266 Professor Fuchs on Opal, and 



employment as gun-flint, much more so, it is well known, than 

 hornstone, which seems to want opal entii-ely, except when it 

 approaches flint or calcedony. It deserves to be remembered, 

 that the specific gravity of these minerals is less than that of 

 rock-crystal, a circumstance which is also in favour of their con- 

 taining opal. 



Opal does not always consist of pure opal alone, but not un- 

 frcquently includes some finely subdivided quartz. A milk- 

 white opal which had remained half-a-year in a potash solution, 

 left a skeleton, which, on being shaken, fell to powder, which 

 was similar to quartz powder. Probably the not yet sufficiently 

 explained phenomenon of the play of colours in precious opal 

 has its origin in a certain arrangement of fine mixed portions 

 of quartz, which causes the light to be differently refracted. 

 This view is rendei'ed more probable by the circumstance, that 

 no transparent opal has a play of colours, but only translucent 

 or feebly transparent varieties, which probably are rendered thus 

 opake by the quartz mixed with the opal. More accurate re- 

 sults might be obtained by treating 'the opal with potash. By 

 this process, also, we might ascertain which of the friable mine- 

 rals that have sometimes been considered as quartz, sometimes 

 as opal, belong to the former, and which to the latter of these 

 substances. Prepared silica is to be regarded as an opal, since, 

 even when it has been strongly heated, it agrees with that mine- 

 ral in chemical characters. 



From the comparison of the opal and quartz with one another, 

 we obtain the important result, that the same material substror- 

 him, — in this case silica, — can occur sometimes crystallized, 

 sometimes amorphous, and, at the same time, apai't from the 

 form, can possess extremely different properties. I consider this 

 condition not less worthy of attention than dimorphism, or the 

 occurrence of the same substance in two distinct forms; and the 

 more so, as amorphism is, as I shall afterwards shew, by no 

 means a rare phenomenon. We must accordingly distinguish 

 two conditions of consolidation, viz. the crystalline and the non- 

 crystalline, and we nuisi not regard as the same consolidation 

 and crystallization. 



We are acquainted with many bodies only in a crystalline, 

 and many only in an uncrystalline condition. Many allow a 



