Air, earth, water and fire

Air, earth, fire and water - sounds familiar doesn't it? Elementary, or archaic chemistry we might say.

Rare earth

What is a rare earth? Is it a special kind of soil? And what is rarefied air and is it different from a rare gas? Rare is well understood as meaning uncommon, though this is not its meaning in rare beef. Earth is used both for soil and for the whole planet. But at one time 'earth' was a common term for a metal oxide, the earthy substance produced when a metal is burnt. Soil is made up mainly of a mixture of oxides. The lanthanides occur as a mixture of oxides and they are relatively uncommon, so they came to be called the 'rare earths'. The rare gases (alas no longer noble) are still uncommon, found as trace gases in the atmosphere. Rarefied air, on the other hand, is common on the top of mountains and refers to air that has become rarer, ie thinner, with lower density and pressure. Thus rarefaction means to make rare, the process of reducing the pressure of a gas.