178 Dr. Faraday's Experimental Researches in Electricity, 



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, dimi- 

 nished in brilliancy, but very regular both on making and 

 breaking contact. Whwi 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 sur- 

 faces of mercury are used, it is the combustion of the metal 

 which produces the greater part of the light. The circum- 

 stances connected with the burning of the mercury are most 

 favourable on breaking contact; for the act of separation ex- 

 poses clean surfaces of metal, whereas, on making contact, a 

 thin film of oxide, or soiling matter, often interferes. Hence 

 the origin of the general opinion that it is only when the con- 

 tact is broken that the spark passes. 



959. With reference to the other set of cases, namely, 

 those in which chemical affinity is exerted (94'7.), 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 

 Berzelius, I believe, who considered the heat and light evolved 

 in cases of combustion as the consequences of this mode of 

 exertion of the electric powers of the combining particles. 

 But it will require a much more exact and extensive know- 

 ledge of the nature of electricity, and the manner in which it is 

 associated with the atoms of matter, before we can understand 

 accurately the action of this power in thus causing their union, 

 or comprehend the nature of the great difference which it pre- 

 sents in the two modes of action just distinguished. We may 

 imagine, but such imaginations must for the time be classed 

 with the great mass of doubtful knowledge (876.) which we 

 ought rather to strive to diminish than to increase; for the 

 very extensive contradictions of this knowledge of itself shows 

 that but a small portion of it can ultimately prove true. 



960. Of the two modes of action in which chemical affinity 

 is exerted, it is important to remark, that that which pro- 

 duces the electric current is as definite as that which causes 

 ordinary chemical combination; so that in examining the^ro- 

 duction or evolution of electricity in cases of combination or 



