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



flow to the electrode, its efficiency decreases somewhat after 

 it has been going for some time, and its velocity must be 

 increased in order that the former efficiency maybe obtained. 

 The stirrer was fixed at its top to the end of the shaft 

 from which it derived its motion by means of a short piece 

 of rubber tube. It was also in connexion with a speed- 

 counter. 



In the experiments the apparatus was nearly filled with 

 600 c.c. of solution. Before use, a rapid current of hydrogen 

 was passed through it for about half an hour by means of the 

 tubes d, b, r, the gas escaping through a tap in the gas- 

 burette. After this, mercury w T as poured down the tube d, 

 which made a gas-tight joint and besides formed metallic 

 contact for the wire leading to the auode. Either before or 

 after each experiment, the apparatus was tested for leakage, 

 the stirrer always rotating at about the same speed as it did 

 in the experiment. The apparatus was mounted inside a 

 large water-bath, to which also the pulleys <fcc. for the 

 stirrer were fixed. It was made to order by Messrs. (J, 

 E. MuTler & Co., of High Holborn, W.C. 



The measuring-instrument for the current was in these 

 experiments always the millivoltmeter mentioned above, the 

 electrical arrangements being in principle exactly the same 

 as those employed for the experiments described above. 



The solutions employed are Nos. A and B of the previous 

 experiments, which have also been examined by Schrader*; 

 the minimum current-density employed by me being greater 

 than the maximum value of 36 milliamperes per sq. cm. used 

 by Schrader. Some results are given in the following tables. 

 The quantities of hydrogen observed are corrected to 0° and 

 760 mm. The percentage ratio of the hydrogen given off 

 to the total number of equivalents liberated has been calcu- 

 lated from the current employed. The time required for the 

 concentration of the copper to go down to zero, calculated 

 by means of the empirical equation 16, has been given in 

 each case. 



It will be seen from the results that when the solution is 

 not artificially stirred, the quantity of hydrogen liberated 

 decreases rapidly after the experiments have been going on 

 for some time. This is evidently due to the rapid convection- 

 currents brought about in the liquid by the electrolysis, and 

 not to slight chancres in the average concentration of the 

 liquid ; for when it was allowed to stand for about a day, the 



* Taken from his thesis kindly lent me by Prof. Warburg, in whose 

 laboratory the experiments were carried out. 



