186 History of Luminescence 



decomposition of dead animal and plant remains. They both burn 

 in air but neither of them are self-inflammable. 



The Phlogiston Doctrine and Luminescence 



The closing years of the eighteenth century were notable for the 

 passing of the phlogiston doctrine ^^ of Johann Joachim Becher 

 (1635-1682) and his pupil, George Ernst Stahl (1660-1734) . Al- 

 though Becher and Stahl were both chemists and physicians they con- 

 tributed nothing to the history of luminescence, but they did influ- 

 ence later thinking on the subject because of the attractive idea 

 that fire resulted from the escape of a material substance, phlogiston. 

 Becher thought that bodies were made up of air, water, and three 

 earths, one of which was " terra pinguis," a fatty or inflammable 

 earth, ^*^ really a fire-principle, corresponding to the sulphur of 

 Paracelsus. 



Stahl's views ^' were more definite and his name is more generally 

 and rightly associated with the phlogiston theory of combustion. He 

 held that the fire-principle, which he called " phlogiston," ^^ later 

 thought of as a definite chemical compound, escaped when a sub- 

 stance was burned or oxidized. Phlogiston thus accounted for the 

 flame and heat. The element sulphur was almost pure phlogiston 

 since it left no residue on burning. Wood or carbon was believed to 

 be made up of phlogiston and ash, phosphorus of phlogiston and 

 white ash (P2O5) , metals of phlogiston and a calx (a metal oxide) . 

 When the calces of metals were heated with carbon (rich in phlogis- 

 ton) the carbon gave up its phlogiston to the calx and the metal 

 containing the phlogiston was formed. 



While, it was perfectly apparent that wood or carbon or a candle 

 became lighter on burning, it had been recognized very early that 

 calces were heavier than the corresponding metals. How then could 

 additions of phlogiston to a calx make a metal which weighed 

 less? Such discrepancies led to the suggestion of levity or negative 

 weight,^^ finally to abandonment of the phlogiston doctrine, pri- 



'^ See J. H. White, The hislory of the phlogiston theory, London, 1932. 



*" Geber (died a. d. 765) had spoken of sulphur as " pingueodo terrae," fatness of the 

 earth. Becher's other two earths were the vitreous and the mercurial earth, described 

 in Physica subterranea (1669) . 



^' G. E. Stahl, Zymotechnia fundamentalis (1697) ; Fundamenta chymiae dogmaticae 

 et experimentalis (1723), translated by P. Shaw (1730). "Phlogiston" comes from 

 the Greek phlogistos, meaning burnt. 



^^ Boerhaave spoke of " oil " rather than " phlogiston " or " fatty earth," but the 

 three words all stood for the quality of combustibility. 



^^ An idea of Johann Juncker, Conspectus chemiae theoretico-practicae, etc., 2v., 

 Halle, 1730. 



