Electric Spark direct from Chemical Action. 177 



956. There is, however, one beautiful experimental proof 

 of a state of tension acquired by the metals and the electro- 

 lyte before the electric current is produced, and before contact 

 of the different metals is made (915.) ; in fact, at that moment 

 when chemical forces only are efficient as a cause of action. 

 I took a voltaic apparatus, consisting of a single pair of large 

 plates, namely, a cylinder of amalgamated zinc, and a double 

 cylinder of copper. These were put into ajar containing di- 

 lute sulphuric acid*, and could at pleasure be placed in me- 

 tallic communication by a copper wire adjusted so as to dip 

 at the extremities into two cups of mercury connected with 

 the two plates. 



957. Being thus arranged, there was no chemical action 

 whilst the plates were not connected. On making the con- 

 nexion, a spark was obtained f, and the solution was imme- 

 diately decomposed. On breaking it, the usual spark was 

 obtained, and the decomposition ceased. In this case it is 

 evident that the first spark must have occurred before metallic 

 contact was made, for it passed through an interval of air, and 

 also that it must have tended to pass before the electrolytic 

 action began ; for the latter could not take place until the 

 current passed, and the current could not pass before the 

 spark appeared. Hence I think there is sufficient proof, that 

 as it is the zinc and water which by their mutual action pro- 

 duce the electricity of this apparatus, so these, by their first 

 contact with each other, were placed in a state of powerful 

 tension (951.), which, though it could not produce the actual 

 decomposition of the water, was able to make a spark of elec- 

 tricity pass between the zinc and a fit discharger as soon as 

 the interval was rendered sufficiently small The experiment 

 demonstrates the direct production of the electric spark from 

 pure chemical forces. 



958. There are a few circumstances connected with the 

 production of this spark by a single pair of plates, which 

 should be known, to ensure success to the experiment. When 

 the amalgamated surfaces of contact are quite clean and dry, 

 the spark, on making contact, is quite as brilliant as on 

 breaking it, if not even more so. When a film of oxide or 



♦ When nitro-sulphuric acid is used, the spark is more powerful, but 

 local chemical action can then commence, and proceed without requiring 

 metallic contact. 



f It has been universally supposed that no spark is produced on making 

 the contact between a single pair of plates. 1 was led to expect one from 

 the considerations already advanced in this paper. The wire of communi- 

 cation should be short; for with a long wire, circumstances strongly af- 

 fecting the spark are introduced. 



Third Series. Vol. 6. No. 33. March 1835. 2 A 



