258 On the ExcitemetU of Foliate Plates. 



I have already said ; — the second, the result of his own experi- 

 ments, wliich I shall presently consider. 



Before I proceed, let me observe, that although I certainly 

 have used the word electric in opposition to condiiclor, it was 

 hy no means my intention to imply that " the former only had 

 the faculty to be excited by friction ;" and lest my mode of ex- 

 pression sliould a])pear to tjthers, as well as to Mr. De Luc, to 

 involve such an hypothesis, I would wish substituted the word 

 Tion-conduclor for electric wherever the latter occurs in either of 

 n)y jiapers. 



The experiments, from which is formed Mr. De Luc's second 

 grourid of objection, are contained in a paper on the electric 

 effects of friction*, and tend to prove that Iriction between si- 

 milar bodies will produce excitement. Certaiidy as excitement 

 takes phice from the contact and separation of dissimilar bodies, 

 ^tnd docs not from the contact and separation of similar bodiesf, 

 the analogy I have endeavoured to establish will not hold, ex- 

 cept it appears that electricity is excited by the friction of dis- 

 similar bodies, and is not by the friction of similar bodies. 

 That no considerable degree of excitement can be obtained by 

 the friction of similar bodies, will 1 suppose be readilv admitted; 

 and that the fact is generally true, that it is the friction between 

 dissimilar bodies that produces excitement, the few instances 

 to the contrary being only exceptions, will also be granted ; and 

 I might therefore urge, that it is not fair to bring a few anoma- 

 lous exceptions to a general rule, in opposition to strong analogies. 

 Let me however observe, that if two bodies be jnecisely similar 

 in relation to an experiment,— and if they be made to act on each 

 other, their action must be reciprocal, — in that action the acting 

 surfaces must both be submitted to a like operation, and it is 

 altogether impossible to conceive how the effect on one can differ 

 from the effect on the other. Moreover, electrical excitement is, 

 as far as we know, confined to the surface of bodies. The in- 

 fluence of dissimilarity of surface in refeicnce to electricity is 

 fculhciently known, a difference of colour will occasion a body to 

 assume a positive or a negative charge. If a polished tube of 

 glass be excited by friction with flannel, it will be positively elec- 

 triiied ; but if the surface of the same glaL^s tube be ground and 

 submitted to the same operation, it will be negatively electrified. 

 Supposing Mr. De I-uc's experiment to have been performed 

 ^vhen the apparatus was new, and when the polish on tbe glass 

 rubber and on the glass cylinder was perfect, a very little dif- 

 ference \i\ the hardness and fineness of the two pieces of glass 

 would occasion the surfaces to be differently affected when he 



* Phjt. Journal, vol. j(xviii. f See my Exj^iciiincnts. Ibid. vol. xxix. 



*^' turned 



