Sentence:  The photochemical reaction cells consisted of 10 mm. i.d. Pyrex <WSD>tubing</WSD> , 5.5 cm. long , diffraction effects being minimized by the fact that the light passed through only liquid glass interfaces and not gas glass interfaces . 
Meaning of the word marked with <WSD> tags:  conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases