Alchemists work mainly with three substances: sulphur, mercury and salt. But these are not the sulphur, mercury and salt we know, and alchemists explain their origin in the following way: sulphur was formed by fire acting on air, mercury by air acting on water, and salt by water acting on earth. Sulphur corresponds to the masculine, outgoing principle; mercury to the feminine, receptive principle, and salt is the result of their union - their child. Each is represented geometrically: sulphur by a triangle and cross, mercury by a moon and cross, and salt by a cross in a circle. Sulphur, mercury and salt are also symbols for the three worlds - the spiritual, psychic and physical. Sulphur is the symbol for the spirit, mercury for the soul, and salt for the physical body. When you possess the knowledge of these three substances - sulphur, mercury and salt - and also of the four elements - fire, air, water and earth - you will know the principles of true alchemical science.