= Rayleigh-Ramsay argon density experiment
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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.<Argon by Lord Rayleigh>[Argon, Royal Institution Proceedings 14 (1895), 524–538: gas-weight table; storage test; magnesium circulation; nitrogen spectrum test and comparison of isolation routes]{ref}
\Image[/argon-density-comparison-and-separation.png]
{title=Different preparation routes, a controlled gas weighing, and residual-gas separation}
{description=Original functional diagram of the <Rayleigh-Ramsay argon density experiment>, not a scale reconstruction. The weighing comparison and subsequent chemical separation answer different questions. The <gas density> comparison requires common <pressure> and <temperature>; the hot-<magnesium> treatment concentrates the unconsumed component. The diagram shows operations, not a laboratory construction plan.}
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.
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