What is another word for "leaven"?

Roget's Thesaurus (1911) lists these synonyms for "leaven": air, airy, and, appertain, appurtenance, astatic, balloon, barm, belong, bubble, buoy, buoyancy, buoyant, cobweb, combine, component, compose, composition, constituent, constitute, contents, convection, cork, down, draft, dust, element, enter, ether, ethereal, feather, feature, ferment, float, floating, flue, form, forming, gossamer, gossamery.