24 CORRELATION OF PHYSICAL FORCES. 



If this class of phenomena be electrical, it is electricity 

 determined as it is generated ; there is no dual character 

 impressed on the matter acting, the flash is electrical, as a 

 spark from the percussion of flint is electrical, or as the slow 

 combustion of phosphorus, or any other case of the develop- 

 ment of heat and light. It seems to be better to class this 

 phenomenon under the categories of heat and light than 

 under that of electricity, the latter word being retained for 

 those cases where a dual or polar character of force is mani- 

 fested. In experiments which have been made by the friction 

 of similar substances where the one appears positively and 

 the other negatively electrical, there will be found some 

 difference in the mode of rubbing, by which the molecular 

 state of the bodies is in all probability changed, making one 

 a dissimilar substance from the ether ; thus it is said by 

 Bergman n, that when two pieces of glass are rubbed so that 

 all the parts of one pass over one part of the other, the former 

 is positive and the latter negative. It is obvious that in this 

 case the rubbing in one is confined to a line, and that must 

 be more altered in molecular structure at the line of friction 

 than the one where the friction is spread over the whole 

 surface : so if a ribbon be drawn transversely over another 

 ribbon, the substances are not, qua the rubbing action, identi- 

 cal ; so again, in the rupture of crystals, we are dealing with 

 substances having a polar arrangement of particles the sur- 

 faces of the fragments cannot be assumed to be molecularly 

 identical. 



The development of electricity by the common electrical 

 machine arises, as far as I can understand it, from the separa- 

 tion 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 vibrating 

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



