GriLL — Effect of Electric Current on Photographic Plates. 77 



Gold coin anode. — The result obtaiaed in this way is very difiereufc from 

 tliose ah-eady noticed. The effect produced around the coin is extremely 

 regular (Plate I., fig. 5). In this case the two reactions described in the 

 beginning of this paper are to be observed. There is the outside ring 

 indicating the distance to which the swelling of the gelatine has extended. 

 The space inside this I'iug does not blacken on development. Around the coin 

 is the blackened region stretching out from the coin in ray-like prominences. 

 The distance to which these rays extend is limited by the outer ring. The 

 blackening effect seems to travel out at a slower rate than the other reaction, 

 until it reaches the limiting ring, beyond which it does not proceed. This 

 splash-like effect is characteristic of gold coins, and may be in some way 

 connected with the copper which is in the alloy. The regularity of the 

 stain produced by gold coins is also noticeable. Plate II., fig. 6, represents a 

 plate on which were placed copper, gold, and silver coins as anodes ; the 

 kathode being a gold coin. 



Plathmm anode. — When platinum is employed, no blackening is produced 

 on the development of the plate. Some platinum foil was folded round a 

 coin and the exposure made in the usual way. There was no effect in addition 

 to the slight swelling of the gelatine, which could be noticed even after the 

 plate had been developed, but not as a black stain. It would seem from this 

 result that the reaction resulting in the blackening of the plate is a compli- 

 cated one necessitating the presence of the two reactions to which I have 

 referred. A very instructive result was obtained which will be of value in 

 helping to explain tlie phenomenon. Plate II., fig. 7, represents the effect 

 produced by a copper coin covered with platinum foil, as already described ; 

 but in this instance there happened to be some small holes in the platinum 

 where it was bent round the edge of the coin. As will be noticed in the 

 figure, there is no blackening produced by the platinum as a whole, but at a 

 few points black lines project out radially. These lines correspond to the 

 holes in the platinum foil, and are evidently due to the copper of the coin 

 around which the platinum was folded. It will be noticed that these lines 

 terminate on the circumference of the outer circle already referred to, 

 pointing to a connexion between the two reactions. 



Bismuth anode. — Produces the result shown in Plate II., fig. 8. 



Carbon anode. — A piece of eleetric-liglit carbon, ground smooth, gave the 

 result shown in Plate II., fig. 9. 



Cobalt and cadmium anodes gave rise to no darkening of tlie plate, thougli 

 the other effect was produced. 



Zinc anode did not produce blackening of plate except at actual point of 

 contact. The non-blackening effect was verj' marked, and it was noticed that 



