353 
LETTER XXXVII. 
SEMI-OPAL EXPERIMENT ON SEMI-OPAL OF TELKEBANYA OPAL 
jrpg WONDERFUL PROPERTIES ANALYSIS BY KLAPROTH 
HORN-STONE AND FLINT. 
The semi-opal is the next substance which, possessing this 
kind of lustre, demands our attention : but, previous to making any 
remarks on the nature of its constituent parts, it will be necessary 
to determine the propriety of placing it among those substances 
that possess the peculiar lustre, which I have supposed to belong 
to those substances, which are formed by an intermixture of bitu- 
men with silex. The reason for instituting this inquiry is, that 
Mr. Kirwan describes the lustre of the semi-opal, and of the opal 
itself, as being of the glassy kind ; which, being admitted, would pro- 
hibit both these substances from preserving the rank which is here 
allotted them. This character, however, a glassy lustre, I believe, 
with the utmost deference to Mr. Kirwan, is not the legitimate lustre, 
at least, of the real external surface of the opal, or of the semi- 
opal. The lustre of its fragments, indeed, may be, and that which 
it receives from the polish of art generally is, of that description . 
but in an hydrophanous opal now before me, which, when dry, 
resembles a piece of white wax, and when in water is transparent, 
and resplendent with purple and a vivid green, the lustre, even of 
its polished surface, is decidedly resinous ; and, most undoubtedly, 
the lustre of the actual external surface of the opal, that which was 
in contact with the matrix, is almost always of the waxy kind. The 
matrices, also, both of the opal, and of the semi-opal, frequently pre 
sent appearances, very strongly indicative of the presence of bitumen . 
