35.3 



LETTER XXXV1L 



SEMI-OPAL EXPERIMENT OX SEMI-OPAL OF TKLKEBANY 



OPAL. ...ITS WONDERFUL PROPERTIES.... A NA LYSIS BY KLAP- 

 ROTH.... HORN-STONE AND FLINT. 



1 HE 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 neces- 

 sary 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. Kir wan 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, 1 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 hyclrophanous 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 : 

 VOL. i. z z 



