Peculiar Properties of Glass. 43 



Both Professor Bonney and Professor Zirkel, with whom I 

 also conversed respecting them, were of opinion that if they 

 did not polarize light, and were not of the regular system, 

 they could not be regarded as crystals, however perfect in 

 form they might be. 



Lately, I have given more attention to this subject, and 

 by the aid of Dr. Alexander Hodgkinson, of Manchester, I 

 have been able to demonstrate that these crystalline forms 

 actually do polarize light. The most distinct effect pro- 

 duced on them was by the employment of circularly 

 polarized light. When the microscope stage was rotated 

 with one of these crystals in focus, the regular changing of 

 colours was very distinctly seen on each crystal, thus 

 proving that the crystalline forms developed by the alkaline 

 fluorides possessed also the polarizing properties of the 

 irregular system to which most of them belong. 



It is remarkable that these crystals are only seen near 

 the edges of etchings by the alkaline fluorides, or only 

 where the immediate surface of the glass has been removed. 

 In the deeper parts of the etchings an irregular surface is 

 presented, resembling to the naked eye a crop of small 

 crystals, but on microscopical examination shewing no 

 distinct crystalline form. It was somewhat difficult to 

 determine whether the crystals were indentations in the 

 glass or whether they stood in elevation, but after careful 

 microscopical examination both Dr. Hodgkinson and I 

 came to the same conclusion, that they stood in elevation. 

 In a large thick glass vessel, capable of holding ten gallons, 

 I placed six or seven gallons of fluosilicic acid solution con- 

 taining a little hydrofluoric acid. After some months the 

 vessel became deeply etched and, viewed from the outside, 

 the surface seemed to be covered by a crop of well-formed 

 crystals of considerable size. This vessel cracked in different 

 places, which I find usually results in time from dissolving 

 the inner surface of a glass vessel by hydrofluoric acid or the 



