The Structure of Amher. By 11. G. Sorbij and P. J, Butler. 227 
light is transmitted through only a comparatively small central 
circular spot, as shown by Fig. 4, which is nothing more than the 
image of the opening under the condenser seen out of focus. 
Except in a very few cases, there is no difficulty in distin- 
guishing these various cavities from one another, but still there are 
cases in which an incautious observer might be led into error. 
Those filled with liquid act like small lenses, and at certain adjust- 
ment of the focus the images of other cavities lying below them can 
be seen, and often look extremely Hke enclosed bubbles. The true 
nature of the case may, however, be generally ascertained by a 
little attention to detail. Thus, for example. Fig. 5 shows what 
might easily be mistaken for an enclosed bubble, but when the focus 
is adjusted to see it distinctly the somewhat obscure outline of 
another cavity, at a lower level, is visible, and by altering the focus 
there is no difficulty in proving that the apparently enclosed bubble 
is only the image of the lower-lying cavity. In some cases we may 
see as it were many bubbles, as shown by Fig. 6, due to the images 
of several subjacent cavities. If such a deceptive appearance were 
mistaken for a cavity containing several bubbles, an observer might 
be led to conclude most erroneously that the cavity was filled with 
some substance now no longer liquid, since the permanent existence 
of more than one bubble is impossible in fluid cavities, and only 
met with in glass cavities, as described and figured in Mr. Sorby's 
paper on the microscopical structure of crystals.* 
Though it is almost certain that the liquid in the cavities of 
amber is water, yet it cannot be said to be completely proved by 
any facts hitherto observed. In the case of anhydrous minerals, 
like quartz, the water can be expelled by heating in a glass tube, 
and condensed as hoar-frost in another part of the tube surrounded 
by a freezing mixture, and proved to be water by thawing at 0° C. 
This method, however, cannot be applied in studying amber, since 
on heating portions devoid of fluid cavities it is decomposed, and 
amongst other products yields water. 
It appears to be almost certain that a very large proportion of 
the cavities, hke Fig. 4, which may be said to be empty, are only 
fluid cavities which have lost all their liquid. On examining some 
sections of amber it is easy to see that in the centre of the thickness 
of the slice nearly all the cavities are full of liquid, and by gradually 
changing the focus so as to examine the cavities nearer the surface, 
there is a larger and larger proportion of the empty, until near to 
the surface none retain any liquid. In such a case it appears almost 
certain that the liquid has been lost during or since the preparation 
of the section. But independent of this there appears to be good 
evidence to prove that the liquid has also sometimes been lost from 
the exterior part of the amber, before being cut into a thin slice, 
* ' Quarterly Journal of the Geological Society,' 1858, vol. xiv. p. 453. 
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