Deflexion of a. Particles through Large Angles. 623 



atoms, but probably also by the electronic charges distributed 

 throughout the remainder of their volumes. 



Summary* 



The experiments described in the foregoing paper were 

 carried out to test a theory of the atom proposed by Prof. 

 Rutherford, the main feature of which is that there exists 

 at the centre of the atom an intense highly concentrated 

 electrical charge. The verification is based on the laws of 

 .scattering which were deduced from this theory. The 

 following relations have been verified experimentally : — 



(1) The number of a particles emerging from a scattering 

 foil at an angle (/> with the original beam varies as l/sin 4 <£/2, 

 when the <x particles are counted on a definite area at a con- 

 stant distance from the foil. This relation has been tested 

 for angles varying from 5° to 150°, and over this range th« 

 number of a particles varied from 1 to 250,000 in good 

 agreement with the theory. 



(2) The number of a particles scattered in a definite 

 direction is directly proportional to the thickness of tbe 

 scattering foil for small thicknesses. For larger thicknesses 

 the decrease of velocity of the a particles in the foil causes a 

 somewhat more rapid increase in the amount of scattering. 



(3) The scattering per atom of foils of different materials 

 varies approximately as the square of the atomic weight. 

 This relation was tested for foils of atomic weight from that 

 of carbon to that of gold. 



(4) The amount of scattering by a given foil is approxi- 

 mately proportional to the inverse fourth power of the 

 velocity of the incident a particles. This relation was tested 

 over a range of velocities such that the number of scattered 

 particles varied as 1 : 10. 



(5) Quantitative experiments show that the fraction of 

 a particles of Ra C, which is scattered through an angle of 

 45° by a gold foil of 1 mm. air equivalent (2*lxl0~ 5 cm.), 

 is 3*7 x 10~ 7 when the scattered particles are counted on a 

 screen of 1 sq. mm. area placed at a distance of 1 cm. from 

 the scattering foil. From this figure and the foregoing 

 results, it can be calculated that the number of elementary- 

 charges composing the centre of the atom is equal to half the 

 atomic weight. 



We are indebted to Prof. Rutherford for his kind interest 

 in these experiments, and for placing at our disposal the 

 large quantities of radium emanation necessary. We are 

 also indebted to the Government Grant Committee of the 

 Royal Society for a grant to one of us, out of which part of 

 the expenses has been paid. 



