Effects of atmospheric gases on the change of the crystal structure or colour of various cadmium pigments in the course of heat treatment were studied. The samples used in this experiments were precipitates having the compositions of the following four kinds, that is, CdS, j9-CdS, 2 CdS•ZnS and 1.5 CdS • (Cd008•Se). As atmospheric gases, dried oxygen of high purity, nitrogen and helium were used. The precipitates of the various samples, after outgassing for 1 hr. under the conditions of 200°C

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... d 10-3 mmHg, were heat-treated at 400, 500, 600 or 700°C in atmospheric gases. The changes in the physico-chemical properties due to heat treatment, despite the difference in the compositions of the various samples, showed a similar tendency. The oxygen atmosphere which was active toward the pigments improved the crystallinity and increased the brightness of colour, while nitrogen and helium, inert atmosphere, had little effects. Crystallization and g-->a phase transition took place preferentially on the crystal surface and in fine particles. t Studies of Cadmium series pigments. IV.