378 Dr. W. J. Russell. The Action of Resin and [Mar. 24, 
alcoholic solution containing 0°25 per cent. of resin will give a dark picture, 
and with solutions of only 0°125 and even 0-086 per cent. of resin, faint 
pictures may be obtained. Again, another way of using resin is to pour 
the alcoholic solution on to a glass plate and allow it to dry there. There 
are, of course, other solvents which may. be used in place of alcohol. 
A card prepared with an alcoholic solution of resin was placed in the 
dark slide of a camera, and the lght of an arc lamp focussed upon it for 
five minutes. The card was then put up with a photo plate at 55° for one 
hour. A good and dark picture of the are was obtained. 
If resin be heated to a temperature of 40° to 50° for a short time it does 
not affect its activity, but if the heating be continued for 20 or 30 hours it 
slightly diminishes it. At higher temperatures the action is more marked ; 
for instance, at 140° the activity of the resin is much decreased after only 
four hours’ heating, and, although resin may be fused without appreciably 
diminishing its activity, still, if it be kept in a liquid state for three or 
four hours, its activity 1s much decreased. An interesting experiment is 
easily made with a slab of resin owing to its brittleness. A weight placed 
on the slab cracks it in all directions; this can be slightly warmed on the 
under side so as to prevent its falling to pieces, and then on putting it up 
with a photo plate for a short time a dark picture of the cracks is obtained 
(fig. 2, Plate 11). Another interesting experiment shows that the activity 
existing in resin can be transferred to a non-active body, making it as active 
as the original resin. A glass vessel was nearly filled with crushed resin, 
and a piece of inactive Bristol board placed on the top of it, at a distance of 
5 mm. above the resin. This was left for a week at ordinary temperature, 
then on putting the Bristol board in contact with a photo plate at 55° C. for 
five hours a dark picture was obtained. | 
Theré are other ways in which this action of resin may be diminished or 
destroyed ; for instance, by the action of sulphur dioxide. A slab of resin 
was broken into two pieces; one was placed for five minutes in a saturated 
solution of sulphur dioxide, then very thoroughly washed and dried; the 
other piece was treated in the same way, but with water alone, and both 
pieces were put up at 40° for 18 hours. The one which had been washed 
with water alone gave a dark picture, and the one treated with the sulphur 
dioxide gave no picture. 
It has been shown in a former paper that wood, after exposure to sunlight, 
has its power of acting on a photographic plate in the dark much increased, 
and that this increase of activity is not permanent, but gradually passes 
away. Resin acts in the same way: expose it to sunlight or to the are 
light, and then bring it in contact or proximity to a photo plate, and it will be 
