6 Lapworth, Latent Polarities of Atoms 



illustrates the far-reaching- influence of the polarising " key- 

 atoms" on the properties of the hydrogen atoms both in the 

 meta position and in the methyl group of ^-nitrotoluene and 

 the relation between this arrangement and the less extended 

 one for ethyl crotonate strengthens the case for concluding 

 that the influence of any " key-atom " on a chain of other 

 atoms is to influence their latent polarities in the sense which 

 the writer attaches to the alternating + and - signs of the 

 preceding schemes. 



It is this kind of influence which the writer considers to 

 have the greatest importance in the development of the prin- 

 ciple of latent polarities and which should always be expressed 



* ,6 

 in employing- it. The expression H+— C , adopted by 



Fry and other authors of this school, for formic acid (compare 

 /. Amer. Chem. Sot., 1914, 36, 1035) is used by them to 

 express several, probably unrelated, properties of the acid 

 simultaneously. It is doubtless quite consistent with Thomp- 

 son's theory of a bond; but this theory provides no explana- 

 tion of the principle which the present writer desires to 

 emphasise and with which principle such formulae are incon- 

 sistent. 



Formulae A and B indicate the application of the principle 

 of alternating latent polar influences as expressing the influ- 

 ence of the methin hydrogen atom and the hydroxylic oxygen 

 respectively in modifying the latent polarities of the other 

 atoms in the chain of four : — 



H-cf h - c<; 



+ _ x O-H - + x O-H 



+ - -r + 



(A) (B) 



There are possibly two isodynamic phases of the molecule or,, 

 much less probably, the effects are superimposed, in which 

 case an expression 



- + x O - H 



- + 



