abandoned phlogiston altogether as it provided the only explanation for redox
phenomena not involving oxygen. Thus, the role of revolutionising chemistry was
left to Lavoisier.
Since Scheele believed he had proved that the consumed oxygen (or “fire air” as
he called it) was neither present in the remaining air, nor in the oxidised product, the
only remaining option for Scheele was that the oxygen escaped the vessel during
the reaction. The logic conclusion was that oxygen took up phlogiston from the
burning or oxidising material to form heat. As the concept of energy was not yet
invented, the view of heat as a form of matter was well established among chemists,
and heat was one of the elements listed by Lavoisier [41]. Not until about 1850 was
the nature of heat reasonably well understood. Scheele proved that heat was
evolved during every—what we would call—oxidation reaction, not only combustions. Scheele found further proof as he believed he could decompose heat by
substances with a strong affinity for phlogiston. When mercury(II) oxide (mercury
calx) decomposed to mercury and oxygen on heating, Scheele thought that the heat
was decomposed by mercury(II) oxide into phlogiston and oxygen, the phlogiston
reacting with the mercury calx to form mercury, while the oxygen (fire air) was
liberated. Already in Malmö, Scheele had demonstrated that he could reduce silver
nitrate and potassium nitrate by heat alone (Sect. 8.2). Scheele also reported a series
of experiments that showed the difference between heat transfer by radiation and
convection [42].
Light was also considered by Scheele as a compound of oxygen and phlogiston
but richer in phlogiston than heat. He explained the decomposition of silver
chloride and nitric acid by light by suggesting that silver chloride and nitric acid
absorbed phlogiston from light. Thus, Scheele’s theory of 1775 was that oxygen
was a substance which readily absorbed phlogiston to form heat and light.
Bergman was convinced that heat was a substance with a small, almost
immeasurable weight. In 1784, in one of his last contributions to science, Bergman
proposed a method for determining the weight of heat [43]. The reason, according
to Bergman, why previous attempts to determine the weight of heat by trying to
measure the weight difference between a piece of metal when cold and when
red-hot had failed was that the weight ratio of heat and metal is very small. Instead,
Bergman suggested sealing ice in a metal container and determining the weight
increase on melting. In this way, it would have been possible to measure the large
amount of latent heat absorbed on melting. Although based on a false, material,
view of heat, the suggestion was ingenious. Bergman introduced the term eteric to
denote substances such as heat which could not be retained in any container, not
even those made of dense gold [44].
In spring 1774, Lavoisier sent a copy of his Opuscules physiques et chimiques
(Physical and chemical essays) to Scheele. In that book, he reported experiments
conducted with a large burning glass. Scheele wrote a letter to Lavoisier, suggesting
that he should heat silver carbonate (silver calx) by means of this burning glass.
Lavoisier received the letter [45], but no letter from Lavoisier to Scheele is known,
and nothing suggests that Lavoisier carried out the suggested experiment. Scheele’s
letter was published and discussed by Éduard Grimaux in 1890, but Grimaux’s
21.5 Scheele’s and Bergman’s Views on Oxygen, Heat and Phlogiston
291
phenomena not involving oxygen. Thus, the role of revolutionising chemistry was
left to Lavoisier.
Since Scheele believed he had proved that the consumed oxygen (or “fire air” as
he called it) was neither present in the remaining air, nor in the oxidised product, the
only remaining option for Scheele was that the oxygen escaped the vessel during
the reaction. The logic conclusion was that oxygen took up phlogiston from the
burning or oxidising material to form heat. As the concept of energy was not yet
invented, the view of heat as a form of matter was well established among chemists,
and heat was one of the elements listed by Lavoisier [41]. Not until about 1850 was
the nature of heat reasonably well understood. Scheele proved that heat was
evolved during every—what we would call—oxidation reaction, not only combustions. Scheele found further proof as he believed he could decompose heat by
substances with a strong affinity for phlogiston. When mercury(II) oxide (mercury
calx) decomposed to mercury and oxygen on heating, Scheele thought that the heat
was decomposed by mercury(II) oxide into phlogiston and oxygen, the phlogiston
reacting with the mercury calx to form mercury, while the oxygen (fire air) was
liberated. Already in Malmö, Scheele had demonstrated that he could reduce silver
nitrate and potassium nitrate by heat alone (Sect. 8.2). Scheele also reported a series
of experiments that showed the difference between heat transfer by radiation and
convection [42].
Light was also considered by Scheele as a compound of oxygen and phlogiston
but richer in phlogiston than heat. He explained the decomposition of silver
chloride and nitric acid by light by suggesting that silver chloride and nitric acid
absorbed phlogiston from light. Thus, Scheele’s theory of 1775 was that oxygen
was a substance which readily absorbed phlogiston to form heat and light.
Bergman was convinced that heat was a substance with a small, almost
immeasurable weight. In 1784, in one of his last contributions to science, Bergman
proposed a method for determining the weight of heat [43]. The reason, according
to Bergman, why previous attempts to determine the weight of heat by trying to
measure the weight difference between a piece of metal when cold and when
red-hot had failed was that the weight ratio of heat and metal is very small. Instead,
Bergman suggested sealing ice in a metal container and determining the weight
increase on melting. In this way, it would have been possible to measure the large
amount of latent heat absorbed on melting. Although based on a false, material,
view of heat, the suggestion was ingenious. Bergman introduced the term eteric to
denote substances such as heat which could not be retained in any container, not
even those made of dense gold [44].
In spring 1774, Lavoisier sent a copy of his Opuscules physiques et chimiques
(Physical and chemical essays) to Scheele. In that book, he reported experiments
conducted with a large burning glass. Scheele wrote a letter to Lavoisier, suggesting
that he should heat silver carbonate (silver calx) by means of this burning glass.
Lavoisier received the letter [45], but no letter from Lavoisier to Scheele is known,
and nothing suggests that Lavoisier carried out the suggested experiment. Scheele’s
letter was published and discussed by Éduard Grimaux in 1890, but Grimaux’s
21.5 Scheele’s and Bergman’s Views on Oxygen, Heat and Phlogiston
291