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 <WSD>series</WSD> of reaction tubes for the purpose of obtaining information on the effect of chlorine concentration and of carbon tetrachloride concentration on the reaction rate . 
Meaning of the word marked with <WSD> tags:  similar things placed in order or happening one after another