t m 3 
A kitid of Trapp is found of that hardnefs, iii 
which no particles of Feltipat are to be feeno 
Coloured glaffes refemble true gems j but as they 
are very foft in proportion to thefe, they are eafily 
difcovered by the means of the file t The common 
quartz-cryftals are harder than coloured glafifes, 
but fofter than the gems. The loadfiione difcovers 
the prefence of iron, when it is not mixed in too 
fmall a quantity in the ftone, and often before 
the ftone is roafted* Some kinds of H^matiteSi 
and particularly the Cosrulefcens^ is very like fome 
other iron ores, but diftinguifties itfelf from thefe 
by a red colour, when pounded, the others giving 
a blackifh powder, and fo forth. 
SECT. XIIl. 
To manage the Blow-pipe with eafe requires 
fdme pradice. A beginner blows generally too 
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 nofe, 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 conftantly taking in air 
through the nofe, and with the tongue moderat- 
ing its blowing out*, 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 a6lion 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 can never fail the ope- 
rator. The only inconveniency attending, is, that 
