The chemist uses initials (or other letters) to designate the elements, when he makes a formula to express the composition of any compound; and he adds a figure to each symbol when more than one part or atom (by atomic weight) enters into the make-up of the molecule. Thus H20 represents a compound in which the molecules are two parts hydrogen and one part oxygen; in common language, this particular compound is known as water. K20 is potash (or potassium oxid) — two parts potassium and one part oxygen. Gypsum or land-plaster is calcium sulfate, — CaS04, which means calcium one part, sulfur one part, oxygen four parts. Quartz is SiO2. Quicklime is CaO.

Phosphoric acid is P2O5. Common table salt is NaCl (sodium and chlorin). Following are the formulas for various common substances: —