76 Scientific Proceedings, Royal Dublin Society. 



observed in the red light of the dark room during progress of the exposure. 

 This reaction consists in a slight elevation or swelling of the gelatine surface 

 of the plate, which is- seen to spread out more or less uniformly around the 

 anode. This reaction does not seem to give rise to a blackening of the plate, 

 as will appear immediately The other anode effect is apparently connected 

 closely with, or influenced by, that just referred to, but is not always present. 

 This second reaction results in a blackening of the plate on development. 

 The former of these reactions would appear to limit the distance to which 

 the latter extends. Sometimes the blackening of the plate extends to the 

 limits of the former, and sometimes not so far. 



The rate at which these effects spread out gradually falls off, and at the 

 same time the current decreases. In one experiment the current fell from an 

 initial value of 4 to 8 milliamperes in ten minutes. The falling off of the, 

 current seems to be due to some polarization effect, and explains the fact 

 that an exposure of ten minutes produces almost as marked a result as one 

 of an hour's duration. Sometimes there is a metallic deposit at the point of 

 contact of the anode. 



It will be necessary to examine certain oases in detail. In some of these 

 eases it will be sufficient to illustrate the anode effect, as that at the kathode 

 is practically the same in every case. 



Copper anode. — In the case of copper the two effects seem to be produced 

 and the blackening of the plate extends from the metal to the limiting 

 boundary of the other reaction. Plate L, fig. 2, shows the resiilt obtained by 

 using a piece of copper plating. It will be noticed that the kathode effect is 

 much less marked, and is most evident at the corners. This effect seems to 

 follow the direction of the lines of force. The result obtained by making use 

 of copper coins is not so intense as when purer copper is employed. 



Silver anode. — The result obtained when a silver coin is employed is 

 characteristic. The space round the ooin is divided into two clearly defined 

 regions. Nearer the coin the plate is not blackened ; in fact, sometimes, as in 

 the case of Plate I., fig. 3, the developed plate is extraordinarily clear in this 

 region, and far more transparent than in any other portion of the plate, even 

 than in those parts which were not exposed to any luminous or electric 

 influences. Outside this transparent ring is a region of intense blackening. 

 This ring extends around the whole of the inner ring, but it is generally 

 further on the side facing the kathode. 



Iron anode. — The result obtained in this case is similar to that produced 

 by silver (Plate I., fig. 4). 



Nickel anode. — Produces a slightly different effect, though comparable to 

 that obtained when iron is used. 



