TRANSACTIONS OF SECTION B. 3G1 



The anomalous dispersion effect takes place at all these lines. Absolute measure- 

 ments have been made by inference methods by Wood and St. Loria in the case 

 of sodium, and from these, if we know the density of the vapour, we can obtain 

 an estimate of the number of electrons taking part in the effect on light, 

 assuming the dispersion theory of Drude to hold. The fact that Drude's theory 

 gives excellent agreement with experiment for all relative measurements is 

 strong evidence in support of its main correctness. Loria obtained the result 

 from his experiments with sodium vapour at 380° C. that the ratio of the total 

 number of atoms engaged in optical effects to the whole number of atoms was 

 400 : 3. I calculated from Wood's experiments at 644° C. that the same ratio 

 was about 12 : 1. This depends on a very rough estimate of the density of the 

 vapour, but is of the right order. From measurements at other lines it appears 

 that each line of the series is probably due to a special set of atoms, so that there 

 are indications that the complexity of a spectrum is not due to complexity of 

 each individual atom, but to differences actually existent in the atom. This 

 view has been in the minds of physicists for some time, and there is hope that 

 work on the lines described may enable us to find out something of the nature 

 of the differences. A good deal of evidence of other kinds leads in the same 

 direction — the Zeeman effect, the magnetic rotation of the plane of polarisa- 

 tion, and so on'; but this is not the occasion to discuss these problems. 



An interesting method of preparing rubidium and caesium may be mentioned. 

 Hackspill found they could be prepared by heating the chlorides with calcium. 

 I found in some early experiments that the metals could be obtained by heating 

 the chlorides with sodium, and later that if lithium be used the vapours are 

 obtained practically pure owing to the much greater temperature required for 

 vaporising lithium than for the other metals. 



5. Optically Active Substances which contain no Asymmetric Atom in the 

 Molecule. By Professor William Henry Perkin, F.R.S., and Pro- 

 fessor William Jackson Pope, F.R.S. 



Whilst pointing out the relation between molecular configuration and optical 

 activity van't Hoff clearly indicated that substances which exhibit optical 

 activity in solution do not necessarily contain an asymmetric atom in the mole- 

 cule ; whilst he pointed out that optical activity results from enantiomorphism 

 of molecular configuration and showed that in general the latter type of enantio- 

 morphism is recognisable by the presence of an asymmetric carbon atom in the 

 graphic formula, he also made it quite clear that cases of optical activity might arise 

 amongst compounds which contain no asymmetric carbon atom in the molecule. 



It is now known that a substance may exhibit optical activity in the amor- 

 phous or dissolved condition if its molecule contains an atom of quinquevalent 

 nitrogen or phosphorus or of tetravalent sulphur, selenium, silicon, or tin. The 

 most general case of optical activity distinguished by van't Hoff, that exhibited 

 by a substance of enantiomorphous molecular configuration, but of which the 

 molecule contains no asymmetric atom, has, however, not been realised until quite 

 recently. 



Van't Hoff referred to substances of the general constitution : — 



\j : C : o( 

 b/ M 



as being capable of existing in two enantiomorphously related molecular con- 

 figurations. The symmetrical dimethylallene, CHMe : C : CHMe, which belongs 

 to this class, should exist in two enantiomorphously related, and hence optically 

 active, isomerides; although this substance has been prepared it is obvious that 

 experimental difficulties will render difficult its resolution into optically active 

 components. 



As the most general case of optical activity is of profound theoretical import- 

 ance, we have for a number of years devoted ourselves to its realisation. After 



