Niels Bohr and John A. Wheeler applied the liquid drop model developed by Bohr and Fritz Kalckar to explain the mechanism of nuclear fission. Bohr had an epiphany that the fission at low energies was principally due to the uranium-235 isotope, while at high energies it was mainly due to the more abundant uranium-238 isotope. The former makes up just 0.7% of natural uranium, while the latter accounts for 99.3%. Frédéric Joliot-Curie and his Paris colleagues Hans von Halban and Lew Kowarski raised the possibility of a nuclear chain reaction in a paper published in "Nature" in April 1939. It was apparent to many scientists that, in theory at least, an extremely powerful explosive could be created, although most still considered an atomic bomb an impossibility. The term was already familiar to the British public through the writings of H. G. Wells, in his 1913 novel "The World Set Free".