1887 Hertz, experimenting on production of electromagnetic waves by oscillating charges, observed that ultraviolet light could assist a detector to pick up a transmitter signal.
1888 Hallwachs showed that UV light caused a negatively charged zinc plate to lose its charge. He concluded that this was due to freeing of negative charges from the plate. He also showed that visible light did not have the same effect as UV light on the zinc. Read about whether Hertz or Hallwachs should be given credit for discovering the photoelectric effect.
1889 Elster and Geitel showed that the photoelectric effect (discharge of negative charges from a metal exposed to light) occurred with visible light on alkali metals whereas most other metals needed ultra-violet (higher energy) radiation for discharge of electrons.
1899 J.J.Thomson and Lenard independently showed that the discharged negative particles had the same charge/mass ratio as cathode ray particles.
1905 Albert Einstein provided a daring extension of Planck's quantum hypothesis and was able to explain the photoelectric effect in detail. It was officially for this explanation of the photoelectric effect that Einstein received the Nobel Prize in 1921.
So now electrons were seen to be obtainable from metals, as well as passing through gases.