Its been awhile, so I consulted the UHS page:
"Go to Mordra's place and take the three potions she mentioned:
red (antidote)
black (invisibility)
glowing (mandrake).
Go to alchemists' and take the fourth (empty) vial.
Place the potions and the vial beside the burner.
The caps of the bottles should be covered by the nozzles of the burner.
Then, turn it on (double-click on its center).
If it doesn't work, one of the potions is out of place.
It doesn't matter in which order you set them, but the empty vial should be at the separated tube."
http://www.uhs-hints.com/uhsweb/hints/ultima7/776.php
Make sure they are lined up PERFECTLY and that each is the right potion type.