the Corundum Stone , and its Varieties , &c„ 287 
of adularia, made by Mr. Vauquelin ; yet it differs very essen- 
tially from that, by the want of potash, and by the proportion 
of lime being more considerable.* The presence of the last- 
mentioned earth is sometimes rendered evident, in the parts 
which are slightly decomposed, by the weak and momentary 
effervescence that takes place in those parts, when the substance 
is put into nitric acid. 
On the other hand, many of its external characters are such 
as naturally lead to its being ranged with adularia. It differs 
from it, however, in the facility with which the latter may be 
broken in two different directions ; while, in the substance here 
treated of, fractures can never be obtained, except in one of 
those directions ; nor have I ever been able to observe on the 
fractures of any other kind of felspar, those fine striae which, 
# The analyses made by Mr. Vauqjtelin, of the different kinds of felspar, 
naturally lead me to make some further remarks upon that substance ; which, indeed, 
may be equally applied to many other substances. The able chemist above mentioned, 
found sq parts ot potash in ioo of adularia, and 13 in 100 of the green felspar of 
Siberia ; whereas, he did not find an atom of that substance in another kind of felspar, 
which was in a laminated mass ; nor in that decomposed felspar which is known by 
the name of kaolin. Potash may therefore be considered as not being one of the con- 
stituent parts of felspar, but merely as a foreign substance, accidentally interposed 
therein. Adularia, in that case, would be nothing but an impure kind of felspar; and 
wouid present the astonishing phenomenon of a substance constantly impure, in its 
most perfect state of transparency and crystallization. It is indeed difficult to con- 
ceive that the potash is merely interposed, in such very considerable proportion, in 
the kind of felspar called adularia; yet, if it really formed one of its constituent 
parts, it would necessarily produce a substance totally different from those which do 
not contain any of it; whereas, all the mineralogical characters of felspar and adularia, 
evidently demonstrate that these two substances are perfectly similar in their nature. 
There still remain, in my opinion, many discoveries to be inade, in that part of 
chemistry which relates to the composition of mineral substances, before the chemist 
and the mineralogist shall be enabled to proceed together, with a certainty of agree- 
ment respecting the object of their inquiries. 
P p 2 
