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 effect of chlorine concentration and of <WSD>carbon</WSD> tetrachloride concentration on the reaction rate . 
Meaning of the word marked with <WSD> tags:  a colorless nonflammable liquid used as a solvent for fats and oils; because of its toxicity its use as a cleaning fluid or fire extinguisher has declined