Microscopic Characters of Denitrified Glass. 



97 



are very irregular in form, and this has already been attributed to 

 the presence of the flaws emanating directly and indirectly from the 

 fissure. There appeared to be no reason why in such a solid the 

 devitrification should not proceed steadily inwards until the three sets 

 of prisms arrested one another along three lines passing from the 

 three angles of the triangular section, and meeting in its centre. 



With a view to settling this point another trigonal prism (Specimen 

 No. 143), free from any flaws, was devitrified. A transverse fracture 

 through this devitrified prism shows three distinct and similar areas 

 of crystallisation ; each is an isosceles triangle. These triangular areas 

 are bounded by the three sides of the prism and by three straight lines 

 of arrest, which accurately join the centre or axis of the prism with 

 its three angles or edges, fig. 9, Plate 3. This demonstrates conclu- 

 sively that the irregular devitrification seen in Section K is due to 

 simultaneous crystallisation along flaws. 



Specimen D heated twice under same conditions as Specimens I and 

 K, is part of a completely devitrified six-sided prism of plate-glass. 

 The surface has a glaze like that of pottery. The transverse section 

 of the prism is not a perfect hexagon, and it has not been cut quite 

 at right angles to the principal axis. There is a well-marked crust of 

 divergent crystalline fasciculi due to the first short heating, prismatic 

 joints being also present, but they are not well defined. Devitrifica- 

 tion has then proceeded inwards in directions approximately normal 

 to the lateral faces of the prism in broad divergent crystalline groups, 

 separated by joint planes, which preserve at the best a very imperfect 

 parallelism. In fact the prismatic structure which they indicate seems 

 very irregular, and in the section a prism is often represented by a 

 lanceolate or an irregularly shaped area, while the divergent crystal- 

 lisations do not all seem to be formed in directions parallel to the 

 plane of section. These crystallisations show brindled markings, 

 similar to those seen in Specimen ISTo. 115. There is strong chromatic 

 depolarisation in this, as also in the preceding Sections I and K. 



Specimen E is part of a six-sided prism of plate-glass, 2 cm. in 

 diameter, which has been devitrified to a depth of barely 1J mm. 

 under precisely the same conditions as Specimen H. The devitrified 

 crust is yellowish-white, and has a glazed surface like that of pottery. 

 Two sections have been cut from this specimen, B being taken 

 transversely to the principal axis, and E 2 parallel to it and to one of 

 the faces of the prism. The latter section consists, in fact, only of 

 the devitrified crust of one of the faces of the prism. 



Section E, taken transversely to the principal axis of the six-sided 

 prism, shows a devitrified crust, which by reflected light looks white, 

 while by transmitted light it appears under the microscope of a brown 

 or yellowish-brown tint. It consists of divergent groups of very 

 delicate acicular crystals, but even under high powers their termina- 



VOL. XXXIX. H 



