362 James Croll — Mechanics of Glaciers. 



years ago.^ There is, however, another element which must he taken into account. 

 It is one which will help to cast additional light on some obscure points connected 

 with glacial phenomena. 



" Ice is evidently not absolutely solid throughout. It is composed of crystalline 

 particles, which, though in contact with one another, are, however, not packed 

 together so as to occupy the least possible space, and, even though they were, the par- 

 ticles would not fit so closely together as to exclude interstices. The crystalline 

 particles are, however, united to one another at special points determined by their 

 polarity, and on this account they require more space ; and this in all probability is 

 the reason, as Professor Tyndall remarks, why ice, volume for volume, is less dense 

 than water. 



" ' They (the molecules), like the magnets,' says Professor Tyndall, 'are acted upon 

 by two distinct forces ; for a time, while the liquid is being cooled, they approach 

 each other, in obedience to their general attraction for each other. But at a certain 

 point new forces, some attractive, some repulsive, emanating from special points of 

 the molecules, come into play. The attracted points close up, the repelled points 

 retreat. Thus the molecules turn and rearrange themselves, demanding as they do 

 so more space, and overcoming all ordinary resistance by the energy of their demand. 

 This, in general terms, is an explanation of the expansion of water in solidifying. '- 



" It will be obvious, then, that when a crystalline molecule melts, it will not merely 

 descend in the manner already described, but capillary attraction will cause it to flow 

 into the interstices between the adjoining molecules. The moment that it parts with 

 the heat received, it will of course resolidify, as has been shown, but it wiU not 

 solidify so as to fit the cavity which it occupied when in the fluid state. For the 

 liquid molecule in solidifying assumes the crystalline form, and of course there will 

 be a definite proportion between the length, breadth, and thickness of the crystal ; 

 consequently it will always happen that the interstice in which it solidifies will be 

 too narrow to contain it. The result will be that the fluid molecule in passing into 

 the crystalline form will press the two adjoining molecules aside in order to make 

 sufficient room for itself between them, and this it will do, no matter what amount 

 of space it may possess in all other directions. The crystal will not form to suit the 

 cavity, the cavity must be made to contain the crystal. And what holds true of one 

 molecule, holds true of every molecule which melts and resolidifies. This process is 

 therefore going on incessantly in every part of the glacier, and in proportion to the 

 amount of heat which the glacier is receiving. This internal molecular pressure, 

 resulting from the solidifying of the fluid molecules in the interstices of the ice, acts 

 on the mass of the ice as an expansive force, tending to cause the glacier to widen out 

 laterally in all directions. 



" Conceive amass of ice lying on a flat horizontal surface, and receiving heat on its 

 upper surface, say from the sun ; as the heat passes downwards through the mass, 

 the molecules, acting as conductors, melt and resolidify. Each fluid molecule solidi- 

 fies in an interstice, which has to be widened in order to contain it. The pressure 

 thus exerted by the continual resolidifying of the molecules will cause the mass to 

 widen out laterally, and of course as the mass widens out it will grow thinner and 

 thinner if it does not receive fresh acquisition on its surface. In the case of a glacier 

 lying in a valley, motion, however, will only take place in one direction. The sides 

 of the valley prevent the glacier from widening ; and as gravitation opposes the 

 motion of the ice up, and favours its motion down the valley, the path of least 

 resistance to molecular pressure will always be down the slope, and consequently in 

 this direction molecular displacement will take place. Molecular pressure will there- 

 fore produce motion in the same direction as that of gravity. In other words, it will 

 tend to cause the glacier to descend the valley. 



" The lateral expansion of the ice from internal molecular pressure explains in a 

 clear and satisfactory manner how rock-basins may be excavated by means of land- 

 ice. It also removes the difficulties which have been felt m accounting for the ascent 

 of ice up a steep slope. The main difficulty besetting the theory of the excavation 

 of rock-basins by ice is to explain how the ice after entering the basin manages to 

 get out again — how the ice at the bottom is made to ascend the sloping sides of 

 the basin. Pressure acting from behind, it has been argued by some, if the basin 

 be deep and its sides steep, will simply cause the ice lying above the level of the 

 basin to move forward over the surface of the mass filling it. This conclusion is, 



> Phil. Mag., March, 1869; September, 1870. « "Forms of Water," p. 127. 



