Account of a remarkable Fluid Cavity in Topaz. 235 



cavity should burst. I found, however, that it was not very 

 expansible by heat ; and that it was so evaporable, that at ordi- 

 nary temperatures the sides of the cavity were covered with 

 vapour, whose particles or small globules were about the y^^dth 

 of an inch in diameter. 



St. Leonard's College, St. Andrews, 

 Februaiy 28, 1853. 



XXXVIII. Account of a remarkable Fluid Cavity in Topaz. 

 By Sir David Brewster, K.H., D.C.L., F.R.S., V.P.R.S. 



Edinb.j and Associate of the Institute of France^, 



THE remarkable fluid cavity in Topaz which I propose to 

 describe is shown in the annexed diagram. The cavity 

 itself is of a very irregular form ; its greatest length, AB, being 



0*18 of an inch, and its greatest breadth, CD, 0*10 of an inch. 

 It is filled with a fluid in which there is a large vacuity V. The 

 fluid does not expand with heat, and is therefore quite dificrent 

 from either of the two new fluids which I discovered in Topaz and 

 other minerals. The vacuity V does not change its place by 

 holding the crystal in different positions, but by a violent jerk it 

 may be moved to the other end of the cavity, and even broken 

 into several separate vacuities. From the extreme sluggishness 

 of its motion it is very probable that it has a considerable degree 

 of viscidity; and that it is a fluid of low refractive power may be 

 inferred from the refraction of light through the broken surface 

 of one of the crystals in the cavity. 



The cavity contains several crystals, as shown in the figure. 

 Some of them have perfectly formed and brilliant facets, and are 

 * Communicated by the Author. ' 



R2 



