Page:Outlines of Physical Chemistry - 1899.djvu/133

 sulphate solution containing the cell filled with f erroc yanide solution, is placed under the exhausted receiver of an air- pump for two or three days. The air is thus drawn from the pores of the cell, and the semipermeable wall is formed. The semipermeable walls prepared in this way become sufficiently dense to withstand a pressure of from four to four and a half atmospheres (25 to 30 per cent, of the apparatus thus made are good). By long and continued washing the cell is then thoroughly cleaned, and the mano- meter b is sealed on.

The following osmotic experiments can be made with this apparatus.

After the apparatus has been set up, the cell is filled with a 1 per cent, solution of sugar, then the tube a is drawn out and sealed before the blowpipe (an operation which requires a little practice, but after all is not difficult). The apparatus is then immersed in distilled water. The water will penetrate into the cell, and there produce an increased pressure which will be indicated by the manometer. In the course of a few days (or weeks) the maximum pressure will be reached and remain constant for a considerable time. This pressure is subject to the following laws :

1. It is proportional to the concentration of the solution employed. This corresponds to Boyle 1 s law. At a tem- perature of 15°

a 1 per cent, solution of sugar gives a pressure of 58*5 cm. of mercury.

a 2 per cent, solution of sugar gives a pressure of 101'6 cm. of mercury.

a 4 per cent, solution of sugar gives a pressure of 208*2 cm. of mercury.

2. The pressure increases with the temperature accord- ing to the equation,

Pt = PoO- + at),

where " a = -— = 000867. This corresponds to Gay- Lussac's law.

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