or, better, citric acid in concentrated solution. This paper,
after desiccation and exposure to light, possesses the property of
reducing the solution of silver nitrate and that of chloride of gold, and
of turning blue with a solution of potassium ferncyanate in the parts
where light has reduced the iron sesquichloride into the oxide at the
minimum."

"To coat the paper with an equal layer of iron sesqnioxide, I brush it
with a tuft of fine linen dipped in a solution of iron perchloride at 10
or 12 per cent. of water, and dry the sheets in the dark. I immerse
afterwards these sheets, one after the other, in a tray containing aqueous
ammonia, in such a manner as to well wet each sheet successively. A
sufficient number of sheets being immersed, I pour off the ammonia in a
vial, and, in the tray, I wash them several times, and remove them one by
one to hang them up to dry, even in full light, the iron sesquioxide not
being sensitive to light."

"The paper can be prepared in quantities beforehand. To use it I apply
upon each sheet a solution of citric acid at 30 or 35 per cent. of
water(44)--which may be done by daylight--and let them dry in the dark."

"Exposed under a negative of the ordinary intensity, the paper is
impressed in sunshine in a few minutes; in the shade it requires about the
same time as chloride of silver paper."

"After exposure the image is not visible, and without being obliged to
shelter it from light, I immerse the print in a solution containing about
1 per cent. of silver nitrate. This solution can be used over and over
again, by adding to it a little of the silver salt. It does not become
turpid by use; it simply turns slightly green from formation of iron
nitrate. The image appears soon and rapidly becomes vigorous; in half an
hour it will be completely developed. When the exposure is sufficient the
color is deep sepia, but not so intense if the quantity of citric acid is
feeble. No fixing is necessary; it suffices to wash in several changes of
waters."

"The image can be toned with great facility by a weak solution of gold or
of platinum chloride, or, better, by a mixture of these two salts. If the
impressed paper be treated by a very diluted solution of potassium
ferrocyanate, one obtains very pretty blue proofs."

"A weak solution of gold chloride develops a violet image. A solution of
platinum chloride has no effect."

"All the various phases of this printing method can be foll