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Messrs. Sorby and Butler on the Structure of [Feb. 18, 



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 crys- 

 tals, 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 compression 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 cavitiesr 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 pro- 

 ceed 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 radia- 

 ting cracks nearly in one plane, terminating in a transverse 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 nature of the in- 

 cluded 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 referred 

 to unequal contraction in cooling from a high temperature ; and, if so, the 

 results would necessarily depend on a variety of circumstances. Now 

 that attention has been directed to it, it will probably be found to be a 

 very common peculiarity of certain classes of minerals, and serve to throw 

 a good deal of light on their origin. 



Crystals surrounded by radiating cracks on a much larger scale have 



