Mr. keir on the 
536 
greater proportion of calcareous and other earthy parti- 
cles may difpofe thefe glades to cryftallize more than 
others, which contain a larger quantity of faline and 
metallic fluxes. 
Flint-glals, when long expofed to a dull red-heat, ac- 
quires a cloudinefs, which, probably, proceeds from a 
number of fmall white particles, concreted by means of 
cryftallization ; but thefe cryftals are too minute for ob- 
fervation. I lufpecl alio, that the opaque whitenefs, given 
to glals by arfenic, is the effect of a cryftallization, to 
which this fubftance difpofes certain kinds of glals ; for 
the opacity given to fuch glafs by arfenic, being greater 
than the opacity of the arfenic itfelf, cannot be commu- 
nicated to a large proportion of tranfparent glafs, merely 
by the mechanical interpofttion of this opaque, and fome- 
rtimes only femi-opaque fubftance. 
Mr. reaumur has obferved, that fome kinds of glafs, 
by long expofure to certain degrees of heat, acquire a 
white opaque cruft on their furface ; and that this change 
of colour and texture, by a longer continuance of the 
heat, penetrates farther, till at length the whole fubftance 
of the glals is converted into a white, opaque body, 
which, from fome fuppoled refemblance to porcelain, 
has been diftinguifhed by the name of Reaumur’s 
porcelain, hut is really nothing elfe than glafs indiftindlly 
cryftallized. 
Some of the properties of glafs are confiderably changed 
by cryftallization; its tranfparency is deftroyed, and it ac- 
quires an opaque or femi-opaque whitenefs; its denlity is 
2 incrcafcd, 
