So called from the circumstance of cyanogen in its combinations with
iron performing a leading part in the process. It was discovered by
Sir John Herschel. The process is a simple one, and the resulting
pictures are blue.
Brush the paper over with a solution of the ammonio-citrate of iron.
This solution should be sufficiently strong to resemble sherry wine in
color. Expose the paper in the usual way, and pass over it very
sparingly and evenly a wash of the common yellow ferro-cyanate of
potass. As soon as the liquid is applied, the negative picture
vanishes, and is replaced by a positive one, of a violet blue color, on
a greenish yellow ground, which at a certain time possesses a high
degree of sharpness, and singular beauty of tint.
A curious process was discovered by Sir John Herschel, by which dormant
pictures are produced capable of developement by the breath, or by
keeping in a moist atmosphere. It is as follows.
If nitrate of silver, specific gravity 1.200 be added to ferro-tartaric
acid, specific gravity 1.023, a precipitate falls, which is in a great
measure redissolved by a gentle heat, leaving a black sediment, which,
being cleared by subsidence, a liquid of a pale yellow color is
obtained, in which the further addition of the nitrate causes no
turbidness. When the total quantity of the nitrated solution added
amounts to about half the bulk of the ferro-tartaric acid, it is
enough. The liquid so prepared does not alter if kept in the dark.
Spread on paper, and exposed wet to the sunshine (partly shaded) for a
few seconds, no impression seems to be made, but by degrees, although
withdrawn from the action of light, it developes itself spontaneously,
and at length becomes very intense. But if the paper be thoroughly
dried in the dark, (in which state it is of a very pale greenish yellow
color,) it possesses the singular property of receiving a dormant or
invisible picture, to produce which from thirty to sixty seconds'
exposure to sunshine is requisite. It should not be exposed too long,
as not only is the ultimate effect less striking, but a picture begins
to be visibly produced, which darkens spontaneously after it is
withdrawn. But if the exposure be discontinued before this effect
comes on, an invisible impression is the result, to develope which all
that is necessary is to breathe upon it, when it immediately appears,
and very speedily acquires an extraordinary intensity and sharpness, as
if by magic. Instead of the breath, it may be subject to the regular
action of aqueous vapor, by laying it in a blotting paper book, of
which some of the outer leaves on both sides have been dampened, or by
holding over warm water.