http://agathe.gr/democracy/the_boule.html
The Boule (The Senate) The Athenian legislature also included a deliberative body known as the Boule. It was made up of 500 members -- 50 from each of the 10 tribes -- who were chosen by lot and served ... A fragmentary marble basin or perirrhanterion, marked as belonging to the Bouleuterion, presumably held the holy water in which the Athenians were accustomed to wash or dip their hands before entering any sacred space.