Hydro- Electrical Currents, 95 



Hydro-Electrical Currents. 



24:. As the decomposing cell, fig. 4, attached to a pair of zinc and 

 copper plates of half an inch superficies each, excited by water, pure 

 at the commencement of the experiment, could perform a greater variety 

 of chemical decompositions^ than a thermo- electrical current derived 

 from a bismuth and antimony joint heated till the bismuth fuses, it ap- 

 peared to me to be readily applicable for some experiments with hydro- 

 electrical currents. 



Platina poles, sheathed with glass that they might present a very 

 small surface to the fluid to be decomposed, were inserted in the vol- 

 tameter, which was afterwards filled with water acidulated by sulphuric 

 acid. The lowest electro-motive-force capable of efiecting decomposi- 

 tion in the above arrangement, was derived from 3 pair of zinc and cop- 

 per plates excited by water only ; with a saturated solution of common 

 salt in the decomposing cell, instead of sulphuric acid and water, 4 pairs 

 of those plates were necessary. Although the size of each plate was 

 only half a superficial inch, the minute bells of gas might be seen fop 

 hours to rise from the surfaces of both poles. 



25. The difi*erence betwixt the intensity of electricity from hydro and 

 thermo sources is well shewn in two experiments with the voltameter, 

 fig. 4, filled with a saturated solution of iodide of potassium in water, 

 and a pair of pure silver poles for the first experiment, and copper poles 

 for the second. The iodides of silver and copper, although insoluble in 

 water, are readily dissolved in a solution of the iodide of potassium. 

 The most intense thermo-electrical current I could command produced 

 not a trace of decomposition in the voltameter with silver poles and 

 iodide of potassium in solution. But if a hydro-current from a single 

 pair is passed through the same voltameter for a few minutes, a new 

 compound is formed, the iodide of silver dissolved in iodide of potas- 

 sium ; this a thermo-current can readily decompose, so that in the vol- 

 tameter, where before no effect 'could be produced by this agent, silver 

 now rapidly deposits on the negative pole through its influence. With 

 copper poles instead of silver, the second experiment differs from the 

 first, in so far, that there is a minute action by a thermo-current, before 

 a hydro- current has been passed through the voltameter ; but after the 

 application of a hydro-current, the rate of deposition of the metal un- 

 dergoes a marked change, and the iodide of copper is now found in the 

 solution. 



26. With the voltameter, fig. 4, arranged with glass-sheathed platina 

 poles, so as to expose a surface of ^^55 of a superficial inch to the elec- 

 trolite, the electrical spark appears, whenever the gas, disengaged at 

 the poles, is given off with sufficient rapidity, to envelope them in its at- 

 mosphere. Thus a surface small enough to give a spark on the pole 

 where the hydrogen appears, is often too large for the development of 

 the spark where the oxygen appears; this is clearly shewn in a volta- 

 meter, where the superficies of one pole is greater than the other, for 



