Microscopic Characters of Devitrified Glass. 



89 



a small quantity of ferruginous material. The plate, measuring about 

 6 feet by 3 feet 8 inches, was devitrified in the following manner. A 

 layer of sea sand, previously washed, sifted, and dried, was spread to 

 a depth of 2\ inches on the floor of a kiln used for annealing glass- 

 house pots. On this sand the plate was carefully bedded and covered 

 with more sand to a depth of G inches, the whole being kept together 

 by a low brick wall. The object of bedding in sand was to prevent 

 the bending and fusion of the plate when the kiln attained its highest 

 temperature. The kiln was then lighted, and the heat slowly raised. 

 In six days it had attained a dull red, and in six days more was at its 

 full heat, a very bright red, sufficient to soften, but not enough to 

 melt, the glass through its covering of sand. This temperature was 

 maintained for twelve days, when the kiln was let out, and quickly 

 cooled by opening the door. The mass of sand, however, retained its 

 heat for a considerable time after it was possible to enter the kiln, 

 for, on removing the dwarf retaining wall four days after the door 

 was opened, the plate broke in consequence of cold air coming in 

 contact with one of the edges, whilst the other parts were at a com- 

 paratively high temperature. The glass was thus in the kiln altogether 

 for twenty-eight days, during six of which it was gradually heated to 

 dull redness, during six more the temperature was increased to bright 

 red, maintained at this for twelve days, and cooled during four. It 

 was found to be thoroughly devitrified, and large pieces were ground 

 with sand and water to a fine smooth surface on both sides, by which 

 the thickness was reduced to seven-eighths of an inch. The portion 

 selected for microscopical examination was broken off a corner of one 

 of the large pieces. It is opaque, dull, and porcellanous externally 

 on the ground surfaces, in places which have not been ground the 

 surface is rougher, and has a glazed appearance. The parallel faces 

 are of a pale green colour with a reticulation of white lines, enclosing 

 areas which range from about -~ to \ inch in diameter, mostly poly- 

 gonal in form. The specimen was of irregular triangular shape, and 

 the sides and edges formed by cracks, probably produced at an early 

 period of the heating process, are of a uniform greenish- white or pale 

 greenish -grey tint. The hardness appears to be slightly greater than 

 that of the same glass before devitrification, upon which it produces 

 feeble scratches. A cross fracture, revealing the devitrified interior, 

 shows very delicate, slightly undulating bands, which agree in direc- 

 tion with the parallel faces of the plate, and the alternate bands, when 

 viewed in this direction, exhibit a silky lustre like that of chrysotile, 

 satin-spar, and other fibrous substances, only rather feebly. At 

 the marginal extremities of this fractured surface similar bands 

 are seen running in a direction at right angles to the others, 

 and these transverse bands occupy two triangular areas, as shown 

 in fig. 1. 



