296 VOLTAIC ELECTRICITY. SECT. XXIX. 



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 color 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 invisisible state through a considerable 

 space or quantity of liquid to the poles, where they 

 come into evidence. Numerous instances might be 

 given, but the decomposition of water is perhaps the 

 most simple and elegant. Suppose a glass tube filled 

 with water and corked at both ends ; if one of the wires 

 of an active Voltaic battery be made to pass through 

 one cork and the other through the other cork, into the 

 water, so that the extremities of the two wires shall be 

 opposite and about a quarter of an inch asunder, chemi- 

 cal action will immediately take place, and gas will con- 

 tinue to rise from the extremities of both wires till the 

 water has vanished. If an electric spark j^e then sent 

 through the tube, the water will reappear. By arrang- 

 ing the experiment so as to have the gas^iven out by 

 each wire separately, it is found that water consists of 

 two volumes of hydrogen and one of oxygen. The hy- 

 drogen is given out at the positive wire of the battery, 

 and the oxygen at the negative. The oxides are also 

 decomposed ; the oxygen appears at the positive pole, 

 and the metal at the negative. The decomposition of 

 the alkalies and earths by Sir Humphry Davy formed 

 a remarkable era in the history of Science. Soda, 

 potass, lime, magnesia, and other substances heretofore 



