the minute amounts he had obtained were merely a mixture of material [31] (presumably originating from his crucibles or reagents).
In a section of his book on Air and fire, Chemische Abhandlung von der Luft und
dem Feuer, written in 1775, Scheele discussed the composition of combustible
materials. He expressed the idea that they consist of an acidic element combined
with phlogiston. Both sulphur and organic substances gave acids (SO 2 and CO 2 ,
respectively) on combustion [32]. Strangely enough, he did not mention phosphorus but included zinc in the discussion.
2 He then turned to arsenic (actually
arsenic(III) oxide) which he had found could not only be reduced to metallic arsenic
but also deprived of phlogiston (oxidised) to arsenic acid. Scheele now suggested
that all metallic calces and earths perhaps consisted of acids and phlogiston.
Scheele’s 1781 paper on tungstic acid (Sect. 15.5) was followed by a paper by
Bergman [4], where he suggested that molybdic and tungstic acid could be reduced
to metals. He also saw a similarity with arsenic and arsenic acid, and put forward a
similar idea on the composition of metals. In his phlogistic terminology, Bergman
suggested that metal calces (metal oxides) were not true elements but were actually
composed of metallic acids and phlogiston. In modern terminology, Bergman had
realised that metals could occur in higher oxidation states than those frequently
encountered in the common oxides. High-valent metal oxides are indeed often
acidic, e.g. CrO 3 , Mn 2 O 7 , WO 3 and MoO 3 . Bergman developed a special interest in
“acid of iron”, and on doing so he was far ahead of his time. Still today, iron (IV) and
iron(V) compounds are hot research topics, not least as such species constitute
reactive intermediates during enzymatic oxidation reactions. Neither Bergman, nor
Scheele, realised that Scheele had actually prepared salts of manganic and permanganic acids (H 2 MnO 4 and HMnO 4 , respectively) by oxidation of MnO 2 in his
pyrolusite paper (Sect. 15.1). The following year, Bergman discussed the elemental
acids in his Meditationes de systemate fossilium naturali, where he predicted that
also vegetables and perhaps even animals contained a number of unknown acids.
Regarding the alkalis, there were clear indications that at least one of them,
volatile alkali (ammonia), was not a true element. In 1779, Bergman wrote about
alkaline air (gaseous ammonia): “when electric explosions go through this air, the
volume is increased each time. In contrast to ammonia, this gas [a mixture of
nitrogen and hydrogen] is not absorbed by water, and is found to be very flammable” [33].
22.5 Phlogiston Content of Metals—Determination
of Equivalent Weights
As will be discussed in Chap. 23, Bergman was a firm believer in stoichiometry and
the constant composition of chemical bodies. In a dissertation defended on
December 13, 1780, by Andreas Tunborg, Bergman set out to determine the relative
2
Zinc oxide is amphoteric, having both acidic and basic properties.
22.4 Scheele’s and Bergman’s Views on the Earths, Metals, Acids and Alkalis
307
In a section of his book on Air and fire, Chemische Abhandlung von der Luft und
dem Feuer, written in 1775, Scheele discussed the composition of combustible
materials. He expressed the idea that they consist of an acidic element combined
with phlogiston. Both sulphur and organic substances gave acids (SO 2 and CO 2 ,
respectively) on combustion [32]. Strangely enough, he did not mention phosphorus but included zinc in the discussion.
2 He then turned to arsenic (actually
arsenic(III) oxide) which he had found could not only be reduced to metallic arsenic
but also deprived of phlogiston (oxidised) to arsenic acid. Scheele now suggested
that all metallic calces and earths perhaps consisted of acids and phlogiston.
Scheele’s 1781 paper on tungstic acid (Sect. 15.5) was followed by a paper by
Bergman [4], where he suggested that molybdic and tungstic acid could be reduced
to metals. He also saw a similarity with arsenic and arsenic acid, and put forward a
similar idea on the composition of metals. In his phlogistic terminology, Bergman
suggested that metal calces (metal oxides) were not true elements but were actually
composed of metallic acids and phlogiston. In modern terminology, Bergman had
realised that metals could occur in higher oxidation states than those frequently
encountered in the common oxides. High-valent metal oxides are indeed often
acidic, e.g. CrO 3 , Mn 2 O 7 , WO 3 and MoO 3 . Bergman developed a special interest in
“acid of iron”, and on doing so he was far ahead of his time. Still today, iron (IV) and
iron(V) compounds are hot research topics, not least as such species constitute
reactive intermediates during enzymatic oxidation reactions. Neither Bergman, nor
Scheele, realised that Scheele had actually prepared salts of manganic and permanganic acids (H 2 MnO 4 and HMnO 4 , respectively) by oxidation of MnO 2 in his
pyrolusite paper (Sect. 15.1). The following year, Bergman discussed the elemental
acids in his Meditationes de systemate fossilium naturali, where he predicted that
also vegetables and perhaps even animals contained a number of unknown acids.
Regarding the alkalis, there were clear indications that at least one of them,
volatile alkali (ammonia), was not a true element. In 1779, Bergman wrote about
alkaline air (gaseous ammonia): “when electric explosions go through this air, the
volume is increased each time. In contrast to ammonia, this gas [a mixture of
nitrogen and hydrogen] is not absorbed by water, and is found to be very flammable” [33].
22.5 Phlogiston Content of Metals—Determination
of Equivalent Weights
As will be discussed in Chap. 23, Bergman was a firm believer in stoichiometry and
the constant composition of chemical bodies. In a dissertation defended on
December 13, 1780, by Andreas Tunborg, Bergman set out to determine the relative
2
Zinc oxide is amphoteric, having both acidic and basic properties.
22.4 Scheele’s and Bergman’s Views on the Earths, Metals, Acids and Alkalis
307