Mr. 3. Drummoud on the Molecular Structure of Ice. 103 



fringes will be positive or negative, according as the slip is bent 

 to the one side or the other — the negative appearing always 

 upon the convex side, and the positive upon the concave, — show- 

 ing that the molecular structure, whatever may be its nature, is 

 the same on each side of the dark neutral line. If we now turn 

 to the change produced upon glass during the process of devitri- 

 fication, some light will, I think, be thrown upon the nature of 

 this structure. Reaumur was the first to observe and describe 

 this phrenomenou, which he had produced by exposing a vessel 

 of common bottle or green glass, filled and enveloped on all sides 

 by a mixture of sulphate of lime and sand, to the heat of a pot- 

 tery furnace during the whole course of a firing. When after- 

 wards examined, the glass appeared to have exactly in its centre 

 a dark line, from which, on each side and parallel to each other, 

 an infinite number of small needle-shaped crystals proceeded to 

 the exterior surfaces. Now in this phfenomenon it will be ob- 

 served that we have the same dark central line found in homo- 

 geneous glass by polarized light, on each side of which the cry- 

 stals are arranged at right angles. That this is the mode of 

 arrangement which the molecules assume in their homogeneous 

 state previous to the crystalline metamorphosis, we have no direct 

 reason to conclude ; neither have we any to reject the idea. Ij; 

 seems to me, however, the most probable hypothesis ; and I shall 

 therefore assume it. Again, when glass has been cooled rapidly, 

 as in the case of what are termed Prince Rupert drops and the 

 Bologna or philosophical phial, we have another class of pheeno- 

 mena closely connected with molecular structure. In these cases 

 a peculiar tendency is induced to subvert the force of cohesion 

 by which the molecules are held together, in consequence of 

 which the slightest force is sufficient instantly to cause each 

 portion to fly in pieces, or be converted into powder. The cause 

 of this has been investigated by Dumas ; but his conclusions it 

 were unnecessary to repeat. Keeping, however, all these phse- 

 nomena in view, we shall be enabled, I think, to elucidate those 

 observed by Prof. Erman. 



And first of all, reverting to his description, I would point out 

 the opposite character of the fractures in the two classes of ice 

 examined by him. In the case of that which formed the cover- 

 ing of the lake, the cracks proceeded from the outer, or rather 

 the upper and lower surfaces of the sheet, towards its centre; 

 while in the ice with which the great fracture was filled, the 

 princijial crack ran through its centre, the others proceeding 

 laterally from it towards the sides. From these phrenomcna the 

 following conclusions ma}'^, I think, be drawn : — 



1st. That the molecular structure in both cases was the same. 



2nd. That the molecular slate m each was entirely difl'ercnt. 



