318 ELECTRICAL LIGHT. [SECT. xxvm. 



of its course. The heat produced by the electric shock is 

 intense, fusing metals, and even volatilizing substances, 

 though it is only accompanied by light when the fluid is 

 obstructed in its passage. 



Electrical light, when analyzed by the prism, presents 

 very different appearances to the solar light. Frauenhofer 

 found that, instead of the fixed dark lines of the solar spec- 

 trum, the spectrum of an electric spark was crossed by very 

 numerous bright lines; and Professor Wheatstone has ob- 

 served 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, and varies both 

 in intensity and colour 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 colour. The ex- 

 periments of Sir Humphry Davy, however, seem to be at 

 variance with this opinion. He passed the electric spark 

 through a vacuum over mercury, which, from green, became 

 successively sea-green, blue, and purple, on admitting di- 

 ferent 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 vapour, 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. Pro- 

 fessor Wheatstone has been led to conclude that electrical 

 light results from the volatilization and ignition of the pon- 

 derable matter of the conductor itself. 



Pressure is a source of electricity which M. Becquerel has 

 found to be commo'h to all bodies ; but it is necessary to in- 



