Microscopic Characters of Devitrified Glass. 



105 



glass contained in other pots in the same kiln, that no appreciable 

 change had taken place in the glass np to this time, we propose to 

 reckon, in this and subsequent experiments, what may be called the 

 active period of devitrification, from the first attainment of 650°, 

 neglecting altogether the time required to bring the specimen up to this 

 temperature, which necessarily varies in different cases, and is known 

 to be without appreciable effect on the glass. The pot containing Speci- 

 men 126 was withdrawn from the kiln 29 hours after its first attaining 

 the temperature of 650°, by which time the heat had slightly increased. 

 The pot with its contents was allowed to cool during about four hours, 

 when the glass was removed from its covering of sand, which had 

 cooled down almost to the atmospheric temperature. This specimen 

 shows devitrification only on the surfaces, the alteration being so 

 slight that writing can be clearly read through the glass when it is 

 placed close over it, but when raised an inch above the writing the 

 latter appears blurred and illegible. The devitrification, which is- 

 quite incipient, consists in the segregation of vast numbers of minute 

 granules and globulites about various points on the surface of the 

 glass, and in very many cases small stellate crystallites lie in the midst 

 of these segregations. They are colourless and translucent, but too 

 small to show any double refraction, even if they possess the property. 

 Under an amplification of 120 linear the specimen shows portions of 

 its surface which are still quite clear and unaltered. The margins of 

 the unaltered areas show some fine nebulous segregations which 

 envelope no crystallites, but the majority contain the stellate forms 

 already alluded to. Of these, the simplest form is a four-rayed star 

 or cross, the arms of the cross being apparently at right angles, but 

 most of these crystallites are many-rayed, as shown in fig. 13, Plate 3, 

 which was drawn with a magnifying power of 820 linear. On the top 

 and left hand margin of this drawing portions of a crack are shown, 

 and on certain parts of the surface of this specimen the nebulous and 

 crystallite-bearing spots are separated by a network of irregular 

 cracks. 



Specimen 127, a piece of polished plate-glass, J inch thick, treated 

 in the same pot as Specimen 126, and under exactly the same con- 

 ditions. This is another instance of incipient and purely superficial 

 devitrification. The general appearance is somewhat like that of 

 No. 126, but in this case, although a few imperfectly developed 

 spiculse are present, there are no distinct stellate crystallites, possibly 

 because in glass of this kind, containing a considerable quantity of 

 lime, stellate crystallites do not occur so frequently as in the quality 

 represented by Specimen 126, and the alteration of the glass consists 

 merely of delicate nebulous spots, which under a power of 820 linear 

 are seen to be composed wholly of globulites, and this is the most 

 rudimentary phase of devitrification touched upon in this paper. 



