CONSTANT, a quantity which does not vary. In mathe matics and physics a quantity may be constant with respect to one variable but not with respect to another. Many so-called physical constants are constant under certain conditions only; e.g., the boiling point of a pure substance is constant only if the atmospheric pressure is constant (see VAPORIZATION). On the other hand there are constants which express invariable ratios, such as the gas constant R (see THERMODYNAMICS), the gravita tional constant (see GRAVITATION), and Avogadro's constant (q.v.) N. The elementary electric charge e (see ELECTRON) is an example of an invariable physical quantity. Another type of physical constant is the universal constant, e.g., Plank's con stant h, the element of action (see QUANTUM THEORY).