calx

Calx is a term with several related meanings in Latin and the history of science. In Latin, calx means heel as well as lime or stone. The word is the source of modern terms such as calcaneus (the heel bone) and the chemical word calx, though contemporary senses differ. In older scientific literature, calx referred to a metal oxide or to a lime-like material produced by calcination.

In chemistry and metallurgy, calx describes an oxide formed by heating a metal or mineral. For example, the calx of lime is calcium oxide (CaO), produced by calcination of calcium carbonate. When water is added, it forms calcium hydroxide, historically called calx or slaked lime. The calces of other metals, such as copper oxide or lead oxide, were also described as calces in medieval and early modern chemistry.

In anatomy, calx appears in terms such as calcaneus, the heel bone, reflecting the original meaning of the word as "heel." In modern usage, calx is largely historical; in scientific writing, oxide or lime is preferred. The term survives chiefly in etymology and in historic discussions of chemistry and medicine.