1. For the two motile bacilli, B. coli and B. aertryke, the absorption of a single 155 K.V. electron is sufficient to cause death. Furthermore, all, or nearly all, the electrons absorbed are lethal. The same is undoubtedly true of Staphylococcus aureus. In addition to providing a quantitative picture of the interaction of bacteria and cathode rays, these results suggest that radiation of the energy content used in our experiments is not suitable for altering the inheritable characteristics of bacteria.

2. The differences in sensitivity to cathode rays shown by the bacteria studied can be explained by the purely physical factor of size.

3. Counts giving significant conclusions concerning killing rates can be obtained only if there is no clumping of the cells when spread and only if the cells are not allowed to multiply before irradiation. Both these precautions seem rarely to have been met in the experiments that have in the past been made with X-rays and other forms of radiation.

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