410 Dr. W. J. Eussell. On Hydrogen Peroxide as the 



these experiments, but as many of the pictures are only formed after 

 a long exposure, it is well to use rapid plates. In the following ex- 

 periments the plate used has been in almost all cases the " Ilford 

 special rapid," and the process of development has in every case been 

 that recommended for their ordinary use. 



The first step towards demonstrating that hydrogen peroxide is the 

 active agent in producing these pictures, is to show that all the results 

 produced both by metals and by organic bodies on a photographic 

 plate, can be produced by hydrogen peroxide. This body is now made 

 in considerable quantities and sold in aqueous solution of a given 

 strength. This commercial article appears to act equally well to a 

 carefully prepared and pure specimen of the same strength. 



A convenient way of testing the action of any liquid on a photo- 

 graphic plate is to use a small circular glass dish, such as is made for 

 bacteriological experiments, the photographic plate resting on the top 

 of the dish, and the amount of the liquid used determines the distance 

 the plate is from the active surface, the experiment being carried on in 

 complete darkness. If pure water be tested in this way, it is found 

 that no picture, that is no darkening of the plate, occurs on its being 

 treated with the developing solution. The plate can be left over the 

 water for eighteen to twenty hours, but if left longer than this, the film 

 is destroyed by the aqueous vapour. If to the pure water in the dish 

 a mere trace of hydrogen peroxide be added, a darkening of the plate 

 will quickly occur. For instance, if the liquid contains only one part 

 of the peroxide in a million of water, and the plate be exposed to its 

 action for eighteen hours, a faint picture is produced. Bearing in 

 mind the small amount of evaporation which takes place under these 

 conditions, and consequently the minute amount of the peroxide which 

 comes in contact with the plate, it clearly shows the exceeding delicacy 

 of the reaction. 



Again, if a piece of Ford blotting paper, which by itself is inactive, 

 after being wetted with a solution of one part of peroxide in 500,000 of 

 water, and hung up in a warm room for three quarters of an hour to dry, 

 is placed in contact with a photographic plate for two hours at a tem- 

 perature of 55 C., on subjecting the plate to development a distinct 

 picture is produced. In fact, moistening good blotting-paper with a 

 solution which may be strong or weak, and allowing it to dry for a 

 long or short time, is a very good way of applying the peroxide. In 

 place of blotting paper any inactive porous substance may be used. 



Plaster of Paris wetted with a peroxide solution and allowed to set, 

 continues for a long time to be an active body. If by any of these means 

 a large, in place of a small, amount of the peroxide be allowed to act 

 on a plate, then in place of a dark, a light picture is obtained, a pheno- 

 menon similar to what is known to photographers as reversal. 



The conditions under which certain metals and certain organic bodies 



