Full Idea
We have discovered a phenomenon (heat) which in all possible worlds will be molecular motion - which could not have failed to be molecular motion, because that's what the phenomenon is.
Gist of Idea
Once we've found that heat is molecular motion, then that's what it is, in all possible worlds
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.133
A Reaction
He refers to the identification as an 'essential property' of the phenomenon (and not merely a necessity). For my taste, Kripke uses the word 'property' too widely.