100 Prof. H. E. Armstrong on tlie Determination of 



atom of the radical such a formula is impossible, and the 

 ordinary formula must be assigned to aniline. The heat of 

 formation would then be 



5r + 9v + N . C + 2N. H-p" . N = 210770 units. 



The value found experimentally is 211090 units, thus con- 

 firming the formula NH 2 . C 6 H 5 . 



(17) Heat of Formation of Pyridine. — Pyridine is usually 

 represented by the formula 



HC=CH— CH 



I II 



HC= N — CH 



The heat of formation of such a compound would be 



5r -t 2v x + 2v 2 + C=N— C = 131800 + 0=N— C. 



The difference between the heat of formation calculated from 

 the heat of combustion, P = 171370 units, and 131800 units, 

 viz. 39570 units, should represent the amount of heat deve- 

 loped in the fixation of the nitrogen atom ; but it so exceeds 

 the values found in the case of all other nitrogen compounds 

 that the assumption that doubly-linked carbon atoms are 

 present in pyridine must be abandoned. If a formula similar 

 to the prism formula for benzene be adopted, the heat of for- 

 mation may be expressed as follows : — 



5r + 6t* + N = C 8 = 160200 + N = C 8 . 



The value of N = C 3 thus found, 11170 units, is also unusually 

 high ; pyridine therefore must differ from benzene in consti- 

 tution ; Thomsen suggests the formula 



„CH — CH 



/ \/ 

 N— CH V 

 \ / \ 

 \CH — CH 



the calculated heat of formation of such a compound being 

 very nearly that found for pyridine (171370 units), thus: 



5r + Iv + C . N= 170490 units. 



(18) JYitro-paraffins. — Nitromethane and nitroethane are 

 commonly regarded as compounds of the form R . N0 2 . De- 

 ducting the heats of formation of the hydrocarbon radicals 

 from the observed heats of formation of the compounds, the 

 difference will be the heat developed in the formation of the 

 radical C . N0 2 . 



