|French physicist Charles Cagniard's 1822 "curved tube" apparatus, filled with air (G), mercury (B) and liquid (A), with which he discovered the phenomenon of critical state and critical temperature. |
In early experiments by Charles Cagniard and a person Drion (1859), peculiar effects were obtained, such that a substance was obtained in an intermediate condition, when it was neither liquid nor gas.
Cagniard employed a curved tube, ABG (figure), nearly full of mercury. He confined a liquid at A in a space very little larger than itself, and inclosed a quantity of air at G to serve as a manometer The tube was placed in a suitable bath, and as the temperature rose the vapour of the liquid in A exerted a continually increasing pressure, which was registered by the compression of the air in G. When the temperature reached a certain value the surface of separation between liquid and vapour ceased to exist; the whole became homogeneous. The pressure was very great, but at the somewhat high temperatures employed neither ether, alcohol, nor water could be retained in the liquid state.
In 1859, a person by the name of Drion carried out similar experiments. 
The critical point studies of carbon dioxide was continued in the later 1869 work of Thomas Andrews. 
1. Shachtman, Tom. (1999). Absolute Zero and the Quest for Absolute Cold (Charles Cagniard de la Tour, pg. 67). Mariner Books.
2. Draper, Charles H. (1906). Heat and the Principles of Thermodynamics (pg. 140). Blackie and Son Limitied.
3. Drion. (1859). “Article”, Ann. de Chim. Et de Phys. Pg. 221; and Comptes Rendus, t. xlviii.
4. Byran, George H. (1907). Thermodynamics: An Introductory Treatise Dealing Mainly with First Principles (§20. Critical Point, pg. 12). B.G. Teubner.
● Critical temperature – IUPAC Goldbook.