a cup for use with the handpiece of an apparatus for applying gas to a portion of a tooth comprising a liner configured and arranged to engage a gas dispensing handpiece , a body defining a gas exposure chamber , the body having a higher resiliency to deformation than the liner . the body may be so deformable that it forms against the surface of a tooth to effect a seal and to expose a portion of the tooth to gas circulated in the chamber . the body may overmold the liner and may be formed of thermal - plastic rubber , and the liner may be formed of a plastic material . the gas exposure chamber may be cylindrical .