The discovery that the β rays of the radio-elements were similar to the cathode rays produced in a vacuum tube was an important advance, and has formed the basis of several subsequent theories. J. Perrin[333], in 1901, following the views of J. J. Thomson and others, suggested that the atoms of bodies consisted of parts and might be likened to a miniature planetary system. In the atoms of the radio-elements, the parts composing the atoms more distant from the centre might be able to escape from the central attraction and thus give rise to the radiation of energy observed. In December 1901, Becquerel[334] put forward the following hypothesis, which, he stated, had served him as a guide in his investigations. According to the view of J. J. Thomson, radio-active matter consists of negatively and positively charged particles. The former have a mass about ¹⁄₁₀₀₀ of the mass of the hydrogen atom, while the latter have a mass about one thousand times greater than that of the negative particle. The negatively charged particles (the β rays) would be projected with great velocity, but the larger positive particles with a much lower velocity forming a sort of gas (the emanation) which deposits itself on the surface of bodies. This in turn would subdivide, giving rise to rays (excited activity). In a paper communicated to the Royal Society in June 1900, Rutherford and McClung[335] estimated that the energy, radiated in the form of ionizing rays into the gas, was 3000 gram-calories per year for radium of activity 100,000 times that of uranium. Taking the latest estimate of the activity of a pure radium compound as 2,000,000, this would correspond to an emission of energy into the gas in the form of α rays of about 66,000 gram-calories per gram per year. The suggestion was made that this energy might be derived from a re-grouping of the constituents of the atom of the radio-elements, and it was pointed out that the possible energy to be derived from a greater concentration of the components of the atom was large compared with that given out in molecular reactions. In the original papers[336] giving an account of the discovery of the emanation of thorium and the excited radio-activity produced by it, the view was taken that both of these manifestations were due to radio-active material. The emanation behaved like a gas, while the matter which caused excited activity attached itself to solids and could be dissolved in some acids but not in others. Rutherford and Miss Brooks showed that the radium emanation diffused through air like a gas of heavy molecular weight. At a later date Rutherford and Soddy showed that the radium and thorium emanations behaved like chemically inert gases, since they were unaffected by the most drastic physical and chemical treatment.