turns green, then brown, and if, before the
latter coloration is obtained, gallic acid or pyrogallol be added, the
image becomes bluish-black or brown-black. In the same circumstances
tannin (gallo-tannin) produces a blue-black image; catechu-tannin(43) and
quino-tannin give green, etc. Employed as a developer, potassium
ferricyanate develops an image in prussian blue, and auric chloride one in
the charact