380 Dr. W. J. Russell. The Action of Resin and [Mar. 24, 
exposure of one hour in strong diffused lght, the resin which had been 
exposed to the blue hght had been strongly acted on, and gave a good dark 
picture, while the one exposed to the red light gave no increase of activity. 
The amber resin acted in the same way. A beam of blue light or of red light 
thrown on a slab of resin or a card saturated with resin gave the same results. 
On exposing resin under different coloured glasses the same effects were 
produced: after an exposure of one hour to bright daylight, under a blue glass, 
the resin became very active, under a red glass no change took place, and 
under a green glass there was only a very slight increase of activity. 
It thus appears that the action of light in this respect, on resin, is similar 
to its action on wood, as described in a former paper. This increased activity 
of the resin slowly passes off, even at ordinary temperatures, on keeping in the 
dark or in red light. A slab was exposed to the are light for one hour and 
then cut up into eight pieces. One piece was put up with a photo plate at once 
at 40° for two hours: it gave a good dark picture; the other pieces were kept 
in the dark, at ordinary temperatures: after three days a piece was tested, the 
picture 1t gave was only slightly lighter than the former one. After nine 
days, again, a loss of activity had occurred and the same was the case after 
18 days. The experiment was carried on for nine months, and at the end of 
this time although the picture it gave was much fainter than the first one 
still it was slightly darker than the picture it would have given before 
exposure. If the resin be only slightly stimulated by exposure to bright 
daylight, the same gradual decrease of activity was traced. In red light the 
decrease was apparently the same as in darkness. 
Although glass and some other bodies are opaque to the action of resin, 
porous bodies, of course, allow the action to pass through ; for instance, with 
ordinary paper, if a slab of resin be placed behind it, a very good sharp 
picture is obtained ; if, however, the paper be highly glazed and dressed, it is 
perfectly opaque. With ordinary papers interesting pictures, showing their 
structure and water-mark, and stencil pictures are easily obtained. 
If paper be treated with different substances in solution, it is made more or 
less transparent. As a general rule it would seem that acid salts, such as the 
sulphates, which do not act on the photographic film, make a paper opaque, 
but that neutral salts do not alter its transparency. If a paper be dried by 
warming it, 1t becomes rather less transparent. 
The principal constituent of resin‘is said to be an acid, known as abietic 
acid. It is not a body which has been very thoroughly examined, but it 
has the property of acting on a photographic plate in the dark to a remark- 
able extent. It can be obtained by dissolving resin in alcohol and passing 
hydrochloric acid gas into the solution; the acid then separates out in a 
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