490 



SCIENCE. 



[N. S. Vol. XI. No. 274. 



action to pass through it, as is seen in this 

 picture of a piece of perforated zinc, a pic- 

 ture which was produced at ordinary tem- 

 peratures. Gold-beaters' skin, albumen, 

 collodion, gutta-percha, are also bodies 

 which are transparent to the action of the 

 zinc and the other active bodies. On the 

 other hand, many bodies do not allow the 

 transmission of the action through them ; 

 for instance paraflBn does not, and among 

 common substances writing-ink does not, as 

 is easily shown by placing ordinary paper 

 with writing on it between the active body 

 and the photographic plate. The active 

 body may conveniently be either a plate of 

 zinc or a card painted with copal varnish 

 and allowed to dry, or a dish of drj'ing oil. 

 The picture of an ordinarily directed en- 

 velope shows this opacity of ink well. It 

 is a propertj' long retained by the ink, as 

 this picture of the direction of a letter, writ- 

 ten in 1801, shows ; also this letter of Dr. 

 Priestley's, dated 1795; and here is also 

 some very faded writing of 1810, which 

 still gives a very good and clear picture. 

 Even if the writing be on parchment, the 

 action passes through the parchment, but 

 not through the ink, and hence a picture is 

 formed. 



With bodies which are porous, such as 

 most papers, for instance, the action passes 

 gradually through the interstices, and im- 

 presses the plate with a picture of the gen- 

 eral structure of the intervening substance. 

 For instance, the following pictures show 

 the structure and the water- mark of certain 

 old and modern writing-papers. Some 

 modern writing-papers are, however, quite 

 opaque ; but usually paper allows the action 

 to take place through it, and combining 

 this fact with the fact of strong activity of 

 the printing-ink, the apparently confused 

 appearance produced on obtaining a picture 

 from paper with printing on both sides is 

 accounted for, as the printing on the side 

 away from the photographic plate, as well 



as that next to it,'prints through the paper, 

 and is, of course, reversed. 



I hope I have now given you a clear idea 

 how a picture can be produced on a photo- 

 graphic plate in the dark, and the general 

 character and appearance of such pictures. 

 I now pass on to the important question of 

 how they are produced. Moser suggested 

 fifty years ago that there was ' dark light,' 

 which gave rise to pictures on polished me- 

 tallic plates, and lately it was suggested 

 that pictures were produced by vapor given 

 off by the metals themselves; the explana- 

 tion, however, which I have to offer you is, 

 I think, simpler than either of these views, 

 for I believe that the action on the photo- 

 graphic plate is due to the formation of a 

 well-known chemical compound, hydrogen 

 peroxide, which undergoing decomposition 

 acts upon the plate and is the immediate 

 cause of the pictures formed. The compli- 

 cated changes which take place on the sen- 

 sitive plate I have nothing to say about on 

 the present occasion, but I desire to con- 

 vince you, that this body, hydrogen perox- 

 ide, is the direct cause of these pictures pro- 

 duced in the dark. Indirect proof has to 

 be resorted to. Water cannot be entirely 

 excluded, for an absolutely dry photo- 

 graphic plate would probably be perfectly 

 inactive, and as long as water is present 

 peroxide of hydrogen may be there also. 

 But what are the conditions under which 

 these pictures are formed? Only certain 

 metals are capable of producing them. This 

 list of active metals which I have men- 

 tioned to you was determined solely by ex- 

 periment, and when completed it was not 

 evident what common property bound them 

 together. Now, however, the explanation 

 has come for these are the very metals 

 which most readily cause, when exposed 

 to air and moisture, the formation of this 

 body peroxide of hydrogen. Schonbein 

 showed as long ago as 1860 that when zinc 

 turnings were shaken up in a bottle with a 



