_The developing solutions should be as little acid as possible, but not
neutral, for then the proofs would be veiled and grayish._

The image can also be developed in a solution of gold, or in a very weak
solution of mercuric chloride at 1 per 10,000. The proof must be
extremely well printed and left for from two to five minutes in the
mercuric solution. If the time of exposure is right, the image will
change but little in the solution, and will take, when treated with silver
nitrate, the most splendid tones.

The proofs should be carefully washed when finished. If they were
developed with silver, they must be immersed in diluted aqueous ammonia,
which will perfectly clear the whites. If developed with chloride of
gold, the water should be heated to 60 to 80 deg. C. (140 to 176 deg.
Fahr.)

HOUDOY'S PROCESS (1858.)

The paper is floated upon a lukewarm solution of gelatine at 5:100, and
when dry, on a bath of uranium at 10 or 15 per 100 of water. After
exposure to the sun the image is developed with a solution of silver
nitrate acidified with acetic acid. The exposure varies, according to the
nature of the negative, from one to ten minutes; it must be long enough
for the image being developed in from thirty to forty seconds. It is then
removed from the silver bath and placed in the following:

Ferrous sulphate 3 to 8 parts
Acetic acid 2 parts
Water 100 parts

In this bath the image takes a great vigor and appears entirely on the
surface of the paper. When the proof has been too long exposed it should
be washed slightly before placing it in the iron bath. Developed, the
image is, generally, of a sepia tone, which can be turned to black by a
solution of chloride of gold, 1:1,000, washing afterwards as usual.

NIEPCE DE ST. VICTOR'S PROCESS (1859).

Red Prints.

Float the paper for fifteen or twenty seconds on a 20 per cent. solution
of nitrate of uranium and dry before the fire in the dark room. This
paper can be prepared many days before use. Expose in sunshine from eight
to ten minutes, according to the intensity of the light and the quality of
the negative, then wash in moderately warm water (50 to 60 deg. C.) for a
few seconds. This done, immerse in a solution of red prussiate of potash
at 2 per cent. of water; in a few moments the proof will become of a fine
blood-red color, like "sanguine." Wash, etc.

Green Prints.

Make a red print as above des