ingly and evenly_, a wash of the solution of the common yellow
ferrocyanate (prussiate) of potash. The latent picture, if not so faint
as to be quite invisible (and for this purpose it should not be so), is
negative. As soon as the liquid is applied, which cannot be in too thin a
film, the negative picture vanishes, and by very slow degrees is replaced
by a positive one of a violet blue color on a greenish yellow ground,
which at a certain moment possesses a high degree of sharpness and
singular beauty and delicacy of tint. If at this instant it be thrown
into water, it passes immediately to a Prussian blue, losing, at the same
time, however, much of its sharpness, and sometimes indeed becoming quite
blotty and confused."

"To prevent this confusion gum arabic may be added to the prussiated
solution, by which it is hindered from spreading unmanageably within the
pores of the paper, and the precipitated Prussian blue allowed time to
agglomerate and fix itself on the fibers. By the use of this ingredient
also, a much thinner and more equal film may be spread over the surface,
and _when perfectly dry_, if not sufficiently developed, the application
may be repeated. By operating thus I have occasionally (though rarely)
succeeded in producing pictures of great beauty and richness of effect,
which they retain (if not thrown in water) between the leaves of a
portfolio, and have a certain degree of fixity--fading in strong light and
recovering their tone in the dark. * * *"

"If paper be washed with a mixture of the solutions of ammonio-citrate of
iron and ferrosesquicyanate (red prussiate) of potash, so as to contain
the two salts in about equal proportions, and being then impressed with a
picture, be thrown into water and dried, a negative blue picture will be
produced. This picture I have found to be susceptible of a very curious
transformation. To effect this it must be washed with a solution of
protonitrate of mercury, which in a little time entirely discharges it.
The nitrate being thoroughly washed out and the picture dried, a smooth
iron is passed over it, somewhat hotter than is used for ironing linen,
but not sufficiently so to scorch or injure the paper. The obliterated
picture immediately reappears, not blue, but brown. If kept for some weeks
in this state between the leaves of a portfolio, in complete darkness, it
fades, and at length almost disappears. But what is very singular, a
fresh application of heat