In 1826 his work showed that the fulminates, and the very different cyanates (made by Wöhler) had the same molecular formulae. This sort of phenomenon (isomerism) could not then be explained, but it showed that a molecule was not merely a collection of atoms; they were arranged in particular ways, with each arrangement corresponding to one compound and one set of properties. The work led also to his friendship with Wöhler and their valuable joint work on the benzoyl group. The friendship survived when Liebig’s combative nature had eventually spoiled all his other chemical friendships.

By 1830, Liebig had developed a method for the analysis of organic compounds which was quick and accurate, by burning them in a stream of air and oxidizing the products fully to CO2 and H2O; collecting and weighing this CO2 and H2O gave a direct way to find the percentages of carbon and hydrogen in the organic compound. Liebig and his students used this method to analyse hundreds of organic compounds, and the results were basic for the great advances to be made (notably by Kekulé) in organic chemistry after about 1850.

In his middle age, from 1840, Liebig worked on what we would now call biochemistry. He argued (correctly) that carbohydrates and fats are the fuel of the animal body and (incorrectly) that fermentation did not involve living cells. In agriculture, he argued (rightly) for the use of potassium- and phosphorus-containing fertilizers but underrated the importance of nitrogen, and of soil structure, in fertility. He always played a vigorous – sometimes ferocious – part in debates on chemical theory; his pupils dominated organic chemical teaching; and his views moved agriculture towards chemistry. He made a good deal of money out of his scientific work and attracted criticism for this.

Justus von Liebig