March 30, 1900.] 



SCIENCE 



491 



little water hj'drogen peroxide was formed, 

 and the delicate tests which we now know 

 for this body show that all the metals I 

 named to you not only can in the presence 

 of moisture produce it, but that their power 

 of doing so follows the same order as their 

 power of acting on a photographic plate. 

 Again, what happened with regard to the 

 organic bodies which act on the photo- 

 graphic plates ? I have already mentioned 

 that in experimenting with the metals it 

 was accidentally observed that copal var- 

 nish was an active substance producing a 

 picture like that produced by zinc, and 

 that the action was traced to the turpentine 

 present ; again a process very much like 

 groping in the dark had to be carried on in 

 order to determine which were active and 

 which inactive organic bodies, and the re- 

 sult obtained was that the active substances 

 essentially belonged to the class of bodies 

 known to chemists as terpenes. Now a 

 most characteristic property of this class of 

 bodies is that in presence of moisture and 

 air they cause the formation of hydrogen 

 peroxide, so that whether a metal or an or- 

 ganic body be used to produce a picture, it 

 is in both cases a body capable, under the 

 circumstances, of causing the formation of 

 hydrogen peroxide. Passing now to ex- 

 perimental facts, which confirm this view 

 of the action on sensitive plates, I may at 

 once say that every result obtained by a 

 metal or by an organic body can be exactly 

 imitated by using the peroxide itself. It is 

 a body now made in considerable quantity, 

 and sold in solution in water. Even when 

 in a vei'y dilute condition it is extremely 

 active. One part of the peroxide diluted 

 with a million parts of water is capable 

 of giving a picture. It can, of course, be 

 used in the glass dishes like any other 

 liquid, but it is often convenient not to 

 have so much water present ; and then it is 

 best to take white blotting paper, wet it in 

 the solution of the peroxide, and let it dry 



in the air. The paper remains active for 

 about twenty- four hours ; or, what is still 

 better, take ordinary plaster of Paris, wet 

 it with the peroxide solution, and let it set 

 ' in a mould' so as to get a slab of it. 

 This slab increases in activity for the first 

 day or two after making, and retains its 

 activity for a fortnight or more. Such a 

 slab will give a good and dark picture in 

 three or four seconds. 



To show how similar the pictures pro- 

 duced by the peroxide and those by zinc 

 are, pictures of a Japanese paper stencil, 

 which had been paraffined to make it quite 

 opaque, have been made by both processes, 

 and are shown with other instances in 

 which turpentine was used in the following 

 slides. It is also very easy to obtain good 

 pictures with the peroxide alone of the 

 structure of paper, etc. ; see, for instance, 

 this one of a five-pound note and these of 

 lace. Again, the strict similarity between 

 the action of the peroxide and that of the 

 metals and organic bodies is further shown 

 by the fact that its action passes through 

 the same media as their action does ; and 

 here are good pictures formed by the action 

 of the peroxide after passing through a 

 sheet of these substances. How this sing- 

 ular transmission can be explained, I have 

 treated of elsewhere, and time does not 

 allow of my discussing the matter to-night. 



There are many ways in which the 

 bright, active zinc surface can be modified. 

 Draw your finger across it, press your 

 thumb upon it, and you stop its activity, as 

 is shown by the picture it will give. Lay 

 a printed paper on the zinc, and let the 

 contact continue for three-quarters of an 

 hour, at a temperature of 55°, then bring 

 the zinc in contact with a sensitive plate, a 

 picture of the printing is formed, but allow 

 the contact between the zinc and printing 

 to continue for eighteen hours at the salne 

 temperature, and the picture then given by 

 the zinc is the reverse of the former one. 



