Josiah Parsons Cooke by William Shaw Warren
Not surprisingly, the many articles celebrating the recent 150th anniversary of the periodic table featured heavily on the Russian chemist Dmitri Mendeleev. One inquirer not mentioned was the North American chemist Josiah Parsons Cooke (1827 – 1894). Notably, Mendeleev (1872:50) cited Cooke, in addition to a number of others working on an arrangement of the chemical elements, such as Gladstone, Pettenkofer, Kremers, Dumas, Lenssen and Odling, as ‘having observed the regular relationship between the atomic weights of elements belonging to a particular group’. Whilst the recent focus on Mendeleev is not surprising, this is an opportunity to highlight the contribution of Josiah Cooke, a North American, in what is otherwise often a very Europe-centric account of the periodic table. The history of the periodic table was not a solely European enterprise!
Before saying something of Cooke’s contribution to the arrangement of the elements by atomic weight, a few words on his formation as a chemist: the young Josiah showed an early interest in chemistry, performing experiments in his home laboratory. He was accepted by Harvard College to study mathematics – by no means a forgone conclusion as he struggled with Greek and Latin, both essential for matriculation. He graduated in 1848 and was later appointed a tutor in mathematics. At that time Harvard’s chemistry department had rather stagnated. In 1850, Cooke was appointed to teach chemistry. As a child, Cooke had attended chemistry lectures given by Yale’s Benjamin Silliman (1779 – 1864), and this interest in chemistry continued into his young adulthood. Thus, it was as an amateur chemist that Cooke added teaching chemistry to his duties at Harvard. By the end of that year, at the very young age of 23, Cooke was appointed as Erving Professor of Chemistry – his predecessor, John Webster, having been hanged for murder. Apart from his childhood experiments, Cooke’s entire education in chemistry came from a year in Paris attending a number of lectures delivered by the French chemist Jean Baptiste Dumas. Cooke often referred to himself as a ‘self-taught chemist’. On returning to Harvard, Cooke revitalised the chemistry department by introducing teaching methods pioneered by Liebig at Giessen and using apparatus he purchased himself.
In 1851, Dumas (later to be acknowledged by Mendeleev) introduced homology into organic chemistry, where compounds with both similar properties and formulae are grouped together – e.g. the carboxylic acids. Dumas applied the same principle to a number of the chemical elements by suggesting four sets of triads: (Li, Na, K); (Ca, Sr, Ba); (Cl, Br, I); (S, Se, Te). In 1855 Cooke published his own scheme: The Numerical Relation between the Atomic Weights, with Some Thoughts on the Classification of the Chemical Elements. Cooke (1855:33) paid tribute to Dumas’s Law of Triads but considered the scheme to be only ‘a partial view of this subject’. As with Mendeleev some fifteen years later, Cooke’s (1855:236) initial reason for devising his system was to simplify the acquisition of chemistry for his students, adding that the scheme ‘never would have been published had it not led to the discovery of the numerical relation between the atomic weights’. Unlike Dumas’s system, Cooke’s scheme included all the fifty-five known elements arranged into six series. At the time of publishing, Cooke’s classification offered the most complete system based on the atomic weights of the elements. Unlike methods, such as Dumas’s, based on simple triads of elements, Cooke’s scheme was more sophisticated in bringing together seemingly dissimilar elements – such as metals and non-metals - that were shown to exhibit similar chemical relations.
Cooke classified the elements into six series, each with its own characteristic general formula: atomic weight = a + nb, where the value of b gives its name to the series and n is an integer unique to each element. Something of Cooke’s system can be glimpsed by looking at The Six Series:
Cooke found the existing methods of classification unsatisfactory, being too focussed on one set of properties. Text books would often separate non-metallic elements, such as nitrogen and phosphorous, from the more metallic arsenic, antimony and bismuth, even though their chemistries are analogous. As Cooke (1855:237) writes: ‘For a zoologist to separate the ostrich from the class of birds because it cannot fly, would not be more absurd, than it is for a chemist to separate two essentially allied elements, because one has a metallic lustre and the other has not.’ Cooke’s six series each exhibited similarities and trends in chemical and physical properties. This offered a way of teaching chemistry that reduced the need to learn a number of seemingly disconnected facts.
In Series Six for example, the electronegativities of the elements (affinity for O, Cl, S, etc) increase down the series. The student, states Cooke (1855:237), ‘will be pleased to find confirmed’ that phosphorus being more electropositive than nitrogen has a stronger affinity for chlorine and sulphur forms not only PCl3 and PS3, but also PCl5 and PS5. In addition, the decreasing affinity towards hydrogen down the series corresponds to there being no bismuth-hydrogen compound and explains the decreasing basicity down the series, NH3, PH3, AsH3 and SbH3, in terms of the decreasing affinity between the elements.
Cooke’s intention was to demonstrate that, for each series, ‘the properties of the elements follow a law of progression [and] that the atomic weights vary according to a similar law, which may be expressed by a simple algebraic formula’. It was Cooke’s hope that over time his system might be refined so that by means of a general formula it would be possible both to place an element and to predict its properties with a degree of certainty. As George Kauffman (1969:231) states, Cooke’s hopes ‘were to be fulfilled to a remarkable degree within his own lifetime by another scientific prophet, one who hailed from the steppes of Siberia - Dmitri Ivanovich Mendeleev’.
Dr Chris Campbell
Honorary Research Associate
Department of Science and Technology Studies
University College London
References
Cooke, J. 1854. ‘The Numerical Relation between the Atomic Weights, with Some Thoughts on the Classification of the Chemical Elements’, Memoirs of the American Academy of Arts and Sciences, New Series, 5 (2): 235-256.
Kauffman, G. 1969. ‘American Forerunners of the Periodic Law’, Journal of Chemical Education, 46(3):(128-135).
Mendeleev, D. 1872. ‘On the Periodic Regularity of the Chemical Elements’ in Jensen, W.B., 2002. Mendeleev on the Periodic Law: Selected Writings, 1869–1905. Dover Publications, Mineola, NY, pp43 – 101.
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