SECT, xxix.] VOLTAIC ELECTKICITY. 337 



visible 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, chemical 

 action will immediately take place, and gas will continue 

 to rise from the extremities of both wires till the water has 

 vanished. If an electric spark be then sent through the tube, 

 the water will reappear. By arranging the experiment so as 

 to have the gas given out by each wire separately, it is found 

 that water consists of two volumes of hydrogen and one of 

 oxygen. The hydrogen 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 re- 

 markable era in the history of science. Soda, potass, lime* 

 magnesia, and other substances heretofore considered to be 

 simple bodies incapable of decomposition, were resolved by 

 electric agency into their constituent parts, and proved to 

 be metallic oxides, by that illustrious philosopher. All 

 chemical changes produced by the electric fluid are accom- 

 plished on the same principle; and it appears that, in 

 general, combustible substances, metals, and alkalies go to 

 the negative wire, while acids and oxygen are evolved at 

 the positive. The transfer of these substances to the poles 

 is not the least wonderful effect of the Voltaic battery. 

 Though the poles be at a considerable distance from one 

 another, nay, even in separate vessels, if a communication be 

 only established by a quantity of wet thread, as the decom- 

 position proceeds, the component parts pass through the 

 thread in an invisible state, and arrange themselves at their 



