Washington — Copper Crystals in Aventurine Glass. 417 



mass of glass, which is much cracked, are many smaller clus- 

 ters of the same trichites. 



The blue color of this layer is obviously due to the oxidiz- 

 ing action of the atmospheric air on the copper, analogous to 

 the action of the oxidizing flame of the blowpipe on a borax 

 bead containing copper, and the black trichites may be with 

 great probability supposed to be Tenorite, CaO. 



Although some writers have thought that the crystals 

 described in the preceding pages are not metallic copper but a 

 silicate of copper or Cu 2 0, yet Wdhler's original determina- 

 tion has been since confirmed,* and their color, luster, opacity, 

 crystalline form and habit, and twinning forms leave no doubt 

 of their nature, even apart from chemical tests. 



Yogt has already pointed out the fact that the copper has 

 evidently crystallized from solution in the molten glass exactly 

 like a salt from water, and this view is confirmed by such facts 

 as the blue upper layer, the presence of the clear zones around 

 the crystals of the second group, as well as the fact that they 

 seem to follow the laws of crystal growth in solutions as laid 

 down by O. Lehmann.f 



This author shows by numerous examples that crystals tend 

 to grow most rapidly at the most sharply pointed parts, and 

 this we find beautifully exemplified in the projections at the 

 angles of the hexagonal and triangular plates as well as in the 

 salient ridges along the thin edges. Also in accordance with 

 this law is the generally observed fact that the projections on 

 the triangles are on the whole relatively longer than those on 

 the hexagons. 



He also shows;}: that the crystal habit varies with the con- 

 centration and temperature of the solution and with the 

 rapidity of crystallization, thre more irregular forms, such as 

 skeleton crystals, being produced by greater concentration and 

 quicker crystallization among other conditions. The formation 

 of twins seems to be due to disturbing influences on th.e regu- 

 lar crystallization. § 



On such grounds the diverse habits of the various crystal 

 groups are to be explained ; the tabular phenocrysts being 

 first formed from a more concentrated solution, some of these, 

 as the conditions changed, tending to grow by additions at the 

 angles and edges, while more copper crystallized out as the 

 smaller phenocrysts, till finally in the last stages only the 

 minute regular octahedral microlites were formed. 



The manufacture of this glass being a trade secret I could 

 extract no information from the foreman who gave me the 



* Vogt, op. cit., p. 231, note 5. 



f Molecular physik. Leipzig, 1888, i, pp. 33*7 S. 



% Op. cit., p. 303. § Op. cit, p. 416. 



