608 . JOHN JOHNSTON AND PAUL NIGGLI 
lowered by uniform pressure. This phenomenon cannot in general 
be produced by uniform pressure because uniform pressure raises 
the melting-point of most substances; so that we shall with advan- 
tage consider unequal pressure to be the effective factor in the 
case of water as in that of other substances. Consider a block of 
ice at o° supporting a loaded wire. The pressure exerted by the 
wire depresses the melting-point of the ice by an amount Ai; 
immediately beneath the wire, therefore, we have at any instant a 
thin layer of ice at —At¢ in equilibrium with water at —Aé. The 
water, however, escapes round the wire, and so comes into contact 
with ice at 0°; such a system is, however, unstable, for under these 
conditions water cannot remain subcooled. Consequently, the 
water freezes again and forms a solid block above the wire. 
The process by which a mass of loose snow is compacted into a 
block of ice is identical with this. The pressure, due to the super- 
incumbent material, lowers the melting-point at the surface of 
contact of adjacent grains; the water at —At? flows into the inte. 
stices where the pressure is smaller and freezes again. ‘This pro- 
cess continues until the interstices are all filled up, that is, until 
a solid block of ice is formed. This again, we believe, is the general 
mode in which consolidation of a mass of originally loose material 
takes place, as for instance in the welding of metals and in pyro- 
metamorphic processes. The consolidation of loose mineral ma- 
terial, however, takes place more usually through a process of 
solution, the general principle being identical in both cases; this 
we shall now consider. 
' Influence of unequal pressure upon the solubility of solid sub- 
stances.—Considerations in every respect analogous to the foregoing 
are applicable to systems of a solid in contact with water or other 
solvent; in such cases, p. 2ssure acting in excess on the solid increases 
its solubility, and thus renders the solutions supersaturated with 
respect to the unstressed solid. Le Chatelier? accounts in this 
way for the consolidation of natural beds of rock-salt, gypsum, 
calcium carbonate, and clay. To test this matter directly he 
* The amount of this increase of solubility can be computed from equations analo- 
gous to those applicable to melting-points. 
2Z. physik. Chem., 1X (1892), 335. 
