Liquefaction of Gases

There seems to be no definite evidence as to when the idea that a pure liquid possessed a definite vapour pressure at a defi nite temperature first originated. Dalton, early in the century, had shown that the vapour pressure of water was independent of the presence of air in the space above it, and had measured the vapour pressures between the ice point and boiling point. Davy (loc. cit.) had observed that "the elasticity of vapours in contact with the liquids from which they are produced, under high pressures increases in higher ratio than the arithmetical one of temperature ; but the exact law is not determined," and he had determined the relation of temperature and pressure for certain condensed gases. Faraday now took the subject up at the point at which Davy dropped it, and he determined the vapour pressures of all the gases which could be liquefied at temperatures above — I To° C, and at pressures up to 5o atmospheres. (Phil. Trans., 1845.) Faraday was unable to liquefy hydrogen, oxygen, nitrogen, nitric oxide, and carbonic oxide, and the failure of others to succeed by using higher pressures gave rise to the idea that these gases were permanent, the term being still used, with recognized qualifications. An interesting investigation with the same object was that of Natterer (Sitzungsberichte d. Wiener Akad., Ann. d. Plays., 1844-55). Gases were compressed into a wrought iron vessel, the pressure, which approached 3,00o at mospheres, being transmitted through mercury to a loaded piston, which acted as a manometer. The experiments failed in their object ; but indicated the extent to which the permanent gases departed from Boyle's Law regarding the relation be tween pressure, volume and temperature.

The Critical State.

We must now go back to 1822 when Cagniard de la Tour (An. de Ch. et de Ph. 21 and 22) proved that when a liquid was heated above a certain temperature it was completely converted into vapour. He used the glass apparatus shown in fig. 2, the liquid being contained in the wide limb of the U-tube over mercury, the narrow limb serving as a manometer, the pressure being calcu lated from the volume of the air. The ap paratus was heated in an air bath. He obtained the following results: The experiments of Cagniard de la Tour are an example of the excellent work carried out by French physicists of the period. They were, however, completely lost sight of, and it was not till 186o that the experiment was repeated by Andrews, and the fun damental principle associated with it was discovered. Andrews' discovery that for each substance there is a critical temperature above which it cannot be liquefied was first made public in the 1863 edition of Miller's Chemical Physics and in 1869 formed the subject of the Bakerian lecture of the Royal Society under the title, "On the continuity of the gaseous and liquid states of matter." Research of Andrews.—Andrews had attempted to liquefy the permanent gases by applying high pressures at temperatures down to — I I0° C. Failing to liquefy them, he turned his atten

tion to the study of the process of liquefying carbon dioxide, with a view to throwing light on the general problem of the liquefaction of gases. For this purpose he used an apparatus, now in the South Kensington museum, of which a diagram is shown in fig. 3. It consisted of twin steel tubes, flanged and capped at both ends, and connected laterally. Through glands in the upper caps glass tubes passed, of capillary bore above and closed at the top, the part inside the steel apparatus being 2.5 mm. bore, and open below. The tubes contained respectively carbon dioxide and air, over mercury with which the steel apparatus was filled, the air tube serving as a manometer, the pressure being assumed to be in versely proportional to the volume. Steel screw plungers passing through glands in the bottom of the apparatus served to regulate the volume and pressure. The glass tubes projecting outside the apparatus were surrounded by a water bath with glass sides. He found that at temperatures below 31° C liquid appeared in the tube containing the carbon dioxide when the air manometer indicated a definite pressure for each particular temperature, and that when liquefaction had commenced, the gas could be completely condensed without increase of pressure, except such as could be attributed to the presence of traces of air in it. Near 30, which he was the first to call the critical point, the space which had been occupied by the liquid at lower temperatures became filled with a "homogeneous fluid, which, when the pressure was slightly reduced, or the temperature slightly lowered, exhibited a peculiar appearance of moving or flicker ing striae throughout its entire mass." At temperatures above 31° he found that the gas could be reduced in volume to that which it should occupy as liquid with com plete continuity, and without exhibiting a visible surface, hence the title under which the results of the work were pub lished. The results obtained are repre sented by the simple curves in fig. 4.

These curves, called isothermals, repre senting the relationship of pressure and volume for a series of constant tempera tures, are in heavy line. An isothermal for a temperature below the critical temperature represents the volume of the gas as diminishing as the pressure increases till the vapour pressure is reached, and the gas begins to liquefy. The volume then decreases without increase in pressure till the gas is completely liquefied, this part of the curve being a horizontal straight line. As the liquid is only slightly compressible the curve then ascends steeply. The part of the diagram within and below the dotted line represents conditions under which liquid and vapour can co-exist. The critical isothermal has no horizontal por tion, since the volumes of liquid and vapour are identical. Above the critical point the curves approach rapidly to the form of the hyperbola.