126 PROCEEDINGS OP THE AMERICAN ACADEMY. 



tion ; and if an acid solution of cupric sulphate is thus made to circulate 

 over copper placed in the hot side, considerable amounts of copper may 

 be dissolved with ease. The crystals deposited upon cooling in the cold 

 limb cling loosely to the glass. A more advantageous form of this device 

 is shown in the foregoing illustration. 



From these facts concerning the action of copper on a solution of 

 cupric sulphate, the precautions necessary for obtaining the weight of 

 deposit which most nearly represents the true electrochemical equivalent 

 are evidently the following : — 



1. The solution must be as cold as possible. 



2. The solution must be acidified, to avoid hydrolysis. 



3. The solution must be as dilute as is compatible with the transpor- 

 tation of the required strength of current to a given electrode without 

 risk of deionizing hydrogen. The smaller the electrode, the stronger 

 is the necessary solution. 



4. Air must be excluded. 



Since even at the freezing point of the saturated solution the action 

 still takes place, although to a greatly diminished degree, the complete 

 fulfilment of these conditions will still yield too small a deposit of copper. 

 In order to overcome the difficulty, Foerster and Seidel * have recom- 

 mended the saturation of the solution with cuprous ions in the first place ; 

 but this plan oversteps its mark, and forms almost the only jsiece of faulty 

 logic to be found in their paper. Obviously if cuprous ions were the only 

 ones present, twice as much copper would be deposited with a given 

 current as with pure cupric ions. It is well known that when a mixture 

 of metals is present, that substance is deposited which gives the lowest 

 contact-potential with the solution, provided the current is weak and the 

 solution strong. It is easy, however, to deposit brass from a suitable 

 solution of zinc and copper sulphates with a strong current. The present 

 case presents some analogies to this example ; but to it is added the pro- 

 viso that the ions are convertible into one another, and that the cupric 

 and cuprous salts must be present in a ratio definite for each concentra- 

 tion. If some of the cupric salt is removed, for example, less cuprous 

 salt is capable of existing in the solution, and extra copper must neces- 

 sarily be deposited in order to establish the equilibrmra, according to the 

 equation 2 Cu+l;? Cu"*""^ + Cu. Experimental verification of this rea- 

 soning will be given later. 



We are then between Scylla and Charybdis, — from a cupric solution 



* Zeitschrift fur anorg. Chem., 14, 137 (1897). 



