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; 
