22.4 Scheele’s and Bergman’s Views on the Earths,
Metals, Acids and Alkalis
Scheele strongly believed in the elemental nature of the primitive earths: siliceous
earth, clay, magnesia, lime and earth of heavy spar. In his papers, Scheele never
speculated over the possibility of these earths being composed of simpler bodies.
Bergman, as will be seen, believed that the earths, or at least silica, could be
decomposed into more primitive elements.
Bergman seems to have agreed with Scheele that the earths could not be
interconverted. Frequently, however, there were reports from chemists claiming to
have proven otherwise. Bergman and Scheele made great efforts in trying to prove
or disprove these reports in order to reveal the true nature of the earths.
In 1776, Scheele published a paper proving the individual nature of aluminium
and silicon oxides [20]. The reason was the opinion of Baumé that earth of alum
(Al 2 O 3 ) was actually siliceous earth (SiO 2 ), that clay was the same earth combined
with sulphuric acid and that alum (KAl(SO 4 ) 2 ) was the same earth over-saturated
with sulphuric acid. Scheele was sceptic: “With regard to chemical opinions, it is
my custom not to credit any, till I have brought them to the test of experiment” [21].
Actually, Scheele discussed this issue with Gahn in a letter dated November 1772,
the writing of this manuscript being the next task for Scheele after writing together
the paper on arsenic acid and the book on air and fire.
At first, it seemed to Scheele that Baumé was correct: he melted powdered rock
crystal (pure SiO 2 ) with potassium hydroxide in a crucible, extracted the residue
(potassium silicates) with water and added sulphuric acid [22]. Siliceous earth
precipitated and Scheele found small amounts of alum in the mother liquor. Still,
this alum could have been a contaminant in the rock crystal or the alkali, and
consequently Scheele repeated the procedure with the precipitated siliceous earth
seven times, and each time he obtained alum. At this point Scheele noticed that the
crucibles were corroded, and he found that the source of the alum was clay from the
crucible: “But, behold! on examining the crucibles employed for these repeated
fusions, I found them everywhere uneven in the inside, and full of little excavations,
which they had not before the experiment” [23]. When the operation was performed
in an iron crucible, no alum was obtained and Scheele could (probably to his relief)
establish that “The siliceous earth, therefore, still remains a peculiar earth” [23]. In a
similar fashion, Bergman criticised a paper claiming that cobalt could be obtained
from iron and arsenic [24]. Bergman concluded that the cobalt must have been
present as an impurity in the reagents.
Despite the work of Scheele, Bergman was not convinced of the elemental
nature of siliceous earth (SiO 2 ), but in lack of evidence he had to consider it as
elemental. What made Bergman suspicious were Scheele’s experiments with
hydrofluoric acid (Sect. 14.1), where siliceous earth appeared to arise upon contact
of the acid with water, while it was in fact formed by the hydrolysis of volatile
silicon tetrafluoride and condensation of the silicic acid formed:
22.4 Scheele’s and Bergman’s Views on the Earths, Metals, Acids and Alkalis
305
Metals, Acids and Alkalis
Scheele strongly believed in the elemental nature of the primitive earths: siliceous
earth, clay, magnesia, lime and earth of heavy spar. In his papers, Scheele never
speculated over the possibility of these earths being composed of simpler bodies.
Bergman, as will be seen, believed that the earths, or at least silica, could be
decomposed into more primitive elements.
Bergman seems to have agreed with Scheele that the earths could not be
interconverted. Frequently, however, there were reports from chemists claiming to
have proven otherwise. Bergman and Scheele made great efforts in trying to prove
or disprove these reports in order to reveal the true nature of the earths.
In 1776, Scheele published a paper proving the individual nature of aluminium
and silicon oxides [20]. The reason was the opinion of Baumé that earth of alum
(Al 2 O 3 ) was actually siliceous earth (SiO 2 ), that clay was the same earth combined
with sulphuric acid and that alum (KAl(SO 4 ) 2 ) was the same earth over-saturated
with sulphuric acid. Scheele was sceptic: “With regard to chemical opinions, it is
my custom not to credit any, till I have brought them to the test of experiment” [21].
Actually, Scheele discussed this issue with Gahn in a letter dated November 1772,
the writing of this manuscript being the next task for Scheele after writing together
the paper on arsenic acid and the book on air and fire.
At first, it seemed to Scheele that Baumé was correct: he melted powdered rock
crystal (pure SiO 2 ) with potassium hydroxide in a crucible, extracted the residue
(potassium silicates) with water and added sulphuric acid [22]. Siliceous earth
precipitated and Scheele found small amounts of alum in the mother liquor. Still,
this alum could have been a contaminant in the rock crystal or the alkali, and
consequently Scheele repeated the procedure with the precipitated siliceous earth
seven times, and each time he obtained alum. At this point Scheele noticed that the
crucibles were corroded, and he found that the source of the alum was clay from the
crucible: “But, behold! on examining the crucibles employed for these repeated
fusions, I found them everywhere uneven in the inside, and full of little excavations,
which they had not before the experiment” [23]. When the operation was performed
in an iron crucible, no alum was obtained and Scheele could (probably to his relief)
establish that “The siliceous earth, therefore, still remains a peculiar earth” [23]. In a
similar fashion, Bergman criticised a paper claiming that cobalt could be obtained
from iron and arsenic [24]. Bergman concluded that the cobalt must have been
present as an impurity in the reagents.
Despite the work of Scheele, Bergman was not convinced of the elemental
nature of siliceous earth (SiO 2 ), but in lack of evidence he had to consider it as
elemental. What made Bergman suspicious were Scheele’s experiments with
hydrofluoric acid (Sect. 14.1), where siliceous earth appeared to arise upon contact
of the acid with water, while it was in fact formed by the hydrolysis of volatile
silicon tetrafluoride and condensation of the silicic acid formed:
22.4 Scheele’s and Bergman’s Views on the Earths, Metals, Acids and Alkalis
305