6 decay relationships of their half-lives, when the series is in internal radioactive equilibrium. The product of the number of existing atoms and the decay constant (which is proportional to the reciprocal of the half-life) must have the same value for each member in a stationary state. Therefore, the total activity of a short-lived member of a series (e.g., radon) can, in principle, be measured just as well as the total activity of a long-lived member of the same series (e.g., radium) with which it is in radioactive equilibrium, even though the amount of substance present may be extraordinarily smaller. The only requirement is that the half-life is not so short that the substance "slips through the chemist's fingers." Under this condition, in practice, short-lived substances can even be measured more easily than long-lived substances.