Chemical formulas, those small icons which chemists are wont to scribble in their notebooks and in odd places, such as the back of an envelope, and which to the general public have become emblems of their profession, are an excellent topic for history. These artefacts remain today tools for communication within the community of chemists. They continue serving as didactic instruments in teaching. The establishment of an individual formula for a chemical compound or a substance chronicles the laboratory methods, both routine and specific, which came into play in order for it to be written down and to assume the status of the analog of a word, to be stored within the growing lexicon of chemistry.
When addressing this topic, the historical narrative, besides its usual needs for accuracy and for an unerring sense of the strange and original taste of the bygone, demands the twin crutches of philosophical and linguistic inquiries. I wish to provide these complements if not in full, at least in a manner suggestive of some of the main issues.
I shall concern myself with the period of consolidation, when formulas entered the language of organic chemistry and started becoming sterotyped, the approximate period 1865-1905. 1 Why choose such a periodization? Because it brackets, approximately, the birth of the modern chemistry journal, JACS was started in 1879, and that of the modern comprehensive repertory of new chemical compounds, Chemical Abstracts were launched in 1907. Kekulé announced in 1865 his cyclic structure of benzene. The Chemical Society published in London, in 1882, Nomenclature and Notation, the first guidelines for establishing systematic and uniform practice. And the American Chemical Society followed suit in 1884 by establishing its Committee on Nomenclature and Notation. The international conference convened in Geneva in 1892 established norms for chemical nomenclature. 2 And Alfred Werner, in 1895, gave a systematic nomenclature for coordination complexes. Key milestones in the history of molecular formulas - so-called "structural formulas"; I favor the adjective "molecular" since the meaning of "structural" has changed considerably over the twentieth century - include the serendipitous synthesis of mauveine (1857), the first synthesis of alizarin (1868) and the identification of ibogaine (1905); Gomberg first free radical appeared in print in 1900. The forty years 1865-1905 were thus for molecular formulas of organic compounds those of the rise in their practical use, of their standardization and also of the first challenges to the rules governing them.
As always in history of science, the risk of Whig history lurks at every corner of the retrodictive narrative. The danger is to read into the structural formulas, as they were used at the end of the nineteenth-beginning of the twentieth century, meanings which they had yet to acquire in the post-Gilbert N. Lewis and post-Linus C. Pauling eras. Examples of such potential anachronisms are: (i.) viewing benzene rings as ipso facto synonyms of "aromaticity;" (ii.) reading double bonds as implying shorter and stronger interatomic linkages; (iii.) interpreting loss of a water molecule in a dehydration process as a thermodynamic driving force for the observed conversion. The eerie superficial similarity of these late nineteenth formulas to our early twenty-first century formulas can easily become misleading.