104 Mr. J. Drummond on the Molecular Structure of Ice, 



3rd. That the structure in both cases was closely related to, 

 if not identical with that of ordinary sheet glass. And 



4th. That the ice formed in the fracture possessed a disrup- 

 tive structure, analogous in some respects to that of glass when 

 cooled rapidly. 



It is only, of course, from the direction of the cracks that we 

 can trace by the eye the nature of the structure. But conclu- 

 sions drawn from these would be of little value, could we not at 

 the same time trace them to the action of molecular forces. 

 Thus considered, however, the value of the cracks as evidence of 

 structure will be at once apparent. In the ice with which the 

 great fracture was filled, the principal crack passed through the 

 centre, others proceeding from it laterally. Now the line of this 

 crack is exactly that of the molecular axis of sheet glass, the 

 principal characteristic of its structure to which I have drawn 

 attention. Again, in the ease of the great Lake sheet, Prof. 

 Erman states that at a depth of about a third of the entire thick- 

 ness, the cracks proceeding from the upper and lower surfaces 

 met, and that this depth was the same for all. Now if this 

 point be taken as that of the position of the central molecular 

 axis, we have here also the same structural characteristic. True, 

 we have not here any direct evidence of its existence; but the 

 considerations to follow will render it more conclusive. 



The cracks I suppose to be the result of molecular forces act- 

 ing in each case in an entirely opposite manner. We have still, 

 therefore, to trace the causes which produced this opposite mole- 

 cular action ; and these, I think, are to be found in the opposite 

 conditions under which the two descriptions of ice were formed. 

 First of all we may take it for granted that the great Lake sheet 

 was solidified under ordinary circumstances, that is to say, when 

 the temperature could not be much lower than the freezing- 

 point. Subsequently, when the temperature was considerably 

 reduced, the great fracture was formed. Further, when this rent 

 was newly made, the water must have rushed up, filled it, and 

 become instantly solid. But when solidification was thus going 

 on at a temperature within the mass of 32°, the walls of the 

 fracture to which it was immediately exposed were at a tempera- 

 ture lower by many degrees than the freezing-point. The sheet 

 therefore was solidified under circumstances analogous to the 

 process of rapid cooling in the case of glass, and by which a dis- 

 ruptive structure is produced. In this condition, therefore, we 

 have, it seems to me, the cause of the difference between the 

 molecular states of the two classes of ice, — a diff'erence in ac- 

 cordance with which they appear to have strictly acted. For 

 when the temperature subsequently became further reduced, 

 those of the Lake sheet shrank towards its molecular axis, thereby 



