JOSEPH PRIESTLEY

Joseph Priestley was born near Leeds in 1733 and worked in an era when alchemy was beginning to be discredited and replaced by logical scientific thought.  Previously, chemical enquiry had been prompted by the dream of making gold from substances of little value and very little true chemistry was known at the time.

Joseph Priestley and his contemporaries, Lavoisier, Black, Scheele and Cavendish took part in the amassing of fact and preparation of many new chemical substances.  As has happened so often in the history of science, several times their work overlapped and was duplicated when two people produced the same result quite independently.  For example, Joseph Priestley and Scheele both laid claim to the discovery of the gas now known as oxygen.

Many chemical laws were later derived from the bank of chemical knowledge these men helped to build and a general picture of chemistry began to evolve.

Joseph Priestley is chiefly remembered for his work with gases.  Air and its life-giving qualities were his chief interest.  He experimented with mice, putting them in a confined air space to discover how long they would live.  He noted that after a mouse had died a candle would not burn in the air remaining.  But if he kept a plant in the ‘used air’ somehow the air seemed to revive and the candle could burn again.  This was the first record of the reaction now known as photosynthesis, but Joseph Priestley failed to notice that sunlight was an essential factor in the ‘revival’ of the air.

In 1774, Joseph Priestley made and collected a sample of reasonably pure oxygen by covering some red mercuric oxide with an airtight glass dome and heating it with a burning glass.  He noticed that a candle burned very brightly in the new gas and a mouse could survive much longer in a jar of it than it could in the same sized jar of air. Joseph  Priestley even tried sniffing the gas himself to discover what effect it had.  He said he felt full of energy and his lungs felt soothed.  Perhaps the gas would be useful in Medicine.

It is surprising that with this accumulation of data, Joseph Priestley did not realize that his gas obtained from the red powder was the life-giving component of air.  Instead, he clung to the phlogiston theory.  He thought that when substances burned, they gave out phlogiston which was absorbed by something else.  Things could burn in air because the air was not saturated with phlogiston and could accept some more.  When the air became saturated, it would no longer support burning.

In 1772 Joseph Priestley published an important paper called ‘Observation on Different Kinds of Air’ describing in it the laboratory preparation of several new gases including nitrous dioxide and hydrogen chloride.  The insoluble gases he collected over water, and those which were soluble in water, he collected over mercury.  His ‘shelf’ used in gas collection is the fore-runner of the beehive shelf used in the laboratory today.

Although he was somewhat conservative in his scientific views, his political views were the opposite.  His violent support of the French Revolution incited a Birmingham mob to pillage his house and hold it to siege for three days.

Joseph Priestley, scientist and non-conformist minister died in 1804

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