On Tuesday, September 1st, 2009 at 20:43:19h +0100, Java Jive pondered:

I wonder what the different formulae were/are.

Town gas (synthetic gas) consists primarily of hydrogen (H2) and carbon
monoxide (C0), plus varying proportions of nitrogen (N2) and carbon
dioxide (C02).

Natural gas (earth gas) consists primarily of methane (CH4).

A mercaptan (foul smelling sulfur compound) is added to both town gas
and natural gas as a means to quickly alert people of gas leaks.

Town gas is sold by the therm and natural gas is often sold by the
cubic metre, since town gas may have differing energy
properties due to fluctuations in its composition.

http://en.wikipedia.org/wiki/Therm