Eobeetson. — On certain Maxima and Minima. 469 

 association of the vapour at the boiling-point, for the follow- 



ing reasons 



(1.) It is less noticeable in the case of the esters, which 

 are only slightly associated. 



(2.) The minimum is less marked when the boiling-points 

 under reduced pressure are considered, and under reduced 

 pressure the amount of association is reduced. 



(3.) It is not exhibited by series of compounds which are 

 associated neither in the liquid nor gaseous state, nor, on the 

 other hand, by the alcohols which form liquid (not gaseous) 

 molecular complexes. 



Among the esters the successive differences are much 

 more regular. In the following table the boiling-points of 

 the methyl, ethyl, and propyl esters of the fatty acids are 

 given : — 



Table VII. 



For each series the first set of differences shows a clear 

 maximum between the esters derived from the C 4 and C 5 acids, 

 the number being in all cases practically the same (25). The 

 acids themselves show a maximum at the same place, al- 

 though it is much less marked. In the other set of differences 

 there is a general tendency for the numbers to decrease, but 

 the characteristic minimum observed in the acids finds no 

 parallel among their ethereal salts. 



In general the methyl esters bear a much closer resem- 

 blance to the acids themselves than do the esters with larger 

 alkyl radicals. This is seen, for instance, in their higher 

 melting-point and the greater complexity of the liquid mole- 

 cules. At the boiling-point such differences disappear. This 

 is only another example showing that the boiling-point is a 

 comparable temperature for physical data. 



Much more clearly than with the esters is a similar rela- 

 tion exhibited by the acid chlorides. The following table is 

 given by Henry,* who pointed out the large variations in 

 successive differences of the boiling-points, but did not ap- 



Rec. Trav. Chim., 1899, 18, 247. 



