36 Professor Ernest Rutherford [Jan. >\1, 



The a particle decreases in velocity as it passes through matter. 

 Tliis result is clearly brought out by photographs showing the deflec- 

 tion of a homogeneous pencil of a rays in a magnetic field before and 

 after passing through an absorbing screen. The greater divergence 

 of the trace of the a rays on the plate, after passing through the 

 screen, shows that their velocity is reduced, while the sharpness of 

 the band shows that the a particles still move at an identical speed. 



In order to make an accurate determination of the constants of 

 the tt particles, it is necessary to work with homogeneous rays, and we 

 consequently require to use a thin layer of matter of one kind. For 

 experiments of this character, a wire coated with a thin film of radium 

 C by exposure to the radium emanation is very suitable. The 

 velocity of the a particle and the value e/m^ the ratio of the charge 

 carried by the a particle to its mass, can be deduced by observing the 

 deflections of a pencil of a rays exposed in a magnetic and in an 

 electric field of known strengths. The deflection of a pencil of a rays 

 in an electric field is small under normal conditions, and special care 

 is needed to determine it with accuracy. 



In this way I have calculated the velocity and value of ejm for a 

 number of a-ray products. The velocity of expulsion varies foi' 

 different products, but is connected by a simi)le relation with the 

 range of the a particle in air. Tlie value of elm has been determined 

 for selected products of radium, thorium, and actinium, and in each 

 case the same value has been found. This shows that the a particles 

 expelled from radio-active substances in general are identical in con- 

 stitution. They have all the same mass, but differ from one another 

 in the initial \'elocity of their projection. Although we are sure tliat 

 the a particles, from whatever soui'ce, are identical atoms of matter, 

 we are still unable to settle definitely the true nature of the a particle. 

 The value of e/ni found by experiment is nearly 5 x 10=^. Xow the 

 value of eJm for the hydrogen atom in the electi'olysis of water is 10'. 

 If the charge carried by the a particle and the hydrogen atom is the 

 same, the mass of the a j)article is twice that of the hydrogen atom, 

 i.e. a mass equal to the hydrogen molecule. But we are not certain 

 that they do carry the same charge. Here we are, unfortunately, 

 confronted by a number of possibilities, for the magnitude of m for 

 the a particle is conditioned by the value assumed for e. If the 

 charge of the a particle is assumed to be twice the value of the 

 hydrogen atom, the mass comes out four times the hydrogen atom — 

 the value found for the helium atom. The weight of evidence still 

 supports the view that the a particle is in some way (X)nnected with 

 the helium atom. If the a particle is a helium atom Avith twice the 

 ionic charge, we must regard the helium produced by radio-active 

 bodies as actually the collected a particles the charges of which have 

 been neutralised. This at once offers a reasonable explanation of the 

 production of heUum by actinium as well as by radium. In addition, 

 Strutt uas recently contributed strong evidence that helium is a 



