n of the acid, and
the acid by the destruction of another). After half an hour or an hour's
exposure to sunshine, a very beautiful negative photograph is the result,
to fix which, all that is necessary is to soak it in water in which a
little sulphate of soda is dissolved. While dry the impression is of a
dove color or lavender blue, which has a curious and striking effect on
the greenish yellow ground of the paper produced by the saline solution.
After washing the ground color disappears and the photograph becomes
bright blue on a white ground. If too long exposed, it gets
'over-sunned,' and the tint has a brownish or yellowish tendency, which,
however, is removed in fixing; but no increase of intensity beyond a
certain point is obtained by the continuance of exposure."

"If paper be washed with a solution of ammonio-citrate of iron and dried
and then a wash passed over it of the yellow ferro-cyanuret of potassium,
there is no immediate formation of true Prussian blue, but the paper
rapidly acquires a violet-purple color, which deepens after a few minutes,
as it dries, to almost absolute blackness. In this state it is a positive
photographic paper of high sensibility, and gives pictures of great depth
and sharpness, but with this peculiarity, that they darken again
spontaneously on exposure to the air in darkness, and are soon
obliterated. The paper, however, remains susceptible to light, and
capable of receiving other pictures, which in their turn fade, without any
possibility (so far as I can see) of arresting them, which is to be
regretted, as they are very beautiful, and the paper of such easy
preparation. If washed with ammonia or its carbonate, they are for a few
moments entirely obliterated, _but presently reappear with reversed lights
and shades_. In this state they are fixed, and the ammonia, with all that
it will dissolve, being removed by washing in water, their color becomes a
pure Prussian blue, which deepens much by keeping. If the solution be
mixed there results a very dark violet-colored ink, which may be kept
uninjured in an opaque bottle, and will readily furnish by a single wash
at a moment's notice the positive paper in question, which is most
sensitive when wet."

"It seems at first sight natural to refer these curious and complex
changes to the instability of the cyanic compounds; and that this opinion
is to a certain extent correct is proved by the photographic impressions
obtained on pa