500 L. H. G R A Y 



understanding of the genetic control of biochemical processes. X 

 radiation is convenient for producing these mutations, but it is only 

 one among a number of agents including ultraviolet light, the ordi- 

 nary mustards, the nitrogen mustards, and other chemicals. Al- 

 though ultraviolet radiation generally produces more mutations than 

 an equally lethal dose of X radiation it is of the two a less quantitative 

 agent on account of the fact that the absorption of ultraviolet energy 

 depends critically on chemical structure. It is usually impossible to 

 estimate at all accurately the amount of ultraviolet energy that must 

 be absorbed per cubic micron of the material of the gene in order to 

 produce a mutation. Ionizing radiations, on the other hand, dissi- 

 pate an amount of energy per unit mass that is almost independent 

 of chemical structure. Thus, X-ray experiments of Demerec and 

 Latarjet show that 2.5 X 10^° ergs per gram, of energy in the form of 

 ionization, is needed to produce the mutation that leads to bacterio- 

 phage resistance in E. coli. Since at least ten times this amount of 

 ultraviolet energy is required, and probably much more, even at wave- 

 lengths near the peak of mutagenic efficiency, it seems safe to conclude 

 in this case at least that ionization has a much higher mutagenic ef- 

 ficiency than excitation. Since, however, the efficiency of ionizing 

 radiation relative to ultraviolet light for killing the organisms is still 

 greater, a larger proportion of survivors carrying mutations can gener- 

 ally be obtained by the use of ultraviolet light. 



We shall endeavor to make clear in subsequent sections how the 

 use of several different ionizing radiations, for example, fast electrons 

 and a particles, in addition to X rays, can throw light on the question 

 of the size and disposition of the material that has to be transformed — 

 questions that cannot be answered if only one ionizing radiation is 

 employed. Lea and Salaman {21) by this means have been led to dis- 

 tinguish bacteriophages such as S 13, which behave as macromole- 

 cules, from others such as C 36 and Staphylococcus K, in which the 

 genetic material appears to occupy only part of the whole virus, and 

 to be distributed, thus resembling, though on a simpler scale, the 

 structure they had previously postulated for vaccinia virus. 



B. PHYSICAL CHARACTERISTICS OF DIFFERENT 



IONIZING RADIATIONS AND THEIR RELATION TO 



EFFECTS PRODUCED IN LIVING CELLS 



1. Available Sources of Ionizing Radiation 



Table I (p. 502) lists some seventeen different sources of ionizing 

 radiation, all of which have been employed in radiobiological investi- 



