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
Potential temperature 1st law simplifies to: For approximately ideal gases, such as the dry air in the Earth's atmosphere, the equation of state, formula_18 can be substituted into the 1st law yielding, after some rearrangement: where the formula_20 was used and both terms were divided by the product formula_21 Integrating yields: and solving for formula_23, the temperature a parcel would acquire if moved adiabatically to the pressure level formula_24, you get: The potential virtual temperature formula_26, defined by is the theoretical potential temperature of the dry air which would have the same density as the humid air at a standard pressure P.