SORBY — STRUCTURE OF CRYSTALS. 491 



elusion that sometimes they did not solidify from a state of fusion 

 until the temperature was as low as that of boiling water. In the 

 present state of the inquiry (July 1858), it therefore appears to me 

 that the best course is to suppose that the quartz of the various igne- 

 ous rocks crystallized at about the same temperature, and that the 

 greatest value of v yet observed, viz. in the trachyte of Ponza, was 

 when p was not so great as to prevent the value *3 being a sufficiently 

 accurate approximation to Y to be substituted in equation (6), so as 

 to enable us to calculate the value of p from the observed value of v 

 by the equation 



i>=369,000 pT| equation (10). 



Considering the very strong analogy between the structure of this 

 trachyte and that of elvans and granites, this supposition appears to 

 me quite admissible as an approximation until a more correct is known. 

 Of course the results deduced from the equation are the amounts of 

 pressure in feet of rock, and not the actual depth. In some cases 

 the pressure was probably much greater than that of the superin- 

 cumbent rocks, for otherwise they could not have been fractured and 

 elevated ; whereas in other cases it may have been much less, if the 

 internal pressures had been in any way relieved. Fortunately, a con- 

 siderable variation in the strength of the saline solutions in the fluid- 

 cavities would be to a great extent compensated for, because, though 

 a more dilute solution would expand more by heat, it would be more 

 compressed by pressure. Moreover, according to the principles de- 

 scribed by Mr. James Thompson (Transactions of the lloyal Society 

 of Edinburgh, vol. xvi. p. 575), which have been strikingly verified 

 by the experiments of Professor Win. Thompson on the thawing of 

 ice (Philosophical Magazine, 3rd ser. vol. xxxvii. p. 123), by those 

 of Bunsen on spermaceti and paraffine (Poggendorff's Annalen der 

 Physik und Chemie, 1850, vol. lxxxi. p. 562), and by those of Hop- 

 kins and Fairbairn on spermaceti, paraffine, sulphur, and stearine 

 (Report of the British Association for 1854, p. 57), — if a substance 

 expands in solidifying, it would become solid at a lower temperature 

 when under a greater pressure ; whereas if it contracts, it would 

 solidify at a higher temperature. Therefore if, as I have already 

 shown, the stone-cavities in the quartz of granite indicate that the 

 general fused mass from which the quartz crystallized expanded in 

 the act of solidification, it would probably become solid at a lower 

 temperature when under greater pressure. We may, however, be 

 nearly certain, that at very great pressures the compression of water 

 would be relatively less than at moderate ; for, if not, a finite pressure 

 would compress it into nothing ; and therefore, since the force re- 

 quired to produce this relatively less compression of the liquid, which 

 was not so much expanded by the lower temperature, might be nearly 

 the same as would produce the greater compression of the more 

 heated and expanded liquid assumed in the above equation, it is ob- 

 vious that these two sources of error have a tendency to counteract 

 one another, and therefore perhaps the equation would give a tole- 



VOL. XIV. — PART I. 2 K 



