Amedeo Avogadro's 1811 essay, in the standard Alembic Club English translation, proposing that equal volumes of gases contain equal numbers of molecules and distinguishing, for the first time, the atom from the molecule. Almost wholly neglected for half a century, the hypothesis was revived by Cannizzaro at the 1860 Karlsruhe Congress and became a foundation of modern chemistry. Reproduced here with a full editorial apparatus.
Avogadro published his essay in 1811 into a chemistry that had just been handed two powerful but apparently incompatible tools. The first was John Dalton's atomic theory, set out from 1808, which held that each element consists of identical atoms of a characteristic weight and that compounds form by the union of small whole numbers of these atoms. The second was Joseph Louis Gay-Lussac's law of combining volumes, announced in 1808, which observed that gases combine in simple whole-number ratios by volume - two volumes of hydrogen to one of oxygen yielding water, equal volumes of hydrogen and chlorine yielding hydrogen chloride, and so on. Each was a triumph of measurement and generalization. Together they seemed to point toward a contradiction that neither Dalton nor Gay-Lussac could resolve, and that for a generation stood in the way of a consistent table of atomic weights.
The difficulty was this. If equal volumes of gases contained equal numbers of atoms - the natural reading of Gay-Lussac's law - then one volume of oxygen combining with two of hydrogen to give two volumes of water vapor implied that each oxygen atom had been split in half, since two volumes of product cannot contain more oxygen particles than the single volume of reactant supplied. Dalton, whose atoms were by definition indivisible, rejected the inference and with it Gay-Lussac's law as anything more than a rough approximation. The combining volumes refused to reconcile with the indivisible atom, and chemistry was left without a reliable way to pass from the weights in which substances combined to the weights of the particles themselves.
Avogadro's resolution turned on a distinction his predecessors had not drawn. He proposed that equal volumes of gases at the same temperature and pressure contain equal numbers not of atoms but of molecules - and that the molecule of an elementary gas need not be a single atom. The smallest particle of hydrogen or oxygen gas, he argued, is itself divisible, a compound of two like half-particles that come apart in chemical combination. On this view the awkward arithmetic resolves at once: a volume of oxygen molecules, each splitting into two parts, can supply oxygen to two volumes of water vapor without any atom being divided, because the molecule, not the atom, is what divides. The integrant molecule of the compound is built from the elementary molecules of its constituents, and the elementary molecule of a simple gas may contain more than one atom.
This distinction between the atom - the ultimate particle that enters into combination - and the molecule - the particle that exists free in the gas and determines its volume - is the conceptual core of the essay and the foundation of all later structural chemistry. With it, the relative weights of the molecules of different gases could be read directly from their densities, since equal volumes hold equal numbers, and the relative weights of the atoms could be inferred in turn. Avogadro had, in principle, supplied chemistry with the missing bridge from the laboratory measurements of density and combining volume to a consistent system of atomic and molecular weights.
The essay was almost entirely neglected.
