272 



Development of Feeble Currents. [Dec. 15, 



ing suitable resistances that the silver deposition in the voltameter 

 was brought down to 0*1 to 0'2 gram of silver per 24 hours, the silver 

 deposited always exceeded that dissolved by 0*001 to 0*003 gram. 

 Similarly two duplicate cells set up with silver plates immersed in 

 ammonia solution containing sal-ammoniac of strength, about 



5NH 3 ,5NH 4 Cl,100H 2 O, 



and opposed to platinum immersed in sulphuric acid solution con- 

 taining chromic anhydride, gave the following figures, much more 

 resistance being in circuit in the second experiment than in the first. 



Time. 



Silver deposited in 

 voltameter. 



Silver dissolved from 

 plate. 



Difference. 



1 



18 hours 

 46 „ 



'514 gram 

 0*107 „ 



0-510 

 0*106 



004 

 0*001 



A similar cell containing ammonia solution without sal-ammoniac, 

 and consequently having a very large internal resistance, caused 

 only 0*013 gram of silver to be dissolved in eighteen hours, whilst 

 0*015 gram was deposited; in this case a visible film of silver 

 peroxide was formed on the silver plate (a wire of pure metal). 



Just as silver is capable of being dissolved in an appropriate fluid 

 when opposed to an aeration plate, so may several other metals not 

 ordinarily prone to atmospheric oxidation; thus mercury with dilute 

 sulphuric acid as fluid, and an aeration plate of platinum sponge, 

 generates a measurable continuous current, forming mercurous sulphate 

 in so doing, so that after some time the liquid becomes turbid through 

 separation of that sparingly soluble salt, and the filtered fluid precipi- 

 tates calomel on addition of dilute hydrochloric acid. Acetic acid acts 

 similarly, but far less energetically. Potassium cyanide solution, on 

 the other hand, causes a much more rapid solution of mercury, form- 

 ing mercuric potassiocyanide ; it is noticeable that in this case only 

 100 parts of mercury go into solution for 108 of silver deposited in 

 the voltameter, whereas when sulphuric acid is used 200 parts of 

 mercury become sulphate per 108 of silver deposited. 



If gold be substituted for. mercury in this latter arrangement, rapid 

 solution takes place with formation of aurocyanide of potassium, 196 

 parts of gold being dissolved per 108 of silver thrown down in the 

 voltameter ; the rate of action here, as in other analogous cases, can 

 be notably increased by placing the gold plate and potassium cyanide 

 solution in one basin and the aeration plate (platinum sponge) in 

 another with sulphuric acid, uniting the two fluids by a wide siphon, 



