RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 15 
impregnated with nitrate of silver. But if, in place of the nitrate of silver, a wash of 
hydriodate of potash be superadded, the effect is remarkable. If the hydriodic so- 
lution be strong and plentiful, the paper is immediately coloured dark brown, whether 
in light or darkness. If very weak, no effect ; but if applied of a certain interme- 
diate strength, though not immediately affected in the shade, yet, if held (while wet) 
in the sun, it darkens with extraordinary rapidity to the same deep brown hue, and 
presently after , the exposure to the sun continuing , whitens again. A fresh dose of the 
hydriodate being applied, it again darkens, but is no longer capable of restoration, 
and the darkness then goes on increasing to a fine deep chocolate brown. The paper 
with which I have found this experiment to succeed best is called “ smooth wove 
demy.” It requires a clear and steady sun. If gold be substituted for platina, the 
hydriodic salt exercises, in virtue of its ordinary chemical relations, so powerful a 
darkening effect, that the influence of light can hardly be traced ; but, as far as can 
be ascertained, something of the same sort obtains here also. 
48. In the experiment of the last article we see that the influence of the hydriodic 
salt, contrary to its usual habitudes (see Art. 17), is to exalt the deoxidizing action 
of the light, and even to call that action into evidence where it either did not exist, 
or was masked before. This is not, however, by any means a singular instance. 
Hydriodate of potash, when very weak, acts in combination with nitrate of silver to 
produce a paper much more sensitive than the nitrate alone, the remarkable pecu- 
liarities of which will be noticed further on. A paper endowed with a pretty high 
degree of sensibility may also be prepared with the following triple application, viz. 
1st, Acetate of lead; 2nd, Hydriodate of potash; 3rd, Nitrate of silver. And here 
also a phenomenon, something of the same kind as was described in the subsequent 
part of the experiment of the last article, occurs ; for if paper so prepared and dark- 
ened in the sun be washed over with a fresh dose of the hydriodate, the exposure to 
sunshine being continued, it whitens with great rapidity* ; and were it practicable 
(which I have not found it) to ensure precisely the same ingredient-proportions and 
the same degree of blackening in the sun to start from, I should not hesitate to pro- 
pose this as an excellent process for a positive photographic paper*. A still more re- 
markable example of the power of this salt to stimulate the darkening action of the 
sun’s rays, is to be found in its combination with the tartrate of silver. But of this 
more hereafter, when we have become better acquainted with the separate action of the 
different elementary rays, which it is necessary to study before we can understand the 
complex relations which enter into these phenomena. To this therefore I hasten. 
IV. Chemical Analysis of the Solar Spectrum. 
49. That rays of different colours and refrangibilities have not all an equally ener- 
getic action in effecting chemical changes, has long been understood ; and that the 
* Another paper which possesses this property in an eminent degree is prepared with 1st, acetate of lead ; 
2nd, sulphate of soda; 3rd, nitrate of silver, darkened in the sun and washed with the hydriodate. 
