or Mir.aosnopTO OBJECT*. t*iT 



rings vrith a cross, the long line of which passes through 

 both, the short line dividing it in the miad^e. 



The process of cutting these sections is rather difis<ak, 

 but a little care and persererance will conquer ail this. The- 

 following is extracted from the Enm/olopadia Metropolitana : 

 " Nitre crystallizes 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 considerable quantity of the ordinary 

 saltpetre* of the shops specimens are readily found which 

 have perfectly transparent portions of some extent. Select- 

 ing 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 colco- 

 thar 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 friction 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 precau- 

 tions a perfect vitreous polish is easily obtained. We may 

 here remark, that hardly any two salts can be polished by 

 the same process. Thus, Eochelle salt must be finished wet 

 on the silk, and instantly transferred to soft bibulous linen 

 and rapidly rubbed dry. Experience alone can teach these 

 peculiarities, and it is necessary to resort to contrivances 

 (sometimes very strange ones) for the purpose of obtaining 

 good polished sections of soft crystals, especially of those 

 easily soluble in water. 



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

 this is slightly deliquescent, it is well to be certain that we hare the 

 nitrate of potash, which is free from this defect. 



