of Mr. C. J. Burnett, on the salts of
uranium, etc., as an Introduction. It will be seen that the process by
which blue prints are to-day obtained is exactly that Sir John Herschel
devised in 1840.

"It is no longer an insulated and anomalous affection of certain salts of
silver or gold, but one which, doubtless, in a greater or less degree,
pervades all nature, and connects itself intimately with the mechanism by
which chemical combination and decomposition is operated. The general
instability of organic combinations might lead us to expect the occurrence
of numerous and remarkable cases of this affection among bodies of that
class, but among metallic and other elements inorganically arranged,
instances enough have already appeared, and more are daily presenting
themselves, to justify its extension to all cases in which chemical
elements may be supposed combined with a certain degree of laxity, and so
to speak in a _tottering equilibrium_. There can be no doubt that the
process, in a great majority, if not in all cases, which have been noticed
among inorganic substances, is a deoxidizing one, so far as the more
refrangible rays are concerned. It is obviously so in the cases of gold
and silver. In the case of the bichromate of potash it is most probable
that an atom of oxygen is parted with, and so of many others. A beautiful
example of such deoxidizing action on a non-argentine compound has lately
occurred to me in the examination of that interesting salt, the
ferrosesquicyanuret of potassium described by Mr. Smee in the
_Philosophical Magazine_, No. 109, September, 1840, and he has shown how
to manufacture in abundance and purity, by voltaic action on the common or
yellow ferrocyanuret. In this process nascent oxygen is absorbed,
hydrogen given off, and the characters of the resulting compound in
respect of the oxides of iron, forming as it does Prussian blue with proto
salts, indicate an excess of electro-negative energy, a disposition to
part with oxygen, or which is the same thing, to absorb hydrogen (in the
presence of moisture), and thereby to return to its pristine state, under
circumstances of moderate solicitation, such as the affinity of protoxide
of iron (for instance) for an additional dose of oxygen, etc."

"Paper simply washed with a solution of this salt is highly sensitive to
the action of the light. Prussian blue is deposited (the base being
necessarily supplied by the destruction of one portio