THE PHYSICS OF ICE. 407 



molecule to molecule, particle to particle, mass to mass, and a ball or 

 other fio^ure of solid ice is the result. If there be no moisture, there 

 can be no refreezing until moisture is produced or applied. Thus 

 pieces of ice below the freezing temperature will not adhere because 

 the surfaces* are dry. The same is true of dry, granular snow in very 

 cold weather — only by long-continued moulding and pressure in the 

 hands can it be compacted. But in this case some liquefaction has 

 been produced, and then the surfaces in contact freeze together. Snow, 

 in the upper Alps, often covers gorges in the glaciers, and if moist can 

 be trodden into bridges sufficiently comjoact to pass safely over. 



This property of ice and snow Prof. Tyndall calls regelation. It 

 was discovered by Faraday in 1850, who found that moist surfaces 

 of ice adhered if brought together. This occurs under water as well 

 as in the air ; at summer heat, and beneath water so hot as to be 

 painful to the hands. The phenomenon may be explained in this 

 way : If we hold in our hands two cubes of ice, their outer surfaces 

 are exposed to the atmosphere, and, if it be warm enough, some lique- 

 faction at the surfaces takes place, and they become moist. Now, if 

 the cubes be brought together, two of the outer surfaces become inner 

 ones, and the moisture, chilled to the temperature of the ice, freezes, 

 and the two cubes become one mass. It is because the molecules of 

 ice may be continually crowded into new positions that the mass may 

 be changed in form without its continuity being broken. A slab of 

 ice placed in a suitable position will bend by its own weight. In this 

 case the molecules throughout undergo gradually a change of position ; 

 but, if the stress be too raj^idly applied, fracture occurs. 



All the properties and phenomena of ice which we have considered, 

 and in a marked degree that of regelation, are shown in the growth 

 and movement of glaciers. In these we see the development of ice on 

 its grandest scale. Equally in structure and form in molecular and 

 molar motion they are an expression of energies that are irresistible 

 and sublime. " To produce from aqueous vapor," observes Prof. Tyn- 

 dall, "the little mass of snow which a child can carry, demands an 

 exertion of energy competent to gather up the blocks of the largest 

 stone avalanche I have ever seen, and pitch them to twice the height 

 from which they fell." Who, then, shall estimate the potential energy 

 of the great ice-rivers of the Alps, or of the glaciers of the Arctic 

 Zone ? 



The motion of the Mer de Glace, and of other glaciers, is so slow as 

 to be ascertained only by persistent observation, or by careful measure- 

 ment. In 1827 a hut was erected by Huji on the glacier of the Unter- 

 aar for purposes of observation, but the hut was found to move down 

 the valley. In fourteen years it was nearly a mile below its first po- 

 sition. In 1820 three mountain-guides were plunged by an avalanche 

 into a gorge of a glacier on the side of Mont Blanc. After a burial of 

 forty years in the ice, they were found several miles below the spot 



