104 PREPARATION AND MOUNTING 



light and its beautiful effects. Such would not deem these efforts 

 to aid the student in cutting sections complete without some 

 notice of those which are taken from various crystals, in order to 

 display that curious and beautiful phenomenon, the rings with a 

 cross. The arrangement of these is somewhat changed by the 

 crystal which affords the section ; but nitrate of potash gives two 

 sets of rings with a cross, the long line of which passes through 

 both, the short line dividing it in the middle. 



The process of cutting these sections is rather difficult, but a 

 little care and perseverance will conquer all this. The following 

 is extracted from the Encyclopaedia Metropolitana : " Nitre crys- 

 tallizes in long six-sided prisms whose section, perpendicular to 

 their sides, is the regular hexagon. They are generally very much 

 interrupted in their structure ; but by turning over a considera- 

 ble quantity of the ordinary saltpetre* of the shops specimens 

 are readily found which have perfectly transparent portions of 

 some extent. Selecting one of these, cut it with a knife into a 

 plate above a quarter of an inch thick, directly across the axis of 

 the prism, and then grind it down on a broad wet file till it is 

 reduced to about one quarter or a sixth of an inch thick, smooth 

 the surface on a wet piece of emeried glass, and polish on a piece 

 of silk strained very tight over a strip of plate-glass, and rubbed 

 with a mixture of tallow and colcothar of vitriol. This operation 

 requires practice. It cannot be effected unless the nitre be applied 

 wet and rubbed till quite dry, increasing the rapidity of the fric- 

 tion as the moisture evaporates. It must be performed in gloves, 

 as the vapour from the fingers, as well as the slightest breath, 

 dims the polished surface effectually. With these precautions a 

 perfect vitreous polish is easily obtained. We may here remark, 

 that hardly any two salts can be polished by the same process. 

 Thus, Rochelle-salt must be finished wet on the silk, and instantly 

 transferred to soft bibulous linen and rapidly rubbed dry. Experi- 

 ence alone can teach these peculiarities, and it is necessary to 

 resort to contrivances (sometimes very strange ones) for the pur- 



* Sometimes the saltpetre of the shops is nitrate of soda, and as this is 

 slightly deliquescent, it is well to be certain that we have the nitrate of potash, 

 which is free from this defect. 



