IONIZATION AND BIOLOGICAL EFFECTS 121 



consideration, 2^ and of the quality of radiation as the factor which deter- 

 mines the number and relative lengths of the ionization loci. In many 

 instances one needs only the relative values of these data, while in others 

 absolute values are required. Unfortunately, this field of investigation 

 has received very little attention heretofore, from the point of view 

 presented here. The biologist, therefore, will have considerable difficulty 

 in finding the necessary data in the desired form. However, the effort 

 will be amply repaid not only by the result but by the grasp of the 

 fundamental principles of the subject which will be acquired through 

 such a search. 



REFERENCES 



1. HoLWECK, F., and A. Lacassagne. Action sur les levures des rayons-X mous 

 (K du fer). Compt. Rend. Soc. Biol. [Paris] 103: 60-62. 1930. 



2. LiND, S. C. The chemical effects of alpha particles and electrons. 2nd Ed. 

 252 pp. Chemical Catalog Company, Inc.; New York, 1928. 



3. QuiMBY, Edith H., and Helen R. Downes. A chemical method for the measure- 

 ment of quantity of radiation. Radiology 14 : 468-481. 1930. 



4. Thoraeus, Robert. A study of the ionization method for measuring the inten- 

 sity and absorption of roentgen rays and of the efficiency of different filters used in 

 therapy. Acta Radiol. Suppl. 15: 88. 1932. 



5. WooDARD, Helen Q. Factors influencing the decomposition of iodides by roentgen 

 and gamma rays. Amer. Jour. Roent. and Radium Ther. 33: 227. 1935. 



OTHER REFERENCE HANDBOOKS 



1. Bachem, a. Principles of X-ray and radium dosage. 274 pp. A. Bachem; 

 Chicago, 1923. 



2. Broglie, M. de. X-rays. (Transl. by J. R. Clarke.) E. P. Button & Com- 

 pany, Inc.; New York, 1926. 



2^ It has been tacitly assumed that the number of ions produced in one gram of 

 living tissue is the same as the number which would be produced in one gram of air 

 under the same conditions. The similarity in the atomic composition of air and living 

 tissues really allows us to conclude that substantially the same amount of energy is 

 abstracted from a beam of ordinary X-rays by the same mass of air or living tissue 

 irradiated under identical conditions. This energy makes its appearance as high- 

 speed electrons in both cases, and is then available to produce ions. In the case of air 

 it is known that the average amount of energy required to produce an ion pair in 

 this way is approximately 32 electron volts. Hence knowing the total amount of 

 energy abstracted from the radiation, one can calculate the number of ions which will 

 be produced in the air. On the other hand, the average energy required to produce 

 an ion pair in living tissue is not knowTi and, therefore, the number of ions produced in 

 the tissue by a certain known amount of energy, carried by the secondary electrons, 

 cannot be determined. Fortunately, however, for the same amount of radiant energy 

 absorbed by one gram of air or tissue, the relation between the respective numbers of 

 ions produced in the two materials is probably the same for all wave-lengths, and the 

 two are therefore related by a simple multiplying factor. In the absence of definite 

 information on this point, and as a first approximation, one may assume this multi- 

 plying factor to be one. 



