Full Idea
It looks as if there is a cyclical process whereby the elements generate one another.
Gist of Idea
The elements seem able to transmute into each other
Source
Plato (Timaeus [c.362 BCE], 49c)
Book Reference
Plato: 'Timaeus and Critias', ed/tr. Waterfield,Robin [OUP 2008], p.41
A Reaction
He gives examples of elements turning into one another, such as air turning into rain. But originally it sounds as if they were immutable, since creation was from the distinct four elements, and not from a mixture of them. This is Timaeus speculating.