SCIENCE AND PRACTICE. 217 



circuit. If we close the circuit by connecting the upper 

 end of a by a wire, c, with the upper end of b, a combina- 

 tion of their opposite electricities takes place in consequence. 

 The positive electricity goes from the copper pole to the zinc, 

 and the negative from the zinc pole to the copper, outside tl\e 

 clement. 



At the same time the component atom of oxygen is 

 separated from the first atom of water and combines with 

 an atom of zinc ; the atom of hydrogen of No. 1 combines 

 with its neighbouring atom of oxygen of No. 2 ; the 

 hydrogen of 2 with the oxygen of 3 ; and so on, until 

 the copper plate is reached, where an atom of hydrogen 

 is liberated and, being positive, is attracted by the copper. 

 The oxygen liberated from atom No. 1, combining with 

 the more powerful electro^motor, zinc, forms oxide of zinc ; 

 and the hydrogen liberated at the copper collects on the 

 plate in the form of gas until the bubbles are large enough 

 to rise to the surface of the liquid. An essential condition 

 to the formation of such an element is, therefore, mobility 

 of the medium in which the plates are plunged. Were 

 they, for example, contained in dry ice, or water in a solid 

 state, no electro-motion could occur. 



The polarisation of the atoms of water depends upon the 

 difference in the degree of polarisation of the zinc and 

 copper plates ; and the rate of transfer of the atoms, or 

 the galvanic current, depends upon the affinity of the more 

 powerful electro-motor for the liberated oxygen. 



The oxidation of the zinc plate is therefore a measure of 

 the current as is also the volume of hydrogen liberated at the 

 copper plate. 



The hydrogen liberated at the copper plate collects here 

 by degrees until it totally covers the immersed surface, 

 attracted to it by its opposite polarity. Now Buff has 

 proved that hydrogen is more positive than zinc ; the con- 

 sequence is that, after a time, the positive polarity of the 

 zinc plate is opposed on the other side of the element by 

 the greater positive polarisation of a large surface of 

 hydrogen gas. The atoms, 1, 2, 3, &c., cease, therefore, to 



