3 CORRELATION OF PHYSICAL FORCES. 



for instance, the ends of a broken bar. Such experiments as 

 these will, indeed, be seldom free from slight electrical cur- 

 rents, on account of the practical difficulty of fulfilling the 

 condition of perfect homogeneity in the substances themselves, 

 their size, their temperature, &c. ; but the effects produced 

 are very trifling and vary in direction, and the resultant effect 

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

 trary. How could we possibly image to the mind or de- 

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

 same body, or give instructions for a repetition of the exper- 

 iment? 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 become 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, that when the motion is contin- 

 uous in a definite direction, electricity may result from the 

 friction of homogeneous bodies. If A and B nib against 

 each other, revolving in opposite directions, concentric cur- 

 rents of positive and negative electricity may be conceived 

 circulating within the metals, and be described by reference 

 to the direction of their motion ; this indeed would be a dif- 

 ferent phenomenon from those we have been considering ; but 

 without some distinction between the two substances in qual- 

 ity or direction, the electrical effects are indescribable, if not 

 inconceivable. 



When, however, homogeneous bodies are fractured or 



