Text: Lord Rayleigh's discovery of argon Lord Rayleigh's interest in the density of nitrogen, which led to the discovery of argon, was prompted by William Prout. In 1815, Prout had stated that atomic weights were exact multiples of hydrogen. If this were true, the exact density of oxygen compared to hydrogen should be 16, and not 15.96 (the commonly accepted value for the time). Rayleigh investigated the oxygen and hydrogen relationship and then moved on to the nitrogen and hydrogen relationship. He prepared two samples of nitrogen: one from the air and another, at Lord Ramsay's suggestion, from ammonia. In the first method, carbon dioxide is removed from the air using potash, and oxygen removed by being passed over heated copper (itself reduced from its oxide by hydrogen). The gas is then dried by being bubbled through a wash-bottle containing sulphuric acid and then passed over heated copper oxide (to remove any hydrogen present). Finally the gas is passed again through sulphuric acid. In the second method, the process remains unchanged, except that the hot copper is replaced by a wash-bottle containing liquid ammonia, through which the air was allowed to bubble. The nitrogen prepared by the second method was lighter than that obtained by the first method. In order to obtain information from research chemists, he wrote a letter to Nature, dated 29th September 1882 stating, "I am puzzled by some results on the density of nitrogen, and I shall be obliged if any of your chemical readers can offer suggestions as to the cause. According to the method of preparation I obtain two quite distinct values. The relative difference, amounting to 1/1000 part, is small in itself, but it lies entirely outside the errors of experiment, and can only be attributed to a variation in the character of the gas." After repeating similar experiments during a two year period, and obtaining consistent results that nitrogen obtained from the air was heavier by 0.5%, Rayleigh reasoned that this discrepancy must be due to another constituent of the air. He therefore decided to repeat an experiment previously carried out by Cavendish to oxidise the nitrogen in the air with the aid of an induction coil, but the process of accumulating enough of the gas to test its properties was a slow one. On 31st January 1895, Rayleigh and Ramsay presented a joint paper entitled Argon, a New Constituent of the Atmosphere to a meeting of the Royal Society. However, the findings were not universally accepted, as some argued that so heavy an element could not be a gas. Rayleigh's reply was characteristic: "The result is no doubt very awkward... and all we can do is apologise for ourselves and the gas." Rayleigh was awarded the Nobel Prize for Physics in 1904 for this work. In the same year, Ramsay was given the Nobel Prize in Chemistry for his contribution to the discovery of the so-called inert gases. ---------------------------------------------------------------------------- Now read about naturalist and explorer Sir Joseph Banks --------------------------------------------------------- | Main Menu | This week | About the School | Features | Famous Harrovians | | Boys' Stuff | Teaching Resources | Sport | 'The World Outside...' | editors@harrowschool.org.uk

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