1) The head of a bolt 
straight-necked distillation vessel or flask
• (n.) A long, straight-necked, glass vessel for chemical distillations; -- called also a matrass or receiver. • (n.) The head of a bolt.
Bolt'head` noun 1. (Chemistry) A long, straight-necked, glass vessel for chemical distillations; -- called also a matrass or receiver. 2. The head of a bolt. 
the head of a bolt. · (formerly) a matrass.