On the Excitement of Foltaic Plates. 259 



" turned rapidly the winch for a little while," I doubt not that 

 a polished glass surface would easily be excited by a gioinid gla>,s 

 surface : for as the griudingof glass so alters the relation it bears 

 to woollen cloth, that in its poHshed state it becomes positive, 

 and in its rough state it becomes negative, by friction with it, — 

 so do I suppose that the grinding of one of two pieces of glass will 

 so alter their relations, in point of electricity, to one another, 

 that friction between them may occasion excitement. Again: 

 there is a great variety in glass, arising from the materials of 

 which it is manufactured, and we are not satisfied that the 

 pieces were in the first instance precisely similar. 1 think also 

 that it might be shown that in the experiment with the ribbons 

 (analogous to those performed a long time since by Mr. Berg- 

 man with skains of silk), from the manner they were made to act 

 on each other, there was some difference as to siirface. I will 

 not, however, urge this point any further: I would not wish that 

 the analogy I have attempted to draw, should be supported by 

 finely drawn ingenious suppositions ; but that, if it be found to 

 hold generally true, it should not be rejected on account of one 

 or two anomalous exceptions:— and if it be ascertaineU that all 

 considerable degrees of excitement by friction are obtained by the 

 action on each other of dissimilar bodies, it will not be alto- 

 gether unphilosophical to suspect that some minute causes, not 

 cognisable to our senses, or not easily understood, have operated, 

 and tended to produce the anomaly in those cases, in which very 

 minute degrees of excitement are observable from the action 

 on each other of bodies apparently similar. 



In explaining the views I had taken of some points of electri- 

 cal science, 1 cautiously avoided using any expressions which 

 would involve an hypothesis ; and vvlicn I was constrained from 

 long usage and the want of better terns to do so, 1 accom- 

 panied those expressions with one or two remarks, which I have 

 been rather surprised to find has been the object of censure. 

 Mr. De Luc considers not only that " the existence of an electric 

 fluid has been demonstrated by a long series of experiments," but 

 that it is composed of " many ingredients," that "besides light, 

 fire, and an odorate substance, there arc other ingredients in 

 the electric fluid, one of which well determined is alnost tenu- 

 ous fluid, which imparts its strong expansibility to the others, 

 and is the cause of the phenomena called electric influences." 

 This fluid is called "vector, as giving motion to the inexpansive 

 substance which constitutes the density." Filitlicrujore, that 

 it has been " demonstrated that electric motiuus are produced 

 only by the substance con-,tiluting the density, without ;inv par- 

 ticipation of the fluid producing the electric 'influences." ' I am 

 Killy aware, sir, that Mr. Do Luc has written all this, and has 

 I^ ^ instituted 



