Thomas Andrews

Thomas Andrews was born in Belfast in 1813, the son of a linen merchant. He studied chemistry in Glasgow, then medicine in Paris and Edinburgh, taking his MD in 1835, and practised medicine in Belfast while pursuing chemical research in his own time. In 1849 he was appointed vice-president and professor of chemistry at the newly founded Queen's College, Belfast, where he remained for the rest of his career.

His early work established the heat evolved in chemical combinations with an accuracy that made him one of the founders of thermochemistry, and he settled a long-running dispute by demonstrating in 1856 that ozone is an allotrope of oxygen rather than a compound. But his enduring work began in the 1860s, when he took up the behaviour of carbon dioxide under pressure — a substance chosen because its interesting region lay conveniently near room temperature.

In his 1869 Bakerian Lecture, 'On the Continuity of the Gaseous and Liquid States of Matter', Andrews reported that above 31 °C no amount of pressure would liquefy carbon dioxide. Below that temperature a meniscus separated liquid from vapour; above it, the distinction simply ceased to exist. He named the temperature the critical point and drew the conclusion in his title: liquid and gas are not two states but one continuum, and a substance can be carried from one to the other without ever crossing a boundary. He also recorded the milky glow now called critical opalescence.

The result reoriented the subject. It explained decades of failure to liquefy the 'permanent' gases — nobody had gone below their critical temperatures — and pointed directly to the work that made the cold frontier possible. Van der Waals credited Andrews's isotherms as the stimulus for his 1873 equation of state, the first theory to predict a critical point rather than merely observe one.