Sentence:  Although there was some variation in results which must be attributed either to trace impurities or to <WSD>variation</WSD> 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 . 
Meaning of the word marked with <WSD> tags:  an instance of change; the rate or magnitude of change