If, now, transformation occurred immediately the transition point was reached, the sudden expansion and contraction would take place at the same temperature. It is, however, generally found that there is a lag, and that with rising temperature the relatively large expansion does not take place until a temperature somewhat higher than the transition point; and with falling temperature the contraction occurs at a temperature somewhat below the transition point. This is represented in Fig. 130 by the lines BC and EF. The amount of lag will vary from case to case, and will {332} also depend on the length of time during which the dilatometer is maintained at constant temperature. As an example, there may be given the results obtained in the determination of the transition point at which sodium sulphate and magnesium sulphate form astracanite (p. 268).[399] The dilatometer was charged with a mixture of the two sulphates. -------------------------------------------------------- Temperature. | Level of oil in capillary. | Rise per 1°. -------------------------------------------------------- 15.6° | 134 | 16.6° | 141 | 7 17.6° | 148 | 7 18.6° | 154 | 6 19.6° | 161 | 7 20.6° | 168 | 7 21.6° | 241 | 73 22.6° | 243 | 2 23.6° | 251 | 8 24.6° | 259 | 8 -------------------------------------------------------- The transition point, therefore, lies about 21.6° (p. 268).