The situation in 1909 is well summarised by Kurz and by Cline. Later investigations showed that the ionisation in a closed vessel was due to “penetrating radiation” partly from the walls of the vessel and partly from outside.
This showed that part of the radiation came from outside. In 1903 Rutherford and Cooke and also McLennan and Burton showed that the ionisation was significantly reduced when the closed vessel was surrounded by shields of metal kept free from radioactive impurity. An extra-terrestrial origin, though now and then discussed (for example by Richardson ), was dropped for many years. in ionisation and could therefore not support an extra-terrestrial origin.
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