for mea furlng the higher Degrees of Heat, &cc. 325* 
That it is not calcareous, g^pfeous, or argillaceous, is ma- 
nifeft from the experiments. — It is not jafper ; as this confifts, 
in great part, of argillaceous earth, which would have been 
extrabled by the vitriolic acid. — It is not fluor; as this, by the 
fame acid, would have been decompofed, its own acid expelled, 
and a gypfeous earth left. — It is not of the micaceous kind ; as 
the peculiar afpebt of thefe earths would readily betray them 
to the eye. — It is not granite; for ftrong fire, which granite, 
melts in, has no effebt upon this. 
Nor is there any known kind of earth to which it is in any 
degree fmilar, except thole of the filiceous order ; and with 
thele it perfebtly agrees in all the properties, I am acquainted 
with, that they poffefs in a fate of powder. 
It does not vitrify or foften with pure clay, in the frongef 
fire I have been able to produce. Nor is it difpofed to melt 
with the matter of Hefiian crucibles ; for a little of it rubbed 
on the infide of a crucible, and urged with frong fire, conti- 
nued white, powdery, and unaltered. Thirty grains of this 
earth were mixed with an equal weight of dry foflil alkali, 
and the fame quantity of a fine white quartzy find was mixed 
with the fame proportion of the lame alkali : the two mixtures 
were put into two fmall crucibles, which were furrounded 
with find in a larger one, that both might be expoied to an 
equal heat. They both began to melt at the fame time ; and 
at about 8o° of the thermometer they had formed pertebl 
tranfparent glades.. 
Though thefe properties may not, perhaps,, be thought fuf- 
ficient of themfelves, for determining with certainty that this 
lubfi ance is of the filiceous kind ; yet, when joined to the ne- 
gative proofs, of its not belonging to any other known order 
r 
or 
