Is in use for separating a dissolution formed by two liquids depending on her different boiling-points.
When one makes boil the dissolution contained in the flask, the volatile solvent that has a minor boiling-point, it evaporates and leaves in the flask a residue of soluto not volatile.
To gather the solvent like that evaporated, one makes him happen for a condenser in the one that circulates cold water. There there becomes condensed the steam, which falls down in a glass.
