The Collaborative International Dictionary of English v.0.48:

Investigable \In*ves"ti*ga*ble\, a. [L. investigabilis. See
   Investigate.]
   Capable or susceptible of being investigated; admitting
   research. --Hooker.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Investigable \In*ves"ti*ga*ble\, a. [L. investigabilis. See
   In- not, and Vestigate.]
   Unsearchable; inscrutable. [Obs.]
   [1913 Webster]

         So unsearchable the judgment and so investigable the
         ways thereof.                            --Bale.
   [1913 Webster]