418 The active Agent in producing Photographs in the Dark. 



were purified, so that when placed in the dish with the sensitive plate 

 above them no action after a week's exposure took place. However, 

 when a zinc disc was introduced below the inactive liquids the photo- 

 graphic plate was generally acted on, but with the benzene and petro- 

 leum spirit sometimes no action occurred. These rather singular results 

 were next tested in another way. Portions of these inactive liquids 

 were put into stoppered bottles with polished strips of zinc foil, and 

 allowed to remain there for a week, and it was then found that the 

 liquid had become active, for on testing it by putting it into a dish 

 with a photographic plate above it, a dark picture was formed, so that 

 the action of the zinc was to make the whole of the liquid active. 

 Magnesium, cadmium, aluminium, fusible metal, and bismuth, all pro- 

 duced effects similar to those obtained with the zinc, but nickel, lead, 

 tin, &c., produced no such effects. Further it was proved that a very 

 small amount of peroxide rendered alcohol, for instance, very active ; 

 O'l c.c. of a 2 per cent, solution of the peroxide added to 10 c.c. of 

 alcohol gave it the power of acting on a sensitive plate i inch above its 

 surface, so as to produce in a few hours a dark picture. The still more 

 careful purification of these liquids, and especially the exclusion of 

 moisture, was undertaken, and in every case it was found when all 

 moisture was excluded that the zinc had no longer the power, when 

 below a liquid, of acting on a photographic plate. Specimens of alcohol, 

 ether, and chloroform were prepared, and these when placed in a dish 

 with zinc at the bottom of it (standing over sulphuric acid) allowed no 

 action to pass through them, and when treated for a week or more 

 with bright zinc in a bottle still retained their perfect inactivity. To a 

 sample of the alcohol which in a dish with zinc allowed no action on 

 the sensitive plate above to occur, a trace of water was added, as much 

 as adhered to the end of a thin glass rod, and now with the same length 

 of exposure a dark picture was formed. From these experiments, as 

 well as those previously mentioned, it appears that this action on the 

 photographic plate is one of extreme delicacy. 



The action of water alone on zinc is interesting, and appears to con- 

 firm the view that hydrogen peroxide is the active agent in all these 

 reactions. It has already been shown that although bright zinc is 

 active, dull zinc is inactive. However, if a piece of bright zinc be 

 placed in water and remains there for twenty-four hours or so, it, of 

 course, oxidises, white spots or lines appear, and, in fact, in time the 

 whole surface would become covered with oxide. Now the oxide thus 

 formed is strongly active. Take the plate out of the water, let it 

 dry, place it in contact with a photographic plate, and a strong picture 

 of the spots of oxide is obtained. No doubt peroxide of hydrogen is 

 formed, and remains entangled in this porous oxide ; in fact it is difficult 

 entirely to remove it. The plate, with this oxide on it, may be dried 

 at ordinary temperatures and exposed to the air for a day or two, and 



