September 2, 1909] 



NATURE 



285 



degree of concentration — presumably at thi- point at whicli 

 the two forms of the distributed materials most nearly 

 balance — is also in no way identical with maximum mole- 

 cular hydrolytic activity. On these assumptions not a few 

 of the deductions based on the ionic dissociation hypothesis 

 are clearly fallacious. 



It has been asserted that the association hypothesis does 

 not admit of quantitative treatment, and that therefore it 

 is at a disadvantage ; but if the quantitative meaning given 

 to various results in accordance with the tenets of the 

 dissociation hypothesis be more often than not one which is 

 inadmissible, little is gained by applying the speculation 

 quantitatively. As already remarked, the only cases in 

 which chemical and electrolytic activity can be compared by 

 the methods proposed are those in which the comparison 

 is made between solutions of comparable or equivalent 

 strength — that is to say, between compounds arranged in 

 vertical series in the order of their activity.' Electrolytes 

 are comparable in most, if not in all, their properties when 

 the comparison is made in this way ; but order of activity 

 is one thing, actual activity another. It is in this sense, 

 and this sense only, that we may agree with Arrhcnius in 

 his statement, " L'activit^ ^lectrolytique se confond avec 

 I'activit^ chimique." 



The ionic dissociation hypothesis is a beautiful mare's- 

 nest, which fails apparently to fit the facts whenever it is 

 examined. "And the moral of that is," to quote the 

 words of the classical Duchess so well known to children, 

 " we must not use the words ion and ionisation in any 

 speculative sense but confine their application to cases 

 such as were contemplated by Faraday when he introduced 

 the term ion ; the conception of activity, whether electro- 

 lytic or chemical, should alone be attached to such words ; 

 no idea of actual, separate, individual existence should 

 enter into our minds in using them : the ion is to be 

 thought of merely as the potentially active, transferable 

 radicle in a compound, not as a separated particle enjoying 

 independent existence." It is so easy to speak of dissocia- 

 tion when it is desired to give expression to the idea ; the 

 first thing the scientific speaker or writer should guard 

 against is ambiguity. 



The subject of gaseous interchanges must not be left out 

 of account, although it is impossible to do justice to it. 

 Mendelejeff's contention that gaseous interchanges are 

 usually bimolecular has been defended by Dixon and 

 Larmor of late. But the facts must be faced. The almost 

 inconceivable frequency of the molecular impacts must not 

 be forgotten. The extraordinary attractive power of the 

 hydrone molecule is also to be remembered — this would 

 tend to promote the formation of aggregates with which 

 the necessary third substance w^ould every now and then 

 form a bimolecular system — which, however, would in 

 realitv be at least trimolecular. The proportion of hydrone 

 molecules in a dried gas has probably been under-estimated 

 — the density of hydrone being very low (q) — as no dehy- 

 drating agent can be supposed to remove all such molecules 

 or even nearly all ; the hydrated substance must have a 

 certain pressure of dissociation. Sir James Dewar's appears 

 to be the only method which is in any way deserving of 

 the epithet absolute ; the results he has obtained with 

 helium in a radiometer are strongly in favour of my view. 

 Lastly, the gradual growth in velocity of the explosive wave 

 up to the point of detonation as the compression becomes 

 greater is clear indication that reduction in volume and 

 increase of opportunity for the formation of systems of the 

 proper degree of complexity is a matter of great conse- 

 quence. Even the behaviour of cordite is significant, par- 

 ticularly the projection of unburnt rodlets of the material 

 from the gun : apparently it is not decomposed by shock 

 intramolecularly but is decomposed by heat into gases, 

 which interact explosively. 



Having dealt with the subjects of chemical change and 

 the nature of solutions, however inadequately, I must now 



1 Solutions of acids and alkalies have maximum conducting power at cer- 

 tain relatively hijjh degrees of concentration. Hydrolytic activity also in- 

 creases steadily in the case of acids as the solution becomes more concen- 

 trated ; whether it attains to a maximum and whether this coincides with the 

 conductivity maximum is uncertain at present ; it is very difficult to decide 

 this point experimentally, as the rate of change is so rapid in strong solu- 

 tion ; moreover, the action takes another course in strong solutions, as com- 

 pounds are formed by the interaction of the hydrolyte and hydrolist, so that 

 two changes are superposed which cannot well be followed separately. 



NO. 2079, VOL. 81] 



endeavour to justify my opening reference to the importance 

 of the organic side of our science. 



The province of organic chemistry is so vast that it mav 

 appear to be difficult- to distinguish the main lines of 

 advance from the by-paths which intersect the field of 

 inquiry in every direction. In reality this is not the case; 

 certain salient features stand out which inust attract atten- 

 tion if once attention be directed to them. The efforts of the 

 chemist to elucidate structure and to correlate structure 

 with function have been extraordinarily successful. In the 

 first place, as already remarked, the student of the subject 

 now has his attention concentrated on the tetrahedron as 

 the symbol of the functional activity of carbon ; however 

 numerous the compounds, he knows that certain simple 

 rules can be laid down as applicable to all. It is estab- 

 lished beyond question that carbon atoms have a remark- 

 able tendency to form ring systems. The affinities of the 

 atom seem to act almost rigidly in certain directions, 

 which appear to be those of lines drawn froin the middle 

 point of a regular tetrahedron to its angular points. 

 Rings containing either five or six atoms of carbon are 

 therefore those which are most readily formed and of 

 maximum stability ; carbon atoins may and do unite in 

 pairs, threes and fours, but compounds of this order are 

 far less permanent than those containing either five or six 

 atoms, as the affinities appear to meet in such a manner 

 that they do not satisfy each other and consequently the 

 compounds enter somewhat readily into combination with 

 other substances. When the number of carbon atoms 

 exceeds six, not only is there less tendency to form a ring 

 system but the stability of the system is slight ; when the 

 number is considerable, stability is attained by the forma- 

 tion of complex systems, consisting of several rings con- 

 joined (camphor, naphthalene, anthracene, Sec). 



The behaviour of carbon compounds generally, in so far 

 as this may be regarded as dependent on the condition of 

 the carbon, is extraordinarily simple, and may be summed 

 up in the statement that it is either paraffinoid, ethenoid or 

 benzenoid.' Paraffinoid carbon is incapable of combining 

 with other substances and but slightly attractive, so that 

 the hydrogen atoms are by no means easily displaced from 

 paraffinoid compounds ' ; ethenoid carbon combines readily 

 with various substances, forming compounds of paraffinoid 

 type : in the benzenoid state, carbon appears to combine 

 somewhat readily with a variety of substances, but the 

 products enjoy only an ephemeral existence and usually 

 escape notice, as they at once break down, giving rise to 

 benzenoid substitution derivatives, so that in this last form 

 carbon simulates the paraffinoid state but is more active. 



In earlier years our attention was concentrated on 

 benzene and the benzenoid compounds ; much was done 

 to elucidate the structure of these hydrocarbons and of 

 their derivatives ; meanwhile these latter have proved to be 

 of extraordinary significance technically, notably as dye- 

 stuffs, but also on account of their medicinal value, as 

 perfumes and in photography. 



The structure of benzene has been the subject of much 

 discussion during the period under consideration. I trust 

 I shall not be accused of parental bias if I urge that the 

 centric ■* formula is the best expression of the functional 

 activity of 'the hydrocarbon benzene and its iminediate 

 derivatives ; the attempts which have been made of late 

 years to resuscitate the KekuM oscillation hypothesis in 

 one form or another appear to me to be devoid of practical 

 significance. Any formula which represents benzene as 

 an ethenoid must be regarded as contrary to fact. But in 

 considering the properties of benzenoid compounds gener- 

 ally, it is necessary to make use of the Kekul6 conception 

 as well as the centric expression. The model of benzene 

 devised by Barlow and Pope subserves a somewhat different 

 and complementary purpose, being primarily of importance 



1 A fourth condition requiring recognition is that of the carbon in acetyl- 

 ene ; at present the acetylene compounds are so few in number, however, 

 that this form may be left out of account. 



- The displacement of the hydrogen associated with carbon is in all 

 probability a secondary' phenomenon ; it is likely that this is true generally 

 and that hydrogen is never merely removed or attracted away but always has 

 its place taken by a radicle which becomes temporarily attached to the 

 multivalent atom with which the hydrogen is associated. 



3 I have discussed this matter somewhat fully in a recent essay, with 

 reference to the nature of amorphous carbon, in connection with the remark- 

 able work of Sir James Dewar on the absorption of gases by charcoal at low 

 temperatures {Journal of the Royal litstititticn). 



