What is another word for "yeast"?

Roget's Thesaurus (1911) lists these synonyms for "yeast": air, airy, astatic, balloon, barm, bubble, buoy, buoyancy, buoyant, cobweb, convection, cork, down, draft, dust, ether, ethereal, feather, ferment, float, floating, flue, gossamer, gossamery, helium, hot, hydrogen, imponderability, imponderable, imponderous, leaven, levity, light, lighten, lighter, lightness, like, mote, portable, render.