SORBY — STRUCTURE OF CRYSTALS. 471 



that of the veins in limestone, and in many trappean rocks ; also 

 in fluor spar, in the sulphates of baryta and strontia, and in several 

 other minerals found in ordinary veins, as if they had been deposited 

 from solution in water. In most of the cases I have examined, the 

 vacuities in the normal fluid-cavities are very small ; and, unless they 

 were formed under great pressure, the temperature must have varied 

 from that of the atmosphere up to about that of boiling water. 

 Much, however, remains to be determined ; and the variations in tem- 

 perature have too local a connexion to be considered in this paper. 



b. Quartz-veins. 



Since the facts to be learned from the study of the fluid-cavities 

 in quartz are extremely interesting, I must describe them in some 

 detail. As is well known, they are occasionally of considerable size, 

 so as to be perfectly visible to the naked eye, and contain bubbles 

 that move about like those in spirit-levels. The fluid contained in 

 them was proved by Sir H. Davy (Philosophical Transactions, 1822, 

 p. 367) to be nearly pure water; and my own experiments confirm 

 that conclusion. I froze the fluid in a cavity about -1-th of an inch in 

 diameter in a transparent crystal, and found that it thawed exactly 

 at the thawing-point of ice. When clear quartz containing no fluid- 

 cavities is heated in a tube, no water is given off; but that with 

 fluid-cavities gives off a fluid condensing, at a low temperature, into 

 crystals whose form and thawing-point are the same as those of ice. 

 Besides this water, there is often another substance given off, which 

 condenses as a solid nearer to the hot end of the tube than where the 

 water is deposited. I have ascertained that this is chloride of potas- 

 sium or sodium. The water also often has a strong acid-reaction, 

 due to hydrochloric acid, either derived from the decomposition of 

 the above-named salts by the heated quartz, or, as is certainly the 

 case in some instances, existing in a free state in the fluid-cavities. 



In order to ascertain the nature of the salts dissolved in the fluid 

 in the cavities, I reduce the carefully-washed crystals to powder, so 

 as to break open the cavities, and then dissolve out the soluble salts 

 with distilled water. When rendered quite clear by filtering and 

 standing for some days, on evaporating this solution to dryness, the 

 nature of the salts can be ascertained by the microscope and appro- 

 priate chemical tests. In this manner I have found (July, 1858) 

 that the fluid in the cavities often contains a very considerable quan- 

 tity of the chlorides of potassium and sodium, the sulphates of pot- 

 ash, soda, and lime, and sometimes free acids. This explains why 

 I was not able to freeze the fluid in some rock-crystal from Ceylon, 

 containing very excellent fluid-cavities of about T ^)th of an inch in 

 diameter, at a temperature of about —20° C. (—4° F.) ; for though, 

 according to my own observations, pure water in tubes less than -g-o-yth 

 of an inch in diameter does not freeze till the temperature is reduced 

 to about — 15° C. (3°F.), it freezes at once at that temperature in 

 those of the diameter of these fluid-cavities. It also serves to explain 

 the amount of expansion by heat. I had ascertained from most ex- 



