1880.] 



On the Critical Point. 



199 



It may be remembered that Dr. Andrews, in his "Research on 

 Carbonic Anhydride," never obtained the gas absolutely free from air, 

 consequently, in diminishing the volume of his gas, in contact with its 

 liquid, he always noticed that a slight increase of pressure was neces- 

 sary. If the curves representing the behaviour of benzene under 

 similar circumstances be referred to, it will be noticed that the 

 pressure actually is reduced, in producing diminution of volume, in 

 Curves I, II, III, and IV. A relic of the same form of curve remains 

 in the behaviour of a mixture of ether and benzene, but no trace is 

 observable in the behaviour of ether. The explanation appears to be 

 that the molecules, when the gas has been compressed to a certain 

 extent (very shortly before all gas is condensed to liquid), begin to 

 exert some attraction for each other, and cod sequent ly relieve the 

 pressure. The explanation of the fact that this phenomenon is notice- 

 able in the case of benzene, but not with ether, is perhaps connected 

 with their different behaviour at higher temperatures : the meniscus of 

 benzene is always easily distinguished, even up to its vanishing point ; 

 whereas that of ether soon becomes extremely mist-like and hazy. I 

 have little doubt that many other substances, when heated under 

 pressure in a condition absolutely free from the admixture of any other 

 gas, will show similar results. 



Probably, closely connected with this observation, is another: — 

 namely, that it is possible, after condensing all gas to liquid by pres- 

 sure, to lower the pressure very considerably without ebullition of the 

 liquid, and consequently without formation of gas. At a temperature 

 of 228° for instance, it is possible gradually to reduce the pressure 

 from 29 to 22*4 atmospheres without any evolution of gas in the case 

 of benzene ; sudden ebullition then takes place, and the pressure rises 

 to 29*3 atmospheres, the volume at the same time suddenly increasing 

 to 65. This behaviour is represented by a dotted line in the diagram 

 representing the isotherm for benzene. The dotted line in the 

 second isotherm represents a similar phenomenon ; the pressure could 

 be reduced to 35*4 atmospheres before sudden ebullition took place. 

 At higher temperatures, a very slight reduction of pressure caused 

 ebullition, but the phenomenon could still be noticed, although no 

 attempt to measure it was made. The same phenomena were noticed 

 with ether, and are also exhibited on the diagram. 



It is necessary, before discussing the results of these observations, 

 to give some tables, showing the relative proportions of gas to liquid 

 at the various temperatures chosen ; always, of course such that it 

 was possible to distinguish the two states of matter easily from each 

 other. I have thought it sufficient to reproduce merely those of 

 benzene and of its mixture with ether ; for the behaviour of ether does 

 not materially differ from that of the mixture. The curves con- 

 structed to exhibit these relations graphically have for their ordinates 



