pierre laszlo

L'Histoire du concept de 'molécule'

The molecule is central to structural chemistry; while the reaction holds a similar position for chemical dynamics. The trap for any history of the molecule is to swell into a history of chemistry. True, there is considerable overlap. But there is all the more need for focus that the molecule has just risen again to the apex of chemistry. The advent of molecular computers is imminent, some believe. Single molecule chemistry has become a research boulevard. ‘Vizualizing’ individual molecules is a related hype-producing topic. Duplicating by synthesis molecules of nature, however formidable, is now routine.

A molecule is a microscopic object with a shape (Ampère, 1814). This imaginative leap has become so ingrained that even TV commercials nowadays display molecular models. But this was neither an easy nor a quick achievement. Disbelief was the prevailing attitude still at the turn of the twentieth century. Because of the positivist bias and its prohibition against conjectures rooted in factual bedrock, a whole century was lost before molecules became real to scientists—from Ampère until Jean Perrin and Theodor Svedberg. Indeed, conceptualizing molecules is inseparable from laboratory techniques (purification). It stemmed from the study of crystals and crystallography starting with Kepler. Another major step forward was taken by René-Just Haüy towards the end of the eighteenth century.


Would this be the rare science history book that doesn’t trade insight for comprehensiveness ? Big ideas make good targets. Without them, on the other hand, one risks muddling through. Kubbinga does not offer a single organizing idea, but rather a wealth of historical parallels to support his intuition of a ‘molecularism’ informing the history of science during the modern period. Is the disproportionate book length justified ? Is the tale well told ? And is it a gripping story ? One has to answer these in the negative. This eruditely informed book leaves the reader wishing the author had spent more time defining and clarifying terms such as ‘molecule,’ ‘molecular theory,’ and ‘molecularism.’

The structural theory of chemistry, as a self-consistent web of interrelationships, forms a language—the lingua franca of chemistry. When stripped of unessentials, the current notion equates such a unique assembly of atoms—the analogs of letters or of phonemes—to a word, with its central role in language. Moreover and to put it in a nutshell, since the concept of molecule is intertwined with the issue of its representation—graphic formulas, if you will—the intellectual history here has to go hand in hand with an art history of sorts. Which these volumes do not offer.
How valid is the analogy of molecules to billiard balls, upon which rests the kinetic theory of gases? Can we see individual molecules? When was a molecule first weighed? Who first counted molecules? Individual chapters ought to have been devoted to such issues, and to giving clear and balanced answers.

A good start in that direction is to look-up the word ‘molecule’ in a dictionary such as the OED and to trace its semantic history from the eighteenth century on. This shows the fully formed modern notion of an assembly of atoms already present  in Henry Enfield Roscoe’s Lessons in Elementary Chemistry, 1871 edition. It points also to the divergence in the nineteenth century between physical and chemical molecules, the latter with its atoms held together by chemical affinity (Tyndall, 1882).

The author mistakenly credits Jean Perrin for the notion of a gram-molecule. It had originated a generation earlier with Alexander Crum Brown.