BOILING POINT AND AFFINITY 55 



relations at the boiling point are the less marked, on 

 account of the divergence of boiling points from being 

 corresponding temperatures : so that along side of the rules 

 depending on the additive character, others arise depend- 

 ing on constitution, and therefore referring in the first place 

 to isomers. 



(a) Else of Boiling Point under the Influence of 

 Strong Affinities. 



A leading empirical rule is, that compounds formed under 

 the action of strong affinities, and so with a great evolution 

 of heat, have high boiling points, and vice versa. 



This fact appears even in the table on p. 51 since only 

 the compounds formed under the action of weak affinities, 

 such as C1 2 O, NO, and HI, show boiling points in fair accord 

 with those calculated by addition from the elements, whilst 

 in other cases, such as hydrochloric acid and ammonia, the 

 boiling point often lies 100 higher. 



Very striking instances of this may be found, such as 

 the in volatility of potassium chloride as compared with the 

 relative volatility of its components, and on the other hand 

 the volatility of a compound with weak affinities, Ni(CO) 4 

 despite the involatility of one of its components. 



The view suggests itself that this rise of boiling point 

 caused- by affinity implies that in the expression 



p _ RT a 



- v-b " F 2 ' 



I is small, and a large, and consequently P small. 



Accumulation of Negative Atoms or Groups. With the 

 general rule mentioned above may be associated one as to 

 the influence on the boiling point of the accumulation of 

 negative groups, or atoms, such as will be described more 

 exactly later, the halogens, oxy-groups, cyanogen and the 

 like. We have e. g. 



H 2 CC1H 2 CC1, boiling point 84, and 



58, Diff. +26. 



