triple Sulphuret, of Lead, Antimony , and Copper, See. 43 
This subject requires that I should enter into it more minutely. 
In so doing, I shall make calcedony the basis of my observa- 
tions, which may be afterwards easily applied to such other 
substances as are similarly circumstanced. I shall first observe, 
that calcedony, by the purity and homogeneity it is frequently 
seen to possess, as well as by its nature and the grain of its 
fracture, positively announces a state of crystallization ; yet, 
although we frequently observe it in circumstances exactly 
similar to those which, in other substances, give rise to deter- 
minate crystals, (such as actual solution in a fluid, and that 
fluid inclosed in a cavity where it is in a state of rest,) it always 
offers itself to us with the same appearance, in which there 
cannot be perceived any tendency to a regular form. Every 
mineralogist is acquainted with the beautiful geodes of Oberstein, 
in the cavities of which are seen crystals of quartz, also of 
carbonate of lime, and very frequently, likewise, fine crystals of 
the substance called by Werner Krentzstein, and by the Abb6 
Hauy Harmotome. Calcedony is frequently found with the above 
substances ; but, when that happens, although it is very pure, 
and is in the same circumstances as those substances, it always 
appears either in the form of layers, or of mamilke, or in the 
ramose form of stalactites ; and never shows the least tendency 
to a determinate crystalline form. The same remark may be 
applied to other geodes, composed of agate, of flint, &c. Indeed, 
if calcedony were capable of assuming a determinate crystalline 
form, how does it happen that this substance, (which is one of 
the most common in nature, and is found in so many different 
forms, and so variously circumstanced,) has never yet been met 
with in a state that shewed any appearance of that form. It is 
true, that many mineralogists have spoken of crystals of calce- 
G 2 
