SORBY — STRUCTURE OF CRYSTALS. 487 



Fig. 1 12 is one in this mineral, but they are undoubtedly very rare. 

 Few also occur in this mica, but they are certainly sometimes met 

 with, as shown by fig. 113, which is very flat and shallow. The 

 presence of so many in the quartz, and of so few in the felspar and 

 mica, is analogous to what occurs when a mixed solution of common 

 salt and alum is evaporated, as already described ; and, when solu- 

 tions of alum and chloride of potassium are evaporated side by side, 

 the crystals of chloride of potassium are even more loaded with fluid- 

 cavities than the quartz in granite, whilst some of the crystals of 

 alum contain none. 



By many experiments, I have proved most conclusively that the 

 fluid in the cavities in the quartz of granites and elvans is water, 

 holding in solution the chlorides of potassium and sodium, the sul- 

 phates of potash, soda, and lime, sometimes one, and sometimes the 

 other salt predominating, Since the solution has often a most 

 decided acid reaction before, or even after, having been evaporated 

 to dryness, there must be an excess of the acids present. This oc- 

 currence of free hydrochloric and sulphuric acid is, I think, a very 

 interesting fact, when we bear in mind how very characteristic they 

 are of modern volcanic activity. Sometimes the amount of salts dis- 

 solved in the heated water was greater than could be retained in so- 

 lution at the ordinary temperature, and cubic crystals of the chlorides 

 have heen deposited, as shown by fig. 114. Near the granite, 

 this is also sometimes the case with the fluid-cavities in the quartz 

 of metamorphic schists and quartz-veins, which cavities contain the 

 same saline solution as those in the quartz of the granite itself, as 

 if in all these cases the quartz had been deposited from the same 

 liquid, which, at a greater distance from the granite, became more 

 dilute, on account of being mixed with pure water. Besides the 

 cubic crystals, the fluid-cavities in the quartz of granite occasionally 

 also contain prismatic crystals, as seen in fig. 115, and therefore agree 

 very closely with those in the blocks ejected from Vesuvius. There is 

 often a considerable variation in the amount of crystals contained in 

 the fluid-cavities in the same portion of quartz, as if the strength of 

 the solution had varied during the consolidation of the rock ; and 

 there is also sometimes a passage from fluid- to vapour-cavities, as if 

 there had been an alternation of liquid and vapour or gases ; both of 

 which circumstances would be likely to occur. 



The stone-cavities are not well developed, except in granites whose 

 structure approximates somewhat to that of elvans. The most dis- 

 tinct I have yet found are in the quartz of the granite of St. Austel, 

 containing the fluid-cavities, fig. 111. They are entirely similar to 

 those in the quartz of the trachyte of Ponza or of elvans, as will be 

 seen on comparing fig. 117 with figs. 103 and 104. As I have 

 already remarked, when some substances pass into the crystalline 

 state they occupy more space than when melted, and therefore, if 

 entirely enclosed in a solid substance, they might expand so much as 

 to crack it, like we all know often happens when water freezes. This 

 appears to have occurred in the cavity fig. 119, there being three 

 cracks radiating from it, as drawn. This increase in the bulk of the 



