SORBY — STRUCTURE OF CRYSTALS. 473 



into a globular form. Since the bubble moves about in the central 

 fluid, and this also moves in the exterior fluid, both must be liquids ; 

 and I very strongly suspect that further research will prove that one 

 is water and the other a condensed gas. 



In determining the relative size of the vacuities in fluid-cavities, 

 of course, care must be taken not to make use of such as have caught 

 up bubbles of gas along with the fluid, which is more likely to hap- 

 pen with large cavities than with small. There is also a greater risk 

 of the large coming across flaws in the crystal, so as to lose fluid. 

 The very minute should, however, be avoided, as being too much 

 affected by the cohesion of the liquid to the sides. It is therefore 

 best to select those of moderate size, which have vacuities of very 

 uniform relative magnitude, in parts where vapour- or gas- cavities 

 do not occur and the crystal is very solid. Sometimes we may dis- 

 tinctly see that the quartz has been cracked, and the cracks after- 

 wards filled up with quartz. This, like the formation of the large 

 veins described below, appears in some cases to have taken place at 

 a lower temperature, and explains why bands of cavities occasionally 

 occur with vacuities relatively less than those in the fluid-cavities of 

 the general mass. As already mentioned, whenever it is possible, 

 such tubular cavities should be chosen as that represented by fig. 50. 



In the trachyte of Ponza there occur veins of quartz, as described 

 by Scrope (Transactions of the Geol. Soc. 2nd ser. vol. ii. p. 208). 

 These contain many fluid- cavities with water holding in solution the 

 chlorides of potassium and sodium, the sulphates of potash, soda, 

 and lime, and free hydrochloric acid. In this case we may, I think, 

 conclude that the pressure was not very great, so that 0= V, and the 

 relative size of the vacuities would indicate the temperature at 

 which the crystals were deposited from the aqueous solution. The 

 mean of many good observations is #=*143, which, when substituted 

 in equation (8), gives t= about 220° C. (428° F.). At this tempe- 

 rature the elastic force of the vapour of water is, from (7), equal to 

 292 feet of rock. 



The quartz of the veins in Cornwall has precisely the same 

 structure as the above in every respect ; the fluid-cavities contain 

 the same salts in solution ; and at a great distance from the 

 granite, making no allowance for pressure, the relative size of the 

 vacuities indicates the same temperature, but if the pressure was 

 great, a still higher. On approaching the granite, the temperature 

 and pressure appear to have been much greater ; for the relative size 

 of the vacuities in the fluid-cavities in the quartz of the veins is nearly 

 the same as in those in that of the granite itself. Thus, the mean 

 of the means for the quartz of the granite at St. Michael's Mount 

 and Mousehole is v=*148, and for the quartz of the associated 

 quartz-veins, also containing mica, tin-ore, wolfram, and other mine- 

 rals, v='133. In cases like this, 1 think we may consider the pres- 

 sure equal for both, so that the difference of the temperature may 

 be calculated by means of equation (9). If the pressure was no 

 greater than the elastic force of the vapour, these facts indicate that 

 the quartz of the veins crystallized at a temperature not more than 



