348 PRINCIPAL HENRY A. MIERS AND MISS FLORENCE ISAAC ON THE 



figure. After the index had fallen to 1 '433260 at 28, another, much denser, shower 

 occurred at this temperature quite suddenly, the temperature rising immediately to 

 32. The crystals were now found to have been transformed, and under the 

 microscope no /3-crystals were to be seen, but only long a-needles having an angle of 

 43 and extinction parallel to the oblique end. The shower of a-crystals rendered 

 the solution quite opaque and solid, arid no further readings were possible. At 25 

 therefore the solution apparently passed into the labile state for the modification ft. 

 At this temperature a shower of /3-crystals appears, and the figure shows the 

 concentration to fall somewhat and approach the solubility curve for ft. At 28 the 

 solution is within 12 of the /3-solubility curve, and here the much denser shower of 

 a-crystals takes place, owing to the fact that although the solution is now metastable 

 with regard to ft it is still labile with regard to a. This second shower causes the 

 concentration to fall still further, and had not the density of the shower rendered the 

 whole solution almost solid and prevented further readings for the index, it would 

 doubtless have been possible to trace the index-temperature curve down to the point 

 where it touches the solubility curve for a. The shower of /3-crystals in this 

 experiment was not nearly so dense as the a-shower, but this is probably due to the 

 positions of the solubility curves for a and ft, which show that at any given 

 temperature a supersaturated solution is much more strongly supersaturated with 

 respect to a than with respect to ft. 



In another solution (experiment 26) the concentration of the acid was again 

 approximately 88 per cent,, as in the first experiment. It was also heated to 70 and 

 placed in the trough of the smaller goniometer. The index rose from 1 '422974 at 

 53 to 1-435442 at 21'5, and no crystals appeared as the solution cooled. From 50 

 to 27 the index-temperature curve for the solution practically coincides with that of 

 the first experiment described. At 21'5, however, a dense shower of /3-crystals, with 

 the usual plane angle and extinction, suddenly occurred. During the shower the 

 temperature rose to 25 and the index fell to 1 '432862. The temperature then began 

 to fall again, but the index continued to fall, reaching 1 '431870 at 22. The index- 

 temperature curve is now within 1 of the solubility curve for ft, as may be seen from 

 fig. 6, and the concentration lias fallen considerably from its original value. The 

 solution was now inoculated with a minute crystal of the a-modification. The effect 

 of the inoculation was to cause an immediate transformation of the /3-crystals and 

 another" si lower of a-crystals. The crystals are now all the characteristic needles of 

 angle 43 and extinction parallel to the oblique end. The temperature rose from 22 

 to 24'5, and the index fell rapidly from 1-431870 at 22 to 1-429680 at 24'5. The 

 temperature then fell again and the index also continued to fall, reaching 1 '428092 

 at 18'5, a point coinciding with the solubility curve for a. Had the solution not 

 been inoculated with a, it is possible that the index-temperature curve for the solution 

 would have followed the /J-solubility curve down the diagram as the temperature fell. 

 Had this been the case, the index at 18'5 would have been approximately T430 



