The Discovery of Argon: a Case Study in Scientific Method

Carmen J. Giunta, Department of Chemistry, Le Moyne College, Syracuse, NY 13214

Presented at the 211^th ACS National Meeting, New Orleans, LA, March 24, 1996.
Copyright © 1996

Abstract

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.

Pedagogical Background:
Le Moyne College CHM 203, "Scientific Thought"

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.

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.^[1] 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 discoveries.

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.^[2] 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.^[3] 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.^[4] Rayleigh was led into the investigation by small anomalies he found in measurements of the density of nitrogen purified by different methods.^[5] 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.^[6]

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^[5] 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,^[7] 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.^[6] 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."^[6] 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 19^th 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^[3] 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 background.

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.^[8] 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.^[9]

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.^[10] I have not found any similar sites, but I would love to hear of any.

Notes

[1] Sheldon J. Lachman, The Foundations of Science (Ann Arbor, MI: George Wahr, 1956, 1992).

[2] James Bryant Conant, On Understanding Science (New Haven: Yale, 1947).

[3] James Bryant Conant, ed., Harvard Case Histories in Experimental Science (Cambridge, MA: Harvard, 1957). Topics from chemistry, physics, and biology are represented.

[4] Lord Rayleigh and William Ramsay, "Argon, a New Constituent of the Atmosphere", Philosophical Transactions 186A, 187 (1895).

[5] 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).

[6] 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).

[7] Henry Cavendish, "Experiments on air", Philosophical Transactions 74, 372 (1785).

[8] Aaron John Ihde, The Development of Modern Chemistry (New York: Dover, 1984).

[9] 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).

[10] http://webserver.lemoyne.edu/faculty/giunta