472 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



cellent data, that the vacuities were uniformly very nearly *141 of the 

 enclosed liquid at 0° C. ; and therefore, calculating from the results 

 of my experiments, if the fluid had been pure water, it would have 

 expanded so as to fill the cavities at 189° C. However, on heating a 

 portion of the quartz in a bath of paraffine, so arranged that it could 

 be examined with the microscope, I found that at 217° C. very mi- 

 nute bubbles were still visible in the fluid-cavities ; but at 220° they 

 had most certainly disappeared. We may therefore conclude that 

 the fluid expands so as to fill the cavities at a temperature of from 

 218° to 219°. On reducing some of the crystal to powder, I obtained 

 so much of alkaline chlorides and sulphates that I do not think they 

 could amount to less than 15 per cent, of the fluid in the cavities. 

 They could not amount to above 30 per cent. ; or else crystals would 

 have been deposited in the cavities. If in equation (3) we substi- 

 tute the values of B and C, previously determined for a solution 

 containing 25 per cent, of mixed alkaline chlorides and sulphates, we 

 obtain 



t— -V/684462V+ 12144- 110 equation (8) 



and substituting in this the value, V=*141, we find £=219 0, 4. Cal- 

 culating from the laws of the variation due to a difference in the 

 amount of salt in solution, if less or more than 25 per cent., the value 

 of t would be reduced ; so that, if 20 or 30 per cent., it would be 

 218°. If therefore, as is probable, the fluid in the cavities is a 

 strong solution, the temperature determined by calculation almost 

 exactly agrees with that previously ascertained by actual experiment. 

 It is seldom that the size of the cavities is sufficiently large to enable 

 me to verify my calculations in this manner ; for the experiment can- 

 not be made when very high magnifying powers are requisite ; but 

 the agreement in this case is so remarkable as to cause me to have 

 very considerable confidence in those that cannot thus be verified. 



Equation (8) of course gives the temperature requisite to expand 

 the fluid so as to fill the cavity, and does not indicate the tempera* 

 ture at which the crystal was formed, unless the pressure was only 

 equal to the elastic force of the vapour. This true temperature is 

 expressed by substituting the value of V given by equation (2), in 

 (8), when we obtain 



\/ 



v+-00000271p 

 684462 i _. 0Q000271 ^ + 12144-110 . . equation (9) 



which, of course, becomes (8) whenj3=0. 



Some quartz contains cavities enclosing two immiscible fluids, like 

 those occurring in Brazilian topaz, described by Sir David Brewster 

 (Transactions of the Royal Society of Edinburgh, vol. x. pp. 1 and 

 407). Since, however, their peculiarities have been so well explained 

 by him, and they occur so rarely in quartz as to be quite an excep- 

 tion to the general rule, I need not do more than refer to fig. 52 as 

 an illustration of their general character. They appear as if they 

 contained two bubbles, one inside the other, owing to the fluid which 

 has a less refractive power containing a bubble, and collecting itself 



