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THE POPULAR SCIENCE MONTHLY, 



becomes solid. But, ice may be chilled to any attainable degree of 

 cold. Prof. Tyndall reduced its temperature 100°, during which pro- 

 cess it shrank in volume and became intensely hard. From the blow 

 of a hammer it broke with a vitreous ring. 



We often witness the fact that water will not freeze if in rapid 

 motion, although it be much colder than the freezing-point ; but in 

 this case will freeze at the bottom where its motion is retarded, form- 

 ing what is called " ground, or anchor ice." The sandy bottom be- 

 neath swiftly-flowing streams is sometimes frozen solid by radiation 

 of its heat to the cold water flowing over it. But no ice will form at 

 the bottom of a pond or lake if the water be at rest ; it then forms 

 upon the surface only. The particles of water, as they become chilled 

 to near the freezing-point, expand, become lighter, and continually 

 rise to the surface, where they solidify, forming a roof of ice. This 

 phenomenon opens a most interesting chapter of physical science, and 

 we will presently recur to it. 



The freezing-point of water may be changed by pressure, that is, 

 water under pressure will not solidify at a temperature of 32° ; nor 

 is it known how great a degree of cold it can resist if a corresponding 

 degree of pressure be brought to bear upon it. The lowering of the 

 freezing-point of water by pressure is one-seventieth of a degree Fahr. 

 for a whole atmosphere. Under a pressure of several thousand atmos- 

 pheres, ice has been liquefied at or near the temperature of zero ; so 



Fio. 1. 



Liquid Planes in Ice from Pressure. 



that the freezing-point was zero, instead of 32°. The tendency of 

 pressure is, therefore, to keep water liquid, and to render it so after 

 being frozen. The efiect of pressure on a cube of transparent ice is 

 well shown in Fig. 1. It is no longer transparent, but is traversed by 

 hazy lines which come into view as the strain is applied. These hazy 



