30 LAWS OF AVOGADRO AND VAN'T HOFF. chap.
particularly at high pressures, is not quite impermeable for the salt.
Pfeflfer further proved that the osmotic pressure increases slowly with rise of temperature as the following table shows : —
��Tempentare.
�Osmotic pressure in cm. Hg.
�Observed.
�Calculated.
�6-8° 13-6° 14-2^ 22-0°
��The numbers in the last column have been calculated on the assumption that the osmotic pressure, just as the gas pressure according to the law of Gay-Lussac, increases pro- portionally to the absolute temperature, and it will be shown later that this must be the case. Pfeffer's numbers do not justify this conclusion, but they at least show that the direction of the influence of temperature is in agreement with the assumption.
Lastly, we may try to find if Avogadix)*s law also obtains for the osmotic pressure-^that is, whether for dissolved sub- stances the constant R in the equation ^v = RT has the same value as for gases.
At the absolute temperature 279*8° cane sugar in a 1 per cent, solution has a pressure of 505 mm. Hg. As the molecular weight of sugar is 342, if 1 gram is contained in 100 c.c. of solution, 1 gram-molecule is contained in 34,200 C.C.
From the equation — R = 61,720,
instead of the value 62,265 found for gases (see p. 26).
This calculation was first made by van't Hoff (4), who
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