MOTION. 23 



is nought. Indeed, it would be difficult to conceive the con- 

 trary. How could we possibly image to the mind or describe 

 the direction of a current from the same body to the same 

 body, or give instructions for a repetition of the experiment ? 

 It would be unintelligible to say that in rubbing to and fro 

 two pieces of bismuth, iron, or glass a current of electricity 

 circulated from bismuth to bismuth, or from iron to iron, or 

 from glass to glass ; for the question immediately occurs 

 from which bismuth to which does it circulate ? And should 

 this question be answered by calling one piece A, and the 

 other B, this would only apply to the particular specimens 

 employed, the distinctive appellation denoting a distinction 

 in fact, as otherwise A could be substituted for B, and the 

 bar to which the positive electricity flowed would in turn be- 

 come the bar to which the negative electricity flowed. We 

 may say that it circulates from rough glass to smooth, from 

 cast iron to wrought, for here there is not homogeneity. It 

 is moreover conceivable, though not experimentally proved, 

 that when the motion is continuous in a definite direction, 

 electricity may result from the friction of homogeneous bodies. 

 If A and B rub against each other, revolving in opposite 

 directions, concentric currents of positive and negative elec- 

 tricity may be conceived circulating within the metals, and 

 be described by reference to the direction of their motion ; 

 this indeed would be a different phenomenon from those we 

 have been considering ; but without some distinction between 

 the two substances in quality or direction, the electrical effects 

 are indescribable, if not inconceivable. 



When, however, homogeneous bodies are fractured or even 

 rubbed together, phenomena are observed to which the term 

 electricity is applied ; a flash or line of light appears at the 

 point of friction, which by some is called electrical, by others 

 phosphorescent. 



I have myself observed a remarkable case of the kind in 

 the caoutchouc fabric now commonly used for waterproof 

 clothing : if two folds of this substance be allowed to cohere 

 so as partly to unite and present a difficulty of separation, 

 then, on stripping the one from the other, or tearing them 

 asunder, a line of light will follow the line of separation. 



