36 CORRELATION OF PHYSICAL FORCES. 



of the fragments cannot be assumed to be molecularly identi 

 cal. 



The developement of electricity by the common electrical 

 machine arises, as far as I can understand it, from the sepa* 

 ration or rupture of contiguity between dissimilar bodies ; a 

 metallic surface, the amalgam of the cushion, is in contact 

 with glass ; these two bodies act upon each other by the force 

 of cohesion ; and when, by an external mechanical force, 

 this is ruptured, as it is at each moment of the motion of the 

 glass plate or cylinder, electricity is developed in each ; were 

 they similar bodies, heat only would be developed. 



According to the experiments of Mr. Sullivan electricity 

 may be produced by vibration alone if the substance vibra 

 ting be composed either of dissimilar metals, as a wire partly 

 of iron and partly of brass caused to emit a musical sound ; 

 or of the same metal, if its parts be not homogeneous, as a 

 piece of iron, one portion of which is hard and crystallised 

 and the other soft and fibrous ; the current resulting appears 

 to be due to the vibration, and not to heat engendered, as it 

 ceases immediately with the vibration. 



We may say, then, that in our present state of knowledge, 

 where the mutually impinging bodies are homogeneous, heat 

 and not electricity is the result of friction and percussion ; 

 where the bodies impinging are heterogeneous, we may safely 

 state that electricity is always produced by friction or percus 

 sion, although heat in a greater or less degree accompanies 

 it ; but when we come to the question of ratio in which fric- 

 tional electricity is produced, as determined by the different 

 characters of the substances employed, we find very complex 

 results. Bodies may differ in so many particulars which in 

 fluence more or less the developement of electricity, such as 

 their chemical constitution, the state of their surfaces, their 

 state of aggregation, their transparency or opacity, their 

 power of conducting electricity, &c., that the normce of their 

 action are very difficult of attainment. As a general rule, it 



