PRESSURE OF TIDAL GLACIERS 213 



mercury; and, as glass is heavier than water, the glass 

 block was replaced by a block of cork, to the bottom of 

 which a thin plate of glass was cemented. When the 

 cork was pushed down through the water and its glass 

 face squeezed against the slab of glass at the bottom, it was 

 found to adhere, but not permanently. The pressure ap- 

 plied did not force all the water from between the glass 

 faces, and the remaining water film gradually thickened, 

 until, in a few seconds or a few minutes, the cork was 

 freed and rose to the surface. 



As regards conditions, the only essential difference be- 

 tween the two experiments was in the liquids employed, 

 and the properties of the liquids which determined the 

 diverse results were the relations of internal to external 

 molecular forces. Because the cohesive force of water is 

 less strong than its force of adhesion to glass, water ' wets ' 

 a glass surface; it is able to spread and interpose itself 

 as a film between two glass surfaces pressed tightly to- 

 gether; and when thus interposed it can not be forced 

 out by pressure (at least, by such pressure as is involved 

 in the problem of the tidal glacier). Because the cohe- 

 sion of mercury is stronger than its adhesion to glass, it 

 does not wet glass; and it does not tend to insinuate itself 

 between closely approximated glass plates, but tends rather 

 to withdraw from the interspace. 



These experiments indicate that the ability of the sea 

 to penetrate, and communicate its pressure, along the 

 contact surfaces of a tidal glacier and its bed, may depend 

 on the cohesive force of water, as compared with its ad- 

 hesive force in relation to ice and rock. It is a familiar 

 fact that water wets both ice and rock; and the problem 

 of the glacier is therefore better represented by the second 

 of Day's experiments than by the first. 



Another factor of the problem should now be consid- 

 ered, the temperature of the bottom ice and adjacent rock; 



