Even then a higher pressure of electricity is required than is used for lighting the filament in the ordinary way. Instead of having the two platinum wires side by side, they are usually placed at opposite ends of the tube when the ordinary experiments with vacuum tubes are being performed. Also, instead of ending in sharp points the platinum wires usually end in small discs or caps. When a current is sent through a tube of this kind the trace of gas in the tube shines with a strange light. The light usually shows some beautiful colours, and is arranged in layers. These layers are generally in the neighbourhood of the positive wire, which is called the anode. At the negative wire we find a layer of white, or violet, light, and next to this a dark space, which separates it from the glowing space beyond. When special care is taken to get rid of as much gas as possible, the dark space expands until it fills the whole of the tube. Then a strange thing happens. A green glow plays on the whole of the tube opposite the cathode, and it is in this green glow that the X-rays are born. But before we describe how the X-rays originate, we may say a few words of what happens in the vacuum tube, and gives rise to the green glow. If we could see the cathode through a microscope of a magnifying power greatly beyond that of any existing microscope, we should find it to be the scene of a very intense activity. We should see great jets of small particles start from the cathode at a tremendous speed, and shoot away in straight lines until they reached the glass wall opposite, there producing the green glow. These small particles are thought to be the real particles of electricity which constitute the electric current. An electric current consists of a stream of these extremely small particles, which are called “electrons.” They are much smaller than the atoms of a wire; and thread their way slowly between the atoms, knocking against them every now and then, and producing a great deal of heat. The same number of electrons passes through every section of the whole circuit in the same time, and naturally, when a wire in the electric circuit is very thin the electrons must pass at a greater speed in order to get through and make room for those that come after them. In a thin wire, therefore, the heat is very great, and when the wire becomes a filament, the heat is great enough to make it shine with a bright light. This gives us an electric glow-lamp.