Metals and other Bodies on a Photographic Plate. 109 



cardboard has to be exposed to the zinc for fully six weeks. This 

 changing of the Bristol board does not take place satisfactorily 

 above ordinary temperatures. With other metals than zinc, these 

 changing effects have not as yet been obtained. 



Another experiment, which illustrates the way in which the 

 metals can act, is to take a piece of ordinary perforated zinc, polish 

 one side, and lay this polished side against a plate of plain glass in 

 a printing frame, then place the photographic plate against the dull 

 side of the perforated zinc, and leave it in the dark for three or four 

 days ; then, on developing the plate, a reversed picture is obtained, 

 that is, the holes in the zinc will be represented by dark spaces, and 

 the zinc itself by light ones. If the holes in the perforated zinc are 

 large, they are represented by shaded circles, so that these pictures 

 are produced by the vapour emitted by the polished zinc which has 

 crept into the open spaces and thus gained access to the photo- 

 graphic plate. It has already been shown that the action exerted 

 by zinc passes more readily down a glass than down a paper tube of 

 the same size ; this has been strikingly confirmed by taking two 

 pieces of glass tubing 1 inch long f inch in diameter ; inside one 

 a single coil of inactive paper was placed, and both tubes stood on 

 a sheet of polished zinc, and a photographic plate rested on the 

 top of them. They were then left for a week, and on developing the 

 plate, a black patch appeared above the tube without the paper, 

 and no action was visible above the one with the paper. Without 

 removing the paper, it was painted over with melted paraffin, and 

 again a photographic plate put on the top of the two tubes ; now 

 two circular dark patches were produced of equal intensity. 



If the activity of the zinc depends on a vapour which it emits, it 

 seemed possible that it could be carried along by a stream of air. 

 In order to^try whether this was the case, a tube a foot long was 

 packed with zinc turnings which had been amalgamated, and a 

 stream of pure air Fent through it. The end of the tube was fixed 

 into the side of a dark box and a sensitive plate with a screen upon 

 it suspended above it, thus no direct action could be exerted by the 

 amalgamated zinc on the plate. The experiment was continued for 

 four days, then, on developing the plate, a picture of the screen 

 above where the tube entered the box was obtained, but at the other 

 end of the plate there was no action. 



The presence of mercury in this experiment was unsatisfactory, 

 and might account for the result obtained, therefore an exactly 

 similar experiment was made, and zinc turnings alone were used and 

 a plate without a screen. The experiment was carried on for a week, 

 and it was then found that a black patch had been produced imme- 

 diately above the end of the tube. To be sure that this darkening 

 did not arise from the action of the air, the whole of the zinc was 



