MINERALOGY. 55 
ftones, among which they elfe might have been 
ranked equally as here, as they are already in 
other fyftems : to which 1 venture to add, that, 
as a naturalift, I cannot conquer that general 
weaknefs of valuing them fo highly ^ for befides 
their furpriiing hardnefs, and fine colours, that 
pleafe the eye, it is not without foundation, that 
they might be thought applicable to every ufe 
for which the filiceous kind is employed, if they 
were to be had in large quantities : and by this 
alone it is they deferve to have the preference of 
the other ftones of this order. In regard to the 
o 
colours, it is to be obferved, that thofe of 
the ruby and emerald are faid to remain in the 
fire, but that the colour of the topaz flies off : 
whence it is ufual to burn that gem on purpofe 
that it may be made ufe of inftead of the diamond, 
as it is harder than the quartz cryftal. The 
colours of gems are commonly fuppofed to de- 
pend on metallic vapours : but may they not 
more juftly be imagined to arife from a plilo- 
gifton, united with a little metallic or fome 
other earth ,? becaufe we find that metallic earths, 
which are perfectly v/ell calcined, give no colour 
to any glafs, and that the manganefe, on the 
other hand, gives more colour than can be 
afcribed to the, fmall quantity of metal which 
is to be extrafeed from it. (Seflion cxiii.) 
The phlogifton may perhaps have more diffi- 
culty to efcape through the pores of the hardeft 
of the precious ftones, if it is true, that the pro- 
perty of lofing the colour is in proportion to 
their hardnefs, as fome authors feern to in- 
dicate, by affuring us, that none but the co- 
loured diamonds and the rubies keep their co- 
lours in the fire , but in this circiimftance I like- 
wife want experience, but hope to fee it illuftrated 
E 4 by 
