MOTION. 35 



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. 



If this class of phenomena be electrical, it is electricity 

 determined as it is generated ; there is no dual character im- 

 pressed on the matter acting, the flash is electrical as a spark 

 from the percussion of flint is electrical, or as the slow com- 

 bustion of phosphorus, or any other case of the development 

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

 nomenon 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 manifested. 

 In experiments which have been made by the friction of sim- 

 ilar substances where the one appears positively and the 

 other negatively electrical, there will be found some differr 

 ence in the mode of rubbing by which the molecular state of 

 the bodies is in all probability changed, making one a dissim- 

 ilar substance from the other ; thus it is said by Bergmann, 

 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, identical; so 

 again, in the rupture of crystals, we are dealing with sub- 

 stances having a polar arrangement of particles the surfaces 



