oxygen this cell was then immersed in a thermostatically controlled bath maintained at 145 u00b0 c u00b1 0 01 u00b0 c the induction period was about 30 hours the same test repeated on the powder before molding gave an induction period of 42 hours see curve 1 fig i a few cut pieces of the film were mounted on special supports in a xenotest 450 apparatus and exposed therein to the light of a xenon lamp of 1600 w filtered in such a way as to obtain an emission spectrum as close as possible to the solar spectrum of xenon the temperature in the exposure chamber was maintained at 50 u00b0 u00b1 2 u00b0 c while the relative humidity was kept at 35 u00b1 5 film samples were drawn at intervals and the ir absorption at a wave length 1720 cm sup 1 corresponding to the absorption of the carbonyl groups was measured in order to evaluate the concentration of the carbonyl groups there was assumed a conventional number for the molar absorption coefficient equal to 300 1 mole cm curve 1 in fig ii represents the index of carbonyls in the film with the passage of time the induction period was about 60 hours.