6$ 



Dr. H. J. S. Sand on the Concentration 



was passed through it. This current was measured, as will 

 be sufficiently understood from the diagram, by determining 

 the voltage on the ends of the known resistance by means of 

 a separate potentiometer circuit. 



As regards the degree of accuracy obtained in the experi- 

 ments, the upper limit is probably reached by those arranged 

 in Table V. The experiments here having all been repeated, 

 this table shows the degree of coincidence which can be 

 obtained in a series of determinations. The readings have 

 in general been taken to within five seconds. The most 

 serious errors are no doubt brought about by changes of 

 temperature which cause convection-currents in the liquid. 

 The accuracy of the experiments contained in Tables B and C, 

 which were carried out in winter, has certainly been con- 

 siderably reduced, owing to the heating arrangements of the 

 building. Indeed, differences of as much as 10 per cent, have 

 here in single cases been obtained in successive experiments 

 which could not be accounted for in any other way. I have 

 nevertheless not hesitated to give these earlier values as the 

 ranges of the times obtained are so great that even differences 

 of as much as 10 per cent, do not preclude a comparison with 

 the theoretical values. These being calculated for 18°, should 

 be multiplied by 1+0*026 (0 — 18) if the experimental values 

 have been obtained at 6°. I have instead made the corrections 

 at the experimental numbers themselves, dividing them by 

 1 + 0*026 (0—18). It must be acknowledged that some 

 uncertainty is introduced by this huge temperature correction. 

 The temperature-coefficient for the diffusion of copper 

 sulphate never having been experimentally determined has, 

 as was also done by Wiedeburg, been assumed to be the same 

 as that of other substances. 



From the tables it will be seen that the experimental values 

 are in all cases contained between the theoretical limits, and 

 their differences from those calculated by the intermediate 

 formula are in most cases not great. When we consider the 

 extremely great ranges in which the times can be made to 

 vary by varying the current-density, and also the numerous 

 causes of experimental error, I think the medium formula 

 No. 16 will be considered sufficient as an empirical expression 

 of the results. From the fact that the values are always con- 

 tained between the theoretical limits, and, besides, are far 

 distant from the lower limit, I think we may conclude that 

 these experiments prove within the limits which our theoretical 

 knowledge of the processes takiug place in the electrolyte 

 allows, that liberation of hydrogen from an acid solution of 

 copper sulphate takes place only after the concentration of 

 the copper at the electrode has gone down to zero. 



