WHAT IS ELECTRICITY? 199 



liquid. The electrons which they have gained have 

 followed the arrows from the zinc plate, along the copper 

 wire and down the copper plate. 



A zinc atom minus its two electrons has left the zinc 

 plate ; it is now a zinc ion. These two electrons have dis- 

 placed other electrons from their combination with zinc 

 and copper ; and it is these electrons, or their substitutes, 

 which have attached themselves to the hydrogen ions. 

 There are hydrogen and chlorine ions in the liquid. The 

 hydrogen ions move toward the copper plate, and the 

 chlorine ions toward the zinc plate, but less rapidly. 



Some of these will touch the zinc plate ; and if they 

 could pass round the circuit, through the wire, there 

 would be no electric pressure ; but it is because the plates 

 and the connecting wire are impervious to matter, while 

 they are pervious to electrons, that electric pressure or 

 to give it the usual name, electromotive force or potential 

 is developed. In fact, the metals and the wire are semi- 

 permeable membranes ; they allow electrons to pass, while 

 they block the passage of matter. 



Perhaps the idea may be somewhat more easily grasped 

 if it is put in another form. Electrons do not pass 

 through water ; probably because the treble combination 

 of electrons, hydrogen, and oxygen is too firm to allow 

 of the transference of electrons from one molecule to 

 another. But when a salt is dissolved in the water elec- 

 trons can pass, for they easily transfer themselves from one 

 place to another, carrying along with them atoms such as 

 chlorine. Their progress is much impeded thereby ; but, 

 as explained before, they are easily transmitted through 

 metals, and thus, again, electric pressure is developed. 



The analogy with ' osmotic pressure,' as the pressure of 

 the sugar molecules dissolved in water against a semi- 

 permeable membrane is called, is obvious; just as the 

 water in which the sugar is dissolved can pass in and out 



