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XVIII. On some points of analogy behveen the Molecular Structure 

 of Ice and Glass ; icith special reference to Professor Erman's 

 observations on the Structural Divisions of Ice on Lake Baikal. 

 By James Drummond, Esq.'^ 



PROF. ERMAN'S interesting observations have caused me 

 to recur to some pha;nomena bearing upon the molecular 

 structure of glass, which seem to me to throw considerable light 

 on that of ice (to which he has drawn attention), leading especially 

 to conclusions more lucid and precise than those at which he has 

 arrived. And I am induced thus to notice them from the con- 

 viction that, although neither new nor recondite, any facts whic^i 

 from being placed in a new light may tend to the elucidation of 

 this subject, either directly or by analogy, M'ill be acceptable. 



I shall first of all, however, restate from Prof. Erman's paper 

 the nature of the phseuomena observed by him f: — "The ice 

 [upon the lake] was perfectly transparent, but traversed by pe:f- 



pendicular cracks These cracks were all extremely narrow, 



and filled with air. Many of them reached from the surface only 

 to a certain depth, which was the same for all, and seemed to be 

 a third of the entire thickness of the ice. The other cracks then 



began at this depth, and reached down to the water Quite 



different in look and origin from the ci'acks here described were 

 the much wider fissures which are formed by the cooling and 

 contraction of the ice subsequent to its perfect congelation. J. 

 found one of these at the place where we were stopping. ..... 



It had throughout a uniform width of 4 inches, an4 reached 

 from the upper surface to the water. It was filled with new ice, 

 which gave it the look of a vein or dyke in rock. What added to 

 this resemblance was, that the ice filling the crack was always 

 much whiter than the surrounding. It was traversed by fine 

 cracks or flaws in a very regular and remarkable manner. One 

 of these cracks formed a continuous and somewhat waving line, 

 which occupied almost exactly the middle of the vein, and from 

 that proceeded, at an acute angle, an immense number of smfiUer 

 flaws to each side, just hke the lateral ribs of a leaf issuing from 

 the central rib." 



Such, then, are the phsenomena; but before proceeding to 

 their consideration, I shall briefly notice those bearing upon the 

 molecular structure of glass, to which I have referred. If we cut 

 a thin slip from a piece of sheet or crown glass, and hold it, 

 while slightly bent, in a polarizing apparatus, coloured fi'inges 

 will be observed, one set being separated from the other by a 

 dark neutral line running through the centre of the glass. The 



* Communicated by the Author, 

 t Phil. Mag. for June 1859, p. 406. 



