Both Uses

atom
in
Song of Myself
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I celebrate myself, and sing myself,
And what I assume you shall assume,
For every atom belonging to me as good belongs to you.†
*

atom = the smallest part of any material that cannot be broken up by chemical means
My tongue, every atom of my blood, form'd from this soil, this air,
Born here of parents born here from parents the same, and their
parents the same,
I, now thirty-seven years old in perfect health begin,
Hoping to cease not till death.†

Definitions:

(1) (atom)
the smallest part of any material that cannot be broken up by chemical means; comprised of a nucleus of protons and neutrons surrounded by orbiting electrons
(2) (meaning too rare to warrant focus)