I 



TRANSACTIONS OF SECTION B. 647 



exist in combination with hydrocarbon radicles. The bisdiazoamides of H. v. Pech- 

 mann and Frobenius ^ are derivatives of the 5-atom chain HjN.NH.NH.NH.NH.j or 

 HN:N.NH.N:NH,a type which hardly seems likely to be of sufficient stability to 

 exist. The tetrazones of Emil Fischer have for their type the 4-atom chain 

 IljN.N:N.NH., or HjN.NH.NH.NH,,, of which the free existence is equally pro- 

 blematical, although a derivative containing the chain — N:N.NII.NH— has been 

 obtained by Curtius.- Hydrazoic acid may be regarded as a derivative of triimide, 



NH 

 IIN<^ I , but this type appears to be also incapable of isolation.^ The bydra- 



zidines or formazyls of Pinner ■* and of H. v. Pechmann,' have for their parent 

 compound the hypothetical substance H,,N.N:CH.N:NH. In 1888 Limpricht 

 described certain azo-compounds® which, if possessing the structure assigned by 

 that author, must be regarded as derivatives of diamirfotetrimide : 



HN-NH H..N.N-N 



II ' I II 



HN-NH HjN.N-N 



Both these types are at present imaginary; whether it is possible for cyclic 

 nitrogen systems to exist we have no means of knowing — all that can be said is 

 that they have never yet been obtained. It is possible, as I pointed out in 1890 

 at the Leeds meeting of the British Association, that mixed diazoamides may be 

 derivatives of such a 4-atom ring. 



Any chemist who has followed the later developments of the chemistry of 

 nitrogen could supply numerous other instances of undiscovered types. A chapter 

 on the imknown compounds of this element would furnish quite an exciting addi- 

 tion to many of those books which are turned out at the present time in such 

 profusion to meet the requirements of this or that examining body. I have 

 selected my examples from these compounds simply because I can claim some of 

 them as personal acquaintances. It would be easy to make use of carbon com- 

 pounds lor the same purpose, but it is unnecessary to multiply details. It has 

 frequently happened in the history of science that a well-considered statement of 

 the shortcomings of a theory has led to its much-desired extension. This is my 

 hope in venturing to point out one of the chief deficiencies in the structural 

 chemistry of the present time. I am afraid that I have handled the case badly, 

 but I am bound to confess that I am influenced by the same feelings as those 

 which prevent us from judging an old and well-tried friend too severely. 



The theory of types to which we have reverted as the outcome of the study of 

 molecular structure is capable of almost indefinite extension if, as there is good 

 reason for doing, we replace atoms or groups by their valency analogues in the 

 way of other atoms or groups of atoms. The facts that in cyclic systems N can 

 replace CH (benzene and pyridine), that 0, S, and NH are analogues in furfurane, 

 thiophene, and pyrrole, are among the most familiar examples. The remarkable 

 iodo- and iodoso-compounds recently discovered by Victor Meyer and his colleagues 

 are the first known instances in which the trivalent atom of iodine has been shown 

 to be the valency analogue of nitrogen in organic combination. Pushing this 

 principle to the extreme we get further suggestions for new groupings, but, as 

 before, no certainty gi prevision. Thus, if nitrogen formed the oxide N^O^ the 

 series might be written : 



^\r> n/^\r> N:0 n/N-O ^ /N : o . 



• • >0 0<^ ■ >0 or . 0< • . or 0< ^.^ . ,-, &c. 



n/ \n/ N:o \n_o \N:0 



Of course these formulae are more or less conjectural, being based on valency 

 only. But since nitrous oxide is the analogue of hydrazoic acid, they hint at the 



» Ber., 27, 898. - Hid., 26, 1263 » Curtius, Ber., 26, 407. 



f Ber., 17, 182. » Ibid., 25, 3175. « Ibid., 21, 3422. 



