52 PHYSICAL PROPERTIES AND COMPOSITION 



There is therefore no question of strictly additive rela- 

 tions, which is not surprising, since the boiling points are 

 not corresponding temperatures the critical pressures vary 

 widely (15 atm. for hydrogen, 200 for water) so that we 

 must give up Kopp's attempt to find indirectly from the 

 compounds a number for each element, and to calculate 

 the boiling point of compounds by addition of these. Still 

 the additive character is not so completely hidden, but that 

 certain important rules can be deduced from it : we will 

 consider these next. 



(a) Equal Boiling Points of Isomers. 



Equality in the boiling points of isomers is an im- 

 mediate consequence of additivity. It is often found in 

 practice. We may take as example the isomers of the 

 formulaCgH^Og: 1 



, . 184.8 

 . 191-3 

 . 1981 



(b) Increase of Soiling Point with Molecular Weight. 



If the compounds are not isomeric, but polymeric, or 

 more generally if the one molecule is, according to the 

 kind and number of its atoms, larger than the other, then 

 mostly the higher boiling point is found for the larger 

 molecule, in accordance with the additive character of 

 that property. Consider the compounds of nitrogen and 

 oxygen. 



The smallest diatomic molecules correspond to the 

 lowest boiling points, and to gases that were long known 

 as permanent. 



O 2 - 181 N 2 - 194 NO - 154. 



The triatomic boil a good deal higher : 



O 3 - 106 N 2 O - 87, 



1 For this and the following the collection in Graham-Otto, 1898, by 

 Marckwald (ist vol., part 2) has mostly been used. 



