RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 13 
more sensitive than if similarly covered with the chloride, and that if both be washed 
with one and the same solution of nitrate, there is no comparison in respect of this 
valuable quality, the iodide being far superior, and of course to be adopted in prefer- 
ence for the use of the camera. It is however more difficult to fix, the action of the 
hyposulphites on this compound of silver being comparatively slow and feeble. 
38. When the glass is coated with bromide of silver, the action, per se, is very slow, 
and the discoloration ultimately produced far short of blackness ; but when moistened 
with nitrate of silver, S. G. l'l, it is still more rapid than in the case of the iodide, 
turning quite black in the course of a very few seconds’ exposure to sunshine. Plates 
of glass thus coated may be easily preserved for the use of the camera, and have the 
advantage of being ready at a moment’s notice, requiring nothing but awash over with 
the nitrate, which may be delayed till the image is actually thrown on the plate and 
adjusted to the correct focus with all deliberation. The sensitive wash being then ap- 
plied with a soft flat camel-hair brush, the box may be closed and the picture impressed, 
after which it requires only to be thrown into water and dried in the dark to be ren- 
dered comparatively insensible, and may be finally fixed with hyposulphite of soda, 
which must be applied hot, its solvent power on the bromide being even less than on 
the iodide. 
39. Analogous to the chloride, iodide, and bromide of silver, is the fluoride. I have 
not experimented on it, but should it be decomposed by light, either per se, or aided 
by the nitrate, it seems extremely probable that the nascent fluorine or hydrofluoric 
acid disengaged, finding itself in contact with glass, might corrode it, and thus pro- 
duce an etching, a result well worth some trouble to obtain. 
40. Of organic salts of silver precipitated on glass, I have tried several. The results 
are very widely different from those obtained by washing over papers, and will form 
a separate subject of study. Some of them (as the oxalate) form very even and most 
delicate coatings, highly sensible to light, and, what is remarkable, differing greatly 
in this respect according as the side next the glass, or that in contact with air, is ex- 
posed to it. 
41. Light has long been known to reduce the salts of gold as well as silver, and I 
have shown* that platina in some of its combinations is also very powerfully affected 
by the same agent. From the use of these metals it was reasonable to look for results 
of an interesting nature. It might, for instance, be expected that by applying the 
chloride of one or other of them as a ground or mordant for the reception of a wash 
of an argentine solution, the reduction of both the silver and the mordant metal 
would take place, and thus a more rapid and intense blackening would be produced 
than in the case of silver alone. The well-known instability of the oxide of gold under 
deoxidizing influences generally, and the intense colouring pow T er of this metal 
would, a priori, render it probable that cases of unstable equilibrium might occur, 
which the action of a very feeble light might overset. 
* See London and Edinburgh Philosophical Magazine, New Series, No. 1 , 1S32. 
