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something about its internal structure as revealed by the microscope. 

 The method of study is this. The fragments of quartz, or the 

 rocks containing quartz, are sliced very thin and mounted; the 

 slice being reduced to such a degree of thinness as to allow of its 

 ready examination by transmitted light. 



At the outset let us find out what can be the difference 

 between a piece of opaque white quartz and clear transparent rock 

 crystal. The difference is somewhat similar to that between a 

 piece of opaque ice and a piece of glassy transparent ice ; for if 

 water be frozen very quickly, a large number of air-bubbles will be 

 entrapped; while slow freezing enables the air to escape. Or again, 

 take a handful of snow, it is white but decidely opaque. Though 

 formed of clear crystals of ice, air is so mixed up with them, 

 that the rays of light constantly suffering reflection from the limiting 

 surfaces of the two, are unable to pass through the mass. Subject 

 such a mass of snow crystals, however, to powerful pressure, the 

 air will be squeezed out, and the formerly opaque snow may be 

 converted into a block of transparent ice. Reverse the experiment, 

 pound up your block of clear ice, and once more you have an 

 opaque white powder, the minute icy particles being separated from 

 one another by air. The same may be done with a transparent 

 quartz crystal : grind it to powder, and the powder is found to be 

 opaque and white. 



Hence may we not conclude that the difference between clear 

 rock crystal and opaque white vein quartz is that, while in the case 

 of the transparent quartz, there is nothing, or but Httle, to reflect 

 the rays of light, or to prevent their passing uninterruptedly through 

 the mass ; in the case of the opaque white quartz, there must be 

 something included which acts in the same manner, as the air- 

 bubbles in opaque or cats' ice, as it is called. The microscope 

 reveals what this something is. 



Take a piece of ordinary vein-quartz, and examine a thin 

 slice of the same ; every part of the field of view is seen to be full 

 of little cells of very various form, some round, some long dra^vn 

 out, some branched, and some even having a regular and what 

 appears like a crystalline outline. Examine them more closely, 



