XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 515 



particular atoms ionized are picked out at random but the numbers of 

 each kind are roughly proportional to the proportion by weight of this 

 element in the medium. 



The figures for the actual number of atoms that would be ionized 

 by a 1 m.e.v. proton crossing a chromatid thread, regarded as a fila- 

 ment of hydrated nucleoprotein 0.1 /x in diameter, are: H, 14; C, 

 49; N, 16; 0, 25; and P, 1, respectively. A sulfur atom would be 

 ionized on an average once in every two transits. The location of 

 the primary positive ions may, therefore, be specified rather pre- 

 cisely. We are much less well informed about the negative ions. 



Wilson chamber photographs by Klemperer (38) show that in 

 moist hydrogen gas the negative ions, formed by the attachment of 

 the ejected electrons after they have been brought to rest, are situ- 

 ated at a distance equivalent to about 15 m/j. of tissue from the posi- 

 tive ions. Jaffe, from electrical measurements in compressed air 

 and in liquid hexane, arrived at comparable figures for the mean dis- 

 tance separating positive and negative ions in each of these media. 

 No data are available for water but it seems likely that it is at least 

 on the order of magnitude of 15 m/x. 



It is important not to overlook the fact that the picture presented 

 of the location of the ions formed by the various types of particle 

 refers only to the state of affairs at the instant of ion formation. 

 Many facts, such as, for example, the reversal by a second irradiation 

 of a gene mutation, itself produced by irradiation, suggest that either 

 the potential energy acquired by a molecule when it is ionized, or 

 the ion itself, can travel considerable distances within a macromole- 

 cule. The movement of such energy would certainly repay careful 

 experimental investigation. 



When the molecules ionized are small, such as the water molecule, 

 and are formed free in an aqueous medium, their subsequent move- 

 ments up to the moment of neutralization will be the result of electri- 

 cal attractions and repulsions, and of diffusion. Since, from the 

 point of view of the interpretation of the biological effects of radia- 

 tion, we are mainly interested in aqueous solutions, it may well be 

 found necessary to take account, also, of the spontaneous ionization 

 of the medium and the electrical condition at the surface of the larger 

 solute molecules, whose inactivation is of special interest. 



The inactivation of pure solutes in aqueous solution has revealed 

 a number of striking differences between the radiochemical yield ob- 

 tained with j8, 7, or X radiation on the one hand and protons or a 



