WHAT IS ELECTRICITY ? 203 



are retained by uniting with the copper ions, when copper 

 atoms will be deposited on the copper plate. 



Just as the surface of the water forms a diaphragm 

 through which salt cannot pass, while steam can, so the 

 surface of the zinc plate forms a diaphragm through 

 which matter such as zinc, hydrogen, or chlorine ions 

 cannot pass, while electrons can, and they are also able to 

 be conveyed by the wire, as steam is conveyed through 

 the pipe. The motive power both of steam and electricity, 

 in a word, is due to their passing from a region where 

 their pressure is high to where it is low. 



