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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: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. The useful answer pairs each proposed constituent with a collection method, a specific reaction or observation, and the limits of that test.

The historical name fermentation covered several transformations of organic matter: alcoholic, lactic, butyric and acetous changes, as well as some processes now classified separately. In modern biochemistry, fermentation more narrowly means energy-yielding metabolism that regenerates oxidized cofactors without requiring an external respiratory electron acceptor. The older use of the word should not be forced into that modern definition.

In alcoholic fermentation, suitable yeast converts a fermentable sugar into ethanol and carbon dioxide. For glucose, the principal net balance isA vessel with an outlet for the evolved gas and a separate receiver illustrates the change: test the carbon dioxide with limewater and identify the volatile alcohol in a separated distillate. Gas evolution alone does not prove that the liquid contains ethanol. Actual yeast cultures also make biomass and minor products; the chemical equation describes the principal conversion. In modern terms, glycolysis supplies ATP, and the reduction leading to ethanol regenerates NAD+ from NADH.

In lactic acid fermentation, an ideal homolactic balance isThe souring of suitable carbohydrate-containing materials, including milk, supplied a familiar historical example. Hydrolysis of lactose supplies monosaccharides, while acid production lowers the pH and can precipitate casein. Casein coagulation is a consequence of the changed conditions, not evidence that the protein has itself been converted into lactic acid. Some organisms instead make additional products, so not every lactic fermentation obeys only this homolactic balance.

In butyric acid fermentation, carbohydrates or lactate can give butyric acid, carbon dioxide and molecular hydrogen. An idealized balance from lactic acid isIt contrasts with the simple homolactic balance because both gaseous products and a different organic acid appear. Mixed cultures and alternative pathways can alter the product distribution; odour is neither a sufficiently specific nor a necessary laboratory test.

Acetous fermentation denotes the conversion of ethanol into acetic acid, as in vinegar production:It depends on oxygen and is now understood as an aerobic microbial oxidation, rather than a fermentation in the narrow biochemical sense. This explains why access to air matters differently here than in the main alcohol-producing stage.

Other older extensions of the word include saccharification or diastatic changes, such as the hydrolysis of starch into smaller sugars by malt enzymes, and sugar inversion, in which sucrose takes up water to form glucose and fructose:The two hexose products have the same elemental formula but different structures. Such hydrolyses prepare substrates for later metabolism; they do not by themselves constitute the alcohol-producing conversion. Viscous fermentation or mucilaginous fermentation described transformations yielding gum-like material and sometimes mannitol; it is an operational historical category without one universal balanced chemical equation. Putrefactive fermentation referred to the decomposition of nitrogen-containing organic matter, including proteolysis and subsequent reactions producing Ammonia, amines and sometimes hydrogen sulfide. It likewise has no single formula covering every mixed substrate and product.

The unifying historical interest is that small amounts of a “ferment” bring about major changes in a large amount of organic substrate. Modern explanation distinguishes organisms, enzymes and metabolic pathways. The later demonstration of fermentation in cell-free yeast juice showed that alcoholic conversion need not require intact living yeast cells in the reacting liquid; that result belongs to 1897, not to knowledge to demand of an 1866 candidate.^[ref]

Use a qualitative separation into aqueous extract, lipid-soluble material, protein-rich residue and mineral matter, and test each fraction independently. A tubercle is a lesion containing several kinds of material, not a pure chemical compound with one diagnostic formula. The question asks how to investigate its composition; no observations or numerical composition are supplied.

First record the lesion's physical appearance and take a portion for microscopy before destructive treatment. Separate the lesion from surrounding tissue as far as practicable, while recording that contamination by blood, connective tissue or adherent salts can affect the analysis. Different stages of a lesion need not have the same proportions of organic material and calcified matter. A small portion gently dried in suitable apparatus shows whether much water is lost; collecting condensation supports identification of the volatile fraction as containing water, although mass loss alone is not a chemically specific water test.

On separate portions, or by a documented extraction sequence, examine the main fractions:

Test soluble salts using a fresh portion or a suitable water extract: for example, silver nitrate can detect chloride, while barium under controlled acidic conditions can detect sulfate. A chloride test should not be performed on material already contaminated with hydrochloric acid. The results identify constituents or operational fractions, not necessarily the precise salts originally present in intact tissue; ashing can also change their chemical forms.

If the intended composition is elemental rather than proximate, investigate an isolated organic fraction separately: oxidation with heated copper(II) oxide can convert its carbon into carbon dioxide and its hydrogen into water, which are trapped and identified independently. A suitable nitrogen determination can convert organic nitrogen into Ammonia for identification in a separate receiver. Mineral carbonate and pre-existing moisture must be controlled before attributing these products to the organic material. Such tests answer a different question from identifying the particular proteins or lipids.

This approach has a historical basis. Original analyses by Robert Mortimer Glover, reproduced in an 1847 review, separately extracted material with ethanol, diethyl ether and water, then examined protein-rich residues and mineral salts. They reported variation between lesions and did not consistently find casein. This evidence supports fractionation and caution about specificity, not one universal numerical recipe for all tubercles.^[ref]

A modern conclusion must separate composition from cause. A protein-, lipid- and phosphate-containing lesion is not thereby proved to be tuberculosis. Histological interpretation and appropriate microbiological examination are different investigations. Current confirmation of Mycobacterium tuberculosis involves validated tests such as nucleic acid amplification and culture, rather than the extraction reactions above; these are modern qualifications and are not attributed to the 1866 examination.^[ref]

Prepare carbon dioxide, make measured mixtures with air, and test a standardized lighted taper in fresh mixtures to find an extinction boundary. In this taper extinction experiment, “carbonic acid gas” means gaseous carbon dioxide, not liquid carbonic acid. The proportion should be reported as a fraction of the final mixture, with the conditions of the test.

A documented sufficient mixture is available from a later first-hand experiment: one volume of carbon dioxide with two volumes of air extinguished the introduced taper in the apparatus used. ThusThis is a conditional literature result, not an invented observation from the Oxford examination. In The True Position of Oxygen as a Restorative in Carbonic Acid Poisoning by W. Ernest Thomson, Experiment XLVIII directly compares one-third carbon dioxide mixed with two-thirds oxygen and with two-thirds air; the latter extinguished the taper. The experiment was published in 1894, so it supplies a later physical comparison rather than evidence of what the 1866 candidate measured.^[ref]

The question asks for an approximately sufficient proportion; it does not explicitly require a universal minimum. The cited condition supplies one sufficient point. To find approximately the least sufficient proportion for the proposed apparatus, determine the lower boundary by the following procedure instead of treating one-third as that boundary.

Prepare the carbon dioxide in a small gas-generating flask by adding dilute hydrochloric acid to calcium carbonate:Fit a delivery tube, keep the outlet unobstructed, and discard the first gas while the original air is being displaced from the apparatus. A wash bottle and droplet trap can reduce acid carry-over before collection in a freely moving gas syringe or suitable gas holder. This acid-carbonate preparation is also used in a current Royal Society of Chemistry demonstration.^[ref] A separate sample may be checked with limewater:The turbidity is a qualitative check for carbon dioxide; it is not a measurement of the fraction in the flame-testing mixture. Avoid using a water-filled collecting vessel for the quantitative mixing stage without allowing for dissolved carbon dioxide and water vapour.

For the mixing stage, use two calibrated gas syringes and a closed mixing vessel with a means of transferring the mixture into a larger flame-testing jar. Measure the component gas volumes at the same temperature and pressure. Mix the gases thoroughly before the flame test. If the gases behave approximately as an ideal gas mixture, their measured volume fractions then approximate their mole fractions. For example, an intended one-third mixture can be prepared from one measured portion of carbon dioxide and twice that volume of air. These are preparation instructions, not recorded trial outcomes.

Keep the taper's material, exposed wick, initial candle flame, insertion depth, gas volume and test duration fixed. Give the flame a reproducible exposure, using a gas-filled jar and a narrow insertion opening with a loose collar to limit entry of fresh air. The observation period must be specified before testing; a fixed five-second exposure is one possible chosen protocol, not a reported condition of the historical reference experiment. Use a large mixture volume relative to the small taper, introduce it briefly, and use a fresh mixture for every trial; prolonged burning changes the mixture by consuming oxygen and producing carbon dioxide and water vapour. Do not seal a burning vessel into an improvised pressure-tight apparatus.

Start with an air control that sustains the taper. Test successively richer, separately prepared carbon dioxide mixtures until extinction occurs within the specified observation period. Retest with a less rich mixture and subdivide the interval. Repeat mixtures near the boundary to detect inconsistent results. A completed experiment would report, for example, the greatest tested fraction $x_b$ at which burning consistently persists and the smallest tested fraction $x_e$ that consistently extinguishes the taper. If the responses are ordered and reproducible under the fixed protocol, reportA midpoint and half-interval may describe the resolution of that bracket; they are not a statistical confidence interval. No numerical values of $x_b$ or $x_e$ are supplied by the source, and none are asserted here.

For a useful model check, let $y_{{\mathrm{O}}_2,0}$ denote the initial oxygen fraction in air, and let $x$ denote the fraction of added pure carbon dioxide in the final mixture. Before the taper changes the gas, an ideal, uniformly mixed replacement model givesTaking dry air to have $y_{{\mathrm{O}}_2,0}=0.2095$ is an explicit model input. For the documented one-third mixture, this gives $y_{{\mathrm{O}}_2}\approx 0.140$, or about $14\%$ oxygen. It is a calculation from an assumed starting composition, not a measured oxygen analysis or a general extinction rule. Also distinguish a fraction of the final mixture from an addition relative to the original air: the volume ratio $r=V_{\mathrm{C}{\mathrm{O}}_2}/V_{\mathrm{air}}$ satisfies $x=r/(1+r)$, so a 1:2 addition ratio means $x=1/3$, not $x=1/2$.

Carbon dioxide affects the thermal balance of a diffusion flame as well as the oxidizer fraction. Extinction therefore depends on the taper, mixing, gas flow, temperature, pressure and heat loss; an oxygen-dilution calculation alone cannot predict its value. The official account of carbon dioxide suppression describes its predominantly thermophysical action.^[ref]

Finally, distinguish this experiment from covering a burning candle and waiting for it to go out. That procedure simultaneously changes gas composition, temperature and spatial distribution. A modern experimental study using gas and temperature sensors found different conditions at different heights and extinction before all oxygen was consumed.^[ref] A pouring demonstration with unmixed carbon dioxide likewise does not determine a uniform bulk fraction. A sufficient, condition-labelled literature mixture and a reproducible bracketing procedure answer the available task; an actual examination threshold or a universal minimum is not claimed.

The wick supplies liquid wax by capillary action; wax vapour reacts with atmospheric oxygen, releasing the heat that keeps the supply going. The wick is a delivery system, rather than the principal fuel.

First, an external flame warms the wick and adjacent wax. Melting produces a small liquid reservoir, and capillary action draws it into the spaces between the wick fibres. Near the hot tip, evaporation and some pyrolysis supply gaseous fuel. Melting and evaporation are physical changes; pyrolysis and combustion change the chemical substances present. Once established, the candle flame transfers enough heat back to the wax to maintain this sequence.^[ref]

The candle flame is a diffusion flame. Its relatively dark inner region contains newly supplied fuel with limited access to oxygen. In the yellow luminous region, incomplete conversion of the hydrocarbons produces small soot particles, which become incandescent. Farther out, mixing supplies enough oxygen to consume most of the remaining fuel, carbon monoxide and soot. Thus the yellow light is largely thermal emission from hot particles, rather than a yellow gas.^[ref]

For a representative alkane component of paraffin wax, complete combustion has the chemical equationReal candle materials are mixtures, and some, such as fatty materials, already contain oxygen; the displayed chemical equation is an illustrative wax model. The principal products of complete combustion are carbon dioxide and water vapour. Convection carries the hot gases upward and brings fresh air into the flame. A cold collector can reveal condensed water; collecting the gases and passing them into limewater gives a calcium carbonate cloudiness from carbon dioxide.

Removing the oxygen supply, cooling the reaction zone, or cutting off wax delivery extinguishes the flame. The smoking wick immediately afterwards can still emit combustible vapour; without sufficient heat at the mixing zone, that vapour need not sustain a flame.