It depends on which TD you have. There is oil dispersable and Water dispersable. I've even seen some places that claim it can be done in water or oil. I would just take a bit and put it in some water, if it mixes well it's the water type. If it just clumps together it's oil tyep.
Thanks everyone. They reminded me of the story in Exodus of the pillar of cloud by day and fire by night. Didn't start off with a religious theme, just sort of a Rorscach effect.