IONIZATION AND BIOLOGICAL EFFECTS 89 



the way. Furthermore, when a positive ion is formed, the electron which 

 leaves the atom or molecule attaches itself to any atom or molecule which 

 happens to be in its path. There is some evidence also that ions are apt 

 to form clusters with neutral atoms or molecules. The final result of 

 all these chance encounters is that some atoms or molecules are brought 

 together under conditions which favor a regrouping of the atoms, and 

 consequently chemical changes take place. A great many chemical 

 changes produced by ionizing radiations have been observed (2). The 

 change may be a breaking up of complex molecules into simpler ones, or 

 vice versa, depending on the conditions of the experiment. But, in 

 general, the tendency is to transform complex molecules into simpler 

 compounds or elements. Thus volatile hydrocarbons and carbon 

 dioxide are liberated from paraffin under the action of very intense radia- 

 tion. Water is decomposed into hydrogen and oxygen, but at the same 

 time some hydrogen peroxide is formed. Organic compounds with 

 complex molecules are affected more readily. 



It should be emphasized at this point that the chemical changes 

 brought about by ionizing radiations are always very slight when the 

 commonly available sources are employed. It has already been men- 

 tioned that only a small proportion of the atoms in an irradiated material 

 are ionized. Of the ions thus formed, only some make subsequent 

 encounters which fulfil the conditions conducive to chemical changes. 

 On the other hand, with powerful beams of three-million-volt cathode 

 rays,^ very marked alterations have been produced in substances which 

 are influenced only slightly by even long exposure to ordinary sources 

 of radiation. 



The biological action of radiation may be accounted for by the 

 chemical effects which radiation produces. The sequence of events may 

 be assumed to be : (a) ionization, (6) chemical changes, and (c) biological 

 changes. There may then be further chemical changes as a consequence 

 of the first biological changes, and so forth. This view of the process 

 accounts, at least to some extent, for the delay (latent period) in the 

 appearance of the effects of radiation on living organisms. The fore- 

 going "picture" of the biological action of radiation is very rudimentary, 

 and it is no more than a statement of certain steps in the process which 

 we either know or surmise. It does not provide the modus operandi 

 necessary for the proper interpretation and correlation of the known 

 biological effects of ionizing radiations. 



Whatever the mechanism of the biological action of radiation may be, 

 we know definitely that ions are produced in the material and that the 



1 The cathode-ray effects referred to in this sentence are due in part to the heat 

 liberated when the electrons are stopped. (Personal communication from A. Brasch 

 and Fritz Lange.) 



