Page:Outlines of Physical Chemistry - 1899.djvu/145

 BECKMANN THERMOMETEB

��of the following table of corrections by Chrutzmacher and multiplying the increase in the boiling point or the depres- sion of the freezing point by the given factor :

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��Temperature

� �Factor

�- 35° to -30° ...

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�0°to 5° ...

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�45° to 50° ...

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�95° to 100° ...

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�145° to 150° ...

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�195° to 200° ...

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�245° to 250° ...

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��Theoretical Relations

A. The Osmotic Pressure of a solution and its Vapour Tension (Arrhe'nius).

Let us imagine a vessel of the form of a tube terminated at the lower end by a funnel which is closed by a semipermeable wall, as shown in fig. 33. Let this vessel be filled with a solution s and stand in another vessel containing pure solvent d. Let the whole be covered with a bell-jar and completely exhausted.

Equilibrium will be established when the osmotic pressure has forced the solution to a height h, and the free space in the bell- jar is saturated by the vapour d\

At the surface h the vapour given out by the solution has a tension /', and at the same height the vapour given out by the solvent has a tension (/ — hd), h being the height of the liquid column from the sur- face d, and d the density of the vapour in the bell-jar. We say then that equilibrium exists when

/ = (/-&<*)

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��Because, if /' were smaller than {f - hd), the following would occur. The vapour given out from d would condense at h> and would there be de- posited as liquid which would increase the weight of the osmotic column and would dilute the solution. The equilibrium being thus broken, a certain quantity of solvent would be forced through the

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