Control strategies in the discovery of Argon

In 1904, the renowned British physicist Lord Rayleigh (1842-1919) was awarded the Nobel Prize in Physics for his groundbreaking research on gas densities and the discovery of Argon, a previously unknown element and a constituent of the Earth's atmosphere. That same year, the Scottish chemist William Ramsay (1852-1916), was likewise awarded the Nobel Prize in Chemistry for his work on inert gaseous elements, with Argon first and foremost. Rayleigh and Ramsay initially conducted their work separately, but then they joined forces to identify the new gas and study its properties.

Lord Rayleigh (1842-1919)

In 1882, Rayleigh had expressed his eagerness to redetermine the densities of the principal gases, such as hydrogen, oxygen, and nitrogen, in order to test the hypothesis, first expressed by William Prout, according to which the atomic weights of the elements were integral multiples of the atomic weight of hydrogen. In the case of nitrogen, Rayleigh noticed a discrepancy between the densities of nitrogen prepared by two different methods. In the first method, nitrogen was obtained by freeing atmospheric air from oxygen, carbon dioxide, and other impurities. In contrast, in the second method, one-seventh of the prepared nitrogen was obtained with the aid of ammonia. Rayleigh named those two kinds of nitrogen "atmospheric" and "chemical," respectively. Further investigation into this discrepancy led to the discovery of Argon, which turned out to be an extended process encompassing the identification and subsequent incorporation of Argon into the conceptual framework of nineteenth-century chemistry.[1]

Isolation of Argon by means of red-hot magnesium (Rayleigh, 1895, p. 144)

Throughout his research on argon, Rayleigh implemented a range of control strategies: control of experimental conditions, variation of experimental parameters, and multiple determination of experimental results. The role of the scientific community was also vital, providing supplementary control of his experimental results. For instance, upon observing the above-mentioned discrepancy, Rayleigh sought feedback from the scientific community. He published a letter in Nature, inviting suggestions for elucidating the observed inconsistency (Rayleigh 1892). This move was a form of communal control, since it aimed at engaging the broader scientific community in the scrutiny and assessment of his experimental results.

One of Rayleigh's first actions was to magnify the discrepancy by preparing all the "chemical" nitrogen using ammonia, rather than only a part of it, as he had done in his initial experiments. He advocated this form of control, referring to it as a “principle” (Rayleigh 1895, 189) or “rule” (Rayleigh 1904, 213) for conducting experiments in general. The magnification of a discrepancy, rather than an attempt to eliminate it, was a "guided manipulation" aimed at its explanation.[2]

Furthermore, Rayleigh advocated multiple determination of experimental results. That methodological approach was evident at every stage of his research on Argon. To begin with, multiple determination revealed the initial discrepancy. Furthermore, Rayleigh verified almost all of his subsequent experimental outcomes using different methods or a combination of theory and experiment. He consistently compared his findings with the work of other chemists. For instance, he drew upon the research of the Belgian chemist Jean Stas in order to disprove the idea that chemical nitrogen could be a mixture.

In their endeavor to isolate the new substance, Rayleigh and Ramsay employed two distinct methods. In the first method, they removed nitrogen by using oxygen and subjecting the mixture to an electric spark, as Cavendish had done more than a century earlier. In the second method, they isolated the new gas with the assistance of red-hot magnesium. In their paper introducing Argon, Rayleigh and Ramsay argued that “atmospheric” nitrogen was a mixture, giving multiple reasons for their conclusion. Among those was that the double isolation guaranteed the existence of a new constituent of the atmosphere. Their argument was grounded in the notion that the likelihood of two dissimilar processes yielding the same result was exceedingly small (Rayleigh and Ramsay 1895, 180).[3]

Isolation of Argon with the aid of oxygen subjecting the mixture to an electric spark (Rayleigh, 1895, p. 142)

Rayleigh and Ramsay employed multiple determination again when experimenting on the properties of Argon; for instance, in determining its density and the ratio of its specific heats. Employing previous experiments by other scientists as standards of reference, they conducted comparisons to specify the density of the new element and establish its atomicity. Again, they employed different methods and multiple experiments.

In all, control strategies were integral parts of the research that led to the discovery of Argon. It was a marked feature of Rayleigh's initial experiments and of the joint investigations of Rayleigh and Ramsay regarding the isolation of the new gas and the exploration of its properties. The case of Argon indicates that Jean Perrin's utilization of multiple determination for demonstrating the existence of atoms had a worthy precedent in the work of Rayleigh.

[1]    For the discovery of Argon as an extended process, see Arabatzis & Gavroglu 2016.

[2]    Jutta Schickore has used this term in order to describe a targeted, precise intervention (Schickore 2019). Rayleigh applied this form of control in other experiments too. For instance, he magnified disturbances in his experiments concerning the determination of the Ohm (Rayleigh and Schuster 1881).

[3]    Their argument is similar to Ian Hacking's “argument from coincidence.” See Hacking 1983, pp. 200-202.

Further Reading

Arabatzis, Theodore, and Kostas Gavroglu. "From discrepancy to discovery: How argon became an element." In T. Sauer and R. Scholl (eds.), The Philosophy of Historical Case Studies. Cham: Springer, 2016, pp. 203-222.

Hacking, Ian. Representing and Intervening: Introductory Topics in the Philosophy of Natural Science. Cambridge University Press, 1983.

Rayleigh, Lord, and Arthur Schuster. "On the determination of the Ohm in absolute measure." Proceedings of the Royal Society of London 32, 1881: 104-141. Reprinted in

Rayleigh, Scientific Papers Vol. II, Cambridge University Press, pp. 1-37.

Rayleigh, Lord. "Density of Nitrogen." Nature 46, 1892: 512–513. Reprinted in Rayleigh, Scientific Papers Vol. IV, 1903, Cambridge University Press, pp. 1-2.

Rayleigh, Lord, and William Ramsay. "Argon, a new constituent of the atmosphere." Philosophical Transactions of the Royal Society of London A 186, 1895: 187-241. Reprinted in Rayleigh, Scientific Papers Vol. IV, 1903, Cambridge University Press, pp. 130-187.

Rayleigh, Lord. "Argon." Science 1/26, 1895: 701-712. Reprinted in Rayleigh, Scientific Papers Vol. IV, 1903, Cambridge University Press, pp. 188-202.

Rayleigh, Lord. "Extracts from Nobel Lecture” (1904). Reprinted in Rayleigh, Scientific Papers, Vol. V, 1912, Cambridge University Press, pp. 212-213.

Schickore, Jutta. "The structure and function of experimental control in the life sciences." Philosophy of Science 86/2, 2019: 203-218.

• This blog post is based on the following publication: Christopoulou V., & Arabatzis T. (2024). "From the Determination of the Ohm to the Discovery of Argon: Lord Rayleigh's Strategies of Experimental Control." In Elusive Phenomena, Unwieldy Things: Historical Perspectives on Experimental Control (pp. 243- 267). Cham: Springer Nature Switzerland. DOI: https://doi.org/10.1007/978-3-031-52954-2_9