202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



atomic structure, which I first suggested in 1911. The essential 

 controlling feature of all atoms was found to reside in a very minute 

 central nucleus which carried a positive charge and contained most 

 of the mass of the atom. A relation of unexpected simplicity was 

 found to connect the atoms of all the elements. The ordinary prop- 

 erties of an atom are defined by a whole number, which represents 

 the number of units of resultant positive charge carried by the 

 nucleus. This varies from 1 for hydrogen to 92 for the heaviest 

 element uranium, and, with few exceptions, all the intervening 

 numbers correspond to known elements. 



On this view of atomic structure it was evident that, to bring about 

 the transmutation of an atom, it was necessary in some way to alter 

 the charge or mass of the nucleus or both together. Since the nucleus 

 of an atom must be held together by very powerful forces of some kind, 

 this could only be effected by bringing a concentrated source of energy 

 of some kind to bear on the individual nucleus. The most energetic 

 projectile available at that time was the swift a-particle spontaneously 

 ejected from radioactive substances. If a large number of a-particles 

 were fired at random at a sheet of matter, it was to be expected that one 

 of them must occasionally approach very closely to the nucleus of 

 any light atom in its path. In such a close encounter, the nucleus 

 must be violently disturbed, and possibly under favorable conditions 

 the a-particle might actually enter the nuclear structure. 



This mode of attack upon the nucleus at once proved successful. I 

 found in 1919 that nitrogen could be transformed by bombardment 

 with fast a-particles. The process of transmutation is now clear. 

 Occasionally an a-particle actually enters the nitrogen nucleus and 

 forms with it a new unstable nucleus which instantly breaks up with 

 the emission of a fast proton (hydrogen nucleus) and the formation of a 

 stable isotope of oxygen of mass 17. About a dozen of the light ele- 

 ments were found to be transformed in a similar way. The protons 

 liberated in the nuclear explosions were at first counted by observing 

 the flashes of light (scintillations) produced in phosphorescent zinc 

 sulphide. This method was slow and very trying to the eyes of the 

 observers. Progress, however, became more rapid and definite when 

 electrical methods of counting individual fast particles were developed. 

 These electrical counters, mainly depending on the use of electron 

 tubes for magnifying small currents, have now reached such a stage of 

 perfection that we are able to count automatically individual fast 

 particles like a-particles and protons even though they enter the detect- 

 ing chamber at a rate as fast as 10,000 per minute. By other special 

 devices, we are in like manner able to count individual /3-particles. 

 In this connection, I must not omit to mention that wonderful instru- 

 ment, the Wilson expansion chamber, which makes visible to us the 

 actual tracks of flying fragments of atoms resulting from an atomic 



