3. To punish its members for disorderly behavior, and, with the
concurrence of two thirds, to expel a member.
In the execution of these powers, the Senate has the attributes of
a court, and, according to established precedents, it may summon
witnesses and compel their testimony, although it may well be doubted
if a law be not necessary even to the execution of this power.
Besides these three cases, expressly named in the Constitution, there
are two others, where it has already undertaken to exercise _judicial
powers_, not by virtue of express words, but in _self-defence_:--