428 Dr. W. J. Russell. On the Action exerted by certain 



list between aluminium and lead. That certain alloys should act in 

 the same way as the metals is certainly of interest, and probably of 

 considerable importance. The oxide and sulphate, both of zinc and 

 cadmium, were found to be devoid of any power of acting on the 

 photographic plate. Iron, gold, and platinum, are not active, and 

 copper only very slightly. All the above results are founded on 

 experiments in which the exposure lasted for one week ; with longer 

 exposure other metals will probably produce some action. 



In order to determine whether moisture was an active agent either 

 directly by affecting the medium or indirectly by affecting the photo- 

 graphic plate, experiments were made by exposing the plates under 

 bell-jars, in which in one case there was water, and in the other 

 sulphuric acid or calcium chloride, and even in these extreme cases 

 no appreciable difference was found to occur, and even if the 

 membrane was purposely damped it did not appear to aid the action 

 except by bringing it closer to the metal, so that aqueous vapour is 

 not apparently an active agent in producing these reactions. In an 

 atmosphere of hydrogen the action takes place as it does in air. 

 Carbon dioxide, under ordinary conditions, does produce an effect, 

 but this probably arises from its action on the zinc plate. Altera- 

 tion of temperature produces very marked effects ; increase the 

 temperature, and the action of the zinc is greatly increased ; for 

 instance, two similar plates, both wrapped in tinfoil and the 

 plates separated from the zinc by means of a cardboard frame. 

 One was placed on a water bath and exposed to a temperature of 

 about 70° C.j and the other placed in a vessel of ice at 0° C. After 

 five hours the one which had been exposed to the high temperature 

 had given a black picture, while the one at the low temperature gave 

 a picture barely visible. A similar experiment was also made with 

 nickel, and this gave, after heating to about 70°, a good dark picture, 

 but the corresponding experiment, when the metal was kept at C 

 for five hours, gave no picture at all. Aluminium, when treated in 

 the same way, gave at the higher temperature only a faint picture, 

 but at the lower temperature, even after two days, no picture at all. 

 It has already been mentioned that this action of the metals cannot 

 pass through even thin glass, nor can it pass through selenite, nor a 

 layer of gum arabic, nor one of paraffin. Glass being impervious to 

 the action, renders it somewhat difficult to try satisfactorily the 

 action exerted by liquids, but celluloid may be used for this purpose ; 

 also mercury may be covered with a thin layer of water, and then its 

 action entirely ceases. The action of certain salts in the dry state 

 has, however, been tried by soaking non-glazed paper in different 

 solutions, drying it, and then placing it, either with or without a 

 screen, between the zinc and the photographic plate. These experi- 

 ments have given some interesting results ; for instance, paper 



