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.
The progress of the solution towards saturation can be very well tested by determining the density of the solution from time to
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.
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.
This method, it will be remembered, was employed by Richards for the determination of the transition point of sodium sulphate decahydrate (p. [136]). The following figures give the results obtained by Meyerhoffer in the case of the transformation:—
CuK2Cl4,2H2O
