Page:Outlines of Physical Chemistry - 1899.djvu/144

 OUTLINES OF PHYSICAL CHEMISTRY

��The Beckmann Thermometer

In boiling point and cryoscopic determinations the thermometer used is generally of the pattern devised by Beckmann. This instrument oonsista of a very fine capillary tube terminating in two mercury reservoirs. The lower reservoir is comparatively large and corresponds to the bulb of an ordinary thermometer. The upper one ia constructed as shown in fig. 82, and contains an auxiliary quantity of mercury.

The scale extends only over five or six degrees, each divided into hundredths, but the mercury column can be so adjusted that it terminates at a convenient part of the scale at any desired temperature.

Suppose that a cryoscopie determination has to be made in acetic acid, that ia, about the temperature 16°C. To adjust the thermo- meter to this temperature, it ia placed in a bath and heated until the mercury column risea and joins the mercury in the upper reservoir. It is then placed in a second bath whose tem- perature, indicated by an ordinary thermometer, is about 18° or 19°. When this temperature Fio. 32. ' s reached, the upper reservoir is tapped with the hand and the mercury thread is broken at the point marked a in the figure. Further cooling to about 16° brings the end of the mercury column to a convenient position opposite the graduated scale.

The same thermometer can be used at various tempera- tures — 0°, 16°, 100°, &c. For very exact measurements it is best to consider the divisions on the scale as arbitrary and to ascertain the value of the degree at the time, by comparison with a reliable standard thermometer. We may also obtain sufficiently accurate results by making use

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