328 MR. ROBERT HUNT ON THE INFLUENCE OF IODINE IN RENDERING 
in front of a clear fire, to a point just below that at which it would be scorched, a 
perfect formation of the olive oxide of silver results. 
The iodine vapour requires an equally long time to produce any visible effect upon 
this paper, as upon those before mentioned (8. 9.). But half an hour is sufficient to 
render this preparation so far iodidated as is necessary, or indeed as is possible, with- 
out impairing its sensibility. 
This iodidated oxide requires a strong light to yield any decided effect, and a 
greater heat than is directed by Daguerre must be applied to the mercury to bring 
out the impression, which is but faint at last. 
10. The protoxide of silver, and some of its salts, now engaged my attention. 
Taking advantage of the facts pointed out by Wohler*, I exposed papers saturated 
with a solution of the nitrate of silver, and dried in a cylinder which was kept hot by 
boiling water, to a current of very pure heated hydrogen gas. By this means I ob- 
tained a paper of a brick-red colour. This appears to be a nitrate of the protoxide of 
silver ; by soaking this paper in rain water for some time, the free acid was removed 
from it, and on drying it became darker than before. 
This paper is not very readily attacked by the iodine, but it is, by the exposure of 
an hour to a full volume of its vapour, rendered slightly sensitive. 
11. By immersing a paper prepared as above (10.) in a solution of pure potassa, 
the salt is decomposed, and the paper on drying becomes a full black, which on 
being passed through steel rollers, assumes a fine lustre, resembling that given by 
Plumbago. 
This paper is more readily attacked by the iodine than the former (10.), but it is 
not more sensitive. In bright sunshine I have produced a tolerable copy of a leaf 
with its delicate vernations, but in the camera I have never succeeded as I desired, or 
indeed as I expected. 
12. The deutoxide of silver is reduced to the metallic state by hydrogen at high 
temperatures. By the same arrangement as above (10.) I was enabled to get a very 
perfect silver paper, which in all respects was similar to the paper which is mecha- 
nically silvered (3.). 
13. Having proved that iodine has the power of rendering sensitive, not only the 
oxides of silver, but a salt of the protoxide, it became an interesting inquiry to as- 
certain whether it would produce any like effect upon other salts of silver. The only 
one, however, I have yet discovered which admits of the desired influence, is the 
yellow-brown phosphate of silver. The iodine in two hours does not produce the 
slightest change of colour, but exposure to light and the vapour of mercury shows 
that the iodine imparts some sensitiveness to this salt. 
14. A careful examination of the silver surfaces produced by the action of the per- 
phosphuretted hydrogen gas on various salts of that metal, now occupied my atten- 
tion, and undividedly engaged it for a very considerable time. 
* Journal de Pharm., Juillet, 1839. 
