The crust of the earth, so far as it is permeable and above the sea-level, receives from rainfall its supply of fresh water.
On the upheaval of such rocks above the sea-level, fresh water from rainfall began to flow over their exposed surfaces, and, so far as the strata were permeable, to lie in their interstices upon the salt water.
Jacquerod and Perrot have found that quartz-glass is freely permeable to helium below a red-heat (Comet.
The conception of a semi-permeable membrane, permeable to the solvent only, was used by van't Hoff as a means of applying the principles of thermodynamics to the theory of solution.
The conceptions of osmotic pressure and ideal semi-permeable membranes enable us to deduce other thermodynamic relations between the different properties of solutions.