The melting point of the α modification is 34-35°; the melting point of the unstable β-modification being 130°. The freezing curves AC and BC were obtained by determining the freezing points of different mixtures of known composition, and the numbers so obtained are given in the following table.
| Grams of the α modification in 100 gm. of mixture. | Freezing point. |
| 26.2 | 101° |
| 49.2 | 79° |
| 73.7 | 46° |
| 91.7 | 26.2° |
| 95.0 | 28.6° |
| 96.0 | 30.0° |
The eutectic point C was found to lie at 25-26°, and the natural freezing point D was found to be 27.7°. The equilibrium curve DE was determined by heating the liquid mixtures at different temperatures until equilibrium was attained, and then rapidly cooling the liquid. In all cases the freezing point was practically that of the point D. From this it is seen that the equilibrium curve must be a straight line parallel to the temperature axis; and, therefore, isomeric transformation in the case of the two benzaldoximes is not accompanied by any heat effect (p. [197]). This behaviour has also been found in the case of acetaldoxime.[[287]]
The isomeric benzaldoximes are also of interest from the fact that the stable modification has the lower melting point (v. p. 202).
Acetaldehyde and Paraldehyde.—As a second example of the equilibria between two isomerides, we shall take the two isomeric (polymeric) forms of acetaldehyde, which have recently been exhaustively studied.[[288]]
In the case of these two substances the reaction
3CH3.CHO