592 Mr. Mallet on Glacier Ice. 



am strongly impressed have a crystalline origin) by experi- 

 ments made on a large scale with such masses of crystallized 

 salts as I have above alluded to, subjected for some time to 

 alternations of temperature, pressure, &c. The analogy would 

 be closer if salts belonging to the same crystalline system as 

 water were chosen for experiment. In a paper of mine lately 

 published by the Geological Society of Dublin, I have drawn 

 attention to a fact of crystallization which seems to bear di- 

 rectly on this point, viz. that in all crystallized or crystalli- 

 zable bodies, if they are suddenly cooled, or chilled as it is 

 technically called, from a state of fusion or solution, by a plane 

 surface of low temperature, the crystals in forming arrange 

 themselves perpendicularly to the refrigerating plane: thus, 

 if speculum metal, or cast iron, or indeed any crystallizing 

 metal be cast in a thick metallic mould, so as suddenly to chill 

 them, their crystals are all found or fractured perpendicular 

 to the faces of the mould. 



The same has been remarked of the crystals of trap rock in 

 dykes, and generally of vein stones. It is also true of highly 

 concentrated solutions of salts, which on crystallizing form one 

 mass, like water in freezing into ice ; and conversely, if a cry- 

 stal lizable body be heated near to but not up to its fusing- 

 point, by the application of heat in one plane, a crystalline 

 structure perpendicular to the plane is immediately developed. 

 Thus, if a cube of lead be laid on or held against a hot plate 

 of any substance until it is heated to within a few degrees of 

 fusion, it becomes brittle (not plastic, observe), and on break- 

 ing it is found to have a crystalline structure, the direction of 

 the crystals being perpendicular to the heating surface; so 

 that in general change of temperature beyond certain limits de- 

 velopes in crystallizable bodies a crystalline structure in the di- 

 rection of transmission of the "wave of heat, whether ijito or out 

 of the mass of the body. 



This it seems to me will probably be ultimately found to be 

 concerned in the formation of the coloured bands (viz. bands 

 of variable crystallization) in glacier ice ; and if this be true, 

 it renders the plasticity of the mass the less likely, in propor- 

 tion as its perfect crystallization becomes more certain. 



The perfect brittleness and looseness of cohesion of crystal- 

 line bodies when heated nearly to their melting points, as above 

 illustrated in the case of metals, and as observed in thawing 

 icebergs, suggests also one circumstance of rapid degradation, 

 and of motion of translation in glaciers, which connects itself 

 with Mr. Hopkins's views, but has not, that I am aware of, 

 been noticed by him; namely, that the mass of glacier ice at 

 the bottom, either in contact with the glacier valley, with the 



