150 



Prof. H. L. Callendar and Mr. H. T. Barnes. 



The differences from the linear formula given in the last column of 

 Table VI are shown by the crosses on the curve in fig. 5. This figure 

 is plotted, as in the previous case, to exhibit not the whole change of 

 the B.M.F., but only the defect of the change from lineality. For 

 instance, at 39° C, the E.M.F. does not suddenly begin to increase, 

 but continues to diminish at a slower rate. 



It was evident on plotting these observations that there was a 

 break in the curve, and that the observations after 41° C. belonged 

 to a different branch. To investigate this point more closely, a con- 

 tinuous set of readings was taken on a pair of very sensitive cells. 

 Starting at a temperature of 35° C, the temperature of the bath was 



Fig. 5. 



7 



very slowly and continuously raised by disconnecting the regulator, 

 and readings were taken of the temperature and E.M.F. of the cells 

 alternately every few minutes. The E.M.F, of the cells was observed 

 to fall at a steady rate, accurately following the curve already found 

 for the observations at steady temperatures, until a temperature of 

 42*4° was reached. At this point there was a sudden increase of 

 more than 2 millivolts in the E.M.F., due to rapid crystallisation of 

 the hexahydrate, and in less than five minutes the cells had reached 

 a point on the other branch of the curve. 



After remaining at this point for some hours, the bath was slowly 

 and continuously cooled, observations being taken in the same manner 

 during the cooling. As had been expected, the cells were found to 

 follow the branch of the curve shown in fig. 5, corresponding to the 

 solubility of the hexahydrate. The solution remained supersaturated 

 with respect to the heptahydrate until a temperature of 31° was 

 reached, at which point the E.M.F. had fallen nearly five millivolts 

 below the normal. At this point there was a sudden crystallisation 

 of the heptahydrate, and the E.M.F. rose in a few minutes to its 

 normal value. It vvill be observed that the two branches of the curve 



