RADIOACTIVITY— RUTHERFORD lg3 



data based on the kinetic theory of gases. There was, however, little 

 definite information to form any idea of the structure of atoms, 

 although theoretical physicists like Larmor and Lorentz, in order to 

 account for the vibrating properties of the atom as shown by its line 

 spectrum, had suggested that the atom must consist of charged 

 particles, but there was no evidence of the nature of the particles 

 concerned. This difficulty, as we now know, was in part resolved 

 by the discovery of the electron and the interpretation of the 2^e- 

 man effect. At this stage, although the relative atomic weights of 

 many of the elements had been accurately measured, the ideas of 

 atoms were very vague and uncertain. Although an enormous 

 amount of information on the combining properties of atoms had 

 been collected, and simple and useful working rules had been applied 

 in explanation, more definite ideas of the underlying meaning of 

 chemical combination had to await a much clearer conception of the 

 electronic structure of atoms. 



EARLY EXPERIMENTS IN RADIOACTIVITY 



My introduction to the subject of radioactivity began in a natural 

 way in the Cavendish Laboratory, Cambridge, in 1897 as the result 

 of earlier experiments on the ionization produced in gases by X-rays. 

 Becquerel had shown that the radiation from uranium caused the 

 discharge of an electroscope, but had concluded that the radiation 

 was different from X-rays in showing some evidences of refraction 

 and polarization. I proceeded to examine whether the ionization 

 was of the same type as that produced by X-rays and in the course 

 of the work found that the rays were of two types, one easily ab- 

 sorbed, called the a-rays, and a more penetrating type, named the 

 /?-rays. A few observations were also made on the rays from 

 thorium, which Schmidt in 1898 had found to be radioactive. Soon 

 after my appointment to McGill University, Montreal, in 1898, Prof. 

 R. B. Owens and I began some experiments on the radiations from 

 thorium, using the electrical method. We found that the effects 

 produced by some thorium compounds, and particularly the oxide, 

 appeared to be very capricious and much influenced by slight 

 draughts of air in the testing vessel. Strong ionizing effects were 

 observed when thoria was covered witli several sheets of paper, but 

 only weak effects when the preparation was completely covered over 

 by a thin sheet of mica. This peculiar inconsistency of the effects 

 from thorium was at first very puzzling, as under the same condi- 

 tions the radioactivity shown by uranium was quite constant. 



In order to investigate the matter further, I arranged to pass a 

 current of air over the thoria down a long tube and to examine the 

 conductivity of the air in a large ionization chamber by means of 



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