1885.] NEW-YORK MICROSCOPICAL SOCIETY. 133 



Sorby's description, given in the latter work, in which he calls 

 special attention to the necessity of avoiding the use of polishing 

 powder. For grinding a thin section of a compact mineral, 

 polishing powder is appropriate, because there are no cavities 

 which will absorb it. It is not necessary, therefore, to use a 

 cover-glass upon a thin section of smoky quartz : it is prefer- 

 able to polish the upper surface. In the preparation of thin 

 sections of a granular mineral or of a true rock, however, 

 polishing powder must not be used, since the scales of mica and 

 the cleavage planes and other crevices, which occur in a rock 

 section, would be likely to become filled with the powder, and 

 the usefulnes of the section would thereby be destroyed. Nor 

 should such thin section be of too great thinness, lest the larger 

 cavities be emptied. For the proper examination of these, a 

 thickness of one millimetre, or even much more, will not be too 

 great, so long as the cavities of the size desired are preserved 

 and their position is sufficiently near the upper surface to be 

 within the focal distance of the objective employed. 



Another and simpler method of preparation is applicable to 

 many compact specimens of the mineral ; viz., chipping off thin 

 flakes by a quick sharp blow of a small hammer, and mounting 

 them in thickened balsam under a cover-glass, or in ordinary 

 balsam or damar in a cell. Unfortunately, in many cases, 

 especially where the liquid contents consist largely of liquid 

 carbon dioxide, and a condition of extreme tension prevails, the 

 material is apt to be so extremely brittle that the least jar causes 

 it to crumble into angular fragments, unsuitable for mounting, 

 and with the largest and most interesting cavities emptied. 

 Such a material will also decrepitate when heated, sometimes 

 with very great violence, flying into powder and projecting the 

 particles out of the test-tube or vessel. Special precaution must 

 be taken, in mounting either a flake or a thin section of so 

 fragile a material, lest it be fractured by heat. The thin section 

 may be cemented upon the slide by a film of balsam, previously 

 thickened by heating ; and, immediately after pressing down 

 the section, the manipulator may quickly cool the whole mount 

 by blowing upon it, or by resting it upon a cold metal plate. 



Thin sections which contain very minute cavities, and which 

 may therefore need examination under high-power objectives, 

 should, of course, be mounted under the thinnest covers. If an 



