revives and restores it to its full intensity."

"This curious transformation is instructive in another way. It is not
operated by light, at least not by light alone. _A certain temperature_
must be attained, and that temperature suffices in complete darkness.
Nevertheless, I find that on exposing to a very concentrated spectrum
(collected by a lens of short focus) a slip of paper prepared as above
(that is to say, by washing with the mixed solutions, exposure to
sunshine, washing and discharging the uniform blue color so induced, as in
the last article), its whiteness is changed to a brown over the whole
region of the red and orange rays, _but not beyond_ the luminous spectrum.
Three conclusions seem unavoidable: first--that it is the heat of these
rays, not their light, which operates the change; second--that this heat
possesses a peculiar chemical quality which is not possessed by the purely
calorific rays outside of the visible spectrum, though far more intense;
and third--that the heat radiated from obscurely hot iron abounds
especially in rays analogous to those of the region of the spectrum above
indicated."

Sir John Herschel then proceeds to show that whatever be the state of the
iron in the double salts in question, its reduction by blue light to the
state of protoxide is indicated by many other agents. "Thus, for
example," says Robert Hunt, "if a slip of paper prepared with the
ammonio-citrate of iron be exposed partially to sunshine, and then washed
with the bichromate of potash, the bichromate is deoxidized and
precipitated upon the sunned portion, just as it would be if directly
exposed to the sun's rays."

"I have proved this fact with a great number of preparations of cobalt,
nickel, bismuth, platinum and other salts which have been thought hitherto
to be insensitive to the solar agency; but if they are partially sunned
and then washed with nitrate of silver and put aside in the dark, the
metallic silver is slowly reduced upon the sunned portion. In many
instances days were required to produce the visible picture; and in one
case paper being washed in the dark with neutral chloride of platinum was
sunned and then washed in the dark with nitrate of silver; it was some
weeks before the image made its appearance, but it was eventually
perfectly developed, and, when quite so, remained permanently impressed
upon the paper."

The following process, discovered at the same time as the cyanotype, and
termed