The Action of Resin, etc., on a Photographic Plate. 377 
indication of where the air struck the plate was produced in two hours. 
On continuing to pass air through the tubes the activity of the resin 
gradually decreases, but if the resin be taken out and again broken up its 
activity is restored. 
If the tube containing the resin instead of being straight is bent at a 
right angle, and a photo plate be placed below the bend, on passing a slow 
current of air through the tube a large amount of action is produced upon 
the plate. | 
The presence of oxygen appears to be necessary for the action to take 
place. 
Two slabs of resin were placed separately in two desiccators: one was 
filed with dry air and the other with dry carbon dioxide; a photographic 
plate was fixed at 1 mm. below the resin plate, and both desiccators were 
kept at 40° C. for 18 hours. It was then found that, although considerable 
action had taken place in the desiccator filled with air, none had occurred 
in the one filled with carbon dioxide. 
A very marked and important character of this action of resin is that it is 
not able to pass through the thinnest sheet of glass or mica or aluminium. 
Glass 1/200 inch thick and mica 1/750 inch thick absolutely prevents the 
action passing through. This seems to separate this action from others of a 
somewhat similar character. 
Another important point with regard to the action of resin and other allied 
bodies is the form of the shadow which they produce. If, for instance, a 
glass screen is placed in front of a piece of resin on a photo plate the shadow 
is not bounded by straight lines, but the action, like that of a vapour, creeps 
in behind the screen, and in time meets from both sides. To prevent this 
action arising from any side action of the resin plate, a glass tube was filled 
with resin and directed against the centre of the screen. The experiment 
was repeated with the same apparatus, and a copper screen and ordinary light. 
Fig. 1 (Plate 10) shows the effect produced in the two cases. 
- With regard to other properties of the resin plate, a thin plate acts as 
energetically asa thick one; thus a plate only 0-017 inch thick gave a picture 
‘of the same density as one 0°29 inch thick. : 
To obtain a suitable slab of resin for experiment it is best to melt the resin 
and cast it on a bright metal plate, and afterwards, to free the surface which 
has been in contact with the metal from air-bubbles, to pass a gas flame 
over it. 
Another way of using resin for experiments is to dissolve it in alcohol 
and saturate a card or paper with the solution and allow it to dry. Even 
very dilute solutions may be used: a card which has been soaked in an 
