Page:Text-book of Electrochemistry.djvu/50

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the copper sulphate solution, the liquid rises in the tube r against the external pressure (Fig. 8b). After some time, however, the level of the liquid in the tube sinks to that of the liquid outside, because the membrane R does not completely prevent the diflfusion of the copper sulphate.

The Abb6 de Nollet in 1750 had performed the same experiment, using alcohol instead of copper sulphate.

The so-called "cliemical garden" is an osmotic pheno- menon from which much may be learned. If a crystal of ferric chloride be thrown into a dilute solution of potassium ferrocyanide, it sinks and becomes enveloped in a film of Prussian blue, winch is permeable by water but not by ferric chloride or potassium ferrocyanide. Consequently water forces its way into this semi-permeable cell of Prussian blue and expands it. Further quantities of the ferric chloride will be dissolved by the water which has entered, so that the osmotic pressure is always kept liigh. If the inflowing water bursts the membrane, a new precipitation takes place at the same spot, and so the cell at once closes. The small air-bubbles originally attached to the ferric chloride crystal exert an upward pull on the cell, and a more or less tree-like formation is noticed; at the higher extremities the small air-bubbles are frequently visible (Fig. 9).

Nature of Osmotic Pressure. — Eamsay's application of palladium as a semi-permeable membrane teaches us much. If we imagine the hydrogen replaced by water, the nitrogen by sugar, and the sheet of palladium by a film of copper ferrocyanide, then we have Pfeffer's experiment.

The water forces its way into B (Fig. 7), dissolves the sugar there, and fills B with the solution until the manometer indicates an excess of pressure, which corresponds with the osmotic pressure of the sugar. In Kamsay*s experiment the pressure of the hydrogen was the same on both sides of

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