The salt of chromium (bichromate of potash) is
dissolved, and paper steeped in the solution.
The salt thus brought into contact with organic
matter in the paper, enters into chemical union
with it where it is touched by light, and forms
an insoluble compound. So much of it as light
has not touched is washed away after the
picture has been taken on this paper, which is, in
the next place, soaked for a few minutes in the
solution of a salt of iron. The iron adheres
firmly to the mordant image, but is removed
from the rest of the paper by another washing.
Now dip the paper in a solution of gallic acid,
add galls to the iron, and a picture comes out
with fine violet-black tints, which is, in fact, a
picture in writing-ink, as permanent as writing-ink
is known to be. This process has held its
ground, standing the test of wider practice, and
by it photographic pictures can be made that
may be cheap as well as permanent.

A still newer discovery, of which the value is
still open to contest, is M. Niepce de St.
Victor's Uranium process. Its value is said to lie
in its simplicity, its rapidity, and in the
permanence of its results. A piece of paper
washed over with a solution of nitrate of
uranium, and left to dry, printed in a quarter
of an hour, from a negative picture, an intense
positive, which was brought out by dipping into
the nitrate of silver bath. A few small changes
made in the details of Niepce's process, such as
the use of boiled paper, strict attention to the
purity of the uranium salt, and the addition of a
little alcohol to the solution of silver, have
enabled M. O. Hagen to produce by means of
uranium an intense positive picture in half a
minute, or even, by the use of bibulous paper, in
a quarter of a minute. The method is so simple
and rapid that the photographer who uses it in
a good light can readily take from his negative
sixty copies in an hour.

This, again, is not a cheap process, and is only
of value in the printing of photographs in the
ordinary manner; and these photographs, from
the expensive nature of some of the substances
used, and from the time occupied in printing
them, must of necessity remain at a price which
places them beyond the reach of the masses.
There is, however, a recent discovery of a
method of copying photographs upon a metal
plate, which promises to place them within
everybody's reach. The discoverer was Mr.
Fox Talbot, who fully described his method last
year in the Photographic News of the 22nd of
October. A new result is there obtained by the
use of salts of chromium and iron. After coating
a plate of copper, steel, or zinc with a solution
of gelatine in which there is a due proportion
of bichromate of potash, the plate so
prepared is placed in the photographic printing
frame, under the object to be copied. On
exposure to light a minutely delicate fac-simile is
reproduced on the gelatine surface. This is
next covered with a thin film of gum copal,
melted by a spirit lamp, forming what engravers
term an aquatint ground. Over this there is
next spread, with a camel-hair brush, the etching
liquid, a solution of perchloride of iron. Where
the gelatinised surface had been protected from
light by the dark shades of the object to be
copied, the solution penetrates with ease to the
metallic surface, and by its corrosive power will
engrave the dark lines of the picture, while the
gelatine, which had been made insoluble by its
exposure to the light, prevents white surfaces
from being bitten into.

This process is similar in its main features to
that patented by Mr. Fox Talbot in 1852, but
differs from it in several important respects, for
it is able to give half-tones with an accuracy
which is perfectly surprising. We have seen
engravings from plates etched in this way, in
which the microscopic names of tradesmen on
the fronts of their houses were distinctly visible
by help of a good magnifying-glass, though to
the naked eye not only unreadable, but even
invisible. In such a case, the engraving was,
indeed, the copy of a reduced photograph, but this
does not lessen the evidence afforded by it.

A similar contrivance is that which has been
devised by M. Fizeau, of Paris. He takes a
"Daguerrean" silver plate, and uses on it a
mixture of nitrous, nitric, and hydrochloric
acids. This mixture does not attack the whites
of the picture, but the blacks are acted upon
immediately. The resulting chloride of silver,
as it impedes the action of the acid, is removed
with a solution of ammonia, so that the action
may continue. It is complete when a finely-
engraved plate has been produced. The lines
are then filled up with drying-oil, and the
surface electrotyped with gold. The varnish then
having been removed out of the engraved lines,
by means of caustic potash, the surface has
grains of resin sprinkled over it, for the purpose
of producing the engraver's aquatint ground,
and the action of the acid is renewed until the
lines shall have acquired sufficient depth. The
plate being of silver, is too soft to print from,
a copy is therefore taken in copper, by electrotype.
Not long ago there was shown at the
meeting of a scientific society, by Mr. Malone,
a paper covered with representations of coins,
printed from a plate engraved in this manner.
The engraving was so exquisite that each coin
seemed to be presented actually in relief.

M. Charles Nègre's process appears to be a
fresh modification of the same idea. It is asserted
that his plates are "touched" by the hand; but,
however this may be, few people who visited the
Exhibition of Photographs, at South Kensington,
last year, will have torgotten the beautiful plates
he contributed.

These are all plans for the engraving of
photographs on metal plates. The plan of Messrs,
Salmon and Garnier, which, though very
ingenious, we must not stop to describe, produces
a heliographic engraving on brass, which may be
printed from in a lithographic press. But there
is also a distinct process of photo-lithography, in
which a lithographic stone is used. It is coated
with a mixture of gelatine and bichromate of
potash, in a dark room, and, when dry, a
negative—that is, a photograph in which the lights