SUPERFUSION AND REGELATION. 



35 



exactly the same point as that where it stands when immersed in 



melting snow, i.e., exactly at zero if the thermometer be accurately 



graduated. If, however, sea-water be used, or if some common 



salt, Epsom salts, sugar, or other solid 



soluble matter be dissolved in the 



water, it will be found that ice will 



not be formed until the temperature 



falls perceptibly below zero. As 



shown in Expt. 27, the ice thus pro- 



duced contains but little of the 



dissolved salt, &c., as compared with 



the water from which it froze. 



If water be allowed to stand per- 

 fectly at rest in an atmosphere a good 

 deal below freezing-point, it sometimes 

 happens that the whole can be cooled 

 down several degrees below zero 

 before any ice forms ; but the least 

 degree of agitation, such as the shak- 

 ing of a passing cart or train, entirely 

 prevents this. If water cooled down 

 in quietude below its normal freezing- 

 point be stirred, some of it at once 

 turns to ice and the temperature of 

 the remainder at once rises to zero. 

 This property of liquids being capable 

 of cooling below their melting-points 

 without solidifying under certain cir- Fi g- 19 "Verification of Boiling- 

 cumstances is called supervision, Point, 



and is closely connected with the analogous peculiarity termed 

 super saturation exhibited by solutions under similar conditions 

 (Chapter V.). 



Water subjected to great pressure requires more chilling to 

 freeze it than when not compressed; conversely ice strongly 

 pressed begins to melt on the surface, but the water thus formed 

 instantly becomes ice again on relieving the pressure. In con- 

 sequence a quantity of snow or of crushed ice fragments can be 

 cemented together into a solid block by simple compression and 

 relaxation of the pressure. This property is termed regelation, 

 and is one of the principal causes forming glaciers, which are loose 

 white opaque snow at the top and clear solid transparent ice at the 

 bottom : it is difficult to illustrate it without a powerful press, but 

 to some extent the action may be noticed when a handful of snow 

 is strongly squeezed in the hands to form a snowball ; the separate 



