When two liquids are separated by a porous membrane, each may pass through the membrane into the other with more or less freedom. This process is called osmose.

Demonstration. - Tie a piece of parchment paper over the end of a thistle tube and fill the tube part way up the stem with a strong solution of copper sulphate. Thrust it into a beaker containing water, and fix it in such a position that the liquids stand at the same height, both inside and outside the tube. Set the beaker aside for some time. It will soon be seen that the height of the liquid within the tube is increasing, and if the experiment is carried on for some time, the water in the beaker outside the tube will become colored with the copper sulphate solution. This indicates that molecules have passed through the parchment in both directions, but more rapidly from the pure water than from the solution.

 If in the preceding demonstration the upper end of the thistle tube is closed, there will be produced an osmotic pressure on the inside of the tube. This occurs because water molecules pass through the membrane more readily than do copper sulphate molecules. Since copper sulphate molecules occupy part of the space inside, more water molecules, which are in a state of constant vibration, strike the membrane from the outside than from the inside, and for this reason more of them pass into the tube than out of it.

High osmotic pressures have been produced by the use of a semipermeable membrane made by depositing a thin surface of copper ferrocyanide within the walls of a porous cup. The simplest method of doing this is to place the cup in a solution of copper sulphate and then fill it with a solution of potassium ferrocyanide. The two substances enter the cup from opposite sides and on coming in contact form a semipermeable membrane. This acts as a molecule sieve, allowing the water molecules to pass, but not those of a solution. In one experiment of this kind a solution of sugar was used, and a pressure of 31 atmospheres (about 465 lb. per square inch) was obtained in one hour and forty-five minutes. Before another reading could be taken, the pressure became so great as to shatter the apparatus.

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