106 Dr Brewster on the existence of Two New Fluids 
tween this class of phenomena and the two contending Geologi- 
cal Theories. The existence of highly rarified gas in the cavi- 
ties of crystals, has been regarded by the distinguished Presi- 
dent of the Royal Society of London, as “ seeming to afford a 
decisive argument in favour of the igneous origin of crystalline 
rocks and the fact of almost a perfect vacuum existing in a 
cavity containing an expansible but difficultly volatile substance,' ” 
(as naphtha), he likewise considers as highly favourable to the 
same theory. The discovery of compressed gas in similar ca- 
vities might have been regarded as neutralising in some degree 
the first of these arguments : but Sir Humphry Davy remarks, 
that it may be explained, by supposing the crystal to have been 
formed under a compression much more than adequate to com- 
pensate for the expansive effects of heat. 
Without presuming to combat these deductions, or to sug- 
gest any of the numerous explanations by which the Neptunist 
might reconcile with his own system the compressed and di- 
lated condition of the included air, I shall content myself with 
stating, that the facts described in the preceding paper appear 
to me decidedly hostile to the igneous origin of crystals, and, 
in some points of view, favourable to their aqueous formation. 
The existence of a fluid which entirely fills the cavities of crys- 
tals, 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 atmosphere, while the fact of a perfect vacuity existing in 
sulphate of barytes , and capable of being filled up by the ex- 
pansion 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 
which 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 faith- 
less auxiliaries which are so ready to enlist themselves in the 
service of either power. 
■* The phenomena here alluded to will be described in a subsequent number. 
They are fully explained in the original memoir. 
