XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 



505 



this period is less than two units. Four scales of abscissae are given. 

 The uppermost is particle speed and is common to all particles. 

 The remainder are scales of particle energy. From a study of these 

 it is at once evident that the reason we customarily think of protons 

 as densel}^ ionizing particles and electrons as only producing a sparse 

 disti-ibution of ions is that an electron having even a few electron 

 kilovolts of energy is a much faster particle than the most energetic 

 protons and a particles usually encountered. 



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Reference to Table I, in which the mean linear ion density of all 

 the ionizing particles generated by each type of radiation is listed, 

 shows, however, that there can be overlapping as between electron 

 and proton ionization, making it possible to test the equivalence of 

 these two particles biologically. Experimental data required for such 

 a test are provided by the observations of Lea, Haines, and Bretscher 

 (24) and of Spear (25) on the lethal action of very soft X rays and 

 neutrons on vegetative forms of Escherichia coli and spores of Bacillus 

 mesentericus. The dose of a radiation required to inhibit colony for- 

 mation is about the same for the two organisms but the /3-ray dose is 

 nearly 28 times greater for the spores. When relative sensitivities 

 of the two organisms are plotted against ion density for a variety of 



