ARGON, symbol A (or Ar), a gaseous element, was first actually obtained in 1785 by Cavendish who observed that °not more than 1/120th part° of the atmosphere remains after treating the air for removal of oxygen and nitrogen. Attention was not called to this statement by Cavendish until 1894 when Lord Rayleigh found "atmospheric nitrogen" to • be per cent heavier than "chemical nitrogen." For some years previous to, this discovery Lord Rayleigh had been engaged in a careful determination of the densities of certain gases and consistent results had been obtained for all of them save nitrogen, which, when prepared from air by the abstraction of all other known components, was found to be heavier by one part in 200 than the nitrogen prepared from nitrogen compounds such as ammonia, ammo nium nitrite, urea, nitric or nitrous oxides. There could be no doubt about the reality of the difference, because the same experimental methods, when applied to other gases, gave re sults that were consistent with one another to about one part in 10,000. Lord Rayleigh pub lished a letter in Nature narrating these facts and calling for suggestions from chemists as to the cause of the consistent difference in den sity. No ideas of value were elicited. The possibility that the °chemical nitrogen" was contaminated with hydrogen or that the "at mospheric nitrogen" had polymerized into a state analogous to °ozone" was then disproved by the most careful experiments. At this stage in the investigation Professor Ramsay asked permission to co-operate in the work and his services were gladly accepted.
The hypothesis was made that "chemical nitrogen" contains an unknown gas, than true nitrogen; or, that °atmospheric ni trogen" contains some similar gas that is heavier than true nitrogen. In spite of the many anal yses that had been made of the air, it was thought more probable that the unknown gas would be found in °atmospheric" rather than in "chemical" nitrogen.
It was known that at a red heat nitrogen will combine with metallic magnesium, with the formation of magnesium nitride. "Atmos
pheric" nitrogen, carefully freed from all known impurities, was therefore passed through a long tube of hard glass filled with magnesium shavings and heated in a furnace. The first experiment of this sort was made in May 1894, and gave encouraging results, the °atmos pheric" nitrogen showing a slight but unmis takable increase in density. A more elaborate experiment of the same sort followed in which °atmospheric" nitrogen was caused to pass over hot magnesium for more than two weeks. By this means, its density, originally about 14 (that of hydrogen being I), was increased to 19.09, and the bulk of the gas under examina tion was diminished until not much more than 1 per cent of it remained. Plainly a great concen tration of the unknown gas had been effected. To remove the last traces of true nitrogen, pure oxygen was next added, and the mixture exposed to a rain of electric sparks in the pres ence of caustic soda. When so treated, the ex perimental gas contracted, indicating that the nitrogen was being withdrawn in the form of nitrate of sodium. When contraction was no longer noted, the nitrate of sodium and the excess of oxygen were removed and it was found that the remaining gas was about 20 times as heavy as hydrogen.
When subjected to the electric spark and examined, by the spectroscope, this residual gas was found to exhibit certain characteristic groups of red and green lines that did not cor respond to any element previously known. The experimenters therefore felt reasonably sure that a new element had been discovered and this conclusion has been borne out by all sub sequent investigations.
The discovery of this element (to which the name °argon,* meaning "inactive," was given) was formally announced to the public in August 1895, and for it Lord Rayleigh and Professor Ramsay were awarded the Hodgkins prize and also the grand prize of the Institution.