Paul Becquerel was a French plant physiologist who, working in the cryogenic laboratory of Kamerlingh Onnes in Leiden and inspired by Arrhenius's ideas on panspermia, showed that bacteria retained their germinating power for as long as two years under extreme cold in a vacuum. He also found, however, that the same strains were quickly killed by ultraviolet radiation, thus contributing to the experimental data which cast serious doubt of the panspermia hypothesis.