In the Classroom
Using History To Teach Scientific Method: The Case of Argon Carmen J. Giunta Department of Chemistry, Le Moyne College, Syracuse, NY 13214
Pedagogical Background While developing a general-education science course for college students majoring in fields outside natural science, I came across James Bryant Conant’s idea of using case histories in science to teach how scientists approach and conduct scientific research (1). Conant believed that studying the work of great scientists can illustrate the “tactics and strategy of science”. Selected seminal cases from the early days of a science require the least amount of factual or technical background on the part of the students; at the same time, these early cases are the best examples of the intellectual groping involved in scientific research. Conant and some of his colleagues at Harvard (including Leonard Nash) went on to develop a two-volume set of eight case histories (2). I believed that a historical approach would appeal to students interested in history and philosophy, and I set about designing a course based on studying scientific developments of the past. The goal of the class, which I have taught for three years so far, is to teach non-science majors how scientists think (and some chemistry content if I’m lucky). It 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 that employs controlled experiments, quantifiable measurements, logical inferences, testable hypotheses, and the like. It begins with students reading and discussing a brief monograph on scientific methodology (3). The students then read and discuss case histories of scientific discoveries. The class also has a very limited exposure to collecting data themselves in a simple experiment testing the hypothesis that bodies fall to earth at a rate proportional to their weight. Morals of the Story: Lessons from Studying the Discovery of Argon The story that I use here to illustrate the case-history approach is the discovery of argon. This discovery is not quite so hoary or fundamental as the oxygen theory of combustion, Dalton’s atomic theory, or the periodic table—any of which could also be the subject of equally illustrative cases. Still, the discovery of argon is 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 samples prepared by different methods (5–7). Some of the methods mixed nitrogen from nitrogen-containing compounds with ostensibly pure nitrogen of atmospheric origin. Thus different samples contained different proportions of nitrogen and
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a hitherto unsuspected atmospheric gas, eventually isolated and named argon. Argon was the first noble gas isolated, so 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. In the remainder of this section I list a few of the lessons that can be gleaned from detailed study of the discovery of argon as told by Rayleigh in a public lecture at the Royal Institution (8). Of course listing lessons is not the same as teaching them. In the next section, I describe how I present the details from which these and other lessons can be drawn.
Lesson 1 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 was added to nitrogen from the atmospheric sample. Because the atmosphere also contains argon (unbeknownst to anyone at that time), the addition of nitrogen (as from ammonia) to an atmospheric sample altered the relative proportions of nitrogen and argon. As a result of this slight difference in composition, samples of equal volumes prepared by different methods at identical temperature and pressure had slightly different masses and therefore different densities. At any rate, the lesson 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.
Lesson 2 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
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In the Classroom
Lesson 3 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.” Rayleigh, a physicist, addressed his appeal for suggestions to chemists, that is, to scientists whose expertise was different from his and who might have ideas that did not occur to him. Today’s scientific journals have no room for such communications, but cross-disciplinary consultations and collaborations are widespread in modern science. Lesson 4 Henry Cavendish had probably encountered argon a century earlier (9), but he could not follow through the way Rayleigh could. Rayleigh not only consulted 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 interested in characterizing 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 makeshift “pneumatic trough”. (The figure that accompanies Cavendish’s account of his experiment depicts an inverted U -tube connecting a pair of mercury-filled wine glasses.) 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 (8). This episode offers an excellent example of how scientific discoveries depend at least in part on the current state of science and technology. Even Rayleigh’s public lecture on argon (8) provides a fairly detailed description of his apparatus for isolating argon; still more details and diagrams can be found in the formal report of Rayleigh and Ramsay (4 ). Lesson 5 Answering one question begets new questions: characterizing argon. Once the researchers isolated their inert residue in sufficient quantity to study it, the focus of the investigation clearly changed from Rayleigh’s original concern with the density of nitrogen. Having found the cause of the anomaly, they now set about characterizing it. As happens so often in science, answering one question leads to many other questions. Comparison of argon’s spectrum to known spectra helped establish that the gas was previously unknown. (The mention of spectra provides an opportunity to discuss the visible arc spectrum used in this case and 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. Some tests were natural outgrowths of the investigation up to that point. For example, Rayleigh had tried to
Nitrogen analyses (after Rayleigh, ref. 8) copper iron ferrous
method
of Rayleigh’s measurements of samples prepared by a variety of methods reveals a clear difference between two sets of data. Figure 1 displays the mass of several samples of nitrogen obtained by various methods; each mass is the mean of several individual measurements, all under the same conditions of temperature and pressure. The first three samples are of what Rayleigh called “atmospheric nitrogen”, and the last five are “chemical nitrogen”. Rayleigh suspected that the anomaly he had noticed was due to the fact that some samples contained nitrogen from chemical compounds (chemical nitrogen), whereas others contained only nitrogen from the atmosphere. (In reality, “atmospheric nitrogen” was mainly nitrogen mixed with a small amount of argon.) In order to test whether the anomaly was due to a difference between chemical and atmospheric nitrogen, Rayleigh prepared atmospheric nitrogen and chemical nitrogen each in several different ways. Removing oxygen by reacting it with hot copper, hot iron, or ferrous sulfate in alkaline solution yielded atmospheric nitrogen. Rayleigh prepared samples of chemical nitrogen (including no atmospheric air) derived from several nitrogenous compounds: nitric oxide, nitrous oxide, urea, and ammonium nitrate (this last, purified at red heat or at room temperature), but not ammonia. Even a cursory inspection shows that the first three results cluster around one value while the last five cluster around another, albeit with a bit more scatter. The difference in mass between chemical nitrogen and atmospheric nitrogen under identical conditions is small (as a look at the horizontal scale shows), but real. The lesson here is that Rayleigh noticed that small difference and demonstrated that it was a real difference through careful multiple measurements. Until he found the cause of the difference, he could not determine the density of nitrogen as accurately as his instruments would permit. With a mathematically sophisticated class, this incident could be a springboard to a treatment of statistical significance. With a chemically sophisticated class, additional details about the apparatus or about difficulties in obtaining such precise measurements (such as buoyancy) could be profitably presented; such details are found in ref 6.
nitric nitrous urea
NH4NO3 hot NH4NO3 cold 2.295
2.300
2.305
2.310
2.315
mass / g Figure 1. Mass of nitrogen samples prepared by a variety of methods. See text for explanation of method labels.
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In the Classroom
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, phosphorus 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). In addition to making the methodological point about the original investigation changing into a new one, there are several opportunities for teaching content here which can employ detailed comparisons between the physical and chemical properties of argon and nitrogen.
Lesson 6 Rayleigh recognized that the claim of elemental status for the newly discovered gas was controversial. Rayleigh told his audience that the assertion that argon is an element “is difficult, and one that has given rise to some difference of opinion among physicists” (8). 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 over one more new element? One reason was surely the periodic table, which had become established during the preceding quarter of a century. There was no place for argon in that table as a relative of similar elements, for no element similar to argon was yet known. There was an additional difficulty: in atomic mass argon lies between potassium and calcium, elements to which it bears not the slightest resemblance. (Of course, in atomic number it precedes potassium, but no physical basis for atomic number had yet been discovered.) 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. This is the conclusion Ramsay reached after finding that an inert gas found associated with certain uranium-containing minerals was neither nitrogen nor argon. Already in 1896, Ramsay added a new column to the periodic table after the halogens (10). He set about looking for other members of the family of inert gases. In 1897, he made a very public prediction of the discovery of an inert monatomic gas of atomic weight 20 to the Chemistry section of the British Association for the Advancement of Science (11). The methodological 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 Form of the Case History: Bringing the Lessons to the Classroom The vehicles I use for presenting case histories in my nonmajors class are pieces written by the original researcher and heavily annotated by me. Some of the notes gloss technical terms, some provide context for the investigation in light of contemporary and later knowledge, and some fill in historical or technical details. Most importantly, some pose questions that lead to lessons like the ones mentioned above. For in-
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stance, “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 (at the top of the page) and annotations (at the bottom). Students can apply to such cases the same analytical skills required for close reading of literature elsewhere in the curriculum. My treatment of the argon case is available from me upon request. There are, of course, other ways of presenting case histories. For example, the Harvard Case Histories in Experimental Science (2) are expository articles containing copious excerpts of writings by the original researchers. Their format places the explanatory commentary in the foreground and the original texts in the background. Presumably chemistry majors can also benefit from the study of important discoveries in their field. Curricula in the natural sciences tend to be so packed with content about the current state of knowledge that they severely shortchange their history. Chemistry majors ought to be capable of analyzing historical cases in greater detail than the humanities students for whom I have developed case histories. I believe the format I have described makes these cases suitable even for chemistry majors, who can still read and delve into the words of the original researcher even while skipping over annotations that explain material they already know. Not many case studies in scientific method exist in a form that can be imported directly into the classroom; however, sources of information for presenting or putting together case histories are more readily available. These include works on the history of science in general and the history of chemistry in particular, as well as papers and lectures by original researchers. I found Aaron Ihde’s history of chemistry, originally written in the 1960s and currently available in a Dover paperback edition, particularly helpful (12). 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 (13). David Knight’s two volumes of thematically organized facsimile articles are particularly valuable (14). Unfortunately, if understandably, Knight’s volumes are no longer in print. The Internet provides an opportunity to make classic scientific papers more readily available than they are in old journals and out-of-print anthologies. I have begun a very modest effort to put the texts of a few papers and excerpts on my World Wide Web site (15). Conclusion Detailed examination of significant developments in science can be used to teach scientific method to non-science majors. Using the discovery of argon as an example, I have described how the words of an original researcher can be supplemented to bring out the methodology of the investigation and to make the account of a discovery intelligible to students with little scientific background. I have also suggested some sources of important classic papers and supplementary historical material from which other case studies may be constructed. Literature Cited 1. Conant, J. B. On Understanding Science; Yale University Press: New Haven, CT, 1947.
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In the Classroom 2. Harvard Case Histories in Experimental Science; Conant, J. B., Ed.; Harvard University Press: Cambridge, MA, 1957. Topics from chemistry, physics, and biology are represented. 3. Lachman, S. J. The Foundations of Science, 3rd ed.; George Wahr: Detroit, 1956, 1992. 4. Lord Rayleigh; Ramsay, W. Philos. Trans. 1895, 186A, 187. 5. Lord Rayleigh. Nature 1892, 46, 512. 6. Lord Rayleigh. Proc. R. Soc. London 1893, 53, 134. 7. Lord Rayleigh. Proc. R. Soc. London 1894, 55, 340. 8. Lord Rayleigh. R. Inst. Proc. 1895, 14, 524. This is an excellent, nontechnical account. 9. Cavendish, H. Philos. Trans. 1785, 74, 372. 10. Ramsay, W. Gases of the Atmosphere; Macmillan: London, 1896. 11. Ramsay, W. Nature 1897, 56, 378. 12. Ihde, A. J. The Development of Modern Chemistry; Dover: New York, 1984. 13. Leicester, H. M.; Klickstein, H. S. A Source Book in Chemistry 1400–1900; McGraw-Hill: New York, 1952; this is an excellent example in chemistry. 14. Classical Scientific Papers: Chemistry; Knight, D. M., Ed.; American Elsevier: New York, 1968. Classical Scientific Papers: Chemistry, 2nd Series; Knight, D. M., Ed.; American Elsevier: New York, 1970. 15. Giunta, C. Carmen Giunta’s Classic Chemistry Page. http://maple. lemoyne.edu/~giunta; accessed August 1998.
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