52 BOILING AND FREEZING POINT. chap.
with a cover of sheet metal and the stirrer H, The ther- mometer scale extends only over five or six degrees, in order not to require to be of inconvenient length. In order, how- ever, to make this thermometer available for the registration of temperature over a large interval, the capillary is bent at the top, and enters a reservoir, as shown in Fig. 18 (see ^^^ also Fig. 19). When the bulb of the thermometer
\Ko is warmed, the mercury rises in the capillary
stem, and overflows into the top of the reservoir.
By adhesion to the glass, however, the mercury ^ is prevented from falling off into the bottom of Fig 18. ^^ reservoir. By gently tapping, the thread can
be broken, and the excess of mercury drops into the reservoir. In this way the quantity of mercury in the thermometer can be varied at pleasure, and the quantity is so arranged that at the freezing point of the solvent the meniscus will stand near the top of the scale. The tem- perature of the freezing mixture should be only very little lower than the freezing point to be determined, and all disturbances due to radiation should be avoided.
After the freezing point of the solvent has been determined, a weighed quantity of substance is introduced through B, and dissolved by stirring with the wire G, The temperature is now allowed to sink a little below the freezing point, and a small crystal of the solidified solvent is dropped in. This causes deposition of solid from the super-cooled solution, and the mixture is now vigorously stirred when the temperature lises to a maximum (the freezing point) and remains constant for a considerable time, then falls slowly on account of the solution becoming more concentrated because of the separation of ice, whereby the freezing point is continually decreasing.
Experimental Determination of the Boiling Point — The boiling point apparatus devised by Beckmann (-7) is very similar to that used for the detennination of the freezing point. The inner tube A (Fig. 19) is the same as that described above, but a short platinum wire, a^ is sealed into the bottom of it.
�� �
