26 SCIENCE PROGRESS 



then emitted differ slightly from those corresponding to the 

 natural periods of the electrons. 



According to the model now favoured by the leading 

 physicists the atom consists essentially of two strongly contrasted 

 portions. There is a central system or nucleus surrounded 

 by an intense electric field, and it is in the nucleus that the 

 mass of the atom is mainly concentrated. Here, also, the 

 positive charge of the atom is situated. Only a few negative 

 electrons are present in the nucleus, and it is they which deter- 

 mine the column occupied by an element in the Periodic Table. 1 

 Around the nucleus is an outer shell of electrons, and it is to 

 the latter that chemical and all the common physical phenomena 

 are to be referred. Such phenomena are in general reversible, 

 because the atom may lose those electrons and regain them from 

 external sources. The above-mentioned experimental results, 

 from which it is deduced that the atom is affected by increase of 

 density and by the impact of ultra-violet waves, are concerned 

 only with the outer ring of electrons. They therefore leave the 

 question at issue untouched, for radioactivity is primarily con- 

 trolled by changes in the central nucleus. 



In the uranium atom the nucleus has, according to the theory 

 developed by Rutherford, a diameter of the order of i/io,oooth 

 of that of the whole atom. 2 For some reason this central 

 system becomes actively unstable and a component a-particle 

 or positively charged helium atom escapes. In passing through 

 the electric field, it rapidly gains kinetic energy, and finally is 

 violently expelled from the parent atom with a velocity of about 

 2 x io° cms. (12,000 miles) per second. The temperature equiva- 

 lent of this velocity in the case of helium is easily calculated to 

 be about 65,000,000,000° C. 3 This result, of course, is meaning- 

 less, but it serves a useful purpose in suggesting the intense 

 concentration of energy which is involved, and further, it 

 indicates that radioactive processes are controlled more by 

 electric than by thermal phenomena. Moreover, if pressure is 

 due to the translatory motions of molecules, it seems impossible 

 that the impulsive forces thus set up should ever prove equal to 

 the task of imposing inactivity upon a radioactive atom. 



It has been suggested as a possible effect of high pressure 



1 Soddy, Chemistry of the Radio Elements, part ii. p. 41, 1914. 



2 Rutherford, Phil. Mag., vol. xxi. p. 669, 191 1. 



3 Ramsay, Elements and Electrons, p. 149, 191 2. 



