500 



Transactions. — Chemistry and Physics. 



which owes its wide application in cryoscopy to its slight 

 tendency to cause bodies to associate, the experiments of the 

 authors have shown that, while the alcohols and other ali- 

 phatic compounds remain fairly normal, the fatty acids asso- 

 ciate strongly with rising concentration. This, however, will 

 be discussed in a future communication. 



1. Acetic acid has already been examined in detail. The 

 following are a few of the numbers : — * 



Temperature. 



(Boiling- 

 point, 119° C). 

 125° 



130° 

 140° 

 160° 



190° 

 219° 

 230° 

 250° 



Density. 

 (Normal Molecular 

 Weight, 2 08). 

 320 



312 

 2-90 

 2-48 

 2-30 

 2-17 

 209 

 2-08 



On plotting these results as a curve it is found that the 

 rate of dissociation of these molecular complexes is a direct 

 function of the temperature for 50° above the boiling-point. 

 Then the rate is gradually decreased till 230° C, when the 

 vapour density becomes constant. 



2. We can find no record of any determinations made with 

 propionic acid, the next member of the fatty acids. Accord- 

 ingly we performed several experiments by the ordinary 

 method of Dumas. 



(a.) At 146° C. and at 760 mm. the vapour density was 

 found to be 51. The normal value is 37 ; consequently at 5° 

 above the boiling-point the vapour is associated 38 per cent. 



(b.) At 192° C. and at 755 mm. the vapour density was 

 found to be 45, indicating 22 per cent, association. 



3. Normal butyric has already had its vapour density 

 determined at different temperatures. f A normal value is 

 reached at a temperature of about 100° above the boiling-point 

 of the compound. By continuing the curve for butyric acid it 

 is found that the vapour density 5° above its boiling-point 

 would be 38 — i.e., at that temperature it is associated 25 per 

 cent. Thus for the three acids at 5° above their boiling- 

 points — 



Vapour associated. 

 Acetic acid ... ... .. 54 per cent. 



Propionic acid ... ... 38 „ 



Butyric acid ... ... 25 „ 



* Cahours, " Comptes Rendus," xx., 51. 

 t Cahours, loc. cit. 



