Any question answered...

You fail at physics, but the explanation for the fast leakage of carbon dioxide turns out to be a question of chemistry. If it were simple gas physics, the big CO2 molecule should leak slower than the smaller N2 molecule, and so it would through any other barrier. Unfortunately for cyclists, CO2 goes through butyl rubber very easily because the one is a solvent for the other - I can't remember which way round, and I'm not about to get sidetracked while I should be working.

Oh come on, you could have left that for a while. I liked the idea of 2 nitrogen molecules being bigger than a carbon molecule and 2 oxygen molecules.