Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Unsubstantial

Unsubstantial as a ghost.
—Anonymous

Unsubstantial as a mirage.
—Buddha

Unsubstantial, like the teasing phantoms of a half-conscious slumber.
—Nathaniel Hawthorne

Unsubstantial as a bag of money in a looking-glass.
—Telugu Proverb

Unsubstantial as a dream.
—Thomas Wade