Full Idea
I might say that flame colours are a characteristic feature of metals, but this is an empirical proposition which is in part about the unobserved, and stands in need of justification.
Gist of Idea
If flame colour is characteristic of a metal, that is an empirical claim needing justification
Source
Alexander Bird (Philosophy of Science [1998], Ch.5)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.168
A Reaction
This draws attention to the fact that essentialism is not just a metaphysical theory, but is also part of the scientific enterprise. Among things to research about metals is the reason why they have a characteristic flame.