The undiscovered element

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Across
  1. 2. From the apothecary
  2. 4. The hornets
  3. 5. Exploding infant
  4. 6. Down under devil
  5. 7. 32 not out
  6. 11. Leavers' supper entertainment
  7. 12. A favourite topic
  8. 15. Mr Jackson's favourite pasttime
  9. 16. Do this round the backs
  10. 18. Doesn't do much
  11. 19. The Science Department has 3
Down
  1. 1. Plants do this
  2. 3. New head
  3. 7. Real science?
  4. 8. Not too many planets like this
  5. 9. The first balloon goes off with a bang
  6. 10. A chemist might do this on two wheels
  7. 13. The Queen breathes this
  8. 14. Chemistry on the move
  9. 17. Not a banjo