Fire
denote that fire was
Dickinson uses the past tense—the fire already happened. She's writing about evidence, not the thing itself.
departed creature's sake
Fire becomes a living thing that dies. The ashes aren't residue—they're remains of something that was alive.
exists the first in light
Fire's life cycle: starts as light (visible, immaterial), then becomes solid matter (consolidates). She's reversing the expected sequence.
Only the chemist
The turn to science. What looked like metaphysics is actually chemistry—fire literally does become carbonates (carbon compounds).