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



Experi- 

 ment. 



Concen- 

 tration. 



Remarks. 



14 



15 



16 



per cent. 



85-5 



84-933 



84-5 



17 



82 988 



The index rose from 1-421368 at 50 to 1-433160 at 20. A dense shower of 

 a-crystals occurred at 20 and the temperature rose to 25 -5, the index falling 

 rapidly to 1-429183, corresponding to a point on the solubility curve for a. The 

 temperature then fell again and the index also continued to fall. 



The index rose from 1-424081 at 41 -5 to 1-433260 at 19, no crystals having 

 appeared in the solution. At 18 -5 a dense shower of a-crystals occurred, 

 causing a rise in temperature to 24. No further readings for the index were 

 possible. 



The index rose from 1-421066 at 46'5 to 1-432862 at 18. At 18 a single 

 a crystal appeared in the solution, and at 17 -8 an opaque shower of a-crystals 

 occurred, the temperature rising to 23 and the index falling very suddenly to 

 1-428589. The temperature then fell again and the index rose slightly, but 

 finally fell to 1-428390 at 19', corresponding to a point on the a-solubility curve. 



The index rose from 1-4201G2 at 45 to 1-432367 at 14-5. At 14 -5 a single 

 long a-needle grow up from the bottom of the solution and a shower of a-crystals 

 followed. The temperature rose to 19 and the index fell to 1- 427892. The 

 temperature then fell again and the index rose slightly, reaching 1 428192 at 18, 



this corresponding to a point on the solubility curve for a. 



The actual readings are plotted on fig. o and show that at the points where the solutions 

 attain their highest index all the index -temperature curves for the various experiments 

 very approximately touch a curve parallel to the solubility curve for the a-modification. 

 This is the a-supersolubility curve for the aqueous solutions, and is separated from 

 the a-solubility curve by an interval of about 9 of temperature. From 82'5 per cent. 

 to 88 per cent, of the acid the index-temperature curves give points lying more 

 consistently on a continuous curve than in more concentrated solutions. At higher 

 points the figure shows the behaviour of the solutions to be more irregular. These 

 stronger solutions do not seem to give a-showers so readily as those with concentration 

 below 88 per cent., the majority of the solutions examined with concentration above 

 88 per cent, giving /3-showers. It will be seen from fig. 5 that in some of the 

 experiments, such as 9 and 10, the index-temperature curves have crossed the super- 

 solubility curve slightly. It is possible that these solutions became inoculated with 

 a at these temperatures and the inoculation caused the labile a-showers, but had this 

 not occurred these solutions might have yielded /J-showers and given points on the 

 /3-supersolubility curve. The effect of inoculation with a-crystals even at high 

 temperatures is very marked. In an inoculated solution the index-temperature curve 

 usually fails to reach the supersolubility curve, but as soon as it reaches the solubility 

 curve a dense shower occurs, and it continues to follow the solubility curve down the 

 diagram as the solution cools. In fig. 5 the a-solubility curve is plotted from 

 PICKERING'S results. 



