Bergman (Sect. 22.5). Such reactions would not get a satisfactory explanation until
the discovery of valence electrons in the twentieth century.
Initially, Scheele assumed that the amount of oxygen in the atmosphere was
constantly decreasing, as large amounts of oxygen were consumed by respiration
and combustion. In 1778, he measured [57] the oxygen content in the atmosphere
during a whole year in order to detect any variations, but found it to be constant
[37]. He found this
a very remarkable fact; and to assign the cause of it seems difficult, as a quantity of pure air
[oxygen], in supporting fire, daily enters into a new union; and a considerable quantity of it
is likewise corrupted, or changed into aerial acid, as well by plants as by respiration;
another fresh proof of the great care of our Creator for all that lives. [58]
This view was of course rather naïve, as it should have been apparent to Scheele
that a detectable decline in the atmosphere’s oxygen content during a single year
would have extinguished all life within a few years. It was Priestley who discovered
that green plants could restore foul air [59]. This was before he discovered oxygen,
but he concluded that “plants […] reverse the effect of breathing and tend to keep
the atmosphere sweet and wholesome”. Scheele failed to reproduce Priestley’s
results; perhaps this was due to lack of sunlight in Scheele’s laboratory.
21.6 Priestley’s Theories of Oxygen and Combustion
Priestley’s views on oxygen and air were fundamentally different from those of
Scheele. While Scheele clearly stated that air is a mechanical mixture of oxygen,
nitrogen, carbon dioxide and water vapour, Priestley seems to have regarded
oxygen as air deprived of phlogiston, and nitrogen as phlogiston saturated air. He
seems to have regarded oxygen and nitrogen as the two end points on a continuous
scale, rather than two discrete substances. Throughout his life he called oxygen
“dephlogisticated air” and nitrogen “phlogisticated air”. In different sections of his
book, Experiments and Observations on Different Kinds of Air, he gave different
explanations of the nature of oxygen and, as Partington put it, “It is not easy to
extract from his conflicting statements Priestley’s considered views (if he had any)
on the nature of oxygen and nitrogen” [60]. Since oxygen was easily obtained from
nitrates, which in turn could be obtained from metal oxides, he arrived at the
conclusion (unclear how) that air was composed of an earth (of undisclosed nature)
combined with nitric acid and phlogiston:
[…] there remained no doubt in my mind, but that atmospherical air, or the thing that we
breathe, consists of the nitrous acid and earth, with so much phlogiston as is necessary to
its elasticity; and likewise so much more as is required to bring it from its state of purity to
the mean condition in which we find it. [61]
In some contexts, he seems to have expressed views closer to that of Scheele, i.e.
that air would be a mixture of dephlogisticated (oxygen) and phlogisticated air
(nitrogen).
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21 The Discovery of Oxygen
the discovery of valence electrons in the twentieth century.
Initially, Scheele assumed that the amount of oxygen in the atmosphere was
constantly decreasing, as large amounts of oxygen were consumed by respiration
and combustion. In 1778, he measured [57] the oxygen content in the atmosphere
during a whole year in order to detect any variations, but found it to be constant
[37]. He found this
a very remarkable fact; and to assign the cause of it seems difficult, as a quantity of pure air
[oxygen], in supporting fire, daily enters into a new union; and a considerable quantity of it
is likewise corrupted, or changed into aerial acid, as well by plants as by respiration;
another fresh proof of the great care of our Creator for all that lives. [58]
This view was of course rather naïve, as it should have been apparent to Scheele
that a detectable decline in the atmosphere’s oxygen content during a single year
would have extinguished all life within a few years. It was Priestley who discovered
that green plants could restore foul air [59]. This was before he discovered oxygen,
but he concluded that “plants […] reverse the effect of breathing and tend to keep
the atmosphere sweet and wholesome”. Scheele failed to reproduce Priestley’s
results; perhaps this was due to lack of sunlight in Scheele’s laboratory.
21.6 Priestley’s Theories of Oxygen and Combustion
Priestley’s views on oxygen and air were fundamentally different from those of
Scheele. While Scheele clearly stated that air is a mechanical mixture of oxygen,
nitrogen, carbon dioxide and water vapour, Priestley seems to have regarded
oxygen as air deprived of phlogiston, and nitrogen as phlogiston saturated air. He
seems to have regarded oxygen and nitrogen as the two end points on a continuous
scale, rather than two discrete substances. Throughout his life he called oxygen
“dephlogisticated air” and nitrogen “phlogisticated air”. In different sections of his
book, Experiments and Observations on Different Kinds of Air, he gave different
explanations of the nature of oxygen and, as Partington put it, “It is not easy to
extract from his conflicting statements Priestley’s considered views (if he had any)
on the nature of oxygen and nitrogen” [60]. Since oxygen was easily obtained from
nitrates, which in turn could be obtained from metal oxides, he arrived at the
conclusion (unclear how) that air was composed of an earth (of undisclosed nature)
combined with nitric acid and phlogiston:
[…] there remained no doubt in my mind, but that atmospherical air, or the thing that we
breathe, consists of the nitrous acid and earth, with so much phlogiston as is necessary to
its elasticity; and likewise so much more as is required to bring it from its state of purity to
the mean condition in which we find it. [61]
In some contexts, he seems to have expressed views closer to that of Scheele, i.e.
that air would be a mixture of dephlogisticated (oxygen) and phlogisticated air
(nitrogen).
294
21 The Discovery of Oxygen