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appreciable. Van der Waals, correcting the pressure and volume for these circumstances, arrived at the formula —
��(^p + l^yv - I) = RT.
��containing two new constants, a and 6, which are functions of the "internal pressure" and of the molecular volume. Experimental results show a high degree of agreement with this formula. •
Isotonic Solutions. — Certain parts of plants, when in a more or less dehydrated condition, are able when placed in water to absorb some of it without losing any of the cell- content — a fact which has long been known to physiologists. Further experiments with salt solutions and plant cells showed that at a certain concentration of the solution an equilibrium is established between it and the cell-contents. If the solution is too dilute, water passes into the cell ; if too concentrated, water passes out from the cell. Those salt solutions which are in equilibrium with the cell-sap are said to be isotonic or isosmotic with it.
By using the same or quite similar cells — cells are used which lie close together in a homogeneously developed part of a plant — ^these could be compared with solutions of different substances, and the concentrations of the various dissolved substances in solutions which are isotonic with the cells could be determined. These solutions are, of course, isotonic with respect to each other.
De Vries {1) found, in a series of experiments with Iradescantia discolor and Begonia manicata, that solutions which contained in a litre equivalent quantities of potassium nitrate, sodium nitrate, and potassium chloride were isotonic with each other. But a solution containing 1 gram-molecxde of potassium chloride had the same effect as a solution con- taining 1*7 gram-molecules of cane Sugar or glycerol.
The cell preparations in the salt solution to be investi- gated are examined under the microscope. Each cell (Fig. 5 a) is surrounded by a solid cell-membrane, which allows both
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