t m 1 



A kind of Trapp is found of that hardnefs, iri 

 which no particles of Feltfpat are to be feen. 

 Coloured glaffes referable true gems ; but as they 

 are very foft in proportion to thefe, they are eafily 

 difcovered by the means of the file : The common 

 quartz-cryftals are harder than coloured glaffes, 

 but fofter than the gems. The loadftone difcovers 

 the pjrefence of iron, when it is hot mixed in too 

 fmall a quantity in the ftone, and often before 

 the ftone is roafted; Some kinds of Htematites^ 

 and particularly the Ccerulefcens, is very like fome 

 other iron ores, but diftinguifhes itfelf from thefe 

 by a red colour, when pounded, the others giving 

 a blackifli powder, and fo forth. 



SECT. XIII. 



To manage the Blow-pipe with cafe requires 

 fome practice. A beginner blows generally tod 

 ftrongly, which forces him to take breath very 

 often, and then he draws the flame at the fame 

 time along into the Blow-pipe : This is trouble- 

 fome for himfelf, and the experiment cools always 

 a little at the fame time. But the more expe- 

 rienced can breathe in, through the riofe, and yet 

 at the fame time blow through the pipe, whereby 

 a conftant flame from the candle is kept up. The 

 whole art confifts in conflantly taking in air 

 through the nofe, and with the tongue moderate 

 ing its blowing cut; fo that the tongue performs 

 nearly the office of a fucker in a pump ; or rather, 

 the action of the nofe, lungs, and mouth, refem- 

 bles here the action of bellows with double parti- 

 tions. In this manner there is no need of blowing 

 violently, but only with a moderate and equal 

 force, and thus the breath cnn never fail the ope- 

 rator. The only inconveniency attending, is, that 



the 



