This Day in Chemistry: Joseph Priestley

Lucy Bell-Young

by Lucy Bell-Young

1st August 2018

On this day in 1774, Joseph Priestley first isolated one of the most crucial elements of our time: oxygen. A chemist, philosopher, famous dissenter and eventual expatriate, Priestley provided the scientific work that modern-day chemistry hinges on – but he didn’t necessarily agree with the theories his work facilitated.

About Joseph Priestley

Born in 1733, Joseph Priestley occupied many positions during his lifetime. He was a theologian, a natural philosopher, a Rational Christian, a chemist, an educator as well as a liberal political theorist.

Science played an integral role in Priestley’s theology of and commitment to Rational Christianity. He believed that gaining an awareness of the natural world was essential to human progress and bringing about the Christian Millennium.

Unfortunately, this belief also manifested itself in his narrow-minded rejection of the Chemical Revolution; and this, combined with his unwavering Christian theism and support of dissenters, eventually isolated him from the scientific community and even his own country.

Writing more than 150 published works, Priestley contributed many important things to the world of chemistry despite his stance on theology and religion, including:

  • The invention of soda water
  • Investigations on electricity
  • Discovery of different gases
4 glasses of different coloured soda drinks

Joseph Priestley discovered soda water when he suspended a bowl of water over a beer vat. This infused the water with carbon dioxide, which was then known as ‘fixed air,’ and he realised that it created a fresh-tasting drink.

Discovery of Different Gases

Priestley’s lifetime fascination with the chemistry of gases was sparked in 1767 when he managed to infuse water with carbon dioxide. He did this by suspending a bowl of water over a beer vat. This is how soda water was first discovered. But in the mid-eighteenth century, only three types of gases had been identified:

  • Air
  • Fixed air, now known as carbon dioxide
  • Inflammable air, now known as hydrogen

Then known as ‘airs,’ Priestley carried out investigations to explain the chemistry behind these three gases. He concluded that all three were examples of the phlogiston theory in action. This was a seventeenth-century, obsolete theory that claimed every combustible substance is made of and releases phlogiston, a hypothetical and immaterial principle of fire, when burned.

Despite his belief in this erroneous theory, Priestley is widely credited as one of the founding fathers of chemistry for his discovery of several new gases. By designing ingenious apparatus, like his improvement on the pneumatic trough, and using them with great skill and precision, Priestley was able to discover 10 new gases. These are now known by different names:

  1. ‘Phlogisticated Air’ a.k.a. Nitrogen (N)
  2. ‘Nitrous Air’ a.k.a. Nitric Oxide (NO)
  3. ‘Dephlogisticated Nitrous Air’ a.k.a. Nitrous Oxide (N2O) or laughing gas
  4. ‘Red Nitrous Vapour’ a.k.a. Nitrogen Dioxide (NO2)
  5. ‘Marine Acid Air’ a.k.a. Anhydrous Hydrogen Chloride (HCl)
  6. ‘Alkaline Air’ a.k.a. Ammonia (NH3)
  7. ‘Vitriolic Acid Air’ a.k.a. Sulphur Dioxide (SO2)
  8. ‘Fluor Acid Air’ a.k.a. Silicone Tetrafluoride (SiF4)
  9. A new gas that was later identified as Carbon Monoxide (CO)

Number 10 on the list is perhaps his most famous discovery yet: ‘dephlogisticated air,’ more commonly known as oxygen (O2).

Clouds with sun behind them

Joseph Priestley discovered 10 new gases, including nitrogen, ammonia, carbon monoxide and oxygen. When he first discovered oxygen, he called is ‘dephlogisticated air’ and didn’t realise the exact behaviour or importance of this gas.

Discovering Oxygen

On 1st August 1774, Joseph Priestley isolated a new type of air by using a magnifying glass to concentrate sun rays on a sample of mercuric oxide (HgO). When this sample had heated up, Priestley noticed that it emitted a colourless gas he had never seen before.

Unable to pursue it due to a European tour he was embarking on, Priestley could only continue the experiment in January 1775. After many investigations, he concluded in a letter that year that:

“This air is of exalted nature. A candle burned in this air with an amazing strength of flame; and a bit of red hot wood crackled and burned with a prodigious rapidity, exhibiting an appearance something like that of iron glowing with a white heat, and throwing sparks in all directions.”

He also entrapped a mouse in the newly discovered substance in order to determine ‘the superior quality of this gas,’ concluding that the mouse became ‘quite vigorous’ and lived a lot longer than they would have ‘had it been common air.’

Finally, when he tested this new substance on himself, he noted that it was:

“Five or six times better than common air for the purpose of respiration, inflammation, and, I believe, every other use of common atmospherical air.”

While he referred to this new gas as dephlogisticated air, maintaining the phlogiston theory by claiming that this air was saturated with phlogiston, what he had in fact discovered was good old oxygen.

A magnifying glass on top of an old open book

Joseph Priestley discovered oxygen by heating up a sample of mercuric oxide. He did this using a magnifying glass, then known as a burning lens, to focus the sun’s rays on the sample. When heated, a colourless gas that Priestley had never seen before was emitted.

A New System of Chemistry

While travelling across Europe in 1774, Priestley showed Antoine Lavoisier, a French Chemist, how he had come by this new ‘air.’ This was an extremely important meeting for the future of chemistry, but not so much for the future of Joseph Priestley.

Between 1775 and 1780, Lavoisier repeated Priestley’s experiments and conducted multiple investigations in order to determine the nature of this newly discovered gas. Lavoisier:

  • Recognised it as an important part of the atmosphere
  • Discovered how it participated in combustion and respiration
  • Renamed the substance ‘oxygen’

The phlogiston theory was quickly replaced by what Lavoisier identified as the oxidation theory of combustion and respiration. Therefore, while Priestley discovered oxygen, it was Lavoisier’s findings on its properties and behaviour that revolutionised chemistry.

However, Priestley did not accept this new system of chemistry proposed by Lavoisier and instead continued to proclaim that the phlogiston theory was the correct conclusion. He even published a pamphlet where he laid out all of the shortcomings of Lavoisier’s oxygen theory, and this was the beginning of Priestley’s downfall.

Priestley’s Downfall

Priestley’s biggest downfall was his unwavering commitment to Rational Christianity. In fact, it is a popular opinion among historians that he was more of an obsessive natural philosopher than he was ever a chemist or scientist.

While Priestley did believe in the importance of understanding the world around him, and how this was integral to his religion, he thought that chemistry could only support religion in this way if it avoided speculation or theorisation.

For Priestley, Lavoisier was in direct conflict with this Rational Christian belief. In this way, it is clear to see how Priestley’s theology and religion both contributed to and hindered his scientific work.

As well as rejecting the Chemical Revolution, Priestley was also an open supporter of dissenters. He argued that scientific and human progress was hinged on the freedom of speech and worship. For these reasons, he supported the American and French Revolutions and believed that the government should have a limited role in favour of the welfare of individuals.

In 1791, Priestley was a main target during the Birmingham Riots, a Church-and-Crown mob that destroyed his home and laboratory. In 1794, he was forced to flee to America where he spent the rest of his life, dying in 1804.

A black and white portrait of Antoine Lavoisier

Antoine Lavoisier built on Priestley’s findings to subsequently discover the nature of ‘dephlogisticated air,’ which Lavoisier renamed as oxygen. His new oxidation theory of combustion and respiration involving oxygen paved the way for the Chemical Revolution, which Priestley completely rejected.

Despite his controversy, today, on the anniversary of his most important discovery, ReAgent tip their hats off to Joseph Priestley. Without him, who knows when oxygen would have been discovered?


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