When typical reaction cells to which 0.3 of an atmosphere of oxygen had been added were illuminated, chlorine and phosgene were produced. Exchange was also observed in these cells, which had chlorine present at ** f.

Although there was some variation in results which must be attributed either to trace impurities or to variation in wall effects, the photochemical exchange in the gas phase was sufficiently reproducible so that it seemed meaningful to compare the reaction rates in different series of reaction tubes for the purpose of obtaining information on the effect of chlorine concentration and of carbon tetrachloride concentration on the reaction rate. Data on such comparisons together with data on the effect of light intensity are given in Table /1. .

In series /1, the relative light intensity was varied by varying the distance of the lamp from the reaction cell over the range from 14.7 to 29.2 cm.. The last column shows the rate of exchange that would have been observed at a relative intensity of 4 (14.7 cm. distance) calculated on the assumptions that the incident light intensity is inversely proportional to the square of the distance of the lamp from the cell and that the rate is directly proportional to the incident light intensity. Direct proportionality of the rate to the incident intensity has also been assumed in obtaining the value in the last column for the fourth sample of series /2, where the light intensity was reduced by use of a screen.