134 Drs. Ramsay and Young on 



It appears to us that these results negative the " chemical " 

 explanation of the constitution of liquids, or, to confine our- 

 selves to known cases, of the liquids alcohol and ether. The 

 molecules of these liquids cannot, we think, be regarded as 

 complex, consisting of gaseous molecules in chemical combi- 

 nation with each other, as, for example, n(C 2 H 6 0), where n is 

 any definite number. We believe, rather, that the physical 

 explanation of the nature of liquids is the correct one, and 

 that the difference between liquids and gases lies merely in 

 the relative proximity of their molecules. 



The chief argument for this view is that it is difficult to 

 conceive that the rise of vapour-density of acetic acid, both 

 at high and at low temperatures, can be due to the same 

 cause, under conditions so radically different ; for at high 

 temperatures we have conditions unfavourable to chemical 

 combination, but owing to the necessarily high pressure, the 

 molecules are in close proximity ; whereas, at low tempera- 

 tures, the conditions are favourable to chemical combination, 

 while the molecules, owing to the corresponding low pres- 

 sures, are very far apart. Now we have shown that, with 

 alcohol and with ether, a rise of density does not accompany 

 fall of temperature ; indeed, the saturated vapour of alcohol, 

 at low temperatures, obeys the laws of Boyle and Gay-Lussac; 

 while the rise of vapour-density at high temperatures is com- 

 mon to all bodies. But with acetic acid, the lower the tem- 

 perature the higher the density of its saturated vapour — a fact 

 which indicates the formation of complex molecules ; at high 

 temperatures, however, it forms no exception in behaviour to 

 ordinary liquids. 



We have shown that with stable substances there is proof 

 of the absence of complex molecules in their vapours ; but it 

 might be asserted that in the passage from the gaseous to the 

 liquid state, combination might occur. That this cannot be 

 the case, is evident from a consideration of the behaviour of 

 liquids near their critical point. 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 mole- 

 cules, there would necessarily be fewer molecules in unit 

 volume of the liquid than in unit volume of the gas — an im- 

 probable conception. 



It is impossible to decide from our experiments whether 

 the higher limit of vapour-density of acetic acid is 60 ; and 

 the difficulty of measuring small pressures with sufficient 

 accuracy renders an answer to this question apparently im- 

 possible ; but it is a remarkable circumstance that our observa- 

 tions, as well as those of Bineau, should so closely approximate 



