284 CONDUCTION. SECT. XXVIII. 



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 the thumb, and retains it for a long time. All these cir- 

 cumstances are modified by the temperature of the substances, 

 the state of their surfaces and that of the atmosphere. Several 

 crystalline bodies become electric when heated, especially tour- 

 maline, one end of which acquires positive, and the other nega- 

 tive electricity, while the intermediate part is neutral. If the 

 tourmaline be broken through the middle, each fragment is found 

 to possess positive electricity at one end and negative at the 

 other. Electricity is evolved by substances passing from a liquid 

 to a solid state, and by chemical action during the production and 

 condensation of vapour, which is a great source of atmospheric 

 electricity. In short, it may be generally stated, that when any 

 cause whatever tends to destroy molecular attraction there is a 

 development of electricity ; if, however, the substances be not 

 immediately separated, there will be an instantaneous restoration 

 of equilibrium. 



Electricity may be transferred from one body to another in the 

 same manner as heat is communicated, and like it too the body 

 loses by the transmission. 



Although no substance is altogether impervious to electricity, 

 nor is there any that does not offer some resistance to its passage, 

 yet it moves with more facility through a certain class of sub- 

 stances called conductors, such as metals, water, the human 

 body, &c., than through atmospheric air, glass, silk, &c., which 

 are therefore called non-conductors. The conducting power is 

 affected both by temperature and moisture. The terrestrial 

 globe is a conductor on account of its moisture, though dry 

 earth is not. Though metals are the best conductors of electricity, 

 it affects their molecular structure, for the heat which accom- 

 panies its passage acts as a transverse expansive force, which 

 increases their breadth by diminishing their length, as may be 

 seen by passing electricity through a platinum wire sufficiently 

 thick to resist fusion. Through air the force is disruptive on 

 account of its non-conducting quality, and it seems to act chemi- 

 cally on the oxygen, producing the substance known as ozone 

 during its passage through the atmosphere. If a conductor be 

 good and of sufficient size the electricity passes imperceptibly 



