Vacuum technology

Fifth in the list stands il libero cammino medio, and the noun in the middle is the one to stop at. English says « path », and a path is a thing: a line on the ground, something that can be swept, fenced or blocked. Italian keeps that thing under another name, il sentiero, the track through a wood, and saves il cammino for the walking itself — the going, the route as it is actually covered. That is why mettersi in cammino is to set off, why a pilgrimage is il Cammino di Santiago, and why the most quoted line in the language opens nel mezzo del cammin di nostra vita, halfway along the journey of our life. Applied to a molecule the choice is exact. What is measured is not a corridor between two walls but the distance one particle gets through before it strikes another, which is a journey and nothing else. One letter separates the whole of this from il camino, with a single m, which is a chimney and has no part in any of it.