October 13, 192 1] 



NATURE 



219 



one or to the other it is possible to bring- the 

 majority of compounds within the scheme- This 

 possibility, whilst making- it easy to find an ex- 

 planation for a variety of facts, is an obstacle to 

 the establishment of the theory on a firm physical 

 basis. However, Dr. Langmuir is engag^ed on 

 the quantitative examination of the consequences 

 of the suggested distribution of electrons, and the 

 progress made since the original publication in 

 1919 is so great that a still closer approximation 

 to chemical facts may be exp>ected with some con- 

 fidence. One theoretical prediction, that of the 

 salt-like character of lithium hydride, was men- 

 tioned as having been confirmed by experiment, 

 and this confirmation is of some importance. 



Unlike the Bohr atom, which is based mainly 

 on the physical study of radiation, the Langmuir 

 atom finds its chief justification in its power of 

 accounting for chemical facts, and its 

 presentation is almost entirely non-mathe- 

 matical, so that it should make a si>ecial 

 appeal to chemists. However, the discussion 

 which followed the opening address approached 

 the subject mainly from the point of view of 

 physics and electro-chemistry, few chemists 

 having yet considered in detail the bearing of the 

 theon.' on organic or structural chemistry- The 

 awakening of a greater interest in the subject of 

 valency among chemists is likely to be one of the 

 most useful results of the discussion, since the 

 unsatisfactory position of the existing theories is 

 well known, esp>ecially in regard to inorganic com- 

 pounds, and the difficulties have usually to be 

 evaded rather than met in the presentation of the 

 subject. On Dr. Langmuir's view, it is incorrect 

 to write structural formulae for inorganic com- 

 pounds such as salts, acids, or silicates, by using 

 the same system of bonds as for organic com- 

 pounds, electro-valency, which is represented by 

 an electron passing from the sheath of one atom 

 to that of another, being essentiallv different from 

 co-valency, represented by the sharing of a 

 " duplet " of electrons by two atoms. This dis- 

 tinction is an important feature of the theor}- , and 

 is very ingeniously applied in the explanation ot 

 the structure of molecules of diverse kinds. 



Prof. Smithells, whose interest in the matter is 

 that of a chemist, exhibited a series of models of 

 atoms and molecules which went far to make the 

 suggested arrangements of electrons clear to the 

 audience, models or photographs of models being 

 almost essential to an elementary- explanation. 

 Prof. W. L. Bragg- showed how the X-ray 

 analysis of crystal structure leads to the result that 

 each atom may be regarded as occupying a 

 sphere of measurable and constant radius, a con- 

 clusion which is entirely in harmony with the 

 structure of the Langmuir atom. On the other 

 hand, he mentioned new evidence of an important 

 character, derived from the diffraction of homo- 

 geneous X-rays by atoms. As X-ray analysis may 

 be used to determine the position of atoms in a 

 space lattice, so a further refinement of the 

 method leads to conclusions as to the arrangement 

 of the electrons within the atom. The results 

 NO. 271 I. VOL. I08I 



appear to show that most of the electrons are 

 clustered together in the inner portion of the 

 sphere occupied by the atom, that is, close to the 

 nucleus, whereas the Langmuir structure, as at 

 present assumed, requires that the number of 

 electrons should be greatest in the outer shells. 



Another diflSculty arises from the static char- 

 acter of the Langmuir atom. Prof. Partington 

 brought forward evidence from the molecular 

 heats, which may be derived theoretically from 

 a dynamic atom of the Bohr type, but are 

 inconsistent with the arrangement which has been 

 adopted to account for chemical valency. Sir 

 Oliver Lodge, however, w^elcomed the remark of 

 Dr. Lang^muir that a static arrangement was not 

 a fundamental condition of the theory, and sug- 

 gested that chemists and physicists would find a 

 dynamic atom the most suitable for the explana- 

 tion of both classes of phenomena. A way out 

 of the difficulty was suggested by Dr. E. K. 

 Rideal, his view being that the atoms might be 

 regarded as static except during- the actual emis- 

 sion or absorption of energy, oscillation of the 

 electrons under those conditions being more pro- 

 bable than rotation. Members of an assemblage 

 of apparently like molecules differ in reactivity, 

 this difference being attributed to an alteration of 

 the position of one of the valency electrons 

 relatively to the nucleus. This view awaits ex- 

 perim.ental confirmation. 



Direct evidence in favour of the theory was 

 produced by Prof. Rankine from experiments on 

 the viscosity of gases. These lead to values for 

 the dimensions of the chlorine molecule, for 

 instance, corresponding with two argon atoms 

 with their outer electron shells contiguous, and 

 to similar relations between bromine and kryp- 

 ton, and between iodine and xenon. Further, 

 on the assumption of the Lang-muir structures, 

 methane bears to ammonium the same relation as 

 krypton bears to rubidium. Rubidium and 

 ammonium are known from cn,'stallographic evi- 

 dence to have nearly equal molecular volumes, and 

 an atom of krj'pton should therefore have the 

 same volume as a molecule of methane. Deter- 

 minations of viscosity prove this to be the case. 

 Dr. S. H. C. Briggs gave reasons for writing 

 elements as compounds of a nucleus with elec- 

 trons, and for applying the ordinar\- type of equa- 

 tion for dissociation and similar reactions to 

 elements as well as to compounds. 



In the informal discussions which took place 

 among members of Section B after the meeting 

 it was possible to learn somethingf of the attitude 

 of chemists towards the theory. The distinction 

 between electro-valency and co-valency is valuable, 

 and the harmony between the new conceptions 

 of the grouping of atoms and that of the space 

 lattice in crystals is very- attractive. The least 

 satisfacton," aspect of the theory is seen in its 

 application to carbon comp>ound^. The tetra- 

 hedral arrangement of the carbon atoms, con- 

 firmed by the X-ray analysis of the diamond, has 

 not only accounted with wonderful success for the 

 known facts of organic chemistry, but also has 



