Formative time lags have been measured in mercury vapour for E_s/p₀=138-422 V cm⁻¹ Torr⁻¹, and have been compared with time lags calculated from models based on Davidson's theory of the temporal growth of a gas discharge. The observed times were compatible with a model having one slow process and one faster secondary ionization process: that is, a process in which delayed photons are produced by the destruction of metastable atoms by collisions in the vapour (coefficient δ₁/α), and a faster process such as positive ion bombardment (coefficient γ). The analysis could not distinguish between the various faster processes so all were included in the coefficient γ. Data are given for the appropriate values of both these secondary ionization coefficients as functions of E_s/p₀. The curve for δ₁/α as a function of E_s/p₀ resembles the excitation probability curve for the ³P₂ metastable state of mercury and the corresponding delayed photon process is predominant for E_s/p₀<220 V cm⁻¹ Torr⁻¹.