The Collaborative International Dictionary
Polychrest salt
Polychrest \Pol"y*chrest\, n. [Gr. ? useful for many purposes; poly`s many + ? useful, fr. ? to use: cf. F. polychreste.] (Med.) A medicine that serves for many uses, or that cures many diseases. [Obs.]
Polychrest salt (Old Med. Chem.), potassium sulphate, specifically obtained by fusing niter with sulphur.
Wiktionary
polychrest salt
n. (context obsolete medicine chemistry English) potassium sulfate obtained by fuse niter with sulfur