= Solution
\b[<Carl Wilhelm Scheele> and <Joseph Priestley> independently prepared and investigated the <gas> now called <oxygen>; <Antoine Lavoisier> was central to interpreting it within a new account of <combustion> and chemical composition.] Assigning a unique discoverer requires deciding whether discovery means first preparation, first public report, recognition of a distinct kind of <gas>, or a successful theoretical interpretation. The historical controversy is largely about why one of these achievements should be privileged over the others.
<Carl Wilhelm Scheele>'s investigations, generally dated to the early 1770s, preceded publication of <Chemical Treatise on Air and Fire by Carl Wilhelm Scheele> in 1777. His work distinguished the component that sustained burning from the remainder of ordinary air. He obtained the active component through several chemical preparations, including heating suitable substances, and interpreted the results in terms of <phlogiston>. Delayed publication matters to public priority, but it does not show that the experimental work had not occurred. Equally, a publication date alone does not establish the precise date of every earlier trial.<Chemical Treatise on Air and Fire by Carl Wilhelm Scheele>[Experimental sections on the two constituents of air and preparations of fire-air; publication in 1777, distinct from the earlier experimental chronology]{ref}
<Joseph Priestley>'s 1775 account is unusually revealing because it records hesitation and revision. On 1 August 1774 he heated red <mercury(II) oxide> with a large burning lens and collected the released <gas>. A candle burned particularly strongly in it; later trials involving animals and comparison with <nitric oxide> helped him distinguish its properties from ordinary air. He initially lacked a settled understanding of what he had obtained. Calling the product <dephlogisticated air> eventually made it intelligible within his own framework: it had an unusually great capacity to accept the <phlogiston> supposedly released in burning and animal breathing.<Experiments and Observations on Different Kinds of Air by Joseph Priestley (1775)>[Volume II, section III: experiment of 1 August 1774, initial uncertainty and subsequent trials of the new air]{ref}
The modern <chemical equation>,
$$
\boxed{2\mathrm{HgO}\longrightarrow2\mathrm{Hg}+\mathrm{O_2},}
$$
is a useful reconstruction, but it must not be passed off as <Joseph Priestley>'s own explanation. In modern terms, heating decomposes <mercury(II) oxide> into <mercury (element)> and molecular <oxygen>. The eighteenth-century experimenter instead worked with such categories as calces and differently modified airs. The material operation can be recognized across the change in theory while the meaning of its product remains historically contested.
<Antoine Lavoisier> made a different contribution. His programme tracked what entered and left substances in <calcination> and <combustion>, placing <gases> within quantitative material balances. In the mercury experiment described in <Elements of Chemistry by Antoine Lavoisier>, a retort communicated with an enclosed supply of air over <mercury (element)>. Prolonged heating formed a red calx and diminished the enclosed air. Stronger heating of the recovered calx released an especially combustion-supporting <gas>; adding it back to the residual air restored the properties of the original mixture. The persuasive feature was the linked decomposition and reconstruction, not just a single spectacular flame.<Elements of Chemistry by Antoine Lavoisier>[Part I, chapter III: apparatus, twelve-day mercury calcination, examination of residual air and recombination with the gas released from the red calx]{ref}
That sequence supported the claim that air was a mixture and that <calcination> involved combining a metal with part of it. A metal could gain <mass> because material entered it from the air, without requiring an invisible substance to leave with anomalous properties. <Conservation of mass> made the <weighing scale> a constraint on the explanation. However, <Antoine Lavoisier>'s entire system was not modern <chemistry>. He treated <caloric> as an explanatory principle and associated the newly named <oxygen> with the formation of all <chemical acids>. The latter universal claim is false: <hydrochloric acid> is an <chemical acid> whose solute, <hydrogen chloride>, contains <hydrogen> and <chlorine> but no <oxygen>. This concrete counterexample prevents us from defining discovery as possession of today's complete theory.<Elements of Chemistry by Antoine Lavoisier>[Part I, chapter IV, on naming oxygen as the acidifying principle; chapter V, on the composition of acids; preliminary discussion and table of simple substances]{ref}
There are consequently several legitimate allocations of credit. An investigator concerned with earliest preparation may emphasize <Carl Wilhelm Scheele>; one concerned with the well-documented 1774 experiment and its rapid communication may emphasize <Joseph Priestley>; one concerned with the explanatory reconstruction of <combustion> may emphasize <Antoine Lavoisier>. The last achievement depended on experimental results that the others had produced. Correct interpretation was neither a substitute for preparing the material nor an achievement wholly independent of communication between investigators.
Historians disagree because their accounts embody different pictures of scientific change. A simple accumulation story can make discovery look like identifying one additional item in a ready-made list of <chemical elements>. A conceptual-change story emphasizes that the list and its criteria were themselves changing: <oxygen> was not just another variety of modified air in the new <chemistry>. National priority narratives and the dramatic reputation of the <Chemical Revolution> can further encourage a single heroic winner. Yet replacing that winner with a collective label is insufficient unless the different experimental and theoretical contributions are explained.
\b[The most defensible answer distributes credit while specifying the achievement.] <Carl Wilhelm Scheele> and <Joseph Priestley> deserve independent experimental discovery credit; <Antoine Lavoisier> deserves central credit for the chemical interpretation and reorganization that gave <oxygen> its place in a new system. Discovery was a sequence of preparation, discrimination, communication and reinterpretation, rather than one instantaneous encounter with a theory-free fact.
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