Metals and other Bodies on a Photographic Plate. 105 



stance used to charge the Bristol board the exposure need only be for 

 a few hours, but if after this charging it be exposed to the air for a 

 day or two, its activity will be found to have gone. There is 

 obviously no visible indication of this activity of the Bristol board, 

 and consequently if a device be cut out on a screen which is placed in 

 front of a sheet of cardboard, or any inactive paper, and it is exposed 

 to turpentine or to oil, or if the vapour of these bodies be in any 

 other way brought in contact with the paper, a dark picture of such 

 device, which is not visible, may be produced. Some unexpected and 

 curious cases of ghostly pictures thus formed have been met with, they 

 are, however, all produced in this way, and need not be described now. 

 The above experiments have been made at ordinary temperatures, but 

 if the temperature be increased the activity of these organic bodies is 

 also greatly increased. High temperatures cannot, of course, be used, 

 but a temperature of 55 C. does not appear to alter the photographic 

 plate. With drying oil which is one of the most active substances 

 that has been used it is easy to obtain a picture in thirty minutes at 

 the above temperature. The interesting pictures which in the former 

 communication it was shown could be produced simply by laying a 

 piece of dry wood or the section of the branch of a tree on a plate 

 are produced by the volatile matter contained in the wood. These 

 pictures appear at first sight very accurate and complete, but this is 

 not really BO, for some parts of the structure of the wood are not 

 shown and other parts are too strongly developed, depending on the 

 amount of volatile substance present in the different parts. It is, 

 however, very remarkable that so small a quantity of the volatile 

 body as exists in a piece of dry wood should be able to produce a 

 picture; the activity of the wood is increased by the pre2ence of 

 moisture. This property of acting on the photographic plate, pos- 

 sessed by the linseed oil, belongs apparently to the vapour and not to 

 the oil itself, for if a sheet of thin gelatin be placed on a photo- 

 graphic plate, and on it a thick glass ring nearly filled with oil, and 

 over the top of the ring another sheet of gelatin, not in contact with 

 the oil, and another sensitive plate, it will be found that after a 

 week's exposure no action has taken place below the oil, but that a 

 large amount has occurred above it where the vapour has penetrated 

 the upper sheet of gelatin. A similar result is also obtained by 

 simply floating a piece of the thin gelatin on a dish of oil and placing 

 a sensitive plate above. At the sides where the vapour can form and 

 get away there is action on the plate ; in the centre there is none. 



In addition to glass, mica, and bodies of that kind, the action does 

 not take place through a layer, except it be very thin, of either gum 

 arabic or of paraffin. If a piece of Bristol board or a glass plate 

 has hardened drying oil on it, and be painted over with a strong 

 solution of gum arabic which is allowed to dry, then the delicate 



