SORBY — STRUCTURE OF CRYSTALS. 481 



calcite must have been formed at a temperature of about 340° C. 

 (644° F.). Of course, if the pressure was greater than supposed, the 

 temperature must have been still higher. Since then (Gmelin's 

 Handbook of Chemistry, Cavendish Society's Translation, vol. i. 

 p. 167) solid bodies begin to be dull red in the dark at 335° C, and 

 bright red at 400°, this temperature would be that of a very dull 

 red heat only just visible in the dark, at which, from equation (7), 

 the elastic force of the vapour would be equal to a pressure of 1954 

 feet of rock. 



On heating the fragment containing the cavity shown by fig. 80 

 to a very dull red heat visible in the dark, it became as fig. 81. The 

 small crystals had disappeared, and the vacuity and crystal had 

 changed their places ; thus proving that the cavity contained a 

 liquid, and that the crystals were soluble in it. On heating to a 

 very decided red heat, the cavity became as fig. 82 ; the fluid had 

 disappeared, and the nepheline had partially fused and collapsed 

 over the altered crystal. When another fragment containing the 

 cavity, fig. 83, was heated to a dull red heat, the cavity lost its fluid, 

 and the crystal melted into a globule, as shown in iig. 84. All these 

 results agree perfectly with the supposition, that the fluid is an 

 aqueous solution, and the crystals chloride of potassium or sodium ; 

 and it will be seen that the expansion of the liquid is not sufficient 

 to burst the cavities until the heat is that of redness, which agrees 

 perfectly well with calculation, if we consider that the enclosed 

 crystal was not all dissolved on account of being exposed to a high 

 temperature for only a short time. All, or nearly all, the fluid- 

 cavities contain the crystals, which are on an average equal to about 

 -|rd of the bulk of the liquid, or about four times as much as is de- 

 posited from a solution of chloride of potassium saturated at the 

 heat of boiling water, and many times more than from a solution of 

 chloride of sodium. This entirely confirms the conclusion derived 

 from the size of the vacuity, since, to dissolve so large an additional 

 quantity, a very high temperature would certainly be requisite. Some 

 cavities, as shown by fig. 85, are as though many minute crystals 

 had been deposited over their whole surface, except where prevented 

 by the attached cube. 



Most excellent gas-cavities also occur in the same nepheline, as 

 shown by figs. 86 and 87, being like bubbles of gas enclosed during 

 the growth of the crystal, in the same manner as in some of the 

 artificial crystals already described. Others, like fig. 88, are as if 

 some highly compressed, heated vapour had been enclosed, and on 

 cooling had condensed into small crystals. Such cavities can be 

 distinguished from stone-cavities by the fact of being partially trans- 

 parent in the centre. The same fragment of nepheline also contains 

 excellent glass-cavities, figs. 89 and 90, in all respects analogous to 

 those in crystals formed when melted stony matter is present. As 

 will be seen, the outline is very obscure, and quite different from 

 that of the fluid- cavities, and is rendered apparent chiefly by the 

 small crystals. The difference is also strongly marked by the pre- 

 sence of several bubbles, as shown by fig. 90. This glass-cavity was 



