A curious exception among the alkalis”: metallic ammonium and the role of chemical analogy in the process of epistemic iteration (1807-1820)

Thème : Chemistry

Résumé

‘“A curious exception among the alkalis”: metallic ammonium and the role of chemical analogy in the process of epistemic iteration (1807-1820)’

…we saw the known constituents of ammonia meet at the negative pole and form a metallic body; and in affirming that a metal was composed under our eyes, from a fluid of which the constituents are perfectly known to us…, we relate a miracle…”.1

As this citation shows, the Swedish chemists Magnus Martin Pontin and Jöns Jacob Berzelius immediately realized the significance of their discovery of an amalgam produced from the gaseous compound ammonia in 1808. After all, only metals were known to produce such metallic alloys with mercury, and ammonia was not thought to contain a metal. Similarly excited about this phenomenon, their English colleague Humphry Davy made the prediction that ammonia contained a metallic element that had previously gone unnoticed, which he named ‘ammonium’. However, metallic ammonium could not be isolated and historians have therefore dismissed this inference as an “embarrassing mistake” that was “unhappily based on problematic analyses”.2

I argue, on the contrary, that the story of ammonium provides an interesting case study for the relation between theory, experimentation, classification and analogical reasoning in early 19th-century chemistry.  Since ammonium could not be isolated in the form of a pure chemical substance, Davy and Berzelius relied on indirect, mostly analogical evidence to study its nature. They faced a puzzling contradiction: ammonia behaved in various ways like a metallic oxide, and yet it only produced nitrogen and hydrogen whenever it was decomposed. In this presentation I will show how they attempted to solve this contradiction by adjusting their hypotheses through an iterative process with the aim of establishing a clear link between chemical properties and chemical composition. 

Bibliographie

J. J. Berzelius and M. M. Pontin, “Försök med Alkaliernas och Jordarternas sönderdelning,” Economiska 6 (1808): 121–22.

M. Crosland, “Davy and Gay-Lussac: Competition and Contrast,” in Science and the Sons of Genius: Studies on Humphry Davy, ed. S. Forgan (London: Science Reviews Ltd, 1980), 113; T. Levere, “Coleridge, Chemistry, and the Philosophy of Nature,” Studies in Romanticism 16 (1977): 374.

Livestream : https://youtube.com/live/QT3Tc3q0Uww