644 , REPORT — 1895. 



especially within the limits of isomerism, have been incalculable. It is the ladder 

 by which we have climbed to the present brilliant achievements in chemical 

 synthesis, and we are not in a position to perform the ungracious task of kicking it 

 away. In recalling attention to its weaknesses I am only putting myself in the 

 position of the physician who diagnoses his patient's case with the ulterior object 

 of getting him stiengthened. There can be no doubt that renewed vitality has 

 been given to the doctrine by the conceptions of tautomerism and desmotropy, 

 formulated by Conrad Laar in 1885, and by Paul Jacobson in 1887. The import- 

 ance of these ideas is becoming more evident with the advancement of chemical 

 discovery. Any attempt to break down the rigidly statical conception of our 

 structural formulae appears to me to be a step in the right direction. Then, again, 

 I will remind you of the prolific development of the doctrine in the hands of Le Bel 

 and van't HoU by the introduction of the stereochemical hypothesis in 1874 — un- 

 questionably the greatest advance in structural chemistry since the recognition of the 

 quadrivalent character of the carbon atom. If evidence be required that there i» 

 a physical reality underlying the conception of valency, we need only point to the 

 close accordance of this notion of the asymmetric carbon atom with the facts of 

 so-called ' physical isomerism ' and the splendid results that have followed from its 

 introduction into our science, especially in the field of the carbohydrates through 

 the investigations of Emil Fischer and his pupils. In other directions the stereo- 

 chemical hypothesis has proved to be a most suggestive guide. It was applied by 

 Professor v. Baeyer in 1885 ^ to explain the conditions of stability or instability of 

 certain atomic groupings, such as the explosiveness of polyacetylene compounds 

 and the stability of penta- and hexa-cyclic systems. Again, in 1888 this eminent 

 chemist showed its fertility in a series of brilliant researches upon benzene deriva- 

 tives.* Nor can I omit to mention the great impetus given in this field by the 

 classical work of Wislicenus, who in 1887 applied the hypothesis to unsaturated 

 compounds and to cyclic systems with remarkable success.^ Quite recently Victor 

 Meyer and J. Sudborough have shown that the ability of certain derivatives of 

 benzoic and naphthoic acids to form ethers is governed by stereochemical consider- 

 ations.'' But I must avoid the temptation to enlarge upon this theme because the 

 whole subject has been recently brought together by C. A. Bischofl" in his ' Hand- 

 buch der Stereochemie ' (Frankfurt, 1893-94), a work to which all who are 

 interested in the subject will naturally turn for reference. 



While the present advanced state of structural chemistry may thus be looked 

 upon as the outcome of the conceptions of Frankland and Kekul6, it may be well 

 to bear in mind that the idea of structure is not necessarily bound up with the 

 hypothesis of valency in its present form. Indeed, some advance had been made 

 in representing ' constitution,' especially by Kolbe, before the formal introduction 

 of this hypothesis. The two ideas have grown up together, but the experimental 

 evidence that in any molecule the atoms are grouped togetiaer in a particular way 

 is really independent of any theory of valency. It is only after this evidence has 

 been acquired, either by analysis or synthesis, that we proceed to apply the hypo- 

 thesis in building up the structural formula. It is of course legitimate to assume 

 the truth of the hypothesis, and to endeavour by its use to convert an empirical 

 into a rational formula ; but this method generally gives us a choice of formulae 

 from which the true one can only be selected by further experimental investigation. 

 Even within the narrower limits of isomerism it is by no means certain that all the 

 modifications of a compound indicated by hypothesis are actually capable of exist- 

 ence. There is, for example, evidence that some of the ' position isomerides ' among- 

 the derivatives of mono- and poly cyclic compounds are too unstable to exist ; a fact 

 which in itself is sufficient to indicate the necessity for a revision and extension of 

 our notions of valency. Thus, by way of illustration, there is nothing in the 

 hypothesis to indicate why orthoquinones of the benzene series should not be 

 capable of existence ; yet it is a fact that in spite of all efforts such compounds 



' Ber., 18, 2277. ^ Ann., 137, 158, and subsequent papers. 



" Ueher die rdumliclie Anordmmr/ der Atome in organischeji, MoleMlcn, &c. 

 ♦ Ber., 27, 510, 1580. 3146, and 28, 182, 1254. ' 



