SORBY — STRUCTURE OF CRYSTALS. 459 



but the more common forms are like figs. 28, 30, and 31. When 

 deposited at a heat of 50° C. (122° F.), the cohesion of the sides is 

 sufficient to entirely counteract the small amount of contraction, and 

 prevents the formation of vacuities. Besides fluid-cavities, a few most 

 interesting larger cavities full of air were formed, which appear as if 

 they had been bubbles, given off from the solution, that were enclosed 

 in the growing crystal. Small quantities of this air are also in some 

 cases caught up in the fluid-cavities ; so that a few contain bubbles 

 of considerable size in proportion to that of the cavity. The dif- 

 ference between the cavities full of fluid and those full of air is most 

 striking. The refractive power of the fluid being nearly the 

 same as that of the crystal, the cavities containing it are almost 

 invisible by reflected light, and give only a narrow outline by trans- 

 mitted, whilst, the refractive power of the air being so much less, 

 the cavities containing it shine brilliantly by reflected light, and by 

 transmitted light have a very broad and dark outline, as shown by 

 fig. 29. 



Hitherto all my descriptions refer to crystals that were mounted 

 in liquid and never dried. When, however, exposed for some time 

 to dry air, it is as if some of the cavities were not so completely 

 closed as to prevent the slow passage of liquid from them ; and there- 

 fore bubbles make their appearance, gradually increasing in size, 

 and becoming quite large, as shown by figs. 30 and 31, which are 

 cavities in alum, originally quite full of fluid at the ordinary tempe- 

 rature. In the case of some crystals, especially those like alum, 

 containing chemically combined water, perhaps the fluid may actually 

 pass off through their solid substance ; but this is apparently con- 

 fined to cavities near the surface. In many crystals, however, and 

 especially in the more solid parts, where the fluid has been completely 

 shut up, it appears to remain nearly or quite permanently — at all 

 events for many years — even when they have been kept quite dry. 

 These facts must be carefully borne in mind when attempting to 

 deduce the temperature at which crystals were formed; and care 

 must be taken not to confound cavities that have lost some fluid by 

 drying, with those in a normal state enclosing bubbles that have been 

 produced by the contraction of the liquid on cooling. They may 

 often be distinguished without much difficulty, because when fluid is 

 lost there is a great inequality in the relative size of the bubbles in 

 different cavities, whereas in the other case it is nearly uniform. 



If the planes of a fluid-cavity are inclined at certain angles to the 

 line of vision, they may totally reflect the transmitted light, and the 

 cavity appear like a fragment of some black and opake substance 

 enclosed in the crystal. This is often the case in sulphate of potash ; 

 and an example from that salt is represented by fig. 33. 



When a solution of common salt is evaporated at 100° C. in an 

 open-mouthed flask, a crust is formed on the sides, above the level of 

 the solution. In this case, during the growth, the crystalline crust 

 is alternately exposed to the solution and the air ; and when a portion 

 is mounted in fluid and examined with the microscope, it is seen to 

 have a peculiar and very interesting structure. Some of the cavities 



VOL. XIV. PART I. 2 H 



