A glass phial filled with genuine Paris air, this Readymade acts as a type of portable souvenir, which Duchamp originally gave to his friend Walter Arensberg. Duchamp had a pharmacist empty the ampoule, which originally contained serum, and then seal it again once air replaced its original contents. Accordingly, (and in typical Duchampian fashion) Duchamp attached a label with "Serum Physiologique" ("Physiological Serum") printed on it (Schwarz 676). As Ramirez explains, in this state Paris Air, unlike most other Readymades, is "extremely fragile" (46).
Image and text from toufait.com. More to read. Link here.