Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies
Equal volumes of any gas, at the same heat and pressure, hold equal numbers of molecules.
Weigh two balloons of gas the same size, and — if you trust one daring idea — you have just compared the weights of their molecules.
One bold guess
Avogadro's guess is short: any two gases that fill the same volume, at the same warmth and pressure, hold the same number of molecules — no matter how heavy or light the gas.
If that is true, something wonderful follows. You can never see a single molecule, let alone weigh one. But if equal volumes hold equal numbers, then weighing two equal volumes of gas compares their molecules directly: the heavier volume has heavier molecules, in exactly the same ratio. Counting the uncountable becomes a matter of putting gases on a balance.
A count who worked alone
Amedeo Avogadro was an Italian nobleman, trained in law before he turned to physics, working quietly in Turin. He published the idea in 1811 — and almost no one listened, for fifty years. The giants of chemistry, Dalton and Berzelius among them, could not accept his second claim: that two identical atoms would pair into a molecule. Only in 1860, at a great congress in Karlsruhe, did Stanislao Cannizzaro show that Avogadro's idea untangled the whole knotted question of atomic weights — two years after Avogadro had died, never knowing he had been right.
Why it mattered
It gave chemistry a ruler. Before Avogadro, the same compound was written with different formulas by different chemists, because no one could agree how many atoms sat in a molecule. After him, the densities of gases pinned the molecular masses down, and the formulas fell into place. His hypothesis is the hidden hinge between “atoms exist” and “here is the formula for water.”
A car-park rule
Imagine a rule that says any car park of a given size always holds the same number of vehicles, whatever the model parked there. Now weigh the whole park. A heavy total means heavy cars; a light total means light cars — and the ratio of the two totals is exactly the ratio of the weights of single cars. Avogadro says gases obey just this rule, with molecules playing the part of the cars.
Its place in the story
Avogadro's idea completes Dalton's atoms (see dalton-1808) and Gay-Lussac's volumes, and it is the argument Cannizzaro carried to Karlsruhe to finally settle the atomic weights. The number that bears Avogadro's name — about a 6 followed by twenty-three zeros — was only pinned down much later, when Jean Perrin watched tiny grains jitter under a microscope and counted his way to it.
The first hypothesis to present itself in this connection, and apparently even the only admissible one, is the supposition that the number of integral molecules in any gases is always the same for equal volumes, or always proportional to the volumes.
Thus in all these cases there must be a division of the molecule into two; but it is possible that in other cases the division might be into four, eight, &c.