156 Professoi' M. 0. Forster [May 5, 



Thus the physical evidence supports the chemical indications, and 

 serves to classify the triazo-group as a complex radicle with a strong 

 family resemblance to the halogens. 



Bearing this resemblance in mind, it was natural to anticipate the 

 possibility of constructing a composite molecule in which the azoimide 

 nucleus is united with one atom of a halogen, for example chlorine, 

 just as molecular chlorine is composed of two similar atoms. Such a 

 synthesis has been lately accomphshed by Raschig (1909), by the 

 interaction of hypochlorous and hydrazoic acids, 



HCIO + HNg = CIN3 + H^O, 



but the union between the two components is very easily broken, 

 and chloroazoimide is highly explosive. 



Proceeding now to study the behaviour of the triazo-group when 

 placed in an organic environment, it is necessary first to examine the 

 operations by which typical organic azoimides may be prepared. 

 Although too numerous to be even summarised on the present 

 occasion, some points in connection with the changes involved 

 require attention. We have seen that the action of nitrous acid on 

 ammonia sets free completely the nitrogen present in both materials. 

 If, instead of ammonia there is taken its benzene derivative, aniline, 

 in the form of the hydrochloride, nitrogen becomes added to that of 

 the aniline, and although it is true that the product, when heated, 

 gives up all its nitrogen just as ammonium nitrite does, the inter- 

 mediate compound is perfectly stable at the temperature of melting 

 ice : — 



(cold) CgHs-NH^, HCl + HNOo = CgHs-No-Cl + 2H2O 



(hot) CeHs • NH2, HCl + HNO2 = CeHs • OH + N^ + HCl + H,0. 



This intermediate compound and related substances are among the 

 most important, from both practical and theoretical standpoints, 

 known to chemists. They are called diazonium salts, and their 

 discovery, with that of many remarkable changes undergone by them, 

 is due to Peter Griess (1858). It is not the least illuminating feature 

 of Griess's work that the study of the diazo-compounds, underlying 

 as it does the immensely valual)le coal-tar colour industry, was 

 pursued by its author while employed as chemist in a well-known firm 

 of brewers, to whose particular undertaking the work in question had 

 no application ; such far-sighted encouragement of apparently useless 

 research for its own sake is an object lesson which is not, unhappily, 

 without value even in these days. 



Fascinating as are the problems, solved and unsolved, within the 

 boundaries of diazo-chemistry, it would be inappropriate to encroach 

 on them this evening, but the diazo-compounds forming a natural 

 stepping-stone to the triazo -derivatives, it is relevant to draw passing 

 attention to one recent application of their activity. The great class 



