Fortunately the movie leaves it vague as to what's actually inside there and let's us use our imagination. Personally I like to think Lex just put that label on to fuck with Finch. Inside is regular old Peach tea.I'm still of the opinion that wasn't actually Lex's urine in that jar, cause there's no way it could have been smuggled onto the table of a Senate committee hearing without the smell permeating in the room since there wasn't a lid. It would have been an even crueler joke had Lex just put in some sort of Peach tea drink to get one over on Senator Finch.
Lex is a great villain. I'll never understand how people can compare him to Ledger's Joker. The similarities end at their eccentricities if you ask me.