The Discovery of Argon: a Case Study in Scientific Method
Carmen J. Giunta, Department of Chemistry, Le Moyne College, Syracuse, NY
Presented at the 211th ACS National Meeting, New Orleans, LA, March 24, 1996.
Copyright © 1996
The history of science is full of stories which exhibit scientific methodology
to an exemplary degree. Such stories can be vehicles for the teaching of
scientific thought to non-science majors in general education science courses,
particularly if they do not involve much technical background and are told in
ordinary language. The discovery of argon is a story replete with lessons in
how scientists pursue knowledge: Lord Rayleigh's use of multiple methods to
measure the density of nitrogen; his persistent tracking down of a small but
real anomaly in those measurements; his and William Ramsay's eventual
realization that the anomaly was due to a previously unknown but relatively
plentiful component of the atmosphere, an inert, monatomic gas; and Ramsay's
subsequent successful search for other members of the inert gas family. I
present this story in Rayleigh's words, annotated to supply background and to
pose questions to the student reader.
Introduction and Outline
Good morning. My topic this morning is using case histories to teach
scientific method to non-science majors. The particular case on which I'll
focus is the discovery of argon. First, I'll provide a bit of context on the
role of case studies in a general-education science course I teach at Le Moyne
College. Next, I'll very briefly put the discovery of argon in its historical
context, and then I'll describe that discovery, paying particular attention to
the lessons it teaches about scientific method. Finally, I'll say a few words
about the form in which I present this case to my class.
Le Moyne College is a small, undergraduate-oriented institution with a Core
curriculum founded on the liberal arts. For the last few years, I had a vague
idea of designing a course based on seminal papers in chemistry as an option
for the natural science part of the Core. My intent was to appeal to the
background in history and philosophy that Le Moyne students receive elsewhere
in the Core curriculum.
Le Moyne College CHM 203, "Scientific Thought"
The goal of the class is to teach non-science majors how scientists thin--and
also some content if I'm lucky! CHM 203 approaches science primarily as a way
of knowing rather than as a body of knowledge. Essentially, it is a course on
the "scientific method", conveying the nature of science as an empirical
endeavor which employs controlled experiments, quantifiable measurements,
logical inferences, testable hypotheses, and the like. The course begins with
students reading and discussing a brief monograph describing the scientific
method. The class also carries out an experiment testing the
hypothesis that bodies fall to earth at a rate proportional to their weight.
The students then read and discuss case histories of scientific
Scripture says there is no new thing under the sun, and that is certainly true
of the idea of using case histories to teach scientific method. I didn't get
very far in my preliminary thinking about this course before I came across
James Bryant Conant's ideas on the subject. Conant and some of his
colleagues at Harvard (including Leonard Nash) went on to develop some of
those ideas into a two-volume set of eight case histories. Conant recognized
case histories as an effective pedagogical tool whose study was used to great
effect in law and business schools at that time. Following the work of great
scientists through their own words can illustrate the "tactics and strategy of
science", as Conant put it. Selection of seminal cases from the early days of
a science requires the least amount of factual background on the part of the
students; at the same time, these early cases are the best examples of the
intellectual struggles involved in scientific research.
Historical Background: The Discovery of Argon
The case I'll speak about today, the discovery of argon, isn't quite so hoary
or fundamental as such developments in chemistry as the oxygen theory of
combustion, Dalton's atomic theory, or the periodic table. Still, the
discovery of argon was an event of sufficient scientific importance to merit
detailed study. Sir William Ramsay and Lord Rayleigh (born John William
Strutt) published their discovery of argon in 1895. Rayleigh was led into
the investigation by small anomalies he found in measurements of the density
of nitrogen purified by different methods. Those different methods led to
different quantities of nitrogen, and thus to different proportions of
nitrogen and a hitherto unsuspected atmospheric gas. Argon was the first
noble gas isolated. Naturally there was no place for it in the periodic table
as it then existed. Ramsay's subsequent work isolated helium and discovered
neon, krypton, and xenon by the end of the century. Ramsay and Rayleigh were
awarded Nobel Prizes in 1904. Note the plural "Prizes": Rayleigh was awarded
the physics prize for argon, while Ramsay was awarded the chemistry prize for
argon and the family of noble gases.
Morals of the Story: Lessons Learned from the Discovery of Argon
In this section I list a few of object lessons which can be garnered from
detailed study of the discovery of argon.
Rayleigh was put on the trail of argon because he used more than one method to
measure the density of nitrogen.
Purifying nitrogen was mainly a matter of removing oxygen from atmospheric
air. One way of doing so was to pass the air over hot copper, thus removing
the oxygen as copper oxide:
O2 + 2 Cu --> 2 CuO .
Another was to bubble air through liquid ammonia and then through a hot tube:
3 O2 + 4 NH3 --> 6 H2O + 2 N2 .
The water produced in this reaction could then be removed by drying agents,
and the nitrogen product joined nitrogen from the atmospheric sample. Because
the atmosphere also contains argon (unbeknownst to anyone at that time), the
proportions of nitrogen and argon were different in samples in which
additional nitrogen was produced. At any rate, the moral of the story is that
scientists often use more than one method to make measurements of the same
quantity in order to be more confident that they really are measuring what
they think they are measuring.
Rayleigh noticed small differences between methods only because of the high
precision of his measurements.
When is a difference between measurements big enough to bother about? When
the difference is greater than the experimental error of the measurement. A
look at the results of Rayleigh's measurements from a variety of methods
reveals a clear difference between two sets of data. This
observation can be a springboard to a treatment of statistical significance.
Rayleigh turned to experts in disciplines outside his own to attempt to
explain his anomalous results.
Rayleigh's 1892 note in Nature was an admission that he was stumped by the
anomalies he encountered in measuring the density of nitrogen: "I am much
puzzled by some recent results as to the density of nitrogen, and shall be
obliged if any of your chemical readers can offer suggestions as to the
cause." Trained as a physicist, Rayleigh addressed his appeal for suggestions
to chemists, that is to scientists whose expertise was different from his and
who might have ideas which did not occur to him. Today's scientific journals
don't have room for such communications, but cross-disciplinary consultations
and collaborations are widespread in modern science.
Henry Cavendish had probably encountered argon a century earlier, but he
could not follow through the way Rayleigh could.
Rayleigh didn't just consult current opinion; he went back to the literature.
Cavendish had passed electricity though air, absorbing the reaction products
(nitrogen oxides) with a piece of potash. He was left with a residue of just
under 1% of his original sample. But Cavendish was in no position to follow
through on characterizing this residue for a number of reasons, both
theoretical and technological. Cavendish was still operating under the
phlogiston theory and was trying to characterize the principal components of
the atmosphere. Furthermore, isolation of enough of the residue to study
would have faced enormous technological obstacles, given that his source of
electricity was a friction machine and his gas-handling apparatus was a
mercury "pneumatic trough". A century later, Rayleigh used Cavendish's method
"with the advantage of modern appliances", noting that, "In this Institution
we have the advantage of a public supply" of electricity. This episode
offers an excellent example of how scientific discoveries depend at least in
part on the current state of science and technology.
Rayleigh and Ramsay conducted a battery of tests to characterize the new gas
physically and chemically.
Once the investigators isolated their inert residue in sufficient quantities
to study it, how did they characterize it? Comparison of argon's spectrum to
known spectra helped establish that the gas was previously unknown. (This
mention of spectra provides an opportunity to discuss the plethora of
spectroscopic characterization techniques currently in widespread use.)
Measurements of constant-pressure and constant-volume heat capacities
established the monatomic nature of the new substance. Finally, some tests
were natural outgrowths of the investigation up to that point. For example,
Rayleigh had tried to measure the density of nitrogen in the first place, so
of course he measured the density of argon. Both researchers isolated argon
as an unreactive residue of air, so naturally Ramsay tried to get it to react
with a laundry list of reactive substances (elements, acids, bases, oxidants,
and reducing agents including hydrogen, chlorine, phosphorous vapor, sulfur
vapor, tellurium vapor, sodium vapor, molten sodium hydroxide, molten
potassium nitrate, potassium permanganate in hydrochloric acid, sodium
peroxide, bromine water, and a cocktail of nitric and hydrochloric acids).
Rayleigh recognized that the claim of elemental status for the newly
discovered gas was controversial.
Rayleigh told his audience that, "the subject [the assertion that argon is an
element] is difficult, and one that has given rise to some difference of
opinion among physicists." In light of the evidence that Ramsay and
Rayleigh marshaled for the elemental status of argon, and given that more than
40 elements had already been discovered in the 19th century, why was there
controversy? One reason was surely the periodic table, which had become
established over the preceding quarter of a century. There was no place for
argon in that table. If the periodic law and the discovery of a new inert
elemental gas were both correct, then there must be a family of such elements.
Ramsay arrived at that conclusion, and set about looking for other members of
the family. The lesson here is that new findings must be evaluated in the
context of existing knowledge. Apparent contradictions may cause a new
conclusion to be greeted with skepticism (often warranted). Sometimes,
however, attempts to resolve the contradictions prove scientifically fruitful.
The Case History: Bringing the Lessons to the Classroom
My way of presenting case histories in CHM 203 is to distribute a piece
written by the original researcher and heavily annotated by me. Some of the
footnotes gloss technical terms, some provide context for the investigation in
light of contemporary and current knowledge, and some fill in details. Most
importantly, some pose questions which lead to lessons like the ones I
mentioned above. For example, "Why would Rayleigh use more than one method to
measure the density of nitrogen?" What I put into my students' hands is about
equal parts historical text (in the foreground) and commentary (in the
background). By the way, I would be happy to send a copy of this case to
anyone who requests one; give me your card after the talk or drop me a line.
There are, of course, other ways of presenting case histories. For example,
the Harvard Case Histories in Experimental Science are expository articles
containing copious excerpts of writings by the original researchers. Its
format places the commentary in the foreground and the original texts in the
Sources of information for presenting or putting together case histories in
science include works on the history of science in general and the history of
chemistry in particular. I found Ihde's history of chemistry, originally
written about 30 years ago and currently available in a Dover paperback
edition, particularly helpful. Collections of classic readings in science
and classic readings in chemistry are rich sources of primary material, often
containing brief biographical or other context-setting information in addition
to the primary texts. I particularly enjoyed David Knight's two volumes of
facsimile articles organized by theme.
Unfortunately if understandably, Knight's volumes are no longer in print. The
Internet provides an opportunity to make classic papers in science more
readily available than they are in old journals and out-of-print anthologies.
Inspired by Project Gutenberg, an organization whose goal is to develop a
library of 10,000 public domain electronic texts (mainly literary) by the year
2000, I have begun a very modest effort to put the texts of a few papers and
excerpts on my World Wide Web site. I have not found any similar sites, but
I would love to hear of any.
 Sheldon J. Lachman, The Foundations of Science (Ann Arbor, MI: George Wahr,
 James Bryant Conant, On Understanding Science (New Haven: Yale, 1947).
 James Bryant Conant, ed., Harvard Case Histories in Experimental Science
(Cambridge, MA: Harvard, 1957). Topics from chemistry, physics, and biology
 Lord Rayleigh and William Ramsay, "Argon, a New Constituent of the
Atmosphere", Philosophical Transactions 186A, 187 (1895).
 Lord Rayleigh, "Density of Nitrogen", Nature 46, 512 (1892); Lord Rayleigh,
"On the Densities of the Principal Gases", Proceedings of the Royal Society
53, 134 (1893); Lord Rayleigh, "On an Anomaly Encountered in Determinations of
the Density of Nitrogen Gas", Proceedings of the Royal Society 55, 340 (1894).
 The story as told by Rayleigh in a public lecture at the Royal Institution is
an excellent, non-technical account: Lord Rayleigh, "Argon", Royal
Institution Proceedings 14, 524 (1895).
 Henry Cavendish, "Experiments on air", Philosophical Transactions 74, 372
 Aaron John Ihde, The Development of Modern Chemistry (New York: Dover,
 E. g., David M. Knight, ed., Classical Scientific Papers: Chemistry (New
York: American Elsevier, 1968) and Classical Scientific Papers: Chemistry,
Second Series (New York: American Elsevier, 1970).