Sentence:  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 <WSD>effect</WSD> of chlorine concentration and of carbon tetrachloride concentration on the reaction rate . 
Meaning of the word marked with <WSD> tags:  a phenomenon that follows and is caused by some previous phenomenon