Ill 



,: 



to 



SECT. XXVIII. ELECTRICAL LIGHT. 279 



though it is only accompanied by light when the fluid is 

 obstructed in its passage. 



Electrical light, when analyzed by the prism, pre- 

 sents very different appearances to the solar light. 

 Frauenhofer found that instead of the fixed dark lines 

 of the solar spectrum, the spectrum of an electric spark 

 was crossed by very numerous bright lines ; and Pro- 

 fessor Wheatstone has observed that the number and 

 position of the lines differ with the metal from which 

 the spark is taken. According to M. Biot, electrical 

 light arises from the condensation of the air during the 

 rapid motion of the electricity r and varies both in in- 

 tensity and color with the density of the atmosphere. 

 When the air is dense, it is white and brilliant; whereas 

 in rarefied air it is diffuse and of a reddish color. The 

 experiments of Sir Humphiy Davy, however, seem to 

 be at variance with this opinion. He passed the elec- 

 tric spark through a vacuum over mercury, which, 

 from green, became successively sea-green, blue, and 

 purple, on admitting different quantities of air. When 

 the vacuum was made over a fusible alloy of tin and 

 bismuth, the spark was yellowish and extremely pale. 

 Sir Humphry thence concluded, that electrical light 

 principally depends upon some properties belonging to 

 the ponderable matter through which it passes, and 

 that space is capable of exhibiting luminous appearances, 

 though it does not contain an appreciable quantity of 

 this matter. He thought it not improbable that the 

 superficial particles of bodies which form vapor, when 

 detached by the repulsive power of heat, might be 

 equally separated by the electric forces, and produce 

 luminous appearances in vacuo, by the destruction of 

 their opposite electric states. Professor Wheatstone 

 has been led to conclude that electrical light results 

 from the volatilization and ignition of the ponderable 

 matter of the conductor itself. 



Pressure is a source of electricity which M. Becquerel 

 has found to be common to all bodies ; but it is necessary 

 to insulate them to prevent its escape. J When two sub- 

 stances 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 



