1. residue from smelting operation.   Hint First letter clue   2. observed fact or occurrence subject to scientific study.   First letter clue   3. slope or slanting plane.   First letter clue   4. smallest particle of a substance with all its properties.   First letter clue   5. coloring agent.   First letter clue   6. lowest point in wave; shallow receptacle.   First letter clue   7. outer boundary for closed figure.   First letter clue