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How did Thomas Andrews study the continuity between gaseous and liquid carbon dioxide?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

Thomas Andrews demonstrated the continuity between the gaseous and liquid states through extensive experimental studies on carbon dioxide, which he presented in an 1869 Bakerian lecture to the Royal Society of London.1

By observing the behavior of carbon dioxide under varying conditions of pressure and temperature, Andrews established the concepts of critical temperature and critical pressure. He showed that when a gas is compressed above its critical temperature, no abrupt phase change occurs; instead, the substance transitions continuously into a dense fluid that is indistinguishable from a liquid. Below this critical temperature, however, the gas undergoes an abrupt condensation into a liquid.21

His work proved that there is no fundamental distinction between a gas and a liquid, as they represent extreme stages of a single fluid that can pass from one state to the other without any discontinuity or abrupt change in physical properties.21