[ 2 % ] 
quick heat is felt on the flsin. When the calca*^ 
reous fubilance is mixed with the vitriolic acid^ 
as in the gypfum ^ or with a clay, as in the marie 
it commonly melts by itfelf *, yet more or lefs 
cult in proportion to the differences of the mix- 
tures : The gypfum produces generally a white,, 
and the marie a grey glafe or flag. When there is 
any iron in it, as in white iron ore, it becoixies 
dark, and foniietimes quite black, &c, 
2. The Siliee^ never melt alone, but becpme 
generally more brittle after being burnt : Such of 
them as are coloured become colourlels, and the 
fooner when it does not arife from any contained 
metal; for inftance; the Topazes, Amethifls, cCd 
fome of the precious ftones, hov/ever, exceptexL 
And fuch as are mixed with a quantity of iron^ 
grow dark in the fire, as fome of the Jalpers, &c. 
3. The Garnet-kind melt always into a black 
flag, and that fometimes fo eafy, that it may be 
brought into a round globule upon the charcoal. 
4. The Argillace^, when pure, never melt, 
but become white and hard : The fame effeds 
follow when they are mixed with phloglfton ; for 
inflance, the Soap-rock is eafily cut with the 
knife 5 but, being burnt, it cuts glafs, and would 
ftrike fire with the jfeel, if as large a piece, as 1.9 
neceffary for that purpofe, could be' tried in this 
way. The Soap rocks are fometimes found of a 
dark brown and nearly black colour, but become 
for all that quite white in the fire, as a piece of 
China ware : However, care mufl: be taken not to 
pufh the flame from the top of the wick, there 
being for the mofl: part a footy fmoke, which corn-^ 
moniy will darken all that it touches ; and if this 
is not obferved, a miftake in the experiment might 
eafily happen : But if it is mixed v/ith iron, as it 
is fometimes found, it does not fo eafily part with 
U its 
