SPONTANEOUS CRYSTALLISATION OF MONOCHLORACETIC ACID, ETC. 345 



concentration-temperature diagram the lines of constant refractive index in order to 

 interpret the refractive indices in terms of concentration. 



Seven aqueous solutions of known strength were therefore made up, the concentration 

 of the monochloracetic acid varying from 83 to 90 per cent. The index of these 

 solutions was carefully noted as they cooled from about 50 till crystals first began to 

 separate from them, the solutions being stirred steadily throughout the cooling. Until 

 crystals began to form, the concentration of each solution was very approximately 

 constant during its cooling ; it is, therefore, possible from these experiments to plot 

 on the concentration-temperature diagram lines of constant refractive index, each 

 experiment giving one point on eacli constant-index line as the temperature falls. 

 These constant-index lines were plotted for the following values : 1'422, 1'4'24, 1 '426, 

 &c., up to 1'434. They were found to be very approximately straight lines parallel to, 

 and equidistant from, each other. On the scale chosen for the diagram, in which 5 

 temperature corresponds to 2 per cent, concentration, these lines of constant refractive 

 index are inclined at an angle of 40 15' to the temperature axis. 



These lines having been fixed, it is now possible to plot on the diagram by means of 

 them the changes in index of any monochloracetic .acid solution as it cools, the 

 ordinates giving at the same time the changes in concentration. The solubility curves 

 for solutions of the - and ft- modifications, as determined by PICKERING, are also shown 

 plotted on the diagram (figs. 5 and 6). 



Solutions giving an a-shower. In the first example (experiment 11) the concentra- 

 tion of the acid was approximately 88 per cent. The solution was warmed to about 70 

 to dissolve the crystals, and placed in the trough of the smaller inverted goniometer. 

 The glass prism was immersed in the solution and the changes in the refractive index 

 were watched by the motion of the shadow denoting total reflection. The solution 

 was stirred steadily throughout the experiment by a small platinum vane driven by 

 an electric motor. The index rose steadily from 1'423981 at 50 to 1 '433359 at 27, 

 and no crystals appeared in the solution. At 27 a dense shower of crystals suddenly 

 occurred, causing a fall of index to 1 '430377 at this temperature. The crystals were 

 examined under the microscope and found to be characteristic a-needles with a plane 

 angle of 43 and extinction parallel to the oblique end. After this shower the 

 temperature began to fall again, and the index also continued to fall, though much 

 more slowly, till it reached 1 '429580 at 29?. This point coincides with the solubility 

 curve for a, and the index-temperature curve continues to coincide with the a-solubility 

 curve and follows it down the diagram till it reaches 1 '427590 at 17. This experiment 

 is an example of a solution giving a spontaneous shower of the a-crystals at the labile 

 temperature and fixes a point on the supersolubility curve for the a-modification. 



Solutions giving a /3-shoiver. -In the second example (experiment 24) the concentra- 

 tion of the acid was 8 9 '968 per cent. The solution was warmed to about 60 and 

 placed in the trough of the smaller inverted goniometer. The solution was stirred 

 steadily throughout the experiment. The index rose from 1'429481 at 41 to 



VOL. ccix. A. 2 Y 



