CLOUDS AND RIVERS, ICE AND GLACIERS. 16U 



liquefaction of ice by pressure in the following way : 

 — You remember the beautiful flowers obtained when a 

 sunbeam is sent through lake ice(§ 11), and you have 

 not forgotten that the flowers always form parallel to 

 the surface of freezing. Let us cut a prism, or small 

 column of ice with the planes of freezing running across 

 it at right angles ; we place that prism between two 

 slabs of wood, a.nd bring carefully to bear upon it the 

 squeezing force of a small hydraulic press. 



430. It is well to converge by means of a concave mirror 

 a good light upon the ice, and to view it through a 

 magnifying lens. You already see the result. Hazy 

 surfaces are formed in the very body of the ice, which 

 gradually expand as the pressure is slowly augmented. 

 Here and there you notice something resembling crys- 

 tallisation ; fern-shaped figures run with considerable 

 rapidity through the ice, and when you look carefully at 

 their points and edges you find them in visible motion. 

 These hazy surfaces are spaces of liquefaction, and the 

 motion you see is that of the ice falling to water under 

 the pressure. That water is colder than the ice was 

 before the pressure was applied, and if the pressure be 

 relieved, not only does the liquefaction cease, but the 

 water re-freezes. The cold produced by its liquefaction 

 ander pressure is sufficient to re-congeal it when the 

 pressure is removed. 



431. If instead of diffusing the pressure over sui- 

 E&ees of considerable extent, we concentrate it on a 



