Joseph Priestley was an English chemist who is best known for his isolation and description of several gases, particularly oxygen. He is considered to be one of the founders of modern chemistry because of his contributions to experimental science.

Priestley was born on 13 March, 1733, in Fieldhead, Yorkshire, the son of a Leeds cloth-dresser who was also a Calvinist minister. Priestley himself trained as a minister of the Dissenting Church, which comprised several churches that had separated from the Church of England, and from 1767 to 1772 he held the appointment of Unitarian minister in Leeds. He spent much of his life preaching and writing about religion. He was also greatly interested in education, philosophy and politics but, early on, he had no real interest in the sciences.

This all changed when he met the famous American scientist and statesman, Benjamin Franklin, while on a trip to London in 1766. Franklin's speciality was the new science of electricity and Franklin awakened Priestley's interest in the sciences and encouraged him to conduct experiments in electricity. In particular, in 1766 Priestley performed an elegant experiment to verify Coulomb's Law (that is, the force between two electric charges is inversely proportional to the square of the distance between them).

In 1767, just one year after meeting Franklin, Priestley discovered that graphite, an allotrope of carbon, can conduct electricity. That same year, the same year as he became a pastor in Leeds, and encouraged by Franklin, Priestley published his 'The History and Present State of Electricity'. This led to his election as a Fellow of the Royal Society in 1766. This 'History' proved to be too difficult for the average reader to comprehend and so he prepared a more popular version. This necessitated the inclusion of some drawings and, unable to find anyone prepared to do the work, he taught himself the rules of perspective drawing. Since there was no book to help him, he published one himself. During the course of this work, he stumbled upon the efficacy of India rubber as an eraser of lead-pencil marks. Thus was invented the pencil eraser to which Priestley attached its common name of 'rubber'. The preface to his book contains the first printed reference to India rubber erasers.

Experiments on Gases

Now, it so happened that, while employed as a pastor in Leeds, he lived next to a brewery, and he was intrigued by the 'air' that floated over fermenting grain. From his first experiment, he was able to show that this 'heavier-than-air' gas was able to extinguish burning wood chips. This gas would later be identified as carbon dioxide.
Priestley devised a new way to produce this 'heavy gas', as he called it, in his home laboratory. He poured acid onto chalk (calcium carbonate), and the heavy gas resulted. On dissolving this gas in water he found that it had a pleasant and tangy taste. We now know this as soda water. For this invention of soda water, Priestley was elected to the French Academy of Sciences in 1772, and the following year he received the prestigious Copley Medal from the Royal Society.

Discovery of Photosynthesis

Also in 1772, Priestley made another important discovery. He found that if he placed the shoot of a green plant into a container of water and then covered the container and lit a candle in it, the candle would eventually go out. However, after a while, Priestley was able to both burn the candle again, and to keep mice alive in the air. Priestley was greatly interested in the 'goodness' of air, which is a measure of its respirability. He wrote,

`the injury which is continually done to the atmosphere by the respiration of such a large number of animals ...is, in part at least, repaired by the vegetable creation.'

This balance between animal and plant kingdoms is particularly relevant to present-day environmental concerns over climate change and rainforest destruction.

Thus Priestley became the first person ever to observe photosynthesis in plants - the fact that they take in carbon dioxide and release oxygen.

Discovery of Nitrogen(I) Oxide ('Laughing Gas')

From 1773 to 1780 Priestley was employed as librarian and literary companion to William Petty Fitzmaurice, 2nd Earl of Shelburne¹, and owner of Bowood House, Calne in Wiltshire. This appointment gave Priestley the time and facilities to pursue his own experiments and, he devised a new apparatus, known as a pneumatic trough, that allowed him to collect gases over mercury.

Being a dense liquid, mercury does not absorb gases as easily as water. This device enabled Priestley to float various substances on the surface, the whole being enclosed by a glass vessel into which he could collect any gaseous emanations. He then used a burning lens to focus the sun's rays through the glass and onto the substance under study.

One of his first experiments with this device, in 1772, produced a new gas, nitrogen(I) oxide. It wasn't long before it was discovered that this gas had unusual effects on people, inducing a state of euphoria. Because of these properties it became known as 'Laughing Gas'. In time, this gas would become the first surgical anaesthetic.

Close by Bowood House is a pond known as 'Priestley's Pool', or sometimes 'Doctors Pool' after Priestley, and it was here that Priestley collected gases bubbling naturally from the bottom of the pool for analytical experiments.

Discovery of Oxygen

On 1 August 1774 he discovered that the gas produced by heating a sample of mercury (II) oxide in this way would allow a candle to burn more brightly, and enable a mouse to live up to four times as long as in normal air. Priestley wrote,

'I have discovered an air five or six times as good as common air.'

Thus Priestley discovered oxygen and described its role in combustion and respiration. An advocate of the 'phlogiston theory', however, Priestley called this gas 'dephlogisticated air' and did not fully understand the future importance of this discovery (the Swedish chemist Carl Wilhelm Scheele may have discovered oxygen before Priestley, but did not publicise his work in time to be credited with its discovery). Although Priestley was unable to accurately interpret his results using the scientific knowledge of the time, his work was later used by the French chemist Antoine Lavoisier to construct the theories that now underpin modern chemistry, including the 'Law of Conservation of Matter'. This states that matter can neither be created nor destroyed, but is simply changed from one form to another. It was Lavoisier who also named the gas 'oxygen' and correctly described its role in combustion.

By identifying the gases involved in photosynthesis and plant respiration, Priestley may be credited with unifying the sciences of biology and chemistry.

In addition to carbon dioxide, nitrogen(I) oxide and oxygen, Priestley also isolated and described the properties of several other gases including ammonia, sulphur dioxide, hydrogen sulphide and carbon monoxide.

His Later Life

In 1780 Priestley left his position with Petty at Bowood House because of religious differences. He became a minister in Birmingham, UK. Because of his open support for the American and French revolutions he became unpopular in England during his later years, and his house and effects were burned by a mob in 1791. He went to live in London for a while but the persecution continued. Finally, in 1794, he emigrated with his family to the United States where he settled in Northumberland, Pennsylvania and pursued his writing for the remainder of his life.

Priestley died quietly at his home on 6 February, 1804.

International Chemical Landmark Site

On 7 August, 2000, the Royal Society of Chemistry and the American Chemical Society unveiled an International Historic Chemical Landmark plaque at Bowood House, Calne, Wiltshire to commemorate the discovery of oxygen by Joseph Priestley. on 1 August 1774. The landmark recognises the importance of Priestley's work at Bowood in laying the foundations for modern chemistry.