322 EESEAECHES ON EVArORATION AND PTSSOCTATION. 



gaseous molecules, those of stable liquids consist of like 

 molecules. The opposite view is taken by Van der Waals, 

 and the question is well stated in the preface to his work 

 on the Continuitat dcs gasfiirmigen u. fliissigen Znstandos : 

 " Streng genommen, habe ich noch mehr beweisen wollen, 

 namlich die Identitat beider Aggregatzustande. Findet 

 niimlich die schon zum Theil begrundete Vermuthung, dass 

 audi im fliissigen Zustand, die Molekiile iiicht zusammen- 

 falleu um grossere Atomcomplexe zu bilden ihre voile 

 Bostiitigung, so gibt es zwischen den boiden Zustiindon nur 

 noch den Unterschied der grosseren odcr kleineren Dichte ; 

 mithin, nur einen quantitativen Unterschied." The suppo- 

 sition that the complexity of the molecules is different in 

 the two states, is, we consider, negatived by the following 

 coTisidoratious : — The vapour-density of a gas may become 

 abnormally high by either or both of two causes: the 

 existence of complex molecules (congeries of gaseous mole- 

 cules in the gaseous state) ; or by the greater propin(puty 

 of the molecules of gas, caused by their mutual attraction. 

 Wo found with alcohol, that the density of the saturated 

 vapour was normal at temperatures below 40° or 50°, and 

 remained normal down to 13°, the lowest temperature at 

 which observations could bo made. With ether the vapotir- 

 density was approaching normality at 13°, and from the form 

 of the curve woiild have doubtless become normal at a lower 

 temperature. In both cases, witli increase of temperature, 

 and corresponding increase of pressure, the density of the 

 saturated vapour increased towards the critical point with 

 accelerating rapidity, until, at the critical point, the mass 

 of unit volume of tho saturated vapour was equal to that 

 of the liquid (Fig. 2). 



At the critical point the heat of vaporization of a stable 

 liquid is theoretically zero ; below that temperature wo 



