Graham's law has come to denote an important class of finely divided materials. Around the time Graham did his work, the concept of molecular weight was being established largely through the measurements of gases. Daniel Bernoulli suggested in 1738 in his book Hydrodynamica that heat increases in proportion to the velocity, and thus kinetic energy, of gas particles. Italian physicist Amedeo Avogadro also suggested in 1811 that equal volumes of different gases contain equal numbers of molecules. Thus, the relative molecular weights of two gases are equal to the ratio of weights of equal volumes of the gases. Avogadro's insight together with
Kinetic theory of gases "K": This is a first non-trivial result of the kinetic theory because it relates pressure, a macroscopic property, to the (translational) kinetic energy of the molecules formula_12, which is a microscopic property. Rewriting the above result for the pressure as formula_13, we may combine it with the ideal gas law where formula_14 is the Boltzmann constant and formula_15 the absolute temperature defined by the ideal gas law, to obtain which leads to simplified expression of the average kinetic energy per molecule, The kinetic energy of the system is N times that of a molecule, namely formula_18. Then the temperature formula_15