of the Solar Spectrum on Vegetable Colours. 177 
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 sul- 
phuric acid and well soaked, dried, washed with hydrobro- 
mate of potash, rinsed, and dried again. 
213. Such is the outline of a process to which.I propose 
applying the name of Chrysotype, in order to recal by similarity 
of structure and termination the Calotype processof Mr. Talbot, 
to which in its general effect it affords so close a parallel. Being 
very recent, 1 have not yet (June 10, 1842) obtained a com- 
plete command over all its details, but the termination of the 
Session of the Society being close at hand, I have not thought 
it advisable to suppress its mention. In point of direct sensi- 
bility, the Chrysotype paper is certainly inferior to the Calo- 
type; but it is one of the most remarkable peculiarities of 
gold as a photographic ingredient, that extremely feeble im- 
pressions once made by light, go on afterwards darkening spon- 
taneously, and very slowly, apparently without limit, so long 
as the least vestige of unreduced chloride of gold remains in the 
paper*. ‘Toillustrate this curious and (so far asapplications 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 oxalicacid. It is well 
known to chemists that this acid heated with solutions of gold 
precipitates the metal in its metallic state ; it is upon this pro- 
perty 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 :—Ist, the solution of gold must be neutral, or 
at most very slightly acid ; 2nd, the oxalic acid must be add- 
ed in the form of a neutral oxalate; and 3rdly, 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 these conditions, the gold is pre- 
cipitated 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 blackness of ink; the unsunned portion of the paper 
remaining unaffected, or so slightly as to render it almost 
* Subsequent experiments have convinced me that this property cannot 
be taken advantage of to increase the intensity of the chrysotype impres- 
sion, however it may be available in other processes. Note added during 
the printing, J. F.W. H. 
Phil. Mag. 8. 3. Vol. 22. No. 144. March 1843. N 
