The Romance of Chemistry lii\ 



atoms and chlorine atoms. A lump of salt is built up of a pro- 

 digous number of molecules of salt; and each molecule is "a little 

 building, of which the atoms are the bricks." 



To return to the elements. An ELEMENT has been defined 

 as a substance whose molecule contains only one kind of atom, but 

 this definition must be modified in relation to the fact that the 

 elements uranium and thorium give rise, as we shall see, to other 

 elements different from themselves. Nevertheless the general 

 idea remains that an element is a unique and homogeneous kind 

 of matter. 



When an electric current is passed through water (HzO) it 

 decomposes it into hydrogen and oxygen. Bubbles of oxygen 

 may be collected at one pole, and bubbles of hydrogen at the other 

 pole. This is a fact. There is a theory perhaps more than a 

 theory that free atoms of oxygen travel in one direction and free 

 atoms of hydrogen in the opposite direction through the water. 

 "Ion," the Greek word for a traveller, is the term applied to these 

 particles which travel to the two electrodes during electrolysis. 

 Ions are travelling atoms, or groups of atoms, which are started on 

 their journey by the dissociation of the electrolyte (say, water), 

 and they are believed to carry opposite charges of electricity. 

 Each molecule that is split up gives rise to two kinds of ions 

 (anions, going to the anode pole, and cations, going to the cathode 

 pole) ; and these two kinds of ions are furnished with equal and 

 opposite charges. The ion with a positive charge of electricity is 

 attracted to the cathode, or negatively charged electrode, and the 

 ion with the negative charge is attracted to the anode, or positively 

 charged electrode, and each ion will be then relieved of its charge 

 and become an ordinary atom again. The speeds of the migrating 

 ions have been measured, and it seems that the heaviest ions ( i.e., 

 with the greatest "atomic" weights) move fastest. It has been 

 supposed that this is due to the more slow-going ions dragging 

 along with them a number of molecules of the solvent. 



This is very theoretical, but every one knows the practical 



