36 DR BREYVSTER on the Existence of Two New Fluids 



10. SULPHATE OF BARYTES. The cavities in sulphate of ba- 

 rytes were first pointed out to me by Mr SIVRIGHT. I have 

 since found them in various specimens. They are generally of a 

 very irregular shape, though sometimes they have regular crys- 

 talline forms. 



Many of these cavities are entirely filled with fluid, but in 

 several a very small apparent air-bubble may be seen. This va- 

 cuity does not vanish by the heat of the hand, but it disappears 

 entirely at a temperature of about 150, and again returns when 

 the specimen has cooled. It is therefore a vacuum. 



11. CALCAREOUS SPAR. Cavities filled with water are fre- 

 quently found in calcareous spar. The apparent air-bubbles, 

 which are very small, occur only in some of the cavities. To 

 some of the cavities I applied a heat of about 1 50. The appa- 

 rent air-bubbles entirely vanished, and, what is very remarkable, 

 they have never again reappeared. This singular fact may be 

 ascribed to the strong cohesion between the fluid and the sides 

 of the cavity, which can only be overcome by a greater degree of 

 cold producing a greater degree of contraction than that of the 

 cabinet in which the specimen has been kept. 



12. ROCK-SALT FROM CHESHIRE. In this mineral the cavi- 

 ties assume the most beautiful forms. In one specimen, shewn 

 in Fig. 36., they have the form of regular cubes of various sizes, 

 and with numerous truncations on their sides and angles. In 

 other specimens the cavities have the form of octohedrons ; while 

 in others they have numerous varieties of forms. The cubical 

 hollows above mentioned are in general perfectly filled with 

 fluid ; but some of them have small apparent air-bubbles, which 

 contract to fully one-third of their size by a heat of 120. 



