Page:Outlines of Physical Chemistry - 1899.djvu/136

 116 OUTLINES OF PHYSICAL CHEMISTRY

substances, in quantities proportional to their respective molecular weights, are dissolved, the solutions obtained have the same vapour tension.

These facts, ascertained ex peri men tally by Babo (1848), Wullner (1856), and Baoult (1887), might directly lead to theoretical and practical deductions of the greatest impor- tance. But the determination of a vapour tension is a very delicate operation, and it is preferable to simplify the method of working and start out with three other con-

��1. The boiling point of a solution is higher than that of the pure advent ;

2. The rise in the boiling point is proportional to the concentration of the solution ;

8. The rise in the boiling point 1 / is the same for equimoleonlar solutions

,-^N. in tn « s ftm « solvent.

(f \\ The experimental part of the study

is now completely changed. We do not determine vapour tensions, but boiling points; and this we can do by comparatively easy and simple opera- tions.

Beckmann's Method l

The apparatus consists of a small

flask with three necks and havinga short

platinum wire sealed into the bottom.

The flask rests on an asbestos plate,

and is protected from loss of heat by

radiation by being bound round with

asbestos. One of the side-tubes is used

Fio. 28 f° r introducing the solvent and the

substance, through the other a Beck-

mann thermometer passes, and the central one is connected

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