= Solution
\b[Ordinary <air> contains <water vapour>, <carbon dioxide>, variable trace gases and suspended particles in addition to <nitrogen> and <oxygen>.] Which traces are detected depends on location, sampling volume and test sensitivity. A negative small-sample test is not proof of complete absence.
<Water vapour> is shown by condensation on a sufficiently cold surface or by the colour change of a suitable dry indicator: white anhydrous <copper(II) sulfate> becomes blue on hydration. Passing a known air stream through a drying tube and observing a controlled mass increase gives a more quantitative principle, provided other absorbable vapours are excluded.
<Carbon dioxide> gives a <calcium carbonate> turbidity when a suitable air sample is passed into clear <limewater>:
$$
\boxed{\mathrm{CO}_2+\mathrm{Ca(OH)}_2\longrightarrow\mathrm{CaCO}_3\downarrow+\mathrm H_2\mathrm O.}
$$
Large excess of <carbon dioxide> can redissolve the deposit as dissolved bicarbonate species, so excessive exposure can obscure the initial result. Absorption into an alkaline solution, followed by an appropriate carbonate test, concentrates the trace component.
<Ammonia> and ammonium-bearing particles may be collected by an acid wash and examined after suitable neutralization or liberation of the <Ammonia>. <Nessler's reagent> supplies a sensitive colour test, with a blank to exclude contaminated reagents. Simple moist <litmus> is much less sensitive. Acidic gases and nitrogen oxides may instead be collected in appropriate wash solutions and examined with tests for the resulting anions; their identity cannot be inferred from acidity alone.
Near relevant sources, <hydrogen sulfide> can blacken moist <lead acetate> paper by forming <lead sulfide>. Ozone-sensitive starch–iodide paper can indicate an oxidizing trace constituent through liberation of <iodine>, but other oxidants can give the same response: the historical observation is not a specific proof of <ozone>. Dust and droplets can be collected on a filter or surface and examined by <microscopy>, with chemical tests applied to separated material as needed.
Modern analysis also recognizes <argon> and other noble gases, as well as many low-concentration pollutants, using instrumental separation and <spectroscopy>. Their modern recognition should not be silently included among discoveries available to an 1865 candidate. \b[The useful answer pairs each proposed constituent with a collection method, a specific reaction or observation, and the limits of that test.]
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