IONIZATION AND BIOLOGICAL EFFECTS 109 



quality determines what fraction of the total energy traversing a certain 

 layer of the material is immediately available to produce ions in the same 

 layer. 1^ Theoretically, then, knowing the intensity and the quahty of 

 the radiation at a given point in the material, one would know also the 

 rate at which ions are being produced per unit volume. The quantita- 

 tive relationships involved, however, are such as to preclude their utiliza- 

 tion in the great majority of practical cases. It is almost impossible at 

 the present time to measure with reasonable accuracy the intensity of a 

 beam of hard X-rays in terms of the energy flow per unit area. It is 

 equally difficult to determine the exact quality of radiation at every 

 point of the material (the reasons are of no particular interest to the 

 biologist) . However, many of the practical difficulties may be overcome 

 by a tour de force. 



The argument involved is as follows : As already explained, ionization 

 cannot be measured directly in Hving tissues. Fortunately, however, 

 the interaction of radiation and matter depends (almost) entirely on the 

 atomic numbers of the elements and the respective proportions in which 

 they are present in the material, and not on their state of aggregation. 

 Since the atomic number of hydrogen is 1, its contribution to the ioniza- 

 tion in a living organism is very small. Practically all the ionization is 

 due therefore to the oxygen, nitrogen, and carbon components. Atmos- 

 pheric air, consisting largely of nitrogen and oxygen, is a close approxi- 

 mation to living tissue, insofar as its quantitative reaction with radiation 

 is concerned. Being a gas, the ionization which is produced therein can 

 be measured readily. By virtue of this atomic relation, the number of 

 ions produced in 1 gm. of living matter is substantially the same as the 

 number of ions liberated in 1 gm. of air, under the same conditions of 

 irradiation. Furthermore, the one-to-one correspondence is (approxi- 

 mately) independent of the quahty of radiation. Hence, by measuring 

 ionization in air under suitable conditions, one can determine the ionization 

 produced in a living organism, without a quantitative knowledge of the 

 true (vide infra) intensity of radiation, the quality," and the laws of 

 "absorption" of radiation by organic matter. It should not be assumed, 

 however, that all difficulties have been eliminated. For, the determina- 

 tion of the "suitable conditions" alluded to above, presents a problem of 

 the first magnitude, which has not yet been solved completely. 



1^ The usual presentation of this subject is based on the concept of "absorption" 

 of energy by the material. It is then said that the part of the radiation which is 

 available to produce ions, chemical and biological changes, is that which is absorbed. 

 The fundamental concept is the same in the foregoing discussion, but the writer has 

 purposely avoided the use of the term absorption (in fact, throughout the paper), 

 to circumvent certain difficulties which would complicate the presentation materially. 



1' A knowledge of the quality of radiation is necessary, of course, in order to inter- 

 pret some biological effects in terms of the ionization loci. 



