PNEUMATICS 245 liquid surface separating the two coexisting states of the .substance ; and the isotherm has a corresponding straight line portion. At a temperature of 21 5 C. liquefaction occurs at a pressure of 60 atmospheres. The horizontal portion of the isotherm, which marks the co-existence of the gaseous and liquid states, is considerably shorter than at the former temperature. The isotherm for 31 l C., however, has no such rectilinear characteristic ; and at this and higher temperatures the substance is never during the whole compression in two distinct conditions at once. It is impossible to say when the dense gaseous condition passes into the light liquid condition. The two states are absolutely continuous. The critical temperature that is, the temperature below which there is a distinct separation between the liquid and the gas is fixed by Dr Andrews at 30 92 C. for carbonic acid gas. Above this tempera ture it is impossible to obtain a free liquid surface in a closed vessel. This conclusion had already been arrived at by Faraday in 1826, when he considered himself entitled to state that above a certain temperature no amount of pressure will produce the phenomenon known as condensa tion. Andrews s results also give the true explanation of the observations made by Cagniard-Latour l in 1822 upon the effect of high temperature on liquids enclosed in glass tubes which they nearly filled. He found that at a certain temperature the free liquid surface disappeared, and the tube became filled with a substance of perfectly uniform appearance throughout. He concluded that the whole had become gaseous. In reality he had reached the critical temperature at which the liquid and gaseous con ditions pass continuously the one into the other. The following are Cagniard-Latour s estimated values for the temperature and pressure of various substances at the critical point Temperature. Pressure. Ether 175 C. 248 ,, 258 ,, 38 atmospheres. 119 ,, 1 Alcohol Bisulphide of carbon Avenarius 2 and Drion 3 have studied the critical tem peratures of other substances, such as sulphuric acid, ace- ton, and carbon tetrachloride. The substances, however, which can be so studied are comparatively few, since the greater number of those which are liquid under ordinary conditions have their critical temperatures very high, while the majority of those which are gaseous have theirs very low. >e nie- The necessity for a very low temperature long prevented ctn of ti ie obtaining in a liquid form of the standard gases hydrogen, oxygen, nitrogen, &c. which were accordingly distinguished by the name permanent gases. Faraday 4 proved that these could not be liquefied at a temperature of -110 C., even when subjected to a pressure of 27 atmospheres. Natterer 5 likewise failed to reduce these gases to the liquid state, even at a pressure of 3000 atmo spheres. His means for reducing the temperature were not satisfactory. In 1877 Cailletet and Pictet, working inde pendently, first successfully effected their approximate liquefaction. The former compressed each of the gases oxygen, nitrogen, and carbonic oxide to 300 atmospheres in a glass tube, which was cooled to - 29 C. When the gas was allowed to escape, it did so in the form of a cloud, 1 Annales de Chimie, 2d ser., xxi., xxii. 2 Poggendor/ s Annalen, cli., 1874. 3 Ann. de Chimie et de Physique, 3d series, Ivi. 4 Phil. Trans,, 1845. 5 Wienische lierichte, 1850, 1851. 1854 ; and Pong. Ann., xciv., 1855. 6 Ann. de Chimie et de Physique, 1878 ; Comptes Rendus, 1882. condensing for the moment to the liquid state under the influence of the extreme cold produced by the rapid expansion of the gas. Pictet in a similar way obtained an issuing stream of liquid oxygen. Von Wroblewski and Olzewski 7 have more recently obtained oxygen, nitrogen, and carbonic oxide in a more evident liquid state. They used Cailletet s form of apparatus, and cooled the gas by means of the evaporation of liquid ethylene. Under this extreme cold they observed these substances forming a well-defined liquid in the bottom of the tube. The follow ing table gives the results of five different observations at slightly different temperatures. Temperature, C. .. Pressure, in atmos. -129 6 27-02 -131-6 25-85 -133-4 24-4 -134-8 23-18 -135 -8 22-2 At slightly higher temperatures, the pressure necessary for the liquefaction increased very rapidly. Nitrogen and carbonic oxide were not so easily reduced, remaining still gaseous at - 136 C., and under a pressure of 150 atmo spheres. By a sudden diminution of the pressure to 50 atmospheres there was obtained under the influence of the reduced temperature a rapidly evaporating liquid. The critical point of oxygen has been experimentally fixed by Von Wroblewski 8 at - 113* C. and 50 atmospheres pres sure. With the data given by Amagat s researches, Sarrau 9 has calculated from a formula of Clausius s the following values of the critical temperature and pressure for oxygen, nitrogen, hydrogen. Critical Temperature. Critical Pressure. Oxygen -105 4 C. 48 "7 atmospheres Nitrogen -123-8 42-1 , Hydrogen -174-2 98 9 ,, It will be observed that Von Wroblewski s observed values for oxygen are in remarkably close agreement with the calculated values given here. Apparently oxygen is just at the limit fixed by Faraday. The behaviour of a gas under varying pressure is a Mano- phenomenon of great practical importance, and gives a valu- meter. able method for measuring pressures (see MANOMETER). A modification of the ordinary mercury manometer is used for measuring volumes, and is especially valuable in estimating the densities of substances which cannot be put into water, such for example as liquids or powders. The closed end of the manometer tube, which by means of a stop-cock may be opened to the air at will, is fitted to a flask with which it may be put into connexion when required. Two fiducial marks are then made upon it one at the position where the mercury surfaces in the two limbs of the tube are co-level, and the other somewhat higher at a convenient spot. Between the two marks the tube expands into a bulb, thereby increasing the interven ing volume and minimizing the effect of any slight error in bringing the mercury surface to the higher mark. Let the volume of the flask and tube down to the higher mark be v, and the volume of the rest of the tube down to the lower mark V. At first the volume of air in the flask and tube is V + v, at the atmospheric pressure P. Now pour mercury into the open end till the liquid surface in the closed end reaches the higher mark. The air has been compressed to volume v ; and the corresponding pressure, as measured by the balanced mercury column, is p + P. Hence Let now a volume x be placed in the flask, and let the 7 Comptes Rendus, 1882-83 ; Wiedemann s Annalen, 1883 ; and Annales de Chimie et de Physique, 1S84.
8 Comptes Rend is, 1884. 9 Comptes Rendus, 1882.