1869.] Rubies, Sapphires, Diamonds, and some other Minerals. 297 



been observed by Mr. David Forbes*, and may, we think, be explained in 

 a similar manner. 



The crystals formed in blowpipe beads kept hot for some time over the 

 lamp, also furnish good illustrations of these facts. Phosphate of zirconia 

 is deposited in cubes from a borax bead to which much microcosmic salt 

 has been added ; and when examined with the microscope whilst cooling, 

 cracks like those described in diamond and spinel are seen to be formed 

 round many of the crystals, which are evidently due to the crystals con- 

 tracting less than the surrounding material. On the contrary, the long 

 prisms of borate of baryta deposited from solution in borax are seen to 

 separate from the borax on cooling, and to be filled with transverse cracks, 

 like those in schorl inclosed in quartz, which is clearly owing to their con- 

 tracting more than the borax. 



Fluid-cavities in general. 



Before discussing the nature of fluid-cavities in connexion with the 

 origin of the various minerals, we think it best to describe the remark- 

 able properties of the liquid included in the sapphire, and to point out 

 what it seems to be. Brewster, in his paper on the fluid-cavities in 

 topazt, says that the more expansible liquid contained in them expands 

 one-fourth its size, when heated from 50° to 80° F, or thirty-one and a 

 quarter times as much as water ; and, as already stated, he found that the 

 fluid in sapphire expands about one-half when heated to 82° F. Though 

 this amount of expansion is very remarkable, yet, when the relative 

 expansion at various temperatures is examined, it will be seen to be still 

 more remarkable. Very fortunately the tubular cavity in sapphire, shown 

 by fig. 2, is most admirably fitted for experiment. Mere inspection shows 

 that its general diameter is very uniform ; and that it is really so can be 

 proved by causing the liquid to pass from one end to the other ; for at 

 I7|°C. the length of the column of liquid was of an inch, whether 

 it was at the end A or B. The total effective length of the cavity is 



The specimen inclosing this cavity was fastened to a piece of glass, and 

 this was fixed in a beaker containing water, supported so that the cavity 

 was in the focus of the microscope under a low power. The temperature 

 was raised very slowly, and was maintained for some minutes at each parti- 

 cular degree at which it was thought desirable to measure the volume of 

 the liquid ; and this was usually repeated over and over again when the 

 heat was both rising and falling, so as to obtain as accurate a result as 

 possible. In making the measurements with the micrometer, care was 

 taken to allow for the tapering ends of the cavity and the curved surface 

 of the liquid. The results are given in degrees Centigrade. Though 

 the expansion below 30° was very great, compared with that of any other 

 known substances except liquid carbonic acid and nitrous oxide, when the 



* Ed. New Phil. Journ. July 1857. 

 t Trans. Eoy. Soc. Edin. 1824, vol. x. p. 1. 



