Maclaubin. — Action of Potassium-Cyanide upon Gold. 703 



on the bottom of a beaker partly filled with a saturated solu- 

 tion of potassium-cyanide, the surface of ^Yhich stood about 

 half-way up the varnished portion of this plate. As previously 

 shown, with strips of gold paper varnished in 

 a similar manner there will be practically no 

 action on A. When, however, No. 2 plate is 

 lowered into the solution so that B is partly 

 immersed there is strong deflection of the 

 galvanometer, proving that a considerable cur- 

 rent is passing ; and, from the direction of the 

 deflection, showing that cyanogen is being de- 

 posited electrolytically upon A, and that there- 

 f/ff. J fore aurous cyanide is formed, which dissolves 



in the liquid. Hence the gold plate A is dis- 

 solved. "When No. 2 plate is further lowered, so as to have 

 the surface of the liquid at about the middle of the var- 

 nished strip C, the deflection is hardly perceptible ; whilst 

 on still further lowering the plate, so as to expose part of 

 D to the action of the solution, the deflection is the same as 

 at first. 



Let us consider the course of the current in the case of a 

 straight strip of gold partly immersed. From the experiments 

 just given it is evident that there must be an electromotive 

 force at the surface of the liquid, and that a current will flow 

 down through the metal and back through the solution to its 

 starting-point. We may consider the portion of the strip 

 beneath the surface of the solution as one pole of a battery, 

 and that portion just at the surface as the other pole. Now, 

 it is well known that when a current passes through an 

 electrolyte (in this case potassium-cyanide solution) the latter 

 is decomposed, its- constituents — the ions — appearing at the 

 poles. In the present case cyanogen is liberated at the lower 

 pole, and at once combines with the gold to form aurous 

 cyanide, which unites with potassium cyanide in the solution 

 to form the soluble double salt ; whilst the potassium must 

 pass to the upper pole, where it will either decompose water, 

 liberating hydrogen, or unite at the moment of liberation 

 (whilst nascent) with the oxygen at the surface. In either 

 case potash will be formed at the surface. 



In order to prove that this is the case, the following 

 experiments were made : — 



Three test-tubes (shown in Fig. 4) of the same size were 

 taken. Into No. 1 test-tube lOcc. of a 2-per-cent. solution 

 of potassium cyanide almost free from air were poured. A 

 gold plate was dropped in, and then a thin disc of cork, 

 through the centre of which passed a rod of gold, was 

 placed as shown in the figure. The disc of cork fitted 

 the tube tighdy, but a very small strip was cut away at 



