December 2nd, igij.] Proceedings. xi 



the result of a single encounter with another atom. It was 

 deduced that the number of electrons and consequently the 

 charge on the nucleus was numerically equal to about half the 

 atomic weight. A brief account was given of later experiments 

 in support of this view of the structure of the atom. In an 

 important series of experiments Geiger and Marsden have 

 shown that the large angle scattering of a particles is in very 

 close agreement with this assumption of the constitution of 

 the atom. In particular, they showed that the variation of the 

 number of a particles scattered through different angles by 

 dirterent elements agreed closely with the theory over a range 

 in number of nearly one million times. The deflection of the 

 o particle is due to its passage close to the intense field of the 

 nucleus. As a result of such a close encounter, the atom with 

 which the a particles collides is set in motion, the velocity 

 depending upon its mass. Special interest attaches to the 

 scattering of « particles by the passage through a light gas like 

 hydrogen, since it is to be expected theoretically that a small 

 fraction of the hydrogen atoms should acquire a velocity even 

 greater than the a particle itself. This question is now under 

 experimental examination by Mr. Marsden, and he has found 

 definite evidence that some of the hydrogen atoms actually 

 acquire such a great velocity by their encounters with a i)articles 

 that they are able to travel through hydrogen at least three 

 times the distance of the a particle itself through the same gas. 

 These swift hydrogen atoms are detected by then- property of 

 producing scintillations in a zinc sulphide screen. 



On the nucleus theory it is supposed that the hydrogen atom 

 contains one posidve charge and the helium two. The number 

 of hydrogen atoms which are set in rapid motion is of about the 

 order to be anticipated from this point of view. There is one 

 very interesting deduction that can be made from such experi- 

 ments. It was pointed out two years ago that the diameter of 

 the nucleus even of a heavy atom like gold was exceedingly 

 small, viz., about lo"^- cms., compared with the ordinary 

 accepted diameter of the atom, viz., lo"® cms. In order to 



