1885.] NEW-YORK MICROSCOPICAL SOCIETY. 13.9 



pasteboard or sheet cork, and by inserting plates of cork between 

 the copper plate and the stage. 



The general size, form, and contents of the fluid-cavities have 

 been very fully described by numerous observers, and, in the 

 absence of illustrations, they need be only briefly referred to in 

 this paper. 



The size of the cavities varies greatly. Sometimes they are 

 visible to the eye, rendering the sections of quartz cloudy. In 

 others, when they are held up to the light, one can see the in- 

 dividual cavities. Other thin sections swarm with the most 

 minute forms visible within the reach of our highest magnifying 

 powers, and probably far beyond. In the quartz-grains of most 

 quartzose rocks, — e.g., granyte, gneiss, schists, and sandstones, — 

 the smaller-sized cavities are very common, though invisible to 

 the eye. Those which are large enough to be visible to the eye 

 may be sought in some quartz crystals, in smoky quartz, and in 

 Brazilian topaz ; but they are somewhat rare as yet, even in 

 specimens of these minerals. About a quarter of a century ago 

 all the mineralogical cabinets contained crystals of quartz 

 from Little Falls, N. Y., in which were little cavities containing 

 water and bitumen — the latter often floating on the surface 

 within the bubble. Perhaps the largest liquid-cavity known is 

 that inclosed in the famous specimen of calcite once in the 

 collection of the late Prof. Chilton, a chemist, of this city. This 

 cavity contains nearly two gills of liquid. The specimen is said 

 to be now in the collection of Mr. C. Bement, of Philadelphia. 



The shapes of the cavities are almost infinite in variety, gen- 

 erally rounded, often full of projections, and frequently display- 

 ing crystalline outlines. These have been called negative 

 crystals, since they are simply cavities possessing planes, angles, 

 and general crystalline forms, corresponding to those of the 

 crystal in which they are included. 



The numbers of these cavities in smoky quartz are beyond all 

 statement, and often diminish its specific gravity by several 

 hundredths. In a Cornish granyte the fluid-cavities of the 

 quartz were found on an average not more than 1,000 th of an 

 inch apart, equivalent to a proportion of a thousand millions in 

 a cubic inch of quartz. As this mineral occurs in enormous 

 quantity throughout our rocks, a vast amount of liquid gas must 

 be thus locked up under our feet. In the smoky quartz of 



