SECT, xxvni.] SOURCES OF ELECTRICITY. 319 



sulate them to prevent its escape. When two substances of 

 any kind whatever are insulated and pressed together, they 

 assume different electric states, but they only show contrary 

 electricities when one of them is a good conductor. When 

 both are good conductors, they must be separated with extreme 

 rapidity, to prevent the return to equilibrium. When the 

 separation is very sudden, the tension of the two electricities 

 may be great enough to produce light. M. Becquerel at- 

 tributes the light produced by the collision of icebergs to this 

 cause. Iceland spar is made electric by the smallest pressure 

 between the finger and thumb, and retains it for a long time. 

 All these circumstances are modified by the temperature of 

 the substances, the state of their surfaces, and that of the 

 atmosphere. Several crystalline substances become electric 

 when heated, especially tourmaline, one end of which acquires 

 positive, and the other negative electricity, while the inter- 

 mediate part is neutral. If a tourmaline be broken through 

 the middle, each fragment is found to possess positive electri- 

 city at one end, and negative at the other, like the entire 

 crystal. Electricity is evolved by bodies passing from a 

 liquid to a solid state ; also by chemical action during the 

 production and condensation of vapour, which is consequently 

 a great source of atmospheric electricity. The steam issuing 

 from the valve of an insulated locomotive steam engine pro- 

 duces seven times the quantity of electricity that an electri- 

 fying machine would do with a plate three feet in diameter, 

 and worked at the rate of 70 revolutions in a minute. In 

 short, it may be stated generally, that when any cause what- 

 ever, such as friction, pressure, heat, fracture, chemical action, 

 &c., tends to destroy molecular attraction, there is a develop- 

 ment of electricity. If, however, the molecules be not im- 

 mediately separated, there will be an instantaneous restora- 

 tion of equilibrium. 



The earth possesses a powerful electrical tension, and the 

 atmosphere, when clear, is almost always positively electric. 

 Its electricity is stronger in winter than in summer, during 



