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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.

Consider an original simplified model containing only nitrogen and argon, with argon mole fraction $x$. Let their molar masses be $M_N$ and $M_A$. Adding the component masses gives the mixture's mean molar massFor an ideal gas mixture at the same pressure and temperature as a pure-nitrogen reference, the gas density ratio isThe inversion requires known identities, the binary-mixture assumption and $M_A\ne M_N$. A density ratio cannot uniquely recover an arbitrary multicomponent mixture. Nor should $x$ be interpreted as the argon fraction of untreated air: the model sample has only the two specified components.

Choose round model values $M_N=28\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and $M_A=40\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. For $x=0.010$, the calculated relative density excess isThus a one-percent mole fraction does not mean a one-percent gas density change. In a chosen $2.00\mathrm{L}$ globe at $100\mathrm{kPa}$ and $300\mathrm{K}$, the ideal gas law gives approximately $2.24508\mathrm{g}$ for pure nitrogen and $2.25471\mathrm{g}$ for this mixture: only $9.62\mathrm{mg}$ separates the weighings. These are model calculations with rounded molar masses, not historical measurements.

A direct control calculation explains why the filling conditions matter. At fixed pressure, pure nitrogen athas the same gas density as that mixture at $300\mathrm{K}$. A roughly $1.28\mathrm{K}$ mismatch can therefore imitate the entire stipulated composition signal. For small changes, the ideal gas law givesAn independently measured pressure and temperature, matched weighing procedure, and checks for residual vapour distinguish composition from these alternative causes. The model explains the need for controls; it does not supply the complete corrections for a real gas-weighing instrument.

Gas density is the mass density of a gas. For an ideal gas sample of amount of substance $n$, mean molar mass $\overline{M}$ and volume $V$, combine $m=n\overline{M}$ with $pV=nRT$ to obtainHere $R$ is the molar gas constant. For a homogeneous mixture of gases, $\overline{M}={\sum }_i{x}_i{M}_i$, where $x_i$ is the mole fraction of component $i$. Equal pressure and temperature make a gas density ratio equal to a mean-molar mass ratio within this model. Argon detection in a binary gas mixture gives a worked example and a temperature confounder.

A physicist whose research joined waves, optics, acoustics and precise measurement. He maintained a laboratory at Terling and served as Cavendish professor of experimental physics from 1879 to 1884. At Cambridge he developed laboratory courses; later he donated his Nobel award proceeds toward a Cavendish laboratory extension.^[ref]

The Rayleigh-Ramsay argon density experiment, pursued with William Ramsay, connects a small discrepancy in gas weighing to chemical separation. Its lesson is practical: agreement within one preparation method does not ensure that a second method produces the same chemical substance. The gas density model explains how composition and filling conditions both affect the readout.

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.