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 interfaces and not <WSD>gas</WSD> glass interfaces . 
Meaning of the word marked with <WSD> tags:  a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely