III.
��RELATIVE OSMOTIC PRESSURES.
��be allowed to freeze out of a sugar solution, it is found that only ice (i.e. water) separates, and the sugar remains entirely dissolved.
If a vessel, A, containing water, and another, B, containing an aqueous solution, be placed under a glass globe (Fig. 10), water will pass from A to B, the air acting as semi-j^ermeable wall.
If in a vessel, AX (Fig. 11), one half. A, be filled with
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��A B
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A B
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��water and the other half, B^ with a sugar solution, and if these be separated by a sheet of ice, water can pass from ^ to £ by the thawing of the ice on the side next to the sugar solution and the freezing of the same quantity of water on the other side.
Physiological Measurement of the Relative Osmotic Pressures in Difftrent Solutions. — Physiological experi- ments have been made with isotonic solutions, and these will be discussed in this section. Donders and Hamburger (6') found that two solutions which were isotonic at 0° were isotonic also at 34^ This corresponds with the fact that the pressure varies with the temperature in the same way for all gases (at constant volume), so that they mil have nearly the same pressure at any temperature whatever, if at one particular temperature their pressures are equal.
De Vries (i) showed by means of plant cells that equimolecular solutions of non-electrolytes — that is, solutions containing the same number of molecules in the litre — are isotonic, as exhibited in the following table. For salts this same relationship does not hold good. The table gives the so-called isotonic coefScients, that of potassium nitrate being taken as 3. 1*78 for glycerol indicates, thei^efore, that
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