|An exaggerated depiction (oversized particles) of the mean free path (one average-size straight line) view of molecules moving according to the ideal of the Boltzmann chaos assumption.|
In 1858, German physicist Rudolf Clausius, in his “On the Mean length of the Paths described by the Separate Molecules of Gaseous Bodies on the Occurrence of the Molecular Motion”, introduced the concept of ‘mean free path’ as a rebuttal publication to the "dining room anecdote" (see: thermodynamics anecdotes) objection raised by Danish scientist Christoph Ballot, who stated that since the smell of hot food takes several seconds to traverse a room that particles of air could not be moving at several 100 meters per second as Clausius had previously calculated in his 1857 paper “On the Nature of the Motion which we Call Heat.” 
The average speed of gas particles, at room temperature is on average 500-meters per second or 1200-mph. 
The mean free path of a molecule at 100 km is about 33 mm; at which point the macroscopic approach is already questionable. At 100 km, the mean free path exceeds 3 m, which is comparable to the dimensions of a satellite, such as needs to be taken into account during reentry. 
1. Perrot, Pierre. (1998). A to Z of Thermodynamics (mean free path, pg. 197-98). Oxford University Press.
2. (a) Clausius, Rudolf. (1857), "Über die Art der Bewegung, die wir Wärme nennen", Annalen der Physik, 100:353–379; trans. “On the Nature of the Motion which we call Heat” (PDF), Philosophical Magazine (1857), 14:108-27.
(b) Buys-Ballot, Christoph. (1858). “Ueber die Art von Bewegung welche wir Warme und Elektrizitat nennen” (“On the Kind of Motion we Call Heat and Electricity”), Ann. Phys. 103: 240.
(c) Clausius, Rudolf. (1859) . “On the Mean length of the Paths described by the Separate Molecules of Gaseous Bodies on the Occurrence of the Molecular Motion: together with some other remarks upon the mechanical theory of heat”, Phil. Mag. 17:81-91.
(d) Lindley, David. (2001). Boltzmann’s Atom: the Great Debate that Launched a Revolution in Physics (pg. 25). The Free Press.
3. (a) Clark, John O.E. (2004). The Essential Dictionary of Science. Barnes & Noble.
(b) Air, composition of (Britannica); 2002 Deluxe Edition, CD-ROM.
4. Potter, Merle C. and Somerton, Craig W. (2009). Schaum's Outlines: Thermodynamics for Engineers (pg. 3). McGraw-Hill.
● Mean free path – Wikipedia.