226 



SCIENCE 



[Vol. LV, No. 141S 



the centers of the a particles and those of the 

 nuclei of tlie atoms of gold bombavded. This 

 would go to show that in the ease of the 

 nucleus of an atom of gold, its radius is 'prob- 

 ably not greater than 3 X 10—13 gni. Further 

 evidence in this direction has recently been 

 adduced bj' Chadwiok who found that the dis- 

 tance of approach of high speed a. particles 

 to the nuclei of platinum atoms was about 

 7 X 10—12 (>]jj_ and of low speed a particles 

 about 14 X 10-12 em. 



In order to account for the velocity given 

 to hydrogen atoms by collision with a particles, 

 Rutherford calculated that the centers of the 

 nuclei of helium and hydrogen must approach 

 within a distance of 1.7 X 10—13 cms. of each 

 other, assuming the law of repulsion to be that 

 of the inverse srjuare. 



But the recoil phenomena of hydrogen atoms 

 bombarded by a particles cannot be completely 

 accounted for by assuming an inverse squaie 

 law to hold for all distances between the 

 centers of the a particle and the hydrogen 

 nucleus. Rutherford "suggested that roughly 

 they could be explained hj taking the a par- 

 ticle to be the equivalent of a plate of radius 

 3 X 10—12 ejjj_ and assuming that as long as 

 the a particle did not approach within this 

 distance of the hj'drogeu nucleus, the ordinary 

 inverse square law of repulsion held. If, how- 

 ever, the a. particle did approach within this 

 distance of the hydrogen nucleus a collision 

 ensued which swept the latter straight for- 

 wards. 



An attempt was made by Darwin-^ to work 

 out the collision relations for all possible 

 models of the a. particle for which the electric 

 fields would give integrable orbits. As a basis 

 for this work he assumed the a particle to 

 consist of 4 protons and 2 electrons, and found 

 that a square nucleus in which the protons were 

 arranged at the four coi'ners of the square and 

 the two electrons together at the center of the 

 square, would give a field of force ver j^ similar 

 to that of a bipole with collision relations 

 roughly similar to those deduced from Ruther- 

 ford's experiments. 



^1 Danviu, Fhil. Mag., Vol. 41, p. 4S(5, ilareh, 

 1921. 



This model has beau put to the test by Chad- 

 Ariek and Bieler^^- and by McAuley--' in a new 

 series of investigations on collisions between 

 particles and hydrogen nuclei and has been 

 found lo be not entirely satisfactory. In these 

 experiments the earlier oljservations made by 

 Rutherford were oonflrmed, namely, that a 

 l^articles and liydrogen nuclei in collision do 

 not behave as point charges. Not only is the 

 angular distriljution of the projected hydrogen 

 nuclei diii'ereii!, but the numbers projected 

 at small angles are for a particles of high 

 velocity many times greater than those for 

 point nuclei. For example, the observed nnm- 

 l)er of hydrogen nuclei projected within 30° 

 01 the direction of incident a rays of range S.2 

 em. is more than 100 times as great as the 

 theoretical number. The number projected 

 within the : anie angle by a rays of range 4.3 

 cm. is 15 limes the theoretical number. Also 

 the oljserved variation of the numbers of pro- 

 jected hydi-ogen nuclei with velocity of the a 

 particle is in the opposite direction from that 

 given by the point theory. For example, a. 

 rays of I'ange 8.2 eni. project within an angle 

 of 30° nearly 3 times as many hydrogen 

 nuclei as a lays of range 4.3 cm. On the Ijasis 

 of the point charge theory the a rays of 4.3 

 cm. range should give nearly 3 times as many 

 as the 8.2 em. a rays. It would appear, ac- 

 cording to Chadwiek and Bieler, that as a first 

 approximation the a particle Ijehaves in colli- 

 sion with a hydrogen nucleus as a body with 

 properties intermediate betv^een an elastic 

 sphere and an elastic plate, and more like an 

 elastic oblate spheroid of semi axes about 

 8 X 10—13 cm. and 4 X 10—13 cm., respectively, 

 moving in the direction of its minor axis. On 

 this view a hydrogen nucleus jDrojected towards 

 an a particle would move under the ordinary 

 electrostatic forces governed by the inverse 

 square law until it reached a spheroidal sur- 

 face of the above dimensions. Here it would 

 encounter an extremeh' j)owerful field of force 

 and recoil as from a hard elastic body. The 

 deductions made by Chadwiek and Bieler are 



^- Chadwiek and Bieler, P7ii7. Mag., Vol. 42, 

 p. 923, Dec, 1921. 



=3 McAuluy, Plill Mag., Vol. 42, p. S92, Doc, 

 1921. 



