414 Dr. W. J. Eussell. On Hydrogen Peroxide as the 



would have no longer been visible, or at least only indistinctly so, but 

 experiment shows this is not the case. How then does the peroxide 

 permeate the gelatin 1 Not by the ordinary process of diffusion, for 

 hydrogen cannot diffuse through it, so that it must be by a process of 

 dissolving, or very feebly combining with the medium, or with a con- 

 stituent of it, and thus travelling through escape on the other side. 

 That the action is of this nature seems rendered probable by the 

 following experiments, which show, at least to some extent, what takes 

 place. 



A 2 per cent, solution of hydrogen peroxide was placed in a dish 

 with a sheet of the thinnest gelatin, about one hundredth of an inch 

 thick ; above it and on the gelatin a photographic plate was placed, 

 and allowed to remain there for twenty minutes. No picture was 

 formed. Immediately on removing this first plate from the gelatin, a 

 second one was put in its place, and allowed to remain there also for 

 twenty minutes. This plate gave a faint picture, the third one gave a 

 darker picture, and the fourth one was still darker; but the fifth, 

 sixth, and seventh plates were, as far as could be judged by the eye, of 

 the same degree of darkness. Thus the amount of peroxide given off 

 on the upper surface of the gelatin went on increasing for one hour 

 and twenty minutes, and then the action became uniform. The same 

 kind of action occurs if zinc be used in place of peroxide solution. If 

 a thin sheet of gelatin be laid on a piece of zinc and allowed to remain 

 there for a week, then, on placing above it a sensitive plate, a picture 

 will be produced in about one-third to one-fourth the time which would 

 have been necessary if the previous exposure to the zinc had not 

 taken place. Celluloid was found to act exactly in the same way as 

 the gelatin. The plate, after the first half-hour's exposure, gave no 

 pictures, but a faint one after the second half-hour ; and it was not till 

 after the fourth half-hour that the action became constant. A thicker 

 specimen, 0*011 inch thick, was also examined after intervals of two 

 hours, it acted in the same way as the other specimens, but required 

 ten and a half hours before the action became uniform. If drying oil 

 or copal varnish be used in place of the peroxide of hydrogen solution, 

 analogous results are obtained. This action explains how pictures can 

 be obtained from invisible originals. If, for instance, a piece of white 

 cardboard or paper is placed behind a copper stencil and is exposed to 

 the vapour from peroxide of hydrogen solution, drying oil or copal 

 varnish, &c., the exposed part of the paper becomes active, although 

 not visibly affected, and on placing it on a sensitive plate, a picture of 

 these parts is produced. Zinc acts in the same way, but only slowly. 

 A zinc ornament, laid on a piece of Bristol board for eight months, 

 charged the board only so far as to enable it to give a faint picture. 



Gelatin can be substituted for the paper in these experiments, and 

 can be charged and made to convey a clear picture to a sensitive plate. 



