1860

Cannizzaro at Karlsruhe

Stanislao Cannizzaro (1826–1910)

By the mid-nineteenth century, the scientific world agreed that atomic weights (and thus molecular weights) were extremely important. But what were the right weights? Weighing individual atoms was out of the question, and using the weights of starting materials and products of chemical reactions led to all sorts of arguments about how molecular formulas should be expressed. English chemist John Dalton, a pioneer in atomic theory, was sure that water had the formula HO.

The year 1860 saw the Karlsruhe Congress, the world’s first international meeting of chemists, held to try to settle this and other chemistry questions. A paper from Italian chemist Stanislao Cannizzaro (actually published two years before but little known) made the biggest impression. Building on Avogadro’s work, Cannizzaro’s “Sketch of a Course of Chemical Philosophy” attempted to nail down the real atomic weights. Like Dalton, Cannizzaro started from hydrogen, assigning it a weight of 1, but like French chemist Joseph-Louis Gay-Lussac, he (rightly) took it to be a diatomic molecule, H₂, and he reminded the conference attendees of Avogadro’s evidence that oxygen was diatomic as well. Avogadro had theorized that equal volumes of gases had the same number of molecules in them, and that they weighed different amounts only because the molecules of each gas had their own individual weights.

Based on Gay-Lussac’s measurements, two volumes of hydrogen to one volume of oxygen were needed to create one volume of water vapor, indicating that the actual formula for water was not HO—as Dalton had suggested—but rather H₂O. Weighing the materials showed that oxygen was eight times as heavy as the hydrogen present in water vapor, which led Cannizzaro to conclude that its atomic weight was 16, given the 2:1 ratio of hydrogen to oxygen in water.

Cannizzaro’s paper seems to have almost singlehandedly cleared up most of the confusion that had afflicted chemists about the elements’ fundamental weights. It was so logical and so useful and fit the data so well that it almost had to be correct, and indeed it was.

Stanislao Cannizzaro.

Karlsruhe, Germany, c. 1900, where Cannizzaro put chemical formulas and weights on a solid footing at last.