Alchemical symbols, originally devised as part of alchemy

Alchemy

Alchemy is an influential philosophical tradition whose early practitioners’ claims to profound powers were known from antiquity. The defining objectives of alchemy are varied; these include the creation of the fabled philosopher's stone possessing powers including the capability of turning base...

, were used to denote some elements and some compounds until the 18th century. Note that while notation like this was mostly standardized, style and symbol varied between alchemists, so this page lists the most common.
According to Paracelsus

Paracelsus

Paracelsus was a German-Swiss Renaissance physician, botanist, alchemist, astrologer, and general occultist....

, the Three Primes or Tria Prima are

• Salt (base matter)
• Mercury
  Mercury (element)
  Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
  
   (fluid connection between the High and the Low)
• Sulfur
  Sulfur
  Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
  
   (omnipresent spirit of life)

Western alchemy makes use of the Hellenic elements.