Microscopic Characters of Devitrified Glass. 101 



triclinic crystals. Their angles vary considerably; one gave J.50 , 

 another 116°, but these measurements are of little value, as it is 

 doubtful whether the individuals measured were lying parallel with 

 the planes of section. Some of these forms are shown in fig. 6, 

 Plate 2, not represented as they are actually grouped in the prepara- 

 tion, but selected from various spots. They closely resemble some of 

 the crystallites met with in the slags of blast furnaces. They occur 

 in the white turbid bands in the glass, the transparent portion being 

 almost free from them. 



Specimen M is a piece of clear sheet-glass, about 2*5 mm. thick, 

 from a pot containing somewhat less lime than usual. Owing to the 

 furnace being rather cold during the time the glass from this pot 

 was being worked, devitrification in the form technically known as 

 " ambitty " set in, and increased to such an extent that blowing was 

 stopped and the pot emptied by ladling. The specimen was blown 

 shortly before the ladling operation was commenced ; it contains a 

 few very beautiful crystallites similar to those figured in Plate 8 of 

 Vogelsang's J ' Krystalliten." One of them, which closely resembles 

 one of the usual forms of snow-crystals, being a skeleton hexagon or 

 six-rayed star, gives angles of 60° between the component crystalline 

 needles. These exhibit double refraction, and undergo extinction in 

 directions parallel to and at right angles to their longest axis. Between 

 crossed Nicols depolarisation from strain is visible in the adjacent glass, 

 the minute brushes of light being more intense about the points of 

 the principal needles. It would appear from the depolarisation and 

 directions of extinction that this crystallite may be referred to the 

 rhombic system, twinned somewhat after the manner of chrysoberyl. 

 This seems the more probable, since some of the forms in Section L 

 also resemble certain rhombic forms. The crystallite just described 

 is seen when examined under a power of about 280 linear to be 

 traversed by an irregular network of strong cracks lying in the same 

 plane as the crystallite, and extending nearly to but never beyond its 

 margin. In the centre of the crystallite is a dark spherule. The 

 fact that the reticulating cracks are restricted to the area occupied by 

 the crystallite indicates a relation to the latter, and the depolarisation 

 of the adjacent glass indicates strain. Since this strain- depolarisation 

 only occurs at the margin of the crystallite, we may infer that the 

 strain is connected with its development, and the cracks are no doubt 

 the result of this strain. Had the body been a completely developed 

 crystal and not a skeleton form, the strain would probably have 

 resulted in the development of a perlitic crack, and not in a reti- 

 culating series of cracks which possibly arise from strain about a 

 number of points. Another crystallite in the same piece of glass is 

 very different in appearance to that last described ; its general out- 

 line is that of an irregular hexagon. It is traversed by four well- 



