Still heated by steam

Steam as Source of Heat.—On the large scale, the still is frequently heated by steam under ordinary or increased pressure (Fig. 7). The steam is introduced through the pipe A, and the condensed water is run off at b. 

The Thermometer.—In carrying out a fractional distillation one must be able, not only to rea a constant or nearly constant temperature with great accuracy, but also to take readings of rapidly rising temperatures. These requirements are best fulfilled by the ordinary mercurial thermometer, which is therefore, notwithstanding its m ny drawbacks, used in preference to the air or the platinum resistance thermometer. If accurate results are to be obtained the following points must be attended to. 

Calibration.—The thermometer must be carefully calibrated, and it would be a great advantage if all thermometers were compared with an air thermometer, for two mercurial thermometers, constructed of different varieties of glass, even if correct at 0° and 100 , will give different and incorrect readings at other temperatures, more especially at high ones, for various reasons  

In this way a table, or curve, of con ections may be constructed, and the error at any scale reading of the thermometer may be easily ascertained. 

Redetemination of Zero Point.—The zero point of a thermometer should be redetermined from time to time, as it is subject to changes which, in the case of the cheap soda glass thermometers, may be considerable. These changes are of two kinds  

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