1919] Atomic Projectiles and their Collisions 5G7 



WEEKLY EVENING MEETING, 



Friday, June G, 1919. 



The Hox. Sir Charles A. Parsons, K.C.B. J.P. M.A. LL.D. 

 D.Sc. F.R.S., Manager and Vice-President, in the Chair. 



Professor Sir Ernest Rutherford, LL.D. D.Sc. F.R.S. M.R.I. 

 (Nobel Laureate). 



Atomic Projectiles and their Collisions with Light Atoms. 



The discovery of radio-activity has not only thrown a flood of light 

 on the processes of transformation of radio-active atoms ; it has at 

 the same time provided us with the most powerful natural agencies 

 for probing the inner structure of the atoms of all the elements. 

 The swift a-particles and the high-speed electrons or ^-rays ejected 

 from radio-active bodies are by far the most concentrated sources 

 of energy known to science. The enormous energy of the flying 

 a-particle or helium atom is illustrated by the bright flash of light 

 it produces when it impacts on a crystal of zinc sulphide, and by 

 the dense distribution of ions along its trail through a gas. This 

 great store of energy is due to the rapidity of its motion, which in 

 the case of the a-particle from radium C (range 7 cm. in air) 

 amounts to 19,000 km. per second, or about 20,000 times the speed 

 of a rifle-bullet. It is easily calculated that the energy of motion 

 of an ounce of helium moving with the speed of the a-particle from 

 radium C is equivalent to 10,000 tons of solid shot projected with a 

 velocity of 1 km. per second. 



In consequence of its great energy of motion the charged particle 

 is able to penetrate deeply into the structure of all atoms before it 

 is deflected or turned back, and from a study of the deflection of 

 the path of the a-particle we are able to obtain important evidence 

 on the strength and distribution of the electric fields near the centre 

 or nucleus of the atom. 



Since it is believed that the atom of matter is, in general, complex, 

 consisting of positively and negatively charged parts, it is to be antici- 

 pated that a narrow pencil of a-particles, after passing through a thin 

 plate of matter, should be scattered into a comparatively broad beam. 

 Geiger and Marsden showed not only that much small scattering 

 occurred, but also that in passing through the atoms of a heavy 

 element some of the a-particles w^ere actually turned back in their 

 path. Considering the great energy of motion of the a-particle, this 



Vol. XXII. (No. 113) 2 q 



