Action of Wood on Photographic Plate in the Dark. 131 



argon made from Bath gas gave, when the argon was absorbed in 

 charcoal, a gas residuum giving the helium and neon spectrum, and the 

 same result follows the use of atmospheric argon. In the case, how- 

 ever, of the Bath gas argon the helium spectrum is the stronger, 

 whereas with air argon the neon is the most pronounced. 



In order to further test the method, the crude gases got by heating 

 the mineral Fergusonite were examined. During the cooling of the 

 charcoal the nitrogen and hydrogen spectra were marked, but in a short 

 time nothing could be seen but the lines of hydrogen and helium. 



Great interest will attach to the behaviour of helium, hydrogen 

 and the most volatile part of air, when subjected to the action of 

 charcoal cooled to the temperature of liquid hydrogen. The method 

 promises to open up many avenues for future inquiry. 



I am indebted to Mr. Robert Lennox, F.C.S., for efficient assistance 

 in the conduct of the experiments, and Mr. J. W. Heath, F.C.S., has also 

 helped me in the investigation. 



" On the Action of Wood on a Photographic Plate in the Dark." 

 By WILLIAM J. EUSSELL, Ph.D., F.K.S. Eeceived May 28. 

 Read June 16, 1904. 



(Abstract.) 



[PLATE 7.] 



It has been shown in former papers that many substances are 

 capable of acting on a photographic plate in the dark and producing 

 a picture of themselves. Further investigation shows that this 

 property belongs probably to all woods, some, however, being much more 

 active than others. 



To obtain a picture the wood has to be in contact or at a little 

 distance above the .photographic plate, and has to remain there for 

 times varying from J an hour to 18 hours, and to be at a temperature 

 not higher than 55 C. 



The wood of the conifers is very active, and gives pictures which 

 are very definite. Fig. 1 is a picture of a section of a branch of a 

 Scotch fir, and shows well the rings of spring and autumn growth. 

 It is remarkable that the former are very active, producing in this 

 picture the dark rings, and so with the other pictures, the part which 

 is active in the original is dark in the picture. The rings seen in 

 the wood are very sharp and strongly pronounced in the picture. If 

 the action exerted on the plate be owing to the presence of hydrogen 

 peroxide, as has been previously suggested, no doubt it is produced 



VOL. LXXIV. L 



