September 12, 1895] 



NA TURE 



479 



merits of the two methods; both are necessary for the develop- 

 ment of our science. All methods of attacking the unknown are 

 equally welcomed. In some cases physical methods are avail- 

 ;ible, in other cases purely chemical methods have alone been 

 found of use. There is no antagonism, liut co-operation. If the 

 results of the two methods are sometimes at variance it is simply 

 liecause we have not known how to inter])ret them. The physical 

 chemist has adopted the results of the ai^plication of chem- 

 ical methods of determining "constitution," and is endeavouring 

 10 furnish us with new weapons for attacking this same problem. 

 The chemist who is seeking to unravel the architecture of mole- 

 cules is dependent at the outset upon physical methods of deter- 

 mining the relative weights of his molecules. The worker who 

 is bringing about new atomic groupings is furnishing material for 

 the further development of generalisations from which new 

 methods applicable to the problem of chemical structure may 

 again be evolved. The physical chemist sometimes from the 

 broadness of his view is apt to overlook or to minimise the im- 

 portance of cliemical individuality. On the other hand the 

 chemi.st who is studying the numberless potentialities of combina- 

 tion resident in the atoms, and who has grasped to the full 

 extent their marvellous individualities, is equally liable to forget 

 that there arc connecting relationships as well as specific 

 differences in the properties of elements and compounds. These 

 are but the mental traits — the unconscious bias engendered by 

 the necessar}' specialisation of work to which I have referred, 

 and which is observable in every department of scienti.lc labour. 



The Present State of Structural Chemistry. 



The success attending the application of the doctrine of valency 

 to the compounds of carbon has helped its extension to all com- 

 pounds formed by other elements, and the student of the present 

 day is taught to use structural formula; as the -\ B C of his 

 science. It is, I think, generally recognised among chemists 

 that this doctrine in its present state is empirical, but it does not 

 appear to me that this point is sufficiently insisted upon in 

 chemical teaching. I do not mean to assert that for the last 

 thirty years chemists have lieen pursuing a phantom ; neither do 

 I think that we should be justified in applying to this doctrine 

 the words applied to its forerunner, the "types" of Gerhardt, 

 i>y Lothar Meyer, who says that these "have rendered great 

 service in the development of the science, but they can only be 

 regarded as a part of the scaffolding which was reinoved when 

 I he erection of the system of organic chemistry had made sufii- 

 c ient progress to be able to dispense with it " {" Modern Theories 

 "fChcmistr)-," p. 194.) It appears to me, on the contrary, that 

 there is a physical reality underlj-ing the conception of valency, 

 if for no other reason because of the conformability of this 

 property of the atoms to the periodic law. But the doctrine as it 

 stands is empirical in so far that it is only representative and not 

 explanatory. Krankland and Kekule have given us a great truth, 

 but its \ery success is now making it more and more obvious that it 

 is a truth which is pressing for further development from the phys- 

 ical side. If weare asked why CO exists, and why Cll„and CCU 

 ilo not, together with innumerable similar questions which the 

 inquisitive mind will raise, vvc get no light from this doctrine. If 

 any over-.sanguine disciple goes so far as to assert that all tlie 

 possible compounds of the elements indicated by their valency 

 are capable of existence, and will sooner or later be prepared, 

 he will, I imagine, find himself rapidly travelling away from the 

 1 cgion of fact. 



There is something to be reckoned with besides valency. The 

 I 'ue great desideratum of modern chemistry is unquestionably a 

 physical or mechanical interpretation of the combining capacities 

 ■ if the atimis. Attempts at the con.struction of such theories 

 have liccn made, and thus far only in a tentative way, and the.se 

 I lews cannot be said to have yet come within the domain of 

 practical chemical politics. I have in mind, among other sug- 

 gestions, the dynamical theory of van "t Ilofl' published in 18S1 

 I " Ansichten iiber die organischeChemie"), the theor)- of electric 

 I harges on the atoms broached by Johnstone .Stoney in 1874, 

 and so ably advocated by the late Prof. v. Ilelmholtz in his 

 laraday lecture in 1S81, and the electric polar theory of \'ictor 

 Meyer and Riecke, published in iS88 (" Einige Bemerkungen 

 ubcr den Kohlenstoffatom und die Valenz," Her., 21, pp. 946, 

 1620). 



Pending the rationalisation of the doctrine of valency its jiro- 

 inulgation must continue in its present form. Its services in the 

 ennstruction of rational formuke, especially within the limits of 

 isomerism, have been incalcidable. It is the ladder by which 



NO. 1350, VOL. 52] 



we have climbed to the jiresent brilliant achievements in chemica 

 synthesis, and we are not in a position to perform the un- 

 gracious task of kicking it aw.ay. In recalling attention to its 

 weaknesses I am only putting myself in the position of th 

 physician who diagno.ses his patient's case with the ulterior 

 object of getting him strengthened. There can be no doubt that 

 renewed vitality has been given to the doctrine by the concep- 

 tions of tautomerism and desmotropy, formulated by Conrad 

 Laar in 1885, and by Paul Jacobson in 1887. The importance 

 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 formuloe 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 Be 

 and van 't Hoff by the introduction of the .stereochemical 

 hypothesis in 1874 — unquestionably the greatest advance in 

 structural chemistry since the recognition of the quadrivalent 

 character of the carbon atom. If evidence be required that 

 there is a physical reality underlying the conception of valency, 

 we need only point to the close accordance of this notion of the 

 a.symmetric carbon atom with the facts of so-called " phj'sical 

 isomerism " and the sjilendid results that have followed from its 

 introduction into our science, especially in the field of carbo- 

 hydrates through the investigations of Emil Fischer and his 

 pupils. In other directions the stereochemical hypothesis has 

 ])roved to be a most suggestive guide. It was applied by Prof, 

 v. Baeyer in 1885 (Bcr., 18, '2277) 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 18S8 this eminent 

 chemist showed its fertility in a series of brilliant researches upon 

 benzene derivatives ( Ann., \-yi, 158, and subsequent papers). 

 Nor can I omit to mention the great impetus given in this field 

 by the classical work of Wislicenus, who in 18S7 applied the 

 hypothesis to unsaturated compounds and to cyclic systems with 

 remarkable success ( " Ueber die raumliche Anordnung der .A.tome 

 in organischen Moiekulen," &c.). Quite recently Victor Meyer 

 and J. Sudborough have shown that the ability of certain deriva- 

 tives of benzoic and naphthoic acids to form ethers is governed 

 by stereochemical considerations (Bar., 27, 510, 1580, 3146, and 

 28, 182, 1254). But I must avoid the temptation to enlarge 

 uiion this theme because the whole subject has been recently 

 brought together by C. A. Bischofl" in his " Handbuch der 

 Stereochemie" (Frankfurt, 1893-94), 1 work to which all who 

 are interested in the suliject will naturally turn for reference. 



While the present advanced state of structural chemistry may 

 thus be looked upon as the outcome of the conce])tions of 

 Frankland and Kekule, it may be well to bear in mind that the 

 idea of structure is not necessarily hnanA u]) with the hypothesis 

 of valency in its present form. In leed, 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 together in a par- 

 ticular way is really independent of any theory of valency. It is 

 only after this evidence has been acquired, cither by analysis or 

 synthesis, that we proceed to apply the hypothesis 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 con- 

 vert an empirical into a rational fiirmula ; but this method 

 generally gives us a choice of formuKv 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 hyiwthesis 

 are actually capable of existence. There is, for example, 

 evidence that some of the " iX)sition isonierides" among the 

 derivatives of mono- and poly-cyclic compounds are too unstable to 

 exist ; a fact which in itselt 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 eft'orts such compounds 

 have never been obtained. The conditions essential for the 

 existence of these compounds 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 circum- 

 stances, as Zincke has shown (Ber., 20, 1776), tetrachlor and 

 tetrabrom-orthobenzoquinone are stable compounds. So also the 

 interesting researches of Xietzki have proved that in such a com- 

 pound as rhodizonic acid (/bid., 19, 308, and 23, 3136) ortho- 



