1932] The Search for the T'ltimate Atom 39 



misinterpretation of results Rutherford proved that the spectrum of 

 the alpha (a) particles was the spectrum of helium. Thus he clearly 

 demonstrated that alpha (a) particles are helium atoms which have 

 lost two electrons. 



While still somewhat removed from a complete theory of atomic 

 structure, we can assert with positiveness that the atom is a struc- 

 ture and, in the case of the heavy atoms, a decidedly complicated 

 one. For the light it has thro^vn on the nature of this structure 

 the radioactive compounds have been an invaluable aid. The enor- 

 mous velocities with which alpha (a) and beta (fS) particles are 

 ejected from the nucleus shows the atom is a vast store-house of 

 energy. That the thirty odd radioactive substances are the disinte- 

 gration products of uranium and thorium, that they are in a constant 

 state of disintegration by virtue of forces within the atom, and that 

 lead is the final product of disintegration seems now the general con- 

 clusion from experimental data. 



The earliest determination of — was made in 1897 by J. J. Thom- 



111 



son and Wiechert independently. Thomson's first determination was 

 for the cathode rays, which Crookes claimed to be negatively charged 

 particles. He extended his investigations to the negative ions given 

 off by metal plates when exposed to ultra-violet light, also to the 

 negative ions produced bj- an incandescent carbon-filament in an 

 atmosphere of hydrogen. He found that the value of — for these 

 was the same in all cases, that it was a constant quantity independent 

 of the nature of the gas from which they were produced and of the 

 means used to produce them. These were epoch-making experiments 

 in the development of the present atomic theory. It was with amaze- 

 ment the rank and file of us learned from Thomson that he had dis- 

 covered corpuscles whose mass was not greater than one thousandth 

 that of the hydrogen atom. The first estimate was subsequently re- 

 vised by him to about one eighteen hundredth the hydrogen atom. 

 Bucherer's value, now considered the most accurate, makes the mass 

 of the hydrogen atom 1845 times that of the electron, Thomson's 

 corpuscle. These experiments of Thomson's were followed by many 

 others, some performed by himself, others by his pupils, in the develop- 

 ment of the electronic theory. In the development of this theory the 

 leading role has clearly been played by the Cavendish laboratory, 



