The Eighteenth Century 187 



marily from Lavoisier's studies *° in 1774 on the change of weight 

 when phosphorus, sulphur, and metals burned in air. He also 

 demonstrated that part of the air, the " air vital " or " air pur," 

 which he called " principe oxygine " in 1777, later changed to 

 oxygene, combined with the phosphorus, the sulphur or the metal, 

 when these materials burned. There was left behind, the noxious 

 or unvital air, hence called azote *^ (without life) . 



The phlogiston theory was accepted by most chemists of the 

 eighteenth century— K. Neumann, J. H. Pott, A. S. Marggraf in 

 Germany; S. F. Geoffroy Duhamel de Monceau and P. J. Macquer 

 in France; and the group of men whose names have come to be inti- 

 mately associated with gases and combustion, H. Cavendish, J. Black, 

 and J. Priestley in England, and K. W. Scheele in Sweden. They 

 were all phlogistonists at first, as was T. O. Bergman, Scheele's con- 

 temporary. Marggraf, Macquer, Priestley, and Scheele remained so 

 during their lifetime, but the others were gradually converted to the 

 point of view of Lavoisier, whose terminology was adopted almost 

 from the start by such men as Berthollet, Fourcroy, and Guyton de 

 Morveau. 



Phlogiston doctrine postulated that in the vital processes of com- 

 bustion in the body, phlogiston was constantly removed. To Lavoi- 

 sier also is due the credit of demonstrating*- in 1777, again by 

 quantitative methods, the essential similarity of chemical processes 

 in the burning candle and in the animal— taking up of oxygen and 

 formation of carbon dioxide. Thus the older terms of fire-air or 

 vital air or dephlogisticated air *^ (for oxygen) , spent air or phlogis- 

 ticated air (for nitrogen) and fixed or mephitic air (carbonic acid) 



*" opuscules physiques et chimiqiies, Paris, 1774. 



*^ Some hold azote comes from the alchemistic term " azoth," meaning "mercury of 

 wisdom." Isolation of noxious or injurious air was carried out by Daniel Rutherford 

 (1749-1819) in 1772 at the instigation of J. Black. According to Black, the name 

 " nitrogene " was suggested by J. A. C. Chaptal de Chanteloup {Elemens de chimie 

 1: 128, Paris, 1790) , producing nitre (saltpeter) . 



*^ A. L. Lavoisier, Experiences sur la respiration des animaux, et sur les change- 

 ments qui arrivent a I'air en passant par leur pulmon. Hist. Acad. Roy. des Sciences 

 1777, 185-194, Paris, 1780. 



*^ Priestley noted that oxygen (from heating mercury oxide) caused rapid and 

 intense burning and must therefore be able to take up much phlogiston. Hence it 

 originally contained little phlogiston and was called dephlogisticated air. An air left 

 after a substance had stopped burning was phlogisticated air, for example, nitrogen. 

 Priestley's actual words in a letter to the Royal Society read May 25, 1775 were: " the 

 purest air [oxygen] is that which contains the least phlogiston: that air [nitrogen] is 

 impure (by which I mean that it is unfit for respiration, and for the purpose of 

 supporting flaine) in proportion as it contains more of that principle." Later (1775) 

 he wrote: " I think I have sufficiently proved that the fitness of air for respiration 

 depends on the capacity to receivo the phlogiston exhaled from the lungs, this species 

 of air may not improperly be called, dephlogisticated air." 



