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
Gas blending total number of all molecules in the mixture. For example, a 50% oxygen, 50% helium mixture will contain approximately the same number of molecules of oxygen and helium. As both oxygen and helium approximate ideal gases at pressures below 200 bar, each will occupy the same volume at the same pressure and temperature, so they can be measured by volume at the same pressure, then mixed, or by partial pressure when decanted into the same container. The mass fraction can be calculated from the molar fraction by multiplying the molar fraction by the molecular mass for each constituent, to find