Dr. Faraday's Experimental Researches in Electricity, 125 

 p being the Napierian logarithm of the base. 



log 3/ = log (Npa:) + log /"l + -|^ + |-|- + &c.J- 



= log (Npx) + -|- + &c. 



Also a: = — /^=^+&c.V 



log p = -36221 56887. 



With the help of these expressions, and the table of Brfgg's 

 logarithms to sixty-one places of decimals given in Callet, the 

 approximations may be carried much further if desired. 



XX. Experimental Researches in Electricity, — Eighth Series, 

 By Michael Faraday, D.C.L. F,R.S.Fullerian Prof. Chem, 

 Royal Institution^ Corr, Memb. Royal and Imp, Acadd, of 

 Sciences^ Paris, Petershurgh, Florence, Copenhagen, Berlin^ 

 Sfc, Si^. 



[Contiiiued from p. 45 : with an Engraving.] 



915. r>ETURNING to the consideration of the source of 

 ■•-*• electricity {878, &c.)) there is another proof of the 

 most perfect kind that metallic contact has nothing to do with 

 the production of electricity in the voltaic circuit, and further, 

 that electricity is only another mode of the exertion of che- 

 mical forces. It is, the production of the electric spark before 

 any contact of metals is made, and by the exertion of pure 

 and unmixed chemical forces. The experiment, which will be 

 described further on (956.), consists in obtaining the spark 

 upon making contact between a plate of zinc and a plate of 

 copper plunged into dilute sulphuric acid. In order to make 

 the arrangement as elementary as possible, mercurial surfaces 

 were dismissed, and the contact made by a copper wire con- 

 nected with the copper plate, and then brought to touch a 

 clean part of the zinc plate. The electric spark appeared, 

 and it must of necessity have existed and passed before the 

 zinc and the copper were in contact. 



916. In order to render more distinct the principles which 



