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I got the following extract from a chemistry book (emphasis mine):

It is observed that current does not flow through the gas at ordinary pressure even at high voltage of 5000 volts. When the pressure inside the tube is reduced and a high voltage of 5000–10000 volts is applied, then an electric discharge takes place through the gas producing a uniform glow inside the tube. When the pressure is reduced further to about 0.01 torr, the original glow disappeares. Some rays are produced which create fluorescence on the glass wall opposite to the cathode.

It's about cathode rays discovery in a chapter on atomic structure. Here the author is talking about a uniform glow appearing when the pressure was reduced. Then he says that the rays (which he then refers to as cathode rays) appear after the disappearance of the glow. I have never noticed this glow in any experiment. Does any body know what this is?

The uniform glow is due to ionization and recombination of the residual gas; it's called a glow discharge. At higher pressure, a spark or arc discharge occurs at much higher current density. Fluorescent lamps and neon lamps operate in the glow-discharge region, and high-pressure xenon lamps use an arc discharge.

Elements of the residual gas can be identified by the color of the glow discharge. Nitrogen and argon in air, for example, glow purplish blue.

As pressure decreases further, density is too low (and path too short) to produce a visible glow discharge, but electrons, AKA "cathode rays", hurled off the electrodes impact the walls of the container, which may glow green in borosilicate glass, and in very low pressure vacuum tubes, such as the 1G3GT high-voltage rectifier, the electrons impacting the anode produce X-rays (high energy photons), which may also create fluorescence in the glass shell.

The difference is easy to see in this video, which demonstrates the change in appearance from an arc discharge to glow discharge, and finally to fluorescence of the glass envelope due to X-rays or electron bombardment. The arc starts at ~20 seconds, the glow discharge at ~26 seconds, striations form ~36 seconds, and by ~46 seconds, the glass envelope glows green from electron bombardment (though it might rathet be the green of residual oxygen). This was one of my favorite demonstrations to show students the basis for spectroscopy!