The Structure of Amber. By 11. C. Sorhj and P. J. Butler. 231 
could be some kind of minute spore ; but the evidence for or 
against such an ojDinion is defective. Sometimes minute crystals 
are seen like Fig. 12, having well-marked angles, and a strong 
depolarizing action. These must not be confounded with crystals 
on the surface of sections covered with glass cemented by balsam, 
which have perhaps been formed in situ by the mutual action of 
the balsam and the amber. 
General Conclusions. 
It will thus be seen that amber furnishes us with an excellent 
illustration of the important facts that may be learned by carefully 
studying the minute cavities in minerals. All the circumstances 
connected with the occurrence of amber prove that it cannot have 
been formed in anything like the same manner as the crystalline 
minerals in igneous or stratified rocks. It is interesting to find 
that the general principles of research adopted in Mr. Sorby's paper 
on the microscopical structure of crystals are equally applicable 
in the case of a substance of such an entirely different nature as 
amber. 
Studied in the same manner as has led to such satisfactory 
results in the case of minerals and rocks, amber leads to very 
different but yet equally satisfactory conclusions, which may be 
summed up as follows: — It was originally a soft balsam, which 
gradually passed into a hard resin. During this change water was 
eliminated, which was often retained in the interior as minute 
globules, constituting the fluid cavities ; and gas was also evolved, 
often giving rise to the gas cavities. These various cavities were 
normally spherical, but have sometimes been disturbed by general 
or partial internal movements of the resin whilst it was still more 
or less plastic. The volume of the resin itself was also diminished 
in such a manner as to give rise to internal tension, which has 
modified the optical characters of the separate pieces. 
