382 Dr. W. J. Russell. The Action of Resin and [Mar. 24, 
an insufficient amount of alcohol to dissolve the whole of it. After boiling 
and digesting for a considerable length of time the undissolved acid was 
filtered off, washed, and dried, and was found to be quite as active as before 
this treatment, so that neither process of purification affected the activity 
of the acid. If the fused and inactive acid be simply exposed to light it 
will again become active. If the activity of turpentine depends to any 
appreciable extent on the presence of abietic acid, then if it be treated with 
an alkaline body its activity should be decreased. Turpentine is known to 
be a very active body, and a plate placed about one-eighth of an inch above 
it will, even at ordinary temperatures, in three hours give a black picture. 
Some turpentine was allowed to stand for 18 hours with a small amount of 
solid caustic potash; this was then filtered off, and the liquid distilled and 
put up with a photo plate for three hours; no trace of action was visible. 
Another photo plate was placed above the same turpentine solution and 
allowed to remain for 18 hours; even then only a very faint action took 
place. Another specimen of turpentine was shaken up with magnesium 
oxide and allowed to stand for 24 hours. The clear liquid gave a much 
fainter picture after this treatment. The same occurred when dry sodium 
carbonate was used, but lead acetate had no action on the turpentine. 
Amber, although classed as a resin, differs so much from the substance 
already described that it was of much interest to ascertain how it would 
act under similar conditions. It is a remarkable substance, known from the 
earliest times, and has been used for many purposes. 
Quarried at one time, like a stone, it was naturally looked upon as a 
mineral, but is now known to be of vegetable origin: the exudation of 
certain trees, probably mostly coniferous ones, which have been buried in 
the ground for ages. Even in the Green-sand formation some amber has 
been found. At the present time the principal supply of amber comes 
from the shores of the Baltic, but a small amount is still picked up on the 
east coast of this country. 
If a piece or pebble of amber, either in its rough state or cut so as to give 
it a flat surface, be laid on a photographic plate in the dark, no action takes 
place, even if the contact be continued for 18 hours and the temperature 
be at 40° to 50° C., thus differing from resin. This has been tried with a 
large number of specimens from different parts of the world, and with true 
amber has always been found to be the case. 
There are many bodies closely resembling amber in appearance, chiefly 
resins, which act strongly on the photo plate, and although readily distin- 
guished by an expert in the subject, can easily be mistaken for true amber. 
It often happens that a piece of amber, after long exposure to a plate, will 
