active Agent in producing Plwtocjraplis in the Dark. 415 



It is then evident that the action arising from zinc and other active 

 bodies can, by an intermediate and inactive substance, be carried away 

 and allowed to expend itself at another time and at another place. 



With regard to the transmission of the action through gelatin, the 

 water which it contains is probably the body which enables the per- 

 oxide to pass through. It can also be shown that it aids the trans- 

 mission of the action through other inactive bodies, for instance if 

 Bristol board in its ordinary condition be placed on a polished piece of 

 zinc, the action of the zinc only slowly passes through it, but if the 

 board be damp the transmission takes place much more rapidly. The 

 following comparative experiments illustrate this. Two similar pieces 

 of Bristol board were taken, one was dried and then placed between a 

 piece of perforated zinc and a sensitive plate and put under a bell jar 

 with calcium chloride; the other piece of Bristol board was suspended 

 over water until it was thoroughly damp, and then placed between per- 

 forated zinc and a sensitive plate under a bell jar with a little water 

 present. Both experiments were continued for twelve days, when it waa 

 found that with the dry board there was no picture produced, but 

 with the damp one there was a good and dark one. If copal varnish 

 be used in place of zinc, similar results are obtained, and if parch- 

 ment be substituted for Bristol board the results are the same. 



These experiments are however not conclusive, for it has been shown 

 that with additional amount of water some of it finds its way to the 

 zinc, and there induces the formation of more peroxide which may 

 account for the darker pictures. Even with the terpenes the additional 

 amount of water may induce the additional formation of peroxide. 

 This objection can however be obviated by cutting off the moisture in 

 the damp medium from the active substance, or by using the aqueous 

 solution of the peroxide as the origin of the action. In order to stop 

 the aqueous vapour from either passing from the damp Bristol board 

 or to it from the peroxide solution, a piece of tracing paper is inter* 

 posed which allows the action to pass through it, but not any appreci- 

 able amount of aqueous vapour. On placing a sheet of tracing paper over 

 a glass dish containing the peroxide solution and above it dry Bristol 

 board Vith a photographic plate, in one and a half hours just an indication 

 of a picture was produced, but when under the same conditions Bristol 

 board which had been over water for nineteen hours was used, then a 

 dark picture was formed. Again similar experiments were made using a 

 not highly glazed paper in place of the Bristol board, and the results 

 were the same. 



In place of tracing paper, celluloid was used and the dry Bristol 

 board gave, under similar conditions, no picture, but the damp one gave 

 a very distinct picture. In order to avoid having so much water present, 

 plaster of Paris set by a little of the peroxide solution was used in 

 place of the aqueous solution, and exactly similar results were obtained, 



