March 



1S22] 



SCIENCE 



227 



interesting in (luit they cinphasiKe the view 

 that in dealing witii collision^i between y. par- 

 ticles and livdrogCD nuclei one iiui-t recognize 

 that the inverse square law ol; rcpnl&ion ceases 

 to hold in the innuedia'LO neighborhood or the 

 electric charges carried In- lliese nuclei. What 

 the law of variation of the electric force is 

 very close to an electric charge such as we have 

 in an a particle can not as yet be deduced 

 from the experimental evidence available. It 

 is clear, however, that the electric forces in 

 this region are of great intensity. 



It is of interest to note that Cliadwiek and 

 Bieler have pointed out that their experiments 

 provide the only direct evidence v>-e have as 

 to the size of electrons. Assuming an a pa^i'- 

 ticle to consist of 4 protons and 2 nuclei it can 

 be seen that the dimensions of the model of 

 the cc particle which their experiments have led 

 them to put forward require that the radius 

 of an electron cannot be greater than about 

 4 X lO^ls cm. Hitherto the only information 

 we have had available as to the dimensions of 

 the electron has been that obtained by calcula- 

 tions based on the assumption that its mass is 

 wholly electromagnetic. Such calculations 

 have given the value 2 X 10— i-^ cm. for its 

 diameter. While it is clear that an inverse 

 square law of force does not hold in the 

 region extremely close to a nucleus, the experi- 

 ments of Geiger and Marsden on the angular 

 scattering of alpha particles by gold atoms 

 between 5° and 150° show that it does hold 

 very closely for distance, between 3.1 X 10~'2 

 cm. and 36 X 10— '2 (,m_ from the center of 

 nuclei such as those of gold atoms. In this 

 connection it will be recalled that the agree- 

 ment between the experimental measurements 

 of the X-ray K series spectra and the the- 

 oretical values of Debye-* and Kroo-'' shows 

 that the inverse square law still holds at the K 

 ring of electrons. In the ease of platinum the 

 radius of the K-ring is about 10—'" em. Thus 

 nieasured from any point in the region between 

 3 X IQ- 1^ cm. and IQ- 'f' em. from the nucleus 

 of a heavy atom like gold or platinum, the 

 nuclear charge is equal to the atomic number 



=iDebye, Pliys. Zeit., XVIII, p. 276, 1917. 

 == Kvoo, Fhys. Zeit., SIX, p. 307, 1918. 



and thie law of force is the inverse square. We 

 may therefore conclude that no electrons are 

 present in the region between the nucleus and 

 the K ring. 



This result is of special impoi'tance in con- 

 nection with observations I'eeently made by 

 Barkla-" and White and confirmed to a certain 

 exterit'by Crowther,-" whicli point to the possi- 

 bility of stimulating atoms to emit radiations 

 of wavelengths shorter than those of any of 

 the known K-series. If these experiments 

 should lie corroborated by the I'esults of later 

 woi'k it would appear that we must conclude 

 that these J-rays and j)ossibly, too, the more 

 penetrating gamma rays originate within 

 atomic nuclei and are not produced by dis- 

 turbances of any of the systems of electrons 

 situated within the atoms but outside their 

 nuclei. In this connection it should be pointed 

 out that Richtmyer-*' has failed to find any 

 valid evidence of the existence of X-rays of 

 the J type. 



Vil. The Steuctuee op the Nucleus 



(a) H. particles. 



The study of isotopes which we have briefly 

 outlined above has led to very definite vie^\-s 

 regarding the structure of atomic nuclei. It is 

 clear that all nuclei must be made up of 

 protons and electrons held together by intense 

 fields of force. Direct experimental evidence 

 in support of this view has recently been 

 brought forward by Rutherford-'' and those 

 associated with him."" It is found that when 

 swift alpha particles are made to pass through 

 air or nitrogen a few particles having all the 

 properties of protons are projected forward 

 with velocities which give them a maximum 

 range in air of 40 em. No such long range 

 particles are observed in oxygen or carbon 

 dioxide. When swift alpha particles are made 



-'liBarkla and White, Phil. Mag. (0), XXXIV, 

 p. 270, 1017. 



= ' Crowther, Phil. Mag., (0), Vol. 42, p. 719, 

 IsoY., 1921. 



^s Eiehtmyer, Fhys. Eev., p. 433, March, 1921. 



-'•' Eutherf ord, Bakeriaii Lecture, Proe. Eoy. 

 Soc. (London), A., Vol. 97, p. 375, 1920. 



so Rutherford and Chadwick, Phil. Mag., S. 6, 

 Vol 42, p. 809, Nov., 1921. 



