RADIUM 109 



Rontgen rays in 20 to 30 minutes. Feistmantel (1902) found that irradia- 

 tion for 50 minutes of Actinomyces farcinica, exposed 10 cm. away from the 

 anti-cathode, had apparently no effect. Wyckoff (1930a, b) exposed Bad. coli 

 and Salm. typhi-murium on the surface of agar plates to soft X-rays, obtained 

 either from a tungsten tube operated at low voltage, or by the characteristic 

 K-radiation of copper. Destruction occurred semi-logarithmically, but less rapidly 

 than with cathode rays (see above). Thus after 20 seconds' exposure to filtered 

 copper radiation only 19-6-33-3 per cent, of organisms were dead. Analysis showed 

 that only about one in twenty of the absorbed quanta of these radiations proved 

 lethal. 



According to Wyckoff, the X-rays incident upon a cell either pass through 

 without altering it, or else give up one or more quanta whose energy content is 

 connected with the wave-length A of the rays through the relation 



E = /iv = /ij 



where h is Planck's constant, v is the frequency of the rays, and c is the velocity 

 of light. 



As the result of such an absorption a high-velocity electron is liberated. This 

 electron gives rise to a chain of ions in the matter through which it passes and 

 to X-rays which, in their turn, liberate more ions of less and less energy. The 

 changes caused by X-rays in protoplasm are naturally identified with the physico- 

 chemical changes induced by this ionic shower. The fact that only one in twenty 

 of the absorbed quanta proves fatal suggests that the vital elements capable of 

 being destroyed by a direct quantum hit occupy only about one-twentieth of the 

 cell volume. The harder the X-rays are, the nearer do they approach in their 

 killing effect to the cathode rays. More recent work with Bad. coli by Pugsley, 

 Oddie, and Eddie (1935) yielded results which seemed to show that, provided a 

 correction factor was introduced for lack of uniformity of the X-ray beam, the 

 organisms died in an exponential manner. Discussing th%se results, the authors 

 came to the conclusion that the one-quantum-hit-to-kill explanation, first put 

 forward by Crowther (1926), appeared to account most satisfactorily for the type 

 of curve obtained. 



This explanation is supported by the observations of Lea, Haines and Bretscher (1941). 

 Comparing the bactericidal action of X-rays, neutrons, and radioactive radiations, these 

 workers found that the effect of a given dose was independent of the temperature and o 

 the rate at which the radiation was applied, and that the mean lethal dose was correlated 

 with the ionization density of the radiation, being greatest for those radiations which 

 produced their ionizations closest together. These findings are explained on the assump- 

 tion that each organism contains a number of sensitive " targets," and that the destruction 

 of any one target is sufficient to impair the viability of the organism. Bad. coli is estimated 

 to contain about 1,000 targets, each of a diameter of 8-6 m^u, equivalent in size to a molecule 

 having a molecular weight of 2 x 10^. 



Sub-lethal doses of X-rays may bring about changes in the morphology and 

 growth characteristics of bacteria (see Levin and Lominski 1935, Forfota and Hamori 

 1937). It may be noted that bacteriophages and filtrable viruses have been shown 

 to be susceptible to soft X-rays (Wright and Kersten 1937, Moore and Kersten 1937). 



Radium. — Bruynoghe and Dubois (1925) found that exposure of Leptospira 

 iderohoBmorrhagicB for 26 hours to 8 mgm. of radium, enclosed in a platinum cell -g- mm. 

 in thickness, rendered the organism incapable of growing in vitro or of giving rise 



