292 



Messrs. Sorby and Butler on the Structure of [Feb. 18, 



gate the law of the expansion of the very interesting fluid met with in the 

 cavities of sapphire. 



In describing the various facts, it will be well to consider them in rela- 

 tion to the following general principles : — 



(1) The structure of the various minerals as mere microscopical objects. 



(2) The physical characters of the fluid-cavities, as throwing light on the 

 origin of the minerals. 



(3) The influence of some included crystals on the structure of the sur- 

 rounding mineral. 



Sapphires. 



By far the most interesting objects contained in sapphires are the fluid- 

 cavities. Their occasional presence has been already noticed by Brewster*, 

 who met with one no less than about ^ inch long, two-thirds full of a liquid 

 which expanded so as to fill the whole cavity when heated to 82° F. (28° C). 

 He thought the liquid was less mobile than that described by him in topaz, 

 and could not see a second liquid in the cavity. Though many thousand 

 sapphires have been examined by the authors, no such large cavity has 

 been found ; but several have been met with about y 1 -^ inch in diameter ; the 

 greater number are far less, and some are very minute ; and they seem to 

 contain only the liquid which expands so much when warmed. The size 

 of the included bubble varies much, according to the temperature. At the 

 ordinary heat of a room it is sometimes equal to one-half of the capacity 

 of the cavity, whereas in other cases the cavity is quite full. This is espe- 

 cially the case with the very small cavities, and is to some extent due to 

 the forced dilatation of the liquid. But if we only take into consideration 

 the larger cavities, the temperature required to expand the fluid so as to 

 fill them certainly varies from 20° to 32° C. (G8° to 90° F.), and this not 

 only in different crystals, but also, to a less extent, in the same specimen. As 

 illustrations of the form of such cavities, we refer to Plate VII. figs. 1, 2, 3, 

 and 4, the extent to which they are magnified being shown in each case, At 

 the ordinary temperature the bubble in the cavity shown by fig. 1 is about 

 one-half its diameter, but disappears entirely at 30° C. By carefully mea- 

 suring the size of the cavity in various positions, and comparing it with the 

 diameter of the bubble at 0° C, it appears that the liquid expands from 

 100 to 152 when heated from 0° to 30° C. Fig. 2 is a tubular cavity, and 

 shows in a very excellent manner the boiling of the liquid when it cools 

 after having been made to expand to fill the whole space. At the ordinary 

 temperature the liquid occupies only about half the cavity ; but when heated 

 in a water-bath to 32° C, it fills it entirely. No bubble is formed until 

 the temperature has fallen to 31° ; and then innumerable small bubbles are 

 suddenly formed, which rise to the upper part and unite ; but instead of 

 the liquid merely contracting by further cooling, it still continues to boil 

 for some time, as represented in the drawing. Two other large cavities 



* Sochting's Einschliisse von Mineralien in krystallisirten Mineralien, p. 121, who re- 

 fers to Edin. Journ. of Sc., vol. vi. p. 115. 



