CHAP, i.] ELECTRICITY AND MAGNETISM. 71 



produce electrification in bodies. The most important 

 of these are friction, heat, chemical action, magnetism, 

 and the contact of dissimilar substances. We noted 

 that the production of electricity by friction depended 

 largely upon the molecular condition of the surfaces. 

 We may here add that the difference of potentials pro- 

 duced by contact of dissimilar substances also varies 

 with the temperature and with the nature of the medium 

 (air, vacuum, etc.) in which the experiments are made. 

 Doubtless this source also depends upon the molecular 

 conditions of dissimilar substances being different ; the 

 particles at the surfaces being of different sizes and 

 shapes, and vibrating with different velocities and with 

 different forces. There are (see Art. 10) good reasons 

 for thinking that the electricity of friction is really due 

 to electricity of contact, excited at successive portions of 

 the surfaces as they are moved over one another. But 

 of the molecular conditions of bodies which determine 

 the production of electricity where they come into con- 

 tact, little or nothing is yet known. 



