Argon. The discovery of a new gaseous constituent of the atmosphere, to which the name argon was afterwards given, was first announced at the meeting of the British Association at Oxford in 1894, by Lord Rayleigh and Professor Ramsay, who had worked at the subject, first independently, and afterwards conjointly. This gas, which forms rather less than 1 per cent. by volume of the atmosphere, appears to have been in the hands of Cavendish in 1785, but it was not recognised by him as a separate substance. In making a series of careful determinations of the density of nitrogen, Lord Rayleigh found in the course of experiments extending over several years that the density of nitrogen obtained from the atmosphere was uniformly greater than that of nitrogen prepared from nitrogen compounds. Hence he was led to ex- amine atmospheric nitrogen for the presence in it of traces of a denser gas. For this purpose he employed the method of Cavendish—i.e. passing electric sparks through a mixture of air and a sufficiency of oxygen, absorbing the gases so formed by means of alkaline solutions, and then, when no further diminution of volume could be produced, removing the excess of oxygen. A small residue of argon was always obtained. Ramsay prepared argon from the air by first separating the oxygen and then removing the residual nitrogen by means of red-hot magnesium. Argon is a colourless gas which can be liquefied at a very low temperature by high pressure. It has also been solidified. It appears to be an element, with atomic weight about 40; but the evidence is not yet conclusive. Compounds of argon have not with certainty been obtained, but it seems to combine with carbon in the electric arc, and with benzene vapour under the influence of the silent electrical discharge.
Intimately connected with the discovery of argon is the discovery of helium. Lockyer and Frankland gave the name helium to a supposed element detected spectroscopically in the sun's chromosphere during the eclipse of 1868 (see SUN). In March 1895, Ramsay announced the discovery by means of the spectroscopic of a new gas which he considered identical with helium in the mixture of gases obtained by boiling the mineral clèveite with dilute sulphuric acid. This gaseous mixture is also obtained by simply heating clèveite or bröggerite in an exhausted tube.
Clèveite is a rare mineral found in the Arendal district of Norway. It consists of oxide of uranium associated chiefly with uranates of lead and thorium, and of metals of the yttrium group in smaller quantity. It is considered to be a weathered variety of bröggerite, another Norwegian mineral, from which it does not differ much in composition.
At a later date Ramsay, along with Collie and Travers, found helium to possess the relative density 2.13, and hence to be much less dense than any known gas except hydrogen. They suppose it to be an elementary gas, closely allied chemically to argon, and provisionally assign to it the atomic weight 4.26. This is, however, on the assumption that helium is a single element and not a mixture of elements. The spectrum of helium is characterised by five very brilliant lines; these occur in the red, the yellow, the blue-green, the blue, and the violet.
Considerable discussion has taken place as to what position should be assigned to helium and to argon in the periodic classification of the elements (see ATOMIC THEORY). From a consideration of their whole characters it would appear that both elements belong to the same class, and that helium should be placed before lithium, and argon between chlorine and potassium. This, however, would require the atomic weight of argon to lie below instead of above 39, and would lend support to what Ramsay and his co-workers consider probable, that neither argon nor helium has ever been obtained quite pure, and that both contain an unknown gas of high atomic weight as a common impurity. In this latter connection it is of considerable interest to note that so far as is known atmospheric argon is entirely free from traces of helium, and that helium, derived exclusively from minerals, is entirely free from traces of argon; while the gases, on spectroscopic examination, exhibit some lines which are common to both spectra.