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



way of verification. The cells at 15° C. were still found to preserve 

 their relative differences to within one or two hundredths of a milli- 

 volt, and the mean still agreed with that of the standards. The 

 difference at 30° C. was observed directly in terms of the resistance 

 boxes, as well as in terms of the bridge-wire, at each observation. 

 The fall of E.M.F. found was less by four hundredths of a millivolt 

 in 19*4 millivolts, than on the preceding occasion. After twenty- 

 seven hours at 30° C. the cells, when replaced in the 15° C. bath, 

 returned in twenty hours to within two hundredths of a millivolt of 

 their previous values. 



§ 14. Quickness of Recovery. 



It must not be hastily assumed from the foregoing observations 

 that the cells in each case took nearly a day to recover their original 

 values at 15° 0. The observations were taken in this particular 

 series after the lapse of several hours in order to make sure that the 

 cells had not undergone any permanent change as the result of 

 prolonged exposure to 0° and 30° C. 



We have made several special tests with regard to this point. We 

 find that B.O.T. crystal cells of this size, set up in test-tubes nearly 

 2 cm. in diameter, when suddenly transferred from melting snow, or 

 from the other bath at 30°, or even 40° C, back to the constant tem- 

 perature bath at 15° C, return to within a tenth of a millivolt of their 

 previous values in less than ten minutes. 



For cells of a smaller size, of which we have made several, the 

 recovery is still more rapid. It appeal's to be chiefly a question of 

 the time required for the change of temperature. After exposure to 

 a temperature above 15° C, the cells frequently return, at 15° C, in 

 less than half an hour to within one or two hundredths of a millivolt 

 of their previous values. After exposure to a temperature below 

 15° C, the recovery is a little slower. This might naturally be 

 expected, as crystallisation is generally more rapid than solution. 



We have also taken a series of observations at 40° C, though this 

 temperature lies outside the limits of practical utility. On suddenly 

 raising the temperature to 406° C. the value observed after ten 

 minutes was one millivolt higher than the final value. The next 

 observation was taken after three hours, by which time it was found, 

 on subsequent redaction, that the E.M.F. had become constant. The 

 E.M.F. at 40-60° C. was found to be 35*81 millivolts lower than at 

 15° C. 



The recovery on returning to the 15° bath after a day at 40'6 C. 

 was quite unexpectedly rapid. In ten minutes the value was found 

 to be within a tenth, in two and a half hours within a hundredth of 

 a millivolt. Later observations showed no further change. 



