204 



Dr. W. Ramsay. 



[Dec. 16, 



40 atmospheres ; that at which benzene reaches the critical state 60*5, 

 and that of the mixture 48. The temperatures are: ether, 195 0, 5 ; 

 mixture, 240°'7 ; benzene, 291°'7. Both temperature and pressure 

 thus appear to take a position not far removed from the mean of the 

 two. 



The definition of the words liquid and gas appears to require more 

 accuracy than has hitherto been bestowed. As no known a3riform body 

 absolutely obeys the law of contraction inversely as the pressure, and 

 equal expansion on equal rise of temperature, there is apparently no 

 instance of a perfect gas, although this state is closely approached by 

 such gases as hydrogen, oxygen, and carbonic oxide, especially at high 

 temperatures and not too great pressures. And the definition of a 

 liquid appears to be a fluid exhibiting surface tension. Now, above 

 the critical point, this surface tension disappears as has been repeatedly 

 shown. But I venture to think that the possession of surface tension 

 is not a criterion of the existence of a liquid. And a most striking 

 argument in support of this theory has lately been furnished by M. 

 Cailletet (" Compt. Bend.," xc, 210). He found that carbonic anhydride 

 at a temperature of 5° '5, when the lower portion of his experimental 

 tube was filled with liquid, the upper portion being filled with a mixture 

 of gaseous carbonic anhydride with air, mixed with the air when a 

 pressure of 130 atmospheres was applied. The question is a simple 

 one ; does the gas become liquid, or the liquid become gas ? Or do they 

 both enter a state to be called neither liquid nor gas ? 



I venture to bring forward a theory, with great diffidence, which 

 appears to be supported by numerous observations, viz., that there 

 exists a close analogy between the condition of liquid as compared 

 with its gas, and of a compound as compared with the elements of 

 which it is constituted, and that in the evaporation of a liquid we 

 have to do with a true instance of dissociation, that is, a decomposi- 

 tion of complex molecules into simpler ones. Many compounds, when 

 heated, dissociate into their elements, or into simpler compounds. 

 The extent of dissociation is a direct function of the temperature, and 

 an inverse function of the pressure. Thus, ammonium chloride, when 

 heated, dissociates into ammonia and hydrogen chloride; hydrogen 

 iodide into iodine and hydrogen. It is evidently possible so to regulate 

 temperature and pressure as to obtain a mixture of hydrogen iodide 

 with hydrogen and iodine in any desired proportion. If the analogy 

 holds, it is possible to obtain a mixture of liquid molecules with gas 

 molecules in any desired proportion ; but as surface tension appears to 

 be permanent until liquid and gas reach the same density, mixture 

 does not occur before that point. Still, mixture may be held to exist 

 to some extent, for the vapour is not a perfect gas, and this is pro- 

 bably owing to its containing some liquid molecules among its gaseous 

 ones. 



