Definition of "impalpable"

What is happiness, anyhow? Is this one of its hours, or the like of it?—so impalpable—a mere breath, an evanescent tinge?

c. 1876, Walt Whitman, “The Sky—Days and Nights—Happiness”, in Complete Prose Works‎

On the benches lay figures covered with yellow linen, on which a fine and impalpable dust had gathered in the course of ages, but nothing like to the extent that one would have anticipated, for in these deep-hewn caves there is no material to turn to dust.

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887

And I heard—him—it—this voice—other voices—all of them were so little more than voices—and the memory of that time itself lingers around me, impalpable, like a dying vibration of one immense jabber, silly, atrocious, sordid, savage, or simply mean, without any kind of sense.

1899 March, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number MI, New York, N.Y.: The Leonard Scott Publishing Company, […], part II, page 496

She had an extraordinary sensitiveness to the impalpable elements of happiness, and as she walked at Darrow’s side her imagination flew back and forth, spinning luminous webs of feeling between herself and the scene about her.

1912, Edith Wharton, chapter XII, in The Reef‎, New York, N.Y.: D[aniel] Appleton and Company