Cagniard de la Tour showed in 1822 that there is a critical temperature above which a single substance can only exist as a fluid and not as either a liquid or gas. He heated substances, present as both liquid and vapor, in a sealed cannon, which he rocked back and forth and discovered that, at a certain temperature, the splashing ceased. Later, he constructed a glass apparatus in which the phenomenon could be more directly observed. These phenomena can be put into context by reference to Fig. 1, which is a phase diagram of a single substance. The diagram is schematic, the pressure axis is nonlinear, and the solid phase at high temperatures occurs at very high pressures. Further solid phases and also liquid crystal phases can also occur on a phase diagram. The areas where the substance exists as a single solid, liquid, or gas phase are labeled, as is the triple point where the three phases coexist. The curves represent coexistence between two of the phases. If we move upward along the gas-liquid coexistence curve, which is a plot of vapor pressure vs temperature, both temperature and pressure increase. The liquid becomes less dense because of thermal expansion, and the gas becomes more dense as the pressure rises. At the critical point, the densities of the two phases become identical, the distinction between the gas and the liquid disappears, and the curve comes to an end at the critical point. The substance is now described as a fluid. The critical point has pressure and temperature co-ordinates on the phase diagram, which are referred to as the critical temperature, T _c, and the critical pressure, p _c, and which have particular values for particular substances, as shown by example in Table 1 (1).