| Temperature 95.1°. | |
| Time in minutes. | Level of liquid. |
| 5 | 343.5 |
| 30 | 340.5 |
| 55 | 335.75 |
| 65 | 333 |
| Temperature 96.1°. | |
| Time in minutes. | Level of liquid. |
| 5 | 342.75 |
| 30 | 354.75 |
| 55 | 360.5 |
| 60 | 361.5 |
| Temperature 95.6°. | |
| Time in minutes. | Level of liquid. |
| 5 | 368.75 |
| 100 | 368 |
| 110 | 368.75 |
At a temperature of 95.1° there is a contraction, i.e. monoclinic sulphur passes into the rhombic, the specific volume of the former being greater than that of the latter. At 96.1°, however, there is expansion, showing that at this temperature rhombic sulphur passes into monoclinic; while at 95.6° there is neither expansion nor contraction. This is, therefore, the transition temperature; and since the dilatometer was sealed up to prevent evaporation of the liquid, the pressure within it was 4 atm.
II. Measurement of the Vapour Pressure.—In the preceding pages it has been seen repeatedly that the vapour pressures of the two systems undergoing reciprocal transformation become identical at the transition point (more strictly, at the triple or
multiple point), and the latter can therefore be determined by ascertaining the temperature at which this identity of vapour pressure is established. The apparatus usually employed for this purpose is the Bremer-Frowein tensimeter (p. [91]).
Although this method has not as yet been applied to systems of one component, it has been used to a considerable extent in the case of systems containing water or other volatile component. An example of this has already been given in Glauber's salt (p. [139]).
III. Solubility Measurements.—The temperature of the transition point can also be fixed by means of solubility measurements, for at that point the solubility of the two systems becomes identical. Reference has already been made to several cases in which this method was employed, e.g. ammonium nitrate (p. [112]), Glauber's salt (p. [134]), astracanite and sodium and magnesium sulphates (p. [268]).
The determinations of the solubility can be carried out in various ways. One of the simplest methods, which also gives sufficiently accurate results when the temperature is not high or when the solvent is not very volatile, can be carried out in the following manner. The solid substance is finely powdered (in order to accelerate the process of solution), and placed in sufficient quantity along with the solvent in a tube carefully closed by a glass stopper; the latter is protected by a rubber cap, such as a rubber finger-stall. The tube is then rotated in a thermostat, the temperature of which does not vary more than one or two tenths of a degree, until saturation is produced. The solution is withdrawn by means of a pipette to which a small glass tube, filled with cotton wool to act as a filter, is attached. The solution is then run into a weighing bottle, and weighed; after which the amount of solid in solution is determined in a suitable manner.
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
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.
