732 EVAPORATION its volume. A pellet of mercury was employed to separate the alcohol from some air, the compress! on of which served to measure the pressure in the tube. On heating the alcohol to about 225 C. (according to De la Tour) it ex panded to about twice its volume, and then suddenly disappeared, the pressure being (according to the same authority) about 129 atmospheres. When the quantity of alcohol filled a much greater portion of the tube, the tube burst. The experiment was repeated with ether, naptha, and water, with similar-results ; but in the case of water it was necessary to add a little sodic carbonate to prevent the water dissolving the glass. The experiments have since been repeated by Faraday, and still more recently by Andrews. It was first noticed by Wolf (Ann. de Chlmie, xlix. 230), afterwards by Driun (Ann. de Chimie, Ivi. 221), who examined Wolf s results, experimenting with ether, and with ethylic chloride, and subsequently by Andrews, that the curvature of the surface of the liquid decreases as the tem perature is raised, indicating a diminution in the surface tension, while the surface itself becomes less strongly marked, till it entirely loses its curvature, and then vanishes altogether, only a flickering hazy appearance being visible in different parts of the tube. The temperature at which the liquid and gaseous states merge into one another lias been called by Andrews the critical point. Mendeleef calls it the absolute boiling-point. The temperatures and pres sures corresponding to the critical points of some substances are given in the following table : - Temperature. Pressure in atmospheres. Carbonic anhydride 30-92 C. 75 Ether 187-5 37-5 Alcohol 2587 119-0 Carbonic bisulphide AVater 262-5 411-7 66-5 ? According to Drion, the critical points of ether, ethylic chloride, and sulphurous anhydride are 190 5 C., 184 C., and 157 C. respectively. Wolf experimented upon the diminution of the surface tension of ether, water, and other liquids in capillary tubes, and finding it diminish uniformly as the temperature increased between C. and 100 C., he calculated the temperatures at which the surface tension would entirely vanish, and obtained 217 C. for ether and 537 C. for water. Van der Waals (Over de Continuiteit van den Gas- en Vloeistofloestand, vii.), by taking into account the mutual attraction of the molecules and the volume occupied by the molecules themselves, has arrived at an equation which represents in a somewhat rough manner the relation be tween the volume, temperature, and pressure of a sub stance. When the pressure and temperature are given, there are generally three roots representing the volume in the liquid, gaseous, and unstable states respectively. At the critical point these three roots become equal. From the values of the volume and pressure of water and steam at 0, 100, and 200 C., as deduced by Rankine from the observations of Regnault, Clerk Maxwell has calculated that the critical temperature for water should be about 434 C., the critical pressure about 378 atmo spheres, and the critical volume about 2 52 cubit centi metres per gramme. Dr Andrews has constructed an apparatus for the lique faction of carbonic anhydride, in which the gas is contained in a thermometer tube whose lower portion is much wider than the upper part, and immersed in mercury contained in a test tube, which is placed in a copper cylinder filled with water, to which pressure is applied by inserting a steel screw. The lower end of the glass tube is open,and the upper part projects beyond the copper cylinder. If the carbonic anhydride be heated beyond fhe critical point, pressure being applied so as to keep some of the substance liquid until the critical point is reached, and if the gas be then allowed to cool under this pressure, it will pass con tinuously into the liquid state without any change in the nature of the contents of the tube being apparent. On relieving the pressure the liquid will boil. By the simultaneous application of cold and pressure Faraday succeeded in reducing to the liquid state all known gases except hydrogen, oxygen, nitrogen, nitric oxide, carbonic oxide, and marsh gas, and in solidifying many of them. The cooling was effected by the evapora tion in vacuo of solid carbonic anhydride dissolved in ether, which produced a temperature of about - 110 C. ; and by this means carbonic anhydride, chlorine, nitrous oxide, ammonia, cyanogen, and some other gases were liquefied by cold alone at atmospheric pressure. Faraday was of opinion that 110 C. is above the critical temperature of air, oxygen, hydrogen, nitrogen, carbonic oxide, and marsh gas. Andrews subsequently reduced air to T i T of its volume at ordinary pressure and temperature by means of pressure and the cold produced by the same freezing mixture as was employed by Faraday. Hydrogen was reduced to -^^ of its volume, oxygen to T |- T , and nitric oxide to ^5-5-, but no liquefaction ensued. Towards the close of 1877 Cailletet, at Chatillon-sur- Seine, compressed air and other so-called permanent gases in an apparatus very similar to that of Andrews, but provided with a means of suddenly relieving the pressure. The compressed gases were cooled to - 29 C., and the cold produced by the sudden expansion when the pressure was relieved was so intense that in each case a liquid spray was produced. About the same time Pictet, at Geneva, succeeded, not only in liquefying all the gases which had previously resisted liquefaction, but also in solidifying hydrogen, his method depending on the cold produced by expansion, as in Cailletet s experiment, but the compressed gases being cooled by him to a much lower temperature before expansion than was employed by Cailletet. Some of the laws of evaporation admit of easy explana tion, in accordance with the dynamical theory of the con stitution of bodies. When a particle of liquid in the course of its wanderings reaches the bounding surface with more than a certain normal velocity, it is able to pass through the surface and get quite clear of the liquid, when it becomes a particle of gas or vapour. The number of particles passing through a square centimetre of the surface from the liquid will depend upon the velocity of the liquid particles, and therefore on the temperature of the liquid, but it will be entirely independent of the condition of affairs outside the liquid. Hence, the quantity of liquid which evaporates in a second will not depend upon the pressures of any gases or vapours above the liquid, but only on the temperature. Whenever a particle of vapour moves towards the surface of the liquid and reaches it, it enters the liquid and is condensed. The quantity of vapour so condensed in a second will depend on the velocity of translation of the particles of vapour and the number of such particles in each cubic centimetre of the- space above the liquid, but will not be sensibly affected by the presence of particles of other gases or vapours in the same space. As the density of the vapour increases, the number of particles which enter the liquid per second will increase proportionally, and at length will become equal to the number which leave it. When this is the case evaporation appears to cease : but it is not a cessation of evaporation which actually takes place, but an increase in the rate of condensation which produces a condition of dynamical equilibrium. If there be a quantity of another gas above
the surface of the liquid, its presence will hinder the diffu-