At the start of the episode a man is found dead, floating outside a building, attached to the ground by a rope. It is later determined that he has been given a compound that makes him float. His arms and his legs hang down towards the ground. If he was lighter than air, upon death, his arms and legs would float upwards, as neither gravity or his muscles would be pulling them towards the ground.
Peter Bishop states that the element lutetium is "extremely dense" and has a density "just like" osmium. In fact, lutetium's density is less than half that of osmium and is similar to many other elements including lead, silver and gold.