SPONTANEOUS CRYSTALLISATION OF MONOCHLORACETIC ACID, ETC. 349 



instead of T428, the actual value obtained. This solution was also stirred throughout 

 the experiment. The index-temperature curve in fig. 6 for this experiment shows 

 two distinct breaks at the points where the two showers occurred. 



The last two experiments described are examples of solutions giving a spontaneous 

 shower of ^-crystals before giving the spontaneous a-shower of the first experiment. 

 As ft is not the stable modification of the acid, we get a further shower and a 

 transformation to a. It will be seen that in the last experiment the solution 

 becomes labile at 21'5 with respect to /3, and that the maximum index attained 

 is 1'435442. In the first experiment described a solution of the same concentration 

 gave an a-shower, and became labile for a at 27, the maximum index attained being 

 1 '433359. These two experiments suggest, therefore, that on the concentration- 

 temperature diagram the supersolubility curve for the /8-modification of mono- 

 chloracetic acid lies to the left of the supersolubility curve for the a-modification, 

 just as the /3-solubility curve lies to the left of the a-solubility curve. This is 

 confirmed by all the later experiments on solutions of different concentrations. 

 The later experiments show that in two solutions of equal concentrations a /3-shower 

 always takes place at a lower temperature than an a-shower ; and the maximum 

 index attained by a solution giving a /8-shower is always greater than that attained 

 by a solution of equal concentration which gives an a-shower. 



Solutions giving a y-shower. In a fourth example (experiment 43) it was found 

 possible to obtain a labile shower consisting of crystals of the third modification, y. 

 The concentration of this solution was 89 - 5 per cent., nearly the same as that in the 

 second experiment. It was heated to about 70 and placed in the trough of the 

 large goniometer, which contains about four times the quantity of solution held by 

 the trough of the smaller goniometer. The solution was stirred extremely slowly as 

 it cooled, the stirring being enough to keep the solution from settling in layers, but 

 not enough to cause a violent agitation in the liquid. The index rose from 1 '42401 5 

 at 54 to 1-438212 at 18 without crystals appearing in the trough. At 17 0< 8, 

 however, a shower of transparent glassy-looking crystals formed suddenly and 

 continued to grow in thin sheets, adhering to the sides of the trough and the surface 

 of the liquid. The temperature rose immediately to 21'5, and the index fell to 

 1-435673. It will be shown later that this point really lies on the solubility curve 

 for y, although PICKERING'S observations would not lead to this conclusion. The 

 crystals of this shower, examined under the microscope, showed the characteristic 

 y-rhombs of angle approximately 58, extinction inclined at about 26 to one side of 

 the rhomb, and the usual interference figure. After the temperature had risen to 

 21'5 there was a slight halt, and it remained constant for a minute. After this halt 

 another shower suddenly occurred and a transformation, and the temperature rose 

 further to 24 0- 5. The crystals, when now examined, were found to be of the 

 ^-modification. The density of the shower prevented any more readings for the 

 index. This experiment may be compared with experiment 24 described above, in 



