Jean Perrin, a French scientist, was one of the earlier scientists who attempted to determine the Avogadro constant.

### Who inspired Perrin?

Prior to Perrin’s work in 1909, Amedeo Avogadro, an Italian scientist, published papers between 1811 and 1841 and suggested that

Equal volumes of all gases at the same temperature and pressure have the same number of molecules

This became known as Avogadro’s law. It implies that for a given mass of an ideal gas at constant temperature and pressure, the ideal gas’ volume *V* is directly proportional to its amount *n*, i.e.

*V* = *kn* (1)

where *k* is the proportionality constant. When eq1 is incorporated into the combined gas law, which was developed many years earlier, we have the ideal gas law:

*pV* = *nRT* (2)

*n* at that time is known as ‘amount of gas’ and not ‘number of moles’ as the mole concept has not been developed. Since the amount of gas can be in measured in different ways, the gas constant *R* has different units back then.

Avogadro also investigated the relative mass of different gases, e.g. he deduced from gas density data that the relative molecular weight of nitrogen and hydrogen is in the ratio of 13.2 : 1 and that the ratio of oxygen molecules and hydrogen molecules in water is 0.5 : 1.

Lastly, the research of a botanist (in 1827), Robert Brown, that involves the random motion of particles suspended in a liquid or a gas as a result of their collision with the liquid or gas molecules respectively, also played an important role in Perrin’s calculations.