[ aS7 3 
often ^re thofe which are of a foft confiftence, or ^ 
particular figure, and which preferve the fame figure 
in however minute particles they are broke ^ for 
inftance, the Calcareous Spar, the Sparry Gypfum, 
Sparry Fluor, White Sparry Lead-ore, the Potters 
Ore, (Galena tejjellata) the Teflellated Mock-lead 
or Blende, &c. even all the common fluors which 
have po determinate figure, and moft of the Mi’^ 
fiera metalloTum calciformes cryftallifat^ or fpatof^ ; 
All thefe are not fo compad as common hard 
ftones ; and therefore, when the flame is imme- 
diately pufhed at them, the heat forces itfelf quick- 
ly through and into their clefts or pores, and 
caufes this violent expanfion and difperfion. Many 
of the clays are like wife apt to crack in the fire, 
which may be for the mofl: part afcribed to tho 
humidity, of which they always retain a portion. 
Befides thefe enumerated, there may be found now 
and then other mineral bodies of the fame nature ; 
but it is, however, not fo common. 
The pnly way of preventing this inconvepiency, 
is to heat the body as flowly as pofTible. It is 
be*^ firft of all, to heat that place of the char- 
coal, where the piece is intended to be put on, 
and afterwards lay it thereon ; a little crackling 
will then enfue, but commonly of no great confe- 
quence. After that, the flame is to be blown 
very flowly towards it, in the beginning not di- 
re6lly upon, but fomewhat above it, and fo ap- 
proaching nearer and nearer with the flame until it 
becomes red hot. This will do for the mofl part, 
but there are neverthelefs fome, Vvhich, notwith- 
flanding all thefe precautions, it is almoflc impofii- 
ble to keep on the charcoal. Thus the Fluors are 
generally the moil: difficult j and as one of their 
principal chara6lers is difcovered by their eflefls 
in the five per fe^ (Seft. xviii. 6.) they ought n^'- 
celfarily 
