SECT. XXVIII. ELECTRICAL LIGHT. 289 



from solar light. Fraunhofer found that, instead of the fixed dark 

 lines, the spectrum of an electric spark is crossed by numerous 

 bright lines ; and Professor Wheatstone has observed that the 

 number and position of the lines differ with the metal from 

 which the spark is taken, and believes the spark itself results 

 from the ignition and volatilization of the matter of the con- 

 ductor. 



According to the experiments of Sir Humphry Davy, the 

 density of the air has an influence on the colour. He passed the 

 electric 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 matter. HB thought 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. 



The velocity of electricity is so great that the most rapid 

 motion which can be produced by art appears to be actual rest 

 when compared with it. A wheel revolving with celerity suffi- 

 cient to render its spdkes invisible, when illuminated by a flash 

 of lightning, is seen for 'an instant with all its spokes distinct, as 

 if it were in a state of absolute repose ; because, however rapid 

 the rotation may be, the light has come and already ceased before 

 the wheel has had time to turn through a sensible space. This 

 beautiful experiment is due to Professor Wheatstone, as well as 

 the following variation of it, which is not less striking : If a 

 circular piece of pasteboard be divided into three sectors, one of 

 which is painted blue, another yellow, and a third red, it will 

 appear to be white when revolving quickly, because of the 

 rapidity with which the impressions of the colours succeed each 

 other on the retina. But, the instant it is illuminated by an 

 electric spark, it seems to stand still, and each colour is as distinct 

 as if it were at rest. This transcendent speed of electricity has 



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