24 



PROFESSOR A. FOWLER, F.R.S., ON 



which matter can exist ; and that these are the atomic structures 

 from which all the heavier elements have been derived by an 

 evolutionary process which must be the exact converse of radio- 

 activity. Since the modifications of atoms which occur in radio- 

 active processes consist of the expulsion of a and j3 particles 

 from the nuclei of the atoms, it may be supposed that change of 

 stellar condition proceeds by a progressive modification of the 

 atomic nuclei. Prof essor Nicholson has ventured so far as to 

 give the name "Archonium" to the hypothetical element which 

 he holds responsible for a strong nebular line in the ultra-violet 

 (wave-length 3729), and has even calculated its atomic weight 

 as 2*95. Very substantial support of his deductions is afforded 

 by the work of Fabry and Buisson in quite a different direction, 

 their measurements of the breadth of the line in question having 

 led them to a value also approximating to 3 for the atomic 

 weight of the element producing it. For the other hypothetical 

 element, nebulium, the deduced atomic weight is 1"3. 



Professor Nicholson has further found that some of the 

 unknown lines which are observed in the Wolf-Iiayet stars may 

 be calculated from some of the constants relating to nebulium, 

 such lines representing atoms which are the first products of an 

 evolution from the still simpler atoms of the nebulae. Hydrogen, 

 and perhaps helium, may be the next in order of development. 



The spectrum of a nebula has been aptly described by 

 Professor Nicholson as the " spectrum of chaos." He tells us 

 that, whatever may be the case with terrestrial atoms, the 

 electrons in a nebula are not held firmly in the atoms, and that 

 a continual interchange of electrons must be taking place, with 

 a necessary bombardment of atoms by free electrons, to which 

 the luminosity of nebulae is probably due. The physical state of 

 a nebula must be analogous to that of a highly exhausted 

 vacuum tube of enormous extent. 



If time had permitted I should have referred to the changing 

 spectra of " new stars," which seem to me to give pretty definite 

 evidence that, in spite of its simple spectrum, a nebula either 

 actually contains such substances as iron or the materials from 

 which such elements can be evolved. 



The subject of this lecture is a very large one and could hardly 

 be dealt with adequately in the course of an hour. I hope, how- 

 ever, that I have been able to give some indication of the nature 

 of the evidence in favour of celestial evolution, and of the way 

 in which the efforts of astronomers, experimentalists, and 

 mathematical physicists have been combined in the attempt to 

 trace its course. 



