1908.] Allied Bodies on a Photographic Plate in the Dark, 387 
doubt the lomg exposure of small specimens in a museum may affect their 
activity. 
The foregoing experiments indicate the nature and to some extent the 
results which may be obtained by allowing coal to draw its own picture on a 
photographic plate, and in the hands of a geologist may help to explain the 
process of its formation. 
With regard to the nature of this action on photographic plates in the 
dark, it has been suggested in former papers that it is owing to the presence 
of hydrogen peroxide, and that the effects described can be imitated by 
means of this body. It now seems that actions of this same kind are 
obtainable from many other bodies, but still bodies of the same kind, and 
these additional experiments strongly indicate that the action is produced by 
a vapour rather than by any form of radio-activity. For instance, it is shown 
that the shadows thrown by resin are not bounded by straight lines, but 
curve round a screen; that the action is not capable of passing through 
glass, mica, or aluminium foil, even of extreme thinness, and does not affect 
an electrical field. The action can pass along a glass tube, even when it is 
bent at a right angle, and may be swept out of a tube by a slow current of 
gas; and, further, an experiment described above shows that the activity 
of resin can be transferred to a piece of perfectly inactive Bristol board, 
which will then give a black picture. Further, no action takes place in an 
atmosphere of carbon dioxide. On the other hand, resin dissolved in an 
inactive liquid, such as alcohol or petroleum spirit, causes it to become 
active. | 
The action which strong light has in increasing the activity of many 
bodies is important. For instance, it has been shown that pith may be in 
contact with a photographic plate at 55° for 48 hours and no trace of action 
is visible, but if the pith be exposed to sunlight for two or three hours it 
will then give a dark picture. The same action occurs with old printing, 
with pure india-rubber, etc., and many bodies which under ordinary condi- 
tions are but slightly active become very active after exposure to bright light 
or simply to blue rays. 
My thanks are due to my assistant, Mr. Bloch, who has made all the 
photographs and given me much aid in carrying out the experiments. 
The work has been carried on in the Davy-Faraday Laboratory of the 
Royal Institution. 
Von KXX.—B. Deo 
