54 PHYSICAL PROPERTIES AND COMPOSITION 



the boiling point. There are striking facts in this con- 

 nexion which give indications with regard to molecular 

 weight 1 . 



Let us take first a body like acetic acid, which is probably 

 in part associated even at the critical point. The critical 

 coefficient is then (p. 49) abnormally high, and accord- 

 ingly also the boiling point. To this must be referred 

 most of the difference between the boiling point of acetic 

 acid (118) and that of the isomeric methyl formate (32-5) 

 which is apparently not associated. Thus the boiling point 

 of acetic acid lies halfway between the latter value and 

 that which would be due to the doubled molecule C 4 H 8 O 4 , 

 and which would presumably not be very different from 

 that (164-2) of methyl oxalate (C,H 6 4 ). 



Bodies which, like methyl alcohol, behave normally at 

 the critical point, but become associated in the liquid form 

 at lower temperatures, show according to p. 47 an ab- 

 normal fall in vapour pressure and therefore also a raised 

 boiling point. This may be seen in the case of methyl 

 alcohol (boiling point 66) since (CH 3 ) 2 O boils at 23 and 

 the lower homologue CH 4 O would be expected to have the 

 boiling point 43. Water too boils at far too high a 

 temperature (100) for a compound H 2 O since this, regarded 

 as a lower homologue of methyl oxide, should boil at 63. 



It may be repeated that the cryoscopic methods indicate 

 association in the case of methyl alcohol and water, as of 

 most hydroxylic compounds. 



Finally in the case of carbon which is hardly at all 

 volatile, it appears that there must be a very striking 

 difference between the molecular constitutions C and C.,, 

 since the volatility of C 2 should be about that of a hydro- 

 carbon with two atoms of carbon. 



2. Constitutive Influences. 



It is the more important to discuss the play of con- 

 stitutive influences, since according to p. 52 the additive 



1 Vernon, Chem. News, 64. 54 ; Nernst, Theoretische Chemie, 312. 



