1908.] Alled Bodies on a Photographic Plate in the Dark. 383 
develop on it small spots of action; these local actions are produced by 
fine cracks in the amber, which frequently occur, and it is above the opening 
of these cracks that the action takes place. If the amber be laid for a 
minute on a hot surface the opening of the cracks fills up and the action 
ceases. This resembles the action of resin, and apparently points to the 
collection of volatile matter within the cracks. 
Another way of showing that, although a flat surface of amber does not 
act on a photo plate, still there is a trace of active vapour connected with 
it, for if powdered amber is placed in a glass dish with a plate above it, 
but not necessarily touching the powder, after the usual exposure a dark 
picture is produced. Amber, as is well known, is practically insoluble in 
alcohol, but in all cases a very small amount of some substance dissolves 
out of amber ; now if this substance be collected by filtering the alcoholic 
solution and evaporating it to dryness, the residue is found always. to be 
a very active body and gives a dark picture, thus a lingering indication of 
the amber’s origin seems to be indicated. 
Following the same line of experiments as that applied to resin, amber 
was exposed to sunlight and to the arc light, and its activity was found to 
be much increased. Four pieces of amber were exposed to sunlight for 
different lengths of time, namely, for two, three, five, and seven hours. 
After two hours only a very faint picture was produced; after three hours 
the picture was much darker and strongly outlined; after five hours it was 
still darker, and after seven hours a very dark picture was produced. The 
are light acts in the same way. A specimen of good amber was cut into 
four pieces, and all of them were exposed at the same time, at a distance of 
9 inches from the arc light: one piece for one hour, another for two hours, 
and the other two for respectively four and six hours. All of them were 
afterwards put up with photo plates at 55°C. for 18 hours. The amber 
exposed for one and for two hours did not act on the plate ; the one exposed for 
four hours gave a considerable amount of action, and the one exposed for 
six hours gave a dark picture. Another experiment of the same kind showed 
that the amber became slightly active in two hours, and was much increased 
after four hours, but after six hours and even after ten hours but very 
slight increase of activity occurred. As amber is a body which varies so 
much in constitution and composition, the action of light on it will vary 
slightly with every sample. for instance, five pieces of amber, all from 
different sources, were exposed at the same time for three hours to an arc 
light : two of them gave dark pictures, two only faint pictures, and one no 
picture at all. 
Amber, like resin, if stimulated to increased activity by the action of 
