Page:Text-book of Electrochemistry.djvu/46

 on to the cylinder, and the whole apparatus was placed in a water-bath kept at a constant temperature. The water forced itself into the sugar solution, and the pressure in the cell rose to a maximum value, at which evidently water neither diffused into nor out of the cell. The equilibrium was established more quickly when mercury was poured into the open end of the manometer. If the pressure was in- creased beyond this maximum value, which is the osmotic pressure of the sugar solution in question, water was forced out of the cell into the outer bath.

Pfeffer first investigated the behaviour of solutions of cane sugar of different concentrations, and found the following values : —

Osmotic pressm-e. 535 1016 1513 2082 3075 mm. Hg. Pea'eiitage ot sugar

The numbers in the last line are very nearly equal, and the differences are easily attributable to the errors of experi- ment, which are fairly appreciable. The osmotic pressure is, therefore, proportional to the quantity of substance or the number of molecules in unit volume. This corresponds exactly with gas pressure, which, according to the law of Boyle, is inversely proportional to the volume taken up by the gas — that is, directly proportional to the concentration of the gas.

Pfeffer also carried out experiments with other solutions of such substances as gum, dextrin, potassium sulphate, potassium nitrate, etc. With potassium nitrate he obtained the following results : —

Osmotic pressure 1304 2185 43G8 mm. Ilg.

«noticj«re8m.re ^^3^ ^^3^ ^^

Percentage of K^Oj

In this case the osmotic pressure is not exactly pro- portional to the quantity of salt, but increases more slowly than the concentration. The cause of this deviation lies chieHy in the fact, which Pfeffer proved, that the membrane,

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