1894] Dissociation of the Molecules of Liquids. 173 



c. Cailletet and Matthias have suggested a simple plan for finding 

 the true volume of a substance at its critical point. It consists in 

 mapping the densities in the state of liquid and of gas against 

 temperature (as seen, for example, in the diagram, given by Ramsay 

 and Young in their memoir on alcohol in the ' Philosophical Transac- 

 tions,' 1886, Part I, plate 7), and bisecting the lines of equal pressure, 

 which cross the diagram horizontally. Such lines are lines of equal 

 pressure at constant temperature. On joining the points where the 

 lines are bisected, a straight line is obtained in the case of most 

 liquids, which, when continued vertically, cuts the curve at the 

 critical density. But to this rule Young and Thomas find that water, 

 and methyl, ethyl, and propyl alcohols are exceptions, for they give 

 curved lines. These substances are not associated in the state of 

 gas, although in the liquid state they display association. Acetic 

 acid, however, which displays association both in the state of liquid 

 and of gas, gives a line which is, if not quite, at least very nearly 

 straight. 



It may therefore be concluded that while a curved line implies 

 association in the state of liquid, a straight line implies either no 

 association or association in both conditions. 



d. The heat required to vaporise a dissociating liquid is employed 

 in two ways when the gas, as is always the case, has a simpler 

 molecular formula than the liquid. A portion of the heat is em- 

 ployed in vaporisation alone ; while a portion is absorbed in effecting 

 the decomposition of complex molecular groups. The heat of 

 vaporisation alone diminishes as temperature rises, till at the critical 

 point it is zero ; but the heat required to dissociate molecular groups 

 may increase, if that term is of importance, and may cause the 

 total heat to increase. The researches of Ramsay and Young on 

 ethyl alcohol and on acetic acid have shown that there exist maxima 

 in the heats of vaporisation of these substances. Thus at 0, the 

 heat of vaporisation of ethyl alcohol is 220'9 cal. ; at 10, 221'2 ; 

 at 20, 220-6 ; and at 30, 220'1. The numbers then decrease as 

 usual. With acetic acid at 80, the value is 91'6 cal. ; at 100, 92'3 ; 

 at 110, 92-8 ; at 120, 927 ; at 130, 92-4, and so on. It may be 

 stated, then, that when the numbers representing heats of vaporisa- 

 tion of a compound increase to a maximum, and then diminish, the 

 compound contains complex molecules in the liquid state. It does 

 not follow that all substances which possess complex liquid molecules 

 must exhibit such a maximum, for this peculiarity evidently depends 

 on the relative importance of the heats of dissociation and of vapori- 

 sation. 



e. The curves representing the vapour pressures of non-dissociating 

 liquids do not cut one another at any point in their course. Liquids 

 which associate give vapour-pressure curves which cut some of those 



