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The chemical element with atomic number eighteen. Argon is a colourless noble gas. It can provide a protective atmosphere around hot, reactive materials, for example in welding, where excluding oxygen limits oxidation. Commercial production separates it by distillation of liquid air.^[ref]

Its discovery shows why a small disagreement can matter more than a reassuringly repeatable result. Two preparations can each give consistent measurements while producing different mixtures. The Rayleigh-Ramsay argon density experiment investigated just such a discrepancy. Gas density links the weighing to a mean molar mass; a separate separation process tests which component accounts for it.

A chemical engineer who has worked on separation processes. He is credited as publisher of the Hyper-TVT teaching site; that role does not establish authorship of its individual chapters.

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In his 1895 lecture, Lord Rayleigh compared gas masses in a globe under matched pressure and temperature. Removing oxygen from air left gas about half a percent heavier than nitrogen prepared entirely from chemical sources. Several preparation routes reproduced the difference; hydrogen contamination was a concern. An eight-month storage test re-established the filling conditions: simply reweighing a sealed globe could not test a change in molar mass. William Ramsay concentrated the residual gas by circulating the mixture over hot magnesium, which consumed nitrogen. A spectral test checked remaining nitrogen. Agreement of spectra from this route and the alternative spark-and-alkali separation supported a common residual gas.^[ref]

The density difference alone identifies neither a unique chemical element nor its concentration. A heavier contaminant, a lighter reference contaminant, or unmatched filling conditions can all shift a weighing. A separation process and an optical spectrum provide additional observations. This combination makes the experiment powerful: the anomaly motivates another operation whose result can be checked independently.