The changes in the character of the discharge may be studied by means of an apparatus known as the “electric egg.” This consists of an egg-shaped bulb of glass, having its base connected with an air-pump. Two brass rods project into the bulb, one at each end; the lower rod being a fixture, while the upper one is arranged to slide in and out, so that the distance between the balls can be varied. The outer ends of the rods are connected to an induction coil or to a Wimshurst machine. If the distance between the balls has to be, say, half an inch, to produce a spark with the air at normal pressure, then on slightly reducing the pressure by means of the air-pump it is found that a spark will pass with the balls an inch or more apart. The brilliance of an electric spark is due to the resistance of the air, and as the pressure decreases the resistance becomes smaller, so that the light produced is much less brilliant. If the exhaustion is carried still further the discharge becomes redder in colour, and spreads out wider and wider until it loses all resemblance to a spark, and becomes a luminous glow of a purple or violet colour. At first this glow seems to fill the whole bulb, but at still higher vacua it contracts into layers of definite shape, these layers being alternately light and dark. Finally, when the pressure becomes equal to about one-millionth of an atmosphere, a luminous glow surrounds the cathode or negative rod, beyond this is dark space almost filling the bulb, and the walls of the bulb between the cathode and the anode glow with phosphorescent light. This phosphorescence is produced by rays coming from the cathode and passing through the dark space, and these rays have been given the name of “cathode rays.”