The Collaborative International Dictionary of English v.0.48:

Tetrakosane \Tet"ra*ko*sane`\, n. [Tetra- + Gr. ? twenty.]
   (Chem.)
   A hydrocarbon, C24H50, resembling paraffin, and like it
   belonging to the marsh-gas series; -- so called from having
   twenty-four atoms of carbon in the molecule.
   [1913 Webster]