TRANSACTIONS OF SECTION B. 645 



have never been obtained. The conditions essential for the existence of these com- 

 pounds appear to be that the hydrogen of the benzene ring should be replaced by 

 acid substituents such as oxygen, hydroxyl, chlorine, or bromine. Under these cir- 

 cumstances, as Zincke has shown,' tetrachlor and tetrabrom-orthobenzoquinone are 

 stable compounds. So also the interesting researches of Nietzki have proved that 

 in such a compound as rhodizonic acid ^ orthoquinone oxj'gen atoms are present. 

 But there is nothing in the doctrine of valency which leads us to suspect that these 

 orthoquinone derivatives can exist while their parent compound resists all 

 attempts at isolation. I am aware that it is dangerous to argue from negative 

 evidence, and it would be rash to assert that these orthoquiuones will never be 

 obtained. But even in the present state of knowledge it may be distinctly affirmed 

 that the methods which readily furnish an orthoquinone of naphthalene completely 

 fail in the case of benzene, and it is just on such points as this that the inadequacy 

 of the hypothesis becomes apparent. In other words, the doctrine fails in the fun- 

 damental requirement of a scientific theory ; in its present form it gives us no 

 power of prevision — it hints at possibilities of atomic groupings, but it does not tell 

 us a priori which of these groupings are likely to be stable and which unstable. I 

 am not without hope that the next great advance in the required direction may 

 yet come from the stereochemical extension of the hypothesis, although the 

 attempts which have hitherto been made to supply its deficiencies cannot but be 

 regarded as more or less tentative. 



The New Theoet of Absteaci Types. 



I will venture, in the next place, to direct attention to a modern development 

 •of structural chemistry which will help to illustrate still further some of the points 

 raised. For many years we have been in the habit of abstracting from our struc- 

 tural formultfi certain ideal complexes of atoms which we consider to represent the 

 nucleus or type from which the compound of known constitution is derived. In 

 other words the hypothesis of valency which was developed originally from Ger- 

 hardt's types is now leading us back to another theory of types based upon a more 

 intimate knowledge of atomic grouping within the molecule. In some cases these 

 types have been shown to be capable of existence ; in others they are still ideal. 

 Used in this way the doctrine of valency is most suggestive, but at the same time 

 its lack of prevision is constantly forciug itself upon the attention of chemical 

 investigators. The parent compound has sometimes been known before its deriva- 

 tives, as in the case of ammonia, which was known long before the organic amines 

 and amides. In other instances the derivatives were obtained before the type was 

 isolated, as in the case of the hydrazines, which were characterised by Emil 

 Fischer in 1875, and the hydrazo-compounds, which have been known since 1863, 

 while hydrazine itself was first obtained by Curtius in 1887. Phenylazimide was 

 discovered by Griess in 18G4, and many representatives of this group have been 

 since prepared ; but the parent compound, bydrazoic acid, was only isolated by 

 Curtius in 1890. Derivatives of triazole and tetrazole were obtained by Bladin in 

 1885 ; the types were isolated by this chemist and by Andreocci in 1892. Pyr- 

 azole derivatives were prepared by Knorr in 1883 ; pyrazole itself was not isolated 

 till 1889, by Buchner. Alkyl nitramides were discovered by Franchimont and 

 Klobbie many years before the typical compound, nitramide, NOo.NH.,, which 

 was isolated last year by Thiele and Lachmau.* Examples might be multiplied 

 to a formidable extent, but enough have been given to illustrate the principle 

 of the erection of types, which were at first imaginary, but which have since 

 become real. The utility of the hypothesis is undeniable in these cases, and 

 we are justified in pushing it to its extreme limits. But no chemist, even if 

 endowed with prophetic instinct, could have certainly foretold six years ago that 

 the type of Griess' ' triazobenzene ' would be capable of free existence, and still 

 less that when obtained it would prove to be a strong acid. The fact, established 



k 



Ber., 20, 1776. « Ihid., 19, 308, and 23, 3136. » Ibid., 27, 1909. 



