SPONTANEOUS CRYSTALLISATION OF MONOCHLORACETIC ACID, ETC. 355 



sufficiently stirred to bring down the showers at the labile point. After reaching the 

 highest index a shower of /3-crystals occurs and all the curves drop towards the 

 /3-solubility curve. Unless a transformation from y8 to a takes place, the index- 

 temperature curves do not again cross the y8-solubility curve, but continue to coincide 

 with it as the temperature falls. If, however, a transformation takes place, the 

 temperature rises at once, and the concentration and index drop till the index- 

 temperature curves touch the a-solubility curve. This behaviour is shown in fig. 6 

 by experiments 26 and 28. In several of the other experiments, also, transformation 

 takes place, either spontaneously or by inoculation, but it was not possible to obtain 

 readings for the index owing to the density of the shower. The rise of temperature, 

 however, always showed that the index-temperature curve had fallen approximately 

 to the a-solubility curve after the transformation. 



In experiments 1 and 18 the dried acid was melted and used in the trough of the 

 larger goniometer without adding any water. The acid, however, is so hygroscopic 

 that it probably always picks up a little water during the experiment. This difficulty 

 of keeping the concentration constant recurs throughout, even in the preliminary 

 experiments which fix the lines of constant index. An attempt to keep the solutions 

 covered with oil was not successful owing to the oil mixing very readily with the 

 acid. It is probable, therefore, that some small inaccuracy results from this cause 

 throughout the whole series of experiments, though it is more marked as the 

 concentration becomes higher. 



The /3-crystals were sometimes found to be stable for a considerable time, and in 

 some experiments they remained withoxit transforming for many hours. The solu- 

 bility curve for the a- and /S-modifications of the acid is plotted on fig. G from the 

 values obtained by PICKERING. 



The /3-solubility and supersolubility curves are separated from each other by an 

 interval of about 7'5 of temperature, which is slightly less than the interval 

 separating the a-solubility and supersolubility curves. 



As has already been noticed, the index-temperature curves of experiments 20, 21, 

 22, and 23, in fig. 6, cross the supersolubility curve. In order to ascertain whether it 

 is insufficient stirring which causes these solutions to pass somewhat into the labile 

 state, two other experiments were made, with the same concentrations as in 

 experiments 20 and 21, in which the solutions were stirred more violently. In these 

 the stirring was effected by means of a plunging stirrer of glass, shaped like a horse- 

 shoe, fitting inside the goniometer trough as shown in fig. 4, and driven by the 

 electric motor. This moved rapidly up and down, and kept the whole solution in 

 violent motion. The following results were obtained ; 



2 z 2 



