CLOUDS AND RIVERS, ICE AND GLACIERS. 17.1 



resisted the expansion, the water in consequence re- 

 maining liquid at a temperature of more than 30° Fahr. 

 below the ordinary freezing point. A bullet within the 

 cylinder rattled about at this temperature, showing 

 that the water was still liquid. On opening the tap the 

 liquid, relieved of the pressure, was instantly converted 

 into ice. 



433. It is only substances which expand on solidify- 

 ing that behave in this manner. The metal bismuth, as 

 we know, is an example similar to water; while lead, 

 wax, or sulphur, all of which contract on solidifying, 

 have their point of fusion heightened by pressure. 



434. And now you are prepared to understand Pro- 

 fessor James Thomson's theory of regelation. When 

 two pieces of ice are pressed together liquefaction, he 

 contends, results. The w^ater spreads out around the 

 points of pressure, and when released re-freezes, thus 

 forming a kind of cement between the pieces of ice. 



§ 63. Faraday's View of Regelation. 



435. Faraday's view of regelation is not so easily 

 expressed, still I will try to give you some notion of it, 

 dealing in the first place with admitted facts. Water, 

 even in open vessels, may be lowered many degrees below 

 its freezing temperature, and still remain liquid ; it may 

 also be raised to a temperature far higher than its boiling 

 point, and still resist boiling. This is due to the mutual 

 cohesion of the water particles, which resists the change 



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