Robert Bunsen

Chapter 1 · The Compounds That Cost Him an Eye

Cacodyl, and knowing when to stop

1811 – 1838 · Göttingen · Kassel · Marburg

Robert Wilhelm Eberhard Bunsen was born at Göttingen in March 1811, the youngest of four sons of the university's professor of linguistics. He took his doctorate at nineteen and travelled through Europe visiting laboratories, factories and mines.

His early research was on the *cacodyl* compounds — organic compounds of arsenic, and among the most unpleasant substances a chemist can work with. They are spontaneously inflammable in air, they stink appallingly, and they are extremely poisonous.

The work was genuinely important. Cacodyl was the first *organometallic* compound to be properly characterised, and the group of atoms passed intact through a series of reactions, providing early evidence for the radical theory of organic chemistry.

It was also nearly fatal. In 1843 a sealed tube of cacodyl cyanide exploded and a glass splinter destroyed the sight of his *right eye*, permanently. He also suffered arsenic poisoning severely, twice, and came close to dying.

He found the antidote in the course of it: freshly precipitated *hydrated iron oxide*, which binds arsenic in the gut and prevents its absorption. It is still the basis of treatment for arsenic poisoning.

And then he stopped. He abandoned the cacodyls entirely, on the stated ground that no result was worth what the field was costing him.

That decision is worth noticing. He was not squeamish and he was not risk-averse — he later climbed into volcanic craters in Iceland to collect gases. He made a judgement that a particular line of work had a cost out of proportion to its value, and acted on it.

“A chemist who is not an experimenter is no chemist at all.”

— Attributed to Robert Bunsen

Why this matters

Bunsen found the antidote to arsenic poisoning while being poisoned by his own research, and then abandoned the field on the grounds that the results were not worth the cost.

You have the exploded tube and the decision to stop. What would you ask?

Ask Bunsen

• “What happened when the tube exploded?”
• “How did you find the antidote?”
• “How do you decide a line of work is not worth it?”
• “Why did the cacodyl work matter scientifically?”
• “Were you frightened by any of it?”

Chapter 2 · More Than Half of It Going Up the Chimney

Measuring what a blast furnace throws away

1838 – 1845 · Kassel · Marburg · England · Scotland

In 1838 Bunsen turned to a question nobody had thought to ask. The blast furnaces of Germany and Britain were consuming enormous quantities of charcoal and coke to smelt iron. What was actually coming out of the top?

Nobody knew. The gases were simply vented.

Answering it required *gas analysis* good enough to determine the composition of a complex hot mixture, and Bunsen had to develop the methods himself: collecting the gas, absorbing each constituent selectively, measuring volumes over mercury with corrections for temperature and pressure. His work on gasometry became a standard treatise.

He went to the ironworks and did it, and later collaborated with Lyon Playfair on British furnaces in Staffordshire and Scotland.

The result was startling. More than *half* the heating value of the fuel charged into the furnace was leaving the top unburned. The largest single component was *carbon monoxide* — which is itself a fuel, burning to carbon dioxide with a substantial release of heat, and it was simply being thrown into the air.

So the furnaces were operating at less than half efficiency, and the waste was recoverable: capture the top gas, burn it, and use the heat to preheat the incoming blast.

That is the origin of gas recovery and regenerative preheating in metallurgy, and it saved industrial quantities of fuel.

It is also a demonstration of something more general: that a process nobody has measured is very likely wasting a great deal, and that the way to find out is to go to the works with apparatus rather than to reason about it.

Why this matters

Nobody had measured what a blast furnace emitted, and when Bunsen did, more than half the fuel value was leaving unburned as carbon monoxide.

You have the furnace top and the wasted gas. What is your question?

Ask Bunsen

• “What did you find coming out of the top of a blast furnace?”
• “Why is carbon monoxide in the exhaust such a loss?”
• “How do you analyse a hot mixed gas?”
• “What did the ironmasters make of your figures?”
• “How much of any process is wasted if nobody measures it?”

Chapter 3 · The Collar at the Base

The burner, 1855

1852 – 1855 · Heidelberg

Bunsen took the chair at Heidelberg in 1852, on condition that a new laboratory be built.

He needed a flame he could control. Existing gas lamps burned coal gas at a jet, giving a large, luminous, unsteady yellow flame that deposited soot on everything it touched and was not hot enough for much analytical work. Soot in a flame ruins a measurement, and the luminosity makes it useless for looking at anything else glowing in it.

The solution, worked out with the university's mechanic *Peter Desaga* in 1855, is a single idea: mix the gas with air *before* it burns.

A vertical tube stands over a gas jet. Near the base are holes, and around them a rotating *collar* that opens or closes them. With the collar shut, gas alone reaches the top and burns in whatever air it meets — a large, yellow, sooty flame. Open the collar, and air is drawn in and mixed with the gas as it rises. The mixture burns at the top in a small, almost invisible blue flame, hot enough to melt platinum wire and depositing no soot at all.

One adjustable hole demonstrates the difference between complete and incomplete combustion, and does it visibly, which is why the burner is on every school bench in the world and why the air hole is the first thing a pupil is taught to use.

Neither Bunsen nor Desaga patented it. Bunsen never patented anything.

The design was not wholly original — Faraday had made a similar premixing lamp — but the Bunsen–Desaga version was the practical one and spread everywhere.

Bunsen also invented the ice calorimeter, the grease-spot photometer, a cheap zinc–carbon cell, a filter pump and an effusiometer.

Why this matters

The air hole on a Bunsen burner is the clearest demonstration in chemistry of the difference between complete and incomplete combustion, and it exists because analysis needs a sootless flame.

You have the collar and the two flames. What would you ask?

Ask Bunsen

• “Why does mixing air in first change the flame so much?”
• “What is the yellow colour actually made of?”
• “Why did you need a sootless flame at all?”
• “How much of the burner was Desaga's?”
• “Why never patent anything?”

Chapter 4 · Reading the Flame

Spectroscopy with Kirchhoff, and two new elements

1859 – 1861 · Heidelberg

It had long been known that certain substances colour a flame — sodium yellow, potassium lilac, strontium red. Bunsen tried to use this analytically by viewing flames through coloured glass filters, and could not make it reliable: the colours from a mixture blend and cannot be separated.

His Heidelberg colleague *Gustav Kirchhoff*, a physicist, made the suggestion that solved it. Do not look at the colour. *Disperse* the light through a prism and look at the *spectrum*.

The result is that each element produces a set of *bright lines* at particular positions, and those positions are characteristic and invariant. A mixture gives all the lines of all its components, superimposed but separable, because each line is at its own place.

They built the *spectroscope* — burner, slit, prism, telescope — in 1859, and it is one of the most consequential instruments ever made.

It is extraordinarily sensitive. Bunsen calculated that it could detect around a three-millionth of a milligram of sodium.

They immediately used it to find new elements. In 1860, examining mineral water from Dürkheim, they found two blue lines belonging to nothing known: *caesium*, named for the Latin for sky-blue. In 1861, in the mineral lepidolite, two red lines: *rubidium*, from the Latin for deep red.

To isolate enough caesium to characterise it, Bunsen evaporated something like forty tonnes of mineral water.

Kirchhoff then extended the method to the Sun, showing that the dark Fraunhofer lines in the solar spectrum correspond to bright lines of terrestrial elements — so the composition of a star can be read from its light.

Bunsen's distrust of theory left him with no account of *why* an element emits particular lines. That needed Bohr, in 1913.

Why this matters

Spectroscopy let chemists identify elements in vanishing quantities and, within two years, read the composition of the Sun from its light.

You have the prism, the bright lines and forty tonnes of water. What is your question?

Ask Bunsen

• “Why does a prism succeed where a coloured filter fails?”
• “How sensitive is a flame test with a spectroscope?”
• “Did you really evaporate forty tonnes of water?”
• “Why does each element give its own lines?”
• “How much of this was Kirchhoff's idea?”

Chapter 5 · Every Day at the Bench

Refusing Berlin, and a laboratory full of students

1852 – 1899 · Heidelberg · Iceland

Bunsen refused chairs at Breslau and at Berlin, and stayed at Heidelberg for forty-seven years.

His laboratory was one of the great training grounds of the century. Students at their own benches, doing analyses; Bunsen among them all day, going from one to another. Henry Roscoe, Adolf von Baeyer, Dmitri Mendeleev, Lothar Meyer, Fritz Haber and Adolf Lieben all worked there.

He never married. Asked about it, he is said to have replied that he had no time. He lived simply, spent his days in the laboratory and his evenings reading, and by his own account regarded his students as his household.

He was not a theorist and made a point of it. He distrusted speculation not supported by apparatus, and would not engage with arguments that could not be settled on a bench. That is a real limitation — it is why he had no account of what his own flame or his own spectral lines were doing — and it was a deliberate position.

He travelled to *Iceland* in 1846 with a scientific expedition, collected gases from volcanic vents and geysers, and worked out the mechanism by which a geyser erupts: water at the bottom of the column is under pressure and so superheated above its ordinary boiling point, and when a small amount flashes to steam and lifts the column, the pressure drops and the rest boils explosively. He tested it with a model.

He retired in 1889 and returned to his first love, mineralogy, at eighty.

He died at Heidelberg in August 1899, aged eighty-eight, and is buried there.

Why this matters

Bunsen refused Berlin twice and spent forty-seven years at the same bench, and the students who passed through his laboratory include Mendeleev, Baeyer and Haber.

You have Heidelberg, the students and the geysers. What would you ask him?

Ask Bunsen

• “Why refuse Berlin?”
• “How does a geyser actually work?”
• “Why distrust theory so completely?”
• “What did you teach that Mendeleev took away?”
• “Did you really have no time to marry?”

What Bunsen changed

The burner stands on every school bench in the world, and its air hole is still the clearest demonstration in chemistry of the difference between complete and incomplete combustion. With Kirchhoff he built the spectroscope, discovered caesium and rubidium, and opened the reading of stellar composition from light. The forgotten furnace-gas work was arguably more important still: it established that combustion could be measured, costed and improved.

A debate that continues

The burner design was not wholly original — Faraday had made a similar premixing lamp — and the extent of Peter Desaga's contribution to the practical version is not fully settled.

Keep exploring — ask Bunsen

• “What else has nobody bothered to measure?”
• “Should an invention like the burner ever be owned?”
• “What did the arsenic finally teach you?”

Related lives

• Gustav Kirchhoff — Who Read The Sun's Chemistry
• Dmitri Mendeleev — Author of the Periodic Table
• Justus von Liebig — Who Weighed What Burned Away
• Carl Remigius Fresenius — Who Put the Tests in Order

Related themes

Combustion and flames · Flame tests and spectra · Gas analysis