I 



94 Prof. H. E. Armstrong on the Determination of 



the former temperature, the entire amount may be set down 

 as 13250 units. Boltzmann in a recent paper (Wiedemann's 

 Annalen, vol. xxii. p. 71) gives the value 13920 units : the 

 mean value, in round numbers 13600 units, may be regarded 

 as not far from the truth. But in the conversion of N 2 4 

 into 2N0 2 , the volume becomes doubled, so that the heat of 

 combination of nitrogen atoms at constant volume is 



N.N* = 13600 -580 = 13020 units. 



(14) Heat of Formation of the Oxides of Nitrogen. — From 

 his previous determinations, and taking into account the heat 

 of dissociation of nitrogen peroxide, Thomsen calculates that 

 the heat of formation of pure nitrogen peroxide gas at 18° is 



N 2 , 20 2 =N 2 4 = —3810 units at constant volume, 



while that of nitrogen dioxide is 



N 2 , 20 2 = 2N0 2 = 2( — 8415) units at constant volume. 



On the assumption that nitrogen dioxide has the constitution 



/O 



and that the amount of heat developed by the combination of 

 two similar atoms does not exceed that developed by their 

 union by single affinities, it is possible to determine the 

 heat of formation of oxygen molecules by comparison of the 

 heats of formation of nitrogen monoxide and dioxide at con- 

 stant volume. The manner in which the heat-disturbance 

 which attends the formation of these two oxides is effected 

 may be expressed by the following equations : — 



N 2 , 2 = 2NO = 2(-21575) =2E : O-N . N— O . O, 

 N 2 , 20 2 =2N0 2 = 2(-8415) =4N.O— N.N. 



Or, in words — the heat-disturbance of 2( — 21575) units, which 

 attends the formation of 2 gram-mol. props, of nitric oxide 

 from 1 gram-mol. prop, of nitrogen and 1 gram-mol. prop, of 

 oxygen, is the difference between the heat absorbed in the 

 separation of the nitrogen atoms of the nitrogen molecules, 

 and of the oxygen atoms of the oxygen molecules — the 

 atoms in both cases being united by single affinities — and that 

 given out in the combination of the nitrogen and oxygen 

 atoms by two affinities of each. In the case of the dioxide, 

 as the oxygen atoms remain united by single affinities as they 



* The dot is used merely to indicate the number of affinities of each 

 atom engaged in holding the atoms together. 



