Metals and other Substances on a Photographic Plate. 427 



plate, interesting results are obtained, for the pictures formed 

 show clearly the structure of the papers, and also show that papers 

 have very different powers of transmitting this action. Certain 

 writing papers are quite opaque to the action ; with others pictures 

 of the structure and the watermark are easily obtained. 



The painting a paper with India ink does not destroy its trans- 

 parency. Obviously pictures of bodies, such as skeleton leaves or 

 dried flowers, &c, are easily obtained in this way. 



A mere difference of colour does not appear to alter the absorptive 

 power of a medium ; at least, this is the case with gelatine. The thin 

 sheets of gelatine, whether red, green, or blue, have no difference in 

 their absorptive power, and when gelatine, coloured with aniline dyes, 

 is painted on polished zinc, the colour does not affect the amount of 

 action which takes place. The same thing happens if demy paper 

 he painted with different coloured solutions of gelatine. With ordi- 

 nary pigments different results are obtained, but these results need 

 not be discussed on the present occasion. 



In addition to the metals cadmium and magnesium, mentioned by 

 M. Colson, there are certainly many others which are able to produce 

 effects similar to that produced by zinc. There are also certain 

 alloys which can act in the same wa}-. The following is a rough list 

 of active metallic bodies approximately in the order of their activity : 

 mercury, magnesium, cadmium, zinc, nickel, aluminium, pewter, 

 fusible metal, lead, bismuth, tin, cobalt, antimony. 



The above order, even if not absolutely correct, is sufficiently so 

 to indicate that, although mercury is the most active, the other 

 metals do not follow in the order of their fusibility or exactly accord- 

 ing to any obvious physical property, but most nearly according to 

 their position in the electrical series , Mercury is, then, at ordinary 

 temperatures the most active metal, and its action is exercised not 

 only when the photographic plate is placed half an inch or so above 

 the metal, but when gelatine, gutta-percha, tracing paper, vegetable 

 parchment, are interposed. It appears, however, that the action of 

 the mercury does not take place as readily through gelatine, but more 

 readily through gutta-percha than is the case with zinc. 



Magnesium is also a very active metal, and very good pictures, 

 showing every scratch on its surface, are easily obtained ; also very 

 marked effects are produced when both single and double screens are 

 used. Cadmium also produces very good pictures, and is rather 

 more active than zinc ; nickel and aluminium are not quite so active, 

 but give good pictures ; then follow lead, bismuth, and tin. The 

 last metal is by far the least active. Only a few alloys have at 

 present been experimented with ; brass gives no action, but ordinary 

 pewter, and fusible metal, consisting of lead, bismuth, and tin, 

 were found to have considerable activity, and are placed in the 



