57 
Brewster has shown that the irregular depolarizing action of diamond is 
analogous to that of an irregularly hardened gum ; and this much inter- 
feres with the perfection of the black crosses seen round the inclosed 
crystals, and sometimes even neutralizes this action. Still, as a general 
rule, a black cross is seen ; and, as described by Brewster, when examined 
by means of a plate of selenite which gives the blue of the first order, 
the tints of the sectors in the line of its principal axis are depressed in 
the same manner as when such a black cross is produced by the compres- 
sion of glass— thus proving that the inclosed crystals have exerted a 
pressure on the surrounding diamond. We, however, do not imagine 
that the crystals have increased in size, but that probably they have 
prevented the uniform contraction of the diamond, which, as already 
mentioned, must have been very irregular, even where no such impediment 
was present. A few of the crystals inclosed in rubies give rise to similar 
black crosses, as shown by fig. 11 ; and we are informed by Professor 
Zirkel that his brother-in-law Professor Vogelsang has prepared a thin 
section of a specimen of partially devitrified glass, which also shows black 
crosses round the inclosed crystals. 
Brewster suggested that this phenomenon in diamond was due to the 
elastic force of an inclosed gas or liquid, and compared it with what is 
seen in the case of some cavities in amber. We, however, find that the 
optical character of the crosses seen round the undoubted cavities in 
amber is the very reverse of that in the case of diamond, and cannot be 
explained by the mere mechanical action of an included elastic substance, 
but is similar to the change to a crystalline state which has occurred over 
the whole external surface, and on both sides of cracks passing from it 
inwards. 
The optical properties, however, are not the only evidence of contraction 
round crystals inclosed in diamond ; for actual cracks are often seen to 
proceed from them. These present the striped appearance shown in fig. 
10, owing to more or less perfect total reflection from their waved surface. 
The same kind of phenomenon may be seen in sapphire, and still better in 
spinel, as shown by figs. 12 and 13. Sometimes there is a system of 
radiating cracks nearly in one plane, terminating in a tran verse crack 
which surrounds the whole, as in fig. 12; and in other cases there are 
various complicated wavy cracks in different planes, as in fig. 13. There 
seems to be, some connexion between this structure and the natnre of the 
included minerals ; for round some kinds it is very common, but round 
others very rare or quite absent; and it appears probable' that it may be 
