68 NOTE ON THE PRINCIPLE OF REGELATION, ETC. 
it will be remembered, “‘ as a solution of different substances one in 
another.” Prof. Maskelyne has suggested to me, in conversa- 
tion, that the physical nature of glass most probably nearly 
resembles that of a solution of a crystallizable salt in water, imme- 
diately before crystallizing. These views are evidently coherent, 
and they harmonize with Prof. Graham’s, who defines glass, che- 
mically, as “a mixture of silicates.”* But they all relate to the 
varieties of glass in common use, while we are concerned, at present, 
with the abstract vitreous condition of matter, such as it is repre- 
sented by the phosphoric and boracie acids, probably by the heavy 
optical glass of Faraday, by the simplest glasses of felspar and peri- 
dote obtained by Charles Deville, by the glassy condition of silica, 
natural and artificial, and still more perfectly, perhaps, by the glassy 
form of sugar. 
Bearing in mind then the homogeneous, or comparatively homo- 
geneous, nature of these glasses, and considering the uniformity of 
texture which the acoustic as well as the optical characters of per- 
fect glass in general evince, especially when contrasted with that of 
erystalline plates in the acoustic researches of Savart, and how 
strongly distinguished that texture is from a crystalline texture or 
structure,—a nearer analogy than that of a solution ready to ery- 
stallize, I think, will be found in the condition of water cooled below 
the freezing-point but still remaining liquid, until by a tremor, 
or the percussive contact of a solid body, or the mere contact of a 
erystal of ice, its temperature rises to 32° and it becomes ice. If so, 
glass will be a substance in which this state of arrested liquidity, or 
potential solidity, is permanent. And this inference will harmonize 
with known facts. Gregory Watt proved that heat is evolved when 
mineral glasses crystallize or become (permanently and truly) solid.+ 
The preparation of sugar called barley-sugar is the vitreous condi- 
* These views of Mr. Faraday, Mr. Maskelyne, and Mr. Graham, are confirmed by the 
“experimental evidence of the structure of glass obtained by Leydolt, to whose researches 
Professor W. H. Millar of Cambridge had the kindness to direct me. By etching the sur- 
‘face of glass, he found it to havea porphyritic structure, consisting of crystals imbedded in 
an amorphous substance. Butthe peculiar characters of glass, especially its relations to 
sound and light, evince, as indicated in the sequel, that it is not a congeries of ready-formed 
-erystals, though in all probability crystals will always be found on its surface. The amor- 
phous substance recognized by Leydolt will answer, nearly, to what I shall call “simple 
glass.” Other facts which he observed are’ perfectly in harmony with our previous know- 
‘ledge of the dependence of the texture of glass upon the rate of cooling. See Comptes 
Rendus, tome xxxiv. (1852, April 12) p. 565. 
+ Phil. Trans, 1804, pp. 285-290. 
