SPONTANEOUS CRYSTALLISATION OP MONOCHLORACETIC ACID, ETC. 361 



Sometimes a separates and sometimes y, but most generally ft ; this was also found 

 in the trough experiments on the refractive indices. It is, however, impossible 

 to predict with any certainty which modification will separate from any given 

 tube. 



Showers of a-crystals were not at all general, but were obtained more frequently in 

 tubes containing from 86 per cent, to 88 per cent, of acid than in tubes of higher 

 concentrations, and usually when a solution in any tube first crystallised as a it 

 continued to do so again and again, even though heated several times in boiling 

 water for various lengths of time to dissolve the crystals. This suggests that a 

 particular modification may exist even in the liquid state and that it is ready to 

 crystallise as soon as the labile temperature is reached. Similarly, if a tube had first 

 crystallised in a /3-shower, it usually continued to give /3-showers when heated again 

 to dissolve the crystals and then recooled. The above statement does not invariably 

 hold good, however, for occasionally a tube was found to give both a- and /3-showers 

 in turn after successive heatings and coolings. Friction generally has a special 

 effect in bringing down /3-showers, in fact /B appears to be far the most usual 

 modification to crystallise in the tubes. It was found that when only a few glass 

 fragments were enclosed in the tubes to produce friction the /3-showers did not 

 usually occur until the solutions had passed into the labile state by about 3 of 

 temperature and, therefore, crossed the supersolubility curve defined by the experi- 

 ments on refractive indices. If, however, a few fragments of some heavier material 

 such as corundum or tinstone were enclosed in the tubes to produce the friction, the 

 /3-showers were found to occur as soon as the sohition had reached the labile 

 temperature. Showers of y-crystals usually occurred in tubes containing glass 

 fragments when the solutions have cooled below the labile temperature for a and ft 

 without crystallising. As has been mentioned previously, when y-cvystals are 

 slightly agitated they at once transform to ft. The same was found with the 

 y-crystals in the tubes, for if they are shaken in the solution they at once transform 

 to ft. Occasionally, when a tube gave a shower at the y labile temperature, it was 

 found on examination that the crystals were of the /^-modification. There is little 

 doubt, however, in these cases that the shower first started as y-crystals and that the 

 shaking transformed them at once into ft. 



The tubes were all heated in hot water, to dissolve the crystals, to temperatures 

 varying from 50 to 100, and for lengths of time varying from five minutes to one 

 hour, but neither the length of time nor the temperatures to which the solutions are 

 heated appear to have any effect on the temperature of crystallisation. 



The results of the experiments with tubes are tabulated below ; some tubes 

 contained glass fragments, some corundum, and some tinstone. 



The eight a-points, if plotted on the concentration-temperature diagram of fig. 5, will 

 be seen, in general, to lie not far from the supersolubility curve. Some of the points 

 are from 0'5 to 1'5 to the left of the a-supersolubility curve These tubes have 



YQL. CCIX. 4, 3 A 



