1. The effect of temperature on the reaction time of Mya to light is mainly confined to the latent period. The sensitization period, representing a photochemical process, is changed comparatively little.
2. The relation between the latent period and the temperature is adequately expressed by the Arrhenius equation, for temperatures below 21°C. Above this temperature, the latent period becomes increasingly longer than is required by the Arrhenius formula when µ = 19,680.
3. These deviations, occurring above the highest environmental temperature of Mya, are explained on the assumption that the principal product formed during the latent period is inactivated by heat.
4. Calculation of the velocity of the hypothetical inactivation reaction at different temperatures shows that it also follows the Arrhenius rule when µ = 48,500. This value of µ corresponds to those generally found for spontaneous inactivations and destructions.