The Collaborative International Dictionary of English v.0.48:

Esculin \Es*cu"lin\, n. [See Esculic.] (Chem.)
   A glucoside obtained from the [AE]sculus hippocastanum, or
   horse-chestnut, and characterized by its fine blue
   fluorescent solutions. [Written also [ae]sculin.]
   [1913 Webster]