Andrews, Thomas (1813–1885)

Irish physical chemist. Andrews is best known for postulating the idea of critical temperature and pressure from his experimental work on the liquefaction of gases, which demonstrated the continuity of the liquid and gaseous states. He also studied heats of chemical combination and was the first to establish the composition of ozone, proving it to be an allotrope (see allotropy) of oxygen.

Andrews was born in Belfast and attended five universities, beginning at the age of 15. In 1835 he graduated from Edinburgh as a qualified doctor and surgeon. He was professor of chemistry at Queen's College, Belfast, 1849–79.

Studying chemical combination, Andrews succeeded in the direct determination of heats of neutralization and of formation of halides (chlorides, bromides, and iodides).

Many other scientists had tried to explain the relationship between gases and liquids. Andrews constructed elaborate equipment in which he initially investigated the liquefaction of carbon dioxide, and by 1869 he had concluded that it has a critical temperature (or critical point) of 30.9°C/87.6°F, above which it cannot be condensed into a liquid by any pressure. Hydrogen, nitrogen, and air were then also liquefied by applying pressure to the gases once they had been cooled to below their critical temperatures. Andrews also worked out sets of pressure-volume isotherms at temperatures above and below the critical temperature, and brought a sense of order to what had previously been a chaotic branch of physical chemistry.