ABSTRACT
We have seen that in 1882 Lord Rayleigh, as President of Section A of the meeting of the British Association for that year, had said that he proposed to look further into the validity of Prout's hypothesis that atomic weights were integral multiples of that of hydrogen. To do this he resolved to re-measure, with as great an accuracy as he could, the densities of hydrogen and oxygen, and see if their ratio is an integer. Some years later 1 he obtained the result that it is close to an integer but departs from it by an amount that was outside his experimental error, namely 15.882. Not content with this result, he decided to measure also the density of nitrogen. There are two obvious ways of preparing pure nitrogen. The first is to start from air, dry it, remove any carbon dioxide, and then separate the nitrogen and oxygen by removing the latter by heating the air in contact with copper to form cupric oxide. The second is to take a nitrogeneous compound that can be purified chemically and extract the nitrogen from it. Rayleigh chose first the ‘atmospheric’ route and found that seven weighings were consistent to one part in 10 4 . For the second route he chose a method proposed originally by Vernon Harcourt and suggested to Rayleigh by William Ramsay. Air is bubbled through a strong solution of ammonia before passing over hot copper when the hydrogen in the ammonia is oxidised to water and the gas then dried. Only about one part in seven of the nitrogen comes from the ammonia, but by substituting oxygen for air the whole of the resulting nitrogen could be ‘chemically’ sourced. He found that the two methods disagreed; however careful he was, and whatever variations he tried, the atmospheric nitrogen was always about half a percent heavier than the chemical. This difference was about 50 times the variations found between the weighings and so beyond any experimental error of measurement; something was amiss with the purification achieved in one or both of the routes. Rayleigh was no chemist and had written to Nature in September 1892, before the experiments with ammonia and oxygen, to put the problem before the public and to ask chemists for any suggestions for its resolution. 1 Ramsay replied but, as he said himself, he could do no more than describe at some length the facts as they now stood. 2 The more doubtful of the two procedures seemed to be the first; what is the evidence that dry air, freed of carbon dioxide, contains only oxygen and nitrogen? Rayleigh put the question to Dewar, his chemical colleague at the RI, who replied that he knew of nothing being done on the question since Cavendish's experiments in 1785, in which he had sparked air to induce the two components to react, and had then absorbed the oxides of nitrogen in alkali. More oxygen was added and the sparking continued until, it was assumed, all the nitrogen and oxygen had been removed. Cavendish found that there was a small residue, about one part in 120 of the original air, which defied further oxidation. He attached, however, little importance to this residue noting only that if there were any other gas present in air it was no more than 1/120 of the whole. 3 (A question arose later of whether it was Dewar or Ramsay who first told Rayleigh of Cavendish's experiment. The point would not merit discussion were it not for the later arguments that surrounded all the work of Dewar and Ramsay, but the evidence seems to show that Rayleigh first learnt of Cavendish's work from Dewar in April 1894, although Ramsay may also have mentioned it to him shortly afterwards. 4 )
