(1) Coulomb's experiment
"An insulated charged body exposed to atmospheric air gradually loses its charge." (Coulomb 1785)
However, at the time, it was not clear why it loses the charge.
(2) Theory of air ion conductivity
"Positively or negatively charged tiny particles can be found not only in electrolytic solutions but also in the air. Electricity flows through the air by these particles." (Discovery and Research of Becquerel and X rays)
This theory was applied to explain "Electronic conductivity in the air" by Ester, Geitel and Wilson, and resulted in better understanding of Coulomb's experiment.
These tiny particles are called "Air Ions".
Faraday named these particles "Ions".