S14 On the Composition of Avantunne-Olass. 



I am indebted for the historical information) visited the Vene- 

 tian Glass Manufactories in 1819, it was no longer made, and 

 nothing could be learned of its mode of preparation, which 

 seemed to have remained a secret. The information on the 

 subject, afforded by technological works, according to which 

 avanturine-glass is produced by the fusion of fine leaflets of 

 gold, copper, brass, mica, or talc, with glass, is incorrect, as is 

 distinctly proved by microscopic examination.* 



J. G. Gahn was the first who made the observation that the 

 metallic spangles in avanturine-glass are crystals^ which must 

 have been developed during the cooling of the melted mass. 

 When Hausmann was at Fahlun in 1807, Gahn shewed him 

 these crystals under the microscope, and they then appeared 

 to be regular six-sided and three-sided tables. The view of 

 a piece of avanturine under the microscope, when only a mo- 

 derate magnifying power is employed, is really surprisingly 

 beautiful ; and we perceive that each little spangle is a regularly 

 formed brilliant crystal. The crystals are evidently segments 

 of regular octahedrons, but so thin that we never can observe 

 an entire octahedron. They are quite opaque ; and the glass, 

 in which they are embedded, is transparent when in thin plates, 

 and has a yellowish colour, which in certain directions exhibits 

 a tendency to bluish-green. 



The formation of crystallised bodies, in masses which have 

 been produced at a high temperature, is always deserving of 

 attention, especially with reference to the formation of the 

 crystalUsed compounds of the mineral kingdom ; and hence it 

 was of some interest to ascertain what the crystals in avan- 

 turine really are. The author of this paper, hoping that 

 analysis would afford information on this matter, caused several 

 specimens of avanturine-glass to be examined chemically by 

 Mr Schnedermann. As the substance is not soluble in acids, 

 the decomposition was effected by heat ; partly along with car- 

 bonate of baryta, and in the other cases recourse was had to 

 the usual method of analyzing the silicates. It resulted that 



* We must not confound with avanturine-glass the variety of quartz which, oil 

 account of its similar glittering property, has also heen termed Avanturine. 



