46 Mr. W. Sutherland on the Fundamental 



or — 78, which is still worse. This case is decisive as to the 

 purely arbitrary character of Thomseu's analysis of the thermo- 

 chemistry of the oxides of nitrogen : nor can a less arbitrary 

 interpretation be given to that analysis, and in searching for 

 a more reasonable idea of the thermochemical relationship of 

 these compounds we must start afresh. 



The chief difficulty with the oxides of nitrogen is, that we 

 have so little general chemical knowledge of their constitu- 

 tion on account of the pentad and triad nature of the In atom 

 and the unsaturated state of some of them. But let us make 

 a scheme of the simplest available structural formulae on the 

 supposition that N is always pentad. 



= N-0-N =0 , = N-N =0 , -N = , s=N=0, \ o / 



The first two are saturated, the next two are unsaturated, and 

 the last may be saturated, though not necessarily so. Let us 

 next write in the same order the heats of formation 



-3-8 -8-4 -21-6 -18 



It will be noticed that although N 2 5 and N 2 4 involve 

 more than twice as many molecules in their construction as 

 N0 2 and NO, the numerical values of their heats of formation 

 are much smaller. If for the saturated compounds N 2 5 and 

 N 2 4 we write in our notation the equations for the heat of 

 formation with the assumption (N)=0, to go along with 

 (0) =0, (C1) = and so on, we get 



4/(N:O)+2/(N-O) = 0, (43) 



4/(N:0)+/(N-N) = -3-8; . . . . (44) 

 from which we are entitled to infer that/(N:0), /(N'O), 

 /(N*N), are all small, as it is unlikely that /(N:0) and 

 /(N'O) will have opposite signs. If then we can put 

 /(N:0)=/(N-O) = 0, then the -8*4 for the formation of 

 N0 2 represents the thermal effect of the passage of the N 

 atom from the N molecule to the pentad combined state with 

 one bond unsaturated, and the —21*6 for NO represents the 

 thermal effect for the passage of the N atom from the N 

 molecule to the combined state, with, perhaps, three bonds 

 unsatisfied. In the case of N 2 we can hardly suppose an 

 unsaturated structure to account for the —18, perhaps we 

 have to do with a ring structure such as is indicated above, 

 and has no simple relation to the other structures under con- 

 sideration. From the present point of view then, the numeri- 

 cally larger negative heats of formation of N0 2 and NO as 

 compared with those of N 2 5 and N 2 4 are regarded as due to 

 the unsaturated state of the N atom in them. 



