226 Transactions of the Boyal Microscojpical Society, 
Having now given a general account of the manner of their 
occurrence, we proceed to consider the detailed characters of these 
cavities. One of the first things which strikes an observer accus- 
tomed to study the cavities in minerals is that those in amber are 
usually of almost spherical form. This fact, and the general 
manner in which they are associated where numerous, will be seen 
by reference to Fig. 1, PJ. CLYIII. This contrast between the form 
of the cavities in amber and of those in crystals is easily explained. 
Those in crystals are usually vacant spaces left during the growth 
of the crystal, and very often are characterized by being bounded 
by crystalline planes, and not unfrequently they have exactly the 
same form as minute crystals of the substance in which they occur. 
Thus, for example, in common salt they are often rectangular, and 
in quartz sometimes six-sided, with pyramidal ends. On the 
whole, we may say that the shape of the cavities in crystals, to a 
very great extent, depends on the crystallization of the substance 
enclosing them. On the contrary, the cavities in amber have a 
form quite independent of any structural direction, and are like the 
minute bubbles in a stiff liquid, which naturally assume a spherical 
shape, because a sphere has the largest possible volume in propor- 
tion to the area of the surface. Though this is the general 
character of the cavities in amber, yet there are some very striking 
and interesting exceptions, as shown by Fig. 7 ; but both the 
normal and the abnormal forms combine to prove that the amber 
was originally a stiff liquid, occasionally subjected to internal move- 
ments, which disturbed the naturally spherical shape of the cavities. 
In studying the cavities in detail, it will be convenient to divide 
them into three princi} al groups, as follows : 
1. Cavities filled with liquid. 
2. Cavities containing both a liquid and gas. 
3. Cavities filled with gas. 
There is no practical difficulty in distinguishing these one from 
the other. The refractive power of the hquid is so Kttle less than 
that of the amber itself, that, though the cavities are very well 
marked by a dark outhne, it is comparatively narrow, and when 
illuminated by a condenser with an aperture of considerable angle 
and a wide opening underneath it, light can be seen passing 
through by far the larger part of the entire area, as shown by 
Fig. 2. When a cavity is partially filled with liquid, this same 
character is maintained over a portion of the area ; but in addition 
we see a bubble, characterized by a dark outline, which is nearly 
as broad as one-third of the diameter of the whole bubble, as 
shown by Fig. 3. With the same illumination, those cavities 
which are entirely filled with gas are of course like bubbles, 
extending over the entire area of the cavity, and the breadth of 
their dark outline is about one-third of the entire diameter, and 
