Take a tumbler and fill it with water with a little salt dissolved in it. Next obtain two short pieces of wire and two pieces of pencil lead, which with a pocket lamp battery will complete the apparatus. Connect one piece of wire to each terminal of the battery and twist the other end of it round a piece of pencil lead. Place these so that the ends of the leads dip into the salt water. It is important to keep
the wires out of the solution, the leads alone dipping into the liquid, and the two leads should be an inch or so apart.
In a few moments you will observe that tiny bubbles are collecting upon the leads and these joining together into larger bubbles will soon detach themselves and float up to the surface. Those which arise from one of the leads will be formed of the gas chlorine and the others of hydrogen.
It will be interesting just to enumerate the names of the different parts of this apparatus. First let me say that the process by which these gases are thus obtained is called electrolysis: the liquid is the electrolyte: the two pieces of pencil lead are the electrodes. That electrode by which the current enters the electrolyte is called the an-ode, while the other is the cath-ode. In other words, the current traverses them in alphabetical order.
Now it is familiar to everyone that all matter is supposed to consist of tiny particles called Molecules. These are far too tiny for anyone to see even with the finest microscope, so we do not know for certain that they exist: we assume that they do, however, because the idea seems to fit in with a large number of facts which we can observe and it enables us to talk intelligibly about them. We may, accordingly, speak as if we knew for a certainty that molecules really exist.
Now when we dissolve salt in water it seems as if each molecule splits up into two things which we then call "ions." Salt is not peculiar in this respect, for many other substances do the same when dis
solved in water. All such substances, since they can be "ionized," are called "ionogens."
Now the peculiarity about ions is that they are always strongly electrified or charged with electricity.
At this stage we must make a little excursion into the realm of electricity. You probably know that if a rod of glass be rubbed with a silk handkerchief it becomes able to attract little scraps of paper. That is because the rubbing causes it to become charged with electricity. In like manner a piece of resin if rubbed will become charged and will also attract little pieces of paper. A piece of electrified resin and an electrified glass rod will, moreover, attract each other, but two pieces of resin or two pieces of glass, if electrified, will repel each other. This leads us to believe that there are two kinds of electrification or two kinds of electrical charge. At first these two kinds were spoken of as vitreous or glass electricity and resinous electricity, but after a while the idea arose that there was really one kind of electricity and that everything possessed a certain amount of it, the electrified glass having a little too much of it and the electrified resin a shade too little of it. From this came the idea of calling the charge on the glass a "positive" charge and that on the resin a "negative" charge. Recent investigations seem to show that we have got those two terms the wrong way round, but to avoid confusion we still use them in the old way.
It will be sufficient for our purpose, therefore, if we assume that every molecule of matter has a certain normal amount of electricity associated with it and