Py : 
32 On the Eiffects produced on different Stones 
‘ opal, and its becoming tranfparent in water. I found means 
- to ftop the action of the fluor acid on a cornelian and a dark 
vown jafper, at that flage of the procefs when the ftone was 
fill compact, and the colours only whitened. In water both 
thefe ones recovered their. natural colour, and when dry 
became again white. / 
E expoted another opaque and almoft black piece of flint, 
-with efflorefced white points, and the ufual white cruft, 
without any covering of wax, at the ufual ftove-heat of about 
36°, to the vapour of the fluor! acid for five days in my ap- 
peratus. At the end of that period it had loft almofi 1-8th 
of its weight; for it was reduced from 403 to gz grains, and 
was thoroughly white, fo that the found parts of the nucleus 
had entirely the appearance’ of the efflorefced cruft. Some 
parts were friable; and I at the fame time remarked that the 
parts. of the nucleus, already naturally eforefced, as well as the 
cruft, were much lefs aéted upon than the found black parts. 
It is found alfo, that in regard to the natural efflorefcence 
of flints, the efllorefced cruft ferves the found nucleus as 2 
cover to defend it, and, by theie means, retards the total de- 
compofition of the ftone. Some, from this cruft being formed 
of feemingly foreign materials, whieh is faractinues confi- 
dered as lime and famctinias: as clay, and which in common 
refembles both, have deduced.too much, in regard-to the 
formation of flints, when they explain the phenomenon 
by feparation and fuccefive hardening from one of thefe 
kinds of earth. To. judge from the above expernments, this 
eruft is not properly the matrix, but a refult from the de- 
compofition of the ftone itfelf. 
The efflorefeence of feld-{par inte a granite-like mixhare 
3s, with juttice, confidered as a very remarkable geological 
phenomenon, as.?t announces an endlefs. feries of the total 
decompofition of the prefent combinations. The fueceffive 
degrees of the decompofition, by means of thefe experiments, 
may be exhibited with a fmall and very fimple apparatus. 
Turmalin from the ifland of Ceylon, and Zillerthal in Tyrol, 
ween and black columnar {chorl, and olive-coloured horn. 
blende, after twenty-four hours expofure, had experienced yo 
change. ; 
; White 
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Cees Se ew ee nD 
