Poynting effect
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The ability to combine nitrous oxide and oxygen at high pressure while remaining in the gaseous form is due to the Poynting effect (after John Henry Poynting, an English physicist).
Entonox is a 50:50 combination of the anesthetic gas nitrous oxide and oxygen. This combination is useful because it can provide a sufficient concentration of nitrous oxide to provide analgesia (pain relief) in sufficient oxygen so that the risk of hypoxemia is eliminated. This makes it safe to use by para-medical staff such as ambulance officers. However the ability to combine these two gases at the temperature and pressure in the cylinder while remaining in the gaseous form is unexpected based on the known properties of the two gases.
The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.
