Chapter Words Relating to Matter of Index by Roget's Thesaurus

A feather, dust, mote, down, thistle-down, flue, cobweb, gossamer, straw, cork, bubble; a float, buoy; ether; a feather-weight.

(Verbs). To be light, to float, swim, be buoyed up.

(Adjectives). Light, subtle, airy, vaporous, imponderous, astatic, weightless, imponderable, ethereal, sublimated, floating, swimming, buoyant, portable, uncompressed, volatile.

(Phrases). Light as a feather; light as thistle-down; "trifles light as air."