Every Breath We Take: Understanding Our Atmosphere

These ancient trees have stood here

overlooking the Herefordshire countryside for hundreds of years.

The largest is 35m tall and 3m thick.

This tree is one of Britain's biggest living things.

And if I asked you where the raw materials came from to build it,

you might say from the soil or from the water

that it sucks up through its roots.

But in fact, this tree was built almost entirely from thin air.

The tree uses carbon dioxide from the air

to build the molecules that make up everything

from its mighty trunk to its delicate branches.

Nitrogen that has passed from the air into the soil

nourishes the tree.

And its leaves release oxygen.

The vital, life-giving gas that we need to breathe.

Today, we know the air around us

contains the raw materials from which life is made.

But how did we come to know that this invisible stuff around us

contains anything at all?

It is a remarkable story of heroes and underdogs,

chance encounters and sheer blind luck.

It shaped our modern world.

Unravelled the secrets of life itself.

And it all began with one simple question.

What is air?

I'm Gabrielle Walker and I trained as a chemist

because I love the way chemistry reveals

that ordinary things are full of hidden wonders.

Take this view across the water here in the Solent,

which hasn't changed much in hundreds of years.

But in a sense, I see it differently

from the way that people would have seen it in the past.

I know that the water is made of hydrogen and oxygen.

I know that the sun is a distant star

and I also know that the air around me

is made up of a mixture of gases.

I know these things because today we have the tools

and the technology to unlock the secrets of the natural world.

But for most of human history, there were no tools or technology,

only simple observation and deduction.

But when it comes to the air,

there isn't very much to observe.

There are no clues to suggest that air is made of anything but air.

So ever since the Ancient Greeks,

people had assumed

that air was a single element,

entire and indivisible,

with no constituent parts.

You can see why those beliefs made sense.

In fact, they seemed so reasonable,

that not only did they form the basis

for the way the Ancient Greeks saw the natural the world,

they also continued unchallenged for more than 2,000 years.

It wasn't until the 17th century

that this view of the world began to change,

and the first clues that the air around us contains hidden secrets

began to emerge.

New technology meant that it became possible to go beyond

what could be seen with the naked eye.

The first telescopes revealed entirely new worlds

that had lain hidden beyond our sight.

And the first microscopes uncovered a miniature kingdom

existing right beneath our noses.

A new breed of thinkers emerged,

with an entirely new way of understanding the world.

It was the Age of Enlightenment, the biggest cultural revolution

the world has ever seen.

This was led by men and women

who no longer trusted the traditional views of the world

and their place in it that had been handed down through the generations.

They were no longer content to understand the world

the way the Ancient Greeks had by passive observation.

Instead, they wanted to find the truth.

They probed and tested things.

They conducted experiments.

They called themselves natural philosophers.

We'd call them scientists.

The dawn of science brought remarkable progress.

Including the first big breakthrough in the quest to understand

what air is made of.

It was an extraordinary discovery,

but it was made entirely by accident.

In 1754, a Scottish doctor named Joseph Black

was looking for a cure for an excruciatingly painful condition

that plagued many of his patients...

..bladder stones.

In the 18th century,

the only treatment for bladder stones was surgery

with no anaesthetic.

Patients were held down,

sliced open

and the stones ripped out with metal tongs.

Joseph Black believed there had to be a better solution.

So he set out to make a medicine

that he hoped would dissolve the bladder stones,

eliminating the need for surgery.

The way he went about it was actually very simple,

so it's easy for me to replicate it today.

He started with this stuff, magnesia alba,

which is basically just a kind of salt.

And he intended to heat it

and mix it with water to make a medicine.

Magnesia alba was known for its corrosive properties

and Black thought it might be strong enough to dissolve bladder stones.

He would have heated it using a burning glass,

a sort of magnifying glass to focus the sun,

but I'm going to cheat a bit and use a modern blowtorch.

But as he heated it, Black noticed something odd.

Bubbles of air were released.

The air given off by the magnesia alba looked just like ordinary air,

but Black was not content with passive observation.

He was a natural philosopher, so he tested the air.

And his tests revealed that this air was anything but ordinary.

A candle would not burn in it.

And a mouse, that would last 15 minutes

in a container of ordinary, common air,

died in seconds.

Black had never seen anything like it.

This was the first time that anyone had identified a gas

that was different from common air.

Black called it "fixed air"

because he'd found it fixed inside the magnesia alba.

Today, we know it by its modern name -

carbon dioxide.

The tiny, but vital part of the air, that all plants rely on.

Completely by accident,

Black had found hard evidence

that air is not a single element.

It's made of constituent parts.

The problem was he just didn't see it like that.

Joseph Black had one of the key ingredients of air

right there in front of him, but he didn't realise it.

He couldn't let go of the ancient belief

that air is an element entire and indivisible.

He thought that "fixed air" was some entirely new kind of air,

totally different from the common air that we breathe.

The quest to understand the air had barely begun,

but it was already heading down the wrong path.

Black's discovery hadn't actually got us any closer

to understanding what air is made of.

But it did raise one intriguing question.

Why couldn't a candle or a mouse survive in "fixed air"

when they so easily could in ordinary air?

In other words, what was the relationship between combustion,

respiration and the air?

For months, Black tried in vain

to unravel the mysteries of "fixed air".

But eventually, he admitted defeat

and handed the problem over to his apprentice,

a promising young medical student named Daniel Rutherford.

Fresh-faced, young Rutherford took up the challenge

with great enthusiasm.

But instead of shedding light

on the mysterious properties of "fixed air",

he stumbled across yet another new gas.

It was even more potent than "fixed air".

Nothing would burn in it.

And it killed a mouse in an instant.

Its effects were so striking,

he named it "noxious air".

Rutherford, the apprentice, had discovered nitrogen.

The gas that makes up nearly 80% of the air we breathe.

For a 22-year-old student, this was a remarkable achievement.

But in truth, the discovery of nitrogen

had only added to the confusion.

The relationship between combustion,

respiration and air was still a complete mystery.

But Rutherford wasn't the only one who'd been working on the problem.

Other natural philosophers had come up with an excellent solution.

Or so they thought.

This beautiful manuscript was handwritten in 1783.

And it refers to a bizarre idea

they had at the time about why things burn.

It says, "Inflammable air may be made from liquid substances

"containing phlogiston."

They thought that phlogiston was a sort of fire-like element

existing in anything that could burn,

but this isn't some mystical text.

It's a scientific publication

and in the 18th century,

the theory of phlogiston was right at the cutting edge.

It sounds bizarre to us today,

but at the time, the existence of phlogiston

seemed to make perfect sense.

According to the theory, when the candle burns,

it gives off that fiery substance - phlogiston.

If I cover the candle,

phlogiston builds up inside the jar

until eventually there's no room for any more

and the candle goes out.

It's like trying to fit more people into a crowded room.

There's no room for any more phlogiston to leave the candle

and according to the theory,

that's why the combustion stops.

And it wasn't just candles,

phlogiston was thought to dwell inside living things too.

When I breathe out, phlogiston is released in my breath,

so if you put me in a jar, I'd have the same fate as the candle.

Pretty soon, the jar would fill up with phlogiston,

there would be no more room for any to leave my body and I'd die.

The idea of phlogiston seemed so logical,

it soon became the accepted explanation

for combustion and respiration.

For Rutherford, it was the answer he'd been looking for.

He reasoned that "fixed air" must contain more phlogiston

than common air, and "noxious air" contained even more still.

That would explain why the gases would put out a candle

and kill a mouse.

It was the perfect solution.

Except, of course, it was completely wrong.

Today, we know that if you put a flame

or a living thing in a confined space,

it doesn't die because the space fills up with phlogiston.

It dies because it runs out of oxygen.

Oxygen was the missing piece of the puzzle.

The one vital part of the air, yet to be found.

Its discovery would bury the idea of phlogiston forever...

..give a profound insight into our own physiology...

..and transform our world.

The identification of oxygen was to be

one of science's greatest achievements.

But the question of who deserved the credit for the discovery

caused a bitter dispute.

And it's still a contentious issue today.

There are three contenders, each with a rightful claim to the glory.

An innovative Swedish pharmacist.

A wealthy Parisian aristocrat.

And an ordinary, working-class Englishman named Joseph Priestley.

Priestley was different from other natural philosophers of his day.

He was disorganized.

He lacked focus.

You wouldn't think he had the makings of a great scientist.

But he was creative

and that meant he made connections the others missed.

Typical of his eccentric character,

Priestley's role in the discovery of oxygen

began in the most unlikely of places...

..a brewery.

But it wasn't the beer that drew him here.

It was the thick, heavy air that flowed out of the brewery vats.

This is an experiment that Priestley himself actually did.

He put a flame in the path

of the air that was coming out of the vats...

..and as you can see...

..the flame goes out.

He realised this must be the same "fixed air"

that Joseph Black had discovered a few years earlier.

Priestley decided to experiment with it.

It was the start of an extraordinary journey

that would take him far beyond what Black had discovered

and eventually lead him to oxygen.

On one of his visits to the brewery, Priestley brought some bellows,

because he wanted to try bubbling the gas through water.

My equipment isn't quite as elegant as Priestley's.

I've got this plastic tube,

which is attached to the bottom of the vat here

where all the carbon dioxide has sunk to the bottom

and a bottle of water.

But the principle of the experiment is exactly the same.

So, turn the valve...

..and there it goes!

Once he had tired of mixing the gas with water,

Priestley decided to taste it.

Not bad, it's kind of tingly.

He had invented the world's first fizzy drink.

Soda water.

SODA CAN POPS OPEN

Others would make millions from Priestley's invention.

But he had a different idea that was far more left-field.

He'd heard of a curious observation made by an Irish surgeon,

who had left a piece of meat to rot

and noticed that as it rotted,

it released carbon dioxide, "fixed air".

At the time, nobody had a clue what was causing meat to rot,

but these new experiments gave Priestley an idea.

What if the meat was going bad

because it was losing its "fixed air"?

And if that was the case,

perhaps, he could use this to put the "fixed air" back into the meat

and stop it rotting.

But Priestley wasn't thinking about the type of meat you eat.

He was thinking about people's bodies.

Because in the 18th century,

diseases that caused flesh to rot were rife.

There was gangrene,

yellow fever

and the disease that every sailor feared...

..scurvy.

In Priestley's day,

the British Navy was engaged in almost continuous battle,

defending the Empire at home and overseas.

But more sailors died from scurvy than in battle.

'Medical historian Dr Erica Charters, is going to show me

'why Priestley believed his soda water had the potential

'to be much more than just a fizzy drink.'

So, Erica, these are pretty revolting, what are they?

So, this is an early 19th-century portrayal of scurvy sufferers

and specifically of their legs.

So, people often talk about why they think it's a disease

of putrefaction is because it looks

like people are putrefying

and they can smell the putrefying matter as well.

So, if you look at the images here

and if you kind of imagine what it would've been like to be

on a crowded ship with hundreds of men suffering from scurvy,

you can imagine why people said it's clearly a problem with putrefaction.

I suppose, if...if the flesh is actually, seems like it's rotting,

-it's also giving out "fixed air", carbon dioxide...

-Yeah.

-And that's where Priestley's idea comes in.

-Yeah, that's right.

So, there were all sorts of theories about how you need to

kind of either put the "fixed air" back into the meat

or by having "fixed air", this will somehow preserve your body

from the natural process of putrefaction.

So what Priestley had was "fixed air" actually trapped

-and that would've been a way to get it back into people.

-That's right.

It would have been a very practical solution to curing scurvy.

That would have been a pretty big deal

if he actually had come up with a cure for scurvy.

It would have been. This was seen as being something,

which was important to the British nation

and to the British Empire as well.

So, if you could cure scurvy, you would really make a difference.

There was so much optimism that Priestley's soda water could work,

naval officials instructed Captain James Cook to test it out

on his second voyage.

Cook set sail for the coast of Australia,

but sadly, soda water didn't have the effect everyone was hoping for.

We now know that fresh produce is the key to keeping scurvy at bay.

But as far as Priestley was concerned,

it didn't matter.

Soda water had got him hooked on the study of gases.

He couldn't wait to carry out more experiments

to see what else he could find.

And it was this enthusiasm

that was the key to Priestley's great breakthrough.

What I love about Priestley is he wasn't some great doctor

or scholar, but he had this fiery curiosity.

If an experiment blew up in his face,

he'd just dodge out of the way of the flying glass

and just do it again to see if it would happen again.

For me, Priestley embodies all that's great

about using scientific experiments as a way to explore the world,

because the results aren't always planned.

Sometimes, the best things come

when you're not expecting them.

And that's exactly what Priestley found.

Characteristic of Priestley's disorganised,

scatter-gun approach to experimenting,

one day, for no particular reason,

he tried heating some mercury.

Nothing particularly interesting happens when you heat mercury,

except that you get this - a sort of orangey powder

that at the time they called mercury calyx.

Ever the experimenter, though, Priestley tried heating the calyx

and this turned out to be much more interesting.

To his delight, the calyx released bubbles of gas.

He collected as much of it as he could and decided to test it.

In "fixed air", a flame would go out.

But in this air,

it burned more brightly than anything Priestley had ever seen.

A mouse that would die in "fixed air",

lived quite happily in this new gas.

In fact, the mouse lived twice as long as it would in ordinary air.

Priestley even tried breathing it himself

and he said it made him feel fantastic.

He wrote excitedly, "Up until now,

"only two mice and I have had the privilege of breathing it."

Priestley realised that this new gas was very special indeed.

Priestley had discovered oxygen.

The most fundamental, life-giving part of the air

that we all need to breathe.

He'd seen the profound effects it had on a flame and on living things.

He had witnessed the chemistry of life happening

right there in front of him.

He was so close to breaking down the barriers

that had held Black and Rutherford back.

But he didn't quite make it.

As far as Priestly was concerned,

the fiery, life-giving properties of his new gas

fitted neatly into the fashionable theory of the day -

phlogiston.

This mysterious element was believed to be released during combustion

and respiration.

"Fixed air" and "noxious air" were thought to be full of phlogiston.

That's why they put out a candle and killed a mouse.

But Priestley's gas appeared to have virtually no phlogiston,

so a candle and a mouse would thrive in it.

He gave it the not-exactly-catchy name -

"dephlogisticated air".

Priestley had isolated oxygen,

but that doesn't necessarily mean he deserves the credit

for its discovery.

Because in truth, Priestley had no idea what it was that he'd found.

And it turned out he wasn't even the first person to have come across it.

There was a second contender,

a Swedish pharmacist named Carl Scheele.

In 1771, three years before Priestley's experiment,

Scheele had been busy mixing chemicals in his lab,

when he came across an unusual gas.

He put a candle into it and he wrote,

"That immediately the candle burned with a large flame

"of so vivid a light that it dazzled the eyes."

Obviously, it was oxygen.

So, was Scheele the true discoverer?

Well, unfortunately for him,

although he did the experiment first,

he didn't publish his findings until much later.

So, although Scheele was the first to find oxygen,

Priestley was the first to document it.

But even so, for me,

that was still just the first step towards its discovery.

The next big leap was truly to understand it.

And that required the mind of a visionary.

Across the Channel in France, revolution was in the air.

Taxes were high, famine was rife

and civil unrest was close to boiling point.

Life was harsh for everyday folk,

while the rich were lavished with luxury.

Antoine Lavoisier was the only son

and heir of one of Paris' most distinguished families.

He was extremely bright,

absurdly wealthy and he had one passion -

natural philosophy.

By the age of 29, Lavoisier had published papers on everything -

from the water systems of Paris to the composition of meteorites.

But it wasn't great wealth that made Lavoisier

such a brilliant scientist,

although no doubt it helped.

It was his personality.

Lavoisier was meticulous.

He loved precision and accuracy.

The messy, complicated system of weights and measures

used at the time drove him mad with frustration.

So he developed a new, orderly system of grams and kilograms.

The metric system.

It was Lavoisier's obsession with weights and measures

that would lead him to uncover the true nature of the air around us

and ultimately, transform our world.

Like any 18th-century scientist worth his salt,

Lavoisier spent a great deal of time heating things up

to see what happened.

And just like all the others, Lavoisier started out believing

that substances released phlogiston when they burned.

But precise, meticulous Lavoisier was worried about something

that everyone else had overlooked.

If heating a substance made it lose phlogiston,

it should get lighter,

but that simply wasn't the case.

He carefully weighed a piece of lead,

and then he heated it

until it turned into a brown, powdery substance called lead calyx.

But when he weighed the calyx,

it was heavier than the lead he'd started with.

It was a pivotal moment.

Lavoisier knew the theory of phlogiston had to be wrong.

Something else was going on in combustion.

If lead gets heavier when it's heated,

it can't be losing phlogiston.

It must be gaining something, but what?

This was where Lavoisier's fascination with weights

and measures came into its own.

Lavoisier repeated the experiment.

He took some lead and he put it on a set of scales.

Not modern electronic scales like these, but the principle's the same.

Then, the clever thing was, he put the scales on another set of scales.

Then he sealed it,

so that nothing could get in and nothing could get out.

And now, he heated the lead from the outside.

And just as he expected, as the lead got hotter,

it got heavier.

But when he looked at this scale,

it hadn't moved an inch.

So, because the lead was sealed inside,

whatever was causing it to get heavier

had to come from within the jar.

It had to be the air.

Some mysterious ingredient was passing from the air into the lead.

Lavoisier was determined to get this mysterious ingredient

back out of the lead, so he could study it.

But try as he might,

the lead stubbornly refused to release the air it had absorbed.

The brilliant Lavoisier was stumped.

But then he heard that an English scientist was in town.

Someone who was renowned for his work with new gases.

Joseph Priestley.

Priestley was on a tour of Europe as the guest of an English aristocrat.

When he received an invitation to dinner

from the great Antoine Lavoisier,

he was excited and immediately accepted.

Over dinner, they talked chemistry.

With great enthusiasm,

Priestley told Lavoisier all about his latest discovery.

Lavoisier listened intently

while Priestley described how heating mercury calyx

released vast quantities of "dephlogisticated air".

Mercury was the one thing that Lavoisier hadn't tried.

From the sound of Priestley's experiment,

it could be exactly what he was looking for.

Mercury absorbed air when it was heated,

but crucially, unlike lead,

when it was heated a second time, it released the air it had absorbed.

And not just any air -

"dephlogisticated air".

Lavoisier realised that this could be the missing ingredient

he was looking for.

As soon as Priestley left,

Lavoisier ditched the lead he had been using

and tried heating mercury instead.

Lavoisier heated mercury until it turned into mercury calyx

and he measured how much air went into it.

And then he heated mercury calyx until it turned into mercury

and measured how much air came out.

And found it was exactly the same amount.

The mercury had absorbed a part of ordinary air

when it was heated

and that same air had been released again.

The air released was the gas Priestley called

"dephlogisticated air".

Lavoisier realised that "dephlogisticated air"

must be a part of ordinary air.

It was a ground-breaking discovery.

He ditched the messy, complicated name Priestly used,

and called the gas oxygen.

In 1774, Lavoisier announced the discovery of oxygen to the world.

Scheele was astonished.

Priestly was furious.

He complained bitterly that he was the one who'd discovered it first.

Who deserves to claim the glory still divides opinions today.

For me, it was Lavoisier who truly discovered oxygen.

Because I think a discovery isn't so much about being

the first to find something.

You also have to understand what it is that you've found.

Lavoisier was the one who showed

that oxygen wasn't just a curious new gas.

It is a part of the air we breathe.

This was a huge leap forward in the quest to understand

what air is made of.

And soon, all the other pieces of the puzzle fell into place.

"Fixed air" or carbon dioxide,

and "noxious air", nitrogen,

were found to be parts of the air too.

For the first time,

people understood that air is made of a mixture of gases.

And that knowledge transformed our world.

Because until this point,

the air had been viewed as little more than empty space.

Now, it was seen as an untapped mine, rich in raw materials.

And it wasn't long before those raw materials were extracted and used.

Scientists discovered that oxygen could be bubbled

through molten iron to remove impurities.

And pure iron allowed them to make the steel

that built the railways, ships and factories

that powered the Industrial Revolution.

Identifying the gases that air is made of helped

to build our modern world.

And manufacturing those gases is still big business today.

At this air separation plant in Hampshire,

the individual gases that make up the air

are separated out on an industrial scale.

'At this site,

'2,000 tonnes of air per day

'are sucked in, compressed,

'and then cooled to cryogenic temperatures,

'so the gases become liquids.'

-So it's quite a complicated process?

-Yeah...

'And I'm going to see how it's done.'

-This is Andy.

-Hello, Andy.

-Hello, Gabrielle.

What have you got for us?

I've got a demonstration here to show the process

-that's going on inside our column.

-OK.

OK, what I've got is a balloon full of air

and I've got some liquid nitrogen,

which I'm going to pour into the bowl to make a nice cold bath.

What I'm going to do is drop the balloon in

and the balloon will now get nice and cold.

It will boil up a bit. It's liquid nitrogen.

What happens straightaway is the gas inside the balloon

starts to condense. It's getting cold.

You can see the balloon just starting to shrivel.

It looks like it's freezing on the outside.

-It's not breaking the balloon?

-No, no.

-It will stay in one piece.

-Oh.

A liquid nitrogen bath is about -196 degrees centigrade.

At -185, the oxygen in the balloon should condense

from a gas to a liquid and that's why we are able to condense

the oxygen in our air separation process.

We would draw it to that point as liquid leaving behind nitrogen.

And that's how you separate them?

That's how we separate the air.

SHE GASPS IN SHOCK I'll tip it up now.

You can just make out a level of liquid in the bottom of the balloon.

-There's a small puddle of liquid oxygen.

-So, there it is!

Separated out. There it is. There's the oxygen.

I breathe it and now I see it.

If I leave this out, you can see the balloon is starting to inflate.

It's now getting warm. It's drawing heat from the atmosphere.

If I leave that on my glove,

the balloon will inflate back to its normal size and shape.

The same constituent parts of the air are inside.

So, that's how you separate the air.

That's how we do it.

Because nitrogen becomes a liquid at a lower temperature than oxygen,

cooling the air makes it possible to separate out the two gases.

It's a hi tech process that transforms invisible air

into tangible, raw materials we can use.

Oxygen is used in hospitals to save lives.

It's an ingredient in modern plastics.

And it powers the rockets that have allowed us to explore space.

Nitrogen makes the fertilisers that nourish the crops

we rely on for food.

And it's used in the packaging industry to keep our food fresh.

The discovery that common air around us is made up of a mixture of gases,

each with their own potent properties

was clearly an extraordinary achievement.

There's no doubt that Lavoisier's contribution to our understanding

of air was a vitally important one.

But it still wasn't complete.

There was one question left unanswered.

What was the relationship between the air and living things?

That puzzle would ultimately lead scientists to reveal

why we are able to live at all

and why we must all eventually die.

And the first steps were made by none other than Lavoisier himself.

He had noticed that flames

and living animals had two things in common.

Both radiated heat

and both thrived in oxygen.

But to figure out how burning and breathing

could possibly be connected,

Lavoisier had to overcome a very tricky problem.

He could control the amount of oxygen used by a flame

or an animal, simply by putting them in a confined space.

But how would he measure how much heat they radiated?

The solution was an ingenious piece of apparatus

that still exists today.

Professor Gerard Ferey is going to show it to me.

So, this is actually all Lavoisier's original equipment

-that Lavoisier used himself.

-It is.

So, can...can we look at it? Can we see inside?

Yes, with pleasure, but with respect.

Lavoisier had this piece of apparatus,

called a calorimeter, purpose-built to house a living animal.

A guinea pig was the ideal candidate.

And here was, inside of the calorimeter, the magic machine.

So, where did the animal go?

The animal was put in this sealed cage.

The volume of this cavity was known by Lavoisier.

That means that he knew the amount of oxygen

at the very beginning of the experiment.

That's how he knew. The chamber was sealed.

He knew exactly how much oxygen it was breathing.

'With the guinea pig sealed inside,

'next, Lavoisier packed the outer chambers with ice.'

The animal produces heat...

..during 24 hours

and the ice, which is in this partition of the apparatus,

as soon as it receives heat from the animal,

the ice melts

and when it melts, it becomes water

and water is gathered

at the bottom of the calorimeter

and as soon as you weigh the water,

you know the amount of heat,

which was produced by the animal!

Very simple.

Next, Lavoisier repeated the experiment.

But this time, instead of an animal,

he put a piece of burning charcoal inside.

His findings were astonishing!

When the charcoal had consumed the same amount

of oxygen as the guinea pig,

it had also melted the same amount of ice.

So he realised that the animal was using oxygen

the same way a fire uses oxygen. It was the same thing.

It was a really big breakthrough to make the parallel

between chemistry and life.

With this ingenious experiment,

Lavoisier had discovered that combustion and respiration

are essentially the same process.

Just like coal burning on a fire,

Lavoisier proposed that all animals burn food as their fuel

and it generates heat that warms their bodies.

He speculated that this process takes place in the lungs.

Today, we know it happens deep inside every one of our cells.

Lavoisier's understanding of respiration was only rudimentary,

but it broke new ground.

He had caught a glimpse of the chemistry of life itself.

For me, Lavoisier's contribution to science was as important

as Newton's or Darwin's.

He didn't just identify a new gas,

he changed the whole way that we view the world

by revealing what air really is

and the crucial role it plays in the chemistry of life.

It was the crowning glory of an extraordinary scientific career.

But it was also one of the last experiments Lavoisier

would ever conduct.

He was born into a life of wealth and privilege.

But his findings about respiration changed him.

They showed that the more active a person is, the more fuel,

or food, they needed.

But in 18th-century France, exactly the opposite was happening.

The poor people, who did all the manual labour, had the least food.

Lavoisier saw this imbalance between rich and poor as a great injustice

that had to be stopped.

By the end of the 18th century,

civil unrest in France had grown to full-scale revolution.

But unlike many of his wealthy, aristocratic friends,

Lavoisier didn't flee.

To the rioting masses,

it didn't matter that Lavoisier stayed in Paris

to support the Revolution

any more than it mattered that he was a great scientist.

All they could see was a member of the bourgeois elite.

So, on 8th May 1794, he was brought here

to the Place de La Revolution...

..and guillotined.

A fellow scientist said,

"It took them only an instant to cut off that head,

"and 100 years may not produce another like it."

Lavoisier's life had ended,

but his legacy lived on.

He had steered the quest to understand the air

towards the modern, scientific understanding we have today.

All the major gases -

carbon dioxide,

nitrogen,

oxygen

had been identified.

He had begun to unravel the mysteries

of combustion and respiration.

But there was one problem even the great Lavoisier had overlooked.

A niggling question that would end up revolutionising

the whole of science.

And revealing that oxygen, the gas of life,

has a darker side.

The problem was spotted back in England

by a humble teacher named John Dalton.

When he wasn't in the classroom,

Dalton loved to indulge in a very British passion...

..the weather.

Dalton spent a great deal of his time walking in the hills

and valleys of the English countryside,

trying to understand the weather by measuring the temperature

and humidity of the air.

Lavoisier's revelations about what air is made of captivated Dalton.

But something about it puzzled him.

As he walked among the misty hills,

he wondered how all the different gases that make up the air

could occupy the same space at the same time,

when solid bodies obviously could not.

Dalton reasoned that while air is made up of different gases,

the gases themselves must be made up of individual particles.

That's how they could mix into one another.

Over time, Dalton fine-tuned his theory.

He proposed that the particles of one gas were different

from the particles of another...

..and that it's not just gases -

all things are made of particles.

He called the particles atoms.

Dalton's atomic theory is the foundation

on which our modern understanding of the world is built.

Many of science's greatest achievements

simply wouldn't have been possible if we didn't know about atoms.

It opened the door to a far deeper understanding

of how all things are made,

including the air we breathe.

When you know that air contains atoms and molecules

you can really understand what it's made of.

Carbon dioxide - carbon atoms stuck to two oxygen atoms.

Nitrogen - two atoms of nitrogen tightly bounded together.

And we now know there are traces of other gases in the air -

argon, water vapour...

But when you look at the atomic level, there is one part of air

that stands out from all the others -

oxygen.

An oxygen atom has a nucleus surrounded by electrons.

In the outermost shell has six electrons,

but it really, really wants to have eight.

So, there's a space,

a hole for two electrons that oxygen will do anything to fill.

It will grab those electrons from any passing atom

or molecule that it can.

That's what makes it so different from all the other parts of the air.

Oxygen will react with more or less anything.

It was only when we began to understand air

at the level of the individual atom,

that the true wonder of oxygen was revealed.

Because when oxygen atoms react,

they release the elixir of life itself...

..energy.

I've got here some beautiful liquid oxygen

and I'm going to show you how powerful this stuff really is.

I've also got a perfectly normal digestive biscuit.

I'm going to dunk the biscuit

into the liquid oxygen.

And now, all I need is a spark.

Now, that is an oxygen reaction!

The oxygen is ripping and tearing electrons

from atoms in the digestive biscuit

and releasing all of that energy.

And in the same sort of way, the oxygen is reacting

with the food in our bodies to release the energy we need to live.

Oxygen's spectacularly reactive nature

gives us the huge amount of energy

we need to keep our hearts pumping

and our brains alive.

It allows us to lead active, vigorous lives.

Oxygen gives us everything that's worth living for.

But there is a price to pay.

Because breathing oxygen is like playing with fire.

And just like wood on a bonfire,

we're getting burned.

The oxygen in the air around me

is reacting with the wood to release energy.

That's what fire is.

And that's the same sort of chemical process

that's happening in your body.

But whenever oxygen gets involved in a reaction,

it's not delicate.

It rips and tears to grab the electrons it needs

and in the process, it creates something called free radicals.

Free radicals are some of the most powerful

and destructive particles on the planet.

Flames are full of free radicals

and so are our bodies every time we breathe.

They tear through our molecules and cells.

Over time, that damage accumulates

and in the end, it's that damage that's the reason

why we all grow old and die.

With every breath we take,

oxygen is slowly killing us.

But I still think it's worth it.

Because without oxygen, life would be very different.

If you want to know what life would be like if we didn't breathe oxygen,

you don't have to go very far.

Any fresh body of water will have what I've come here to find.

This may just look like a bucket of mud,

but in fact, there are trillions and trillions of tiny bacteria

living in it.

They can't breathe oxygen the way that we can.

In fact, it's even poisonous to them,

and that's why they have to spend their life

living at the bottom of a pond.

Because they can't count on breathing oxygen,

they have to rely on breathing sulphate from the water,

and that's not nearly as effective at delivering energy.

That means these bacteria simply don't have the energy

to grow any bigger than a single cell.

It's only oxygen-breathers that can release the vast amount of energy

that it takes to grow a big, multicelled body

and power a brain.

And that's why oxygen is so special

because without it,

we'd all be pond slime.

Only oxygen can give living beings the energy they need to walk,

run, fly and think.

It has shaped the course of our evolution.

Without it, we wouldn't be here at all.

The quest to understand what air is made of began as a simple desire

to further our knowledge of the natural world.

But it lead us far beyond that.

It uncovered powerful gases

that have forged the world we live in today.

Gave a profound insight into our own physiology

and the chemistry of life itself.

And it even revealed the existence of atoms.

The fundamental building blocks from which all things are made.

So, it may look as if my hands are empty right now,

but in fact,

they contain the most miraculous stuff in the universe.