AVOGADRO'S CONSTANT is the number of molecules (see MOLECULE) in one gram-molecule, the molecular weight of the substance in grams. It is invariably the same, whatever the substance, its value being approximately 6.16X and it is denoted by the constant N. Avogadro's Law states that equal volumes of different gases at the same temperature and pressure contain the same number of molecules, i.e., the volume of a gram-molecule is constant for all gases. This is correct only for perfect gases (see article GAS), but is approximately true for real gases. See KINETIC THEORY OF MATTER; THERMODY NAMICS ; and ELECTRON, THE.