624 SCIENCE PROGRESS 



there are negative. We now know, as a matter of fact, that 

 in a piece of metal some of the atoms have each lost an electron, 

 so that the atomic residues are positively charged, the charge 

 being necessarily the same in amount as that carried by the 

 detached electron. 



Certain aqueous solutions, notably solutions of acids, bases, 

 and salts, likewise allow electricity to pass through them. 

 They are also called conductors, but the mechanism of the 

 process is known to differ from that in metals. In the case 

 of solutions the carriers of electricity are particles identical 

 in mass with atoms, of which some are charged positively, 

 some negatively. The solution as a whole is electrically neu- 

 tral, there being just as many positively charged atoms, or 

 ions as they are called, as there are negatively charged ions. 

 These ions are present all the time in such solutions. Their 

 presence is shown when we cause them to move by putting on 

 an electrical pressure or potential through the intermediary of 

 electrodes. It is then found that the positive ions move in 

 one direction, the negative in the opposite. Each ion is 

 attracted towards the electrode which has electricity opposite 

 in sign to that carried by the ion. When an ion reaches an 

 electrode it gives up its electricity to the electrode, the ion 

 becoming thereby discharged. It is now simply a neutral 

 atom. Several things may happen to this atom ; e.g. it may 

 be deposited on the electrode as in electro-plating, or it may 

 go off in the form of gas, as happens when we electrolyse a 

 solution of common salt and obtain gaseous chlorine at one 

 of the electrodes — each molecule of chlorine, by the way, con- 

 sisting of two atoms of chlorine which have united with one 

 another after being discharged at the electrode — or finally, 

 the discharged atom may react with the water, as happens in 

 the case of the sodium atom, giving rise to new reactions such 

 as the production of hydrogen gas and caustic soda, the latter 

 in the dissolved state. The essential difference between the 

 conduction of electricity in a metallic solid and that in a solu- 

 tion is the fact that no chemical or other change is produced 

 in the metal, whilst marked chemical changes may be pro 

 duced in the solution. 



Such facts as these lead us to think of electricity as some- 

 thing which may be added to or taken from an atom or mole- 

 cule — the latter being simply two or more atoms linked together. 



