How did Amedeo Avogadro use vapour density in his research?

Amedeo Avogadro utilized vapour density as a physical method to infer the relative weights of atoms and molecules. By comparing the weight of a specific volume of a gas or vapour to the weight of an equal volume of hydrogen gas, he established a relationship that allowed for the calculation of molecular weights relative to a standard.12

Specifically, Avogadro used the following ratio to determine these relative weights:

$$\frac{\text{Weight of 1 volume of gas or vapour}}{\text{Weight of 1 volume of hydrogen}}=\frac{\text{Weight of 1 molecule of gas or vapour}}{\text{Weight of 1 molecule of hydrogen}}$$

This approach was central to his 1811 hypothesis, which proposed that equal volumes of gases at the same temperature and pressure contain an equal number of molecules. While many 19th-century chemists were initially reluctant to adopt such physical methods, Stanislao Cannizzaro later demonstrated that applying Avogadro’s theory—including the use of vapour densities—provided the necessary empirical foundation to resolve the prevailing chaos in chemical notation and atomic weights.231