Pandemonia: Alchemy

So, the Alchemists of Pandemonia have themselves a method of organizing The Elements:

Their 'Periodic' Table is organized into their 5 Cardinal Elements: Air, Earth, Fire, Water and Metal

An Air Element is any element that naturally occurs as a Gas. Hydrogen in Pandemonia is classified as Air 2 (Pandemonians classify what we know as Oxygen as Air 1), Helium is Air 3. And so on.

Water, therefore, is any element that naturally occurs as a liquid. The Alchemists of Pandemonia, while knowing of Hydrogen, don't yet know that Water is made up of Hydrogen and Oxygen. Water is Water 1. Mercury is also a Water Element.

Earth, being somewhat obvious, is any non-metallic solid. That makes Metal even more obvious. 

Fire, where I think it gets particularly interesting, is any radioactive element. On Pandemonia, some radioactive elements are found naturally. Even some that are not naturally occurring in The Real World. Plutonium, known as Hesperium, is found and known as Fire 2. Uranium is Fire 1. Cobalt 60, though not naturally occurring in The Real World, is considered Fire 3 and is found in the Hesperium Range along with Hesperium and renders many geographic areas uninhabitable and/or the home of horrible mutations. Because it's fiction, that's why.

Pandemonian alchemists and philosophers have known of the Fire elements for generations but are only recently beginning to integrate them into their disciplines.