The Str ucture of Amher. By E. G. Sorhy and P. J. Butler. 229 
such cavities as Fig. 8 may be explained. In many cases, however, 
the changes did not end at this stage, but internal movements in 
the mass of the amber detached the car from the balloon, and sepa- 
rated them, as shown by Fig. 9. These facts manifestly throw 
great light on the character of the changes that took place when 
the original soft resin was converted into amber. Some cavities 
have also been much distorted and drawn out by a general change 
in the dimensions of the amber, like the bubbles in glass drawn out 
when melted. 
On examining with polarized light a section of amber cut 
through the centre and exterior crust, it may be seen that the 
central part has no depolarizing action, and has the same optical 
properties as annealed glass. The exterior skin has, however, a well- 
marked depolarizing action, and by carefully studying the character 
of the double refraction, it is found to be such as would result 
from a relative increase in the bulk of the exterior crust or con- 
traction of the internal mass, that is to say, the depolarizing 
action is what would be caused by a pressure acting in the line of 
the circumference, and not in the line of the radius. This is in 
complete accordance with the facts indicated by the balloon-shaped 
cavities. At one time we thought that a similar depolarizing action 
was met with round some of the cavities, giving rise to black 
crosses, as described by Sir David Brewster.* He attributed this 
depolarizing action to pressure exerted by an enclosed gas tending 
to enlarge the cavity ; but the character of the double refraction 
indicates a pressure tending not to increase, but to diminish, the 
size of the cavities. It is the reverse of that met with round 
minute crystals enclosed in diamonds, as described in our paper on 
the microscopical structure of some precious stones,t which is just 
what would result from a contraction of the mass of the diamond 
surrounding the enclosed unyielding crystals. Sir David Brewster 
did in fact admit to us that he had in some way or other been mis- 
led in concluding that the cavities in amber indicate a pressure 
exerted from within. We now very much question the propriety 
of associating the black crosses with cavities as such. 
By far the larger number give no trace of black crosses with 
polarized light, and on the whole the depolarizing action must, we 
think, be attributed to some cause that is independent of the pre- 
sence of any elastic gas. 
In carefully looking over different parts of various sections 
of amber under polarized light, we found that the black crosses 
were by no means uniformly distributed, but occurred in groups, 
which appeared to be to some extent determined by the existence 
of a more general depolarizing action, like that met with in the 
* ' Transactions of the Geological Society,' vol. iii. p. 455. 
t ' Proceediugs of the Royal Society,' 1869, vol. xvii. p. 291. 
