fome chemical Experiments on Tabafheer. 385 
by fufion with alkalies in certain proportions into a white opaque 
mafs, in others into a tranfparent permanent glafs ; and its being 
again feparable from thefe compounds, entirely unchanged by 
acids, &c. feem to afford the ftrongeft reafons to confider it as 
perfedlly identical with common filiceous earth. 
Yet from pure quartz it may be thought to differ in fome 
material particulars ; fuch as in its fufing with calcareous earth, 
in fome of its effeds with liquid alkalies, in its tafte, and its 
Ipecific gravity. 
But its tafte may arife merely from its divided ftate, for 
chalk and powdery magnelia both have taftes, and taftes which 
are very fimilar to that of pure Tabafheer; but when thefe 
earths are taken in the denfer ftate of cryftals, they are found 
to be quite infipid ; fo Tabafheer, when made more folid by 
expofure to a pretty ftrong heat, is no longer perceived, when 
chewed, to ad upon the palate, § IV. (A). 
And, on accurate comparifon, its effeds with liquid alka* 
lies have not appeared peculiar ; for though it was found on 
trial, that the powder of common flints, when boiled in fome 
■of the fame liquid cauftic alkali employed at § IX. (A), was 
fcarcely at all aded upon ; and that the very little which was 
difiblved was foon precipitated again, in the form of minute 
fiocculi, on expofing the folution to the air, and was imme- 
diately thrown down on the admixture of an acid ; yet the 
precipitate obtained from liquor filicum by marine acid was dif- 
covered, even when dry to diffolve readily in this alkali, but 
while ftill moift to do fo, very copioufly, even without the affift* 
ance of heat ; and fome of this folution, thus faturated with 
filiceous matter by ebullition, being expofed to the air in a 
fhallow glafs, became a jelly by the next day, and the day 
after dried, and cracked, &c. exadly like the mixtures § IX. 
Vol. LXXXI. Eee (D and 
