34 



24- TiTANiTES. From the iron mines at Mineville, N. Y. 



Nos. 20 to 24 exhibited by Prof. J. F. Kemp, Co- 

 lumbia University. 



25. Fluid-Inclusions in Mineral Cavities. Among the in- 

 ckisions caught up during the crystallization of minei'als, 

 those of a fluid character are most interesting, consisting 

 of a gas (usually air or carbon dioxide) and a liquid 

 (water, saturated solution of common salt, or liquid car- 

 bon dioxide). In quartz, the enclosing cavities some- 

 times constitute five per cent, of the volume. Three 

 varieties are exhibited. 



a. Liquid Carbon Dioxide, vi^ith gas-bubble, in large cavity 

 in slice of quartz, from North Carolina. Mounted in 

 tank of immersion-apparatus, on microscope stage, for 

 determination of temperature of critical point of the 

 liquid, marked by sudden expansion and disappearance 

 of liquid or bubble, on application of gentle heat. In 

 this apparatus a current of air from the warm breath 

 (temp. 37° C.) is sufficient for the identification of the 

 critical temperature of carbon dioxide (26° to 28° C.) to 

 an accuracy of J^ of a degree. 



b. Brine and Liquid Carbon Dioxide, with gas-bubble, in 

 white topaz, from Minas Geraes, Brazil. The tubular 

 cavities show a parallel disposal to the prismatic faces of 

 the crystal. Within them delicate curves near the extremi- 

 ties show the contact between the denser brine (occasion- 

 ally with salt cubes) which occupies the prongs, and the 

 limpid, colorless carbon dioxide with its floating bubble, 

 w^hich occupies the centre. The bubble adheres, of 

 course, to the uppermost side of the cavity, as in a 

 natural spirit-level. 



c. Gas Bubble in Spontaneous Motion, in the quai'tz of 

 syenyte from Columbia, New Hampshire. This bubble, 

 about I micron (2-5io"Tr ii'^ch) in diameter, dashes from 

 wall to wall of the cavity, in restless motion, like a 

 monad seeking for escape. This motion must have con- 



