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



the cell were not disturbed. In one case, after cooling to 0° C, a 

 cell remained nearly 5 millivolts too high when kept for one hour at 

 15° C. After twenty hours at 15° C, the difference still remained 

 3 millivolts. In the course of the next two hours it was shaken 

 twice, and returned to within one-tenth of a millivolt of its previous 

 value. On further shaking the cell it was noticed that, if the mer- 

 curous sulphate were disturbed so as to come in contact with the zinc, 

 the E.M.F. temporarily fell some 2 or 3 millivolts, but recovered 

 very quickly on the mercurous powder subsiding. This observation 

 illustrates the necessity, now well understood, of keeping the zinc 

 from direct contact with the mercurous paste. 



§ 8. Illustration of Diffusion-lag. 



To illustrate the extreme slowness of the diffusion process, 

 and to show that all saturated cells have really the same coeffi- 

 cient, if sufficient time be allowed for diffusion, the following- 

 experiment was tried. Two exactly similar Board of Trade 

 cells of normal E.M.F. were taken from the constant-tempera- 

 ture bath at 14° C, and placed in the other bath at 25° 0. Two or 

 three observations were taken each day of their subsequent changes 

 of E.M.F. After the lapse of two days, one of the cells was occa- 

 sionally shaken, and rapidly gained the correct value of the E.M.F. 

 of a saturated cell at 25° C. The other cell was left undisturbed > 

 the temperature being maintained constant to one-fiftieth of a degree 

 Centigrade. The E.M.F. of the latter cell fell slowly and almost uni- 

 formly as the diffusion proceeded, but it was not until after the 

 lapse of nearly a fortnight that it reached the correct value. 



The annexed curves (Fig. 1) illustrate the rate of diffusion in 

 these cells. The abscissae represent time in days ; the ordinates, 

 fall of E.M.F. in millivolts from 15° 0. 



For the sake of comparison, a cell of a different type, which we 

 designate "B.O.T. crystal," was submitted to the same treatment at 

 the same time. The straight line BC relates to this " crystal " cell. 

 It will be seen that it shows no appreciable diffusion-lag. In fact 

 its E.M.F. had arrived in twenty minutes within a tenth of a milli- 

 volt of its final value. 



Curve No. (1) relates to the B.O.T. cell which was shaken. The 

 points at which the shaking took place are marked by the sudden, 

 falls of E.M.F. 



Curve No. (2) relates to the cell which was left undisturbed. The 

 rate of diffusion would probably have been much slower and more 

 uniform, but for the slight vibration due to the running of the stirrer 

 in the water-bath. The changes in the rate of diffusion shown at 

 the points 2*5 days and 8*5 days were probably due to excessive 

 stirring about thor.e dates. 



