Microscopic Characters of Devitrijied Glass. 99 



crystalline margin is bounded internally by a sinuous crack, showing 

 the extent of the devitrification produced by the first heating to which 

 the specimen was subjected, while other irregular cracks traverse this 

 portion circumferentially as a rule, but they sometimes pass through 

 the margin radially. The latter are few, penetrate but a short 

 distance, and are mostly fringed by delicate crystalline fibres normal 

 to the crack, and usually terminate in a radial group of fine acicular 

 crystals or fibres. In one or two spots the cracks are seen to follow 

 the contours of small cavities, from which sand-grains have been 

 stripped in the process of grinding the section. Inside the wavy 

 circumferential crack the crystallisations have shot inwards in long 

 divergent groups, which towards the middle portion of the sphere 

 give place to large irregular radiating groups of crystals, so large, 

 in fact, that there does not appear to be more than half-a-dozen of 

 them in the section, and these are in most instances cut through in a 

 plane remote from their centres, thus giving oblique and transverse 

 slices through the crystalline rods. Had these groups been able to 

 crystallise freely they would have resulted in spherules, and this, 

 indeed, might have been the case had the devitrification of the 

 sphere been incomplete ; as it is, they seem to have rudely polygonal 

 boundaries. The devitrification of this specimen seems in part to be 

 of a micro-crystalline-granular character under a magnifying power of 

 18 linear, but under a power giving an amplification of 570 diameters 

 this is seen not to be the case, the mass being resolved into a closely 

 matted aggregate of little acicular crystals with a general tendency 

 to radiate grouping, as shown in fig. 4, Plate 1. In fig. 5 on the 

 same plate the general aspect of a portion of the sphere at and near 

 the margin is shown. The circular hole near the margin is where a 

 sand-grain, around which the glass has fused, has been stripped out 

 in grinding. 



Specimen 'No. 78 is portion of a large hemispherical mass of com- 

 pletely devitrified sheet-glass taken from a mass of many tons which 

 burst from the furnace in the liquid state and ran into a " cave " 

 underneath. The mass solidified rapidly, but owing to its great bulk 

 remained at a high temperature for several days. In the specimen 

 there is a fragment of uncombined lime, indicating that at the spot 

 from which the specimen was taken the fusion of the raw materials 

 composing the glass was not quite complete. This specimen exhibits 

 a curious and very rough concentric scaly or platy structure. It is 

 of a pale greenish-white tint, and the broken surfaces are covered 

 with small glistening hair-like crystals. It feels rough to the touch 

 like a piece of unglazed porcelain, which it rather resembles, and it 

 has a distinctly vesicular structure. The vesicles are spherical. In 

 thin section it is very feebly translucent, and consists of a mat of 

 minute groups of radiating crystals. The aspect of the surface of a 



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