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Salt Evaporation, Portugal
Creative Commons Attribution Share AlikeSalt Evaporation, Portugal - Credit: dynamosquito, Flickr
The three "primal elements" - mercury, sulphur and salt - were considered by alchemists to be the three constituent parts of base matter.  They were also sometimes viewed as spiritual forces.  By alchemical tradition when something is burnt, mercury is the vapour released, sulphur is the combustibility, and salt is the ash that remains.