SORBY — STRUCTURE OF CRYSTALS. 463 



ture increases ; being, for a concentrated solution of common salt, 

 •0000379. In the case of both pure water and saline solutions, the 

 diminution of the volume is in simple and direct proportion to the 

 pressure. Since, then, the amount of the compression of pure water 

 decreases as the heat increases, but that of saline solutions increases, 

 I think it extremely probable that, at very high temperatures, both 

 would converge toward a similar amount ; and, since it would be 

 extremely difficult, or even perhaps impossible, to ascertain the fact 

 by experiment at a very high temperature, we may adopt provision- 

 ally the mean of the two, as being probably not very far from the 

 truth, especially in the case of moderately strong saline solutions. 

 From this, however, must be deducted the amount of the compression 

 of the crystal itself ; and if it be the same as that of glass, we should 

 have an apparent compression of '0000358 for the pressure of each 

 atmosphere, or '00000271 for that of each foot of rock of sp. gr. 

 2*5, which I adopt as a unit because it seems more congenial with 

 the subject before us. In accordance with this and other principles 

 already described, the fluid about to be caught up in a fluid-cavity 

 being expanded by the heat to 1 + V, would have its volume reduced 

 (1 + V) '00000271 for the pressure of each foot of rock. Therefore, 

 when crystals are formed at a high temperature, under a pressure 

 equal to p feet of rock, the real volume of the highly heated but 

 compressed liquid, caught up so as to fill a cavity, would be 

 1+V— (1 +V)'00000271 p ; and when cold and the pressure re- 

 moved, as it must be when a vacuity has been formed, the size of the 

 vacuity would be the difference between this and the volume of the 

 fluid taken as unity, viz. V— (1 + V)'00000271 p. 



e. The elastic force of the vapour of water. 



This has been determined in a very satisfactory manner by Dulong 

 and Arago (Quart. Journal of Science, Jan. to June 1830, p. 191) ; 

 and they give an empirical formula, which they say is particularly 

 applicable to high temperatures. Adopting this, and modifying it 

 so that t represents single degrees above 0° C, and e the elastic force 

 expressed in feet of rock, we have e = 13'2{l + -007153 (7-100)} 5 . 



From the various facts described above, I deduce the following 

 equations. 



Adopting as units, for the temperature, degrees Centrigade ; for 

 the pressure, feet of rock of sp. gr. 2*5, so that 13*2 feet equal one 

 atmosphere ; and for the volume of the vacuities, that of the fluid in 

 the cavities at 0°, — 



If £=the temperature, 

 ^=the pressure beyond that equal to the elastic force of 



the vapour at t, 

 V=the relative size of the vacuity at 0° C, corresponding 



to t, when p = 0, 

 v=the relative size of the vacuity as actually observed 



liable to the influence of the pressure p } 

 e=the elastic force of the vapour of water at t. 



