The Collaborative International Dictionary of English v.0.48:

Saponul \Sap"o*nul\, n. [F. saponule, fr. L. sapo, -onis, soap.]
   (Old Chem.)
   A soapy mixture obtained by treating an essential oil with an
   alkali; hence, any similar compound of an essential oil.
   [Written also saponule.] [Obs.]
   [1913 Webster]