280 SOURCES OF ELECTRICITY. SECT. XXVIII. 



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 attributes 

 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 crys- 

 taline substances become electric when heated, es- 

 pecially tourmaline, one end of which acquires positive 

 and the other negative electricity, while the interme- 

 diate partis neutral. If a tourmaline be broken through 

 the middle, each fragment is found to possess positive 

 electricity at one encH 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 vapor, 

 which is consequently a great source of atmospheric 

 electricity. The steam issuing from the valve of an 

 insulated locomotive steam engine produces seven times 

 the quantity of electricity that an electrifying 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 <4use 

 whatever, such as friction, pressure, heat, fracture, 

 chemical action, &c., tends to destroy molecular attrac- 

 tion, there is a development of electricity. If, however, 

 the molecules be not immediately separated, there will 

 be an instantaneous restoration 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 the day than in the night. The inten- 

 sity increases for two or three hours from the time of 

 sunrise, comes to a maximum between seven and eight, 

 then decreases toward the middle of the day, arrives at 

 its minimum between one and two, and again augments 

 as the sun declines, till about the time of sunset, after 

 which it diminishes, and continues feeble during the 



