Electromotive Force of different Forms of the Clark Cell. 147 



to obtain correct results than might at first sight be expected. In 

 particular, we find it extremely important to maintain the solution 

 for a considerable time with continual stirring at each point at a 

 perfectly constant temperature. This essential condition could be 

 readily secured by the aid of the constant temperature baths already 

 described. 



The method which we adopted at each temperature was to prepare 

 a saturated solution with repeated stirring in a large test-tube in the 

 constant temperature bath. This solution, mixed with crystals, 

 was maintained for several hours at a constant temperature. Samples 

 of solution free from crystals were drawn off in a special pipette at 

 intervals and weighed. They were then evaporated to dryness at 

 100° C, and the percentage of ZnS0 4 in each case was calculated, 

 assuming the residue to be the monohydrate. The different samples 

 at each temperature always agreed very closely, showing the solution 

 to have been saturated and free from crystals. 



The results of our observations are contained in the following 

 table. With the exception of the last four lines, which represent 

 single observations, each line is the mean of two or three determina- 

 tions. Observations in different lines were taken on different days 

 and with different samples of solution. 



Table V. — Solubility of Zinc Sulphate (Callendar and Barnes). 



Temperature 

 centigrade. 

 (*>• 



Percentage of 

 solution. 

 (P)- 



Calculated by 



formula 

 29-5 + -270*. 



Difference 

 (see Fig. IV). 

 Obs.— Calc. 



0° 



29-43 



29-50 



-0-07 







29-53 



29-50 



+ 003 







29 -49 



29-50 



-o-oi 



15 -00° 



33-66 



33-55 



+ 0-11 



15-88 



33 85 



33-78 



+ 0-07 



30-70 



38-46 



37-80 



+ 0-66 



39-92 



41-36 



40-28 



+ 1-08 



39-95 



41-37* 



40 -29 



+ 1-08 



40 -73 



41-43 



40-50 



+ 0-93 



41'49 



41-70 



40 -70 



+ 1-00 



46 -40 



42-68 



42-02 



+ 0-66 



49-97 



43 -51* 



43 -00 



+ 0-51 



49-99 



43-41 



43 -00 



+ -41 



50-00 



43 -50* 



43-00 



+ 0-50 



50 -20 



43 -51 



43-05 



+ -46 



The observations marked with an asterisk were obtained by 

 heating the solution to a temperature ten or twenty degrees above 

 that of the bath and then allowing it to cool down to the bath tem- 

 perature with constant stirring. 



VOL. LXTI. M 



