ELECTRIC SPARK DIRECT FROM CHEMICAL ACTION. 445 



957. Being thus arranged, there was no chemical action whilst the plates were not 

 connected. On mahing the connexion, a spark was obtained *, and the solution was 

 immediately decomposed. On breaking it, the usual spark was obtained, and the de- 

 composition 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 produce the electricity of this apparatus, so these, 

 by their first contact with each other, were placed in a state of powerful tension (95 1 .), 

 which, though it could not produce the actual decomposition of the water, was able 

 to make a spark of electricity 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 dirt was present at either mercurial surface, then the first spark 

 was often feeble, and often failed, the breaking spark, however, continuing very 

 constant and bright. When a little water was put over the mercury, the spark 

 was greatly diminished in brilliancy, but very regular both on making and breaking 

 contact. Wlien the contact was made between clean platina, the spark was also very 

 small, but regular both ways. The true electric spark is, in fact, very small, and 

 when surfaces of mercury are used, it is the combustion of the metal which produces 

 the greater part of the light. The circumstances connected with the burning of the 

 mercury are most favourable on breaking contact ; for the act of separation exposes 

 clean surfaces of metal, whereas, on making contact, a thin film of oxide, or soil- 

 ing matter, often interferes. Hence the origin of the general opinion that it is only 

 when the contact is broken that the spark passes. 



959. With reference to the other set of cases, namely, those in which chemical affinf«.y 

 is exerted {9A7.), but where no transference of the power to a distance takes place, and 

 where no electric current is produced,, it is evident that forces of the most intense kind 

 must be active, and in some way balanced in their activity, during such combinations ; 

 these forces being directed so immediately and exclusively towards each other, that no 

 signs of the powerful electric current they can produce become apparent, although 

 the same final state of things is obtained as if that current had passed. It was Ber- 



* It has been universally supposed that no spark is produced on making the contact between a single pair 

 of plates. I was led to expect one from the considerations already advanced in this paper. The wire of com- 

 munication should be short ; for with a long wire, circumstances strongly affecting the spark are introduced. 



MDCCCXXXIV. 3 M 



