Sentence:  The photochemical reaction cells consisted of 10 mm. i.d. Pyrex tubing , 5.5 cm. long , diffraction effects being minimized by the fact that the light passed through only liquid glass <WSD>interfaces</WSD> and not gas glass interfaces . 
Meaning of the word marked with <WSD> tags:  (chemistry) a surface forming a common boundary between two things (two objects or liquids or chemical phases)