In 1858 Archibald Scott Couper (1831–1892) and August Kekulé von Stradonitz (1829–1896), two young men from different backgrounds—and, as it turned out, entering upon even more disparate career paths—independently recognized that carbon atoms can link directly to one another to form carbon chains. This finding explained the very multiplicity of carbon compounds that had been puzzling chemists. The discovery by these two scientists depended on Kekulé's theory, proposed in 1857, that carbon is tetravalent—valence being defined at the time as the combining capacity of the elements. Couper, in a paper, indicated valence bonds as straight lines linking the symbols for the elements, which is still the practice in most modern structural diagrams.
Couper, a Scot educated in Glasgow, Edinburgh, Berlin, and Paris, came to chemistry from the study of philosophy and classical languages. This background probably helped him make an analogy between letters in words and carbon atoms in molecules, and focus on how carbon atoms combine with other atoms. His invention of an appropriate symbolic language to indicate the order in which the various atoms are joined in molecules may also stem from his philosophical and linguistic training. Sadly, his paper describing carbon linkage was read before the French Academy a few weeks after Kekulé's similar paper was published in Liebig's Annalen der Chemie und Pharmacie. Couper had entrusted his paper to Charles Adolphe Wurtz, in whose laboratory he worked in Paris, and Wurtz had procrastinated in giving it to an Academy member for presentation. It is not known how much Couper's bitterness over his loss of priority and his subsequent fight with Wurtz contributed to his emotional collapse. He soon retreated to his Scottish home and never published another scientific paper for the remaining 30 years of his life.