active Agent in producing Photographs in the Dark. 417 



body related to camphor,* and hence the action passes through it as it 

 does through celluloid, and this is borne out by the fact that if a piece 

 of guttapercha be placed for eighteen hours over the 2 per cent, per- 

 oxide solution, and then placed for twenty minutes on a sensitive plate, 

 it evidently has become active, for it then gives a good picture. 



The above remarks apply also to india-rubber. The thinnest sheet 

 that has been experimented with is O'OIT inch thick ; this allowed the 

 action to pass through it, but was too thick to give a picture, but, like 

 the guttapercha, if placed over the peroxide solution it became active, 

 and produced considerable action on a photographic plate. 



With regard to other substances which allow the action to take place 

 through them, the most interesting are true gold beater's skin, and albu- 

 men. If Bristol board or paper be carefully painted on one side with 

 white of egg and allowed to dry in the air, it forms a medium through 

 which the peroxide can pass. Collodion also allows the action readily 

 to pass through it. In all these cases the tetra-paper may be used to 

 confirm the results obtained. 



Then with regard to bodies which do not allow the action to pass 

 through them. Paraffin is one of them. If paper be painted with 

 melted paraffin and it be placed over a solution of the peroxide, no 

 action passes through, neither is it able to absorb the peroxide like 

 camphor and india-rubber and guttapercha. A piece of paraffin placed 

 over the peroxide solution for twenty hours and then tested by placing 

 it on a sensitive plate produced no action. 



Gum arabic is a body which sometimes is very opaque, but this is 

 simply a question of hydration, and is confirmatory of what has been 

 said before with regard to the action of water. Some unglazed paper 

 was painted on one side with two coats of good gum arabic, and some 

 of it was dried at 55 for some days, and another portion of it was air- 

 dried only for some hours, and both were put over drying oil for three 

 days. The dried paper gave only a very faint picture, but the more 

 moist one a very dark picture. 



When experimenting some time ago on the general nature of these 

 reactions, polished zinc was placed below some inactive liquids to test 

 whether any action took place through them. The small glass dishes 

 were used, and a disc of bright zinc laid inside, and the liquid to be 

 tested poured upon it ; then the photographic plate was placed on the 

 top of the dish. After remaining there for three or four days the plate 

 was generally found acted on as if the zinc had been able to exert its 

 influence upon it. Lately these experiments have been repeated and 

 extended, and, as indicating the extreme delicacy of the reaction with 

 the photographic plates, are of interest. The form of experiment was 

 the same as described above, and the liquids used were alcohol, ether, 

 ethyl acetate, chloroform, benzene, petroleum spirit. All these liquids 

 * Beruthsen, ' Organic Chemistry,' p. 509. 



