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SECTIONAL TRANSACTIONS.—A. 335 
defect and is the only possible standard which appears to offer the same chemical 
and physical scale. But this appearance can only rest on the unproved assumption 
that helium is a truly simple element. Helium cannot be used as a practical chemical 
standard for obvious reasons. Its density is not easy to fix and for mass-spectro- 
graph purposes it is too near the low end of the scale. I question if the advantages 
of its adoption would offset the wholesale alteration of all accepted atomic 
weights. 
The only physical data at present available for comparing the weights of atoms 
with high accuracy are band spectra and mass spectra. For the former an accuracy 
of 10° is already claimed, but as they can only be used to compare atoms of the same 
element with each other the general problem can only be attacked by the mass- 
spectrograph. I hope that in a year or two it will be possible to construct an instru- 
ment capable of an accuracy of 10°. From experience gained in fixing the weights 
of over 80 atoms to an accuracy of about 10! it is possible to outline the general 
methods by which the mass scale will be laid down. First by means of the natural 
bracket O18, C2H,, and the close ratio O'8++: C=C : O'H, two independent 
methods, the ratio C!2 : O' will be fixed with the highest certainty. The molecule 
CO, will now be compared with Hg!*+++, which will extend the scale by means 
of Hg++ and Hg* practically as far as necessary in the upward direction. The 
extension downward to hydrogen may be made by the close ratios C? : Ol+ += 
(2++: Het and He‘: H,=H,:H. The accuracy of the final value of H can be 
further checked by the ratio O16 : C!=He‘: H; and the difference between CH, 
and 018, which is the most certain of all comparisons yet available. It is clear that 
for all this work the standard atom O1* is very much the most convenient. It has the 
additional advantages of lying near the geometrical mean of the series of atomic 
weights and at a convenient position on the packing fraction curve. 
To sum up, I am in favour of retaining the present chemical scale unaltered. For 
the vast majority of his practical operations all the chemist requires is a list of numbers, 
accepted internationally, which can be guaranteed to be within 1 part in 1,000 of the 
true atomic weights. For the more fundamental requirements of physics the neutral 
atum of oxygen 16 appears to me to be the best standard. The disadvantage that the 
two scales differ by one or two parts in ten thousand and that this difference will be 
continually subject to revision I do not regard as very serious. The meaning under- 
lying the chemical scale is so completely different from that underlying the physical 
one that confusion should be easily avoided by speaking of ‘the atomic weight 
of chlorine’ in the one case and ‘the weight of the atom of chlorine 35’ in 
the other. 
Tuesday, September 29. 
Discussion on The Evolution of the Universe. (Sir James JEANS, F.R.S. ; 
Prof. E. A. Mine, M.B.E., F.R.S.; Prof. W. pre Sirrer; Prof. 
Sir A. 8. Epprneton, F.R.S.; Prof. R. A. Miztrcan; Rt. Rev. the 
Lorp BisHor or BirmincHam; Gen. the Rt. Hon. J. C. Smuts, 
P.C., C.H.,F.R.S.; M.1’Aspeé Lemafrre ; Sir Oxtver Lopes, F.R.S.) 
See Appendix. 
Wednesday, September 30. 
Dr. G. C. Suvpson, C.B., F.R.S.—The Second Polar Year. 
The year August 1882—August 1883 has come to be called the First Polar Year 
because during that year twelve nations co-operated to send into the Arctic and 
Antarctic regions special expeditions to make observations, chiefly in meteorology 
and terrestrial magnetism, according to a common plan. Great Britain joined with 
Canada and together they sent a party of Royal Engineers to make observations at 
Fort Rae on the Great Slave Lake. In all, twelve expeditions worked in the Arctic 
and Antarctic. The records obtained were extremely valuable and have been the 
foundation for practically all subsequent investigations into the physics of the 
atmosphere and terrestrial magnetism in polar regions. 
As the fiftieth anniversary of this outstanding epoch in the study of physics 
approached, suggestions were made that it should be commemorated in some suitable 
