330 MR. ROBERT HUNT ON THE INFLUENCE OF IODINE IN RENDERING 
sibility of any admission of atmospheric air; the formation of the black phosphuret 
of silver was going on beautifully, when the large glass vessel burst with such vio- 
lence that the largest piece I could find was but the sixteenth of an inch over. 
I do not at present see any way of preparing those papers with safety, and, much 
against my inclination, I have abandoned the use of this gas. 
18. The exposure of inuriated and simply nitrated photographic paper, darkened by 
light to the vapour of phosphorus, suggested itself. It has been noticed, that most 
vapours have a disposition to attack by the edges; thus it is with that of phosphorus. 
If the paper be suspended, the edges will present a band of silver, slightly coloured, 
long before the other parts exhibit any signs of having been at all acted on. The 
precaution, therefore, of fixing the paper in a frame and exposing its flat surface over 
a large quantity of phosphorus broken into small pieces, is of great importance in the 
preparation. When carefully prepared, this paper stands above the oxides of silver 
for sensibility, but infinitely below the phosphuret. 
19. The vapour of sulphur was next tried, and to a great extent with success. If 
the paper was placed in the vapour while yet wet, the reduction was too rapid, and 
the continuity of the surface was broken. If allowed to dry, a more perfect surface 
was formed, but it was exceedingly apt to become iridescent, and every shade, after 
the iodine was applied, was differently sensitive. It, however, sometimes furnished a 
very perfect paper, capable of producing a fine photographic picture. 
20. The action of sulphurous acid gas was studied with attention, but only when 
the paper was absolutely dry and maintained at a high temperature, could I succeed 
in getting any revival of the silver, which was rapidly attacked by the gas and con- 
verted into a sulphate. 
21. Papers prepared with the nitrate of silver are, when plunged into a vessel con- 
taining sulphuretted hydrogen, speedily covered with sulphuret of silver. The first 
action of the gas is the revival of very white silver, over which a brown shade is 
quickly diffused, which passes into a lead colour if the paper is moist , or becomes va- 
riegated if it is wet. These papers are equal in most respects to those prepared with 
the phosphuretted combination. 
22. If a current of sulphuretted hydrogen be thrown upon a paper still wet with 
solution of nitrate of silver, beautiful coloured rings are produced. When the gas is 
fully saturated with sulphur, these rings are very dark, and of the richest lustre. If 
in this state they are placed between the leaves of a printed volume, a perfect copy of 
the printing is made in twenty-four hours, the letters being white. By passing the 
paper through an alcoholic solution of iodine the letters become a full black. The 
singular nature of this fact will account for its introduction in this place*. 
* On several occasions, when papers variously prepared (but not darkened) and marked and numbered with 
pencil at the back have been laid together, face to back, in the dark for some time, the pencil marks on some 
of them have been found exactly copied (in reverse) in a dark brown impression on their neighbours. This 
