lunar caustic: silver nitrate fused into sticks and formerly used as a caustic
silver nitrate: a nitrate used in making photographic emulsions; also used in medicine as a cautery and as a topical antibacterial agent
metastability: the quality of a physical system that persists in its existing equilibrium when undisturbed (or only slightly disturbed) but able to pass to a more stable equilibrium when sufficiently disturbed