20.1 Bergman’s Essay on Elective Attractions
Bergman’s views on the interactions between bodies were largely inspired by
Newton [9]. Bergman wrote that between all natural bodies, there is an attraction,
the nature of which was unknown. Bergman’s aim was not to seek to explain these
attractions, only to determine their relative strengths. This was definitely a large
enough challenge in the eighteenth century. For celestial bodies, Newton had found
the attractions to be inversely proportional to the square of the distance, but the
short-range attractions between molecules just out of contact seemed to obey different rules. Bergman realised that a main difference was that for celestial bodies,
the diameter could be neglected when compared to the distance between the bodies,
but this was not true for atoms in close proximity to each other. For short-range
attractions, the surrounding environment was, according to Bergman, very important. He divided the short-range attractions into five types; the first three are
described in a very ambiguous way by Bergman, and not further discussed in the
paper:
Attractio aggregationis: homogenous materials are united to form a common
homogenous mass. Here, Bergman possibly meant the attractive forces holding
pure materials together.
Attractio compositionis: this may refer to the attractions between the elements in a
compound.
Attractio solutionis and Attractio fusionis: here Bergman is certainly referring to the
formation of solutions in solvents or melts.
The remaining two types are of more interest, however. Simple elective attractions (Attractio simplex elective) are the attractions between three chemical components, one compound and one simple substance. This is probably better explained
by concrete examples rather than Bergman’s more mathematical description. If, for
example, zinc is exposed to copper(II) sulphate solution, metallic copper precipitates leaving a solution of zinc sulphate. Thus, zinc appeared to have a greater
attraction to (or affinity for) sulphuric acid than copper, and thus appears higher up
in the column than copper in Bergman’s table. Another example would be if
sulphuric acid was added to sodium chloride. This releases hydrogen chloride and
leaves sodium sulphate (actually sodium hydrogen sulphate). Thus mineral alkali
(sodium) appeared to have a stronger attraction to sulphuric acid than to
hydrochloric acid. The column for Alkali minerale is therefore topped by sulphuric
acid, with hydrochloric acid on the third place after nitric acid. Unlike Geoffroy’s
table, Bergman’s table includes empty places. This is where he, by comparing with
neighbouring columns, expected to find substances that he had not experimentally
placed in the table.
In modern chemistry attraction tables are not used, as the elective attractions
determined by Bergman actually is a mixture of several different factors such as
solubility, volatility, acid/base strength and reduction potentials. The apparent
affinity of silver for hydrochloric acid is, for example, due to the low solubility of
272
20 Bergman’s Work on Elective Attractions
Bergman’s views on the interactions between bodies were largely inspired by
Newton [9]. Bergman wrote that between all natural bodies, there is an attraction,
the nature of which was unknown. Bergman’s aim was not to seek to explain these
attractions, only to determine their relative strengths. This was definitely a large
enough challenge in the eighteenth century. For celestial bodies, Newton had found
the attractions to be inversely proportional to the square of the distance, but the
short-range attractions between molecules just out of contact seemed to obey different rules. Bergman realised that a main difference was that for celestial bodies,
the diameter could be neglected when compared to the distance between the bodies,
but this was not true for atoms in close proximity to each other. For short-range
attractions, the surrounding environment was, according to Bergman, very important. He divided the short-range attractions into five types; the first three are
described in a very ambiguous way by Bergman, and not further discussed in the
paper:
Attractio aggregationis: homogenous materials are united to form a common
homogenous mass. Here, Bergman possibly meant the attractive forces holding
pure materials together.
Attractio compositionis: this may refer to the attractions between the elements in a
compound.
Attractio solutionis and Attractio fusionis: here Bergman is certainly referring to the
formation of solutions in solvents or melts.
The remaining two types are of more interest, however. Simple elective attractions (Attractio simplex elective) are the attractions between three chemical components, one compound and one simple substance. This is probably better explained
by concrete examples rather than Bergman’s more mathematical description. If, for
example, zinc is exposed to copper(II) sulphate solution, metallic copper precipitates leaving a solution of zinc sulphate. Thus, zinc appeared to have a greater
attraction to (or affinity for) sulphuric acid than copper, and thus appears higher up
in the column than copper in Bergman’s table. Another example would be if
sulphuric acid was added to sodium chloride. This releases hydrogen chloride and
leaves sodium sulphate (actually sodium hydrogen sulphate). Thus mineral alkali
(sodium) appeared to have a stronger attraction to sulphuric acid than to
hydrochloric acid. The column for Alkali minerale is therefore topped by sulphuric
acid, with hydrochloric acid on the third place after nitric acid. Unlike Geoffroy’s
table, Bergman’s table includes empty places. This is where he, by comparing with
neighbouring columns, expected to find substances that he had not experimentally
placed in the table.
In modern chemistry attraction tables are not used, as the elective attractions
determined by Bergman actually is a mixture of several different factors such as
solubility, volatility, acid/base strength and reduction potentials. The apparent
affinity of silver for hydrochloric acid is, for example, due to the low solubility of
272
20 Bergman’s Work on Elective Attractions