72 



NATURE 



[March i8, 1920 



which is assumed in stereo-chemistry if such a 

 reversal of the magnetic axis were accompanied by 

 a change in the nature of the compound. Such modi- 

 fications, however, might conceivably account for muta- 

 rotation. It was thought that in the case of a double 

 bond, such as exists in cinnamic acid, it might be 

 possible to have a larger number of isomerides than 

 would be given by the ordinary theory, but a close 

 examination of the structural formulae based on the 

 cubical atom has shown that (subject to the limitation 

 already referred to) this is not so. 



The view here suggested appears to afTord ajn 

 adequate basis for a theory of optical activity. Such 

 activity arises from a difference effect, and can be 

 manifested only when there is lack of compensation 

 amongst the electrons associated with the various 

 parts of the molecule. If the chemical bond is to be 

 attributed to a ^air of electrons, it is easy to under- 

 stand how such compensation can be brought about 

 in the great majority of chemical compounds. In the 

 case of a single asymmetric carbon atom, the sym- 

 metrical arrangement of each of the four electron 

 pairs is disturbed by the presence of the adjacent 

 groups, resulting in only partial compensation. Thus 

 in the compound Caibcd, the pair of electrons asso- 

 ciated with group a is under the influence of the 

 unlike groups c and d, and, therefore, cannot be sym- 

 metrical. But if c and d are made alike, the whole 

 molecule will have a plane of symmetry indicated by 

 the broken line in the left half of Fig. 3. Thus the 

 molecule will be inactive through " internal com- 

 pensation " with respect to the electrons v^'hich form 

 the outer shell of the carbon atom. I may add that 

 the ring electron, constrained to move backwards and 

 forwards along its linear axis, is admirably adapted to 

 replace the ordinary electron moving backwards and 

 forwards along a spiral path postulated in Drude's 

 theory of rotatory liquids. 



It may be permissible in this connection to 

 emphasise the remarkable success that has attended 

 Langmuir's development of the "octet" theorv, bv 

 means of which it is possible to predicate the physical 

 and chemical qualities of a substance, and even its 

 crystalline structure. Langmuir states that the theorv 

 seems to explain all the cases of stereo-isomerism with 

 which he is familiar. " For example, in the amine 

 oxides, N R, R, R, O, nitrogen is quadricovalent, so 

 that these substances exist as optical isomers, just as 

 in the case of a carbon atom attached to four different 

 groups." Such a compound is, in fact, represented 

 by the diagram already given. H. S. .^llen. 



The I'niversity, Edinburgh, March 2. 



The Principle of Equivalence and the Notion of Force. 



I SHALL be grateful to be permitted to make an 

 inquiry in connection with the principle of equivalence 

 through the medium of the columns of Nature. 



In the recent forms of the theory of relativity it 

 has been asserted that in the neighbourhood of 

 matter we may alternatively conceive the existence 

 either of a field of gravitational force or of a dis- 

 tortion in the space-time continuum, the two con- 

 ceptions being equivalent. The point then arises, 

 however, as to whether, in arranging the body of ideas 

 and propositions constituting physical science in logical 

 sequence, the idea of force (at any rate, " force " in 

 the sense of "action at a distance") or that of dis- 

 torted space-time should be regarded as logicallv prior. 

 The possibility of adopting the idea of distorted space- 

 time as prior, and hence of finally dispensing with the 

 notion of force from the physical scheme, evidently 

 depends on a furthef generalisation of the principle 

 NO. 262q, VOL. 105] 



of equivalence. For a difficulty seems to arise in 

 connection with other forms of action at a distance, 

 such as the forces in a magnetic or an electrostatic 

 field. Apparently we cannot regard these as equivalent 

 to a space-time distortion, for they lack the uni- 

 versality of gravitation, seeing that only bodies of 

 specific types of material are deflected by them. 



I should therefore like to ask two questions : 



(i) Is it possible to extend the principle of equi- 

 valence in any way so as to include all forms of 

 action at a distance? 



(2) If not, is there anything gained, even from a 

 purely logical point of view, by discarding the notion 

 of gravitational force while we are still unable to 

 discard by the same 'tnethod the notion of certain 

 other forces which in many respects exhibit a close 

 analogy to gravitation? C. A. Richardson. 



4 The Crescent, St. Bees, Cumberland, 

 March i. 



Expenses of Scientific Work. 



A JOINT committee of the British Association of 

 Chemists, the Institute of Chemistry, and the National 

 Union of Scientific Workers is putting forward the 

 claim that the following expenses should be allowed 

 as a charge against income in arriving at the assess- 

 ment of those who earn their living either by purely 

 scientific pursuits or by the application of science to 

 industry : — ■ 



(i) Subscriptions to scientific and technical societies 

 and libraries, and to scientific and technical periodicals. 



(2) Purchase and renewal of scientific and technical 

 books, instruments, apparatus, chemicals, and other 

 materials. 



(3) Rent and expenses of study or laboratory. 



(4) Travelling and other expenses incurred in attend- 

 ing scientific meetings. 



• (5) Provision of special clothing for work and re- 

 newal of clothes damaged in the course of employ- 

 ment. 



(6) Other expenses incurred in the course of 

 research. 



.\ form of memorial to be presented to the Lords of 

 the Treasury is being sent to all bodies representative 

 of scientific workers for their consideration and sup- 

 port. 



Some claims for abatements under the above head- 

 ings have already been made by individuals with 

 varying success. I should be grateful for any informa- 

 tion available in support of the petition. 



^ A. G. Church, 



Secretary. 

 National Union of Scientific Workers, 

 iq Tothill Street, Westminster, 

 London, S.W.i, March 15. 



Scientific Reunions at the Natural History Museum. 



I WRITE to correct a small error that has crept into 

 the note on the meeting of the International Council 

 for the Exploration of the Sea which appeared in 

 Nature for March 11. When the members of that 

 council visited the Natural History Museum on 

 March 2 they were entertained, not by the Trustees, 

 but by the Staflf Association, the occasion being a 

 scientific reunion, as was, indeed, stated in a later 

 note in the same issue. I may add that these reunions 

 are held with the approval and permission of the 

 Trustees. G. F. Herbert Smith, 



Hon. Secretary. 



Natural History Museum Staff .\ssociation. 



