430 Dr. AY. J. Russell. On the Action exerted by certain 



siderable amount of action took place even through the cardboard 

 screen. Many other bodies of the same nature as copal act in the 

 same way. This has been proved to be the case with Damar and 

 with Canada balsam, but copal seems to be the best representative of 

 the class. Certain gums, such as gum arabic, gum Senegal, have not 

 the property of acting in this way. There are, however, a large 

 number of bodies which have the power of acting in a manner 

 similar to the copal ; one of these is wood, and it possesses a very 

 considerable amount of activity. Any ordinary smooth piece of wood 

 laid on a photographic plate will act like zinc in impressing its 

 picture on the plate. 



A section of a young larch tree gave a good picture showing 

 clearly the different rings and the layer of bark, which was the 

 darkest part of the picture. The same section, when a film of 

 gelatine was interposed between it and the plate, still gave a good 

 picture. Wood which is thoroughly dried and hardened is also able 

 to act in the same way. 



A piece of mahogany 3'5 mm. thick, which had been in this 

 form for at least thirty-five years and been carefully preserved in 

 a dark cupboard, gave after a week's exposure a good picture, 

 and the bottom of an old cigar-box acted equally well. Bodies 

 such as straw, hay, bamboo, oiled silk, and, no doubt, many others, 

 act in the same way. If wood, however, be painted with melted 

 paraffin, it is no longer active. Ordinary charcoal also depicts 

 itself on a photographic plate, but if it be heated for some hours 

 in a covered crucible it loses this property. An ordinary piece of 

 wood, if it be charred on one side by heating it with a Bunsen 

 lamp, becomes remarkably active, as shown by placing it behind 

 a screen with a pattern cut out. The action passes readily through 

 different media, such as gelatine, tracing paper, &c, vegetable 

 parchment, &c, and the structure of the charcoal is shown, when 

 the action has taken place, even through a sheet of vegetable 

 parchment. Coal and coke, sulphur, sugar, on the other hand, 

 exert no action of this kind. When trying whether a copy of a 

 lithographic picture could be obtained by placing behind it a plate 

 of zinc, some curious results occurred. It would seem that printer's 

 ink in most cases is not capable of acting;, like copal, on a photo- 

 graphic plate, but that there are many cases in which it is a remark- 

 ably active substance. Specially so is the ink used in printing many 

 of the newspapers. The ' Westminster Gazette,' for instance, is 

 printed with an ink which very readily acts on a photographic plate. 

 A portion of this paper with printing on only one side, laid with 

 the blank side on the photographic plate, in a few days gives a 

 remarkably black and distinct picture. If there be printing on 

 both sides, then two pictures are obtained, the darker printing 



