Throughout history, the search for salt has played an important role in society. Where it was scarce, salt was traded ounce for ounce with gold. Rome′s major highway was called the via salaria, that is the Salt road. Along the road, Roman soldiers transported salt crystals from the salt flats at Ostia up the Tiber River. In return, they received a salarium or salary, which was literally money paid to soldiers to buy salt. The old saying ‘worth their salt′. Which mean to be valuable derives from the custom of payment during the Empire.What does the passage mainly discuss?