Sentence:  The reactants for the gas <WSD>phase</WSD> experiments were first frozen out in a side arm attached to the manifold and then allowed to distill slowly into the manifold of pre-cooled reaction cells before sealing off . 
Meaning of the word marked with <WSD> tags:  (physical chemistry) a distinct state of matter in a system; matter that is identical in chemical composition and physical state and separated from other material by the phase boundary