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
Zeroth law of thermodynamics that provides a continuous ordering of states. The dimensionality of a surface of constant temperature is one less than the number of thermodynamic parameters, thus, for an ideal gas described with three thermodynamic parameters "P", "V" and "N", it is a two-dimensional surface. For example, if two systems of ideal gases are in equilibrium, then = where "P" is the pressure in the "i"th system, "V" is the volume, and "N" is the amount (in moles, or simply the number of atoms) of gas. The surface = constant defines surfaces of equal thermodynamic temperature, and one may label defining "T"