92 Applied Biophysics 



radiation dose depends on the concentration of activated solvent 

 produced (not on the concentration of the sokite) and will, 

 therefore, be relatively small, the concentrations of solute em- 

 ployed must be the smallest consistent with chemical analysis, 

 in order that changes in them may be relatively large. It was 

 the widespread failure to recognize this which led to the supposi- 

 tion that significant chemical changes could not be produced 

 ill vitro by doses within the therapeutic range. For the simplest 

 case, i.e., only one substance in solution, the activation theory 

 would seem a reasonable interpretation of observed facts. 



Dale has recently described some striking experiments in which 

 an apparent loss of radiosensitivity occurs when enzymes are 

 irradiated in the presence of varios protein and other substrates 

 which share the available energy between them and thus "screen" 

 the original solute.-- This work on the protection of one solute 

 by another is a valuable contribution to the interpretation of the 

 chemical effects of radiation in vivo. If the indirect-action theory 

 is applicable under these conditions, then a new light may be 

 thrown on the mechanism of action of radiations. From the 

 point of view of a solute, e.g., an enzyme, its inactivation by 

 energy carriers derived from molecules of aqueous solvent could 

 be regarded as the target theory in reverse ! The possibility of 

 this mechanism operating in vivo, if only under certain condi- 

 tions of dilution, is a further caution against making any gen- 

 eralization prematurely. 



Whether "activated water" is also connected with such physico- 

 chemical effects as the precipitation of positively charged colloids, 

 viscosity changes, and change of electrokinetic potentials remains 

 to be seen.^^ It seems more likely that the physicochemical effects 

 are produced by simple ions.^ 



Biological Indicators 



From time to time, investigators have sought for a simpler 

 biological material with a more definite and convenient reaction 

 than the skin erythema to serve as a biological dose unit. When 

 the irradiated tissue is very small, such as the egg of an insect, 



