Effluvia from Chemical or from Voltaic Reactions. 23 



seems best to adopt an electrolytic hypothesis similar to that 

 of G-rotthus. 



A film of zinc chloride is now by the action interposed 

 between the zinc at a and the hydrochloric acid, as repre- 

 sented in fig. 2, where (since zinc chloride forms at a only, 

 and not at b) we have a little double fluid cell, and the 

 current passes successively through zinc-chloride and hydro- 

 chloric-acid solutions, instead of through the latter only as in 



fig.i. 



The only difference in the two actions seems to be, that in 

 fig. 2 certain atoms of zinc take the place of hydrogen in 

 fig. 1, and are intermediate in the decomposition of the acid 

 by the metallic zinc ; but there seems at first sight no a priori 

 reason why this should cause the negative electrification of 

 the evolved gas. 



It may be pointed out, however, that if we think with 

 Faraday that, while the source of the energy of the current in 

 the cell is the potential chemical energy of the active ions, 

 the production of that energy in the form of current instead 

 of heat is closely related to the decomposition of the electro- 

 lyte, we may presume that the method of the decomposition 

 will probably exercise considerable influence on the electric 

 phenomena. It seemed not improbable that in the case under 

 consideration the reversal of potential in the gas was dtfe to 

 the change in the electrolytic action, as described above. 



4. I therefore thought it worth while to test the above 

 hypothesis, and after repeating Mr. Enright's experiment 

 with zinc dissolving in hydrochloric acid, and observing the 

 very remarkable electrification and reversal of sign which he 

 describes, I determined to make trial of a cell with the b part 

 of the metallic surface in fig. 2 represented by platinum. 



A piece of platinum foil was attached to a small piece of 

 sheet zinc by bending the zinc over it and clipping it tight, 

 and the couple so arranged was placed in a beaker, the zinc 

 lying flat on the bottom, the platinum standing upright. 

 The zinc was covered with strong zinc-chloride solution, on 

 which was carefully placed hydrochloric-acid solution of 

 constant boiling-point, in contact with the platinum only. 

 Gas was given off rather slowly from the platinum. The cell 

 was now placed on an insulated metal plate connected with 

 an electrometer, giving a deflexion of 60 divisions on each 

 side of zero for one Daniell cell. The spot moved very 

 slowly to 10, and showed the cell to be positive. I repeated 

 the experiment, using acid diluted with an equal measure 

 of water, and the spot went to 35, cell positive. In both cases 



