is upon this property
that Berzelius has founded his determination of the atomic weight of gold.
Light, as well as heat, also operates this precipitation; but to render it
effectual, several conditions are necessary:--First--the solution of gold
should be neutral, or at most _very_ slightly acid; secondly--the oxalic
acid must be added in the form of a neutral oxalate; and thirdly--it must
be present in a certain considerable quantity, which quantity must be
greater the greater the amount of free acid present in the chloride.
Under this condition, the gold is precipitated by light as a black powder
if the liquid be in any bulk; and if merely washed over paper, a stain is
produced, which, however feeble at first, under a certain dosage of the
chloride, oxalate and free acid, goes on increasing from day to day and
from week to week, when laid by in the dark and especially in a damp
atmosphere, till it acquires almost the black of ink; the unsunned portion
of the paper remaining unaffected, or so slightly as to render it almost
certain that what little action of the kind exists is due to the effect of
casual dispersed light incident in the preparation of the paper. I have
before me a specimen of paper so treated in which the effect of thirty
seconds' exposure to sunshine was quite invisible at first, and which is
now of so intense a purple as may be well called black, while the unsunned
portion has acquired comparatively but a slight brown. And (what is not a
little remarkable, and indicates that in the time of exposure mentioned
the _maximum_ of effect was attained) other portions of the same paper
exposed in graduated progression for longer times, viz., one minute, two
minutes, and three minutes, are not in the least perceptible degree darker
than the portion on which the light has acted during thirty seconds only."

"If paper prepared as above recommended for the chrysotype, either with
the ammonio-citrate or ammonio-tartrate of iron, and impressed, as in that
process, with a latent picture, be washed with nitrate of silver instead
of a solution of gold, a very sharp and beautiful picture is developed of
great intensity. Its disclosure is not instantaneous; a few moments
elapse without apparent effect; the dark shades are then first touched in,
and by degrees the details appear, but much more slowly than in the case
of gold. In two or three minutes, however, the maximum of distinctness
will not fail to be obtained. Th