Question

How was the Avogadro's law discovered?

Answer 1

Avogadro's Law is one of the early gas laws, and proposed that the volume expressed by a gas was proportional to the number of gaseous molecules given equivalence with respect to temperature and pressure, $V\propto n$. And please note that the behaviour of gases in these scenarios allowed chemists and physicists to get a pretty shrewd idea of molecular behaviour simply by measurement of the macroscopic observables of mass, temperature, pressure, and volume.

Avogadro's original law proposed that $V_{\text{volume of gas}}\propto n_{\text{number of gaseous}}$ particles...i.e. $V=kn$...with $k=$ some constant of proportionality.