PROCESSES OTHER THAN SIMPLE 99 



especially in biological systems, the latter being invariably composed of 

 molecular species of more or less great complexity. 



Much the same difficulty, indeed, also underlies the interpretation of 

 the effects of light on all but the simplest gaseous systems, and this 

 fact has inhibited serious study of such effects in complicated systems, 

 particularly those in the liquid state. For this reason trustworthy infor- 

 mation available from photochemistry is, for complex systems, regretta- 

 bly meager. Hence the important task of interpreting results of radi- 

 ation effects — that is, of analyzing the extremely intricate stages be- 

 tween initial absorption acts and ultimate chemical or biological trans- 

 formation — must usually proceed with inadequate aid (from physics, on 

 the nature and distribution of the primary products, and from chemistry, 

 on their interactions), and is usually exceedingly difficult. In all too 

 many instances in which interpretations have been advanced they in- 

 volve suspicious radicals or ions, endowed with mainly ad hoc properties, 

 which effect remarkably convenient actions under almost no discipli- 

 nary control except perhaps an occasional admonition from the laws of 

 thermodynamics. 



Despite this discouraging basis, some success has been achieved in the 

 understanding of at least a few of the chemical effects of radiation, and 

 notable progress in the field is now being made. It is even possible to 

 understand some of the less intricate reactions in complex systems — at 

 least semiquantitatively, if not in the same satisfying detail achieved by 

 Eyring, Hirschfelder, and Taylor in their uniquely important studies of 

 several gaseous systems some fourteen years ago. Indeed, some striking 

 inferences may often be drawn from an analysis which penetrates no 

 more deeply into the nature of the primary effects than simply to dis- 

 tinguish between excitations and ionizations. These matters are dis- 

 cussed elsewhere in this volume. We may merely note here the familiar 

 conclusion that the experimental evidence thus far interpreted appears 

 to support the identification of excitation and ionization acts with the 

 predominating primary processes. 



Primary Processes Other Than Simple Excitation 

 AND Ionization 



Nevertheless, it would be incautious at the present time to presume 

 that chemical and biological effects of high-energy radiation are caused 

 exclusively by isolated excitation or ionization events in which energy 

 transfers lie in the neighborhood of 10 ev. Whereas these primary proc- 

 esses are surely the ordinarily dominant ones for media composed of 

 simple molecules, complex molecules of high molecular weight may well 



