NOMOS. 23 



the revolution of a plate or cylinder against a 

 rubber, but by the chemical reactions which take 

 place between the metals of the cells and the liquids 

 with which these cells are charged. When the two 

 extremities of the battery are in connexion, the elec- 

 tricity is conducted from the zinc of the first cell 

 through the fluid to the copper of the same cell, 

 from the copper along the intervening metallic 

 bridge to the zinc of the second cell, then to the 

 copper, and so on from zinc to copper, cell after 

 cell, to the copper of the last cell, and thence back 

 again through the connecting wires to the zinc of 

 the first cell. The electricity is conducted from 

 beginning to end. There is a free current. But if 

 the wires connecting the two extremities of the 

 battery are separated, other phenomena are super- 

 added. If the ends of the wires are kept in close 

 proximity, the current continues to pass, and a vivid 

 spark appears between them. The current, in fact, 

 is formed partly by conduction and partly by dis- 

 charge. But if the ends are separated a little more, 

 the spark ceases, and with it the current ; the in- 

 tervening air has become an insulator, and the elec- 

 tricity passes from the state of current to that of 

 tension. 



What, then, is conduction ? What is insulation ? 

 What is charge and discharge ? What are the states 

 of current and tension ? It is still Dr. Faraday who 

 supplies the answer; and this answer is, that there 

 is no real and substantial difference between the 

 states of which these terms are the names. 



It seems strange that the insulating action of the 



o 4 



