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 <WSD>gas</WSD> 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:  the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container