pers to which no iron has been added beyond what exists in
the ferrocyanic salts themselves. Nevertheless, the following experiments
abundantly prove that in several of the changes above described, the
_immediate action_ of the solar rays is not exerted on these salts, but on
the iron contained in the ferruginous solution added to them, which it
deoxidizes or otherwise alters, thereby presenting it to the ferrocyanic
salts in such a form as to precipitate the acids in combination with the
peroxide, or protoxide of iron, as the case may be. To make this evident,
all that is necessary is _simply to leave out the ferrocyanate_ in the
preparation of the paper, which thus becomes reduced to a simple washing
over with the ammonio-citric solution. Paper so washed is of a bright
yellow color, and is apparently little, but in reality highly sensitive to
photographic action. Exposed to strong sunshine, for some time indeed, its
bright yellow tint is dulled into an ochrey hue, or even to gray, but the
change altogether amounts to a moderate percentage of the total light
reflected, and in short exposures is such as would easily escape notice.
Nevertheless, if a slip of this paper be held for only four or five
seconds in the sun (the effect of which is quite imperceptible to the
eye), and when withdrawn into the shade be washed over with the
ferrosesquicyanate of potash, a considerable deposit of Prussian blue
takes place on the sunned part, and none whatever on the rest; so that on
washing the whole with water, a pretty strong blue impression is left,
demonstrating the reduction of iron in that portion of the paper to the
state of protoxide. The effect in question is not, it should be observed,
peculiar to ammonio-nitrate of iron."

"The ammonio and potasso-tartrate fully possess and the perchloride
_exactly neutralized_ partakes of the same property; but the experiment is
far more neatly made and succeeds better with the other salts."

"The varieties of cyanotype processes seem to be innumerable, but that
which I shall now describe deserves particular notice not only for its
pre-eminent beauty while in progress, but as illustrating the peculiar
power of the ammoniacal and other parsalts of iron above-mentioned to
receive a latent picture susceptible of development by a great variety of
stimuli. This process consists in simply passing over the
ammonio-citrated paper on which such a latent picture has been impressed,
_very spar