chrysotype, is thus described by Sir John Herschel:

"In order to ascertain whether any portion of the iron in the double
ammoniacal salt employed has really undergone deoxidation, I had recourse
to a solution of gold, exactly neutralized by carbonate of soda. The
proto-salts of iron, as is well known to chemists, precipitate gold in the
metallic state. The effect proved exceedingly striking, and, as the
experiment will probably be repeated by others, I shall here describe it
ab initio. Paper is to be washed with a moderately concentrated solution
of ammonio-citrate of iron and dried. The strength of solution should be
such as to dry into a good yellow color, not at all brown. In this state
it is ready to receive a photographic image, which may be impressed on it
either from nature in the camera obscura, or from an engraving on a frame
in sunshine. The image so impressed is, however, very faint, and
sometimes hardly perceptible. The moment it is removed from the frame or
camera, it must be washed over with a neutral solution of chloride of gold
of such strength as to have about the color of a sherry wine. Instantly
the picture appears, not, indeed, at once of its full intensity, but
darkening with great rapidity up to a certain point, depending on the
strength of the solutions used, etc. At this point nothing can surpass
the sharpness and perfection of detail of the resulting photograph. To
arrest this process and to fix the picture (so far at least as the further
agency of light is concerned), it is to be thrown into water very slightly
acidulated with sulphuric acid, and well soaked, dried, washed with
hydrobromate of potash, rinsed and dried again. * * *"

"In point of _direct_ sensibility, the chrysotype paper is certainly
inferior to the calotype; but it is one of the most remarkable
peculiarities of gold as a photographic ingredient, that _extremely feeble
impressions once made by light go on afterwards, darkening spontaneously
and very slowly, apparently without limit so long as the least vestige of
unreduced chloride of gold remains in the paper_. To illustrate this
curious and (so far as applications go) highly important property, I shall
mention incidentally the results of some experiments made during the late
fine weather on the habitudes of gold in presence of oxalic acid. It is
well known to chemists that this acid, heated with solutions of gold,
precipitates the metal in its metallic state; it