ELECTRICAL, STRUCTURE OF MATTER RUTHERFORD 169 



to be borne in mind that the actual mass of an atom may be somewhat 

 less than the sum of the masses of component positive and negative 

 electrons when in the free state. On account of the very close 

 proximity of the charged units in the nucleus of an atom, and the 

 consequent disturbance of the electric and magnetic field surrounding 

 them, such a decrease of mass is to be anticipated on general theo- 

 retical grounds. 



We must now look back again to the earlier stages of the present 

 epoch in order to trace the development of our ideas on the detailed 

 structure of the atom. That electrons as such were important con- 

 stituents was clear by 1900, but little real progress followed until the 

 part played by the positive charges was made clear. New light was 

 thrown on this subject by examining the deviation of a particles 

 when they passed through the atoms of matter. It was found that 

 occasionally a swift a particle was deflected from its rectilinear path 

 through more than a right angle by an encounter with a single 

 atom. In such a collision the laws of dynamics ordinarily apply, 

 and the relation between the velocities of the colliding atoms before 

 and after collision are exactly the same as if the two colliding par- 

 ticles are regarded as perfectly elastic spheres of minute dimensions. 

 It must, however, be borne in mind that in these atomic collisions 

 there is no question of mechanical impacts such as we observe with 

 ordinary matter. The reaction between the two particles occurs 

 through the intermediary of the powerful electric fields that sur- 

 round them. Beautiful photographs illustrating the accuracy of 

 these laws of collision between an a particle and an atom have been 

 obtained by Messrs. Wilson, Blackett, and others, while Mr. Wilson 

 has recently obtained many striking illustrations of collisions be- 

 tween two electrons. Remembering the great kinetic energy of the 

 a particle, its deflection through a large angle in a single atomic 

 encounter shows clearly that very intense deflecting forces exist 

 inside the atom. It seemed clear that electric fields of the required 

 magnitude could be obtained only if the main charge of the atom 

 were concentrated in a minute nucleus. From this arose the con- 

 ception of the nuclear atom, now^ so well known, in which the heart 

 of the atom is supposed to consist of a minute but massive nucleus, 

 carrying a positive charge of electricity, and surrounded at a dis- 

 tance by the requisite number of electrons to form a neutral atom. 



A detailed study of the scattering of a particles at different angles 

 by Geiger and Marsden showed that the results were in close accord 

 with this theory, and that the intense forces near the nucleus 

 varied according to the ordinary inverse square law. In addition, 

 the experiments allowed us to fix an upper limit for the dimensions 

 of the nucleus. For a heavy atom like that of gold the radius of 

 the nucleus, if supposed to be spherical, was less than one thousandth 



