4i8 



Supplement to '* Nature," Seplemher 15, 1923 



.C ...wl 



..rwl ll.Ic 1,, 



iini,i>s-iKIc l<.r ir)(',e\en it I liad '!i 



inces made dn; , 



1 1 \ I p ' : ! I n ; 1 1 i ' ' I ' : I ' 



in I ! . il in tin- hiiiM 



(if tin i;( li!i:n !ii!' Iiii , i; ';aj)iinents. Il i^ c.i y 



til < .ilriil.itc liuin ihi-, ! i-s that the ciht-x- >ft 



Irrr in tMiiniii!^' one grain i<l In 1: ' !• (■\'<-ii i om 



panel uitli tli;it liberated in th ;niiilc,uratiun 



of one ; radium. Iht ( \aiaple, calculation 



shows th:i' ' 'K'y released in tlic formation of one 



pound of helium gas i> ii|ni\al( nt to tlu- fn(iLy\' emitted 

 in the complete coiuhustion of .ilnriii 1 i:lii tliousand 

 tons of pure airbon. It has been i !i\ lOMinu- 



ton and Pcrrin that it is mainly to ...... .,i>LiR:c of ciiLr^y 



that we must look to maintain the heat emission of the 

 sun and hot stars over loni: periods of time. Calcula- 

 tions of tlir loss of heat from tlu' sun show that this 

 synthesis of liclium need only take place slowly in order 

 to maintain the present rate of radiation for periods 

 of the order of one thousand million years. It must 

 be acknowledged that these arguments are somewhat 

 speculative in character, for no certain experimental 

 evidence has yet been obtained that helium can be 

 formed from hydrogen. 



The evidence of the slow rate of stellar evolution, 

 however, certainly indicates that the synthesis of 

 helium, and perhaps other elements of higher atomic 

 weight, may take place slowly in the interior of hot 

 stars. While in the electric discharge through hydrogen 

 at low pressure we can easily reproduce the conditions 

 of the interior of the hottest star so far as regards the 

 energy of motion of the electrons and hydrogen nuclei. 

 we cannot hope to reproduce that enormous density of 

 radiation which must exist in the interior o£ a giant 

 star. For this and other reasons it may be very difficult, 

 or even impossible, to produce helium from hydrogen 

 under laboratory conditions. 



If this view of the great heat emission in the forma- 

 tion of helium be correct, it is clear that the helium 

 nucleus is the most stable of all nuclei, for an amount 

 of energy corresponding to three or four a-particles 

 would be required to disrupt it into its components. 

 In addition, since the mass of the proton in nuclei is 

 nearly i-ooo instead of its mass 1-0072 in the free state, 

 it follows that much more energy must be put into the 

 atom than will be liberated by its disintegration into 

 its ultimate units. At the same time, if we consider 

 an atom of oxygen, which may be supposed to be built 

 up of four helium nuclei as secondary units, the change 

 of mass, if any, in its synthesis from already formed 

 helium nuclei is so small that we cannot yet be certain 

 whether there will be a gain or loss of energy by its dis- 

 integration into helium nuclei, but in any case we are 

 certain that the magnitude of the energy will be much 

 less than for the synthesis of hehum from hydrogen. 

 Our information on this subject of energy changes in 

 the formation or disintegration of atoms in general 

 is as yet too uncertain and speculative to give 

 any decided opinion on future possibilities in this 

 direction, but I have endeavoured to outline some of 

 the main arguments which should be taken into 

 account. 



I must now bring to an end my survey, I am afraid 

 all too brief and inadequate, of this great period of 

 advance in physical science. In the short time at mv 



!.,..,, I.... 



i ;i|)|ilicd 



Ml partmcnt 

 with that ot 



til mir knowfcd. mulLcr i . • . 



I i. ill. rd nation 1.. ;, but we in.sy 



[)e justly proud that (ireal linlam lias made ni.iri% 

 fundamental contributions. With this rountr>' I n : t 

 |)rii])trl\ ini hide the Dominions overseas, for ' ' 

 lia\c nut 111 I. 'I 1 x-hindliand in tin ir ( untriliutioii • . 

 till- n.-w kf; ■: ••■ 



piidc Lo ihi., .W1...I.J. V...*;. w.. ,"........■ ... 



Dominions have been responsible for some oi 

 fundamental discoveries of this epoch, parti' wi. my m 

 radioa(ti\ ity. 



This tide of ad\anre was continuous from 1896, but 

 there was an int\iialjle slackening during the War. 

 It is a matter of good omen that, in the last ; 

 the old rate of progress has not only been ni 

 but ( \ en intensified, and there appears to be no ob\ : 

 >v^x\ that this period of great advances has come t' 

 end. There has never iieen a tinn wIk n the enthu.-i. : : 

 of the scientific worker-^ \^' - r, .1, r. ^i when then v..;- 

 a more hopeful feel tt advances wer« 



imminent. This feclin- 1- i" inniuL in part due to the 

 great improvement during this epoch of the technical 

 methods of attack, for problems that at 

 seemed unattackable are now seen to be liki 

 before the new methods. In the main, the epoch i. 

 consideration has luen an age of experiment, w 

 the experimenter has been tin- jjioneer in the a*.: 

 on new problems. At the same time, it iias Ix-en .1 i 

 an age of bold ideas in theory, as tlie quantum theon.' 

 and the theory of relati\'ity so well illustrate. 



I feel it is a great prix ilege to ]ia\e witnessed this 

 period, which may almost be termed the renaissance 

 of physics. It has been of extraordinan' intellectual 

 interest to watch the gradual unfolding of new ideas 

 and the ever-changing methods of attack on difTicult 

 problems. It has been of great interest, too. to note 

 the comparative simplicity of the ideas that ha\f 

 ultimately emerged. For example, no one could have 

 anticipated that the general relation between the 

 elements would prove to be of so simple a character as 

 we now believe it to be. It is an illustration of the fact 

 that Nature appears to work in a simple way, and that 

 the more fundamental the prol)leni, often the simpler 

 are the conceptie)ns needed for its explanation. The 

 rapiditv and certitude of the advance in tliis ef)och 

 ha\e largely depended on the fact that it has been 

 possible to devise experiments so that few vari.i' ' -^ 

 were involved. 'For example, the study of the strui; 

 of the atom has been much facihtated by the possibilit\ 

 of examining the effects due to a single atom of matter, 

 or, as in radioactivity or X-rays, of studying processe> 

 going on in the individual atom which were quite 

 uninfluenced by external conditions. 



In watching the rapidity of this tide of advance in 

 physics I have become more and more impressed by 

 the power of the scientific method of extending our 

 knowledge of Nature. Experiment, directed by the 

 disciplined imagination either of an individual, or still 

 better, of a group of individuals of varied mental 



