116 Applied Biophysics 



the intercomparison of ionizing radiations in general. The beta 



ravs are electrons, i.e., particles having — -- of the mass of a 



^ 1850 



hydrogen atom and carrying unit negative charge, while the 

 alpha particles are helium nuclei having 4 times the mass of the 

 hydrogen atom and carrying two positive charges. Since it was 

 the negatively-charged globulin molecules which were discharged 

 in Hardy's experiments, the effect was at first attributed to the 

 neutralizing action of the positive charge caused by the alpha 

 particles. This now appears in the highest degree improbable.* 

 All the chemical and biological effects ^^o far studied are refer- 

 able to the excitation and ionization of the molecules in the path 

 of the ionizing particle, and it would be impossible to say of any 

 individual excited or ionized molecule whether it had been 

 produced by an electron or an alpha particle. 



The essential difference between the two rays lies in the num- 

 ber and distribution in space of the ions and excited molecules 

 which they produce. Tn the second place, it is important to 

 notice that while the beta and alpha particles emitted by naked 

 radium are comparable in numbers, the beta rays have initially 

 an average energy of about a million volts, which is gradually 

 transformed into ionization and excitation throughout a total 

 path of several millimeters of water or tissue, whereas the 6 

 million volts initial energy of an alpha particle is dissipated in 



less than — millimeter. Within the — ■ miUimeter immediately 

 20 20 



surrounding the radium, the total numl)er of ions formed by the 



alpha rays may therefore be several hundred times as great as 



that produced by the beta rays, and it is not surprising on this 



ground alone that the alpha rays appeared very much more 



effective. 



We shall discuss in detail only experiments in which the 



total number of ions formed by the radiation per unit volume 



* In somewhat analoprous experiments with colloidal graphite, Cray, Read and 

 Liebmann " observed that similar changes in the charged condition of the particles 

 were produced by negatively-charged electrons and positively-charged protons. The 

 two radiations differed only in their numerical efficiency. 



