455 



consistently explained in the terms of Mr. Faraday's theory. 

 And here we seem to find points of coincidence between cohesive 

 force, as ordinarily considered, the principle of regelation, arid that 

 particular view of the former which has been announced by Mr. 

 Faraday in accounting for the phenomena presented by and con- 

 nected with the latter. 



2. But we are led by the preceding facts and considerations to 

 some further inferences, if not indeed to a definite hypothesis, upon 

 the subject of the molecular constitution or physical nature of glass. 

 Mr. Faraday's view of it has been cited already ; he regards glass, 

 it will be remembered, " as a solution of different substances one in 

 another." Professor Maskelyne has suggested to me, in conversa- 

 tion, that the physical nature of glass most probably nearly re- 

 sembles 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 boracic acids, probably by the heavy 

 optical glass of Faraday, by the simple 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 



* 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. Miller of Cambridge had the kindness to 

 direct me. By etching the surface of glass, he found it to have a porphyritic 

 structure, consisting of crystals imbedded in an amorphous substance. But the 

 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 crystals, though 

 in all probability crystals will always be found on its surface. The amorphous 

 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 pre- 

 vious knowledge of the dependence of the texture of glass upon the rate of 

 cooling. See Comptes Rendus, tome xxxiv. (1852, April 12) p. 565. 



