412 DR BREWSTER on the Refractive Powers, and other 



them, so as to exhibit the cavities to the best advantage, he 

 has been enabled to shew most of the phenomena to those who 

 are interested in such pursuits. Several of these specimens are 

 of great value, as will appear from the drawings and descriptions 

 of some of the most remarkable, which will be given in the se- 

 quel of this paper. 



In the additional observations which I have now to submit 

 to the Society on this subject, I shall confine myself to the fol- 

 lowing heads : 



1. On the Number and Arrangement of the Fluid Cavities. 



2. On the Form of the Cavities containing the New Fluids. 



3. On the condition of the Fluids within the Cavities. 



4. On the condition of the Fluids when taken out of the Ca- 



vities ; and 



5. On some miscellaneous phenomena connected with the 



formation of Fluid Cavities. 



1. On the Number and Arrangement of the Fluid Cavities. 



In a former paper I had occasion to mention, that, in a spe- 

 cimen of Cymophane about yth of an inch square, I counted 

 30,000 cavities. Although this statement occasioned great sur- 

 prise, and some expressions of scepticism, yet it was too feeble to 

 convey any idea of their number. So minute are these cavities, 

 that the highest magnifying powers are often necessary to render 

 them visible ; and we might as well attempt to number the grains 

 of sand on the sea-shore, as to count these fluid cavities when 

 they appear in this minute state. 



The strata in which these cavities are arranged, are not so 

 closely related to the primary and secondary planes of the cry- 

 stals as I formerly supposed. I have found them in almost every 



