545 



The Photography of Cavities in Minerals, and the Deter- 

 mination OF the Condensation Points of the Enclosed 

 Gases. 



By a. Ashe. 



{Read June 19tJi, 1903.) 



Plate 28. 



Through the kindness of Mr. Traviss I have been afforded 

 an opportunity of examining a specimen of topaz in which 

 occurred some exceptionally large cavities enclosing apparently 

 liquid carbonic acid gas. 



As the slide presented points of great interest, photographs 

 were taken in the usual manner wdth a horizontal microscope, 

 using a l^-inch objective and No. 2 ocular, the illumination 

 being by means of an oil lamp, except in the case of three 

 instantaneous views, when limelight was employed in con- 

 junction with a rapid shutter. The photograph No. 1 is that 

 of a cavity which at a temperature of 34° F. is filled to the 

 extent of one-third of its volume with condensed carbonic acid. 

 When the temperature of the liquid was raised to 60° or 

 70° F., no perceptible increase in its volume was apparent — in 

 fact, within this range of temperature it followed the ordinary 

 laws of expansion of liquids when under the influence of heat ; 

 but as it approached 82° F., it expanded at a rapid rate until 

 at 84° F. it filled the entire volume of the cavity (see photo- 

 graph No. 2). 



On cooling the liquid quickly, an interesting phenomenon took 

 place. At a particular temperature a number of rapidly moving 

 bubbles were generated in the liquid, and for a moment there 

 was an appearance of ebullition too quick for the eye to follow. 



JouRN. Q. M. C, Series II. — No. 53. 37 



