Kilogram

Measured and measured. Weighed, undivided,
hatched from its bell jars at last, once everywhere
instantiated, and now only here.
There is nothing more exact than a thing
that is itself, to whom all else compares.
Glass shell and metal heart of the changeless,
seed of all matter and the countable,
final human root stuck on the Earth,
let out that breath you've held for centuries.