By JOHN CANNELL GAIN.ALTHOUGH the long controversy between Hantzsch and Bamberger on this subject apparently came to a n end some few years ago, and many chemists have accepted the views of the former, yet it would appear that there is still some uncertainty of opinion with regard to the constitution of these substances.The stereochemical hypothesis of the constitution of the metallic diazo-derivatives (also cyanides, stc.), so stoutly advocated by Hantzsch, has been sharply criticised by Bamberger, von Pechmann, Blomstrand, and Armstrong, but t'he re-introduction of Blomstrand's formula for the diazo-salts by Bamberger in 1895 has met with hardly any opposition.Although this formula, for example, C,H5*NCliN, advocated, as i t has been, by no less tban three chemists independently (Blomstrand, 1869, Strecker, 1871, and Erlenmeyer, 1874, was rejected on the ground that it did not explain the formation of phenylhydrazine from diazobenzene chloride by reduction as did Kekulk's formula, C,H, *N :NCl, i t must be admitted t h a t the reasons given by Bamberger for discarding the latter were slight. This chemist indeed only advanced the old criticism of Blomstrand that Kekuld's formula was t h a t of a chloride of nitrogen and as such was improbable. Apparently Bamberger was not quite sure as to which of the two formulae, C,H,*NCliN and C,H,*NiNCl, was the more preferable, but finally chose the former. Hantzsch did not at first agree with this view ; he regarded diazobenzene chloride as a syn-diazo-compound, but soon rejected this and agreed with Bamberger in adopting the Blomstrand formula.It is necessary to point out here that Blomstrand's formula was