Electricity. 337 



longer to take a photograph with it. The electric arc does not heat. 

 This appears astonishing at first, for all bodies fuse or volatilize when 

 introduced into the arc. " 



" The voltaic arc is the result of the incandescence of a jet of parti- 

 cles detached from the electrodes or poles and thrown from one elec- 

 trode to the other, particularly from the positive pole to the negative 

 pole. The positive electrode has a temperature much higher than the 

 other, the negative electrode being barely red when the positive elec- 

 trode is at a white heat. The positive pole is consumed at double the 

 rate at which the negative pole disappears when the carbons are equal 

 in size." When both electrodes are carbon points, the arc is egg- 

 shaped. u It appears as a flickering flame, and brilliant particles are 

 constantly carried between the two electrodes. " When there are min- 

 eral impurities in the carbons, they are fused and appear as melted glob- 

 ules upon the carbon points. "The voltaic arc behaves precisely as 

 any other portion of the electric circuit. It is attracted or repelled by 

 magnets in an exactly similar manner. Indeed the incandescent particles 

 constitute between the two electrodes a conductor of great mobility ; 

 and the arc may be regarded as a badly conducting chain of these parti- 

 cles, raised to incandescence in consequence of the resistance they of- 

 fer to the passage of the current. " 



We have the following immediate Sources of Electricity: Friction. 

 Percussion: As when one substance is struck violently by another 

 one becomes positive, the other negative. Vibration: As of a metal 

 rod. Disruption and Cleavage: As when a card, a paper collar, &c. , 

 is torn asunder, lumps of sugar crunched, sudden cleavage of a sheet of 

 mica, &c. Crystallization and Solidification: Many substances pass- 

 ing from a liquid to a solid state as sulphur, chocolate, arsenic acid, 

 sulphate of copper and potassium give off electrical action. Combus- 

 tion: As burning charcoal, &c. Evaporation: When evaporation 

 takes place the vapor takes one electrical state, and the remaining li- 

 quid the other. ' < A few drops of a solution of sulphate of copper 

 thrown into a hot platinum crucible produce violent electrification as 

 they evaporate. " When water is turned into vapor the vapor becomes 

 positively electrified, ( hence the charge of positive electricity in the 

 warm, tropically-originated, summer cloud.) Pressure: Many sub- 

 stances become electrified when pressed. Cork becomes positive when 

 pressed against amber, gutta-percha and metals, but negative when 

 pressed against spars and animal substances. A crystal of calc-spar 

 pressed along its blunt edges, between the dry fingers, becomes elec- 

 trical, and remains so for some days. Pyro-electricity is a name given 

 to electricity developed upon the heating or cooling of certain crystals, 

 especially Tourmaline, which, when heated, attracts light bodies. Sili- 



