+ aoe > 
wes ‘ 
= " 
oe’ 
Parr IL. § iv.] MICROSCOPIC CHARACTERS OF ROCKS. 97 
remains still unexplained. The bubble may be made to disappear by 
the application of heat. Sorby pointed out that it can be imitated in 
artificial crystals, in which he explained its existence by diminution 
of volume of the liquid owing to a lowering of temperature after its 
enclosure. By a series of experiments he ascertained thé rate of ex- 
pansion of water and saline solutions up to a temperature of 200° C 
(392° Fahr.), and calculated from them the temperature at which the 
liquid in crystals would entirely fill its enclosing cavities. Thus in 
the nepheline of the ejected blocks of Monte Somma he found that the 
relative size of the vacuities was about °28 of the fluid, and assuming _ 
the pressure under which the crystals were formed to havé been not 
much greater than sufficient to counteract the elastic force of the 
vapour, he concluded that the nepheline may have been formed at a 
temperature of about 340° C. (644° Fahr.), ora very dull red heat only 
just visible in the dark. He estimated also from the fluid cavities in 

Fic. 7.—GAvrites IN CRYSTALS, HIGHLY MAGNIFIED. A, Liquip INCLUSIONS; B, GLASS 
INCLUSIONS; C, CAVITIES SHOWING THE DEVITRIFICATION OF THE ORIGINAL GLASS 
BY THE APPEARANCE OF CRYSTALS, EIC., UNTIL IN THE Lowsst Ficure a Stony 
or LirHom ProptcT Is FORMED, 
the quartz of granite that this rock kas probably consolidated at some- 
what similar temperatures; under a pressure sometimes equal to that 
of 76,000 feet of rock.! Zirkel, however, has pointed out that even in 
contiguous cavities; where there is no evidence of leakage through 
fine fissures, the relative size of the vacuole varies within very wide 
limits, and in such a manner as to indicate no relation whatever 
to the dimensions of the enclosing cavities. Had the vacuole been 
due merély to the contraction of the liquid on cooling, it ought 
to have always been proportionate to the size of the cavity.’ 
MM. De la Vallée Poussin and Renard, attacking the question 
from another side, measured the relative dimensions of the vesicle 
and of its enclosed water and cube of rock-salt, as contained in the 
quartziferous diorite of Quenast in Belgium. The temperature at 
which the ascertained volume of water in the cavity would dissolve 
its salt was found by calculation to be 307° C. (520 Fahr.). But as 
the law of the solubility of common salt has not been experimentally 
determined for high temperatures, this figtire can only be accepted 
1 Sorby, op. cié. pp. 480, 493. 2 Mik. Beschaff. p. 46. 
H 
