The Burning Question - and the nature of science. Dr Bert Sorsby University of Hull

Why do things burn? The modern idea. §Burning is the release of light and heat energy ( and new substances ) when oxygen reacts chemically with a fuel. §A small amount of energy is generally needed to start the reaction. §A flame is the region where this takes place.

Why do things burn? Early ideas §Empedocles ( BC) and Aristotle ( BC) l Four Element Theory: Earth; Air; Fire; Water §Paracelsus ( ) - ‘iatrochemist’ l Three Principles ; Salt; Sulphur; Mercury

Why do things burn? Another idea. §J.J. Becher ( ) & G.E. Stahl ( ) l The Phlogiston Theory §S.Hales ( ); J.Priestley ( ); H. Cavendish ( ) l Different types of ‘air’ - qualitative and quantitative study.

Priestley and ‘dephlogisticated air.(1774) §When mercury is heated in air it produces red calx §When red calx is heated strongly, it produces a new gas, which Priestly called ‘dephlogisticated air’.

Cavendish (1770+) and inflammable air §metal + acid  salt + inflammable air (calx+Ph) + acid  (calx +acid) + phlogiston §inflammable air burns to produce water §Volta and Priestley also recognised this. §Controversy between Priestley, Cavendish and Watt about the composition of water.

Antoine Laurent Lavoisier ( ) and the oxygen theory §Autumn 1774 Priestley met Lavoisier at a dinner in Paris §For the next three years Lavoisier checked and re- interpreted Priesley’s results. §1777 he produced 9 papers for the Academy in Paris.

Lavoisier’s investigations (Part 1) §He heated 4 ounces mercury in a flask with 50 cubic inches of air. l 8 cubic inches of air were used up (weighing 3  grains). l l Red powder on surface of remaining mercury and flask and contests were 3  grains heavier. l New ‘air’ in the flask could not support life or burning.

Lavoisier’s investigations (Part 2) §He removed all the red powder and heated it in another flask l A new gas was produced - volume 8 cubic inches and weight 3  grains l The new gas supported life and burning. §Substances burned in it produced acids therefore ‘oxygen’ or ‘acid maker’ §Published in Traité élémentaire de chimie in 1789 including work on the composition of water etc.

Phlogiston and Oxygen Theories compared §Burning l Metal (Calx +  )  calx +  l Metal + O  Metal oxide §Metals and acid ( Before 1789) l metal (calx+  ) + acid  salt +  l metal + acid  salt (calx?+acid) + inflammable air(?) §After 1789 l Metal + water (hydrogen oxide) + acid  salt (metal oxide +acid) + hydrogen

Reception of Oxygen Theory §In France it was largely accepted l Berthollet 1787 ‘oxymuriatic acid’ l Fourcroy and Guyton de Morveau taught oxygen theory and new nomenclature 1780s. §In Britain only accepted at first in Scotland (Joseph Black 1784). Davy was still sceptical in early 19th century. §In Germany there was resistance. Especially from Gren (  is a form of matter with negative weight)

How do both theories explain other observations? For example:- §Candle(s) under bell jar. §Candle (s) under bell jar over water.