William Crookes

Sir William Crookes contributed to the discovery of the electron. He investigated the flow of electricity through very low pressure gases using the apparatus shown below.

When electricity flows, regions of the gas emit light. The colour of the emitted light is characteristic of the gas being used.

Besides the light produced by the gas, the glass behind the anode glows too (with a different colour to the gas), suggesting that radiation of some sort is travelling from the cathode to the anode that is able to provide the molecules of the gas with energy. The nature of the radiation was not know and they were called cathode rays because they were emitted from the cathode. Cooke did hypothesise, correctly, that the light given off by the gas and glass respectively was as a result of collisions between the particles that constituted the radiation and molecules in the gas or glass.