DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 131 



Sect. 12. Method of viewing moveable Fluids in the Ca- 

 vities of Topaz ', Quartz, and other Crystals. — The existence 

 of small cavities in crystals is of such frequent occur- 

 rence, that few persons who are in the habit of seeing 

 crystals at all, can be unacquainted with the fact. 

 That some of these cavities, however, contain moveable 

 fluids, is a recent discovery, for which we stand indebted 

 to Sir D. Brewster. As I am not aware that the method 

 of observing these under the microscope has ever been 

 explained, a few words upon this subject may not be 

 considered superfluous. 



Let your microscope be placed in a vertical position ; 

 apply a power of 100 or 200 diameters, and put the 

 crystal for examination upon the stage. Bring one of 

 these cavities into the centre of the field, and adjust to 

 distinct vision, when the fluid will be discerned within 

 the cavity, resembling an air-bubble. The proof that 

 this appearance is occasioned by a fluid contained within 

 the crystal is very readily ascertained. Take a metal- 

 heater of a proper size, having a hole in the middle to 

 allow the light to pass through ; warm it, and hold it by 

 its handle under the aperture in the stage, close behind 

 the glass plate upon which the crystal is situated. 

 During this operation you will observe the fluid in 

 the cavity expand, and, in some cases, become vola- 

 tilized ; and then, upon cooling again, return to its fluid 

 condition. 



Many of the cavities in gems, particularly in the 



