336 VOLTAIC ELECTRICITY. [SECT. xxix. 



is requisite to enable the electricity to force its way through 

 the air on bringing the wires together from the opposite 

 poles. Its transit is accompanied by light; and, in con- 

 sequence of the continuous supply of the fluid, sparks occur 

 every time the contact of the wires is either broken or 

 renewed. The most splendid artificial light known is pro- 

 duced by fixing pencils of charcoal at the extremities of the 

 wires, and bringing them into contact. This light is the 

 more remarkable, as it appears to be independent of com- 

 bustion, since the charcoal suffers no change, and, likewise, 

 because it is equally vivid in such gases as do not contain 

 oxygen. Though nearly as bright as solar light, it differs 

 materially from it when analyzed with a prism. Professor 

 Wheatstone has found that the appearance of the spectrum of 

 the Voltaic spark depends upon the metal from whence the 

 spark is taken. The spectrum of that from mercury consists 

 of seven definite rays, separated from each other by dark in- 

 tervals ; these visible rays are two orange lines close together, 

 a bright green line, two blueish green lines near each other, 

 a very bright purple line, and lastly a violet line. The spark 

 taken from zinc, cadmium, tin, bismuth, and lead in the 

 melted state, gives similar results ; but the number, position, 

 and colour of the lines vary so much in each case, and the 

 appearances are so different, that the metals may be easily 

 distinguished from each other by this mode of investigation. 

 It appears, moreover, that the light does not arise from the 

 combustion of the metal ; for the Voltaic spark taken from 

 mercury successively in the vacuum of an air-pump, in the 

 Torricellian vacuum, and in carbonic acid gas, is precisely 

 the same as when the experiment is performed in the air or 

 in oxygen gas. Notwithstanding the difference between 

 electric and solar light, M. Arago is inclined to attribute 

 the intense light and heat of the sun to electrical action. 



Voltaic electricity is a powerful agent in chemical analysis. 

 When transmitted through conducting fluids, it separates 

 them into their constituent parts, which it conveys in an in- 



