IONIZATION AND BIOLOGICAL EFFECTS 107 



generally employed for determining the ionization in biological materials. 

 The term ionization as used in the preceding paragraph is quite 

 indefinite. We must state, therefore, just what it is that we wish to 

 know about the ionization produced in a living organism. Ions are liber- 

 ated in the material during the entire time that it is exposed to radiation. 

 In this time a definite number of ion pairs will have been produced, which 

 represents the total amount of energy transferred to the material by the 

 radiation. Remembering that it takes energy to produce any change, we 

 may readily conclude that one of the factors we wish to know is the 

 total number of ion pairs set free. The same number of ions may be 

 distributed through a small or large volume, and obviously the effect 

 on the individual living cells will be different. Hence it is important to 

 know the number of ion pairs per unit volume liberated during the 

 exposure to radiation. We must know this for every point in the material 

 since, as already stated, the distribution of radiation is seldom strictly 

 uniform. Having this information it is possible to correlate the observed 

 biological change or changes with the amount of energy, in the form of 

 ionization, supphed to the hving cells. If (direct or indirect) means are 

 available for the measurement of the biological changes, the correlation 

 can be quantitative. Investigations of this type provide the fundamental 

 data essential to the development of the science. The next step is the 

 formulation of generalizations. If pursued far enough this will lead to 

 the study of the mechanism by which the ionization produces the bio- 

 logical changes. Here the fact that the ions are concentrated along what 

 we have called ionization loci cannot be ignored. This is so whether one 

 undertakes to develop a theory on the basis of "direct hits" of a hypo- 

 thetical sensitive center of the cell (1) or otherwise. For, even on the 

 simple assumption that the chemical constitution of the medium is altered 

 somewhat, the degree of change may depend on the concentration of ions 

 in the loci (due to recombination or other factors). Then almost cer- 

 tainly the rate of diffusion of the chemically altered substances from the 

 loci outward, must play some part in the biological effectiveness of the 

 ionization. Therefore one should know also the distribution of the ioniza- 

 tion loci and the ion concentration therein. 



The discussion of the phase of the subject given in this chapter, 

 provides the necessary qualitative data. In order to get the quantitative 

 data applicable to any particular practical case, the biologist is referred 

 to some of the textbooks listed at the end of the chapter. ^^ The desired 

 information is not readily available, since little attention has been paid 

 to it in the past, from the biologist's viewpoint. However, enough is 



>* Especially "Radiations from Radioactive Substances," by Rutherford, Chad- 

 wick, and Ellis. Many references to original papers will be found there. 



