(1) X-rays, gamma rays from radioactive elements, cosmic rays which bombard our earth day and night from all sides and have their origin beyond our galaxy, the ultra-violet rays from the sun and the electrons emitted by the sun etc. all have a leading role in the gas ionization. The first three belong to the spectrum of electromagnetic radiations. They are of the same nature as light but as much shorter in wave length. The upper regions of the atmosphere are ionized by ultra-violet and cosmic radiations. In this ionization, these radiations are absorbed to a greater extent, so that the lower regions are protected from their determined effect. No matter, the air is ionized to a certain extent by these natural agents but the ionization in the lower regions of the atmosphere is not sufficient to interfere with ordinary electrostatic experiments. Such a natural ionization is known as spontaneous ionization. Under natural ionization only one in about 10^16 atoms is ionized.
(2) The emanations from radio-active materials, viz alpha particles, beta particles can also ionize a gas through which they pass. If a small chunk of radio active ,material (radium ) be brought near a charged electroscope, it is found to discharge very soon, thus predicting the production of electrons and ions in air by it.
(3) It is also possible to ionize the gases by subjecting them to high temperature, the atoms of a strongly heated gas part with their electrons on account of violent and random collisions they suffer among themselves under intense thermal agitation.
(4) An electric discharge passing through a gas-column makes it partially or wholly ( depending on the strength of the discharge ) ionized. It is seen that after the first spark the discharge takes place more readily ( and with a lower applied P.D ) due to ionization.
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