CHANGE OF STATE SOLID LIQUID. 201 



freezing sets in, and the temperature immediately leaps up to 0, and 

 remains there, during the rest of the process of solidification, the latent 

 heat yielded by the ice first formed being sufficient to raise the tempera- 

 ture of tho mixture to 0. This frequently occurs in jugs of water left 

 standing quietly in cold weather. There is no ice in the water while it 

 is quiet, but the moment the jug is lifted up and its contents poured out, 

 a number of minute crystals of ice form in the water, showing that the 

 latter must have been below 0, for the latent heat of the water given 

 up on freezing can only be accounted for by supposing that it has raised 

 the temperature of the water. If the water be covered with a layer of 

 oil, it is still easier to produce this phenomenon, and without any trouble 

 the temperature may be lowered to - 6 or - 7 without freezing. In 

 this state the water is said to be superfused. If a crystal of ice be 

 dropped into the liquid, freezing at once commences on this as a nucleus, 

 and the superf usion ceases. Or a sudden shock will often put an end to it. 



There is a sudden change of volume on the change from water to ice. 

 As we have already seen, water is most dense at + 4, and it expands 

 slowly as the temperature falls. If the water be kept superfused, the 

 expansion is continuous. But if the water changes to ice at 0, there is 

 a sudden increase of volume, 1 c.c. of water becoming 1-09 c.c. of ice. 

 The ice contracts on cooling below 



Similar phenomena are noticed in the melting and freezing of other 

 substances. For instance, phosphorus melts definitely at 44 '2 ; if kept 

 under water it may be cooled in the superfused condition far below this ; 

 but a sudden shock, or the introduction of a glass rod previously rubbed 

 with phosphorus, is followed by solidification. With sulphur, melting 

 at 115, the superfusion is still more marked. These substances also 

 have definite latent heats on melting, and show a sudden increase of 

 volume -1 c.c. of solid phosphorus melting to 1'034 of liquid and 1 c.c. 

 of solid sulphur melting to 1 - 05 of liquid. 



Regulation. Faraday first directed attention to a remarkable 

 phenomenon, now known as " Regelation." If a number of blocks of ice 

 at be pressed together they adhere at the points of contact. This 

 effect, as remarked by Faraday, is very familiar in another form. Snow 

 near the melting-point will easily bind into snowballs ; but if below 

 it is powdery, and will not form a cohesive mass. Similarly the blocks 

 of ice will not adhere if below 0. 



Tyndall showed that ice at could be moulded by great pressure, 

 the ice breaking up under the pressure, and then freezing together at 

 the points of contact. By continued fracture and regelation, the 

 crevices are filled up, and the ice forms a transparent continuous mass 

 which has the shape of the mould. 



The motion of glaciers has been ascribed to regelation. It has been 

 observed that the whole mass of a glacier flows like a stream of water 

 down the valley in which it lies, but with exceedingly small velocity, the 

 centre of the glacier flowing more rapidly than the edges. Forbes found, 

 for example, that the Aletsch glacier flowed in " mid-stream " at one point 

 about 1 4 inches in 24 hours, while near the edge it flowed only 3 inches in 

 the same time. In winter, the speed is considerably less than in summer. 

 Now the ice of a glacier is pretty nearly at through the penetration 

 into all the crevices of the water melted at its surface. The enormous 



