��OUTLINES OF PHYSICAL CHEMISTRY
��distilled water, gently wiped with bibulous paper, then filled with a 8 per cent, solution of potassium ferrocyanide*. At the same time the cell is immersed in a 3 per cent, solution of copper sulphate. Where the solutions meet inside the walls a deposit of copper ferrocyanide is formed. According to Pfeffer (from whom these details are taken), it is essential that the precipitate be deposited on the inner side of the cell wall and a little inside the wall. And further, the deposit should be thin, adherent, and absolutely continuous. It is not easy to satisfy all these conditions.
Other substances, besides copper ferrocyanide, may be used, and in fact, ferric hydrate, silicic acid, calcium phosphate, gelatine tannate, &c, have been used success- fully.
The cell thus prepared will have become less permeable to water. Water can now pass through only slowly, and only when assisted by a certain pressure. But if, instead of water, we put a solution of an organic or inorganic substance into the cell, a still higher pressure is required
to cause drops to filter through, and what passes through is not the solution, but pure water.
To complete the apparatus and make an osmotic cell, it is only necessary to connect the prepared cell by means of tight joints with a manometer on which the pressure is registered (Pfeffer, 'Osmotische Unter- suchungen,' Leipsic, 1877).
A die has somewhat modified the method.
His apparatus is shown in fig. 27. The
upper part is of glass, and is fixed on to a
porous cell about 3 inches high and 1 inch
internal diameter by means of sealing wax.
Adie produces the precipitate in the wall by dipping
the cell (clean and dry) into a solution of copper sulphate,
and at the same time filling up with a solution of potassium
ferrocyanide. The whole, that is, the bath of copper
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