SECTION A.— MATHEMATICAL AND PHYSICAL SCIENCES. 



SOME PROBLEMS OF COSMICAL 

 PHYSICS, SOLVED AND UNSOLVED. 



ADDRESS BY 



LORD RAYLEIGH, F.R.S., 



PRESIDENT OF THE SECTION. 



Of the activities of our section, the Cape has perhaps been more identified 

 with astronomy than with any other branch. In the middle of the 

 eighteenth century, when exact astronomy of the southern hemisphere 

 may be considered to have begun, there were few, if any, other places in 

 a considerable southern latitude where an astronomer could work in 

 safety with the necessary help of trained artisans. This tradition worthily 

 begins with Lacaille (1750-1751). Other landmarks were the foundation 

 of the Cape Observatory (1821), the expedition of Sir John Herschel 

 (1833-1838), and the forceful and energetic career of Sir David Gill, who 

 was the life and soul of our organisation on its visit to South Africa in 

 1905. Shortly afterwards he retired, and I then had the privilege of 

 friendship with him in London. Indeed I have taken these few facts and 

 dates from the copy of his ' History of the Cape Observatory,' which he 

 gave me very shortly before his death. Although past his prime at the 

 time I knew him, he was still vigorous and keenly interested in scientific 

 developments ; though, if one brought anything new to his notice, a 

 severe cross-examination as to the validity of the evidence had to be faced. 



It is partly on account of this association of South Africa with 

 astronomy that I have chosen to lean as far towards this direction as I 

 feel able, and to pass in review some subjects lying on the border-line 

 between astronomy and physics. 



After the first period of success in identif jang the origin of the spectral 

 lines of the sun and stars with terrestrial materials certain outstanding 

 cases remained which were obviously important, but in which the 

 identification could not readily be made. 



The first of these cases to yield was that of helium, which was un- 

 ravelled while some of the pioneers in astronomical spectroscopy were 

 still active. Although in my youth I was privileged to see the discovery 

 of helium at close quarters, I shall not go back so far. When we hear of 

 the gas being used in millions of cubic feet for inflating large airships, we 

 have to realise that its discovery is an old story. 



Kindred to the hjrpothesis of helium, so triumphantly vindicated by 

 terrestrial experience, were the hypotheses of nebulium, geocoronium and 

 coronium. The problems epitomised by the two former words have now 

 been solved, though the solution has taken quite a different turn from 

 what was expected by the older generation of astrophysicists. 



