active Agent in producing Photographs in the Dark. 411 



act on photographic plates, and how pictures of the structure of paper, 

 skeleton leaves, lace, and other bodies can be obtained, has already 

 been described, so that now it is only necessary to say that substitute 

 for these active bodies peroxide of hydrogen, and exactly correspond- 

 ing results are produced. Writing with ordinary ink, or with a solu- 

 tion of 'ferrous sulphate, or potassium ferrocyanide, has been shown to 

 be opaque to the action of zinc and of turpentine, so is it to the action of 

 the peroxide of hydrogen. Further, the action exerted by the metals 

 and the terpenes, is unable to pass through glass, mica, selenite, &c., 

 but is able to pass through thin sheets of gelatin, celluloid, gutta- 

 percha, india-rubber, tracing paper, gold beaters' skin, parchment, &c. 

 Peroxide of hydrogen acts exactly in the same way ; every body which 

 is known to be either opaque or transparent to the action of the metals 

 or terpenes, is opaque or transparent to the action of the peroxide ; so 

 that a far as the production of similar phenomena goes, the agree- 

 ment is complete. Of the acknowledged tests for the presence of 

 hydrogen peroxide, the one with the titanic acid dissolved in sulphuric 

 acid is exceedingly delicate ; so also appears to be the tetramethylpara- 

 phenylenediamine paper of Dr. Wurster, and both of them have been 

 made use of. 



The next point which naturally suggests itself is, whether peroxide 

 of hydrogen is, or is likely to be, present in all the different cases, when 

 action on the sensitive plate occurs. First, with regard to the metals. 

 The list of the active metals, which has already been given, is as follows, 

 arranged approximately in order of their activity : Magnesium, cadmium, 

 zinc, nickel, aluminium, lead, cobalt, bismuth, tin. Now these are 

 certainly the metals which might be expected to decompose water, and 

 in the presence of oxygen cause the formation of hydrogen peroxide ; 

 and still more the order in which they stand in the above list, judging 

 from their general properties, is that in which they would probably 

 induce the formation of the peroxide. It is also satisfactory to note 

 that this list of metals and their order of activity was drawn up 

 simply from experiment, when there was no idea that hydrogen per- 

 oxide had anything to do with the reaction. Again, as a confirmation 

 that hydrogen peroxide is formed when these metals oxidise in moist 

 air, pieces of Dr. Wurster's tetra-paper were moistened and laid on 

 bright surfaces of the metal. With the metals that head the foregoing 

 list a considerable amount of blue colour was rapidly developed, with 

 the metals at the end of the list the amount of colour was less, and the 

 reaction slower ; and with other metals, such as silver and platinum, 

 there was no action. With copper and with iron a very slight amount 

 of action did occur, but these metals do not appear able to produce 

 definite pictures. Iodide of potassium and starch 'paper, when used in 

 the same way, gave a blue colour with all the active metals, but none 

 with copper nor with iron. 



2 I 2 



