OF MICROSCOPIC OBJECTS. PAZ 
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 Hncyclopedia 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, Rochelle 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 have the 
nitrate of potash, which is free from this defect. 
