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On 6 March 1913, Bohr sent Rutherford the first of a trilogy of papers and asked him to send it on to the Philosophical Magazine. At the time, and for many years to come, every junior scientist like Bohr needed someone of Rutherford's seniority to 'communicate' a paper to a British journal to ensure swift publication. 'I am very anxious to know what you may think of it all', he wrote to Rutherford.26 He was particularly concerned about the reaction to his mixing of the quantum and classical physics. Bohr did not have to wait long for the answer: 'Your ideas as to the mode of origin of spectra in hydrogen are very ingenious and seem to work out well; but the mixture of Planck's ideas with the old mechanics make it very difficult to form a physical idea of what is the basis of it all.'27

Rutherford, as others would, was having trouble picturing how the electron in the hydrogen atom 'jumped' between energy levels. The difficulty lay in the fact that Bohr had violated a cardinal rule of classical physics. An electron moving in a circle is an oscillating system, with one complete orbit being an oscillation and the number of orbits per second being the frequency of oscillation. An oscillating system radiates energy at the frequency of its oscillation, but since two energy levels are involved in an electron making a 'quantum jump', there are two frequencies of oscillation. Rutherford was complaining that there was no link between these frequencies, between the 'old' mechanics and the frequency of the radiation emitted as the electron jumps between energy levels.

He also identified another more serious problem: 'There appears to me one grave difficulty in your hypothesis, which I have no doubt you fully realize, namely, how does an electron decide what frequency it is going to vibrate at when it passes from one stationary state to the other? It seems to me that you would have to assume that the electron knows beforehand where it is going to stop.'28 An electron in the n=3 energy level can jump down to either the n=2 or the n=1 levels. In order to make the jump, the electron appears to 'know' to which energy level it is heading so that it can emit radiation of the correct frequency. These were weakness of the quantum atom to which Bohr had no answer.

There was another, more minor criticism that concerned Bohr far more deeply. Rutherford thought the paper 'really ought to be cut down', since 'long papers have a way of frightening readers, who feel that they have not time to dip into them'.29 After offering to correct Bohr's English where necessary, Rutherford added a postscript: 'I suppose you have no objection to my using my judgement to cut out any matter I may consider unnecessary in your paper? Please reply.'30

When Bohr received the letter he was horrified. For a man who agonised over the choice of every word and went through endless drafts and revisions, the idea that someone else, even Rutherford, would make changes was appalling. Two weeks after posting the original paper, Bohr sent a longer revised manuscript containing alterations and additions. Rutherford agreed that the changes were 'excellent and appear quite reasonable', but he once again urged Bohr to cut the length. Even before he received this latest letter, he wrote to Rutherford telling him that he was coming to Manchester on holiday.31

When Bohr knocked on the front door, Rutherford was busy entertaining his friend Arthur Eve. He later recalled that Rutherford immediately took the 'slight-looking boy' into his study, leaving Mrs Rutherford to explain that the visitor was a young Dane and her husband thought 'very highly indeed of his work'.32 Through hour after hour of discussions over several long evenings during the days that followed, Bohr admitted that Rutherford 'showed an almost angelic patience' as he tried to defend every word in his paper.33

An exhausted Rutherford finally gave in and afterwards began regaling his friends and colleagues with tales of the encounter: 'I could see that he had