For more accurate determinations of the solubility, especially when the solvent is appreciably volatile at the temperature of experiment, other methods are preferable. In Fig. 131 is shown the apparatus employed by H. Goldschmidt,[401] and used to a considerable extent in the laboratory of van't Hoff. This consists essentially of three parts: _a_, a tube in which the solvent and salt are placed; this is closed at the foot by an india-rubber stopper. Through this stopper there passes the bent tube _cb_, which connects the tube _a_ with the weighing-tube d. At _c_ there is a plug of cotton wool. Tube _e_ is open to the air. The wider portion of the tube _cb_, which passes through the rubber stopper in _a_, can be closed by a plug {335} attached to a glass rod _ff_, which passes up through a hollow Witt stirrer, _g_. After being fitted together, the whole apparatus is immersed in the thermostat. After the solution has become saturated, the stopper of the bent tube is raised by means of the rod _ff_ and a suction-pump attached to the end of e. The solution is thereby drawn into the weighing-tube _d_, the undissolved salt being retained by the plug at c. The apparatus is then removed from the thermostat, tube _d_ detached and immediately closed by a ground stopper. It is then carefully dried and weighed. [Illustration: FIG. 131.] Another form of solubility vessel, due to Meyerhoffer and Saunders, is shown in Fig. 132.[402] This consists of a single tube, and the stirring is effected by means of a glass screw. [Illustration: FIG. 132.] The progress of the solution towards saturation can be very well tested by determining the density of the solution from time to {336} time. This is most conveniently carried out by means of the pipette shown in Fig. 133.[403] With this pipette the solution can not only be removed for weighing, but the volume can be determined at the same time. It consists of the wide tube _a_, to which the graduated capillary _b_, furnished with a cap _c_, is attached. To the lower end of the pipette the tube _e_, with plug of cotton wool, can be fixed. After the pipette has been filled by sucking at the end of _b_, the stop-cock _d_ is closed and the cap _c_ placed on the capillary. The apparatus can then be weighed, and the volume of the solution be ascertained by means of the graduations. As has already been insisted, particular care must be paid to the characterization of the solid in contact with the solution. [Illustration: FIG. 133.] IV. Thermometric Method.--If a substance is heated, its temperature will gradually rise until the melting point is reached, and the temperature will then remain constant until all the solid has passed into liquid. Similarly, if a substance which can undergo transformation is heated, the temperature will rise until the transition point is reached, and will then remain constant until complete transformation has taken place.