268 Professor Fuchs 07i Opal, mul 



degree as opal and rock-crystal. However, glass-stone does not 

 always exactly agree in chemical composition with the glass from 

 which it has been produced, for a certain relation in composi- 

 tion is formed during crystallization, which very often does not 

 exist in glass. Hence the mass of glass generally crystallizes 

 only partially, or a mixture of glass and glass-stone is formed. 

 I have, however, also seen glass- stones in large masses, in which 

 no trace of real glass could be detected. That other bodies, 

 having a fixed composition, are capable of being vitrified, is 

 proved to us by many crystallized minerals, especially silicates, 

 which, without suffering any change in their chemical composi- 

 tion, afford perfect glasses by fusion. The same substance, 

 therefore, can appear at one time as glass, and at another as crys- 

 tal. Slags of all descriptions belong also to the glasses, and of 

 these many shew a great tendency to crystallization. 



The bodies capable of vitrification have the peculiarity, that, 

 by fusion, they never become so liquid as others, but always re- 

 main viscid, and may be drawn out in threads. We may there- 

 fore regard glass-fusion as only a condition of great ductility. 

 Hence it is so difficult to fuse into a homogeneous mass the 

 dissimilar bodies existing in this liquid. Hence the chief dif- 

 ficulty of procuring glass free from waves and stripes. And 

 if such a mixture is made as to have a high degree of flui<lity, 

 it is always to be dreaded that it will crystallise, or be converted 

 into glass-stone. 



Of vitrified minerals the following deserve particular notice, — 

 obsidian, pitchstone, and penrlstone, and I have no hesitation in 

 adding leucite. This last substance, which has given its name 

 to a form of the tesseral system, is, in my eyes, no crystal, but, 

 so to say, a crystal-model, enclosed, it is true, by crystalline 

 planes, but uncrystalline and glass-like in its interior. On ac- 

 count of its resistance to fire, it can retain its regular external 

 outline in volcanic fire, while the form of the smallest portions, 

 and therefore the crystalline structure, is lost. The unpreju- 

 diced observer will in vain seek for the cube which HaLiy has 

 assumed as the fundamental form of this mineral ; and if traces 

 of such a structure were found, they would only be remains, and 

 proofs of original structure, such as we meet v.ith in other de- 

 stroyed crystals. If we compare the leucite with analcime, we 



