466 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



enclosing the material of the slag surrounding the crystals, as shown 

 by fig. 40. In one part is a long, straight, open depression on the 

 face of the crystal ; and in another is a tubular cavity of varying 

 width, enclosed in the solid crystal ; whilst in other parts are smaller 

 and shorter cavities, related to the larger in precisely the same 

 manner as those occurring in alum, shown in fig. 26. When more 

 highly magnified, some are seen to be as shown by figs. 41 and 42, 

 which, in containing bubbles, closely resemble fluid-cavities. They 

 were indeed fluid-cavities when first formed ; but it was a fluid that 

 did not remain liquid at the ordinary temperature, being in fact a 

 glass. It of course appears more natural to call a space inside a 

 crystal, containing only a gas or liquid, a cavity ; but if it thus 

 contain a solid glassy substance, since some convenient name is 

 requisite, it appears to me that we cannot do better than adopt a 

 term analogous to that so generally adopted for fluid- filled cavities, 

 and call these glass-filled cavities glass- cavities. 



The most characteristic difference between fluid-cavities and glass- 

 cavities is the movement of the bubbles in the fluid-cavities ; for of 

 course bubbles cannot move in solid glass. However, since many 

 of the bubbles in fluid-cavities do not move, we are sometimes com- 

 pelled to have recourse to other tests. Strongly heating the crystal 

 expels the fluid from fluid-cavities, but produces no effect on glass- 

 cavities unless the heat be sufficient to melt the enclosed glass. 

 They may also often be distinguished, without spoiling the object, by 

 the difference in the relation between the dark exterior and trans- 

 parent centre of the bubbles. The dark exterior is of course due 

 to the refraction and total reflection of the light in passing through 

 the bubble, which vary as the refractive power of the substance. 

 Therefore, since that of the glass is considerably greater than that 

 of the aqueous solutions in the fluid-cavities, the dark zone is con- 

 siderably wider ; and if the bubbles be spheres, the bright central 

 spot, seen with a particular adjustment of the focus of the micro- 

 scope, is relatively nearly twice as large in fluid-cavities as in glass- 

 cavities. 



Sometimes the glass-cavities in the basic silicate of iron contain 

 crystals of a dark colour, that have been deposited from the glass on 

 cooling, as shown by figs. 44 and 45, and are thus analogous to the 

 fluid-cavities in crystals deposited from a strong hot solution of 

 another salt. Others, like fig. 43, are analogous to those in which 

 contemporaneously formed crystals w T ere caught up along with the 

 fluid, and project beyond the general outline of the cavity. In other 

 cases they are entirely filled with the dark substance that became 

 crystalline on cooling, as shown by fig. 46 ; and these, being stone- 

 filled cavities, may be distinguished by the name of stone-cavities, 

 in order to carry out the same general nomenclature. These are 

 analogous to what cavities containing water would be if the ordinary 

 temperature of the atmosphere was so low that the fluid froze, and 

 the whole cavity was filled with minute crystals of ice and of any 

 salt previously in solution in the water. 



A few good glass-cavities occur in the crystals of Humboldtilite, 



