OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
207 
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 ex- 
tremely 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 chlo- 
ride of gold remains in the paper*. To illustrate this curious and (so far as applica- 
tions 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 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 neces- 
sary : — 1st, the solution of gold must be neutral, or at most very slightly acid ; 2nd, 
the oxalic acid must be added 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 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 blackness 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 well be called black, while the unsunned portion has ac- 
quired comparatively but a very slight brown. And (which 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. l m , 2 m , and 3 m , are not in the least perceptible degree darker than 
the portion on which the light had acted during thirty seconds only. 
214. The very remarkable phenomenon described in Art. 208. of a second darken- 
ing, different in character and colour, coming on after the bleaching effect of solar 
light has been fully completed, is not without a parallel among the argentine com- 
pounds. I refer to the action of the hydriodic salts on argentine papers completely 
blackened by exposure to sunshine, an action imperfectly described in § 5. of my 
former paper (Art. 94 et seq.), and signalized as to one of its most striking peculi- 
arities in Note 2, Art. 129. of that communication. To study the phenomena of this 
action in their simplest form, a paper prepared without iodine, and of a positive 
* Subsequent experiments have convinced me that this property cannot he taken advantage of to increase 
the intensity of the chrysotype impression, however it maybe available in other processes. Note added during 
the printing, J. F. W. H. 
