180 Applied Biophysics 



directly, would serve. Obviously, it is desirable to choose as a 

 measure that physical quantity which stands in the closest 

 relationship to the biological effects produced by the radiation. 

 By making a shrewd choice in this matter, the interrelation of 

 physical cause and biological effect will not be obscured by a 

 long chain of essentially irrelevant intermediate processes. 



There is general agreement that the key quantity is the ioniza- 

 tion produced in the biological substance. With a few exceptions, 

 however, it has for technical reasons proved quite impracticable 

 to measure the actual ionization in a solid or liquid, but a 

 quantity which is almost as acceptable as ionization, as a 

 measure of the radiation, is the energy communicated to the 

 medium. The reason for this is that the proportion of this 

 energy which goes to the production of ionization is probably 

 independent of the quality of the radiation — this is certainly 

 almost exactly true for air, where about half the energy goes 

 to the production of ionization, the rest being expended in 

 excitation, and thus the ionization is known apart from a 

 constant of proportionality characteristic of the medium. In one 

 of the very few investigations of a liquid, in this case carbon 

 disulphide, Taylor has shown that the proportion of energy ex- 

 pended in ionization is not greatly different from that expended 

 for air. In actual fact, however, the direct measurement of the 

 energy communicated to the medium is also well-nigh impossible 

 because of the minute amount required even for the most extreme 

 biological effects. We shall see later how it is possible to derive 

 this energy from other measurements. 



The Rbntgen 



With these general ideas in mind, it is easy to see why, in 

 actual historical fact, the ionization produced in air came to be 

 adopted as a measure of radiation, partly as a matter of ex- 

 pediency on account of the relatively simple technical problems, 

 and partly because it was realized that, on account of the 

 general similarity of the atomic types in air and tissue, the 

 energy conversion of X- and gamma radiation in these two 



