ANNIVERSAEY ADDRESS OF THE PRESIDKJfT. 9 1 



and thus illustrate what we may t(^nn primarij devitrification. It 

 represents a segregation of certain crystalline minerals from the 

 body of the glass, which, however, undergoes no change whatever 

 that is visible to the eye in the other part, the free ends of the 

 microliths being bounded by a true glass, apparently identical 

 with that in other parts of the mass. I now turn to some 

 examples illustrative of changes produced by the action of heat 

 upon specimens which have once been perfect glasses. The first, 

 given me by Mr. F. Claudet, is one of great interest. It consists of 

 a number of sheets of window-glass, which, when lying one on 

 another, were exposed to great heat in the noted conflagration at 

 Hamburg, ajid partially fused, so that they now form an almost solid 

 mass about | inch in thickness, with thin alternating bands of 

 opaque-white and of clear glass, not unlike a banded rhyolite. A 

 closer examination shows that, except at top and bottom, each white 

 band is more or less double, half belonging to the underpart of one 

 sheet, and half to the upper part of that in contact with it. Each 

 is composed of a tufted growth of very minute acicular crystals of a 

 pale brownish-grey colour. Usually their close approximation 

 compels them to exhibit a brush-like structure ; but occasionally 

 where there are slight interruptions, we have more or less perfect 

 half-sj)herulites, and in a few cases where the fusion of the surface 

 appears to have been complete, we have a perfect spherulite, whose 

 equatorial plane represents the former junction-faces. As the glass 

 has not been completely melted, it seems probable that the spheru- 

 lites are due to a crystallization of, not from, its material, and they 

 clearly originated at the surface of the sheets. 



A specimen, prepared for me by Mr. F. Siemens, further illustrates 

 this. A group of four pint bottles of different-tinted granulite glass 

 was exposed for twenty-four hours to a temperature of about 600° C. 

 The heat has not sufficed to fuse the bottles, but they have been 

 completely softened, have fallen together and become welded into an 

 irregular fiat cake, though the necks, lips, even the letters stamped on 

 the bottles, can be readily distinguished. We have thus had about 

 the same approach to fusion as in the former case. Here, too. a 

 fracture through one of the bottles shows at either surface a white 

 skin about -gV^h inch thick, duplicated as before, where two surfaces 

 have been welded together; but between these, in the clear glass, are 

 numerous small spherulites from T,\|-th to x\^th of an inch in diameter. 

 Here, again, one would imagine the chemical difi'erences between 

 the crystalline bodies and the glass must be extremely slight. The 

 mineral has a similar aspect with, and is probably the same as, that 

 in the larger spherulites described in the slowly cooled mass of 

 granulite glass. 



I am indebted to Mr. F. 'W. Eudler for a specimen of great in- 

 terest. This is a fragment of a plate of glass, about 1-1 inch thick, 

 devitrified by exposure in a crucible to a bright red heat for three 

 weeks. A slide cut from it exhibits many points of interest on 

 which I have not time to dwell ; but the following have a special 

 bearing on the question before us. So far as I can ascertain, there 



