In the 18th century, scientists became showmen and experiments became entertainment with this elegant instrument that measured the "goodness" of air
It's the 1760s, and you're in a packed coffeehouse in London. A lecturer places a glass tube into a basin of water, produces a spark, and the crowd watches as the gas inside mysteriously contracts. This isn't magic—it's experimental philosophy, and the star of the show is an elegant instrument called a eudiometer. In an age when scientists became showmen and experiments became entertainment, this device didn't just measure gases—it measured the very "goodness" of the air we breathe, turning complex chemistry into compelling theater 8 .
Eudiometers played a crucial role in pneumatic chemistry during the 18th and 19th centuries, a period when scientists were first identifying and understanding different gases.
The name itself reveals its purpose: from the Greek eudios (clear or mild weather), literally a "measure of good air" 1 .
At its core, a eudiometer measures the change in volume of a gas mixture following a physical or chemical reaction. The instrument capitalized on a fundamental insight: different gases behave differently in chemical reactions, and some are more soluble in water than others. By measuring volume changes, scientists could infer the composition of the air they were testing 1 .
The most common test relied on the reaction between nitric oxide (called "nitrous air" in the 18th century) and oxygen. When these two gases combine in the presence of water, they form nitrogen dioxide, which readily dissolves, causing a measurable contraction in volume 1 .
The eudiometer measured the contraction in gas volume after chemical reactions to determine air composition.
The eudiometer wasn't a single, standardized device but rather a family of instruments adapted for different reactions and settings.
| Form | Key Features | Primary Use |
|---|---|---|
| Landriani's Eudiometer | Tall, graduated cylinder over water | Measuring air "salubrity" (healthfulness) 1 |
| Priestley's Pneumatic Trough | Used mercury instead of water to trap water-soluble gases 1 | Isolating and studying gases like oxygen and ammonia |
| Volta's Electrified Eudiometer | Included platinum wires for creating electric sparks inside the tube 1 | Initiating reactions in gas mixtures, studying flammability |
| Volta's Pistol | A sealed, sturdy version designed to contain explosions 1 | Demonstrating the explosive power of gas mixtures |
Among the most significant eudiometer experiments was the one developed by Joseph Priestley around 1772. His "nitrous air test" became the standard method for determining the oxygen content—and thus the "goodness"—of air 1 .
The eudiometer tube is filled with water and inverted into a basin of water, ensuring no air enters the closed end of the tube 1 .
A known volume of atmospheric air (or the air sample being tested) is carefully introduced into the sealed upper part of the tube.
A volume of nitric oxide (NO) gas is added to the same sample. The two gases are allowed to mix.
The nitric oxide reacts with the oxygen in the air sample to form nitrogen dioxide (2 NO + O₂ → 2 NO₂) 1 .
The nitrogen dioxide produced is highly soluble in water and dissolves, causing a marked contraction in the volume of gas remaining in the tube. After waiting for the reaction to complete and the gases to fully dissolve, the final volume is recorded 1 .
English chemist, natural philosopher, and theologian who is credited with discovering oxygen.
The scientific importance of this experiment was profound: the greater the contraction, the richer the air was in oxygen. Air deemed "good" or "healthy" would show a significant volume reduction, while "bad" air (like that from crowded rooms or polluted cities) showed less contraction 1 .
This simple yet elegant test allowed for the comparative analysis of air quality in different environments. Jan Ingenhousz famously used this method to verify that the bubbles released by aquatic plants in sunlight were, in fact, oxygen, a crucial step in understanding photosynthesis 1 . Henry Cavendish also used a eudiometer in his meticulous experiments that determined the exact fraction of oxygen in the Earth's atmosphere 1 .
| Air Sample Source | Initial Volume (mL) | Final Volume (mL) | Contraction (mL) | Interpreted Air "Goodness" |
|---|---|---|---|---|
| Countryside Air | 100 | 79 | 21 | High |
| Crowded Lecture Hall | 100 | 85 | 15 | Moderate |
| Closed Cellar | 100 | 92 | 8 | Low |
Jan Ingenhousz used the eudiometer to demonstrate that plants release oxygen during photosynthesis, a groundbreaking discovery in plant biology.
While Priestley's method was revolutionary, it was Alessandro Volta who gave the eudiometer its dramatic spark—literally. Around 1777, Volta developed an electrified eudiometer that included two platinum wires sealed into the closed end of the glass tube 1 .
This innovation allowed researchers to create an electric spark inside the gas mixture, powerful enough to initiate reactions that were impossible to achieve by simple mixing. This was particularly useful for studying the flammability and explosive limits of various gases, such as the "swamp gases" Volta was investigating 1 .
Volta took this further with his famous "Volta's Pistol"—a robust eudiometer designed to contain small explosions. When an electric spark ignited a mixture of oxygen and another gas inside the sealed pistol, the resulting pressure increase would forcefully eject a cork, dramatically demonstrating the energy stored within chemical bonds 1 .
This device perfectly captured how 18th-century scientists blended rigorous inquiry with public spectacle.
Enabled precise investigation of flammable gas mixtures and combustion reactions.
Provided a method for quantitative analysis of gas composition through controlled reactions.
Created engaging scientific spectacles that popularized chemistry among the public.
Experiments with eudiometers relied on several key materials and reagents, each serving a specific function in the investigative process.
| Material/Reagent | Function in the Experiment | Historical Context |
|---|---|---|
| Graduated Glass Tube | The main body of the eudiometer; allows for precise measurement of gas volume changes 1 | Often 50-100 mL in capacity; similar to a graduated cylinder 1 |
| Water or Mercury | The liquid seal; traps the gas sample in the tube and can dissolve reaction products 1 | Mercury was crucial for trapping gases soluble in water (used by Priestley) 1 |
| Nitric Oxide (NO) | The reactive agent in the "nitrous air test"; combines with oxygen to form soluble NOâ‚‚ 1 | Known as "nitrous air" in the 18th century; prepared through various chemical reactions |
| Platinum Wires | To create an electric spark inside the sealed tube for initiating reactions 1 | Chosen for their non-reactivity; a key feature of Volta's design 1 |
| Oxygen (Oâ‚‚) & Test Gases | The analytes; the gases whose concentration or reactivity was being measured 1 | Prepared from compounds like mercury oxide or obtained from natural sources |
While modern safety protocols didn't exist in the 18th century, working with gases like nitric oxide and using mercury required careful handling. Today, these experiments would be conducted with proper ventilation and protective equipment.
Today, gas chromatography and mass spectrometry have largely replaced eudiometers for precise gas analysis. However, the fundamental principles of gas measurement established by these early instruments remain relevant in modern chemistry.
The eudiometer's journey from laboratory instrument to coffeehouse spectacle illustrates a pivotal moment in the history of science. It was a time when experiments became a form of sociability, when instruments became "conversation pieces," and when the feeling of an electric spark passing between participants in a demonstration mirrored the intellectual excitement connecting them 8 .
While modern gas chromatographs and spectrometers have replaced the eudiometer for precise analysis, its principles remain valid. More importantly, its story teaches us that making science engaging and accessible is not a modern invention. The next time you see a compelling science demonstration, remember the eudiometer and the 18th-century innovators who proved that measuring the air could be just as exciting as the air itself.
Pneumatic Chemistry
Quantitative Analysis
Public Engagement