28 LAWS OF AVOGADRO AND VAN'T HOFF. chap.
water and dissolved substance to pass tliroiigh its pores. Inside this cell-wall there is the real cell-content, the protoplasm. If the cell-content parts with water to the surrounding medium, it contracts and separates from the cell- wall (plasmolysis), at first at the comers as repre- sented in Fiff. 5b. If much
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�����water is lost by the cell, the protoplasm aggregates to a mass which remains con- nected with the cell-wall only by a few fine threads (Fig. 5c). The cell-content can easily be distinguished from the cell-wall by staining (methyl-violet, etc.).
Semi-permeable Membranes.— Living cells have the peculiar property of allowing water but not dissolved material to pass to or from the protoplasm.
After death or by the action of some poisons, the cell loses this property. An artificial cell possessed of this property is naturally of great value. The physiologist Traube (2) succeeded in preparing such a cell by precipitating a thin colloidal film of copper ferrocyanide within the walls of a porous cylinder. With so-called semi-permeable membranes of this nature, Pfeffer (J) carried out a series of striking experiments.
Osmotic Pressure. — Pfeffer filled a porous cylinder, A
(Fig. 6), with copper sulphate solu- tion, and immersed it in a solution of potassium ferrocyanide. Some- where about the middle of the cell- wall the two solutions met, and there a fine membrane of copper ferrocyanide was formed, which gradually grew stronger. The cell A, whose wall only served as a mechanical support, was washed out, and filled quite full with a solution of cane sugar. A cover, Z, fitted with a manometer, M^ was luted
���Fio. G.
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