310 BESEABCHES ON EVArOBATTON AND DISSOCIATTON-. 



pressure has no apparent influence on tlio temperature of 

 volatilization ; and that in which its influence is partial. 



Judging from the determinations of vapour-density which 

 have been made, it may fairly ho stated that those sub- 

 stances which show the greatest amount of dissociation 

 show also the greatest divergence between the curves, and 

 the least connection between temperature of volatilization 

 and pressure. 



If a substance is capable of existing in the gaseous state 

 with only partial dissociation, it exerts pressure which is 

 the sum of the vapour-pressure of the undecomposod sub- 

 stance, and of the bodies resulting from its dissociation. If 

 these two pressures could be determined separately by 

 experiment for a series of temperatures, we should have 

 two separate curves, the resultant of which would bo that 

 representing the pressure determined by the barometer- 

 tube method. Now the combination of two such curves 

 might yield a curve of double flexure, as shown in oar 

 original memoir {loc. cit. p. 120) ; and an indication of this 

 may possibly be seen with succinic acid. Or it may give a 

 curve with an abrupt change of direction, and phthalic acid 

 may afford an example of this. Or, lastly, the curve may 

 be indistinguishable from an ordinary vapour-pressure curve, 

 as is the case with most substances. 



It may be also noticed that a dissociable solid, evaporat- 

 ing from a free surface, undergoes no fractionation, the 

 residue having always the original composition ; while a 

 dissociable liquid undergoes fractionation. 



4. Vapour-pressures of acetic acid — correction of former 

 results (Trans. Cliem. Soc, 188.5, p. 42 ; and 188G, p. 80.5).— 

 The following table summarizes our results, and contrasts 

 them with those of Eegnault (R.), Landolt (Ij.), and Wiill- 

 ner (W.). 



