To describe trying to hold sand, Edgar Allan Poe uses these pair of rhyming words:
And I hold within my hand
Grains of the golden sand-
How few! Yet how they creep
Through my fingers to the deep.
The author is describing his lost of hope "In a night, or in a day / In a vision, or in none". He is talking about a goodbye, a sad moment where he feels the loss of his illusion, like grains of sand falling from his hands, falling deep while he weeps, he cries of sadness for the goodbye.