History of Air-Analysis 5 



was unquestionably much retarded by the general acceptance of the phlo- 

 giston theory advocated by Stahl. According to the supporters of the 

 phlogiston theory, all air had the same composition, but was more or 

 less supplied with the "combustible essence" phlogiston. As a matter 

 of fact, we find that the earliest investigators whose experimental evi- 

 dence subsequently showed air to be composed of two or more gases, 

 namely, Scheele, Priestley, Cavendish, and Lavoisier, all firmly believed 

 in the phlogiston theory. Even after numerous roughly quantitative 

 experiments had been made in which it had been demonstrated that the 

 volume of air decreased after oxidation of material in it, scientists were 

 loath to give up the phlogiston theory, and air was said to be more or less 

 phlogisticated. Although a diminution in volume was observed when air 

 was exposed to certain substances, such as alkaline sulphides, moist iron 

 filings, phosphorus, or nitric oxide, this loss was simply considered as due 

 to a portion of the atmosphere which was not saturated with phlogiston. 



Scheele, noting the fact that the specific gravity of the air after ab- 

 sorption by various reagents had not altered, concluded that the decrease 

 in bulk could not be due, as he first supposed, to the absorption of phlo- 

 giston, and that the atmosphere must of necessity consist of two distinct 

 fluids. Although at first a strong supporter of the phlogiston theory, 

 Lavoisier in 1777 enunciated the belief that the air consisted of two gases, 

 one nitrogen (azote) and the other at first called dephlogisticated air, but 

 finally known as oxygen; thus, for the first time, the definite existence of 

 two distinct elemental components of the atmosphere was made clear. 

 This observation soon led Lavoisier to the belief that all the phenomena of 

 combustion could be explained on the basis of oxygenation without refer- 

 ence to the existence of phlogiston. Cavendish did not accept this new 

 conception of the composition of the air until a number of years later, 

 and even then his acceptance was far from a complete surrender. The 

 one man, Priestley, who, perhaps more than anyone else, illuminated our 

 knowledge of the atmosphere by his discovery of oxygen, advocated the 

 phlogiston theory until his death in 1810. 



It is thus clear that the dephlogisticated air of the earlier scientists was 

 nothing more nor less than what we now call oxygen, and hence, although 

 many of these writers considered the diminution in volume produced by 

 the various reagents as an index of the amount of dephlogisticated air 

 present, their observations have a certain historical value as indicating 

 approximately the estimation of the amount of oxygen in the air by the 

 methods then current. 



The earliest observations of the quantitative relationship between the 

 dephlogisticated air and the phlogisticated air were undoubtedly made 

 simultaneously by Priestley in England and Scheele in Sweden. l Scheele's 



1 According to the published notebooks and laboratory records of Scheele (Carl Wil- 

 helm Scheele, Efterlemnade bref ooh anteckningar, Utgifna af A. E. Nordenskiold, 

 Stockholm, 1892), his experiments must have antedated Priestley by two or three years. 



