Electromotive Force of different Forms of the Clark Cell. 123 



think that they fairly represent the performance of B.O.T. cells 

 under the conditions that he describes. 



§ 7. Temperature Coefficients of B.O.T. Cells. 



Fresh determinations of the temperature coefficients of the old 

 cells, in addition to those of the new cells, were made under different 

 conditions and with greater accuracy by means of the constant 

 temperature bafchs. 



For four of the cells in which the end of the zinc rod was at a 

 small distance from the crystals, the mean coefficients obtained on 

 raising the temperature from 15° to 25° C. varied from 0*00046 to 

 0*00051, and were in practical agreement with previous tests of the 

 same cells under similar conditions. Precisely similar values were 

 obtained after keeping the cells at 25° C. for the night, and then 

 lowering the temperature to 15° C. In another case, after keeping- 

 two of the cells at 25° C. for the night, the changes of their E.M.F. 

 from their values at 15° C. were found to be 9'0 and 8*6 millivolts, 

 giving co-efficients 0*00063 and 0*00060 respectively. The greater 

 change is evidently due to the time allowed for diffusion. 



On cooling the cells down from 15° to 0° C, allowing them to 

 remain for an honr at the latter temperature, the mean coefficients 

 obtained were invariably much larger. The reason is evidently that 

 the zinc becomes partially imbedded in the crystals at the lower 

 temperature, and is necessarily in contact with a normal saturated 

 solution throughout a considerable portion of its surface. In one 

 •case a coefficient as high as 0*00075 was obtained ; in another a co- 

 efficient as low as 0*00059. In the latter case the rod was very long, 

 -and a considerable length was probably exposed to a supersaturated 

 solution. 



That this state of super saturation is likely to occur, and to persist 

 for a considerable time, is also illustrated by another experiment 

 with a cell containing very few crystals. After keeping the cell in 

 question at 25° C. for a day, it was observed that all the crystals had 

 disappeared, whereas the other cells still showed considerable quanti- 

 ties. On cooling the cell down to 15° C, the E.M.F. rose with a 

 coefficient 0*00040, and then remained steady for some time, no 

 crystals reappearing. After a time a sudden rise in the E.M.F. was 

 observed, and the crystals were seen to have reappeared on the 

 -surface of the paste. 



On transferring the cells back from the melting ice to the constant 

 temperature bath at 15° C, the E.M.F. of the B.O.T. cells was 

 almost invariably found to be from 2 to 4 millivolts higher than before 

 cooling. The difference was greatest in those cells which contained 

 the greatest quantity of solution, and persisted for several days if 



