active Agent in producing Photographs in the Dark. 413 



test for the presence of all these bodies, and as the action is cumulative 

 it may even compete with the sense of smell. 



Ip addition to the essential oils, the ordinary vegetable oils, such as 

 linseed oil, which is the most active, and colza and olive, which are 

 much less active and have much less power of absorbing oxygen from 

 the air, can act on a photographic plate. The tetra-paper readily goes 

 blue if suspended in a bell-jar which has a few drops of linseed oil in a 

 dish within it. 



The mineral oils are, on the contrary, devoid of this power of acting 

 on the sensitive plate, and the same applies to bodies such as benzene, 

 phenol, naphthalene, aldehyde, methyl alcohol, coal naphtha, &c. 



It would seem, then, that all the organic bodies capable of acting on 

 the photographic plate are capable of giving rise to the formation of 

 hydrogen peroxide when they oxidise in moist air. 



In former papers it has been shown that the active bodies, both 

 metallic and organic, are able to act on a photographic plate even 

 when thin layers of many different substances are interposed; for 

 instance, if a thin sheet of gelatin be laid on a polished zinc plate it 

 only very slightly modifies either the sharpness of the picture or the 

 time required for its production. If the gelatin plate be thicker the 

 action will still pass through, but the picture will be more indistinct, 

 and the time necessary for its production longer. If a 2 per cent, 

 solution of hydrogen peroxide be poured into one of the small glass 

 dishes, and a sheet of gelatin 0*0013 inch thick be placed over it -J inch 

 above the liquid, a picture will be obtained in fifteen minutes. If the 

 sheet of gelatin be 0*008 inch thick, then the exposure must be for 

 one hour; and if the gelatin be 0*01 inch thick, an exposure of three 

 hours is necessary. If a sheet of celluloid be substituted for the 

 gelatin, and it be 0'005 inch thick, the action still passes through, but 

 more slowly than through the gelatin, and the plate now requires one 

 hour exposure to give a good picture. With a plate of celluloid of 

 double the above thickness, the exposure must be four times as long ; 

 and if the thickness be 0*033 inch, the time of exposure has to 

 be thirty hours. These determinations show well what happens in 

 these cases, but are only good approximations, not standard results. 

 In addition to gelatin and celluloid, guttapercha tissue, india-rubber, 

 tracing paper, collodion, albumin, gold beater's skin, parchment, &c. 

 also allow the action to take place through them, and the obvious 

 question which presents itself is, If hydrogen peroxide be the body 

 which gives rise to the action, how does it pass through these different 

 bodies 1 Take the definite case of zinc ; if a plate of this metal be 

 rubbed with coarse sand paper and placed in contact with a photo- 

 graphic plate, a clear and sharp picture of the scratches is obtained, 

 and it might have been expected that when the action took place 

 through even a very thin sheet of gelatin the picture of the scratches 



