J 880.] 



On the Critical Point. 



205 



The question is also closely connected with that of heat of vaporisa- 

 tion and heat of combination. It is possible to exhibit this point 

 more clearly by help of an example. The heat of vaporisation of 

 water, under a pressure of 760 millims. is 513 calories for 1 cub. 

 centim. at 100°. The expansion which the liquid undergoes in 

 becoming gas is represented by the number 1623. From the known 

 equivalent of heat in work, it is easy to calculate the total work 

 necessary to evaporate water ; and also the work required to expand 

 the substance 1623 times against atmospheric pressure. The work 

 done as heat, in the case of water is 221"1 kilogram-metres ; and as 

 expansion, 16*6 kilogram-metres : hence 221*1 — 16'6 is work done in 

 overcoming molecular resistance. But this work is infinitely more 

 than is necessary to overcome surface tension, and the most probable 

 conjecture is, I venture to think, that the work is employed in dis- 

 sociating the complex molecules of water into simpler molecules of 

 water-gas. 



Now, in the ^ foregoing paper, experiments have been described 

 which show that when a liquid is heated in a certain confined sp,aee, 

 the results of observation, possible while the liquid is still distinguish- 

 able from its gas, lead to the conclusion that at a temperature at 

 which the meniscus of the liquid has disappeared, total expansion of 

 the liquid will take place, and that a certain larger volume, total 

 evaporation will ensue. 



It may be objected that Regnault's measurements of the heat of 

 vaporisation of liquids at high pressures appear to show that it is a 

 quantity diminishing with the temperature. But it has never been 

 shown to be the contrary in the case of heat evolved during chemical 

 combination. Is it not likely that there will be a . less evolution of 

 heat during the combination of hydrogen and iodine at a high than at 

 a low temperature and pressure ? And to return to M. Cailletet's 

 experiment, is it likely that compression, which, as a rule, has the 

 result of turning gas to liquid, should in this case change liquid to gas ? 



To sum up : the views expressed in this paper are : — (1) That a 

 gas may be defined as a body whose molecules are composed of a 

 small number of atoms ; (2) a liquid may be regarded as formed of 

 aggregates of gaseous molecules, forming a more complex molecule ; 

 and (3) that above the critical point, the matter may consist wholly 

 of gas, if a sufficient volume be allowed; wholly of liquid if that 

 volume be diminished sufficiently ; or of a mixture of both at inter- 

 mediate volumes. That mixture is, physically speaking, homogeneous, 

 in the same sense as admixture of oxygen and hydrogen gases may be 

 termed homogeneous ; but chemically heterogeneous, inasmuch as it 

 consists of molecules of two different natures. When prevented 

 from mixing by interposing a capillary tube between the two, the 

 liquid and gas retain their several properties. 



VOL. XXXI. Q 



