58 OUTLINES OF PHYSICAL CHEMISTRY
1. The mercury column of the thermometer must be entirely in the vapour,
2. The barometric pressure must be normal.
3. The operation must be conducted at a moderate rate. It is easy to fulfil the first of these conditions, especially
if a set of short-stem thermometers marking only a limited number of degrees (0° to 60°, 50° to 100°, 100° to 150°, Ac.) be used. But when an ordinary 0° to 860°C. thermometer is used, it is not easy to have the whole of the mercury column immersed in the vapour if the boiling point of the liquid is fairly high. In this case a correction must be introduced. Let t be the observed boiling point, t the mean temperature of the projecting part of the column of mercury, n the length of this column in thermometric degrees, then, since the apparent coefficient of expansion of mercury in glass is 0*000154, it is at once evident that if the whole of the mercury column were surrounded by hot vapour, the « degrees would be prolonged byw(T — <)0"0001 54 degrees. The corrected boiling point would, therefore, be t + n(T — t) b 0*000154 degrees. To deter-
mine t a small auxiliary thermometer is attached to the principal thermometer with its bulb just about the middle point of the projecting mercury column. S If the distillation take place at a pressure other than 760 in.m., another cor- rection must be made. If the barometer indicates b m.m., then the corrected boiling point is obtained by adding to Fio. 8 the temperature denoted by
ol the operation the vapour passes through the lateral tube and can be condensed by meana of an air or a Liebig condenser and collected in a suitable receiver.
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