THE 
MONTHLY MICROSCOPICAL JOURNAL. 
NOYEMBEE 1, 1876. 
I. — On the Microscopical Structure of Amher. 
By H. C. SoRBY, F.K.S., &c., and P. J. Butler, F.R.M.S. 
(Read before the Royal Microscopical Society, Octoher 4, ISTH.) 
Plate CLVIII. 
Anyone examining a number of specimens of amber cannot fail to 
notice that some are more or less clouded, or even quite opaque. 
This is mainly due to the presence of minute cavities, v^hich in 
some cases are so numerous that there would be several thousand 
millions in a cubic inch. Such cavities are comparatively rare in 
the very transparent specimens, and hence their development can- 
not be looked upon as having been a condition essential to the 
change of the original more or less liquid balsam or resin into such 
a brittle substance as amber. Their formation and character must, 
however, be fully taken into account in forming any theory to 
explain the origin of this fossil resin, and the changes which have 
subsequently occurred. 
In order to examine these cavities to advantage, the specimen 
must be cut into a very thin section, polished on both sides, fixed 
to glass with Canada balsam, and also covered by a thin glass 
cemented down with the same material. A moderately high mag- 
nifying power should be used. For general examination a y^th is 
very suitable ; but for more detailed study of the individual cavities 
a ith is much better. 
There is not only a very great variation in the number of the 
cavities in different specimens, but even in different parts of 
the same section, and they often occur in irregular bands and 
streaks, separated by bands containing comparatively few. Large 
cavities may occasionally be seen with the naked eye, and some- 
times they have all the characters of bubbles of air introduced into 
a soft balsam by the enclosed insects when struggling to get free. 
The more common cavities are, however, very small. Those 
so much as yoVo of an inch in diameter are comparatively rare. 
From 2F0 0- to ^oi^o of an inch in diameter is the usual size. 
Occasionally there are specimens in which they are so minute that 
they cannot be accurately defined, and appear to be not more than 
tWo (JTT of an inch in diameter. 
VOL. XVI. R 
