1915'\ Recent Conceptions of the Atom 123 



electrified corpuscles enclosed in a sphere of uniform positive 

 electrification, such a model atom having been worked out by 

 Lord Kelvin. This recalls Dal ton's atom with its enveloping 

 " sphere of heat." 



The Rutherford atom is the last stage in the evolution of the 

 modern conception of this one time indivisible, ultimate particle. 

 This has the advantage over the Thomson atom of an experi- 

 mental as well as a mathematical basis. 



Study of radio-activity phenomena lead Rutherford to an- 

 nounce his Disintegration, or Successive Transformation hypo- 

 thesis. This has been well worked out and is based upon a large 

 number of observations and experiments. As Rutherford says: 

 " The processes occurring in the radio-active elements are of a 

 character quite distinct from any previously observed in chem- 

 istry." 



" One of the most powerful methods of obtaining informa- 

 tion on the internal structure of the atom," writes Rutherford, 

 " lies in the scattering of high speed particles, for example, of 

 a and P particles, in their passage through matter. In conse- 

 quence of this great energy of motion, a high-speed a or y8 par- 

 ticle must pass through the atom which lies in its path. The 

 deflection of the charged particle from its rectilinear path as the 

 result of an atomic encounter throws light on the intensity and 

 distribution of the electrical forces within the atom to which 

 these deflections are due. One of the most noticeable features 

 of the scattering of a particles by thin films of matter is that a 

 small fraction of the a particles is deflected through angles of 

 more than 90°. If, for instance, the a particles are allowed to 

 strike upon a thin film of gold, some 8,000 pass through, as 

 may be seen by the scintillations given on a zinc sulphide screen. 

 These suft'er a certain amount of deflection or scattering, but one 

 is so much deflected as to return to its source." 



There appears to be no doubt that the large deflection some- 

 times suftered by an a particle is the result of a close encounter 

 with only one atom of matter. The type of atom devised by 

 Kelvin and Thomson, says Rutherford, in which the positive 

 electricity is distributed throughout a sphere of radius com- 



