joumlu/ASTim] ^/ ^^^^ Precious Stones. 223 
probably enclosed, not as a liquid, but as a Ligbly-compressed gas, 
which condensed into a liquid on cooling. There is every reason to 
beheve that, when expanded, so as to fill the cavities, it is in the state 
described by Caignard Latour,* characteristic of hquids just before 
they pass into the condition of compressed gases ; and therefore, 
though the cavities are full at a heat no greater than that of 
summer, the mineral may have been formed at the moderately 
elevated temperature apparently necessary to explain the origin of 
metamorphic rocks. I have never seen in sapphire any trace of 
the second liquid (an aqueous solution) which occurs in topaz and 
aquamarina. Fig. 6 ; and hence, probably, the carbonic acid was 
derived from the decomposition of limestone by silica or alumina, 
when little or no water was present. 
Eubies, sapphires, and spinels, also often enclose many small 
crystals, of several different kinds of minerals. Sometimes they are 
arranged parallel to planes of the crystal, so as to give rise to 
symmetrical structures. Fig. 5. Some are so very thin and flat, 
that they show the colours of thin plates by reflected light, and 
look like scales from a butterfly. Others are perfect crystals, re- 
sembling those of spinel ; and others have a curiously curved out- 
line, and can only be recognized as crystals by the aid of polarized 
hght. 
Fluid-cavities are so very common in emeralds, that they may 
often be seen even in the clear gems, mounted in rings. Their 
presence is good evidence that it is a genuine emerald ; since no 
artificial glass would show them, however skilfully the colour might 
be imitated. They contain what seems to be a saturated aqueous 
solution of alkaline chlorides, from which small cubic crystals have 
been deposited. Fig. 8. On applying heat to the specimen, these 
crystals dissolve, and recrystallize when it has become cold. Cavities 
similar to them in all important particulars are very common in the 
minerals of the ejected blocks found at Monte Somma, and also in 
the quartz of some of the Cornish granites, as described by me some 
years ago.f 
The various kinds of garnet cut as jewels are often comparatively 
free from cavities or enclosed crystals; but the small common 
garnets in mica schist are often so full of crystals, that certainly 
only the smaller portion of the whole bulk is made up of the true 
substance of the mineral. After looking at such crystals, it seems no 
wonder that the chemical composition of some minerals is irregular 
and anomalous, when care is not taken to pick out clear and trans- 
parent specimens, which can be examined with the microscope and 
proved to be free from mechanical admixtures. 
The jargons which come to England from the Ceylon market 
♦ ' Annales de Chimie,' 1822, vol. xxi., pp. 127 and 178; xxii., p. 410. 
t ' Quart. Jour, of Geol. Soc.,' 1858, vol. xiv., p. 453. 
