Discovery of Oxygen, Part 2

In Short

This foundational treatise documents the experimental discovery of oxygen—termed "fire-air"—and the systematic breakdown of atmospheric air into distinct gaseous components. Carl Wilhelm Scheele meticulously demonstrates that common air is not a uniform element, but a mixture of two elastic fluids: one that supports combustion and respiration, and another that suffocates flame and life. Through dozens of simple yet ingenious lab setups using retorts, bladders, and inverted jars, he isolates fire-air from various chemical compounds. The work endures as a masterpiece of empirical method, recording a pivotal leap toward modern chemical science.

The Story

The text unfolds as a deliberate, step-by-step investigation into the physical and chemical nature of atmospheric air. Scheele opens by defining the core purpose of chemistry: to skillfully separate substances into their primary constituents and examine their true properties. Setting aside historical assumptions and pure theoretical conjecture, he insists that real understanding requires physical proofs obtained directly by hand.

He begins his narrative arc with basic atmospheric experiments. By placing reactive materials—such as liver of sulphur, animal oils, linseed oil, and nitrous air—into sealed flasks inverted over water, he observes that the standing volume of air consistently decreases by roughly one-fourth to one-third. From these observations, Scheele deduces that common air must consist of two separate elastic fluids: a reactive part that readily combines with inflammable substances ("phlogiston"), and a passive, non-reactive residuum ("vitiated air") that remains behind and extinguishes fire.

Determined to isolate the missing, active component of the atmosphere, Scheele turns to chemical decomposition using heat. He heats various mineral and metallic substances in small glass retorts connected to emptied bladders. Through the distillation of fuming nitric acid, nitre (potassium nitrate), manganese dioxide mixed with acids, red mercuric precipitate, and arsenic acid, he successfully generates a remarkable gas. This liberated gas causes flying coal dust to ignite with brilliant flashes and makes candles burn with expanded, intense brightness. He names this extracted fluid "fire-air."

Having isolated fire-air, Scheele tests its behavior in reverse. He demonstrates that by mixing his newly generated fire-air with the inert "vitiated air" left behind from earlier experiments, he can artificially recombine them to reconstitute ordinary atmospheric air.

The final phase of his investigation expands into biological processes. Scheele examines how living organisms interact with air. By sealing rodents, bees, flies, and plants inside closed vessels with milk of lime, he discovers that animals consume fire-air during respiration, producing "aerial acid" (carbon dioxide), while blood turns bright red upon contact with the atmosphere. Ultimately, he demonstrates that fire-air is the essential fluid sustaining both biological circulation and combustion, while water naturally absorbs and separates this vital gas from the surrounding environment.

How It Unfolds

The voyage begins Scheele outlines the fundamental goal of chemistry and sets out to determine whether atmospheric air is a simple element or a complex compound through rigorous physical testing.

The air is divided By exposing ordinary air to substances like liver of sulphur, linseed oil, and nitrous air inside sealed flasks, he observes a consistent loss of one-fourth of the air's volume, proving the atmosphere contains two distinct gases.

The fire-air is freed Heating nitre, manganese, red mercuric precipitate, and various acids inside small glass retorts, Scheele collects a pure, highly reactive gas in attached animal bladders that causes embers to burn with extraordinary brilliance.

The atmosphere is rebuilt To confirm his analysis, Scheele takes the newly isolated fire-air and mixes it back with the inert, vitiated air left over from combustion, successfully reconstituting ordinary atmospheric air.

The breath of life Testing the gas on living organisms, Scheele reveals that animals, insects, and blood all rely on fire-air for respiration, converting it into aerial acid or vitiated air during biological processes.

The People

Carl Wilhelm Scheele The meticulous investigator at the heart of the text, Scheele wants to uncover the true elementary components of nature through repeatable physical proofs. Confronted by the limitations of traditional chemical theory and the challenge of capturing elusive invisible gases, he works systematically with simple tools like bladders and glass retorts, ultimately discovering fire-air and proving the complex composition of the atmosphere.

Fire-Air (Oxygen) The active, life-giving constituent of the atmosphere that Scheele isolates. It seeks to unite with inflammable substances, support vivid combustion, and sustain animal respiration; when bound within minerals or acids, it remains hidden until liberated by intense heat.

Vitiated Air (Nitrogen) The major inert fluid composing common air. It stands in direct opposition to fire-air, remaining passive in the presence of phlogiston, refusing to combine with water, and promptly extinguishing flame and animal life.

Phlogiston The hypothetical principle of flammability that 18th-century chemists believed was released during burning. In Scheele's framework, phlogiston constantly seeks to unite with fire-air, driving the chemical transformations observed throughout his experiments.

In Its Own Voice

"It is the object and chief business of chemistry to skilfully separate substances into their constituents, to discover their properties, and to compound them in different ways."

Scheele opens his treatise by framing chemistry not as speculative philosophy, but as a practical, laboratory-based discipline.

"Thus much I see from the experiments mentioned, that the air consists of two fluids, differing from each other, the one of which does not manifest in the least the property of attracting phlogiston, while the other... is peculiarly disposed to such attraction."

Following his early trials with liver of sulphur and oils, Scheele clearly states his core deduction regarding the dual nature of atmospheric air.

"I obtained in the bladder the pure fire-air which occupied the space of 50 ounces of water. This is the cheapest and best method of obtaining fire-air."

While heating purified nitre in a glass retort, Scheele celebrates discovering an efficient, repeatable method for isolating oxygen gas in quantity.

What It's Really About

At its core, the text is about replacing speculative natural philosophy with precise empirical measurement. Scheele addresses the fundamental question of what makes up the air we breathe and how matter transforms during burning and respiration. Underneath his experimental procedures lies a quiet rebellion against the classical doctrine of four immutable elements. By demonstrating that air can be split into distinct gases, recombined, and extracted directly from solid minerals, Scheele establishes that invisible elastic fluids are active, measurable chemical agents governed by physical laws.

Why Read It Today

This text will deeply satisfy readers interested in the history of science, classic laboratory chemistry, and the origin of modern ideas about the natural world. Reading it feels like sitting beside an unusually clear-headed, inventive craftsperson at a workbench as he solves one of nature's greatest riddles with basic glassware, sealing wax, and ox bladders.

The primary difficulty for modern readers lies in the historical chemical nomenclature. Scheele interprets all his findings through the framework of the phlogiston theory, referring to oxygen as "fire-air," carbon dioxide as "aerial acid," and nitrogen as "vitiated air." Nitric acid appears as "aqua fortis" or "acid of nitre," while potassium carbonate is called "alkali of tartar."

Once you adjust to this eighteenth-century terminology, the text becomes surprisingly accessible. What remains with the reader is Scheele's infectious curiosity, his transparent honesty regarding imprecise apparatus, and his profound satisfaction in watching bright sparks dance inside a heated retort as a new gas is born.