482 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



in the same fragment as the fluid-cavity, fig. 80, and when heated to 

 a very dull red heat, it remained nearly as drawn. When, however, 

 heated a little higher, the minute crystals disappeared, and the bub- 

 hies changed their places ; whilst at the same temperature the fluid - 

 cavity still retained its fluid. At the temperature at which the fluid 

 was expelled, it became as fig. 91, where all the crystals had disap- 

 peared, and the bubbles not only had changed their places, but two 

 had coalesced. It is therefore clearly proved by experiment (April 

 1858) that, at a heat not sufficient to expand the fluid in the cavities 

 so much as to burst them, the substance in the glass-cavities is 

 melted, so as to dissolve the small crystals it had previously deposited 

 on cooling more slowly, and permit a change in the position of the 

 bubbles, all which results agree most perfectly with the supposition 

 that the crystals were formed at a red heat visible in the dark, when 

 melted stony matter, gases, vapours, and liquid water saturated with 

 soluble alkaline salts, were all present and alternately in contact with 

 the growing crystals, so that the conditions of fusion, sublimation, 

 and solution were all united. 



In the idocrase forming along with calcite the general mass of a 

 block ejected from Vesuvius, many fluid-cavities occur, which often 

 contain so many crystals that it is difficult to determine their form. A 

 very good exampleis givenin fig. 92, with crystals like those in the fluid- 

 cavities in nepheline. Fig. 93 shows clearly that the relative size of 

 the vacuities is very great. On an average, they are equal to one- 

 third of the fluid, and therefore indicate a temperature of 380° C. 

 (71 6° F.), or a decided red heat; at which temperature the elastic 

 force of the vapour of water is, from equation (7), equal to the 

 pressure of 3222 feet of rock. Similar cavities occur in hornblende, 

 indicating a heat of 360° C. There are also very good fluid-cavities 

 in the crystals of felspar found in the ejected masses of ice-spar. 

 These contain many crystals of two or three kinds (as shown by fig. 

 94). On reducing the felspar to powder, I found the usual chlorides 

 and sulphates, but besides these a very considerable quantity of 

 the carbonates of potash and soda, and, therefore, probably the 

 presence of these carbonates is the reason why the crystals in the 

 cavities differ so much from those previously described, though in 

 other specimens they are quite similar, all being cubes. As shown 

 by fig. 95, the vacuities are very large, and of about the same relative 

 size as those in idocrase, indicating a temperature of 380° C. Other 

 cavities, like fig. 96, have caught up vapour or gas along with the 

 fluid, in the same manner as sometimes occurs in artificial crystals ; 

 whilst others are quite full of the gas or vapour, which in some cases 

 has been condensed into crystals on cooling, so as to cover the sur- 

 face, as shown by fig. 97, seen out of focus in the centre. The same 

 crystal also contains most decided and excellent glass-cavities, like 

 fig. 98, and others, as fig. 99, that have become to a great extent 

 crystalline, and contain no bubbles ; both of which are very analogous 

 to cavities in the felspar of the pitchstone of Arran. 



We are thus led to conclude that the peculiar minerals character- 

 istic of the blocks ejected from Vesuvius were formed at a dull red 



