The Hydrogen Atom: Plum-pudding or Planet

The glow of neon lights is a familiar sight in our cities. They tell us where to EAT or which HONEST car or furniture dealer to buy from. They are a part of the very fabric of our society. Perhaps that is why we do not really see them and marvel at their colours. Yet, it is a very interesting phenomenon, this glow of neon lights and the colours they display. Why does the gas in such a tube glow when a current passes through it? Why is the colour of a given gas always the same? This is not the glow of a heated body like the glow of the filament of an incandescent light bulb; it is something totally different. Just how different the two phenomena are is most readily seen by passing the light from a light bulb and from a discharge tube through a prism. The light from the filament produces a more or less continuous spectrum like a rainbow. On closer inspection a few dark lines appear in the spectrum. On the other hand, the light from a discharge tube produces a series of very distinct bright lines of brilliantly pure colours. These colours are more characteristic of the gas in the tube than fingerprints are for an individual person…