Electromotive Force of different Forms of the Clark Cell. 151 



cross at 38*8°, which is in practical agreement with the temperature 

 of equal solubility of the two hydrates, as determined by the observa- 

 tions on the change of solubility. 



§ 25. Permanence and Reproducibility of B.O.T. Crystal Cells. 



It is now more than a year since the completion of the experiments 

 described in the preceding pages, and we are able to add the results 

 of more recent comparisons of the cells as evidence of the order of 

 their permanence and reproducibility. Of the original crystal cells, 

 made more than two years ago, we still have a few remaining. None 

 of the cells of this type have shown any signs of failure, in spite of 

 the treatment to which they have been subjected, but many of them 

 have been taken away by the students who made them. 



Table VII.— Comparisons of Crystal Cells at 15° C. 





Differences from mean of cells in millivolts. 



Number 













of cell. 















Dec. 10, 1895. 



Nov. 28, 1896. 



Dec. 19, 1896. 



Feb. 8, 1897. 



Aug. 2, 1897. 



X 1 



-0-03 



-0-04 



-0-08 



-0-02 



-0-04 



X2 



-0-05 



+ 0-02 



+ 0-06 



- 04 



+ 0-02 



X 3 



+ 0-10 



4-0-04 



+ 0-05 





+ 0-12 . 



X 5 



+ 0-03 



-0-08 



-0-07 



+ 0-02 





X6 



+ 0-08 



+ 0-07 



+- '08 



+ -04 



f <M 



X 10 



-0-07 



+ 04 



+ 0-01 





+ 0-10J 45 



X 11 



-0-08 



-0-0G 



-0-07 





-0-02 < 



As there is no particular reason why these cells should be less 

 permanent than other Clark cells of the B.O.T. type, the above wiil 

 probably be sufficient proof of permanence. It will be observed that 

 the average difference from the mean in each case is nearly one- 

 twentieth of a millivolt. The extreme difference is one-tenth. We 

 have observed that this is about the order of agreement generally 

 attainable with Clark cells set up at different times. 



Over comparatively short intervals of time, such as one month, it 

 would appear from the above list, and from other tests, that the 

 average change in the value of any one cell, as compared with the 

 mean, may be expected not to exceed two or three hundredths of a 

 millivolt, but for longer periods, such as a year, the mean change 

 reaches one- twentieth. 



As a further illustration of the reproducibility of these cells, and 

 of the close agreement in the temperature variation of the E.M.F., 

 under somewhat exacting conditions of testing, we add a list of the 



