Definition of "impalpably"

These books will fill, and well fill, certain stretches of life […] But in old or nervous or solemnest or dying hours, when one needs the impalpably soothing and vitalizing influences of abysmic Nature, or its affinities in literature or human society, and the soul resents the keenest mere intellection, they will not be sought for.

1892, Walt Whitman, “Notes Left Over: Emerson's Books”, in Complete Prose Works‎