CARMEN J. GIUNTA
ARGON AND THE PERIODIC SYSTEM: THE PIECE THAT
WOULD NOT FIT
ABSTRACT. The discovery of the noble gases and their incorporation into the
periodic system are examined in this paper. A chronology of experimental reports
on argon and helium and the properties relevant to their nature and position in
the periodic system is presented. Proposals on the nature of argon and helium
that appeared in the aftermath of their discovery are examined in light of the
various empirical and theoretical considerations that supported and contradicted
them. “The piece that would not fit” refers not only to argon, the element that
at first seemed not to fit into the periodic system, but also to the piece or pieces
of evidence that various researchers and observers were prepared to discard or
discount in coming to terms with the newly discovered gases.
1. INTRODUCTION
The discovery of argon, a previously unknown component of the
atmosphere, was followed quickly by the isolation of helium, previously known only through some otherwise unidentified lines in
the solar spectrum. Within four years, three more members of the
noble gas group, krypton, neon, and xenon were discovered in quick
succession. Barely ten years separated the preliminary announcement of the discovery of argon in 1894 and the recognition of its
discoverers with Nobel Prizes. The Chemistry prize was awarded to
William Ramsay not only for his role in discovering the noble gases
but also for placing them in the periodic system (Nobel Foundation,
1966). The stories of the discoveries of argon and helium have been
often recounted (Travers, 1928; Hiebert, 1963; Weeks and Leicester,
1968), and they are full of instructive lessons (Giunta, 1998). The
incorporation of the noble gases into the periodic table has also been
previously reviewed (van Spronsen, 1969; Wolfenden, 1969; Hirsh,
1981).
Foundations of Chemistry 3: 105–128, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
106
CARMEN J. GIUNTA
The aim of this paper, like that of Hirsh’s, is to examine a variety
of responses to the discoveries of argon and helium (not only those
of the discoverers) and the evidence and assumptions behind those
responses. Although many proposals and speculations with little
or no basis were made and even published in the months immediately after the announcement of argon, several serious attempts
were made to make sense of a set of reported data that appeared
not to be entirely consistent with accepted parts of physical and
chemical knowledge. Examining the pieces of evidence that did not
fit the various proposals can also shed light on the empirical and
theoretical considerations of primary importance to the proponents
of those proposals.
2. CHRONOLOGY OF REPORTED DATA ON ARGON AND HELIUM
Because the events leading to the discoveries of argon and helium
are well known, I present only a brief chronology here. The chronology facilitates comparison between proposals on the nature of argon
and helium and experimental evidence available at the time. Besides
historical accounts of the discoveries of the noble gases (Hiebert,
1963; Weeks and Leicester, 1968; Wolfenden, 1969), an excellent
contemporary source of information was the Chemical News: it
carried reports of all of Rayleigh and Ramsay’s announcements on
argon and helium as well as reports of other important work and
several speculative letters and articles on the subject.
13 August 1894: A new gas of the atmosphere
Lord Rayleigh and William Ramsay made a preliminary oral
announcement of the isolation of a new gas of the atmosphere,
believed to be elementary, at the meeting of the British Association
for the Advancement of Science at Oxford. The gas was isolated
by two methods, both involving the removal of nitrogen and other
components from atmospheric air, eventually leaving only the new
gas in the gas phase. One method removed nitrogen by passing
high-voltage sparks through a mixture of oxygen and air, forming
nitrogen oxides which were separated by reaction with an alkali;
the other method passed air over hot magnesium, removing the
ARGON AND THE PERIODIC SYSTEM
107
nitrogen. Inertness appeared to be its chief characteristic (BAAS,
1894; Nature, 1894; Chemical News, 1894a).1
31 January 1895: Argon
Rayleigh and Ramsay (1895) made a full report to the Royal Society
(London) on the new gas. The lengthy report recounted the turns in
Rayleigh’s investigation of the density of nitrogen that brought him
and later Ramsay to suspect and eventually to isolate an unknown
gas from atmospheric air by the two methods described above. It
described experiments in which nitrogen from the atmosphere was
passed through a series of porous pipes with the object of enriching
the gas in its heavier components by diffusion; a denser gas was
in fact recovered. It described experiments in which samples of
nitrogen derived from chemical compounds were subjected to the
techniques used to isolate argon. In these experiments, substantially
less argon was recovered than from samples of ‘nitrogen’ derived
from atmospheric air. Rayleigh and Ramsay reported that the gas is
more soluble in water than is nitrogen, and that air extracted from
rainwater is denser than atmospheric air. They found that the ratio of
specific heats (constant pressure to constant volume) was 1.66. The
density was 19.7 to 19.9 times that of hydrogen. Attempts to make
argon react with a large variety of reactive substances produced
negative results.
Rayleigh and Ramsay reached the following conclusions:
• Argon is present in the atmosphere, and is not ‘manufactured’
by the processes of isolation.
• The argon recovered from “chemical nitrogen” (i.e., nitrogen
prepared chemically from nitrogenous compounds) is negligible, and probably due to argon dissolved in the water used
in the experiments.
• The ratio of specific heats implies that argon is monatomic;
a highly unlikely alternative is that it is a molecule with
no internal modes of motion, not even rotational. If it is
monatomic, it cannot be a compound: it must be an element
or a mixture of elements. If it is an element, its atomic weight
is about 40.
They addressed the possible position of argon in the periodic
system; their proposals will be examined in greater detail below.
108
CARMEN J. GIUNTA
The same session of the Royal Society heard a report by William
Crookes on the spectra of argon and one by Karol Olszewski on
some of its condensed-phase properties. Both of these papers represented independent and more detailed investigations of phenomena
into which Rayleigh and Ramsay had also delved. Crookes (1895)
reported two distinctly different spectra for argon depending on the
conditions of the electrical discharge through the gas. Olszewski
(1895) reported the boiling point (−186.9 ◦ C at 740.5 mmHg),
melting point (−189.6 ◦ C), and critical temperature (−121 ◦ C) and
pressure (50.6 atm) of the new gas.
Spring 1895: Terrestrial helium
In March 1895, Ramsay announced the isolation of helium from the
mineral cleveite on the 25th to the French Academy of Sciences (in a
telegram sent on the 23rd to Marcellin Berthelot) (Berthelot, 1895b),
on the 27th to the Chemical Society (Chemical Society, 1895), and
in a paper received at the Royal Society on the 26th and read April
25th (Ramsay, 1895a). On the day after the Royal Society announcement of argon, Henry A. Miers of the Mineral Department of the
British Museum had written to Ramsay about reports of nitrogen
obtained from the uranium-containing mineral uraninite (Taylor,
1953). Ramsay subsequently isolated a gas from cleveite, a variety
of uraninite. The spectrum of the gas included the helium line previously observed only in solar spectra. Ramsay’s initial announcements reported that the gas from cleveite also contained argon. J.
Norman Lockyer, a co-discoverer of solar helium, suggested that
the gas obtained from uraninite was a mixture of helium and at least
one other gas (Lockyer, 1895).2 The suspicion or belief that helium
was a mixture persisted over the next few years.
The first reports of physical properties of helium emerged later
in the spring of 1895. In a paper read before the Royal Society on
2 May, Ramsay (1895b) reported that the density of helium is 3.89
times that of hydrogen and the ratio of specific heats is high enough
to suggest it is monatomic. In a letter to Berthelot communicated to
the French Academy on 13 May, Ramsay reported that the density of
helium is 3.88 and the ratio of specific heats 1.66 (Berthelot, 1895c).
Per Cleve (1895), the Swedish chemist for whom the mineral
cleveite was named and who had independently confirmed the pres-
ARGON AND THE PERIODIC SYSTEM
109
ence of helium in cleveite, reported that the density of helium was
2.02, lower than Ramsay’s reported values. On 20 June, Ramsay,
J. Norman Collie, and Morris Travers presented an extensive paper
on sources and properties of helium to the Chemical Society. They
reported that helium is similar to argon in its unreactivity and its
specific heat ratio of 1.66, that the density of helium is 2.13, and
that the solubility of helium in water is lower than that of any other
known gas (Ramsay et al., 1895).
Later 1895: Further research on argon and helium
Argon and helium became object of intense research interest and
speculation as soon as their existence was reported. Many reports
of searches for the gases in animal, vegetable, mineral, atmospheric,
and astronomical sources appeared in 1895. Chemists tried mightily
to make the gases enter into chemical combination, reporting mainly
negative results.3 Rayleigh (1895b) reported viscosity and optical
properties of argon and helium at the British Association meeting
in September 1895. The refractivity numbers he reported (repren −1
senting ngas
) were 0.961 for argon and 0.146 for helium. Ramsay
air −1
(1895c) reported in October that Olszewski was so far unable to
liquefy helium, even after subjecting it to conditions that liquefied
hydrogen.
1898 Postscript: The companions of argon
Although research on the new gases continued, the empirical
evidence that formed the basis of proposals on the nature of argon
and helium and their place among the elements was essentially in
by the end of 1895, at least until the isolation of the companions
of argon from liquid air in 1898 shed new light on the matter.
Ramsay and Travers found the first of these gases on 31 May 1898.
They made a preliminary report about it to the Royal Society on 9
June; however, word of the discovery had already been made public
through a telegram to Henri Moissan (Travers, 1956, p. 174). The
gas was obtained by slowly evaporating liquid air and removing
oxygen chemically from the residue. Ramsay and Travers reported a
lower limit of 22.47 for the density (on a scale of oxygen = 16), and a
specific heat ratio of 1.666. They proposed the name krypton for the
new gas (Ramsay and Travers, 1898a). In mid June they reported
110
CARMEN J. GIUNTA
yet another gas, neon, with a density of 14.67 and specific heat
ratio 1.660 (Ramsay and Travers, 1898b). They announced xenon at
the British Association meeting in September 1898; it has a higher
boiling point and exists in smaller quantities than its companions
(Ramsay and Travers, 1898c). They also announced a twin of argon,
metargon, whose spectrum resembles that of carbon monoxide.
‘Metargon’ was later seen to be argon contaminated with carbon
impurities, some of its spectral lines attributable to C2 (Wolfenden,
1969).
3. INTERPRETATIONS OF ARGON AND HELIUM
Proposals and speculations about the nature of argon and its place
among the elements surfaced quickly after the initial announcement
of the new gas, and they continued after the discovery of helium.
The serious proposals attempted, for the most part, to accommodate
the reported evidence; moreover, the evidence that each proposal
discarded or discounted is not surprising in light of the preconceptions of the person who proposed it. Let us turn first to Rayleigh and
Ramsay’s suggestions concerning the nature of argon and its place
among the elements.
3.1. Initial interpretations of argon by its discoverers
Ramsay was awarded the Nobel Prize for chemistry in 1904 “in
recognition of his services in the discovery of the inert gaseous
elements in air, and his determination of their place in the periodic
system” (Nobel Foundation, 1966). Ramsay’s proposals concerning
argon and helium were, essentially, correct. Yet he too had preconceptions, and not all of the information he had at his disposal seemed
to fit his interpretation.
Even before the isolation of argon, Ramsay speculated to
Rayleigh that there was room in the periodic table after fluorine:
Has it occurred to you that there is room for gaseous elements at the end of the
first column of the periodic table? Thus: –
Li
–
–
–
–
Be
–
–
–
–
B
–
–
–
–
C
–
–
–
–
N
–
–
–
–
etc.
O
–
–
–
–
F
Cl
Mn
Br
?
XXX
–
Fe Co Ni
–
Pd Ru Rh
ARGON AND THE PERIODIC SYSTEM
111
Such elements should have the density of 20 or thereabouts, and 0.8 pc. (1/120th
about) of the nitrogen of the air could so raise the density of nitrogen that it would
stand to pure nitrogen in the ratio 230 ÷ 231. (Travers, 1956, p. 110)
The idea was that a place existed for three elements at the end of
the lithium period, at the head of the three-column group VIII that
separated the odd and even series of elements in the periodic table of
the time (see Table I (Mendeleev, 1897, Vol. 1, p. XV)). Ramsay’s
suggested density of 20 assumed a diatomic molecule of an atom
of atomic weight 20. Indeed, in a draft of a paper dated 7 August
1894, then put aside and never published, Ramsay wrote that it was
curious to find such an inert element with an atomic weight between
such active elements as sodium and fluorine (Travers, 1956, p. 119).
That Ramsay was still thinking of triads while tracking down the
inert gas from cleveite is evident from a letter dated 17 March 1895.
After spectrum analysis showed that the gas contained something
new, Ramsay supposed it was “the sought-for ‘krypton’, an element
which should accompany argon” (Travers, 1956, p. 138).
Ramsay’s public proposals on the place of argon included neither
a three-column group nor argon in the same period as fluorine.
Indeed, claims about the nature of argon by its discoverers kept
very close the experimental evidence, at least until the detection of
terrestrial helium. At the preliminary announcement at the British
Association meeting, they asserted that the gas is a constituent of
the atmosphere and may have supposed it to be an element.4 In
the Royal Society report on argon, Rayleigh and Ramsay (1895)
concluded that the new gas is present in the atmosphere, that it
is monatomic, and that it is an element or possibly a mixture of
elements. If it is an element, its atomic weight is 40, about the
same as calcium. They speculated that the element could represent a
transition between chlorine and potassium, if its atomic weight were
slightly less.
The conclusion that argon is monatomic rested on the specific
heat ratio: according to the kinetic theory of gases, a ratio of 5/3 indicates a ‘molecule’ that has only three degrees of freedom (i.e., has
only translational motion). Because a molecule consisting of more
than one atom has rotational and vibrational as well as translational
degrees of freedom, a specific heat ratio of 5/3 implies a monatomic
gas. The classical kinetic theory predicted a ratio of 4/3 for diatomic
112
CARMEN J. GIUNTA
TABLE I
Mendeleev’s periodic table (1897, Vol. 1, p. XV)
Series
1
2
3
4
5
6
7
8
9
10
11
12
Group
I.
H
II.
III.
IV.
R2 O2 R2 O3 R2 O4 R2 O5 R2 O6 R2 O7 Higher oxides
RO3
RO4
RO
–
RO2 –
–
–
RH4 RH3 RH2 RH
Hydrogen compounds
Al
N
O
Cl
Se
Y
Zr
Nb
Mo
In
Sn
Sb
Te
La
Ce
Di?
–
–
–
–
–
Yb
–
Ta
W
Tl
Pb
Bi
–
–
Th
–
U
Mn
Br
–
I
–
–
–
–
–
Ti
Ga
P
–
F
S
Sc
Si
–
VIII.
R2 O
–
–
K
C
–
VII.
(Cu)
Rb
(Ag)
Cs
–
–
(Au)
–
Na
B
–
VI.
–
Be
Mg
Ca
Zn
Sr
Cd
Ba
–
–
Hg
–
Li
–
V.
V
Ge
Cr
As
Fe Co Ni Cu
Ru Rh Pd Ag
–
–
Os Ir
–
–
Pt Au
gases and 7/6 for triatomic gases. Specific heat ratios for most
common diatomic gases were known to be about 1.4 and triatomic
gases about 1.3. The discrepancy between theory and experiment
had been sufficiently disturbing to make James Clerk Maxwell write
in 1860 that it, “. . . seems decisive against the unqualified acceptation that gases are such systems of hard elastic particles” as the
kinetic theory assumed. By the 1890s the discrepancy was known
and was not understood, but was not regarded by most physicists
as fatal to the kinetic theory. The one gas known at the time to
have a specific heat ratio of 1.67 (mercury vapor) was known to
be monatomic (from its vapor density), as the theory predicted.5
The piece that did not fit at this stage was the atomic weight
of argon, which suggested a place for it in a part of the periodic
table where there was clearly no room for it. An atomic weight of
just under 40 would have squeezed argon between potassium and
calcium, between the alkali metals and alkaline earths, with which
ARGON AND THE PERIODIC SYSTEM
113
it had no properties in common. The discoverers acknowledged the
conflict and that the properties they reported were unusual. Indeed,
Rayleigh (1895a) said in a Royal Institution lecture in early April
1895: “The facts were too much for us; and all that we can do now
is to apologise for ourselves and for the gas”.
3.2. Other early interpretations
The preliminary announcement of the new gas of the atmosphere
received a great deal of attention. Even before the report to the
Royal Society, its president, Lord Kelvin, called the discovery of
a new component of the atmosphere the greatest scientific event
of 1894 (Chemical News, 1894b).6 Critiques and alternatives to the
proposals of the discoverers also preceded the Royal Society report.
Immediately after the British Association report, James Dewar
expressed some skepticism over the claim that the new gas was
a constituent of the atmosphere. He argued that a component of
the atmosphere denser than nitrogen or oxygen ought to condense
with them and be separable from them by distillation of liquid
air; however, his experience with liquid air showed no sign of
such a component comprising anywhere near one percent of the
atmosphere. He suspected that the new gas was an allotropic form
of nitrogen, N3 , ‘manufactured’ by the methods of separation
Rayleigh and Ramsay employed. He suggested that if the new gas
were not manufactured, then it ought not to be obtainable from
nitrogen derived from nitrogenous compounds (Dewar, 1894a, b, c).
Dewar went on to compare the condensation behavior of nitrogen
samples from the atmosphere and from nitric oxide before and after
subjecting them to passage over red-hot magnesium. His preliminary results suggested slight differences in the gases after being
subjected to the magnesium regardless of whether the gas was
prepared from the atmosphere or the nitrogenous compound (Dewar,
1894a, b, c).
It is hardly justifiable to take Dewar to task for discounting
a piece of evidence that did not fit, for there was precious little
evidence yet available to him. If there was such a piece, it was the
density anomaly between atmospheric ‘nitrogen’ (i.e., nitrogen with
argon) and nitrogen derived from chemical sources that led Rayleigh
toward the isolation of argon in the first place. Rayleigh and Ramsay
114
CARMEN J. GIUNTA
did carry out the control experiment Dewar suggested and included
an account of it in their Royal Society report. Dewar seems to have
taken no further part in the controversy over the nature and place of
argon.
In the initial reaction to the Royal Society announcement of
argon, indeed, in the discussion immediately following the reading
of the papers at that meeting, two camps can be discerned, each of
which discounted a different piece of accepted scientific knowledge.
Henry Armstrong, president of the Chemical Society, characterized
as “highly speculative” the inference from specific heat ratios that
argon was monatomic. The new gas, he said, seems to be like
nitrogen only more so. He suggested that, “It is quite likely that
the two atoms exist so firmly locked in each other’s embrace that
there is no possibility for them to take notice of anything outside,
and that they are perfectly content to roll on together without taking
up any of the energy that is put into the molecule”. He noted the
“difficulty of placing an element of that kind,” i.e., with an atomic
weight of 40 (Chemical News, 1895). Apparently his belief in the
periodic law, with which he was well acquainted as a chemist, made
him discount the implications of the specific heat ratio in the kinetic
theory of gases, which was certainly not so central to his expertise.
Arthur Rücker, president of the Physical Society, came down firmly
on the side of the kinetic theory of gases, if there was indeed a
conflict between it and the periodic law. The former, he regarded
as one of “those great mechanical generalisations which could not
be upset without upsetting the whole of our fundamental notions of
science”, while the latter was, “after all, an empirical law which is
based at present upon no dynamical foundation” (Chemical News,
1895). Physicists were to have their fundamental notions upset soon
enough, but not because of argon! Travers reports that most physicists accepted the conclusion that argon was monatomic, although
he speculates that it was out of deference to Rayleigh that the Royal
Society paper did not place greater emphasis on the kinetic theory
and its implications (Travers, 1928, pp. 42–45).
3.3. Protecting the periodic system7
Armstrong’s willingness to discount the evidence of the monatomicity of argon in order to protect the periodic system was shared by
ARGON AND THE PERIODIC SYSTEM
115
several chemists in the months and years after the announcement
of argon. It is instructive to examine the responses of two strong
advocates of the periodic law: Dmitrii Mendeleev and Bohuslav
Brauner.
Mendeleev was, of course, the author of the most influential form
of the periodic law. He presented proposals on the nature of argon
to the Russian Chemical Society in March 1895. His remarks were
reported in the chemical and scientific press all over Europe (e.g.,
Mendeleev, 1895a, b), and were included as an appendix in the
next edition of his textbook (Mendeleev, 1897, vol. II, pp. 491ff).
Mendeleev said that the available evidence pointed toward the
conclusion that argon is an element rather than a compound or a
mixture, although the conclusion required confirmation. He offered
the hypothesis that argon was “polymerized nitrogen”, N3 . He
acknowledged that most triatomic molecules have a specific heat
ratio close to 1.3 rather than 1.66; however, he speculated that as
the ratio for chlorine (1.33) is lower than expected for a diatomic
gas, that of argon might be higher than expected for a triatomic
molecule. He suggested several possible tests of his hypothesis such
as working with argon at as high a temperature as possible (presumably to decompose N3 ), and to look for coincidences between the
spectra of argon and N2 . He acknowledged that the identification
of argon with N3 was conjectural, but he also mentioned alternative hypotheses he thought less likely: argon as X6 , where X was
a yet unknown element between hydrogen and litium with atomic
weight 6.7, or argon as a diatomic molecule of an atom of atomic
weight 20. He thought there was no reason to believe that there is
an element between fluorine and sodium, but even less reason to
believe that there is an element between potassium and calcium, or
even between chlorine and calcium: “The hypothesis A = 40 does
not admit argon into the periodic system”. Mendeleev mentioned
a letter from Ramsay that promised “a fresh proof of the periodic
law” coming out of the discovery of argon, and an editor’s note
refers the reader to papers that include the discovery of helium. In
an afterword, Mendeleev expressed confidence that helium would
somehow furnish fresh proof of the periodicity of the elements.
Even after the discovery of krypton, neon, and xenon, the piece
that would not fit for Mendeleev was the atomic weight of argon.
116
CARMEN J. GIUNTA
Mendeleev (1869) had originally predicted that remeasurement of
atomic weights would remove apparent atomic weight inversions,
and he never did come to terms with inversions in the periodic
system. There are, in fact, two instances of atomic weight inversions among the elements then known: cobalt precedes the lighter
nickel, and tellurium precedes the lighter iodine. The existence of
these inversions, however, was not firmly established at this time.
The atomic weights of nickel and cobalt were the subject of several
determinations in the late 1890s.8 Brauner had measured the atomic
weight of tellurium several times, eventually obtaining a value less
than that of iodine (Brauner, 1883). Mendeleev’s final edition of his
textbook included a group 0 of five noble gases, in which the atomic
weight of argon was listed as 38 (i.e., between those of chlorine and
potassium). The same table gave the same atomic weights to cobalt
and nickel (59) and to tellurium and iodine (127) (Mendeleev, 1905).
John Wolfenden (1969) has called Mendeleev’s treatment as
“far from dogmatic”, in contrast to that of Mendeleev’s disciple,
Brauner, whom he characterized as “plus royaliste que le roi”.
Brauner’s first entry into the fray left no doubt about where his
preconceptions lay: “As an orthodox Mendeleeffian, I find great
difficulty in assuming the existence of a new elementary gas having
the atomic weight 20 or 40 or 80 . . .”. He advocated the N3 hypothesis, noting that atmospheric nitrogen had ample time to form even
larger atomic complexes than N2 . The piece that did not fit was the
specific heat ratio, which he discounted by speculating that, “a close
complex of three nitrogen atoms, weighing only 42, and showing no
internal work, might be assumed to behave physically like a single
atom” (Brauner, 1895a).
After the discovery of helium, Brauner (1895b) addressed its
nature as well. The title of his short paper referred to “the helium
and argon type”, but not in a way that suggested that they represented a group within the periodic law. Instead, he proposed that the
lighter component of helium (thought to be a mixture) was really
H3 and that argon was N3 .9 He acknowledged that the structures
he proposed were at odds with ideas then current on valency and
bonds, but this did not preclude his proposing that oxygen could
also condense into an O3 distinct from ozone and that fluorine probably could as well. He continued to hold this belief, questioning the
ARGON AND THE PERIODIC SYSTEM
117
monatomicity of mercury vapor, the one previously known example
of a gas with a specific heat ratio of 1.66. He indicated that even
a group of elements with atomic weights of 4, 20, 38, 82, etc.
would not fit the periodic law, for those elements would have to
be on the rising parts of an atomic volume curve; however, argon
and helium, if elementary, would have very small atomic volumes
(Brauner, 1896). Even after the discovery of krypton, neon, and
xenon, Brauner (1899) saw no place for argon in the periodic table,
for it did not form compounds.10 Clearly, there was more than one
piece of knowledge that did not fit Brauner’s proposals.
3.4. Diatomic argon
John Hall Gladstone (1895) argued that argon was a diatomic
molecule of atomic weight 20. Such an element would fit into the
periodic table at the top of group VIII, but there was no room in the
table for an element of atomic weight 40. Gladstone acknowledged
that this proposal discounted the “forcible” argument for monatomicity derived from specific heat ratios; however, he wanted to wait
for chemical evidence of argon’s atomic weight (i.e., from analysis
of the compounds of argon he expected would eventually be made).
J. Emerson Reynolds (1895) seconded Gladstone’s proposal, adding
that there was room at the top of group VIII for argon and two
more unknown elements, and suggesting that the two atoms of a
diatomic argon might somehow be “so closely interlinked as to have
a common centre, and therefore to enable the molecule to simulate a monatomic character”. This placement, as we have seen, had
occurred to Ramsay before he or Rayleigh had isolated or tested
argon. Evidently the evidence of monatomicity was sufficiently
compelling to Ramsay to make him discard the idea even as it came
independently to others.
Richard M. Deeley (1895) and William T. Preyer (1895) each
proposed a diatomic molecule for argon, placing it between fluorine
and sodium. In addition, they combined the two short periods
beginning with lithium and sodium into a single long period. The
result was that carbon, nitrogen, oxygen, and fluorine, for example,
headed columns of transition metals rather than the columns that
contain silicon, phosphorus, sulfur, and chlorine respectively (see
Table II for Deeley’s version). C.E. Basevi (1895) also thought
118
CARMEN J. GIUNTA
TABLE II
Deeley’s periodic table (1895)
H
K
Rb
Cs
??
??
He
Ca
Sr
Ba
Yb
?
Li
Sc
Y
La
?
Th
Be
Ti
Zr
Ce
Ta
?
B
V
Nb
Di
W
U
C
Cr
Mo
?
?
?
N
Mn
?
?
Os
?
O
Fe
Ru
?
Ir
?
F
Co
Rh
?
Pt
?
A
Ni
Pd
?
Au
?
Na
Cu
Ag
?
Hg
?
Mg
Zn
Cd
?
Tl
?
Al
Ga
In
?
Pb
?
Si
Ge
Sn
?
Bi
?
P
As
Sb
?
?
?
S
Se
Te
?
?
?
Cl
Br
I
?
(?)
(?)
(?)
(?)
(?)
(?)
that a diatomic argon made more sense than a monatomic one.
He proposed modifications in the kinetic theory that would lead
to that result, complete with a mathematical derivation and a new
parameter.
The first explicit suggestion that argon belonged to a new
family of elements seems to have come from Paul-Émile Lecoq
de Boisbaudran (1895a). (Rayleigh and Ramsay’s suggestion that
argon belongs after chlorine may be supposed to imply the existence of other members of its group; however, it does not explicitly
do so.) As part of a system of which no details were given, Lecoq
de Boisbaudran (1895b) proposed a group of elements of atomic
weight 20.0945, 36.40±0.08, 84.01±0.20, and 132.71±0.15 (on the
scale O = 16). Although his initial note did not make clear which
of these elements was argon, a later communication attributed the
atomic weight near 20 to argon and added an atomic weight near 4
for helium. Once again it is the evidence of monatomicity argon that
did not fit.
3.5. Ideas further afield
George Johnstone Stoney, who is best remembered today for
applying the term electron to a fixed quantity of electrical charge,
speculated that argon might be an analogue of the relatively unreactive paraffins. He suggested that a compound of hydrogen and
infra-carbon, a hypothetical element of atomic weight about 2.5–
3 supposed to lie above carbon in the periodic table, might have
little internal motion (Stoney, 1895). Stoney saw his proposal as
speculative, suggestive of possible lines of inquiry in the aftermath
ARGON AND THE PERIODIC SYSTEM
119
of a new discovery. There was, however, a piece of evidence that did
not fit his speculation, the specific heat ratio.
W.W. Andrews (1895) wrote that he had interpolated curves of
physical properties as a function of atomic weight into the empty
spaces of the periodic table between H and Li. As a result, he
proposed that argon has properties like that of supra-beryllium or
possibly supra-boron (hypothetical elements supposed to lie above
beryllium and boron in the periodic table), suggesting that argon is
supra-Be28 , a very tightly bound and nearly spherical molecule. The
proposal represents a rather imaginative, indeed fantastical, way of
accommodating much of the available evidence on argon: a rigid
spherical molecule would seem to have only translational motion,
and the new element suggested here had a possible place in the
periodic table. Even more fantastical was a note by F. Rang (1895)
that placed helium and argon in the same column – along with
germanium, tin, and lead! Argon was assigned an atomic weight
of 13 and a valence of IV; its molecule was depicted as a threemembered ring of double bonds. Rather than speak of the piece of
evidence that would not fit, we would be harder pressed to look
for evidence that did fit. Rang’s paper serves as an illustration of
two points Wolfenden (1969) made in his overview of the argon
story: the periodic table attracted the attention of “speculative minds
of every degree of competence and fantasy”, and nothing was too
speculative for Crookes to print in the Chemical News!
3.6. A new group
Certainly not all chemists doubted the monatomicity of argon and
helium. Raphael Meldola (1895), in his presidential address to the
chemical section of the British Association, observed that “the case
in favour of argon being an element seems to be now settled by
the discovery that the molecule of the gas is monatomic . . .”. He
went on to characterize the situation a year after the preliminary
announcement of argon by noting, “It seems that two representatives
of a new group of monatomic elements characterised by chemical
inertness have been brought to light”. To be sure, Meldola echoed
Ramsay’s caution that placement of argon and helium in the periodic system must await definitively pure (“homogeneous”) samples.
Still, the remarks display acceptance of the available evidence and
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CARMEN J. GIUNTA
confidence that the new gases would find places within the (possibly
expanded) periodic system.
Like Lecoq de Boisbaudran, William Sedgwick (1895) also
hypothesized a group of zero-valent elements of atomic weight
20, 37, 82, and 130. His prediction was a clear and definite one
from a simple and concrete, although incorrect, model of atoms,
bonding, and valence. He pictured non-metal atoms of different
valences as spheres from which a number of facets, equal to the
valence of the atom, had been chipped. Compounds were formed
when atoms were bound facet to facet with a flat film of chemical force. The model pictured, for example, the atoms of silicon,
phosphorus, sulfur, chlorine, and argon as a series derived from the
same sphere from which four, three, two, one, or no facets had
been removed (each facet representing about two atomic weight
units). C.J. Reed (1895) also found a place for argon and other
unknown and unreactive elements in his system of elements, which
was essentially a plot of valence vs. atomic weight. On the basis
of this classification, up to 15 inert elements were possible with
atomic weights ranging between 4 and 228, but for some reason
he believed that the only elements likely to be found in nature
are those of weight 4, 20, 36, 84, 132, 196, and possibly 180.
Julius Thomsen (1895) argued for a group of inactive elements that
would span the gap between the monovalent electropositive and
electronegative elements and would make the electrical character of
the elements a continuous rather than discontinuous periodic function. (He was serious about mathematical periodicity, invoking some
particular trigonometric functions in his treatment.) He proposed
atomic weights of 4, 20, 36, 84, 132, 212, and 292 for the group.
Sedgwick, Reed, and Thomsen invoked pre-existing systems in
which zero-valent elements were either implicit or easily incorporated; however, none came to terms with the irregular intervals of
atomic weights of successive elements.
Resolution: A new group of elements
We have seen that the notion of a new group of elements was not
all that outlandish in the minds of some. The possibility of entirely
unknown groups in a periodic classification is as old as attempts at
such classification. J.A.R. Newlands’ response to the criticism that
ARGON AND THE PERIODIC SYSTEM
121
his law of octaves left no room for new elements was essentially to
say that new elements, if they were part of new groups, would not
disturb the periodicity (Newlands, 1866).
Ramsay’s statement about the place of argon in the periodic
table grew bolder in 1896 and 1897, after the detection of helium
but before there was any evidence for other noble gases. He was
not reluctant to consider a new group: what gave him pause was
the atomic weight inversion implicit in the placement of argon. In
1896, he published a periodic table that placed helium and argon
in a column next to the halogens. That column contained question
marks next to fluorine, bromine, and iodine, implicitly predicting
the elements now known as neon, krypton, and xenon. At this
stage, the atomic weight of argon was still the piece of evidence
that did not fit, for Ramsay noted that in the periodic table “no
element has an atomic weight lower than that preceding it in the
horizontal line”. (He believed that, “the balance of evidence is in
favour of tellurium having a lower atomic weight than iodine”.)
Ramsay suggested two “methods of escape” from the apparently too
high atomic weight of argon: that argon was a mixture of monatomic
gases (e.g., of atomic weight 37 and 82) or was a diatomic gas
in very high state of dissociation (e.g., 95%). He noted that the
preponderance of evidence contradicted both proposals: argon’s
sharp melting and boiling temperatures argued against a mixture,
and its pressure-temperature behavior showed no sign of a changing
degree of dissociation. Ramsay made a proposal that he knew to
be highly speculative, that chemical affinity could affect atomic
weight; argon, having no chemical affinity, would be influenced by
this hypothetical relationship differently than most other elements
(Ramsay, 1896). There was no basis for this proposal, notwithstanding the mass-energy equivalence that would later come from
Einstein’s theories of relativity.
Ramsay (1897) made a more explicit and public prediction of
another member of the helium-argon group of elements in 1897
in his address as president of the chemistry section of the British
Association. The title of the lecture was “An undiscovered gas”, and
the gas Ramsay had in mind was a monatomic gas of atomic weight
about 20. This was a real prediction, made by Ramsay after he and
Travers had been searching diligently for new noble gases but well
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CARMEN J. GIUNTA
before they turned up any physical evidence for any of them. The
notable shift in Ramsay’s position from the previous year was his
recognition of the inversion in atomic weight order of tellurium and
iodine; here he proposed that argon and potassium represent another
example of the same phenomenon. The absence from the address of
any hypotheses, however unlikely, intended to escape the implication of an inversion represented a rhetorical change that reinforced
that shift. Ramsay proposed a relationship between atomic weight
and chemical affinity in the address as well, but with no suggestion
that it explains or mitigates the argon-potassium inversion.
The discovery, less than a year later, of the undiscovered gas and
two others provided conclusive evidence that these five new gases
were members of a group of unreactive monatomic gases. Indeed,
Crookes (1898) was quick to incorporate krypton and neon into
his novel three-dimensional representation of chemical periodicity
in their proper places even before accurate measurements of their
density or atomic weight were available. He read his paper (on the
position of helium, argon, and krypton in the periodic system) to
the Royal Society immediately after Ramsay and Travers’ paper
announcing krypton. Crookes included neon in the system two
weeks later in an addendum to the paper.
Ramsay was not quite finished with placing inert gases in the
periodic table, however, or with predictions. Ramsay and Robert
Whytlaw Gray (1910) reported that the atomic weight of the
“emanation of radium” (now known as radon) was about 220. “Now
there is no doubt that the emanation is the second member of the
inert gas series after xenon”. They expected another member of that
group to have atomic weight about 175. Predictions of this sort,
vitiated by the poorly understood lanthanides, had been made from
the time of the earliest recognition of chemical periodicity.11
4. SUMMARY AND CONCLUSIONS
The discoveries of argon and helium presented chemists and physicists with new phenomena that did not quite fit into their frameworks
of accepted knowledge because of apparent conflicts with aspects
of the periodic law (important for chemists) or the kinetic theory
of gases (important for physicists). The data, obtained from metic-
ARGON AND THE PERIODIC SYSTEM
123
ulous experiments by prominent scientists, could not be ignored or
dismissed. Attempts to force a fit between the new phenomena and
accepted knowledge took a variety of forms (Hirsh, 1981). Most
of those attempts required discarding, downplaying, or explaining
away one or another piece of either new information or accepted
knowledge.
In the eventual resolution to the place and nature of argon, the
piece that did not fit was the atomic weight of argon; despite a
weight that would place it between potassium and calcium, it found
a place in a new group of the periodic table between halogens and
alkali metals. To many chemists, the piece that did not fit was the
monatomicity of argon as inferred from specific heat ratios and the
kinetic theory of gases. The notion that argon and helium were new
elements and even the possibility that they fit into the periodic table
between halogens and alkali metals was less disturbing to chemists
than the conclusion that argon was a monatomic gas with atomic
weight greater than that of potassium.
Finally, the discovery of terrestrial helium does not by itself
appear to have resolved the problem of the nature of argon. Most
of the unsuccessful speculations about argon came after terrestrial
helium was discovered. The notion that argon was N3 originated
(with Dewar) before helium was known, but it persisted (e.g., with
Brauner) afterward. Furthermore, helium did not even solve the
problem for Ramsay. Recall that Ramsay had speculated about
placing argon between halogens and alkali metals (albeit between
fluorine and sodium) even before isolating argon, let alone helium.
Even after isolating helium, which undoubtedly increased Ramsay’s
confidence that a whole group of elements existed between the halogens and alkali metals, Ramsay looked for ways of avoiding the
argon-potassium atomic weight inversion (Ramsay, 1896).
NOTES
1. The British Association report records only the fact of the announcement but
none of its content. Some details of and reactions to the announcement are
recorded in contemporary reports of the meeting.
2. Lockyer extracted helium from bröggerite, yet another variety of uraninite.
3. Only Berthelot seemed to be successful in coaxing argon into combination with benzene in an electrical discharge (e.g., Berthelot, 1895a, and
124
4.
5.
6.
7.
8.
9.
10.
11.
CARMEN J. GIUNTA
subsequent communications). Ramsay suggested that the combination was
more mechanical than chemical (Ramsay, 1896, chapter VI).
Reports of the announcement in Nature and the Chemical News, and even
the scant information in the British Association report noted that Rayleigh
and Ramsay claimed that the new gas was a constituent of the atmosphere.
The Chemical News report included the supposition (on whose part, it is not
clear) that the new gas was an element; the Nature report included no such
supposition (BAAS, 1894; Chemical News, 1894a; Nature, 1894).
See Laidler (1993), and references therein for the resolution of predicted and
observed specific heat ratios of gases.
Henry Armstrong, president of the Chemical Society, seemed to take umbrage
at this statement, referring to it at a meeting of the Chemical Society a week
later and holding that “chemists could not be expected to remain quiet under
the imputation that they had been eyeless during a whole century” for not
having noticed an element in the atmosphere as abundant as the new gas was
claimed to be (Chemical News, 1894c).
See Scerri and Worrall (2000) on the discovery of argon, the concern that
discovery provoked with respect to the status of the periodic table, and the
eventual accommodation of the noble gases into the table.
One could read about argon and helium and about the atomic weights of
nickel and cobalt in the same publications at the same period of time.
For example, volume 72 of the Chemical News reported results published
by Clemens Winkler (pp. 52–53, 1895) in Zeit. Anorg. Chemie and by
R. Schneider (p. 258, 1895) in Zeitschrift für Analytische Chemie. The
same British Association meeting at which Ramsay would speak of an
undiscovered gas (Ramsay, 1897) included a paper by T.W. Richards and
co-workers on the atomic weights of nickel and cobalt (Report of the
67th Meeting of the British Association for the Advancement of Science,
pp. 609–610, 1895).
Travers would later observe, “It is rather astonishing that the idea that argon
was N3 did not die an early and natural death, but it was kept alive mainly by
Professor Bohuslav Brauner” (Travers, 1928).
The insight was not original with Brauner; he cited Augusto Piccini, “Das
periodische System der Elemente von Mendelejeff und die neuen Bestandtheile der atmosphärischen Luft”, Zeitschrift für Anorganische Chemie 19,
pp. 295–305, 1899.
Both Mendeleev (1872) and J.A.R. Newlands (1864), for example, had
expected that an alkali metal of atomic weight near 170 would someday be
found.
ARGON AND THE PERIODIC SYSTEM
125
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