Microscopic Characters of Devitrified Glass. 



103 



Generalisations. 



From the microscopic examination of the specimens already 

 described,* it seems evident that the devitrification of solids of 

 ■the nature described in this paper takes place in a definite and 

 apparently uniform manner, to which Specimen No. 105 is no excep- 

 tion, for the incipient spherules and the well-developed spherules are 

 but rudimentary phases of the divergent groups which we generally 

 meet with, and which have been already described. In Specimen 105 

 they are essentially superficial, and we can imagine them as hemi- 

 spheres, as represented in fig. 3, ready, as devitrification advances, to be 

 continued inwards, in which case we cease to recognise their spherulitic 



character. In solids free from flaws the devitrification appears then, 

 as a rule, to consist in the development of divergent groups of 

 crystals, the divergence being usually limited by a network of minute 

 joints, which give rise to small polygonal prisms. These crystals and 

 joints extend inwards from the different faces of the solid, and may 

 or may not ultimately meet. The crystalline groups in their respec- 

 tive prisms are banded by arcs of circles, which we may assume are 

 related, but perhaps obscurely, to the initial pseudo-spherulitic struc- 

 ture of the superficial crust of the solid. These arcs indicate succes- 

 sive stages of growth. The crystallisations from the different faces 

 of the solid ultimately, in small masses, arrest one another, and 

 devitrification is then complete. In the case of the sphere, Speci- 

 men F, already described, the process has gone on in much the same 



* With the exceptions of Specimens 78, L and M, in which, devitrification was 

 produced during cooling from the fluid state, and Specimen 105, which was probably 

 still somewhat soft when devitrification commenced, all the specimens described 

 were devitrified whilst in the solid state by more or less prolonged periods of 

 heating. 



