Avogadro's law states that, at equal temperatures and pressures, equal volumes of an ideal gas contain the same number of molecules.
This law was put forth in 1811. Something that I learned in my thermodynamics class this year (Node your homework!) is that actual measurements, or even estimates, of Avogadro's number were not made until the early 1900's (first by Jean-Baptiste Perrin in 1908).
This law was used in constructing the ideal gas law.