146 SUMMARY OF CURRENT RESEARCHES, ETC. 



Glass Crystals.* — The crystal, Fig. 30, is that of ordinary window- 

 glass, which is formed by cooling down large masses of fluid glass 

 for several days to a temperature slightly higher than its point of 

 viscidity. These crystals were discovered about ten years ago, and at 

 intervals up to the present day Mr. W. D, Herman and Mr, G. E. 

 Davis have spent much time in investigating their nature and in 

 photographing them. Their close relationship to rock-structure make 



Fig. 30. 



them exceedingly interesting, as well as the large number of forms 

 which are often found in the same square inch of glass. Some mix- 

 tures of glass give these star-crystals only, while in others there are 

 none found save prisms ; and these latter form excellent polariscope 

 objects. 



The figure was prepared by photographing the crystal with the aid 

 of a ^-inch objective, the collodion-film being then transferred to the 

 wood-block. 



* 'Northern Microscopist,' i, (1881) pp. 21-2. 2 figs. 



