424 Microscopic Characters of Devitrified Glasses. [June 18, 



under which this mode of devitrification would hold good, glass solids 

 of definite geometrical forms were partially and completely devi- 

 trified, and it was seen that in each case the crystalline bundles, sepa- 

 rated by joint planes, extended from the surface of the solids inwards 

 in directions normal to the surfaces. In a trigonal prism it was found 

 that there was an apparent departure from this rule, certain sets of 

 crystals emanating from lines within the prism, and extending out- 

 wards towards the surfaces of the solids, although they did not reach 

 those surfaces. This anomalous procedure was referred to the pre- 

 sence of flaws, and to demonstrate this two fresh specimens were 

 devitrified. One was another trigonal prism free from any flaw, in 

 which devitrification extended uniformly from the surfaces until three 

 straight lines of arrest resulted, meeting in the axis of the prism. 

 The other specimen was a thick piece of flat plate glass. On this a 

 short line was ruled with a diamond, and a tap being administered, a 

 crack was developed from this line, extending rather more than half 

 way through the plate. The plate was then completely devitrified. The 

 result was precisely similar to that seen in the specimen first described, 

 but it was found that the crack emanating from the diamond scratch 

 had acted as a fresh surface, devitrification having taken place at right 

 angles to this crack as far as the median line of arrest, while on the 

 other side of that line, and from the extremity of the crack, a radiating 

 crystalline structure was developed, resulting in a structure analogous 

 to that of a hemisphere, but of course extending the whole length of 

 the crack. 



The remainder of the paper touches upon the incipient development 

 and formation of spherules and crystallites, with other points of inte- 

 rest in connexion with superficial devitrification. The spherules in 

 their most rudimentary stage sometimes appear to consist of nebular 

 segregations of globulites. Instances are also cited in which partial 

 and complete devitrification of a glass plate is brought about by the 

 development of spherules in the interior of the plate in conjunction 

 with the formation of an enveloping crust, which extends to variable 

 distances from the surfaces of the plate, according to the period of 

 devitrification, and also, more especially, according to the chemical 

 composition of the glass. 



Some notes are also given on the relation of the phenomena here 

 described to similar ones met with in naturally devitrified rocks, the 

 paper being intended as the forerunner of others, in which it is hoped 

 that the petrological bearings of the subject may be more fully 

 discussed. 



