THE MICROSCOPE IN MINERALOGY AND GEOLOGY. 85 



that many vitreous specimens (quite transparent) show no- 

 trace of structure until the surface has been carefully acted 

 on by hydrofluoric acid. 



It is generally necessary to have the specimens flat and 

 smooth, and thin enough to transmit light. Sometimes 

 fragments may be thin enough to show structure when 

 mounted in balsam, as in the case of quartz, obsidian, pitch- 

 stone, etc., but usually thin sections must be ground and 

 polished. 



Chip off a fragment of the rock as flat and thin as pos- 

 sible, or cut with a lapidary's wheel, or a toothless saw of 

 sheet-iron with emery. Grind down the specimen on an 

 iron or pewter plate in a lathe until perfectly flat. Then 

 grind with finer emery on a slab of fine-grained marble or 

 slate, and finish with water on a fine hone, avoiding all 

 polishing powders or oil. When perfectly smooth, cement 

 the specimen on a square of glass with Canada balsam, 

 and grind the other side until as thin as necessary, finish 

 as before, remove it from the glass, and mount on a glass 

 slide in balsam. 



In this way, most silicates, chlorides, fluorides, carbo- 

 nates, sulphates, borates, many oxides, sulphides, etc., may 

 be prepared for examination by transmitted light. Very 

 soft rocks may be soaked in turpentine, then in soft balsam, 

 and afterwards heated until quite hard. The deep scratches 

 on hard minerals, like quartz, left by the use of coarse 

 emery, may^ be removed by using fine emery paper held 

 flat on a piece of plate glass, and finally polished with 

 rouge on parchment. Perhaps oxide of chromium from 

 its hardness will be found the best polishing material. 

 Crystals of soluble salts may be ground on emery paper 

 and polished with rouge. Sometimes much may be learned 

 by acting on one side only of a specimen with dilute acid. 



Examination of Specimens. The object of microscopic 

 examination of minerals is to determine not only the nature 

 of the material of which they are composed, but also, and 



