88 



Messrs. D. Herman and F. Rutley. 



Si0 2 68-4 



CaO 12-0 



MgO 0-5 



Na 3 14 7 



Al a b 3 4-9 



100 -5 



while after experiment it contained 



Si0 2 64-5 



CaO 21-9 



MgO 1/2 



Na 2 6-3 



A1 2 3 , . . . 1-4 



H 2 4-2 



99-5 



The observations recorded in the present paper refer to structures 

 developed in specimens of glass which, for the most part, were pre- 

 pared, and all devitrified, at the Glass Works of Messrs. Pilkington 

 Bros., St. Helens, and it is possible that they may have a special 

 interest for those who are studying the natural devitrification of 

 obsidians or other glassy or glass-bearing rocks, since in the cases here 

 recorded the precise conditions of devitrification are known, while in 

 the natural process we do not know the precise conditions. We may, 

 however, assume with considerable safety, that within certain limits 

 there will be a more or less close analogy between the results of the 

 natural and artificial devitrification, allowing of course a margin for 

 certain natural conditions which it would be difficult if not impossible 

 to reproduce experimentally. Thinking that some definite laws might 

 be arrived at by devitrifying solids of various forms, we have operated 

 upon cubes, hexagonal prisms, trigonal prisms, spheres, cylinders, 

 flat plates, and other distinct forms, and these we shall first describe. 

 The difficulties attendant upon the microscopic examination of such 

 materials consist principally in their excessive opacity in some 

 cases, and in others upon the readiness with which the substances 

 disintegrate during the process of grinding. The latter difficulty 

 has in many instances been successfully overcome by Mr. Cuttell, 

 by whom most of the sections have been carefully and admirably 

 cut, so that the boundaries of the solids are preserved without 

 injury. 



Specimen No. 115 originally formed part of a 1-inch-thick piece of 

 plate-glass, which was accidentally coloured green, during fusion, by 



