144 Dr. William James Russell [May 5, 



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 

 mentioned to you was determined solely by experiment, 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. Schon- 

 bein showed as long ago as 1860 that when zinc turnings were shaken 

 up in a bottle with a little water hydrogen 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 

 nroduce 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 varnish 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 result 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 organic body be used to pro- 

 duce a picture, it is in both cases a body capable, under the circum- 

 stances, of causing the formation of hydrogen peroxide. Passing 

 now to experimental 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 very 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 con- 

 venient 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 produced by the peroxide and 

 those by zinc are, pictures of a Japanese paj)er 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 



