in the Cavities of Minerals. 39 



merits : but Sir HUMPHRY DAVY remarks, that it may be ex- 

 plained by supposing the crystal to have been formed under a 

 compression much more than adequate to compensate for the ex- 

 pansive effects of heat *. 



Without presuming to combat these deductions, or to suggest 

 any of the numerous explanations by which the Neptunist might 

 reconcile with his own system the compressed and dilated condi- 

 tion of the included air, I shall content myself with stating, that 

 the facts described in the preceding paper appear to me de- 

 cidedly hostile to the igneous origin of crystals, and, in some 

 points of view, favourable to their aqueous formation. The exist- 

 ence of a fluid which entirely fills the cavities of crystals, at a 

 temperature varying from 74 to 84, may, upon the principles 

 assumed in the opposite argument, be held as a proof that these 

 crystals were formed at the ordinary temperature of the atmo- 

 sphere, while the fact of a perfect vacuity existing in sulphate of 

 barytes, and capable of being filled up by the expansion of the 

 aqueous fluid, at a temperature not exceeding 150, authorises 

 the analogous conclusion, that the crystal could not have been 

 formed at a higher temperature. On the other hand, the filling 

 up of the vacuities in sulphate of iron, and sulphate of nickel, at a 

 temperature much above that at wluch they were formed, may 

 lead geologists to renounce a species of argument which appeals 

 only to our ignorance, and to withdraw from the defence even of 

 their outworks, those faithless auxiliaries, which are so ready to 

 enlist themselves in the service of either power. 



There is one geological relation, however, of the preceding 

 facts, which may deserve some attention. Hitherto the contend- 



I'v.v 



* As the effects of heat and compression might exactly balance each other, 

 the gas would in this case be atmospheric air, in a common state of density ; so 

 that the volcanists are here sheltered against experimental hostilities, amid lh*> 

 generalities of their hypothesis. 



