320 RESEAnfiHES ON EVAPORATION AND BISSOCTATtON. 



bability dno to chemical combination between the simple 

 molecules, while the increase at high temperatures is due to 

 a general attraction (cohesion) of the molecules for each 

 other. In other words, the cause of abnormality at low 

 tem.peratu.res is a chemical one ; at high temperatures, a 

 physical. Now the increase of the density of the saturated 

 vapour at liigli tompej'atures is common to all bodies ; but 

 the increase at low temperatures is only exhibited by such 

 bodies as acetic acid and nitrogen peroxide, which, there 

 is every reason to believe, undergo dissociation. Its absence 

 in the case of the alcohols and ether affords a strong argu- 

 ment against the existence of chemical combination, between 

 tlie simple molecules. But it might bo asserted that in the 

 2)assage from the gaseous to tlie liquid state combination 

 occurs. That this cannot bo the case is evident from a 

 consideration of the behaviour of liquids near their critical 

 points. For the specific volumes of liquid and gas, just 

 below the critical point, are nearly equal ; and were the 

 liquid to consist of congeries of gaseous molecules, t.hoi-o 

 would necessarily be fewer molecules in unit volume of tlio 

 liquid, tlian in unit volume of the gas, --an improbable 

 conception. 



Horstmann has also made experiments on tlie vapour- 

 densities of acetic acid. In his first paper {Berichlf;, 2, 

 p. 299), he suggested that a difference should exist between 

 the forms of the curves representing the densities of the 

 saturated vapour in the case of a true dissociating sub- 

 stance, .and of acetic acid. He supposed that the vapour- 

 density of dissociating substances increases with fall of 

 temperature, but that in the case of acetic acid it do- 

 creases; and in his seciond [)apor {llerichte, 3, p. 78), lie 

 gives experimental determinations of the density of the 

 saturated vapour, which, using his calculations, confirm 



