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On Opal, and the Amorphism of Solid Bodies. By Professor 

 FucHS of Munich,* 



MiKERALOGisTS atid chemists have gradually formed the opi- 

 nion, that solid inorganic bodies are all of crystalline structure, 

 and that those in which we cannot detect any traces of crystal- 

 lization, viz. compact minerals, ffltrst be regarded as aggregates 

 of extremely minute crystals, or, as it were, of crystal germs. 

 Most of the compact minerals are without doubt of this nature ; 

 and we convince ourselves of this more especially by the gra- 

 dual transition of crystalline,— granular and radiated masses, to 

 compact, which we with reason unite, the one with the other, if 

 they exhibit similar physical properties and chemical coniposi- 

 tion. But this view of the subject is not universally vahd ; all 

 compact masses cannot be regarded as such aggregates ; such a 

 connection with crystallized bodies is not shewn by all ; there 

 are several which do not present a trace of crystallization, and 

 which are therefore called formless or amorphoiis bodies. Per- 

 mit me to make some remarks on these substances, and to pre- 

 sent some considerations on their pecuHar condition and impor- 

 tance ; in which I shall refer not only to minerals, but also to 

 other similar substances. I commence with Opal, which seems 

 to me one of the most important of the amorphous substances, 

 as it throws much light on the others. 



Pure opal is essentially nothing else than amorphous and com- 

 pact silica, of which one may convince himself either by regard- 

 ing it with attention, or by comparing it with pure and perfectly 

 formed (juartz, viz. rock-crystal, which represents crystallized sili- 

 ca. But opal, having external crystalline planes, has never been 

 met with, and its internal structure is such that, in regarding it, we 

 cannot imagine that it exhibits the shghtest germ of crystalliza- 

 tion. Its smooth and shining fracture, and generally its whole 

 habit, does not permit us to entertain any idea of crystalline 

 structure, since, when it is not accidentally fissured, it presents 

 a continuous mass similarly connected in all directions, so that 

 in this respect it may be compared to liquid bodies, in which we 

 cannot imagine that there is any thing of a cry stalhne nature, 

 even in the most finely subdivided portion. 



Opal is distinguished from quartz in a remarkable way by its 

 • Read to the Academy of Sciences of Munich the 9th March 1833. 



