384 Dr. W. J. Russell. The Action of Resin and [Mar. 24, 
light, gradually loses this increased activity on keeping it in the dark or in 
dull light, but for a long time retains a slight amount of its increased 
activity. If, however, the amber be heated, this loss of activity takes place 
rapidly, even when heated to only 50° C., and if a flat surtace of it be brought 
in contact with a piece of heated metal for one minute the amber loses 
entirely its activity. It has already been shown that resin is stimulated 
especially by the blue rays of the spectrum; the same thing occurs with 
amber. Specimens of different ambers were exposed both to sunlight and 
tu are light under different coloured glasses, and it was always found that 
under the blue glass it became strongly active and that under the red glass 
it remained quite inactive, and if black glass and colourless glass were used 
the black glass acted like the red glass and the white one like the blue, 
only rather stronger. When double bell jars with coloured liquids were 
used in place of coloured glasses, exactly similar results were obtained. 
One experiment of this kind was continued for four months and gave the 
same result. 
Lignite, jet, and peat have also been tested in the same way as resin and 
amber. Two specimens of lignite from the Museum of Practical Geology, 
Jermyn Street: one an ordinary brown coloured piece, the other a sample from 
Tasmania; both were quite inactive and light did not stimulate them to action ; 
even the alcoholic extract was inactive. Another specimen from Nigeria was 
also inactive, but one from Bovey Tracey was slightly active, and a specimen 
of “ Brown coal” from Victoria, after an exposure of 44 hours, was found to be 
also very slightly active. Several specimens of jet from different sources were 
tried. None of them, if simply laid on a photographic plate and warmed, 
gave any action, but if powdered and a plate placed at 1 mm. above it, at 55° 
for 18 hours, gave a faint picture. Again, if powdered jet was extracted with 
pure alcohol, the small amount of dissolved matter evaporated to dryness 
gave a dark picture. So that jet, although not in ordinary conditions an 
active body, still in the form of powder has the property of acting on - 
a photo plate. Light does not appear to have the power of making it 
active. 
Graphite from Ceylon did not act on a photo plate. A specimen of peat 
was found to have the property of acting on a photo plate, but its activity was 
not increased by exposure to light. 
One other substance belonging to this class of bodies, namely coal, remained 
to be examined, and it was interesting to find that all ordinary coals, if brought 
into contact with a photo plate at a temperature of about 50°, were capable 
of acting upon it and giving a clear and distinct picture; so sharp are these 
pictures that they may be enlarged five or six times and still show clearly all the 
